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Published online 27 February 2006
Published in Soil Sci Soc Am J 70:660-667 (2006)
DOI: 10.2136/sssaj2005.0007
© 2006 Soil Science Society of America
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Soil & Water Management & Conservation

Land Use Effects on the Distribution of Labile Organic Carbon Fractions through Soil Profiles

Zhang Jinboa,b,*, Song Changchuna and Yang Wenyana,b

a Northeast Institute of Geography and Agric. Ecology, Chinese Academic Science, Changchun Jilin, 130012, China
b Graduate School of Chinese Academic Science, Beijing, 10039, China

* Corresponding author (zhangjinbo197901{at}163.com)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Labile fractions of organic matter can respond rapidly to changes in C supply and are considered to be important indicators of soil quality. However, much less is known on the impact of different land use systems and depth on labile organic matter fractions. The objective of this study was to estimate land use effects on a distribution of labile fraction organic C through soil profiles in the Sanjiang Plain of northeast China. Four land-use types were selected: Deyeuxia angustifolia wetland, upland forest, two farmlands (cultivated 9 and 15 yr, respectively) of soils previously under Deyeuxia angustifolia wetland, and abandoned cultivated soil. Soil total organic C (TOC), dissolved organic C, microbial organic C, and hot water-extractable C were measured. The results showed that the intact Deyeuxia angustifolia wetland soil had significantly higher labile fraction organic C contents in the topsoil when compared with upland forest, abandoned cultivated, and cultivated soils. However, there were no significant subsoil differences at all sites. The effects of land use on labile fraction organic C occurred mainly in the topsoil (0–20 cm). The labile fraction organic C contents decreased significantly with increasing soil depth in the intact Deyeuxia angustifolia wetland. However, the upland forest, abandoned cultivated, and cultivated soils showed a considerably smaller decrease in labile fraction organic C contents with increasing soil depth. The proportion of dissolved organic C, hot water-extractable C, and microbial biomass C to TOC increased to a maximum at a depth of about 20 to 30 cm, and then decreased with increasing soil depth in the Deyeuxia angustifolia wetland but not the other land use types.

Abbreviations: TOC, total organic C


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
LABILE FRACTIONS of organic matter such as microbial biomass C, dissolved organic C, and hot water-extractable C can respond rapidly to changes in C supply. Such components have therefore been suggested as early indicators of the effects of land use on soil organic matter quality (Gregorich et al., 1994), as well 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 soils 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 indicators of change in soil quality (Yeates et al., 1998; Saggar et al., 2001). Sparling (1992) and Ghani et al. (2003) reported that hot water-extractable C could be used as an integrated measure of soil quality. However, the majority of studies on the labile organic matter have used only topsoil samples based on the assumption that land use effects occur mainly in the uppermost few decimeters. Much less is known, however, on the impact of different land use systems on labile organic matter fractions, as well as distribution of labile organic matter through the soil profile depth.

The objective of work was to estimate land use effects on the distribution of labile fraction organic C through soil profiles.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sampling
The study site was set up at the Sanjiang Mire Wetland Experimental Station, Chinese Academy of Science, in the Sanjiang Plain of China, at approximately 47° 35' N, 133° 31' E. The average altitude is between 55.4 and 57.9 m, annual mean temperature is 1.9°C, and the non-frost period is 125 d (Song et al., 2003). The study site is in a seasonal frozen zone (Song et al., 2003), with annual precipitation between 550 and 600 mm, concentrated in July and August, which accounts for more than 65% of the annual precipitation.

Four land use types: (i) a relatively intact Deyeuxia angustifolia wetland, (ii) a relatively intact upland forest adjacent to a wetland, (iii) two farm fields (cultivated 9 and 15 yr, respectively) previously Deyeuxia angustifolia wetland, and (iv) a farm field abandoned for 6 yr after being in cultivation for 10 yr and previously a Deyeuxia angustifolia wetland were selected (Fig. 1 ). Deyeuxia angustifolia wetland is a seasonal flooded wetland and submerged only in May. The other sites are not submerged through the season. The C-horizon is Quaternary Period sediment in all study sites. Soils at all sites were classified as Hydric Medihemists. Soil texture is silty clay in all study sites. Soil samples were taken in June and September 2004, respectively, at 0- to 10-, 10- to 20-, 20- to 30- and 30- to 40-cm depths. Three replicates were sampled at each site. Soil samples were sieved (<2 mm) soon after collection and split into two subsamples. One subsample was stored in a field moist condition at 4°C. One subsample was later air-dried for TOC analysis.


Figure 1
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  		Fig. 1. The distribution of observation sites in this study. C9 is a field cultivated for 9 yr, C15 is a field cultivated for 15 yr, A is a field that was abandoned for 6 yr after being cultivated for 10 yr, U is upland forest, D is Deyeuxia angustifolia wetland. All study sites are contiguous within 800 m. The C-horizon is quaternary period sediment. Soils at all sites were Hydric Medihemists.

 
Soil Microbial Biomass Carbon Measurement
Soil microbial biomass C was determined by a fumigation-extraction method on the field moist soils (Vance et al., 1987). Fumigated and nonfumigated soils were extracted with 0.5 mol L–1 K2SO4 by shaking at 30 rpm for 30 min (1:5 soil/extractant ratio), and the extracts were analyzed for C using high temperature combustion (total organic C–VCPH C analyzer, Shimadzu, Kyoto, Japan). The microbial biomass C was calculated using the following equation (Lu, 2000):

Formula 1[1]
where the microbial-C flush was C obtained from the fumigated samples minus the C from nonfumigated samples.

Dissolved Organic Carbon Measurement
Field moist soil samples (equivalent 10 g oven dry weight) 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 30 rpm, and centrifuged for 20 min at 8000 rpm. All the supernate was filtered through 0.45-µm filter into separate vials for C analysis (Ghani et al., 2003). The extracts were analyzed for C using high temperature combustion (total organic C– VCPH C analyzer, Shimadzu, Kyoto, Japan).

Hot Water-Extractable Carbon Measurement
Field moist soil samples (equivalent 10 g oven dry weight) were weighed into 40-mL polypropylene centrifuge tubes. Thirty milliliters of distilled water were added to the tubes and the tubes were shaken for 30 min on an end-over-end shaker at 30 rpm. The tubes were capped and left for 16 h in a hot water bath at 80°C. At the end of the incubation period, each tube was shaken for 10 min on an end-over-end shaker to ensure that hot water-extractable C released from the soil was fully suspended in the extraction medium. The tubes were centrifuged for 20 min at 8000 rpm and all the supernatant was filtered through 0.45-µm filter into separate vials for C analysis (Ghani et al., 2003). The extracts were analyzed for C using high temperature combustion (total organic C– VCPH C analyzer, Shimadzu, Kyoto, Japan).

Above and Belowground Biomass Measurement
At each site, we randomly selected six plots. Green plants at each plot were clipped to the soil surface in the 40 cm x 40 cm area in August, and then oven-dried for 48 h at 70°C. Aboveground biomass was calculated using the followed equation:

Formula 2[2]
where the B is biomass obtained from the 40 cm x 40 cm area.

In April and August, we randomly sampled six cores (diameter = 20 mm, height = 40 cm), washed away the soil carefully, and obtained the roots in the Deyeuxia angustifolia wetland and abandoned cultivated site, then oven-dried for 48 h at 70°C. Belowground biomass was calculated using the following equation:

Formula 3[3]
where B4 and B8 represent root biomass obtained in April and August, respectively. S is the area of core.

In the cultivated site, belowground biomass was measured using a similar procedure with the exception that roots systems from six plants were measured. The below ground biomass was then corrected for a 1-m2 area. Belowground biomass in the cultivated site was calculated using the followed equation:

Formula 4[4]
where the Br is the root biomass obtained from six plants. A is the number of plants in a 1-m2 area.

We did not measure the above and belowground biomass in the upland forest.

Total Organic Carbon Measurement
Air-dried samples were used for the TOC analyses. Soil TOC was analyzed by H2SO4–K2Cr2O7 pyrogenation method (Lu, 2000).

Statistics
Statistical analysis was done with SPSS software package for Windows (Ding et al., 2004). For all analyses where p < 0.05, the factor tested and the relationship were considered to be statistically significant. Because the datum measured in the two cultivated soils were not significantly different (p < 0.05), we calculated a mean to succinctly explain the effects of land use.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Description of the O-Horizon
The profile characteristics, especially in the topsoil, were significantly different, due to different land use at each site. In the intact Deyeuxia angustifolia wetland, the O-horizon is about 6.5-cm thick and there were many roots in the topsoil (0–20 cm). In the upland forest, O-horizon is about 3.5-cm thick and the amount of roots were less than in the intact Deyeuxia angustifolia wetland. After converting intact Deyeuxia angustifolia wetland to cultivated soil, the O-horizon disappeared and roots were far less than in the Deyeuxia angustifolia wetland. On the other hand, in the abandoned cultivated soil, the O-horizon appears again, reaching a 2.0-cm depth and the amount of roots were less than in the intact Deyeuxia angustifolia wetland and the upland forest. The site's detail characteristics and the soil properties are shown in the Tables 1 and 2.


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  		Table 1. Characteristics of study sites in the Sanjiang Plain of northeast China.

 

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  		Table 2. Soils physicochemical properties used in this study.

 
Distribution Characteristics of Total Organic Carbon
The TOC contents in the topsoil (0–20 cm) followed the order: intact Deyeuxia angustifolia wetland soil > upland forest soil > abandoned cultivated soil > cultivated soil (Table 2). The TOC contents of the intact Deyeuxia angustifolia wetland soil rapidly decreased with increasing soil depth. However, upland forest, cultivated, and abandoned cultivated soils all showed a considerably smaller decrease in total organic C contents with increasing soil depth (Table 2). The effects of land use on the TOC occurred mainly in the topsoil, due to significantly higher TOC contents in the topsoil (0–20 cm) in the intact wetland soil when compared with the upland forest, cultivated, and abandoned cultivated soils (Table 2) and similar TOC contents in the subsoil (20–40 cm). The vertical distribution of roots was a major determinant of the total organic C distribution (Liang et al., 2000). In the intact wetland, the roots were mainly distributed in the topsoil (0–20 cm), supplying abundant C to topsoil. Meanwhile, plant litter was also an important source of topsoil organic C in the wetland (Table 1). However, the roots infrequently developed down to a depth at 20 cm and few roots were found in the subsoil. Consequently, the TOC contents rapidly deceased in the subsoil. In the upland forest, O-horizon was thin compared with wetland and the amounts of roots were less than in the intact Deyeuxia angustifolia wetland. Therefore, the TOC contents in the upland forest were lower than in the intact Deyeuxia angustifolia wetland. Conversion of intact wetland to cultivated soil resulted in the rapid decease of TOC contents, because of root layer destruction, exposing more surfaces due to disturbance and biomass dry matter removal during crop harvest (Table 1) (Liu et al., 2000). On the other hand, the abandoned cultivated soil showed an increase in soil organic C when compared with cultivated soil, because the annual input of C to the soil increased due to cultivation disappearance and vegetation restoration (Zhang et al., 2003; Song et al., 2004).

Distribution Characteristics of the Dissolved Organic Carbon
Significantly higher dissolved organic C concentrations occurred in the topsoil (0–20 cm) in the intact wetland when compared with the upland forest, abandoned cultivated, and cultivated soils (Table 3). However, in the subsoil (20–40 cm), the differences were not obvious at all sites. Therefore, effects of land use on dissolved organic C concentrations were only detected in the topsoil (0–20 cm).


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  		Table 3. Distribution characteristics of labile fraction organic C dependent on land use and depth in the Sanjiang Plain of northeast China.{dagger}

 
In the intact wetland, dissolved organic C concentrations decreased significantly with increasing soil depth as expected because of dissolved organic matter adsorption in mineral soil horizons (Kaiser and Zech, 1998; Moore and Matos, 1999). However, the upland forest, abandoned cultivated, and cultivated soils showed a considerably smaller decrease in dissolved organic C concentrations with increasing soil depth (Table 3). We expected that dissolved organic matter adsorption or decomposition should be much higher in intact wetland than upland forest, abandoned cultivated, and cultivated soils, due to higher dissolved organic C concentrations in the intact wetland topsoil when compared with the other sites and similar dissolved organic C concentrations in the subsoil of all sites.

Dissolved organic C concentrations increased linearly with increasing soil TOC content (Fig. 2a ), suggesting that total organic matter content was a major determinant of the amount of dissolved organic matter present. The relationship between dissolved organic C and TOC was more strongly correlated in the topsoil than whole soil profile (Fig. 2b). However, this relationship was very weak in the subsoil (Fig. 2c), suggesting leachate from topsoil could be the primary source of dissolved organic matter in the subsoil. As a result, the distribution of the proportion of dissolved organic C to TOC was different from the distribution of dissolved organic C contents through soil profile. The proportion of dissolved organic C to TOC increased with the increasing soil depth at all sites. However, the differences in the proportion of dissolved organic C to TOC through the soil profile were significant only in the intact wetland (Fig. 3a ).


Figure 2
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  		Fig. 2. Relationship between dissolved organic C and total organic C (TOC) for all land use in the Sanjiang Plain of northeast China. (a) The relationship between dissolved organic C and TOC in the whole soil profile. (b) The relationship between dissolved organic C and TOC in the topsoil (0–20 cm). (c) The relationship between dissolved organic C and TOC in the subsoil (20–40 cm). Points in figure are means (n = 3). DOC is dissolved organic C. Soils at all sites were Hydric Medihemists.

 

Figure 3
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  		Fig. 3. Proportion of labile fraction organic C to total organic C (TOC) through the soil profile in the Sanjiang Plain of northeast China. (a) The proportion of dissolved organic C to TOC through the soil profile; (b) The proportion of microbial biomass C to TOC through the soil profile; (c) The proportion of hot water-extractable C to TOC through the soil profile. Points in figure are means (n = 3). Points in each figure with the same letter are not significantly different at p < 0.05. DOC/TOC is the proportion of dissolved organic C to TOC; MBC/TOC is the proportion of microbial biomass C to TOC; HWC/TOC is the proportion of hot water-extractable C to TOC. Soils at all sites were Hydric Medihemists.

 
Distribution Characteristics of Microbial Biomass Carbon
The microbial biomass C contents were significantly higher in the intact wetland topsoil (0–20 cm) when compared with upland forest, abandoned cultivated, and cultivated soils (Table 3). However, there was no significant difference in the subsoil (20–40 cm) of all sites. Therefore, the effect of land use on the microbial biomass C also occurred mainly in the topsoil (0–20 cm).

The microbial biomass C contents were obviously correlated with the TOC and dissolved organic C (Fig. 4a , Fig. 5a ). Organic C, especially dissolved organic C, is the primary energy source for the microbial biomass, and affects the microbial activity and amount of microbes in the soil (Haynes, 2000; Hofman et al., 2003). The distribution of dissolved organic C controlled the distribution of microbial biomass C. The relationship between microbial biomass C and TOC was more strongly correlated (R2 = 0.89 vs. 0.51) in the topsoil than whole soil profile (Fig. 4b). However, there was no significant relationship between microbial biomass C and TOC in the subsoil (Fig. 4c). On the other hand, the proportion of microbial biomass C to the TOC initially increased to a maximum at a depth of about 20–30 cm, and then decreased with increasing soil depth (Fig. 3b). The differences in the proportion of microbial biomass C to TOC through the soil profile were significant only in the intact wetland (Fig. 3b).


Figure 4
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  		Fig. 4. Relationship between microbial biomass C and total organic C (TOC) for all land use in the Sanjiang Plain of northeast China. (a)The relationship between microbial biomass C and TOC in the whole soil profile. (b)The relationship between microbial biomass C and TOC in the topsoil (0–20 cm). (c) The relationship between microbial biomass C and TOC in the subsoil (20–40 cm). Points in figure are means (n = 3). MBC is microbial biomass C. Soils at all sites were Hydric Medihemists.

 

Figure 5
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  		Fig. 5. Relationship between dissolved organic C, microbial biomass C and hot water-extractable C in the Sanjiang Plain of northeast China. (a) The relationship between dissolved organic C and microbial biomass C; (b) The relationship between dissolved organic C and hot water-extractable C; (c) The relationship between hot water-extractable C and microbial biomass C. Points in figure are means (n = 3). HWC = hot water-extractable C; MBC = microbial biomass C; DOC = dissolved organic C. Soils at all sites were Hydric Medihemists.

 
Distribution Characteristics of Hot Water-Extractable Carbon
During extraction of the hot water-extractable C, the dissolved organic C as well as the microbial biomass, soluble soil carbohydrates, and amines were all extracted from soil (Ghani et al., 2003). Hot water-extractable C was correlated with TOC, dissolved organic C, and microbial biomass C (Fig. 6a , Fig. 5b, and Fig. 5c, respectively). Our results supported the results of Sparling (1992) and Ghani et al. (2003) that the hot water-extractable C could be used as an integrated measure of soil quality.


Figure 6
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  		Fig. 6. Relationship between hot water-extractable C and total organic C (TOC) for all land use in the Sanjiang Plain of northeast China. (a) The relationship between hot water-extractable C and TOC in the whole soil profile. (b) The relationship between hot water-extractable C and TOC in the topsoil (0–20 cm). (c) The relationship between hot water-extractable C and TOC in the subsoil (20–40 cm). Points in figure are means (n = 3). HWC is hot water-extractable C. Soils at all sites were Hydric Medihemists.

 
Distribution characteristics of hot water-extractable C were similar to dissolved organic C and microbial biomass C. The hot water-extractable C contents were significantly higher in the intact wetland topsoil (0–20 cm) than at the other sites. However, there were no significant differences in the subsoils (20–40 cm) when all sites were compared (Table 3). Therefore, the effect of land use on the hot water-extractable C also mainly occurred in the topsoil (0–20 cm). Similarly to the dissolved organic C, the proportion of hot water-extractable C to TOC increased with the increasing soil depth and the differences in the proportion of microbial biomass C to TOC through the soil profile were significant only in the intact wetland (Fig. 3c).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The effects of land use on the TOC and labile fraction organic C were mainly observed in the topsoil (0–20 cm). The labile fraction organic C contents decreased significantly with increasing soil depth in intact Deyeuxia angustifolia wetland. However, the upland forest, abandoned cultivated, and cultivated soils showed a considerable smaller decrease in labile fraction organic C contents with increasing soil depth. The proportion of dissolved organic C, hot water-extractable C, and microbial biomass C to TOC increased to a maximum at a depth of about 20 to 30 cm, and then decreased with increasing soil depth in the Deyeuxia angustifolia wetland but not the other land use types. Leachate from topsoil to the subsoil could be the primary source of dissolved organic matter in the subsoil. The dissolved organic matter eluviation from topsoil to subsoil is much higher in the intact wetland when compared with upland forest, abandoned cultivated, and cultivated soils.


   ACKNOWLEDGMENTS
 
This work was founded by National Natural Science Foundation of China (40471124), Chinese Academy of Sciences (KZCX1-SW-01),(KZCX3-SW-332). We thank the reviewers for their time and suggestions.

Received for publication January 5, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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