Dynamics of soil organic carbon and its fractions after abandonment of cultivated wetlands in northeast China

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  • Dynamics of soil organic carbon and its fractions after

    recovery of cultivated soil that was abandoned. Soil temperature

    Soil & Tillage Research 96 (2an increase in the light-fraction organic C (LF-OC), microbial biomass C (MBC), and dissolved organic C (DOC) concentration.

    The rate of increase in LF-OCwas considerably higher than that in SOC and HF-OC. Similarly, the rate of increase inMBCwas also

    considerably higher than that in SOC in cultivated soils abandoned for 48 years. However, the rate of increase in DOC was far

    lower than that in SOC. The R2 value for the correlation between the increments of the LF-OC and SOC was significantly higher

    than that for the correlation between DOC and MBC (0.99 vs. 0.90), indicating that LF-OC was the most sensitive fraction for

    detecting changes in organic C due to the abandonment of cultivated soil.

    # 2007 Elsevier B.V. All rights reserved.

    Keywords: Agricultural abandonment; Labile organic C; Northeast China; Soil organic C; Wetlands

    1. Introduction

    Soil organic matter (SOM) is a major reservoir of

    organic carbon and is estimated at approximately

    * Corresponding author.

    E-mail addresses: zhangjb@mail.issas.ac.cn (Z. Jinbo),

    songcc@neigae.ac.cn (S. Changchun).

    0167-1987/$ see front matter # 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.still.2007.08.006indirectly influenced the SOC concentration by affecting soil microbial activity. The abandonment of cultivated wetlands resulted inconcentration according to the observation made during theabandonment of cultivated wetlands in northeast China

    Zhang Jinbo a, Song Changchun b,*, Wang Shenmin c

    a State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences,

    No. 71 East Beijing Road, Nanjing 210008, ChinabNortheast Institute of Geography and Agroecology, Chinese Academic Science, Changchun Jilin 130012, China

    cDepartment of Resources and Tourism Sciences, Nanjing University, 210093, China

    Received 23 August 2006; received in revised form 8 June 2007; accepted 11 August 2007

    Abstract

    Soil organic carbon (SOC) and its different labile fractions are important in minimizing negative environmental impacts and

    improving soil quality. However, very little is known of the dynamics of SOC and its labile fractions after the cultivated wetlands

    have been abandoned in northeast China. The objectives of this study were (1) to estimate the dynamics of SOC after the

    abandonment of cultivated soil, (2) to investigate the most sensitive fraction for detecting changes in organic C due to the

    abandonment of cultivated soil, and (3) to explore the key factors affecting the dynamics of soil C after the abandonment of

    cultivated soil in the freshwater marsh region of northeast China. Our results showed that the abandonment of cultivated wetlands

    resulted in an increase in SOC and the availability of C. The SOC content increased to 31, 44, and 107 g kg1 after these cultivatedwetlands were abandoned for 1, 6, and 13 years, respectively, as compared to an SOC content of 28 g kg1 in the soil that had beencultivated on for 9 years. In northeast China, where a cultivated wetland was abandoned, the initial regeneration of SOC pools was

    considerably rapid and in accordance with the Boltzmann equation. An analysis of the stepwise regression indicated that the

    dynamics of SOC (g kg1) can be quantitatively described by a linear combination of the root density and the mean soil temperature5 cm underground in the growing season, as expressed by the following relationship: TOC = 0.008 root density 3.264T + 96.044(R2 = 0.67, n = 9, p < 0.05. T is the mean soil temperature 5 cm underground in the growing season), indicating that approximately67% of the variability in SOC can be explained by these two parameters. The root biomass was the key factor affecting SOCwww.elsevier.com/locate/still

    007) 350360

  • Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350360 3511550 Pg, which is twice the amount of C in the

    atmosphere (Raich and Potter, 1995). Since the pool of

    C in the atmosphere is considerably smaller than in the

    soil, a small relative change in the amount of C in the soil

    will have a substantial influence on the C content in the

    atmosphere (Bruce et al., 1999). Thus, an understanding

    of the dynamics of SOM is fundamental to evaluating the

    role of soil as a C source or sink (Lal, 2004). The change

    in land use has significantly affected the carbon cycles

    both regionally and globally (Kirschbaum, 2000; Lal,

    2002). Much work has focused on the effects of the

    conversion of natural soil to cropland or pasture on C

    storage (Liu and Ma, 2000; Saggar et al., 2001; Ghani

    et al., 2003; Song et al., 2004; Zhang et al., 2003).

    However, very little is known of the dynamics of SOM

    after agricultural abandonment. There is some evidence

    that the abandonment of agriculture and the subsequent

    regeneration of forests may return C storage to the

    preagricultural levels although the rate of recovery

    depends on the time frame one considers and whether the

    land was previously used as cropland or pasture (Moraes

    et al., 1996; Post and Kwon, 2000; Guo and Gifford,

    2002; Templer et al., 2005). However, Gao (1997)

    reported that climate was the controlling factor affecting

    the dynamics of SOC after the abandonment of an

    agricultural land. They reported that abandonment led to

    an increase in SOC in a favorable climate that decreased

    during unfavorable climate conditions.

    SOC contains fractions with a rapid turnover rate as

    well as fractions with a slower turnover rate (Schimel

    et al., 1985). The labile fractions of organic C, such as

    microbial biomass C (MBC) and dissolved organic C

    (DOC), can respond rapidly to changes in C supply.

    These components have therefore been suggested as

    early indicators of the effects of land use on SOM

    quality (Gregorich et al., 1994) and as important

    indicators of soil quality. Dissolved organic matter is an

    important labile fraction since it is the main energy

    source for soil microorganisms a primary source of

    mineralizable N, P, and S and it influences the

    availability of metal ions in the soil by forming soluble

    complexes (Stevenson, 1994). Soil microbial biomass is

    the eye of the needle through which all organic

    material that enters the soil must pass (Martens, 1995).

    Soil microorganisms play a key role in the energy flows,

    nutrient transformations, and element cycles in the

    environment (Tate, 2000). Recently, there has been

    increased interest in the importance of microbiological

    properties as the indicators of change in the soil quality

    (Saggar et al., 2001). However, few studies have

    focused on the dynamics of labile organic C after

    agricultural abandonment.The Sanjiang Plain in northeast China is one of the

    largest freshwater marsh regions and the most

    extensively tilled region in China for the past 50 years.

    Since the 1950s, there have been three periods of

    extensive tillage in this region when approximately

    3.8 Mha of land was tilled. With human interference

    during the past half century, the ecosystem in the

    Sanjiang Plain has changed significantly. Converting

    the nativewetland to agricultural soil resulted in distinct

    changes in the soil water content and temperature (Song

    et al., 2004) leading to a rapid decrease in the labile

    organic C concentration and SOM (Zhang et al., 2006a,

    2007). The losses in SOC were rapid during the initial

    59 years of cultivation. Subsequent losses were

    considerably slower (Zhang et al., 2006a). Fortunately,

    since the 1990s, the government has established a new

    policy that forbids the conversion of intact wetland soil

    to cultivated soil and implements the abandonment of

    cultivated soil. However, little knowledge exists on the

    dynamics of SOC and the labile fractions of organic C

    after the abandonment of cultivated wetlands in

    northeast China. We hypothesized that the SOC content

    increases and that the labile fractions of organic C, such

    as DOC, light fraction (LF) C, and MBC, respond

    rapidly to the abandonment of cultivated soil; these are

    the early indicators of the dynamics of SOM following

    the abandonment of cultivated wetlands.

    The objectives of this study were (1) to estimate the

    dynamics of soil C following the abandonment of

    cultivated soil, (2) to investigate the most sensitive

    fraction for detecting changes in the organic C due to

    the abandonment of cultivated soil, and (3) to explore

    the key factors affecting the dynamics of soil C after the

    abandonment of cultivated soil in this freshwater marsh

    region of northeast China.

    2. Materials and methods

    2.1. Site characteristics and sampling

    The study site is located at the Sanjiang Mire

    Wetland Experimental Station, Chinese Academy of

    Sciences, Tongjiang City, Heilongjiang Province,

    China, at approximately 478350N, 1338310E (Fig. 1).The average altitude is 55.457.9 m. The mean annual

    temperature is 1.9 8C with an average frost-free periodof 125 days. The mean annual precipitation is 550

    560 mm with the precipitation in July and August

    accounting for more than 65% of the total precipitation.

    In May 2003, we selected three adjacent sites within

    a radius of 1 km, in which soybean (Glycine maxMerr)

    was planted continuously before abandonment (Fig. 1).

  • Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350360352

    tch mThe average altitude is between 55.6 and 56 m. Site 1

    was previously a wetland dominated by Deyeuxia

    angustifolia (D. angustifolia); it was converted into a

    farmland and cultivated upon for 9 years, which was

    abandoned after sampled in May, 2003. Site 2 was a

    farm field abandoned for 4 years after being in

    cultivation for 10 years and previously a D. angustifolia

    wetland. Site 3 was abandoned cultivated soil that

    Fig. 1. The position of this study site in the northeast China. The skeabandoned 13 years after converted D. angustifolia

    wetland to cultivated soil for about 8 years. We selected

    D. angustifolia-intact wetland soil neighboring the sites

    1, 2, and 3 for use as reference. The parent material is

    the Quaternary Period sediment at these sites. The soils

    at all sites were classified as Albaquic Paleudalfs with

    silty clay texture. In sites 1, 2, and 3, the sand content is

    19, 23, and 25%, respectively; the silt content is 62, 60,

    and 56%, respectively; and the clay content is 19, 17,

    and 19%, respectively. Our previous results showed that

    the physical, biological, and chemical properties of soil,

    such as the amount of water-stable macroaggregate and

    microaggregate, the bulk density, soil porosity, water

    capacity, pH value, SOC content, soil organic N content,

    DOC, MBC, and basal respiration, reached a new

    equilibrium state after the conversion of natural wetland

    soil to cultivated soil for approximately 915 years

    (Zhang, 2006b). In this study, we selected three adjacent

    sites within a radius of 1 km, in which soybean (Glycine

    max Merr) was planted continuously before abandon-

    ment (Fig. 1) and was cultivated for 8, 9, and 10 years.

    Therefore, we assumed that all the three sites had thesame soil conditions with similar management history

    and that all the changes in the physical, biological, and

    chemical properties of soil were mainly attributable to

    the duration of abandonment.

    Three plots (40 m 40 m) were arbitrarily estab-lished at each field. For each plot, 20 cores (010 cm

    depths) were taken in May 2003, 2004, and 2005,

    respectively. Thus, we gained durations of abandon-

    ap of China was cited from National geomatics center of China site.ment soil samples, which abandoned 0, 1, 2, 4, 5, 6, 13,

    14, 15 years. Meanwhile, we randomly sampled three

    soil cores to measure the bulk density and porosity. The

    field-moist cores in each plot were pooled and sieved

    (

  • Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350360 353resuspended in NaI, and gently shaken by hand. The

    same procedure was repeated twice as described

    above. The three subfractions were combined, oven-

    dried at 50 8C, and stored for analysis. This fractionwas called the LF. The sediment from the centrifuge

    tubes and the beaker was the heavy fraction (HF), and

    it was washed once with 0.01 M CaCl2 and approxi-

    mately 10 times with distilled water, oven-dried at

    50 8C, and weighed (Roscoe and Buurman, 2003). TheC concentration in the total soil and the fractions were

    determined using a FLASH1112 CNS Analyzer. The C

    concentration in the fractions was calculated using the

    followed equation:

    Pw weight of fractionweight of soil

    (1)

    where Pw is the weight of the fraction separated from100 g soil and the weight of soil is 100 g.

    FractionC concentration

    C concentration in fraction Pw (2)

    2.3. Soil MBC

    The soil MBC was determined by a fumigation

    extraction method (Vance et al., 1987). The fumigated

    and non-fumigated soils were extracted with 0.5 mol/l

    K2SO4 by shaking at 30 rpm for 30 min (soil:extractant

    ratio, 1:5), and the extracts were analyzed for C using

    high-temperature combustion (TOC-VCPH analyzer,

    Shimadzu, Kyoto, Japan). The MBC was calculated

    using the following equation (Lu, 2000):

    MBC microbial-C flush0:38

    (3)

    where the microbial-C flush was the C obtained from

    the fumigated samples minus the C from non-fumigated

    samples.

    Microbial quotient MQ MBCSOC

    (4)

    2.4. DOC measurement

    Moist soil samples (equivalent to 10 g oven-dried

    weight) from the field were weighed into 40-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 8000 rpm. All the supernate

    was filtered through a 0.45-mm filter into separate vialsfor C analysis (Ghani et al., 2003). The extracts were

    analyzed for C using high-temperature combustion

    (TOC-VCPH analyzer, Shimadzu, Kyoto, Japan).

    2.5. SOC analyses

    The SOC was determined by wet combustion.

    2.6. Water-stable macroaggregation

    A water-stable macroaggregation of soil was

    separated by passing three 25 g fragmented and air-

    dried soil samples through a 0.25 mm sieve and

    agitating for 60 s with a Ro-tap sieve shaker in water.

    The aggregate remaining on the sieve was collected,

    oven-dried at 50 8C, and weighed.

    2.7. Rate of SOC increase

    According to the equation in Figs. 3, 6 and 7, the rate

    of increase in the organic C fraction was derived using

    the following formula:

    Ri Ci1 CiCi

    Ci+1 Ci is the increment in C concentration; Ci+1 theC concentration in the soil abandoned for i + 1 years; Cithe C concentration in the soil abandoned for i years; Riis the rate of increase in organic C from i to i + 1 years.

    2.8. Statistics

    Statistical analysis was carried out with factor,

    correlation, and regression analyses using the SPSS

    software package for Windows. Figures were drawn

    using the Origin 7.5 software. For all analyses where

    p < 0.05, the stepwise tested values and the correlationwere considered to be statistically significant.

    3. Results

    3.1. Changes in vegetation, biomass, and physical

    properties of soil

    During the initial 12 years following abandonment,

    the vegetation was gramineous. A...

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