Effects of land use on soil water soluble organic C and microbial biomass C concentrations in the Sanjiang Plain in northeast China

Abstract

Four land-use types were selected: Deyeuxia angustifolia wetland, upland forest, two farmlands (cultivated 1 and 9 years, respectively) of soils previously under Deyeuxia angustifolia wetland, and abandoned cultivated soil; and soil organic C, water soluble organic C, microbial biomass C, and hot-water extractable C were measured, to estimate the influence of changing land use and cultivation on organic C fractions, and the distribution of labile fraction organic C through soil profiles in the Sanjiang Plain of northeast China. Results indicated that long-term cultivation caused a significant decline in all examined parameters, while abandonment of cultivated wetlands resulted in an increase in examined parameters. The intact Deyeuxia angustifolia wetland soil had much higher labile fraction organic C concentrations in the topsoil when compared to upland forest, abandoned cultivated and cultivated soils. However, there were no significant subsoil differences at all sites, suggesting the effects of land use on labile fraction organic C occurred mainly in the topsoil (0–20 cm).

Keywords:

• Labile fraction organic C
• land use
• Northeast China
• Sanjiang Plain
• soil organic C

Introduction

Soil organic C (SOC) is viewed as an important factor affecting soil quality and long-term sustainability of agriculture. Decrease in SOC leads to a decline in cation exchange capacity of soils, soil aggregate stability and crop yield (Freixo et al., 2002). Besides being a source and sink of nutrients for plants, SOC plays an important role in the C cycle, as it accounts for the major terrestrial pool of this element (Freixo et al., 2002). Unfortunately, it often takes years for changes in agricultural management to lead to detectable changes in the quantity and quality of SOC (Hassink et al., 1997).

Labile fractions of organic matter such as microbial biomass C (MBC), water soluble organic C (WSOC), and hot-water extractable C (HWC) 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. Water soluble 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 microorganisms play a key role in the energy flows, nutrient transformations, and element cycles in the environment (Tate, 2000). Soil microbiological properties have the potential to be early and sensitive indicators of soil stress or productivity changes, and there is considerable evidence that they can be used to evaluate the influence of management and land use on soils (Saggar et al., 2001).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 measurement of soil quality.

The Sanjiang Plain is located in the eastern part of Heilongjiang province, Northeast China and is bordered by Russia. It covers an area of 10.89 Mha, and is presently the second largest wetland in China (Liu & Ma, 2000). The drainage and use of wetlands for agricultural fields occurred in the past 50 years with the population growth, resulting in the increase in cultivated land from about 0.79×10⁶ ha in 1949 to 4.57×10⁶ ha in 1994 (Liu & Ma, 2000). Now, the Sanjiang Plain has become an intensive area of land use cover change in China. Much less is known, however, on the impact of different land use systems on soil organic carbon (SOC) and labile organic matter fractions, as well as distribution of labile organic matter through the soil profile depth in the northeast China.

The objectives of this study were: (1) to determine the influence of changing land use and cultivation on organic C fractions, and (2) to estimate land use effects on the distribution of labile fraction organic C through soil profiles.

Material and methods

Sampling

The study site is located in Heilongjiang province, northeast China, at approximately 48°36′ N, 130°34′ 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 and accounting for more than 65% of the annual precipitation.

We selected four adjacent sites within 2 km that were originally dominated by Deyeuxia angustifolia. (i) a Deyeuxia angustifolia wetland that is an intact wetland; (ii) an intact upland forest that is adjacent to a wetland; (iii) two farmlands (cultivated for 1 and 9 years, respectively) of soils previously under Deyeuxia angustifolia wetland, crop residue has been mostly removed. The fields were plowed to 15–20 cm by machine using moldboard plows. Soybean (Glycine max Merr) was planted continuously each year in May and harvested in September or October; and (iv) a farm field abandoned into a Deyeuxia angustifolia wetland for 6 yr after being cultivated for 10 yr were selected. 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. The soils at all sites were classified as Albaquic Paleudalfs with silty clay texture (Zhang et al., 2008). The area of each site was not less than 40000 m². Three profiles as replication, with a distance between profiles of not less than 100 m, were sampled at each site. Soil samples were taken at 0–10 cm, 10–20 cm, 20–30 cm and 30–40 cm depths in June before spring cultivation. Soil samples were sieved (< 2 mm) soon after collection and split up into two subsamples. One subsample was stored in a field moist condition at 4°C. One subsample was later air-dried for SOC analysis.

Soil microbial biomass C measurement

Soil microbial biomass C was determined by a fumigation-extracted ion method on the field moist soils (Vance et al., 1987). Fumigated and non-fumigated soils were extracted with 0.5 mol/L K₂SO₄ by shaking at 30 rpm for 30 min (1:5 soil: extracted ant ratio), and C in the extracts was measured using high temperature combustion (total organic C- V_CPH C analyzer, Shimadzu, Kyoto, Japan). The microbial biomass C was calculated using the following equation (Lu, 2000):

where the microbial-C flush was C obtained from the fumigated samples minus the C from non-fumigated samples.

Water soluble organic C measurement

Field moist soil samples (equivalent 10 g oven dry weight) were weighed into 40 ml polypropylene centrifuge tubes. The samples were extracted with 30ml 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 extracted material was analyzed for C using high temperature combustion (total organic C- V_CPH C analyzer, Shimadzu, Kyoto, Japan), and called water soluble organic C.

Field moist soil samples (equivalent to 10 g oven dry weight) were weighed into 40 ml polypropylene centrifuge tubes. Thirty ml distilled water was 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 extracted ion 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 extracted samples were analyzed for C using high temperature combustion (total organic C- V_CPH C analyzer, Shimadzu, Kyoto, Japan), and called hot water-soluble C.

Soil total C analyses

Air-dried samples were used for the SOC analyses. Soil SOC was analyzed by the H₂SO₄-K₂Cr₂O₇ pyrogenation method (Lu, 2000).

Other soil properties

Soil pH was measured in a soil to water ratio of 1:5 (v/v) by a DMP-2 mV/pH detector (Quark Ltd, Nanjing, China). Soil texture was determined with a laser particle characterization analyzer (Beckman Coulter, Los Angeles, California, USA).

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.

Results and discussion

Distribution characteristics of SOC

Data presented in Table I showed that land use change had not significantly changed texture and pH of the subacid, silty clay soil. The bulk density increased significantly after converting intact Deyeuxia angustifolia wetland to cultivated soil for 9 years. Meanwhile, bulk density decreased after the cultivated wetlands were abandoned for 6 years, as compared to cultivated soil.

Table I. Physicochemical properties of the soils used in this study.

						Particle size composition
Site	Depth cm	SOC g/kg	C/N	pH	Bulk Density g/cm^3	>0.05mm g/kg	0.005–0.05mm g/kg	<0.005mm g/kg
Cultivated soil for 1 year	0–10	80.8(9.4)^b	17.8(1.1)^c	5.4(0.5)^a	0.75(0.1)^b	352(62)^a	497(34)^a	151(8)^c
	10–20	81.4(9.9)^b	16.3(1.4)^b	5.3(0.4)^b	0.73(0.4)^b	186(39)^b	533(29)^a	281(35)^b
	20–30	9.9(1.5)^e	16.5(1.2)^b	5.6(0.6)^b	1.09(0.3)^a	115(7)^c	535(55)^a	350(54)^a
	30–40	9.1(0.9)^e	16.7(0.4)^c	6.1(0.5)^b	1.56(0.1)^a	112(31)^c	514(61)^a	374(37)^a
Cultivated soil for 10 years	0–10	26.8(1.4)^d	14.8(1.1)^c	5.6(0.2)^a	1.05(0.1)^b	321(14)^a	533(32)^a	146(6)^c
	10–20	21.4(1.1)^d	16.1(1.4)^b	6.0(0.4)^b	1.13(0.5)^b	213(5)^b	545(95)^a	242(13)^b
	20–30	13.9(0.7)^e	17.5(1.2)^b	6.2(0.6)^b	1.29(0.3)^a	104(16)^c	540(84)^a	356(44)^a
	30–40	9.0(0.5)^e	15.7(0.4)^c	6.3(0.3)^b	1.58(0.1)^a	108(23)^c	521(43)^a	371(31)^a
Abandoned cultivated soil	0–10	39.1(2.0)^c	19.7(1.6)^a	5.3(0.3)^a	0.89(0.04)^b	347(15)^a	510(75)^a	143(07)^c
	10–20	30.2(1.6)^c	18.4(0.6)^a	5.6(0.2)^a	1.01(0.1)^b	192(9)^b	523(32)^a	285(34)^b
	20–30	18.2(0.9)^e	18.7(0.8)^a	6.0(0.4)^b	1.34(0.3)^a	146(24)^c	503(58)^a	351(25)^a
	30–40	10.9(0.6)^e	17.7(0.3)^b	6.1(0.1)^b	1.52(0.2)^a	124(35)^c	515(64)^a	361(51)^a
Upland forest	0–10	76.8(4.1)^b	21.7(2.4)^a	5.6(0.4)^a	0.48(0.2)^c	353(57)^a	502(31)^a	145(09)^c
	10–20	33.2(1.7)^c	21.2(1.4)^a	5.5(0.3)^a	0.65(0.4)^c	203(44)^b	536(35)^a	261(23)^b
	20–30	9.3(0.4)^e	15.3(0.9)^c	6.4(0.6)^b	1.31(0.1)^a	104(7)^c	511(46)^a	385(42)^a
Deyeuxia angustifolia wetland	0–10	119.1(6.8)^a	22.7(3.4)^a	5.4(0.5)^a	0.46(0.2)^c	362(62)^a	482(34)^a	156(8)^c
	10–20	78.2(4.1)^b	15.2(4.4)^c	5.4(0.4)^a	0.53(0.1)^c	183(39)^b	536(29)^a	281(35)^b
	20–30	10.9(0.60)^e	13.3(0.5)^c	5.7(0.3)^a	0.99(0.3)^b	119(7)^c	531(55)^a	350(54)^a
	30–40	11(0.6)^e	16.5(1.4)^b	6.0(0.3)^b	1.54(0.2)^a	113(31)^c	512(61)^a	375(37)^a
Data shown are means with the standard errors in parentheses (n=3). Values in a column with the same letter are not significantly different at p<0.05. SOC is soil organic carbon. C/N is ratio of soil organic carbon to total nitrogen. Soils were classified as Albaquic Paleudalfs in this study.

The SOC concentrations in the topsoil (0–20 cm) followed the order: intact Deyeuxia angustifolia wetland soil>upland forest soil>abandoned cultivated soil> cultivated soil (Table I). The SOC concentrations 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 concentrations with increasing soil depth (Table I). The effects of land use on the SOC occurred mainly in the topsoil, due to significantly higher SOC concentrations in the topsoil (0–20 cm) in the intact wetland soil when compared to the upland forest, cultivated, and abandoned cultivated soils (Table I) and similar SOC concentrations in the subsoil (20–40 cm). The upland forest soil showed a significantly lower organic carbon concentration than native wetland soil in the topsoil. The SOC concentration in the topsoil decreased 32%, after converting wetland to cultivation for 1 year. Long-term cultivation has strongly changed the soil organic C concentration; the soil cultivated for 9 years showed a decrease of about 77% compared to the intact wetland. The SOC concentration increased after the cultivated wetlands were abandoned for 6 years, as compared to soil that had been cultivated for 9 years.

When native wetland land was converted to agricultural soil, SOC was rapidly lost at the first decade. Subsequent losses were substantially slower. The concentration of organic C in soil depends on the balance between C input and decomposition rates (Saggar et al., 2001; Huang et al., 2002). Converting native wetland land to agricultural soil resulted in distinct changes in soil water concentration and temperature (Song et al., 2004). The destruction of soil aggregates exposes previously inaccessible organic matter to microbial attack, favoring decomposition of SOC (Christensen, 2000). Further, the annual input of C to cultivated soils is often lower than that in natural ecosystems such as the wetland that we examined here (Song et al., 2004). Therefore, total SOC is rapidly lost after cultivation. 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 (Liu & Ma, 2000). Meanwhile, plant litter was also an important source of topsoil organic C in the wetland. However, the roots infrequently developed down to a depth at 20 cm and few roots were found in the subsoil (Liu & Ma, 2000). Consequently, the SOC concentrations rapidly deceased in the subsoil. The upland forest soil showed a significantly lower organic carbon concentration than native wetland soil, probably as a result of waterlogging in the wetland which reduces organic matter degradation. The abandoned cultivated soil showed an increase in soil organic C when compared to 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 water soluble organic C and hot-water extractable C

Significantly higher WSOC and HWC concentrations occurred in the topsoil (0–20 cm) in the intact wetland when compared to the upland forest, abandoned cultivated and cultivated soils (Figure 1). However, in the subsoil (20–40 cm), the differences were not obvious at all sites. Therefore, effects of land use on WSOC and HWC concentrations were only detected in the topsoil (0–20 cm). Similarly, the upland forest soil showed a significantly lower WSOC and HWC than native wetland soil in the topsoil. Converting wetland to cultivated soil resulted in a decrease in WSOC and HWC concentrations (Figure 1). The WSOC and HWC concentrations increased after the cultivated wetlands were abandoned for 6 years, as compared to soil that had been cultivated on for 9 years. Although, in this study the WSOC and HWC showed significant change for the different sites, the change of WSOC and HWC were less obvious than that of SOC in the long-term cultivation and upland forest. The WSOC and HWC parameter cannot be identified as a promising soil quality measure in these study sites, consistent with the results of Saviozzi et al. (2001).

Figure 1.  Distribution characteristics of labile fraction organic C dependent on land use and depth in the Sanjiang Plain of northeast China. Points in figure are means (n=3). Points in figure with the same letter are not significantly different at p<0.05. HWC is hot-water extractable C; MBC is microbial biomass C; WSOC is water soluble organic C. Soils were classified as Albaquic Paleudalfs in this study.

In the intact wetland, WSOC and HWC concentrations decreased significantly with increasing soil depth; however, the upland forest, abandoned cultivated, and cultivated soils showed a considerably smaller decrease in water soluble organic C concentrations with increasing soil depth (Figure 1). We expected that water soluble organic matter adsorption should be much higher in intact wetland than upland forest, abandoned cultivated, and cultivated soils, due to higher water soluble organic C concentrations in the intact wetland topsoil when compared to the other sites and similar water soluble organic C concentrations in the subsoil of all sites.

Water soluble organic C concentrations increased linearly with increasing soil SOC concentration (R²=0.80, p<0.001), suggesting that total organic matter concentration was a major determinant of the amount of water soluble organic matter present. The correlation between water soluble organic C and SOC was stronger in the topsoil than whole soil profile (R²=0.94, p<0.001). However, this relationship was very weak in the subsoil (R²=0.02, p>0.05), suggesting leachate from topsoil could be the primary source of water soluble organic matter in the subsoil. As a result, the distribution of the proportion of water soluble organic C to SOC was different from the distribution of water soluble organic C concentrations through the soil profile (Figure 2).

Figure 2.  Proportion of labile fraction organic C to SOC through the soil profile in the Sanjiang Plain of northeast China. (a) The proportion of water soluble organic C to SOC through the soil profile. (b) The proportion of microbial biomass C to SOC through the soil profile. (c) The proportion of hot-water extractable C to soil organic C 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. WSOC/SOC is the proportion of water soluble organic C to SOC; MBC/SOC is the proportion of microbial biomass C to SOC; HWC /SOC is the proportion of hot-water extractable C to SOC. Soils were classified as Albaquic Paleudalfs in this study.

Distribution characteristics of microbial biomass C

The microbial biomass C concentrations were significantly higher in the intact wetland topsoil (0–20 cm) when compared to upland forest, abandoned cultivated and cultivated soils (Figure 1b). 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). MBC concentration in the upland forest soil was significantly lower than that in native wetland soil in the topsoil. Converting wetland to cultivated soil resulted in a decrease in MBC concentration (Figure 1b). The MBC concentration in the topsoil decreased 45% higher than SOC concentration, after wetland was cultivated for 1 year. Compared with the intact wetland, the soil cultivated for 9 years showed a decrease of about 83% in the topsoil, much higher than SOC concentration. The MBC concentration increased after the cultivated wetlands were abandoned for 6 years, as compared to the soil that had been cultivated on for 9 years.

The MBC concentrations were obviously correlated with the SOC and water soluble organic C (R²=0.75, and R²=0.85, p<0.01). Organic C, especially water soluble 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). Meanwhile, the dead microbial material is easily soluble and microorganisms release soluble compounds. The distribution of MBC was similar to water soluble organic C (Figure 1).

Acknowledgements

This work projected by National Basis Research Program of China (2009 C3421103) and National Natural Science Foundation of china (40771189).