soil organic matter in restored rangelands following cessation of rainfed cropping in a mountainous...

ORIGINAL ARTICLE

Soil organic matter in restored rangelands followingcessation of rainfed cropping in a mountainous semi-aridlandscape

Soroosh Salek-Gilani • Fayez Raiesi •

Pejman Tahmasebi • Najmeh Ghorbani

Received: 5 August 2012 / Accepted: 28 September 2013 / Published online: 11 October 2013

� Springer Science+Business Media Dordrecht 2013

Abstract Agricultural abandonment is known to

influence plant cover composition and C inputs into

the soil with a consequence for changes in soil organic

matter (SOM) storage and dynamics in rangeland

ecosystems. This study was conducted on a chrono-

sequence of high altitude rangelands (1) cultivated

with rainfed wheat (CR0), (2) abandoned for 4 (AR4),

12 (AR12) and 45 (AR45) years and (3) uncultivated

(reference) rangelands (UR) with three replicates in

Zagros Mountains, Central Iran. We studied the

changes in the concentrations and stocks of bulk soil

organic carbon (OC), total N, particulate organic C

(POC) and N (PON), dissolved organic C (DOC),

microbial biomass C (MBC), and potentially miner-

alizable C (Min-C) at 0–0.15 and 0.15–0.3 m soil

depths. Results showed that the concentrations and

stocks of OC, N, and labile fractions increased with

the abandonment of agriculture at both soil sampling

depths. After 4–45 years of agricultural abandonment,

soil OC and N stocks increased logarithmically by

3.8–46 % and 2.8–32 % in the whole 0–0.3 m,

respectively. Although, the stocks of labile fractions

decreased slightly 4 years after agricultural abandon-

ment, there were considerable increases (logarithmic)

in these fractions after 12–45 years of abandonment

(POC, 65–148 %; PON, 68–147 %; DOC, 76–139 %;

MBC, 24–62 %). The study shows that rangelands

abandoned for 45 years contained lower soil OC and

N concentrations and stocks compared to uncultivated

rangelands, reflecting 45 years of abandonment would

not be sufficient for SOM to attain the level of

uncultivated rangelands. The present study provided

evidence that the rate of increases in POC and DOC

stocks was greater than that of OC and MBC stocks,

demonstrating POC and DOC fractions of total SOM

pool may be suitable and sensitive indicators for

detecting the effects of agricultural abandonment on

soil OC changes and storage in these restored semi-

arid rangelands. Soil bulk density decreased, while the

mean weight diameter (MWD) and aggregate ratio as

measures of aggregate stability increased consider-

ably within the abandoned rangelands with increasing

time of abandonment. Results from a multivariate

analysis suggested that soil variables such as bulk

density, OC, TN, DOC, POC, PON, MBC, MWD and

metabolic quotient (qCO2) were successful in sepa-

rating land uses. In brief, the abandonment of

agricultural activities in previously cultivated high

altitude rangelands can potentially lead to an increase

of total and labile SOM and also sequestration of C in

these semi-arid rangelands.

S. Salek-Gilani (&) � F. Raiesi � N. Ghorbani

Soil Science Department, Faculty of Agriculture,

Shahrekord University, P.O. Box 115, Shahrekord, Iran

e-mail: [email protected]

P. Tahmasebi

Range and Watershed Department, Faculty of Natural

Resources and Earth Science, Shahrekord University,

P.O. Box 115, Shahrekord, Iran

123

Nutr Cycl Agroecosyst (2013) 96:215–232

DOI 10.1007/s10705-013-9587-4

Keywords SOM storage � Labile C fractions �C sequestration � Rainfed cropping � High

altitude rangelands

Introduction

Understanding the storage and dynamics of soil

organic matter, especially in relation to changing land

use, is fundamental to evaluate the role of soil as a

carbon (C) source or sink (Bruce et al. 1999; Lal

2004). Agricultural practices following rangeland

conversion to cropland would lead to a decrease in C

stored in soils and a net release of C into the

atmosphere, which has strongly influenced the atmo-

spheric CO2 levels and global C balances over the last

centuries (Jinbo et al. 2007; Raiesi 2007; Lal 2008;

Qiu et al. 2012). Nevertheless, agricultural abandon-

ment and re-conversion of croplands to rangelands

might be an alternative practice and management for

restoring rangeland soil conditions and SOM (Post and

Kwon 2000; Templer et al. 2005; Hoshino et al. 2009;

Raiesi 2012a, b).

Land abandonment and subsequent cessation of

agricultural practices occurs worldwide, largely due to

the low soil productivity, the development of indus-

trialization and tourism, and the shortage of water as a

result of drought, particularly in arid and semi-arid

climates (Zhao et al. 2005; Raiesi 2012a, b). There is

evidence that the abandonment of agriculture in the

long-run and the subsequent recovery of natural

vegetation may return soil organic carbon (OC) and

nitrogen (N) storage to the pre-agricultural levels (Post

and Kwon 2000; Templer et al. 2005). Increases of

SOM following the abandonment of cultivation could

be attributed to increased plant productivity and

subsequently higher organic inputs into soil, and to

the formation of macro aggregates, resulting in lower

mineralization of labile SOM (Lopez-Bermudez et al.

1996; Rey Benayas et al. 2007; Hoshino et al. 2009;

Preger et al. 2010; Zhang et al. 2010; Raiesi 2012a, b).

Changes that occur in vegetation cover and environ-

mental conditions after land abandonment may have

an important effect on soil OC and N stocks, microbial

activity, biomass and diversity (Zhao et al. 2005; Rey

Benayas et al. 2007; Preger et al. 2010; Zhang et al.

2010; Raiesi 2012a, b).

Bulk soil OC is heterogeneous and contains labile

fractions with a rapid turnover rate as well as non-

labile fractions with a slower turnover rate (Cambard-

ella and Elliott 1993; Jastrow 1996; Leifeld and

Kogel-Knabner 2005; Zhang et al. 2007). The labile

fractions of soil OC consists of microbial biomass C

(MBC), dissolved organic C (DOC), particulate

organic C (POC) and mineralizable C (Min-C) that

respond rapidly to changes in C supply and to land use

changes, cultivation and abandonment (Jastrow 1996;

Lundquist et al. 1999; Zhang et al. 2006; 2007; Li et al.

2009a, b). These fractions have therefore been

suggested as early indicators of the effects of land

use changes and agricultural practices on SOM pool

and dynamics (Cambardella and Elliott 1993; Leifeld

and Kogel-Knabner 2005; Jinbo et al. 2007), and in

general on soil quality (Gregorich et al. 1994). There

has been an increasing interest in the importance of

labile C fractions as the indicator of changes in soil OC

storage and soil quality (Saggar et al. 2001).

Abandonment of agricultural fields in native range-

lands during the past five decades has been the main

interest for the restoration of rangeland soils in some

arid and semi-arid regions of the world. Agricultural

abandonment, especially rainfed cropping, in these

rangeland ecosystems occurs mainly due to low

rainfall and water shortage and subsequent low

income from rainfed cropping (Rey Benayas et al.

2007; Raiesi 2012a, b). Compared to the study of

effects on vegetation restoration and C storage and

dynamics caused by agricultural abandonment at low

to medium altitude rangelands in arid and semi-arid

regions, few studies have focused on the abandonment

of agriculture in high altitude rangelands of arid and

semi-arid regions of the world (i.e., Raiesi 2012a, b).

This work was designed to test how the agricultural

abandonment in mountainous semi-arid rangelands

affects soil properties and vegetation establishment.

Such information may be useful to assess the resil-

ience of soils and to determine the appropriate time for

soil recovery. The objectives of the present study were

to investigate (1) the effects of different times of

rainfed cropping abandonment on the living plant

above-ground biomass and vegetation restoration, (2)

the effect of natural vegetation recovery on OC, N and

labile C fractions (i.e., DOC, POC, MBC, mineraliz-

able C) along a secondary plant succession, (3) the

most sensitive labile C fractions for detecting changes

in soil total OC following the abandonment of

cultivation and (4) the consequence of changes in

SOM for C sequestration in high altitude rangeland

216 Nutr Cycl Agroecosyst (2013) 96:215–232

123

soils along a chronosequence of cultivation abandon-

ment. To achieve these objectives, we measured bulk

soil OC, N, DOC, POC, MBC and Min-C levels in a

chronosequence of abandoned rangelands in a moun-

tainous semi-arid area of Central Iran.

Materials and methods

Research area

The research area is located in semi-steppe rangelands

of Karsanak region in Chaharmahal and Bakhtiari

Province, Central Iran. The study area (580 ha) is

located about 60 km northwest of Shahrekord city, at

approximately 32� 310 N and 50� 280 E at an altitude of

around 2,574 m above sea level. The climate of the

region is semi-arid with annual mean precipitation of

420 mm, most of which falls during winter and spring.

The annual average temperature is 12 �C with the

average minimum of 1.8 �C and the average maxi-

mum of 21 �C. In general, the soil of the study area is a

silty loam Typic Haploxerept. In the area, landscapes

are mosaic of native grazing (uncultivated) rangelands

(consist of three major vegetation types including

Astragalus adscendense, Agropyron repense and

Bromus tomentelus–A. repense), cultivated and aban-

doned rangelands. Rainfed cropping systems, espe-

cially wheat (Triticum aestivum L.) in grazing

rangelands over the past were common and wheat

cultivation still is continuing at the present time. Over

the last five decades, land abandonment and subse-

quent cessation of agricultural activities occurred in

these rangeland ecosystems largely due to the low

productivity of wheat, the gradual immigration of

local farmers and most likely the shortage of water as a

result of recent droughts and low rainfall. We found

that, during 40 years, many lands of the research area

have been gradually converted to national lands by the

government and the government does not allow

farmers to cultivate and only part of the region was

allocated to agricultural lands. In the area, agricultural

abandonment has promoted woody plant encroach-

ment and expansion during the secondary succession

and thus dense perennial woody and non-woody

species (including shrubs, sub-shrubs, forbs and

legumes) are abundant. We selected those abandoned

and cultivated rangelands with well-known land use

history to assure similar soil conditions and properties

before abandonment and comparable cultivation prac-

tices after land abandonment. Based on data and

information on land use history and current manage-

ment practices obtained from local farmers and sheep

owners, historical records and aerial photos, we made

sure that all abandoned and cultivated rangelands

experienced similar tillage and cultivation practices

(conventional tillage) in the years prior to abandon-

ment time, and they were used for many decades

([100 years) as croplands. Five different land uses

were selected in a long-term chronosequence of

cultivated (CR), abandoned (AR) and uncultivated

(UR) rangelands. They consisted of (1) permanently

cultivated rangelands (CR0) with rainfed wheat crop-

ping with 1 year in cultivation followed by 1–2 years

in fallow, (2) rangelands abandoned for 3–4 years

(AR4), (3) rangelands abandoned for 10–12 years

(AR12), (4) rangeland abandoned for 30–45 years

(AR45) and (5) permanent rangelands uncultivated for

more than 100 years (UR100) as a reference site. The

maximum time of abandonment was considered for

each land use and data analysis. In selecting these land

uses, care was made for the consistency in topograph-

ical features. Soil texture in cultivated rangelands was

silt loam, while abandoned and uncultivated range-

lands had a silt loam or loam soil texture.

Plant biomass, soil sampling and analysis

The experiment was a completely randomized design

with five land uses (i.e., CR0, AR4, AR12, AR45 and

UR100), each replicated three times (n = 3). Each

replicated land use covered a 50 m 9 50 m plot.

Spatially separated rangelands were located within a

distance of 1–3 km that helped avoiding pseudo-

replication. Total above-ground plant biomass

(TAGB) was measured as the sum of above-ground

plant tissues (leaves?stems) harvested for most plant

functional groups. However, large shrubs such as

Astragalus sp. were not sampled due to the difficulty

with harvesting. In brief, a total of five 1 m 9 1 m

sampling quadrates were randomly installed in each

replicated land use (plot) and living above-ground

tissues were harvested by clipping at soil surface at

peak standing biomass in May, oven-dried at 60 �C for

24 h and weighed. The harvested plant tissues (i.e.,

TAGB) were separated into three functional groups

including annual grasses and forbs (AGF), perennial

grasses and forbs (PGF) and small shrubs; and

Nutr Cycl Agroecosyst (2013) 96:215–232 217

123

weighed again for above-ground biomass. For each

plot, plant biomass was estimated based on the

average value of the five quadrates sampled. We used

these values for plant productivity, a fraction of which

annually enters soil as detritus. Dead litter left on the

soil surface from previous growing seasons was

removed before soil sampling and not considered as

organic input. A total of six individual soil samples

were randomly taken at two soil depths of 0–0.15 and

0.15–0.3 m layers in each replicated plot, and mixed

thoroughly to obtain a composite soil sample. In total,

30 composite samples were collected, including 5 land

uses, 2 soil depths and 3 replicates (plots). One portion

of soil samples was air-dried and passed through a 2-

mm sieve and stored before laboratory analysis. Plant

fragments and visible rock fragments larger than

2 mm were removed by hand. The second portion of

soil samples was air-dried and passed through a 4-mm

sieve for aggregate stability analysis. Any visible

stone and plant debris in soil samples sieved to 4 mm

were separated manually and discarded. Soil pH (1:2.5

soil-to-water ratio), carbonate calcium equivalent

(CCE) or CaCO3 content (the titration method), bulk

density (the clod method), total organic C (OC) (the

Walkley & Black method) and total N (N) (the

Kjeldahl method) were all determined following

procedures described in Carter and Gregorich

(2008). Total porosity was calculated assuming a

particle density of 2.65 g cm-3 (Beylich et al. 2010).

All reported values are expressed on oven-dry soil

weight basis (105 �C). Furthermore, OC, N and labile

C fractions stocks (Mg ha-1) for each soil layer were

calculated considering the minimum equivalent soil

mass (ESM) approach instead of simple bulk density

method (Lee et al. 2009; Raiesi 2012b). The lowest

soil mass was considered the amount present in

uncultivated rangelands (2,130 and 2,160 Mg ha-1

at 0–0.15 and 0.15–0.30 m depths, respectively). The

OC, N and labile C fractions concentrations (g kg-1)

were multiplied by the corresponding equivalent soil

mass to obtain the stocks (Mg ha-1). The stocks of

total OC, N and labile C fractions were also calculated

for the whole 0.3 m depth by the sum of stocks at each

sampling depth within the soil profile. Soil aggregate

stability was determined by the mean weight diameter

(MWD) and the aggregate ratio (AR) using the wet

sieving method described in Kemper and Rosenau

(1986). In brief, 100 g of air-dried soil (4 mm) were

wetted using a spraying bottle and soaked in distilled

water for 5 min to allow slaking. The soil samples

were submerged in water and separated by manually

moving (0.03 m up and down) a nest of sieves with

openings 2, 0.25 and 0.053 mm for 2 min at a

frequency of 50 times (Six et al. 2000). After sieving

the soils remained in the basin left undisturbed for

5 min to allow fine particles to settle. Sieves were

slowly removed from the basin and paced onto a catch

pan to collect any remaining water. All the fractions of

aggregates were collected and transferred to pre-

weighed aluminum thins, oven-dried at 50 �C and

weighed. Additionally, sand content ([53 lm) of the

aggregates was determined by soaking of stable

aggregates ([53 lm) in distilled water and crushing

using a forefinger in a bowl, followed by sieving to

obtain the sand particles for correcting the weight of

water stable aggregates in each fraction (Six et al.

2000). Each aggregate analysis was done in two

replicates. The mean weight diameter (MWD), was

calculated as

MWD mmð Þ ¼Xn

i¼1

XiWi

Where Xi is the mean diameter of the soil aggregate

size (mm) fractions i and Wi is the proportion of each

size fraction over the total sample weight (Kemper and

Rosenau 1986). Macro-aggregates were defined as

[250 lm size fractions and micro-aggregates were

defined as\250 lm size fractions. The aggregate ratio

(AR), was calculated as

AR ¼ Macro� aggregates ð%ÞMicro� aggregates ð%Þ

Where percentage of macro-aggregates is the sum

of soil aggregate-size fractions larger than 250 lm and

percentage of micro-aggregates is the sum of soil

aggregate-size fractions smaller than 250 lm (Baker

et al. 2004).

For dissolved organic carbon (DOC) measurement,

soil sub-samples (equivalent to 10 g oven-dried

weight) were weighed into 50-ml polypropylene

centrifuge tubes. The samples were extracted with

30 ml of distilled water for 30 min on an end-over-end

shaker at approximately 230 rpm and centrifuged for

20 min at 8,000 rpm. The supernatant was filtered

through a 0.45-lm filter into separate vials for C

analysis (Ghani et al. 2003; Zhang et al. 2007).

Organic C in the extracts was determined by dichro-

mate digestion method (Vance et al. 1987). The


123

amount of DOC was expressed based on mg C per kg

soil and as the DOC/OC ratio. The measurement of

particulate organic C (POC) by Loss-on-Ignition

(LOI) was done following the procedure described in

Cambardella et al. (2001). In brief, 25 g of air-dried

and sieved (2 mm) soil was dispersed in 75 ml sodium

hexametaphosphate (5 %) for 16 h on a reciprocal

shaker at 120 strokes per minute. After dispersion, the

suspension was sieved through 0.053 mm sieve to

separate sand particles ? floatable macro-organic

matter or POM (particulate organic matter). The

collected sand particles ? POM fractions were dried

at 55 �C and weighed. A subsample was subjected to

450 �C for 4 h to measure POC, using the LOI

method. The amount of POC was expressed based on g

C per kg soil. Particulate organic N (PON) was

determined in another subsample by the Kjeldahl

method and expressed based on g N per kg soil. Soil

microbial biomass C (MBC) was determined by the

fumigation-extraction method (Vance et al. 1987).

The fumigated and non-fumigated soils were extracted

with 0.5 M K2SO4 by shaking at 30 rpm for 30 min

(soil:extractant ratio of 1:4), and organic C in the

extracts was determined by the dichromate digestion

method described in Vance et al. (1987). The MBC

was calculated using the following equation:

MBC mg kg�1� �

¼ ½ðOC fumigated soilÞ � ðOC unfumigated soilÞ�0:38

The amount of MBC was also expressed as the MBC/

OC ratio (qmic) for the reflection of the organic C

variability among the land uses and as an indicator of

the relative C availability for soil biota (Sparling

1992).

Readily mineralizable organic C (Min-C) or soil

microbial respiration as CO2-C evolution was mea-

sured as described in Zibilske (1994). In brief, 100 g

air-dried soil samples (2-mm sieve) were placed in

plastic jars (1,000 ml). The soil moisture content was

adjusted at 60 % water holding capacity (WHC).

Three jars without soil were considered as blanks. All

jars were kept at 25 �C and pre-incubated for 72 h. A

plastic vial containing 10 ml of 0.5 M NaOH for

trapping the respired CO2 was placed in the jars, and

replaced with a freshly prepared NaOH 1, 2, 3, 4, 5, 6,

7, 8, 9, 10 and 11 weeks after the beginning of the

incubation. The CO2 trapped was measured by

titrating the aliquot with 0.25 M HCl following

precipitation of carbonates by BaCl2 solution

(15 %). At each sampling date, the water content

was controlled and jars were randomly distributed in

the incubator. The cumulative CO2-C evolved at the

end of incubation was considered as potential miner-

alizable C. The C mineralization quotient (qM) or C

mineralization coefficient was calculated from Min-C

values divided by the soil initial OC concentration

(Min-C/OC) following Schimel (1986). The C miner-

alization coefficient is a rough estimate of turnover

rate of labile C fraction over the entire incubation

period (11 weeks).

The microbial metabolic quotient (qCO2) was

calculated by dividing soil basal respiration (the

average CO2-C respired during 14 days expressed as

mg CO2-C kg-1 soil day-1) by MBC (g MBC kg-1

soil) as indicated by Anderson and Domsch (1990).

The qCO2 values were expressed as mg CO2-C g-1

MBC per day. The qCO2 reflects the efficiency of soil

microbial populations in consuming organic C

resources during biosynthesis (Anderson 2003).

Statistical analysis

Prior to analysis of variance (ANOVA), plant biomass

and soil data were analyzed for normality and

homogeneity of the variance. Variables without nor-

mal distribution and equal variance (OC, AR, DOC/

OC ratio and PGF) were subjected to a Box–Cox

power transformation to obtain approximately normal

distributions and to stabilize the variances. Differ-

ences in soil characteristics were analyzed using a

two-way ANOVA with land use, depth and their

interaction as the independent variables. Differences

among the mean values were calculated by the

Fisher’s LSD test, when main and interaction effects

were significant. Differences were considered signif-

icant only when p values were lower than 0.05

(p \ 0.05), unless indicated otherwise. However,

untransformed mean values were presented through-

out the paper for simplicity of interpretation. Linear

and nonlinear regression functions were used to

describe the relationships (1) between soil parameters

and the length of abandonment and (2) between soil

parameters and total above-ground biomass. To work

with nonlinear functions, we considered the time of

abandonment 1 year instead of 0 year for cultivated

rangelands (CR0). The functions which accounted for


123

the greater variability within the data were selected as

the best ones. All statistical calculations were carried

out using the software programs SAS 8.02 (SAS

Institute 2005) and Minitab 16.1. Besides, soil data set

from 0 to 0.3 m depth was subjected to principal

component analysis (PCA) to identify major patterns

of variation. PCA technique is commonly used as an

ordination and data reduction technique to discrimi-

nate land uses and to determine the most important soil

parameters to characterize the influence of agricultural

abandonment on soil conditions (Jiang et al. 2009).

Results and discussion

Biomass production, total soil OC and N

Agricultural abandonment strongly enhanced plant

above-ground biomass production across the chrono-

sequence in the studied rangelands (Table 1). Plant

above-ground biomass was significantly greater by

almost 60 % in uncultivated rangelands and 40 % in

rangelands abandoned for 45 years compared to

cultivated rangelands as a result of the presence of

perennial species and shrubs in undisturbed range-

lands (Table 1) and removal of a significant propor-

tion of above-ground biomass for hay in cultivated

rangelands. Increased biomass across the chronose-

quence is consistent with the establishment of

perennial species and rangeland restoration (Zhao

et al. 2005; Li and Shao 2006; Li et al. 2009b).

However, the more recently abandoned rangelands

showed lower biomass production 4 years and higher

12 years after abandonment compared to cultivated

rangelands. This is consistent with the findings of Li

and Shao (2006) in semi-arid grasslands at Ziwuling,

China. The decreased above-ground biomass in the

recently abandoned rangelands (4 years) could be in

part attributed to the re-establishment of annual and

perennial species immediately after the cessation of

agricultural practices (Table 1). Over this short

period, plant growth may rely only on the released

nutrients from native SOM decomposition. It is also

possible that low soil water availability immediately

after the abandonment of cultivated rainfed wheat

cropping may have contributed to the decreased plant

growth and productivity of natural vegetation. The

decreased plant above-ground biomass production in

the most recently abandoned rangelands was clearly

reflected by the decreases in particulate organic matter

(i.e., POC and PON) and DOC, but not in OC and N

stocks in the whole 0–0.3 m soil (Fig. 1). Therefore,

the lack of total SOM changes in the most recently

abandoned rangelands may be related to the lack of

above-ground biomass recovery. In addition, soil

erosion and losses might accelerate after the stop of

agricultural activities, due to the bared surface soil that

may occur before the full restoration of natural

Table 1 Vegetation type and above-ground biomass for different plant functional groups in the cultivated (CR), abandoned (AR)

and uncultivated (UR) rangelands in a semi-arid ecosystem in Central Iran

Land use Dominant type of vegetation Above-ground biomass (kg ha-1)

AGF PGF Shrubs TAGB

CR0 Triticum aestivum and Hordeum vulgar 662 ± 4.10a 23.3 ± 0.93d 4.70 ± 0.33e 690 ± 4.04d

AR4 Heternthelium piliferum and Bromus tectrum 335 ± 13.7b 191 ± 6.10c 46.0 ± 0.46d 572 ± 15.2e

AR12 Heternthelium piliferum, Agropyron repense

and Astragalus sp

298 ± 21.1c 385 ± 30.5b 97 ± 6.86c 780 ± 5.18c

AR45 Bromus tomentellus, Stipa hoenecriana,

Astragalus verus and Astragalus susianus

102 ± 5.80d 684 ± 24.3a 176 ± 8.06b 962 ± 36.5b

UR100 Bromus tomentellus, Stipa hoenecriana,

Astragalus verus and Astragalus susianus

30.0 ± 4.00e 691 ± 14.7a 377 ± 21.5a 1,098 ± 39.3a

Each value represents mean (n = 3) ± SD

Different letters indicate significant difference (p \ 0.05) among land uses by LSD test

CR0 cultivated rangelands, AR4 rangelands abandoned for 4 years, AR12 rangelands abandoned for 12 years, AR45 rangelands

abandoned for 45 years, UR100 uncultivated rangelands, AGF annual grasses and forbs, PGF perennial grasses and forbs, TAGB total

above-ground biomass, SD standard deviation


123

vegetation and soil coverage (Zhao et al. 2005). This

can lead to the depletion of nutrients and SOM,

resulting in lower soil fertility.

Rainfed cropping over the 100-year rainfed crop-

ping period in these semi-arid rangelands resulted in

significant losses in soil total OC (33.5 %) and N

(30.7 %) in the 0–0.3 m depth, with greater losses at

0–0.15 m (37 % C and 33 % N) than 0.15–0.3 m

(29 % C and 28 % N) depths (Table 2). The magni-

tudes of OC and N losses are similar to those observed

by Li et al. (2009a, b) after 30 years of cultivation in

the arid grasslands, north-western China, but substan-

tially lower than those reported by Zhao et al. (2005)

after 50 years of cropping in the semi-arid grasslands,

northern China. Declines of total OC and N are mainly

attributed to less annual C inputs to the rainfed wheat

cropping than native rangelands. In addition, tillage in

native rangelands also stimulates SOM decomposition

through the disruption of soil macro-aggregates

(Cambardella and Elliott 1993; Qiu et al. 2012).

However, our results showed significant (p \ 0.001)

increases in total OC and N concentrations and stocks

in both surface and sub-surface soils with increasing

age of agricultural abandonment (Table 2), suggesting

that the re-accumulation of OC and N may occur with

natural vegetation recovery following agricultural

abandonment in these rangelands. Of course, this

holds true assuming that the cultivated and abandoned

rangelands did not differ in their initial soil OC and N

levels before land use change. Similarly, previous

Fig. 1 Nonlinear (logarithmic) relationships between the

stocks of soil organic C (OC) (a) particulate organic C (POC)

(b), dissolved organic C (DOC) (c), soil total N (d), particulate

organic N (PON) (e) and soil microbial biomass C (MBC) (f) at

0–0.3 m soil depth with the time of abandonment in cultivated,

abandoned and uncultivated rangelands in a semi-arid ecosys-

tem in Central Iran. Each value represents mean (n = 3), Error

bars are ± standard deviation; Bars with different letters are

significantly different at p \ 0.05 among rangelands by LSD

test


123

studies reported higher OC and N contents in aban-

doned than cultivated soils (Zhao et al. 2005; Zhang

et al. 2007; Li et al. 2009b; Raiesi 2012a, b). In

contrast with our results, Raiesi (2012a, b) reported

that the re-accumulation of OC and N may occur only

in the surface soil layers of abandoned lands. The OC

and N contents in a given soil largely depends on the

balance between the C inputs rates through plant

residues and C outputs rates through microbial

decomposition (Post and Kwon 2000). These two

factors are essentially altered by type of land use and

agricultural abandonment (Li et al. 2009a, b). In this

study, the greater soil OC and N contents in abandoned

rangelands are largely attributed to enhanced C inputs

to the soil because a large proportion of above- (plant

residues and dead litters) and below-ground biomass

from natural plant community is annually added to the

soil (Li et al. 2009b). This is supported by the positive,

significant linear correlations between SOM (OC and

N) stocks and above-ground biomass (Fig. 2). The

exclusion of tillage disturbance and recovering aggre-

gate stability over the chronosequence of

abandonment could also have contributed to the

observed decrease in OC losses from abandoned

rangeland soils during decomposition. The recovery of

water-stable macro-aggregates (Table 3) may have

protected the organic matter against microbial decom-

position and facilitated the OC and N accumulation.

Changes in soil total OC and N stocks in the 0-0.3 m

layer were calculated by a logarithmic function

(Fig. 1). There were significant logarithmic relation-

ships between soil total OC (R2 = 0.94; Fig. 1) and N

(R2 = 0.94; Fig. 1) stocks at 0-0.3 m layer and the

time of abandonment. Similarly, several earlier studies

revealed that the SOM stocks increased logarithmi-

cally with the time of abandonment in rangeland (Zhao

et al. 2005; Schipper and Sparling 2011) and forest

ecosystems (Zhang et al. 2010).

In the current study, soil OC accumulated at a rate

of 159, 63 and 18.4 g m-2 yr-1 between 0 and 4,

between 4 and 12, and between 12 and 45 years

following abandonment, respectively. Over the same

time of abandonment, the annual N accumulation rates

were 13.3, 5.25 and 1.53 g m-2 yr-1, respectively

Fig. 2 Linear relationships between the stocks of soil organic C

(OC) and particulate organic C (POC) (a), soil total N and

particulate organic N (PON) (b), dissolved organic C (DOC)

(c) and microbial biomass C (MBC) (d) at 0–0.3 m soil depth

with total plant above-ground biomass (TAGB) in cultivated,

abandoned and uncultivated rangelands in a semi-arid ecosys-

tem in Central Iran. Each value represents mean (n = 3)


123

Ta

ble

2O

rgan

icC

,N

,p

arti

cula

teo

rgan

icC

and

Nco

nce

ntr

atio

ns

and

sto

cks

incu

ltiv

ated

(CR

),ab

and

on

ed(A

R)

and

un

cult

ivat

ed(U

R)

ran

gel

and

sin

ase

mi-

arid

eco

syst

emin

Cen

tral

Iran

Lan

du

seC

on

cen

trat

ion

sS

tock

s

OC

(gk

g-

1)

N (gk

g-

1)

C/N

PO

C

(gk

g-

1)

PO

N

(gk

g-

1)

PO

C/P

ON

OC

(Mg

ha-

1)

N (Mg

ha-

1)

PO

C

(Mg

ha-

1)

PO

N

(Mg

ha-

1)

0–

0.1

5m

CR

09

.50

±0

.01

c0

.96

±0

.04

d9

.91

±0

.43

b2

.90

±0

.04

d0

.21

±0

.01

d1

4.1

±0

.35

a2

0.2

±0

.01

c2

.04

±0

.08

d6

.18

±0

.09

d0

.44

±0

.01

d

AR

49

.67

±0

.28

c0

.97

±0

.03

d9

.90

±0

.14

b1

.83

±0

.02

e0

.13

±0

.01

e1

3.7

±0

.10

a2

0.6

±0

.60

c2

.08

±0

.05

d3

.90

±0

.05

e0

.28

±0

.01

e

AR

12

11

.3±

0.2

9b

1.1

0±

0.0

5c

10

.3±

0.7

2ab

5.0

6±

0.0

4c

0.3

6±

0.0

2c

14

.1±

0.6

7a

24

.1±

0.6

1b

2.3

4±

0.1

1c

10

.8±

0.0

8c

0.7

7±

0.0

3c

AR

45

14

.3±

0.2

9a

1.3

4±

0.0

3b

10

.8±

0.2

2a

6.8

6±

0.0

2b

0.4

9±

0.0

1b

14

.1±

0.0

8a

30

.5±

0.6

1a

2.8

4±

0.0

6b

14

.6±

0.0

4b

1.0

4±

0.0

1b

UR

10

01

5.0

±0

.01

a1

.44

±0

.03

a1

0.4

±0

.20

ab7

.72

±0

.04

a0

.56

±0

.01

a1

3.7

±0

.30

a3

2.0

±0

.01

a3

.05

±0

.06

a1

6.4

±0

.08

a1

.20

±0

.02

a

Mea

n1

2.0

±2

.57

1.1

6±

0.2

21

0.3

±0

.38

4.8

7±

2.5

10

.35

±0

.18

13

.9±

0.2

22

5.5

±5

.49

2.4

7±

0.4

61

0.4

±5

.35

0.7

4±

0.3

9

0.1

5–

0.3

m

CR

08

.16

±0

.55

c0

.82

±0

.06

c1

0.0

±0

.01

b2

.31

±0

.02

d0

.16

±0

.01

d1

4.2

±0

.06

a1

7.3

±1

.25

c1

.76

±0

.13

c4

.99

±0

.03

d0

.35

±0

.01

d

AR

48

.50

±0

.01

c0

.85

±0

.01

c1

0.0

±0

.01

b1

.05

±0

.04

e0

.08

±0

.01

e1

3.6

±0

.08

b1

8.4

±0

.01

c1

.84

±0

.01

c2

.26

±0

.07

e0

.17

±0

.01

e

AR

12

9.6

7±

0.2

9b

0.9

7±

0.0

3b

10

.0±

0.0

1b

3.5

7±

0.0

7c

0.2

6±

0.0

1c

13

.7±

0.0

6b

20

.9±

0.6

2b

2.0

9±

0.0

6b

7.7

0±

0.1

6c

0.5

7±

0.0

1c

AR

45

11

.2±

0.2

8a

1.0

2±

0.0

2b

10

.9±

0.0

3a

6.0

4±

0.0

9b

0.4

2±

0.0

2b

14

.2±

0.0

6a

24

.1±

0.6

1a

2.2

0±

0.0

6b

13

.0±

0.0

4b

0.9

2±

0.0

1b

UR

10

01

1.3

±0

.28

a1

.13

±0

.02

a1

0.0

±0

.01

b6

.95

±0

.02

a0

.52

±0

.01

a1

3.4

±0

.07

c2

4.5

±0

.61

a2

.45

±0

.06

a1

5.0

±0

.03

a1

.12

±0

.03

a

Mea

n9

.77

±1

.46

0.9

6±

0.1

31

0.2

±0

.40

3.9

8±

2.4

80

.29

±0

.18

13

.8±

0.3

62

1.0

±3

.26

2.0

7±

0.2

88

.59

±5

.35

0.6

2±

0.4

0

Su

mm

ary

of

AN

OV

Are

sult

s

Lan

du

se(L

)*

**

**

**

**

**

**

**

**

**

**

**

**

**

**

*

Dep

th(D

)*

**

**

*N

s*

**

**

*N

s*

**

**

**

**

**

*

L9

DN

s*

**

Ns

**

Ns

Ns

**

**

*

Eac

hv

alu

ere

pre

sen

tsm

ean

(n=

3)

±S

D

At

each

soil

sam

pli

ng

dep

th,

dif

fere

nt

lett

ers

ind

icat

esi

gn

ifica

nt

dif

fere

nce

(p\

0.0

5)

amo

ng

ran

gel

and

sb

yL

SD

test

CR

0cu

ltiv

ated

ran

gel

and

s,A

R4

ran

gel

and

sab

and

on

edfo

r4

yea

rs,

AR

12

ran

gel

and

sab

and

on

edfo

r1

2y

ears

,A

R4

5ra

ng

elan

ds

aban

do

ned

for

45

yea

rs,

UR

10

0u

ncu

ltiv

ated

ran

gel

and

s,S

Dst

and

ard

dev

iati

on

,N

sn

ot

sig

nifi

can

t

*p\

0.0

5;

**

p\

0.0

1;

**

*p\

0.0

01


123

(Fig. 1). Rates of OC and N sequestration over the

chronosequence decrease partly due to the limitation

for the amount of OC and N that can accumulate per

unit of area. Generally, the rate of soil OC accumu-

lation was almost 1.2 times greater than that of soil N

accumulation along the chronosequence, showing a

faster rate of OC than N accumulation. Schipper and

Sparling (2011) showed that the mean rates of OC

accumulation were not constant but decreased along a

pasture chronosequence and ranged from 107 to

9 g m-2 yr-1 after establishment of native pastures

on reverted scrublands in New Zealand. Our study

showed soil OC and N stocks increased with increas-

ing time of abandonment, but this effect could be

largely attributed to the increase in plant production

that occurred along the natural vegetation succession.

This means that the time after cultivation abandon-

ment is the only determining factor for changes in

SOM and soil evolution, leading to the hypothesis that

the abandonment of cultivation and soil properties

might be dependent factors.

Our findings showed lower C/N ratios in cultivated

than abandoned and uncultivated soils, indicating

proportionally more depletion of OC than N in

cultivated rangeland soils, and higher rates of OC

sequestration than N in abandoned and uncultivated

rangeland soils. These changes were seen particularly

in rangelands abandoned for 45 years. It is also

possible that greater C/N ratios in long-term aban-

doned than cultivated or short-term abandoned range-

lands might have resulted from perennial woody

species (such as shrubs and semi-shrub) that were the

dominant plant cover in rangelands abandoned for a

longer period of 30–45 years (Table 1). The increase

in C/N ratio of abandoned rangeland soils may also

suggest net N immobilization into SOM and microbial

biomass, although there are no further increases in soil

OC content.

Our results also indicated that stratification ratios of

OC and N (the value of each parameter in the 0–0.15 m

soil layer divided by that in the 0.15–0.3 m layer) were

lower in cultivated rangelands than in abandoned and

uncultivated rangelands, again suggesting greater

organic inputs from natural vegetation with agricul-

tural abandonment. High stratification ratios of soil OC

and N may also display a relatively undisturbed soil

with a higher quality (Franzluebbers 2002). This would

mean that SOM stratification occurs in non-disturbed

abandoned soils and when the mixing of the upper

0.3 m by ploughing is entirely stopped (Raiesi 2012a,

b). Greater OC stratification ratios could be related to

the fact that during land abandonment, soil was

remained undisturbed, which reduces oxidation and

promotes soil OC stratification. In general, high

stratification ratios ([2) would indicate high soil

quality, as ratios \2 are commonly observed in

degraded soils (Franzluebbers 2002).

Agricultural abandonment effect on soil physical

properties

The abandonment of cultivation on these rangeland

soils had a strong influence on soil physical properties.

Results show significantly lower bulk density and

higher total porosity at both soil depths in abandoned

than cultivated rangelands (Table 3). This indicates

that soil compaction was lowered as a result of

cessation of tillage and agricultural practices, and

natural vegetation recovery. Soil bulk density might

increase and consequently total porosity would

decrease as a result of the disruption of soil macro-

aggregates by ploughing and subsequent compaction

in cultivated rangelands (Walker and Desanker 2004;

Raiesi 2012a, b). A lower bulk density and higher total

porosity in the abandoned soils than cultivated ones

could be attributed to the increase of OC after land

abandonment (Zhu et al. 2010; Zhang et al. 2010).

This is supported by the linear regressions showing

that soil bulk density was negatively correlated with

total OC (r = –0.89; p \ 0.001; n = 30) and POC

(r = –0.90; p \ 0.001; n = 30) contents (Table 4).

At both soil depths, aggregate stability measured as

MWD and aggregate ratio (AR) in abandoned and

uncultivated rangelands were higher than that in

cultivated rangelands (Table 3). Ploughing may

weaken soil aggregation and structure (McLauchlan

et al. 2006). Jinbo et al. (2007) and Li and Shao (2006)

also showed that the abandonment of cultivation

increased water-stable macro-aggregates. In the pres-

ent study, both MWD and AR values tended to be

linearly and positively related to total OC

(r = 0.66–0.59; p \ 0.001; n = 30) and POC

(r = 0.86–0.81; p \ 0.001; n = 30) contents

(Table 4). This indicates that soil OC plays an

important role in improving soil aggregate stability,

and that the influence of the POC fraction is more

pronounced than that of total OC. These findings

would suggest that total and labile OC could be the


123

major binding agent for the formation of aggregates

with cultivation abandonment. McLauchlan et al.

(2006) reported that aggregate size (as GMD index)

increased over a chronosequence of abandoned agri-

cultural fields. They suggested that this increase in

aggregate size could be driving the increase in the size

of the labile C pool over time, as aggregates physically

protect labile organic matter from microbial decom-

position. The amount of soil POC is a valuable

indicator of soil structure degradation with cultivation,

and significant losses in this OC fraction could

contribute to the breakdown of large macro-aggre-

gates. It is also possible that the growth of plant roots

(Qiu et al. 2012) and probably the development of

fungal hyphae (Zornoza et al. 2009) may have

contributed to the increased aggregate stability in

abandoned rangelands. These binding agents together

with POM are basically involved in the formation of

larger aggregates (Jastrow 1996).

Agricultural abandonment effect on labile OC

and N fractions

The results of this study demonstrate that 12–45 years

of agricultural abandonment in these rangelands

consistently increased soil POM (i.e., POC and

PON) pools calculated for the whole sampling depth

(Table 2), and that changes in POC and PON stocks

were much higher than that of OC and N stocks

(Table 2; Fig. 3). This observation is in agreement

with those of previous results (McLauchlan et al.

2006; Jinbo et al. 2007), showing an increase in labile

C factions with time of cultivation abandonment.

Changes in POM pools are usually related to the type

of land uses, management and agricultural practices

that all affect the balance between organic matter

inputs from plant biomass (above- and below-ground)

and organic matter losses during microbial activity

and decomposition (Cambardella and Elliott 1993; Li

Table 3 Soil characteristics in cultivated (CR), abandoned (AR) and uncultivated (UR) rangelands in a semi-arid ecosystem in

Central Iran

Land use pH CCE Bulk density Total porosity MWD AR

(g kg-1) (Mg m-3) (%) (mm)

0–0.15 m

CR0 7.08 ± 0.03b 58.8 1.72 ± 0.01a 34.9 ± 0.19e 0.344 ± 0.00e 0.30 ± 0.01e

AR4 7.08 ± 0.03b 30.8 1.65 ± 0.00b 37.6 ± 0.11d 0.359 ± 0.01d 0.33 ± 0.01d

AR12 7.18 ± 0.08a 40.8 1.60 ± 0.00c 39.4 ± 0.00c 0.454 ± 0.00c 0.50 ± 0.01c

AR45 7.26 ± 0.08a 33.3 1.48 ± 0.01d 43.9 ± 0.29b 0.563 ± 0.01b 0.77 ± 0.05b

UR100 7.26 ± 0.03a 27.5 1.46 ± 0.01e 44.9 ± 0.19a 0.632 ± 0.01a 0.99 ± 0.05a

Mean 7.17 ± 0.09 38.3 1.58 ± 0.11 40.1 ± 3.95 0.470 ± 0.13 0.58 ± 0.30

0.15–0.3 m

CR0 7.13 ± 0.02c 72.5 1.75 ± 0.01a 33.9 ± 0.44e 0.396 ± 0.00d 0.38 ± 0.01e

AR4 7.05 ± 0.00d 31.7 1.67 ± 0.01b 37.0 ± 0.19d 0.418 ± 0.01d 0.42 ± 0.02d

AR12 7.33 ± 0.03b 84.2 1.61 ± 0.00c 39.1 ± 0.11c 0.506 ± 0.01c 0.60 ± 0.03c

AR45 7.18 ± 0.03c 67.5 1.51 ± 0.01d 43.0 ± 0.19b 0.647 ± 0.02b 1.05 ± 0.09b

UR100 7.40 ± 0.05a 64.2 1.49 ± 0.01e 43.6 ± 0.29a 0.732 ± 0.01a 1.50 ± 0.03a

Mean 7.22 ± 0.14 64.0 1.61 ± 0.11 39.3 ± 3.90 0.539 ± 0.15 0.79 ± 0.48

Summary of ANOVA results

Land use (L) *** Ns *** *** *** ***

Depth (D) * * ** *** *** ***

L 9 D *** Ns * * ** Ns


At each soil sampling depth, different letters indicate significant difference (p \ 0.05) among rangelands by LSD test

CCE carbonate calcium equivalent, MWD mean weight diameter, AR aggregate ratio, CR0 cultivated rangelands, AR4 rangelands

abandoned for 4 years, AR12 rangelands abandoned for 12 years, AR45 rangelands abandoned for 45 years, UR100 uncultivated

rangelands, SD standard deviation, Ns not significant

* p \ 0.05; ** p \ 0.01; *** p \ 0.001


123

et al. 2009a, b). Greater changes in POC than OC

stocks (Fig. 3) may suggest that soil POC pools are the

most sensitive and suitable C fractions for monitoring

changes in total OC following the abandonment of

cultivation. Jinbo et al. (2007) also reported that the

rate of increase in light fraction C was greater than that

in bulk soil C after abandonment of cultivated

wetlands. With the re-establishment of natural vege-

tation, more plant litter inputs from above-ground

biomass (Table 1) and probably below-ground bio-

mass to soils occurred in abandoned than cultivated

rangelands, which could explain most variation in soil

POC and PON stocks (Figs. 1, 3). The relationship

between POC and PON stocks and the time of

agricultural abandonment was best described by a

logarithmic function (R2 = 0.82–0.83; Fig. 1). We

also observed a linear regression between POC or

PON stocks with total above-ground biomass

(R2 = 0.98–0.99; Fig. 2). As indicated from the

logarithmic fit to data on the labile fractions, the rates

of changes in the POC and PON contents were not

constant, but declined across the chronosequence

(Figs. 1, 3).

In our study, the DOC concentrations and stocks

showed an increasing tendency with time of agricul-

tural abandonment (Table 5; Fig. 1). Probably, the

dissolution of microbial lysates (Li et al. 2009b) and

the presence of active plant roots and root rhizodepo-

sitions (Lundquist et al. 1999) are a source of labile

DOC, which could have increased the amount of DOC

in uncultivated and abandoned rangelands. There was

a logarithmic relationship between DOC stocks and

the time of abandonment (R2 = 0.83; Fig. 1), and a

linear regression between DOC stocks and above-

ground biomass (R2 = 0.97; Fig. 2). The time of

agricultural abandonment and above-ground biomass

together accounted for most of the variation in the

DOC stocks. Similar to POM, greater changes in DOC

than OC stocks (Fig. 3) may suggest that this C

fraction is also sensitive to changes in total OC

following the abandonment of cultivation. In contrary,

Jinbo et al. (2007) found that the rate of increases in

DOC was far lower than that in soil total OC.

However, the DOC stratification ratio was higher in

cultivated (1.72) than abandoned (1.34–1.48) and

uncultivated (1.27) rangelands. It seems that enhanced

C mineralization due to the disruption of soil aggre-

gates in cultivated rangelands, and dissolution of

microbial lysates (Li et al. 2009b) may have increased

the amount of DOC levels in the 0–0.15 m layer

compared to the 0.15–0.3 m layer. Nevertheless, the

whole soil profile showed a tendency toward unifor-

mity of DOC stock with increasing time of abandon-

ment. Overall, in uncultivated rangeland soils DOC

uniformity was the highest and its stratification ratio

was the lowest. Our data further demonstrates that the

rate of increase in the DOC concentration was much

greater than that of the OC concentration after

Fig. 3 Rate of changes in OC and labile OC fractions as a

function of the time of abandonment in cultivated, abandoned

and uncultivated rangelands in a semi-arid ecosystem in Central

Iran. OC organic C; POC particulate organic C; MBC microbial

biomass C; DOC dissolved organic C. Each value represents

mean (n = 3)


123

agricultural abandonment (Fig. 3). Therefore, the ratio

of DOC to OC increased obviously after the abandon-

ment of agricultural activities (Table 5).

Carbon mineralization (Min-C) and MBC are the

most frequently used parameters for quantifying

microbial activities in soils and for detecting changes

in soil C dynamics. Increasing of mineralizable C with

increasing abandonment time (Table 5) may indicate a

greater C availability and subsequently higher micro-

bial activity. In fact, the differences in OC contents

found between cultivated and abandoned rangelands

have led to the differences in the mineralizable C. The

mineralizable C usually enhances with increasing

organic C concentrations (Raiesi 2007). The amount

of Min-C was significantly and positively correlated

(Table 4) with OC (r = 0.79; p \ 0.001), N

(r = 0.82; p \ 0.001), DOC (r = 0.68; p \ 0.001),

POC and PON (r = 0.59; p \ 0.001), consistent with

previous findings (Li et al. 2009a, b). Mineralizable C

was significantly related to AR (r = 0.31; p \ 0.05)

and MWD (r = 0.34; p \ 0.01) (Table 4). Therefore,

the significant increases in the mineralizable C without

reducing the amount of OC found in abandoned

rangelands might be linked to the combined impacts of

high inputs of plant residues to soil and improved soil

aggregate stability and total porosity. It seems that

inputs of OC via plant residues are greater and more

important than losses of OC via microbial C miner-

alization in abandoned rangelands with natural vege-

tation, and that the potential for loss of SOM is at least.

The increases in Min-C associated with agricultural

abandonment correspond well with other studies

which showed increased C mineralization following

cultivation abandonment (Li et al. 2009b; Raiesi

2012b). The recovery of Min-C fraction in the restored

soils was partly due to higher plant biomass inputs

compared to those in the cropped rangeland soils and

probably because of the large amounts of available C

for microbial activity. Soil C supply and availability

are crucial factors regulating C decomposition and

microbial activity in arid and semi-arid rangelands

(Raiesi 2007; Raiesi 2012a, b).

In the current study, the amount of MBC at both soil

depths was higher in abandoned (for 12 and 45 years)

and uncultivated than cultivated rangelands (Table 5),

suggesting increasing time of agricultural abandon-

ment would improve soil biochemical properties and

microbial activity that could be indicative of a shift in

the state of equilibrium of the cultivated rangeland

soils. This is consistent with results reported by Saggar

et al. (2001) who found MBC rapidly increased in

abandoned pasture soils. In general, the natural

logarithmic function gave a better fit to the MBC data

than a linear function (Fig. 1), indicating the rates of

change over time were not uniform (MBC = 0.307 Ln

(time) ?1.49; R2 = 0.88). Similar to OC, MBC stocks

were not identical in uncultivated rangelands and

long-term (45 years) abandoned rangelands (Fig. 1).

This means that soil MBC would not fully recover

45 years after the cessation of cultivation. Correlation

Table 4 Pearson correlation coefficients (r) between concentrations of soil attributes for the whole 0–0.3 m soil depth of cultivated,

abandoned and uncultivated rangelands in a semi-arid ecosystem in Central Iran (n = 30)

Variable Bd OC TN DOC POC PON MBC Min-C qCO2 MWD AR

Bd 1.00

OC -0.89*** 1.00

N -0.84*** 0.96*** 1.00

DOC -0.89*** 0.94*** 0.92*** 1.00

POC -0.90*** 0.90*** 0.86*** 0.98*** 1.00

PON -0.91*** 0.89*** 0.86*** 0.98*** 0.99*** 1.00

MBC -0.91*** 0.97*** 0.95*** 0.98*** 0.96*** 0.95*** 1.00

Min-C -0.62*** 0.79*** 0.82*** 0.68*** 0.59*** 0.59*** 0.72*** 1.00

qCO2 0.77*** -0.63*** -0.58*** -0.78*** -0.85*** -0.85*** -0.75*** -0.20 1.00

MWD -0.89*** 0.66*** 0.60*** 0.77*** 0.86*** 0.87*** 0.75*** 0.34** -0.88*** 1.00

AR -0.83*** 0.59*** 0.54** 0.71*** 0.81*** 0.83*** 0.68*** 0.31* -0.81*** 0.98*** 1.00

Bd bulk density, OC organic C, N total N, DOC dissolved organic C, POC particulate organic C, PON particulate organic N, MBC microbial

biomass C, Min-C mineralizable C, qCO2 metabolic quotient, MWD mean weight diameter, AR aggregate ratio

* p \ 0.05; ** p \ 0.01; *** p \ 0.001


123

analysis (Table 4) showed that MBC was closely

(p \ 0.001) related to OC (r = 0.97), N (r = 0.95),

DOC (r = 0.98), POC (r = 0.96) and PON (r = 0.95)

in the 0–0.3 m layer, and negatively correlated with

soil bulk density (r = -0.91). This means increased

DOC contents measured in uncultivated and aban-

doned rangelands appear to indicate C availability to

soil microflora. Changes in MBC often indicate the

effect of agricultural management practices and

abandonment on soil micro biological and biochem-

ical properties (Nannipieri et al. 1990; Saggar et al.

2001). Similarly, the abandonment of cultivation

resulted in a significant increase of soil MBC in arid

and semi-arid areas (Saggar et al. 2001; Li et al.

2009b; Raiesi 2012b). The effect of agricultural

abandonment on the MBC/OC ratio (qmic) was

consistent in both soil layers and greater MBC/OC

ratios were found in rangelands abandoned for

45 years (Table 5). With agricultural abandonment

in rangelands, the increase in MBC was higher than

that in OC in the 0–0.3 m layer (Fig. 3), resulting in an

increase in the microbial quotient (qmic) (Table 5).

Similarly, Zhang et al. (2007) reported an increase in

the microbial quotient with increasing abandonment

time in freshwater march ecosystems. Saggar et al.

(2001) reported that both soil MBC and MBC/OC

ratios increased with the cessation of agriculture in

pasture ecosystems. The MBC/OC ratio can be a good

measure of the efficiency of OC conversion into

microbial C, the losses of soil OC during decompo-

sition, and is a soil quality indicator allowing the

comparison of microbial biomass across soils with

different organic matter contents (Sparling 1992). If a

soil is degrading, the MBC/OC ratio generally

declines at faster rates than soil total OC (Sparling

1992). Therefore, high MBC/OC ratios in abandoned

Table 5 Dissolved organic C (DOC), soil mineralizable C (Min-C), microbial biomass C (MBC), and microbial metabolic quotient

(qCO2) in cultivated (CR), abandoned (AR) and uncultivated (UR) rangelands in a semi-arid ecosystem in Central Iran

Land use DOC Min-C MBC qCO2

(mg C kg-1

soil)

(g C kg-1 C) (mg CO2-C

kg-1 soil)

(g C kg-1 C) (mg C kg-1

soil)

(g C kg-1 C) (mg CO2-C g-1

MBC day-1)

0–0.15 m

CR0 51.1 ± 0.93d 5.38 ± 0.13c 256 ± 19.7c 27.4 ± 1.86c 451 ± 1.21d 47.6 ± 0.36b 16.4 ± 0.60b

AR4 31.7 ± 0.85e 3.26 ± 0.01d 355 ± 17.8b 36.4 ± 3.12a 407 ± 0.70e 42.1 ± 1.08c 18.2 ± 0.51a

AR12 85.2 ± 0.78c 7.52 ± 0.27b 361 ± 31.8b 32.1 ± 3.37ab 535 ± 1.81c 47.2 ± 1.17b 12.8 ± 1.67c

AR45 111 ± 1.95b 7.77 ± 0.08b 372 ± 6.84b 26.1 ± 1.19c 721 ± 1.69b 50.9 ± 1.23a 5.62 ± 0.31d

UR100 125 ± 2.11a 8.39 ± 0.15a 435 ± 27.4a 28.8 ± 1.71bc 767 ± 0.99a 51.7 ± 0.61a 4.22 ± 0.19d

Mean 80.8 ± 39 6.46 ± 2.11 356 ± 64 30.2 ± 4.1 576 ± 160 47.9 ± 3.8 11.4 ± 6.28

0.15–0.3 m

CR0 29.7 ± 0.51d 3.61 ± 0.19d 191 ± 22c 23.4 ± 3.86a 355 ± 3.03d 43.5 ± 3.06c 10.7 ± 0.52b

AR4 17.8 ± 0.76e 2.08 ± 0.09e 221 ± 37bc 25.8 ± 4.29a 343 ± 4.04e 40.2 ± 0.47d 18.9 ± 0.87a

AR12 57.3 ± 1.38c 5.92 ± 0.30c 229 ± 40bc 23.7 ± 4.42a 466 ± 2.47c 48.2 ± 1.38b 6.11 ± 0.76c

AR45 82.4 ± 2.17b 7.38 ± 0.35b 266 ± 9.7ab 23.9 ± 0.43a 584 ± 2.65b 52.4 ± 1.17a 4.51 ± 0.15d

UR100 97.9 ± 1.03a 8.64 ± 0.19a 285 ± 21a 25.0 ± 2.34a 603 ± 2.31a 53.3 ± 1.66a 3.44 ± 0.17e

Mean 57.0 ± 33 5.52 ± 2.68 238 ± 37 24.4 ± 0.98 470 ± 122 47.5 ± 5.6 8.73 ± 6.32

Summary of ANOVA results

Land use (L) *** *** *** * *** *** ***

Depth (D) *** *** *** *** *** Ns ***

L 9 D *** *** Ns Ns *** ** ***


At each soil sampling depth, different letters indicate significant difference (p \ 0.05) among rangelands by LSD test

CR0 cultivated rangelands, AR4 rangelands abandoned for 4 years, AR12 rangelands abandoned for 12 years, AR45 rangelands

abandoned for 45 years, UR100 uncultivated rangelands, SD standard deviation, Ns not significant

* p \ 0.05; ** p \ 0.01; *** p \ 0.001


123

rangelands may imply the rapid recovery of soil

microbiological properties in the studied ecosystem.

For the soils with low OC contents, however, high

MBC/OC ratio may means that SOM decomposes

rapidly, which would be unfavorable to soil quality

and fertility (Li et al. 2004; Jiang et al. 2006). Besides,

our results indicate that the rates of increase in soil

MBC were less than those in POC and DOC pools

(Fig. 3), suggesting that both soil POC and DOC

fractions might be more responsive to the sequence of

rangeland abandonment than MBC fraction and thus

are more sensitive indicators than MBC in the study

area.

Our results showed that with increasing abandon-

ment time, qCO2 at both soil depths decreased

(Table 5). Over the past three decades, the microbial

qCO2 has been used as an indicator of ecosystem

disturbance and development (Insam and Domsch

1988). The metabolic quotient is also considered as an

index of microbial efficiency in utilizing the available

resources (high efficiency for low values of qCO2)

(Anderson 2003). A decrease in qCO2 value was in

fact related to a larger quantity of MBC with

abandonment period. Similarly, previous studies

reported that the qCO2 decreased with time of

succession stages of restored soil systems (Insam

and Domsch 1988; Insam and Haselwandter 1989).

Furthermore, the lower qCO2 values in uncultivated

and abandoned than cultivated rangelands may be due

to higher C availability and quality, in particular labile

C fractions such as POC and DOC due to the presence

of active plant roots and greater C inputs from plant

biomass. Other factors could also contribute to the low

qCO2. For example, fungi may dominate in abandoned

rangelands (Zornoza et al. 2009) and these soil

microorganisms are more efficient at converting

substrate C into biomass C than bacteria (Anderson

2003). Therefore, improved metabolic efficiency with

abandonment time reflects a tendency toward a rather

stable condition after disturbance by agricultural

practices and the slow recovery of natural vegetation.

Principal component analysis

The results of principal component analysis (PCA) for

12 soil parameters are summarized in Table 6. Prin-

cipal component analysis of the soil properties

Fig. 4 Biplot of PC1 (78.9 % of the variation) and PC2

(13.7 % of the variation) separating five land uses (CR0

cultivated rangelands; AR4 rangelands abandoned for 4 years;

AR12 rangelands abandoned for 12 years; AR45 rangelands

abandoned for 45 years; UR100 uncultivated rangelands) using

12 soil variables (0–0.3 m depth). The variables that each

principal component represents are given in Table 6. The dotted

lines show the sampling points that belong to the same land use

group

Table 6 Variable loading coefficients (eigenvectors) for the

first two principal components (PC) for 12 soil variables

(0–0.3 m layer) from the PCA of the standardized values

(correlation matrix), and their individual and cumulative per-

centage of total variance explained by each PC and eigenvalues

Soil variable PC1 PC2 Total

communality

pH 0.75 0.46 0.90

Bd –0.95 –0.003 0.95

MWD 0.88 0.42 0.99

AR 0.83 0.46 0.84

OC 0.91 –0.38 0.97

N 0.89 –0.39 0.97

DOC 0.97 –0.14 0.97

POC 0.98 0.012 0.97

PON 0.99 0.03 0.97

MBC 0.97 –0.22 0.98

Min-C 0.63 –0.71 0.91

qCO2 –0.85 –0.43 0.93

Eigenvalue 9.47 1.65 11.1

Proportion of variance (%) 78.9 13.7 –

Cumulative variance (%) 78.9 92.6 92.6

Bd bulk density, MWD mean weight diameter, AR aggregate

ratio, OC organic C, N total N, DOC dissolved organic C, POC

particulate organic C, PON particulate organic N, MBC

microbial biomass C, Min-C mineralizable C, qCO2

metabolic quotient, Boldface eigenvectors ([0.60) are

considered as highly loaded


123

resulted in two principal components (PC) with

eigenvalues greater than 1, and these two components

explained 92.6 % of the total variance in the data set

(Table 6). The first component (PC1) explaining

78.9 % of the data variance showed high loadings

for soil pH, bulk density, OC, N, DOC, POC, PON,

Min-C, MBC, qCO2, MWD and AR variables. Soil

OC correlated positively with all parameters except

with bulk density and qCO2. The second component

(PC2) accounting for 13.7 % of the total variation

loaded heavily on Min-C. Soil Min-C was negatively

correlated with qCO2 on PC1. The different locations

of land uses in the plane of the first two principal

components indicate that there were large differences

in soil attributes among land uses (Fig. 4). The

exceptions were cultivated rangelands (CR0) and

short-term abandoned rangelands (AR4), since these

land uses were not separated successfully and clearly.

Results from the PC analysis indicated that the first PC

was successful in separating land uses through soil pH,

bulk density, OC, N, DOC, POC, PON, MBC, qCO2,

MWD and AR parameters (Table 6; Fig. 4). This

means that the eleven soil properties may be used for

assessment of soil response to agricultural abandon-

ment in these semi-arid rangelands in the future

studies.

Conclusions

The current study showed that after cultivated range-

lands were abandoned and converted to grazing

rangelands, soil conditions improve with increasing

time of abandonment. The results showed that (1) soil

OC and N stocks, and structure increased, whereas soil

bulk density decreased with increasing abandonment

time, (2) 45 years of abandonment would not be

sufficient to reach the level of uncultivated rangelands

and (3) rangelands that had once been cultivated but

abandoned for 45 years still showed increasing levels

of labile SOM fractions, particularly POC, PON and

DOC pools. The recovery of POM and DOC fractions

in the restored rangeland soils were mainly due to

higher plant biomass inputs and lower organic matter

decomposition rates compared to those in the culti-

vated rangeland soils. We found evidence that soil

POC and DOC pools are the most sensitive and

appropriate OC fractions for detecting changes in soil

total OC following the abandonment of cultivation.

The positive influence of cultivation abandonment

was particularly evident during the early vegetation

and soil recovery stage (12 years abandoned range-

lands). Furthermore, agricultural abandonment in high

altitude rangelands with semi-arid climate would be a

potential practice for C sequestration in the soil, and

thus lowering CO2 concentration in the atmosphere.

Nevertheless, these rangelands are under heavy to

light grazing pressure by livestock and it is not well-

known whether the cessation of both cultivation and

concomitantly grazing could result in a further capac-

ity for SOM accumulation and C sequestration in soil.

This aspect of rangeland management merits further

research.

Acknowledgments The authors are grateful to Shahrekord

University for providing the financial support. We thank the

anonymous reviewers who provided critical and valuable

suggestions to improve the manuscript.

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