Soil & Tillage Research 136 (2014) 38–50

Long-term tillage systems impacts on soil C dynamics, soil resilienceand agronomic productivity of a Brazilian Oxisol

Joao Carlos de Moraes Sa a,*, Florent Tivet b, Rattan Lal c, Clever Briedis d,Daiani Cruz Hartman e, Juliane Zuffo dos Santos e, Josiane Burkner dos Santos f

a State University of Ponta Grossa, Department of Soil Science and Agricultural Engineering, Av. Carlos Cavalcanti 4748, 84010-330 Ponta Grossa-PR, Brazilb Centre de Cooperation Internationale en Recherche Agronomique pour le Developpement, CIRAD, UPR SIA, F-34398 Montpellier, Francec The Ohio State University, Carbon Management and Sequestration Center, School of Environment and Natural Resources, OARDC/FAES, 2021 Coffey Road,

Columbus, OH 43210, USAd Graduate Program in Agronomy of State University of Ponta Grossa, Av. Carlos Cavalcanti 4748, Campus de Uvaranas, 84030-900 Ponta Grossa-PR, Brazile Undergraduate Program in Agronomy of State University of Ponta Grossa, Av. Carlos Cavalcanti 4748, Campus de Uvaranas, 84030-900 Ponta Grossa-PR,

Brazilf Agronomic Institut of Parana – IAPAR, Polo regional de Ponta Grossa, Av. Presidente Kennedy, s/n8, Ponta Grossa-PR, Brazil

A R T I C L E I N F O

Article history:

Received 4 March 2013

Received in revised form 18 September 2013

Accepted 22 September 2013

Keywords:

Temporal changes

Labile and stable fractions

Resilience

Yield

No-till cropping systems

Biomass-C input

A B S T R A C T

No-till (NT) cropping systems have been widely promoted in many regions as an important tool to

enhance soil quality and improve agronomic productivity. However, knowledge of their long-term

effects on soil organic carbon (SOC) stocks and functional SOC fractions linking soil resilience capacity

and crop yield is still limited. The aims of this study were to: (i) assess the long-term (16 years) effects of

tillage systems (i.e., conventional – CT, minimum – MT, no-till with chisel – NTch, and continuous no-till

cropping systems – CNT) on SOC in bulk soil and functional C fractions isolated by chemical (hot water

extractable organic C – HWEOC, permanganate oxidizable C – POXC) and physical methods (light organic

C – LOC, particulate organic C – POC, mineral-associated organic C – MAOC) of a subtropical Oxisol to

40 cm depth; (ii) evaluate the soil resilience restoration effectiveness of tillage systems, and (iii) assess

the relationship between the SOC stock enhancement and crop yield. The crop rotation comprised a 3-

year cropping sequence involving two crops per year with soybean (Glycine max, L. Merril) and maize

(Zea mays L.) in the summer alternating with winter crops. In 2005, the soil under CNT contained 25.8,

20.9, and 5.3 Mg ha�1 more SOC (P < 0.006) than those under CT, MT, and NTch in 0–40 cm layer,

representing recovery rates of 1.61, 1.31, and 0.33 Mg C ha�1 yr�1, respectively. The relative C conversion

ratio of 0.398 at CNT was more efficient in converting biomass-C input into sequestered soil C than NTch

(0.349), MT (0.136), and CT (0.069). The soil under CNT in 0–10 cm depth contained �1.9 times more

HWEOC and POXC than those under CT (P < 0.05), and concentrations of LOC and POC physical fractions

of SOC were significantly higher throughout the year under CNT. Considering CT as the disturbance

baseline, the resilience index (RI) increased in the order of MT (0.10) < NTch (0.43) < CNT (0.54). Grain

yield was positively affected by increase in SOC stock, and an increase of 1 Mg C ha�1 in 0–20 cm depth

resulted in an increase in yield equal to �11 and 26 kg grain ha�1 of soybean (R2 = 0.97, P = 0.03) and

wheat (R2 = 0.96, P = 0.03), respectively. The data presented emphasizes the role of labile fractions in the

overall SOC accumulation processes in soils managed under CNT and their positive impacts on the soil

resilience restoration and on agronomic productivity.

� 2013 Elsevier B.V. All rights reserved.

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jou r nal h o mep age: w ww.els evier . co m/lo c ate /s t i l l

1. Introduction

Land use change is a major factor that affects soil organic carbon(SOC) and global C balance. It is widely recognized thatconventional plow-based tillage disrupts soil structure and

* Corresponding author. Tel.: +55 42 3220 3090; fax: +55 42 3220 3072.

E-mail addresses: jcmsa@uepg.br, jcmoraessa@yahoo.com.br (J.C. de Moraes Sa).

0167-1987/$ – see front matter � 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.still.2013.09.010

aggregates (Tivet et al., 2013a), induces drastic changes in soilenvironment (i.e., temperature, moisture, and oxygen), decreasesthe overall biological activity (Babujia et al., 2010), exacerbatesCO2 emissions and other GHGs (Elliott, 1986; Polsow et al., 1987),and depletes SOC (Sa et al., 2001). In the main grain productionareas of Brazil (South and Center West, Cerrado), conversion to no-till (NT) has been driven by the adverse environmental impacts,particularly soil degradation by erosion, and high production costsof the traditional plow-based mechanized crop production. Thus,

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–50 39

NT systems have been widely adopted to minimize risks of soildegradation and to sustain the productivity of agroecosystems. By2010, NT systems are used on approximately �26 million ha (Mha)of cropland in Brazil (Febrapdp, 2012). Several studies, includingthose by Diaz-Zorita et al. (1999) and Lal (2006) in differentregions, have emphasized the positive relationship between yieldand SOC content, particularly in highly C-depleted soils of coarsetexture. In addition, recognizing the role of soils as a potential sinkfor atmospheric CO2 (Lal, 2004), there is a growing interest inadopting NT to store atmospheric CO2 in soils for off-settinganthropogenic emissions of CO2, but also to restore SOC pooldepleted by the conversion of native vegetation (NV) intoagroecosystems. Several farming systems have potential toincrease production with a lower footprint on the environment,while contributing to SOC accumulation and reducing emissions ofGHGs (Bernoux et al., 2006; Piva et al., 2012). Production of graincrops and livestock by grazing or mowing within the same farmhave showed positive impacts in terms of SOC accumulation insome regions of Brazil (Cerri et al., 2004; dos Santos et al., 2011)and in temperate conditions (Franzluebbers et al., 2012). However,the magnitude of SOC sequestration depends, among other factors,on the frequency of grazing, mowing or hay management(Franzluebbers and Stuedemann, 2009; Franzluebbers et al.,2001), the edapho-climatic conditions (Nunes Carvalho et al.,2010), and the nature and residence time of forage speciesinfluencing the above and belowground C input (Rezende et al.,1999). A large variability in SOC sequestration rate is also reportedfor grain crop production managed under NT cropping systems(Batlle-Bayer et al., 2010). Textural gradient, climatic conditions,and soil management (i.e., plow-based tillage, no-till) are thebaseline for the soil to function as a sink or a source of atmosphericCO2. However, the nature of the NT cropping systems (i.e., croppingsequence, use of relay/cover crop, frequency of a crop in thesequence) and the variability in biomass-C input (i.e., quantity andquality) for the same textural gradient and region, are the maincontrols of SOC sequestration (Ogle et al., 2005). The NT croppingsystems based on a diversity of cash crops + relay/cover crops, andincluding forage species producing a high biomass-C input (Seguyet al., 2006), maintains a permanent soil cover, supports acontinuous flow of biomass which release organic compounds. Inaddition, supports a highly structured food-web (Djigal et al.,2012) enhances soil microbial communities (Babujia et al., 2010;Kaschuk et al., 2010) and increases SOC storage. Based on severalassumptions, Cerri et al. (2010) estimated the amount in CO2-eqthat can be sequestered in Brazil by the adoption of NT until 2020.Cerri and collegues reported that the conversion from CT to NTsystems may represent in Brazil a net CO2 uptake rate of 1.91 MgCO2-eq ha�1 yr�1.

Since the late 1950s, a significant area of the Campos Gerais inthe Parana State has undergone major land use changes.Agricultural land previously managed under CT has progressivelyconverted to NT cropping systems since the late 1970s (Borges,1993). Based on an extensive survey (1.5 Mha), Sa et al. (2013b)reported that the adoption of NT systems mitigated 33.2 Tg of CO2

and off-set 22.5% (i.e., 7.47 Tg CO2) of total agricultural emissionsin the region. The continuous input of large amounts of biomass-Cto the soil surface creates positive C and N budgets (Ferreira et al.,2012; Sa et al., 2013a), accentuates C and N transformations andflow (Cardoso et al., 2011), and restores different C fractions.Several studies (Sa et al., 2001; Bayer et al., 2004) have emphasizedthe importance of the labile SOC fractions in the processes of SOCsequestration under NT. However, few long-term experimentsexist in the region to assess the impacts of a range of tillagesystems on SOC stock over time, and to evaluate soil resiliency.Based on the conceptual approach of Herrick and Wander (1997),we assumed herein that soil resilience is the capacity of soil to

recover its functional and structural integrity after a disturbanceinduced by CT. Thus, the present study was conducted to test thehypothesis that the NT cropping system by preserving highsurface-soil C fractions and labile pools increases SOC stock,restores soil resilience and enhances agronomic productivity. Thespecific objectives were to: (i) assess long-term (16 years)management effects of tillage systems (i.e., conventional – CT,minimum – MT, no-till with chisel – NTch, and continuous no-tillcropping systems – CNT) on SOC in bulk soil of a subtropical TypicHapludox to 40 cm depth, (ii) assess the magnitude of changes infunctional C fractions isolated by chemical (i.e., hot waterextractable organic C – HWEOC, potassium permanganate oxidiz-able C – POXC) and physical methods (i.e., light organic C – LOC,particulate organic C – POC, mineral-associated organic C –MAOC); (iii) evaluate the resilience of tillage systems whencompared with the native vegetation (NV) used as a baseline, and(iv) assess the relation between SOC stock and agronomicproductivity.

2. Materials and methods

2.1. Site location, characterization and land uses prior to the

experiment

A field experiment was established at the experimental stationof ABC Foundation, located in the city of Ponta Grossa, on thesecond plateau (910 m asl) in the South-central region of the Stateof Parana (2580005400 S and 5080900700 W). The climate of the regionis Cfb (Koppen, 1948), characterized by subtropical humidmesothermal regime with cool summers and frosts in the winter.The mean annual average precipitation and temperature are1545 mm and 18.2 8C (IAPAR, 1994), respectively.

Soil is classified as clayey Red Latosol (Brazilian classification,EMBRAPA, 1999; i.e., Typic Hapludox, Oxisol, Soil Taxonomy, USDAclassification). The soil is very well structured, and freely drained.The parent material was derived from reworked sandstonematerial from the Furnas formation and shale from the PontaGrossa formation of the Devonian period (Maack, 1981).

In 1967, some of the NV area was converted to agricultural landby plowing to 20-cm depth, and then disking twice (Fig. 1). Thisarea was cultivated for three years with rice (Oryza sativa L.) and for17 years with soybean (Glycine max, L. Merril) in the summer andwheat (Triticum aestivum L.) in the winter under conventionaltillage (i.e., plow-based tillage to 20 cm for one followed by two 60-cm diameter of disk harrowing). In 1981, this area was convertedto NT based on a sequence of soybean in the summer and mainlyblack oat (Avena strigosa Schreb) as a cover crop in the winter. In1988, this area was acquired by the ABC Foundation to be theexperimental station. In the winter of 1988, 4.5 Mg ha�1 ofdolomitic lime with 89% of effective calcium carbonate equivalentof the lime material and 400 kg ha�1 of thermal phosphatemagnesium (18% of P2O5 and 7% of magnesium) were appliedand incorporated by moldboard plowing to 30-cm depth and two60-cm narrow disking prior to implementing the experiment. Theobjective was to correct the soil acidity and enhance the P level inthe soil to medium level (e.g., 6 mg kg�1 of P, extracted by anionicresin). Black oat was sown to render the area homogeneous and toform a cover crop for growing soybean in the summer of 1988 andsupport high yield potential in the followed years. The presentexperiment was initiated in the winter of 1989. The situation priorto the establishment of the experiment is herein termed ‘‘PE’’.

2.2. Experimental design and crop rotation

The experiment comprises four tillage systems, implementedon 8.3 m � 25 m plots, according to a Randomized Complete Block

Fig. 1. Chronology of land use in the experimental area at the ABC foundation, city of Ponta Grossa, in the South-central region of the State of Parana, Brazil. The dashed arrow

represented sampling time (i.e., 1967, 1989, 2002, between 2003 and 2004, and in 2005).

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–5040

Design, with three replicates, covered a total area of 2500 m2

(Fig. 1). Tillage treatments consisted of: (i) conventional plow-based tillage (CT) consisting of plowing with a 70-cm disk diameterafter summer harvest and one after winter harvest to 20-cm depthfollowed by two 60-cm diameter of disk harrowing using a heavytractor with 120 hp and working at 7–8 km h�1 speed; (ii)minimum tillage (MT) consisting of one chisel plowing to 25-cmdepth and the tractor (i.e., 120 hp) can pull the chisel at 25 cmdepth and it can pull a scarified twice and one 60-cm diameter ofnarrow disking after summer harvest and one after winter harvest;(iii) no-till with one chisel plowing (NTch) to 25-cm depth onceevery three years; and (iv) continuous no-till without any soildisturbance (CNT). An area under NV, adjacent to the experimentalplots with the same topographic level and landscape position, wasselected as a baseline to assess the management-induced changesin SOC stock.

The crop rotation comprised a 3-year cropping sequence for theentire 16-year period involving two crops per year with soybeangrown in eleven and maize (Zea mays L.) in five summersalternating with oats, wheat and vetch (Vicia sativa L.) in thewinter (Table 1). The rate of fertilizer use every year (e.g., N, P2O5,and K2O) for each crop (e.g., wheat, soybean, and corn) was appliedby subsurface banding (Table 2).

2.3. Total biomass (above and belowground) and C input

The annual inputs of biomass-C in each tillage system wereestimated from detailed historic information about (i) grain yieldof wheat, soybean and maize, and (ii) shoot dry matter yield ofvetch, lupine and oat cover crops collected since the beginning ofthe experiment (Table 1). The shoot dry matter yield of wheat,soybean and maize was collected in 2002 (summer and winter) toestimate the grain yield/shoot ratio and the root/shoot ratio. Fivesub-plots (0.5 m � 0.5 m, aboveground and belowground biomass)were collected on each replicate for soybean, wheat, lupine, oatand vetch at the physiological maturity. Grain (soybean, wheat,lupine, and oat), shoot and root in 0–20 cm depth (Sa et al., 2010)were separated, dried at 65 8C and weighted. For corn, five plantswere collected per sub-plot, grain, shoot and root biomasscalculated following the same procedure than described above.Thus, the ratio grain yield/shoot ratio and the root/shoot ratio werecalculated from the data of above and below ground biomassobtained for each crop (summer and winter), and were estimatedonly once during the experiment. The mean and the standard

deviation of grain yield/shoot ratio for CT, MT, NTch and CNT were:0.95 � 0.01 for wheat, 0.89 � 0.01 for soybean, 1.1 � 0.013 for corn,and for black oat, white oat, vetch and lupine was 1.0 because was notcomputed the grain yield. The mean and the standard deviation ofroot/shoot ratio for CT, MT, NTch and CNT were: 0.15 � 0.012 forwheat, 0.23 � 0.015 for black and white oat, 0.17 � 0.013 for lupine,0.14 � 0.011 for vetch, 0.20 � 0.014 for soybean, and 0.25 � 0.014 forcorn. The ANOVA for grain yield/shoot ratio and root/shoot ratiorecorded no significant difference among tillage treatments, andmeans were used in this study to estimate aboveground and rootbiomass-C (Table 1). The aboveground and root biomass-C weredetermined by multiplying the grain yield:shoot and root:shoot ratiosby grain yield production of wheat, soybean and corn, and byaboveground production of cover crops (e.g., oats, and legumes).

Concentration of C in the crop residues (i.e., g C kg�1 of drymatter), measured by the dry combustion method using anelemental CN analyser (TruSpec CN, LECO, St. Joseph, MI, USA) was395 for soybean, 450 for wheat, 455 for maize, 432 for oat, 387 forvetch, and 402 g C kg�1 for lupine.

Total biomass-C was calculated as the sum of the abovegroundand belowground dry biomasses-C using the equation (Eq. (1)):

Total biomass-C ¼

ðgrain yield � grain yield=shoot ratio �C concentration in abovegroundÞ þðgrain yield � root=shoot ratio �C concentration in abovegroundÞ

1000(1)

Example 1: Total biomass-C = (2.73 � 0.89 � 0.395) + (2.73 �0.20 � 0.395)/1000 = 1.18 Mg ha�1 (see Table 1, for soybean, year89/90).

2.4. Soil sampling

Prior establishing the experiment in 1989, soil samples werecollected from two depths: 0–20 and 20–40 cm to establish thebase line and characterize the experimental site. Towards, the endof September 2002 and 2005, soil samples were obtained from fivedepths: 0–2.5, 2.5–5, 5–10, 10–20, and 20–40 cm. Bulk soilsamples for each replication and for all depths were obtained bydigging 20 cm � 20 cm trenches. Three sub-samples wereobtained per replicate, and were composited. Soil bulk density(rb) for each layer was measured by the core method (Blake andHartge, 1986) using cores of 5.0-cm diameter and 5.0-cm deep for

Table 1Crop sequence from 1989 to 2006, grain production (wheat + soybean or corn), aboveground, root, and total biomass-C input during the experiment.

Year Crop sequence Annual grain production Annual aboveground C inputk Annual root C inputl Total biomass-C inputm

Winter Summer CT

(Mg ha�1)

MT

(Mg ha�1)

NTch

(Mg ha�1)

CNT

(Mg ha�1)

CT

(Mg ha�1)

MT

(Mg ha�1)

NTch

(Mg ha�1)

CNT

(Mg ha�1)

CT

(Mg ha�1)

MT

(Mg ha�1)

NTch

(Mg ha�1)

CNT

(Mg ha�1)

CT

(Mg ha�1)

MT

(Mg ha�1)

NTch

(Mg ha�1)

CNT

(Mg ha�1)

89/90 B. Oata Soybeanb,h 2.7 2.5 2.7 2.6 0.96 0.88 0.95 0.90 0.22 0.20 0.21 0.20 1.18 1.08 1.17 1.11

90/91 Lupinec Cornd,i 10.4 9.8 11.8 10.2 6.20 5.99 6.97 6.21 1.35 1.30 1.52 1.35 7.55 7.29 8.49 7.56

91/92 B. Oat Soybeanh 3.7 4.3 4.3 4.0 2.72 2.93 2.94 2.89 0.62 0.67 0.67 0.66 3.33 3.60 3.61 3.55

92/93 Wheate Soybeanj 5.7 6.2 6.1 6.8 2.19 2.40 2.36 2.68 0.42 0.45 0.45 0.50 2.60 2.85 2.81 3.17

93/94 Vetchf Corni 10.2 10.5 10.4 10.5 6.00 6.15 6.14 6.22 1.28 1.32 1.31 1.33 7.28 7.47 7.45 7.55

94/95 W. Oat Soybeanh 3.5 3.6 3.6 3.6 3.12 3.21 3.19 3.24 0.71 0.73 0.73 0.74 3.83 3.94 3.92 3.98

95/96 Wheat Soybeanj 6.4 6.6 6.6 6.8 2.48 2.55 2.57 2.65 0.47 0.48 0.48 0.50 2.95 3.03 3.05 3.15

96/97 Vetch Corni 10.1 8.4 8.7 8.7 5.95 5.07 5.21 5.27 1.27 1.08 1.11 1.12 7.22 6.15 6.31 6.39

97/98 B. Oat Soybeanh 3.5 3.3 3.0 3.2 2.70 2.71 2.65 2.79 0.61 0.62 0.60 0.64 3.31 3.33 3.24 3.43

98/99 Wheat Soybeanj 6.7 6.7 6.6 6.5 2.58 2.62 2.59 2.56 0.49 0.49 0.48 0.47 3.07 3.11 3.06 3.03

99/00 W. Oat Corni 12.4 12.9 13.1 13.6 5.95 6.18 6.30 6.55 1.36 1.41 1.44 1.49 7.31 7.59 7.73 8.05

00/01 B. Oat Soybeanh 2.8 3.0 3.3 3.6 2.76 2.87 2.87 3.14 0.63 0.66 0.65 0.71 3.39 3.53 3.55 3.85

01/02 Wheat Soybeanj 7.2 7.3 7.8 7.8 2.80 2.83 3.04 3.05 0.52 0.53 0.57 0.57 3.33 3.37 3.61 3.63

02/03 B. Oat Corni 9.7 9.9 10.1 10.3 6.17 6.75 6.67 6.72 1.40 1.54 1.52 1.53 7.57 8.29 8.23 8.25

03/04 W. Oat Soybeanh 3.6 3.7 3.8 3.9 3.97 4.26 4.20 5.11 0.91 0.97 0.96 1.17 4.87 5.23 5.18 6.28

04/05 Wheat Soybeanj 8.3 8.5 8.7 8.9 3.25 3.32 3.39 3.47 0.60 0.62 0.63 0.65 3.85 3.94 4.02 4.12

05/06 B. Oat Corn –g – – – 1.78 1.85 1.86 1.94 0.41 0.43 0.43 0.45 2.19 2.28 2.29 2.38

Total grain

yield or

biomass-C

106.9 107.2 110.6 111.0 61.57 62.60 63.91 65.40 13.28 13.49 13.77 14.08 74.85 76.09 77.72 79.48

Annual

biomass-Cn

3.62ns 3.68 3.76 3.85 0.78ns 0.79 0.81 0.83 4.40ns 4.48 4.57 4.68

a Black oat (Avena sativa Schrub.), biomass-C input of oat in 1989 was not added.b Soybean (Glycine max L.).c Lupine (Lupinus angustifolius L.).d Corn (Zea mays L.).e Wheat (Triticum aestivum L.).f Vetch (Vicia sativa L.).g Yield of corn in 2006 was not added.h Refers the soybean yield after Black oat.i Refers the corn yield after legume.j Refers the sum of wheat and soybean yield for the year.k Aboveground C input was obtained multiplying grain yield/shoot ratio by grain yield of each crop. The grain yield/shoot ratios for cover crops (e.g., oats and legumes) were 1.0).l Root C input was obtained multiplying root/shoot ratio by grain yield for soybean, corn and wheat, and for oats and legumes by total aboveground production.m total biomass-C is the sum of aboveground and root C input. The grain yield/shoot ratio and root/shoot ratio were respectively of 0.89 and 0.19 for soybean, 1.1 and 0.25 for maize, 0.95 and 0.15 for wheat and 1.0 and 0.23 for oats.

The root/shoot ratio of vetch and lupine was 0.14 and 0.17, respectively.n Annual biomass-C input was calculated as the ratio between total biomass-C input divided by 17 years; ns = no significant difference for aboveground, root and annual C input among tillage treatments based on the ANOVA.

J.C.

de

Mo

raes

Saet

al.

/ So

il &

Tilla

ge

Resea

rch 1

36

(20

14

) 3

8–

50

4

1

Table 2Crop sequence from 1989 to 2006 and mineral fertilizer rate applied in winter and

summer crops (wheat, soybean and corn) during the experiment.

Year Crop sequence Annual mineral fertilizer rate

Winter Summer N

(kg ha�1)

P2O5

(kg ha�1)

K2O

(kg ha�1)

89/90 Black oata Soybean 0 60 60

90/91 Lupinea Corn 126 72 48

91/92 Black oat Soybean 0 60 60

92/93 Wheat Soybean 120b 120 120

93/94 Vetcha Corn 120 90 48

94/95 White oata Soybean 0 60 60

95/96 Wheat Soybean 120 120 120

96/97 Vetch Corn 126 72 48

97/98 Black oat Soybean 0 60 60

98/99 Wheat Soybean 120 100 100

99/00 White oat Corn 120 60 60

00/01 Black oat Soybean 0 40 40

01/02 Wheat Soybean 120 100 100

02/03 Black oat Corn 120 60 60

03/04 White oat Soybean 0 40 40

04/05 Wheat Soybean 114 130 100

05/06 Black oat Corn 120 60 60

Total fertilizer input 1326 1304 1184

a For the cover crops was not applied fertilizer.b For the crop sequence wheat and soybean the N rate was applied only for wheat

(30 kg N ha�1 by subsurface banding application, and 90 kg ha�1 by surface

braodcast application).

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–5042

the 5–10-cm, 10–20-cm and 20–40-cm depths (Table 3). Soil corewas obtained in the middle of the layer for the 10–20-cm and 20–40-cm depths. Cores of 5.0-cm diameter by 2.5-cm depth wereused for 0–2.5-cm and 2.5–5-cm depths. Two cores were obtainedfor each depth per replicate. Bulk samples were oven-dried at40 8C, gently ground, sieved through a 2-mm sieve, and homoge-nized. Soil cores were oven-dried at 105 8C.

2.5. Soil sampling for soil organic carbon fractionation

Physical SOC fractionation was done on the bulk samples of2002, and on additional soil samples collected on a monthly basis

Table 3Bulk density (rb), particle size contents (clay and silt), and chemical properties for eac

replicates).

Tillage Soil depth

(cm)

Bulk density

(Mg m�3)

Particle size Chemical pro

Clay

(g kg�1)

Silt

(g kg�1)

pH

CaCl2

H

(c

CT 0–2.5 1.08ns 467 196 5.7 5.

2.5–5 1.20 480 174 5.6 5.

5–10 1.20 493 170 5.4 6.

10–20 1.23 493 169 5.4 6.

20–40 1.12 527 146 5.4 5.

MT 0–2.5 1.09 487 177 5.6 5.

2.5–5 1.24 493 166 5.5 5.

5–10 1.27 493 160 5.4 6.

10–20 1.22 500 156 5.3 6.

20–40 1.13 540 136 5.4 5.

NTch 0–2.5 1.03 487 188 5.5 5.

2.5–5 1.21 493 160 5.3 6.

5–10 1.15 487 157 5.2 6.

10–20 1.16 513 151 5.3 6.

20–40 1.19 487 171 5.0 6.

CNT 0–2.5 1.06 477 184 5.9 4.

2.5–5 1.25 481 176 5.6 6.

5–10 1.31 489 165 5.6 6.

10–20 1.26 501 155 5.4 6.

20–40 1.16 512 154 5.6 5.

nsNo significant difference at P = 0.05 between tillage treatments and for each soil laye

between October 2003 and September 2004 (Fig. 1). For thelatter, twelve sampling zones were identified and three sub-samples were collected per replicate at each date and werecombined. All samples were oven dried at 40 8C and gentlyloosened and passed through a 2-mm sieve. The particle sizefractionation was done according to the method reported by Saet al. (2001). A 40-g oven dry (40 8C) subsample from eachtreatment and each depth was rewetted in 200 mL deionized H2Oand stored overnight at 4 8C. Aggregate disruption was accom-plished by horizontal shaking at a frequency of 100 oscillations/min with three agate balls with 10-mm diameter for 4 h. The soilsuspension was wet-sieved with a 250-mm sieve to obtain the250–2000-mm fraction (i.e., light organic C – LOC). The disruptedsoil suspension that passed through the 250-mm sieve was wet-sieved with a 53-mm sieve to obtain the 53–250-mm (i.e.,particulate organic C – POC) and <53-mm size fractions. Thesuspension in the slurry, representing the silt + clay fraction(<53-mm), was transferred in a 1-L glass cylinder, flocculatedwith 0.8 CaCl2, and it represented the mineral-associated organicC fraction (i.e., MAOC). Each fraction was oven dried at 40 8C andwas finely ground for C determination.

2.6. Hot water extractable organic carbon (HWEOC) and potassium

permanganate oxidizable C (POXC)

The HWEOC was determined by the method of Ghani et al.(2003). Briefly, 3 g of bulk soil was weighed into 15 mlpolypropylene centrifuge tubes. The sample was treated with10 ml of distilled water for 16 h at 80 8C. Each tube was thenshaken to ensure that the HWEOC released from the SOC wasfully suspended in the solution. The tubes were centrifuged for10 min at 4000 rpm. The SOC in the centrifuged extracts wasoxidized by dichromate in sulfuric acid and back titrated withferrous sulfate.

The determination of the POXC is adapted from several studies,including Tirol-Padre and Ladha (2004) and Culman et al. (2012).Briefly, 3 g of bulk soil was weighed into 15-ml polypropylenescrew-top centrifuge tubes. To each tube, 10 ml of a stock solutionof KMnO4 (60 mM) was added. The tubes were shaken at 200 rpm

h sampling layer under CT, MT, NTch or CNT tillage treatments (average of three

perties

+ Al

mol dm�3)

Ca2+

(cmol dm�3)

Mg2+

(cmol dm�3)

K+

(cmol dm�3)

P

(mg dm�3)

0 4.6 2.7 0.80 8.5

7 4.4 2.4 0.51 7.0

0 4.2 2.0 0.35 5.7

1 3.8 2.2 0.19 4.7

5 2.7 1.7 0.14 1.7

4 4.8 2.5 0.74 9.4

9 4.8 2.5 0.40 9.0

4 4.7 2.5 0.28 7.5

5 3.7 2.4 0.20 5.4

9 2.7 1.9 0.14 1.3

8 4.6 2.6 0.80 14.0

6 4.6 2.3 0.42 13.4

9 4.2 1.9 0.27 11.0

7 4.0 2.0 0.24 5.9

9 2.3 1.6 0.12 1.4

8 5.9 3.0 0.91 27.2

5 5.2 2.5 0.55 17.3

6 4.6 2.3 0.40 20.9

7 3.8 2.1 0.32 11.8

5 3.0 1.8 0.22 1.7

r.

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–50 43

for 15 min, and then centrifuged for 10 min at 2851 � g using aSirius 4000 centrifuge model. Of this, 2 mL of the supernatant waswithdrawn with a pipette and transferred to a 125 mL Erlenmeyer,and completed with deionized water. The absorbance of thesolutions was determined at 565 nm and the amount of theoxidized organic C calculated from the KMnO4 consumed.The conversion of the absorbance to POXC concentration (g kg�1)1) was done by using a standard calibration curve, based on thelinear relationship between concentrations of KMnO4 vs. absor-bance at 565 nm. The amount of POXC in the sample was computedas follows (Eq. (2)):

POXC ðmg g�1Þ ¼mM blank � mMsample � ð125=2Þ � 10 � 9

1000 ðmL L�1Þ � wt of sample ðgÞ(2)

where mM blank and mM sample are the concentrations(mmol L�1) of KMnO4 in the blank and sample, respectively,determined from the standard regression curve; 125/2 = thedilution factor (mL mL�1); 10 = the volume (mL) of KMnO4 addedto the soil sample; 9 = the amount of C oxidized for every mole ofKMnO4 (g mol�1 or mg mmol�1). Soil samples used to measureHWEOC and POXC were always taken from the same compositesample as used to measure the physical fractions.

2.7. Soil chemical properties and particle size-distribution analyses

In September 2002, soil pH was measured in 1:1 soil:1 M CaCl2solution (EMBRAPA, 1997), and exchangeable cations (Al3+, Ca2+,Mg2+, K+) and available P were extracted using a cation and anionexchange resin (Raij and Quaggio, 1983). Soil textural analyseswere performed for each replication for all depths (Table 3) usingthe hydrometer with a Bouyoucos’ scale (Gee and Bauder, 1986).

2.8. Soil organic carbon concentration, calculation of C stocks and

resilience index

Sub-samples of <2-mm bulk soil and of aggregate fractions(2000–250-mm; 53–250-mm; <53-mm) were finely ground(<150 mm) for measuring SOC and total N concentrations by thedry combustion method using an elemental CN analyser (TruSpecCN, LECO, St Joseph, MI, USA). Because rb did not vary significantlybetween the treatments within each sampled layer (Table 2), rb ofthe layers were used to calculate the stock of SOC without a masscorrection.

The rates of change of SOC (Mg ha�1 yr�1) among NV and CT,and among CNT and CT, were estimated by using equation (Eqs. (3)and (4)):

Depletion rate ¼ SOCNV � SOCCT

t(3)

Recovery rate ¼ SOCCNT � SOCCT

t(4)

where SOCNV, SOCCNT and SOCCT refers to soil C stock under NV,CNT and CT, respectively, and t is the time (years) since theconversion from NV to CT, and CT to CNT. The resilience index (RI)was computed (Herrick and Wander, 1997) to assess the rate ofSOC recovery for different tillage systems. This index uses NV asthe upper limit and CT as the lower limit of SOC levels (Eq. (5)):

RI ¼SOCCNT;MT;Ntch � SOCCT

SOCNV � SOCCT(5)

where RI is the resilience index.From 1989 to 2005, the amount of C converted from crop

residues to SOC (C-conversion rate, C-CR) was calculated as follows

(Eq. (6)):

C-CR ¼ DSOC2005�1989

Total biomass-C input(6)

where DSOC2005–1989 is the difference in SOC stock (Mg C ha�1)between 2005 and 1989, and Total biomass-C input (Mg C ha�1) isthe accumulated input of biomass-C from 1989 to 2005. Thus, C-CRrepresents the fraction of C from the biomass input converted intoSOC.

2.9. Statistical analyses

Differences among treatments for SOC concentration and stockwere tested for statistical significance through analysis ofvariance (ANOVA). Mean values were compared using the leastsignificant differences (LSD). Analysis was performed by soildepth, and results were considered statistically significant atP < 0.05 (Webster, 2007). Also, bulk density, root mass, root:shootratio, were tested through ANOVA to make sure if the variationcaused by tillage systems on these data would affect thecalculations.

The effects treatment on SOC fractions, measured monthly fromOctober 2003 to September 2004, was assessed through a linearmixed effects model. The relationship between SOC of eachfraction (i.e., LOC, POC, and MAOC) and the explanatory variables(i.e., tillage, days) results in linear mixed effect model wereexpressed as follows (Eq. (7)):

Yi jk ¼ M þ Ti þ R j þ bDi jk þ biDi jk þ bi j þ ei jk (7)

where Yijk is the SOC concentration of each fraction, i refers to thetillage treatments (i.e., Ti: CT, MT, NTch, CNT), j to the replicate (Rj),and k to the days (Dijk); M, Ti, Rj, b, and bi refer to the fixedparameters of the model; M refers to the intercept, when b and bi

are the fixed effect coefficients (like regression coefficients) of thedays and tillage; bij is the random effect coefficient of the tillage;and eijk is the residual effect. The effects of tillage, days, andreplicate, and the interaction between tillage and days on the SOCconcentration were addressed by performing an analysis ofvariance (function anova) on the linear mixed effect model. Thefunction lsmeans was used to compute the least square means oftillage treatments. Tukey test was used, and P values computed byusing the Studentized range distribution with the number ofmeans. All statistical calculations were carried out using RDevelopment Core Team version 2.15.2 (2006), package aov, andspecifically for the linear mixed effects model the packages lme4,lsmeans and pbkrtest. SigmaPlot 12.0 was used for graphicrepresentation.

3. Results and discussion

3.1. Soil organic C changes upon land conversion and tillage

We stated that SOC concentrations under the neighbouringnative vegetation in 2005 were representative of that presentat the time of establishing the experiment in 1989. Thus, thedepletion in SOC concentration in 1989 when compared withNV is mainly the result of the use of heavy mechanization(i.e., disk plowing, moldboard once + disk harrowing) forsoybean production in summer and wheat or fallow in winter(Table 4).

In 1989, the SOC concentration was measured at two depths(0–20 and 20–40 cm depths) and the experiment with differenttillage treatments was initiated. All of the treatments increasedthe SOC stock during the period from 1989 to 2005 (Fig. 3). Thistrend is attributed to the relatively high biomass-C inputreturned to the soil every year (Table 1). The mean annual

Table 4Soil organic carbon (SOC) concentration (g C kg�1) to 40 cm depth managed under

conventional tillage (CT), minimum tillage (MT), no-till chisel (NTch), continuous

no-till (CNT), prior to the establishment of the experiment (PE), and under the

neighbouring native vegetation (NV).

Year Soil layer

(cm)

NV PE CT MT NTch CNT

SOC (g C kg�1)

1967 0–20 40.3

20–40 28.8

1989 0–20 26.70

20–40 19.30

2002 0–2.5 32.4c 37.0b 39.0b 49.2a

2.5–5 31.8b 34.3b 39.0a 39.2a

5–10 31.2ab 29.3b 35.0a 30.6b

10–20 27.7ns 23.7 27.7 25.8

20–40 18.6b 20.9ab 22.3a 23.6a

2005 0–2.5 30.3d 34.0c 37.7b 52.2a

2.5–5 29.2d 32.1c 36.4b 41.2a

5–10 29.3b 31.8b 35.6a 31.9b

10–20 27.1b 25.6b 30.5a 26.6b

20–40 19.5b 20.0b 23.2a 24.1a

Lowercase letters within a same line indicate difference among tillage treatments;

ns: means are not significantly different at P = 0.05.

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–5044

input of biomass-C ranged from 1.71 to 1.94 Mg ha�1 duringthe winter and from 2.55 to 2.78 Mg ha�1 during the summer.The cumulative annual C input was 4.40, 4.48, 4.57, and4.68 Mg ha�1 under CT, MT, NTch, and CNT, respectively(Table 1).

Concentrations of SOC decreased significantly with increase insoil depth, and exhibited different patterns of distributionsamongst tillage treatments. Specifically, SOC concentrations werestratified with depth under CNT (Table 4) when compared with CT.Significant differences in SOC concentrations among tillagetreatments were observed for the different depths with the

Fig. 2. Soil organic C and N stocks (Mg ha�1) to 40 cm depth under conventional tillag

Horizontal bars denote significance (P = 0.05) among tillage. The level of significance f

exception of 10–20 cm depth in 2002. Higher SOC concentration(P < 0.01) in surface layers (0–2.5 and 2.5–5 cm depths) wasobserved under CNT (with the exception in 2.5–5 cm depth in2002) when compared with others tillage treatments. In 2005, soilunder CNT contained 16.9 g kg�1 more SOC in 0–5 cm depth thanthat under CT. Significant differences were also evident in 20–40 cm depth even with lower SOC concentrations under CT and MTwhen compared with CNT. In 2005, higher SOC concentrations(P < 0.01) were observed under NTch in 5–10 and 10–20 cmdepths when compared with others tillage treatments.

Soil rb used for NV and PE to calculate SOC stocks was estimatedfrom Sa et al. (2001) for the same type of soil and textural gradient.Soil rb for NV and PE in 0–20 and 20–40 cm depths were estimatedto 1.18 and 1.12 Mg dm�3, respectively. Further, tillage treatmentssignificantly impacted SOC stocks (Fig. 2). The average SOC stock in0–20 cm depth decreased from 94.9 Mg ha�1 under NV to62.9 Mg ha�1 for CT in 1989 which attained the value of67.8 Mg ha�1 in 2005, a loss of �29% over 38 years since theconversion of NV into cultivated field with the use of CT (Fig. 3).The decline of SOC under CT represents a depletion rate of �0.60and �1.07 Mg C ha�1 yr�1 in 0–20 and 0–40 cm depths, respec-tively. Although soil erosion was not measured, the visualobservation of these plots, sited on about 1–2% slope, showed aminimal erosional impact. Thus, the depletion in SOC is primarilyrelated to a higher oxidation rate of SOC under plowing (Reicoskyet al., 1995). Sa et al. (2001) reported for a Brazilian Oxisol of thesame region that SOC loss due to the conversion of NV during thefirst 20 years was 1.09 Mg ha�1 yr�1 in 0–40 cm depth.

During the period 1989–2005, the SOC stock under CT increasedby 5.2 Mg ha�1 in 0–40 cm depth. The increase, attributed to thehigh annual biomass-C input of �4.4 Mg ha�1 during this period inresponse to the amelioration of soil fertility status, createsfavourable conditions for the CT to improve crop yields throughthe maintenance of SOC stock. This result is likely due to the soiland crop management system practiced for 14 years before 1981,

e (CT), minimum tillage (MT), no-till chisel (NTch), and continuous no-till (CNT).

or each soil depth interval is indicated (*<0.05; **<0.01; ***<0.001).

Fig. 3. Changes in soil organic C stocks (Mg C ha�1) to 0–20 and 0–40 cm depths

under native vegetation (NV), prior the establishment of the experiment in 1989

(PE), conventional tillage (CT), minimum tillage (MT), no-till chisel (NTch), and

continuous no-till (CNT). DSOC = CNT � CT, refers to the difference between the

SOC stock of CNT and CT in 2005.

Table 5Soil organic C concentration (g C kg�1) of the light organic C fraction (LOC, 250–

2000 mm), particulate organic C (POC, 53–250 mm), and mineral-associated organic

C (MAOC, <53 mm) for 0–40 cm depth managed under conventional tillage (CT),

minimum tillage (MT), no-till chisel (NTch), and continuous no-till (CNT) at 2002.

Tillage systems Soil layers (cm)

0–2.5 2.5–5 5–10 10–20 20–40

LOC (g C kg�1)

CT 3.11ns 3.04ns 1.47ns 1.03ns 0.59ns

MT 2.94 2.46 1.69 0.62 0.45

NTch 4.10 2.70 1.81 0.80 0.60

CNT 4.78 3.04 2.07 1.12 0.73

POC (g C kg�1)

CT 5.87b 4.56ns 4.93ns 3.72ns 2.04ns

MT 5.55b 5.41 4.28 2.96 1.17

NTch 10.17a 7.31 5.71 3.32 1.87

CNT 12.21a 6.77 4.82 2.05 1.02

MAOC (g C kg�1)

CT 19.49b 19.61ns 18.77ns 17.04ns 14.73ns

MT 21.16b 21.15 21.46 19.40 15.75

NTch 19.64b 20.75 18.78 17.61 15.79

CNT 24.87a 21.35 18.64 18.16 14.90

Lowercase letters indicate difference among tillage treatments. ns: means within a

same column are not significantly different at P = 0.05.

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–50 45

which included burning the crop residues, driving down the SOCconcentrations. Thus, returning a higher biomass input (from 1989to 2005) to the soil, through more intensified cropping sequencethan prior the establishment of the experiment, may provide someincrease in SOC stock. However, the maintenance of continuous CTinduced SOC depletion and adversely impacted the fertility statusthat jeopardized soil capacity to support this trend on long-termbasis.

3.2. Changes in soil organic C pools: physical fractions, HWEOC and

POXC

The physical fractions >53 mm (i.e., LOC and POC), averagedacross all soil depths, accounted for 45%, 40%, 46% and 43% of thetotal soil mass under CT, MT, NTch and CNT, respectively. Asexpected, clay + silt sized (<53 mm) SOC complexes comprised ofthe largest C concentrations. The MAOC concentration decreasedslightly with depth, regardless of tillage treatments, and rangedfrom 23.1 to 14.9 g C kg�1 of bulk soil in 0–5 and 20–40 cm depths,respectively. Although not significant, the LOC concentrationsappeared in the order MT < CT < NTch < CNT in 0–2.5 cm depth.By contrast, significant differences (P < 0.05) in POC and MAOCconcentrations among tillage treatments were observed in soilsurface layer (Table 5). Both POC and MAOC in 0–2.5 cm depthdecreased from 12.2 and 24.9 g C kg�1 under CNT to 5.9 and19.5 g C kg�1 under CT (P < 0.05), representing a decline of 52%and 22% in POC and MAOC, respectively. Although no significantdifferences were observed, POC concentration was twice as muchunder CT at 20–40 cm depth when compared with CNT, and mayhave resulted from the redistribution of POC by successiveplowing. Although MAOC concentration in CNT was the higheston a soil mass (Table 5), it was significantly lower on SOC basis(%MAOC/SOC) than in CT and MT in 0–2.5 cm depth. In contrast,POC expressed on SOC basis, represented 18% and 25% in CT andCNT, in 0–2.5 cm depth, respectively. These trends emphasize theimportance of C accumulation in the particulate fraction underCNT.

Similarly, monthly changes in SOC physical fractions betweenOctober 2003 and September 2004 were observed in POC in 0–10 cm depth (Fig. 4). Difference in LOC and POC among tillagetreatments (P < 0.05) was observed exclusively in the soil surfacelayer (0–2.5 cm). The SOC concentrations of LOC and POC increasedin the order of CT < MT � NTch � CNT in 0–2.5 cm depth. Inaddition, POC concentration was higher under CNT and NTch thanthat under CT in 2.5–5 and 2.5–5 cm depths.

In all tillage treatments, HWEOC was more than POXC, andranged from 7 to 10% and from 5 to 8% of SOC concentration ofbulk soil amongst tillage treatments and soil depth, respectively.Both HWEOC and POXC concentrations were almost uniformlydistributed in 0–10 cm depth under CT, MT and NTch (Table 6).In contrast, a clear formation of a gradient of HWEOC and POXCfrom surface to sub-soil layers suggests that biomass input underCNT provided higher amount of labile organic compoundscreating a continuous C flow. Independent of the managementpractices, HWEOC and POXC decreased slightly in 10–20 and 20–40 cm depths. Close linear relationships were observed betweenSOC and HWEOC (HWEOC = 0.107SOC � 0.75, R2 = 0.88, P <

0.001), and between SOC and POXC (POXC = 0.081SOC � 0.61,R2 = 0.88, P < 0.001). Among tillage treatments, higher concen-trations of HWEOC and POXC were observed under CNT in 0–2.5and 2.5–5 cm depths (Table 6). Significant differences (P < 0.05)were still evident in 5–10 and 10–20 cm depths with lowerconcentrations under CT when compared with CNT in 2005. Alsoin 2005, the soil under CNT in 0–10 cm depth contained �1.9times more HWEOC (+5.9 g C kg�1) and POXC (+4.3 g C kg�1)than those under CT.

0-2

.5 c

m2.5

-5 c

m5-1

0 c

m10-2

0 c

m

Mineral associated orga nic C

<53 µm

0

5

10

15

20

25

Particulate orga nic C

53-250 µm

0

2

4

6

8

10

12

14

0

5

10

15

20

25

0

2

4

6

8

10

12

14

g C

kg

-1 o

f b

ulk

so

il

0

2

4

6

8

10

0

5

10

15

20

25

0

2

4

6

8

10

12

14

0

2

4

6

8

10

Days

0 100 20 0 30 0 400

0

5

10

15

20

25

Days

0 10 0 200 30 0 40 0

0

2

4

6

8

10

12

14

Days

0 10 0 200 30 0 400

0

2

4

6

8

10

Light particulate orga nic C

250-2000 µm

0

2

4

6

8

10

CNT

NTch

MT

CT

0-2

.5 c

m2.5

-5 c

m5-1

0 c

m10-2

0 c

m

CNT(a)-NTch(ab)-MT(bc)-CT(c) CNT(a)-NTch(b)-MT(cd)-C T(d) ns

Fig. 4. Monthly changes in particle size fractions (i.e., 250–2000 mm: light particulate organic C; 53–250 mm: particulate organic C; <53 mm: mineral associated organic C) to

20 cm depth under conventional tillage (CT = *), minimum tillage (MT = &), no-till chisel (NTch = ~), and continuous no-till (CNT = *). Lowercase letters in brackets

indicate difference among tillage treatments. ns: means are not significantly different at P = 0.05.

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–5046

Haynes and Francis (1993) emphasized that HWEOC isrepresentative of the microbial biomass carbon (MBC), contain-ing more microbial derived than acid hydrolysable carbohy-drates. Also, POXC is significantly related to POC, MBC, and evenmore closely related to smaller-sized (53–250 mm) than largerPOC fractions (250–2000 mm), reflecting a relatively processedpool of labile soil C (Culman et al., 2012). Short-term structural

Table 6Soil organic C concentration of hot-water extractable organic C (HWEOC), and labile SO

under conventional tillage (CT), minimum tillage (MT), no-till chisel (NTch), and conti

Year Soil layer (cm) HWEOC

CT (g C kg�1) MT (g C kg�1) NTch (g C kg�1) CNT (

2002 0–2.5 2.59b 2.94b 3.02b 3.96a

2.5–5 2.33b 2.57b 3.17a 3.25a

5–10 2.31ns 2.19 2.87 2.42

10–20 2.03ns 1.99 2.28 2.14

20–40 1.36ns 1.94 1.79 1.88

2005 0–2.5 2.31b 2.67b 2.92b 5.30a

2.5–5 2.03c 2.38bc 2.98b 3.92a

5–10 2.07b 2.33b 2.92a 3.13a

10–20 1.87b 2.16ab 2.53a 2.36a

20–40 1.39ns 1.87 1.87 2.02

Lowercase letters within a same line indicate difference among tillage treatments. ns:

soil stability is also positively correlated with both fungal andbacterial densities in NT systems (Lienhard et al., 2013), andclosely related to the concentration of HWEOC and other labilefractions as POXC (Haynes and Swift, 1990). Thus, higherconcentrations of HWEOC and POXC under CNT may positivelyinfluence aggregate formation, which may protect SOC (Tivet etal., 2013a).

C pool extracted by potassium permanganate (POXC) for 0–40 cm depth managed

nuous no-till (CNT) at two sampling time (2002 and 2005).

POXC

g C kg�1) CT (g C kg�1) MT (g C kg�1) NTch (g C kg�1) CNT (g C kg�1)

1.91d 2.19c 2.50b 2.87a

1.83c 2.09b 2.39a 2.40a

1.74 1.99 1.98 1.95

1.43 1.58 1.61 1.52

0.97 0.96 1.14 1.15

1.69d 1.99c 2.42b 3.52a

1.59d 1.93c 2.24b 3.05a

1.57c 2.11ab 2.01bc 2.53a

1.37ns 1.73 1.77 1.93

0.98ns 0.92 1.20 1.24

means are not significantly different at P = 0.05.

Fig. 5. Relationship between SOC and N stocks (Mg ha�1) in 0–10 cm depth.

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–50 47

3.3. Soil organic C restoration and soil resilience in response to

biomass-C input and no-till

In 2005, the soil under CNT in 0–5 cm depth contained 9.7, 7.5,and 6.0 Mg ha�1 more SOC stock (P < 0.001) than those under CT,MT, and NTch, respectively (Fig. 2). In addition, there were nosignificant differences in SOC stock among tillage treatments in 5–10 and 10–20 cm depths. However, higher SOC stocks (P < 0.01)were observed under CNT and NTch in 20–40 cm depth whencompared with CT. Although differences were restricted to the 0–5 cm and 20–40 cm depths, the increases in SOC stock caused byCNT influenced the entire soil 0–40 cm depth. Thus, the soil underCNT in 2005 contained 25.8, 20.9, and 5.3 Mg ha�1 more SOC stock(P < 0.006) than those under CT, MT, and NTch in 0–40 cm layer,representing recovery rates of 1.61, 1.31, and 0.33 Mg C ha�1 yr�1,respectively (Fig. 3). In addition, N stock (Mg N ha�1) increasedsignificantly in the order of CT (3.52) < MT (3.77) < NTch(4.33) < CNT (5.21) in 0–20 cm depth (Fig. 2) with significantchanges restricted to the surface layer (0–2.5 cm). As widelyreported in the literature, a strong correlation between SOC andtotal N (Fig. 5) is observed, emphasizing that SOC sequestration isclosely related to N accumulation. Most of the increase in SOC stockobserved in the soil under CNT, as compared with that in the CT,was in the 0–5 cm (38%) and the 20–40 cm (48%) layers. Incontrast, the same comparison showed an increase in SOC stock inthe 2.5–5 cm (11%) and 20–40 cm (58%) layers under NTch whencompared with CT. These variations may result from differentdegree of mechanical disturbance amongst CNT and NTchtreatments. While only the seedling zone is slightly disturbed inCNT, the chisel used in NTch disturbs the entire soil surface,increasing the decomposition rate of crop residues and theoxidation of SOC stock due high N mineralization rate as reportedby Sa et al. (2011) and Cardoso et al. (2011) in the sameexperiment. In addition, the specific results under CNT and NTch

suggest that most of the accumulated SOC stock in 20–40 cm depthis derived from root residues turnover and/or that part of the SOCin surface layers illuviated down the profile and may be attributedto increased faunal activity or water infiltration. Sisti et al. (2004)also observed for a Brazilian Oxisol (63% clay) managed under NTfor 13 years a strong tendency for the 13C abundance of the soilunder NT to be less negative than that under CT, indicating thatthere was more maize derived SOC below 30 cm depth. Sisti andcolleagues, also observed that most of the difference in SOC stockbetween NT and CT occurred in deeper soil layers, and emphasizedthat the decomposition of the antecedent SOM was not affectedunder NT, but was under CT in some rotations (including vetch andmaize) which stimulated the decay of the original native SOM.

The magnitude of SOC fractions emphasized that the smallerfraction (i.e., MAOC) is quantitatively essential to SOC sequestra-tion, representing from 63 to 72% of the C concentration in the 0–5 cm layer and more in deeper layers (Table 5). However,differences of POC among tillage treatments in soil surface layershighlighted the role of coarse fraction in SOC sequestration underCNT. The C stored in the POC fraction represents environmentaland agronomic benefits, which depend on the maintenance of highand diverse biomass-C input under NT on the long-term, topreserve high surface-soil C fractions and to ensure a progressiveprotection of labile moieties within macroaggregates. Thus, SOCmay be relatively more labile in CNT than under CT. Based on SOCand N stocks, the C:N ratio increased in the order of CNT(12.9) < NTch (14.0) < MT (15.4) < CT (19.1) in 0–5 cm depth.Other researches (Martins et al., 2011; Tivet et al., 2013b) havereported that the humification degree of SOC in soil under CT canbe higher than those under NT in 0–5 cm depth, indicating arelease of labile SOC under CT as a result of the continuous andstrong disruption of aggregates. In contrast, the higher protectionof SOC by aggregates in soil under NT enhances labile SOC in eachaggregate size fraction. Tivet et al. (2013a) showed that thedifference in SOC stock among CT and NT systems is largelyattributed to storage in large macroaggregates, which are crucialfor the physical protection of POC. Then, part of this fraction willbecome later stabilized through selective preservation by bio-chemical recalcitrance or interactions with mineral surfaces (vonLutzow et al., 2006).

In comparison with SOC stock at the beginning of theexperiment (1989), changes in the 0–40 cm layer were+5.2 Mg ha�1, +10.1 Mg ha�1, +25.7 Mg ha�1, and +31.0 Mg ha�1,representing sequestration rates of 0.33, 0.63, 1.61, and 1.94 MgC ha�1 yr�1 under CT, MT, NTch and CNT, respectively (Fig. 3).These rates of SOC sequestration are more than those commonlyreported in the literature for the soils of subtropical region of Brazil(Boddey et al., 2010; Amado et al., 2006; Dieckow et al., 2009; Sa etal., 2001) because the biomass-C input in this study is higher thanthose previously mentioned, except the work reported by Amadoet al. (2006). For example, Sa et al. (2001) showed an increase of�1.0 Mg ha�1 yr�1 after 22 years of continuous no-till in the 0–40 cm layer with an annual biomass-C input of � 3.3 Mg ha�1.

The significant increase in SOC stock when comparing CNT toCT contrasted with several studies in southern Brazil thatemphasized that SOC stock were not greater under NT than CTwhen N2-fixing winter legume were not used (Boddey et al., 2010;Freixo et al., 2002; Machado and Silva, 2001). Amado et al. (2006)also noted the role of N status in improving SOC stock insubtropical ecosystems of Brazil. If a negative N balance (i.e.,output due to grain production + losses by sublimation andleaching > input by N2-fixing and N fertilizer) limited SOCsequestration (Sisti et al., 2004), low biomass-C input underNT is also one of the main common situation reported in theliterature given low sequestration rate or no difference betweenCT and NT treatments. Several studies (Bayer et al., 2009; Amado

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–5048

et al., 2006; Sa et al., 2001; Virto et al., 2012) have emphasized thefact that biomass-C input is the cornerstone of the system,explaining much of the difference in SOC stock amongst CT and NTsystems.

In the present study, the systems are fuelled with relativelyhigh amounts of N fertilizer and the use of legumes thrice in thewinter (i.e., lupine once in 1991 and vetch twice in 1994 and1997) between 1989 and 2005 surely improved the N balance.Drinkwater et al. (1998) showed that N for maize derived fromcover crop legumes in rotation, is far more efficient for buildingSOC than fertilizer N applied to maize monocultures. Thedifference in annual biomass-C input (Table 1) did not exceed0.36 Mg ha�1 amongst tillage treatments, with highest inputunder CNT (4.68 Mg C ha�1 yr�1) and the lowest in NTch (4.57 MgC ha�1 yr�1) and cannot explain by itself difference in SOC stocks.An estimative of a global C-CR (i.e., from biomass-C input to SOC)is given herein as the ratio between the difference in SOCbetween 1989 and 2005 and the cumulative biomass-C inputduring the same period. The values ranged from 0.07, 0.14, 0.35,and 0.39 under CT, MT, NTch, and CNT, respectively. This trendemphasizes the differences amongst tillage treatments in themechanisms involved of the stabilization and protection of SOCpools (i.e., different protective capacity through aggregation,spatial inaccessibility of SOC to microbial attacks, and biochemi-cal recalcitrance of organic compounds). Significant differencesin the relation between N and SOC stock were observed amongsttillage systems for 0–10 cm depth (Fig. 5). The contribution ofeach N unit to C stock is lower in CNT than in others tillagetreatments, suggesting lower oxidation rate and higher protec-tive capacity of soil. A lower intercept was observed in CNT(SOC = 10.2N + 2.98, R2 = 0.89, P < 0.0002) followed by CT (SOC =14.4N + 2.92, R2 = 0.79, P < 0.001) and MT (SOC = 18.1 N � 1.20,R2 = 0.71, P < 0.004). However, no significant relationship wasobserved under NTch. In our study, no significant differencesbetween tillage treatments were reported in root/shoot ratio in0–20 cm depth. However, we can also state that the root biomassobtained in 0–20 cm depth does not reflect the belowgroundinputs from roots. Sisti et al. (2004) showed that increased Caccumulation in NT soil below 30 cm depth could be explained bygreater root density when compared with CT. Thus, thecontributions of root systems to SOC accumulation can be

Fig. 6. Yield of wheat and soybean (a) related to total soil organic C stock (Mg C ha�1), and

depth of an Oxisol subjected to 17 years of the following NT cropping system: two crops p

in six summers alternating with oats, wheat and vetch (Vicia sativa L.) in the winter.

underestimated in our study. dos Santos et al. (2011) also showeda close relationship between SOC stock and root C addition, and apoor relationship with total C addition and no relationship withshoot C addition.

The resilience index (RI) based on the SOC stocks of 2005increased in the order MT (0.10) < NTch (0.43) < CNT (0.54).Expectedly, the highest RI is associated with the highest rate ofSOC sequestration (CNT). Further, SOC stocks under CNT in 0–20and 20–40 cm depths represented �86% of the SOC stocksmeasured under NV. Thus, restoration of RI by CNT is evidencethat the soil capability is governed by complex process involvingthe control of oxidation based on the formation of macroaggregates (Tivet et al., 2013a), and also to the continuous Cflow. The latter stimulates the C migration from light organic C toPOC, stabilizes the aggregates, and induces C protection (Six et al.,2002). The RI, as used in the present study, reinstates that NT usedin conjunction with a diverse biomass C-input to keep the soilsurface covered is an important strategy to drive the soil to a newequilibrium and with a high positive impact on the agronomicproductivity.

3.4. Impact of tillage systems on the agronomic productivity

Grain yields were recorded throughout the experiment (Table1). The mean yields of soybean, wheat, and corn amongst thetillage treatments from 1989 to 2005 were 3.55 � 0.08,3.62 � 0.16, and 9.73 � 0.20 Mg ha�1 (mean � std deviation), re-spectively. The ability of CT to sustain such high yields of soybean,wheat and maize is directly related to the high fertility status of thissoil (Tables 2 and 3) with remedial use of deep lime and phosphorusapplied prior to establishment of the experiment. Since 1989, theyield of soybean has increased annually at the rate of �30, 28, 37,and 44 kg ha�1 under CT, MT, NTch, and CNT, respectively. Incomparison, the yield of wheat has increased at rate of �146, 126,136, and 106 kg ha�1, respectively. However, the yield of maize isrelatively stable with no significant increase between 1989 and2005. In 1990, the average yield of maize amongst tillagetreatments already exceeded 10 Mg ha�1, which remains theaverage yield of the region. The increase in yield of wheat andsoybean is related to several operations and managerial inputs suchas improved varieties, pest management, improved harvest

yield of wheat (b) and soybean (c) related to HWEOC, POXC and LOC stocks to 20-cm

er year with soybean (Glycine max, L. Merril) grown in eleven and maize (Zea mays L.)

J.C. de Moraes Sa et al. / Soil & Tillage Research 136 (2014) 38–50 49

technologies, and better soil fertility status. However, a closerelationship was observed between the SOC stock to 20-cm depthand the mean yield from 1989 to 2005 (Fig. 6). Increase in SOC stockby 1 Mg C ha�1 in 0–20 cm depth increased yield by �11 and 26 kggrain ha�1 of soybean (R2 = 0.97, P = 0.03) and wheat (R2 = 0.96,P = 0.03), respectively. The effect of SOC stock on grain yield isprimarily attributed to its ability to act as a source of nutrients forcrops, increase in plant available water capacity, improved soilbiological activities enhancing plant nutrition, and to favourablesoil structure and other physical properties (Lal, 2006). Severalstudies have reported a close relationship between SOC stock andcrop yield. Sa et al. (2013a) observed that for every Mg increase inSOC stock to 1-m depth soybean yield increased by 28 kg ha�1. Inthe semi-arid Pampa (<150 g clay kg�1) in Argentina, Diaz-Zorita etal. (1999) reported that wheat yields were linearly related to TOC(r = 0.68, P < 0.01) when these contents were <17.5 g C kg�1. Inaccord with these, others have reported that depletion of SOC stockby �1 Mg C ha�1 decreased wheat yield of approximately69 kg ha�1 (Lal, 2006). In his review (Lal, 2006) showed that inmost soils the relation between yield and SOC content is linear up toa limit of 20 g C kg�1, and that in some soils an increase in crop yieldis primarily due to an increase in the labile SOC fraction. In thepresent study, yields of wheat and soybean are also closely relatedto HWEOC, POXC and LOC fractions (Fig. 6), but no relationships areobserved for grain yield with POC and MAOC. In addition, betterrelationships of grain yield are observed with HWEOC, followed byPOXC and LOC indicating that the labile C is a sensitive indicatorrelated to agronomic yield. The latter hypothesis is also supportedby the fact that other factors (e.g., rainfall distribution andtemperature) which moderate HWEOC, POXC and LOC also stronglyimpact the agronomic productivity. Additional research, includingsoil aggregation, water infiltration and retention, is needed to drawvalid conclusions on the role of labile SOC fractions (e.g., HWEOC,POXC and LOC) to improve grain yield under CNT. Such research isessential to assess the synergies induced by diversified NT systemsas a foundation of SOC accumulation and to advance in farmsustainability.

4. Conclusions

The data presented support the following conclusions:

� The SOC decreased upon conversion of native vegetation toagroecosystem because of a continuous use of plow-based CT,leading to a depletion of �0.60 and �1.07 Mg C ha�1 yr�1 in 0–20and 0–40 cm depths, respectively.� Conversion to NT cropping systems recovered part of the original

SOC stock, with sequestration rates of 1.61, 1.31, and 0.33 MgC ha�1 yr�1 when compared with CT, MT and NTch, respectively.� Differences in SOC stocks among tillage systems are primarily

due to higher C conversion ratio under CNT resulting from thepreservation of high surface-soil C fractions and labile pools (i.e.,HWEOC, POXC).� There exists a strong relation between SOC stock, labile fractions,

and the grain yield of wheat and soybean.� NT cropping systems offer a vast potential to decrease the

environmental footprint, restoring soil resilience and enhanceagronomic production through continuous flow of C in thepedosphere.

Acknowledgements

We thank the ABC Foundation for allowing access to theexperimental fields. We greatly appreciate the help from Mrs.Jaqueline Aparecida Goncalves and Mr. Romeu Martins Filho forlaboratory analyses. This work was supported by the Agrisus

Foundation (Project # PA 677/10), the Centre de CooperationInternationale en Recherche Agronomique pour le Developpement,and by the Carbon Management and Sequestration Centre at theOhio State University.

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