Soil Organic Carbon Stock and Distribution in Cultivated Land Converted to Grassland in a Subtropical Region of China

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<ul><li><p>Soil Organic Carbon Stock and Distribution in Cultivated LandConverted to Grassland in a Subtropical Region of China</p><p>J. H. Zhang F. C. Li Y. Wang D. H. Xiong</p><p>Received: 8 May 2013 / Accepted: 30 September 2013 / Published online: 13 October 2013</p><p> Springer Science+Business Media New York 2013</p><p>Abstract Land-use change from one type to another affects</p><p>soil carbon (C) stocks which is associated with fluxes of CO2to the atmosphere. The 10-years converted land selected from</p><p>previously cultivated land in hilly areas of Sichuan, China</p><p>was studied to understand the effects of land-use conversion</p><p>on soil organic casrbon (SOC) sequestration under landscape</p><p>position influences in a subtropical region of China. The SOC</p><p>concentrations of the surface soil were greater (P \ 0.001)for converted soils than those for cultivated soils but lower</p><p>(P \ 0.001) than those for original uncultivated soils. TheSOC inventories (1.901.95 kg m-2) in the 015 cm surface</p><p>soils were similar among upper, middle, and lower slope</p><p>positions on the converted land, while the SOC inventories</p><p>(1.411.65 kg m-2) in this soil layer tended to increase from</p><p>upper to lower slope positions on the cultivated slope. On the</p><p>whole, SOC inventories in this soil layer significantly</p><p>increased following the conversion from cultivated land to</p><p>grassland (P \ 0.001). In the upper slope positions, convertedsoils (especially in 05 cm surface soil) exhibited a higher</p><p>C/N ratio than cultivated soils (P = 0.012), implying that</p><p>strong SOC sequestration characteristics exist in upper slope</p><p>areas where severe soil erosion occurred before land con-</p><p>version. It is suggested that landscape position impacts on the</p><p>SOC spatial distribution become insignificant after the con-</p><p>version of cultivated land to grassland, which is conducive to</p><p>the immobilization of organic C. We speculate that the</p><p>conversion of cultivated land to grassland would markedly</p><p>increase SOC stocks in soil and would especially improve the</p><p>potential for SOC sequestration in the surface soil over a</p><p>moderate period of time (10 years).</p><p>Keywords Soil organic carbon Cultivated soil Land conversion Landscape position Soil erosion Sloping field</p><p>Introduction</p><p>During the conversion of natural ecosystems to agricultural</p><p>systems, with an increase in tillage intensity, the soil organic</p><p>carbon (SOC) pool is depleted, and therefore, the CO2 flux to</p><p>the atmosphere increases (Lal et al. 1998). The depletion of</p><p>SOC caused by the conversion of native grassland to culti-</p><p>vated fields is both extensive and well documented (McGill</p><p>et al. 1988; Davidson and Ackerman 1993; Kern and John-</p><p>son 1993; Monreal and Janzen 1993; Mikhailova et al. 2000;</p><p>Saviozzi et al. 2001; Guo and Gifford 2002; Zinn et al.</p><p>2005). Previous studies demonstrated that SOM decompo-</p><p>sition increased through physical disturbance by tillage, as a</p><p>result of macroaggregate disruption and the exposure of</p><p>previously protected soil to microbial processes (Cambard-</p><p>ella and Elliott 1992; Tisdall 1996; Ayoubi et al. 2012).</p><p>These studies emphasized the depletion of SOC pools fol-</p><p>lowing agricultural use, i.e., the conversion of forestland or</p><p>grassland into cultivated land. On the other hand, some</p><p>studies have addressed the reverse processes, i.e., the effects</p><p>of the conversion of cultivated land into grassland on SOC</p><p>stocks. In their review of C changes from 23 different</p><p>studies, Conant et al. (2001) concluded that SOC increased</p><p>in all but one (98 %) of the studies on cultivation-to-pasture</p><p>conversions, with a mean annual increase in SOC stocks of</p><p>J. H. Zhang F. C. Li Y. Wang D. H. XiongKey Laboratory of Mountain Surface Processes and Ecological</p><p>Regulation, CAS, Chengdu 610041, Peoples Republic of China</p><p>J. H. Zhang (&amp;) F. C. Li Y. Wang D. H. XiongInstitute of Mountain Hazards and Environment, Chinese</p><p>Academy of Sciences and Ministry of Water Conservancy,</p><p>P O Box 417, Chengdu 610041, Peoples Republic of China</p><p>e-mail: zjh@imde.ac.cn</p><p>123</p><p>Environmental Management (2014) 53:274283</p><p>DOI 10.1007/s00267-013-0181-y</p></li><li><p>nearly 5 % and a [3 % annual increase in SOC content.However, the increment of SOC stocks varies with different</p><p>climatic conditions. For example, the increased SOC stocks</p><p>in Australia caused by a conversion from cultivation to</p><p>pasture would be well below the values measured in other</p><p>cooler, wetter environments. Therefore, it was noted that</p><p>there is a need for region-specific data regarding the vege-</p><p>tation communities and the full range of land-uses in any</p><p>given environment before accurate and reliable predictions</p><p>of SOC changes can be made (Wilson et al. 2011). In sub-</p><p>tropical regions such as the Sichuan Basin in China, rela-</p><p>tively few studies have contributed to an understanding of</p><p>SOC changes from the impacts of converting cultivated land</p><p>into grassland. In particular, there are scarce data on SOC</p><p>stock changes under the influence of landscape positions</p><p>after a similar land-use conversion.</p><p>After the 1998 Yangtze River Floods, the central gov-</p><p>ernment of China initiated the ecological reconstruction</p><p>in the Upper Yangtze River Basin and enacted the grain-</p><p>for-green policy, where cultivated fields with steep slopes</p><p>or those over 25 were converted into forestland andgrassland in 1999. This project was intended to convert</p><p>14.67 million ha of cultivated land into forestland and</p><p>grassland across China (The National Forestry Adminis-</p><p>tration of China 1999), which was one of the largest eco-</p><p>logical reconstructions worldwide to date. Over the last</p><p>decade, the question as to whether SOC stocks increased in</p><p>converted land remains an issue that must be addressed</p><p>because SOC is associated with CO2 flux to the atmosphere</p><p>and is therefore relevant to global climate change.</p><p>The distribution of SOC can be linked to a number of</p><p>factors at the landscape scale, such as the climate, topog-</p><p>raphy, geological processes, parent material, and vegeta-</p><p>tion (e.g., Zhang et al. 2010). However, given that some</p><p>factors are similar at different locations, other aspects such</p><p>as topography may contribute to spatial variations in the</p><p>distribution of SOC. Most studies to date involve the</p><p>estimation and mapping of SOC stocks and SOC autocor-</p><p>relation, and only a few have addressed the relationship</p><p>between landscape positions and the spatial distribution of</p><p>SOC stocks (Hancock et al. 2010; Zhao et al. 2012). Some</p><p>studies on cultivated soils at the field scale have revealed</p><p>that SOC stocks in footslope and toe slope positions are</p><p>greater than those in shoulder slope and mid-slope posi-</p><p>tions (Pierson and Mulla 1990; Papiernik et al. 2007).</p><p>Small-scale agricultural area studies show that soils at</p><p>lower slope positions have high SOC contents compared to</p><p>soils in upper slope positions (Zhang et al. 2006, 2008).</p><p>However, little is known of the SOC dynamics on the slope</p><p>after the conversion of cultivated land to grassland.</p><p>Previous studies on SOC changes were conducted by</p><p>considering only the land-use conversion or topographic</p><p>impacts. By combining these two aspects, this study</p><p>examined landscape position impacts on SOC sequestration</p><p>under land-use conversion to grassland from cultivated</p><p>land. In view of the geomorphological features of steep</p><p>slopes in this region, where soil property variations may</p><p>occur over a short distance in the line of the slope, SOC</p><p>changes were therefore examined on a hillslope scale (5 m</p><p>intervals). Our objectives for this study were (1) to estimate</p><p>SOC stocks and to quantitatively assess changes in SOC</p><p>stocks after the conversion from agricultural to grassland</p><p>ecosystems and (2) to examine landscape position effects</p><p>on SOC stocks and dynamics under such a conversion.</p><p>Materials and Methods</p><p>Study Site</p><p>This study was carried out in the central part of the Sichuan</p><p>Basin, southwestern China (300402800303900000N and10411034001045303600E; Fig. 1). The climate at the studysite is in the subtropical humid zone, which is characterized</p><p>by four distinct seasons, namely spring, summer, autumn</p><p>and winter. The mean annual temperature is 17.4 C,ranging from a high of 38.7 C to a low of -5.4 C, and the[10 C accumulative temperature reaches 5,421 C peryear. The mean annual precipitation is 872 mm, 90 % of</p><p>which occurs between May and October. The maximum</p><p>monthly rainfall over the course of the year occurs in July,</p><p>which makes up an average of 30 % of the annual rainfall,</p><p>and the minimum occurs in February, with \1 % of theannual rainfall. Sunshine averages 1,241 h annually, with</p><p>mean annual solar radiation of 90 kc cm-2. The elevation</p><p>ranges from 400 to 587 masl, indicating the geomorpho-</p><p>logical characteristic of hilly areas. Local farmers have</p><p>dissected long hillslopes into short slopes to minimize soil</p><p>loss by water and to facilitate field management operations.</p><p>Accordingly, the current hillslopes mostly have a length of</p><p>1025 m, with common slope steepness from 10 to 35 %.</p><p>In 1999, steep slopes used for agriculture were required to</p><p>be converted to forest or grassland by the Chinese</p><p>government.</p><p>The soils were derived from sedimentary rocks from the</p><p>Jurassic Age and were classified as Regosols by FAO soil</p><p>taxonomy (FAO 1988), with strong physical weathering</p><p>but weak chemical weathering. In cultivated soils, the</p><p>dominant crops were wheat (Triticum aestivum L.), corn</p><p>(Zea mays L.), sweet potato (Ipomoea batatas (L.) Lam),</p><p>peanut (Arachis hypogaea L.), and rape (Brassica napus</p><p>L.). The farmers had a uniform crop rotation system on the</p><p>cultivated lands, which typically consisted of wheat, corn</p><p>and sweet potatoes, allowing for the collection of soil from</p><p>three crops to assess SOC changes. Inorganic nitrogen</p><p>fertilizer was generally applied to cultivated lands at a rate</p><p>Environmental Management (2014) 53:274283 275</p><p>123</p></li><li><p>of 330 kg N ha-1 year-1. After the conversion of culti-</p><p>vated land to uncultivated land, the slopes were covered</p><p>with dominant native grass species, including cogongrass</p><p>(Imperata koenigii (Retz.) Beauv.), hairy tare (Vicia hirs-</p><p>uta (Linn.) S. F. Gray), Chinese mugwort (Artemisia argyi</p><p>Levl. et Van), hairy finger (Digitaria sanguinalis (L.)</p><p>Scop.), and others.</p><p>Soil Sampling and Analysis</p><p>Soil samples were collected from three different land types,</p><p>including converted land (previously cultivated on steep</p><p>slopes but converted to forest or grassland), cultivated land,</p><p>and original uncultivated (undisturbed) land. The three</p><p>different land-use types were located within 1 km of each</p><p>other. Two and four slopes were selected to collect soil</p><p>samples for the converted land and cultivated land,</p><p>respectively, and an ancient tomb area was considered the</p><p>original uncultivated land. The coordinates and elevation</p><p>of each sampling point were measured using a survey-</p><p>grade Differential Global Positioning System (DGPS). Soil</p><p>samples were collected at 5 m intervals along a transect of</p><p>the converted and cultivated slopes with slope lengths of 19</p><p>and 21.5 m, respectively. There were 10 and 20 soil profile</p><p>samples for converted land and cultivated land, respec-</p><p>tively. Soil sampling for SOC, total N, physical, and</p><p>chemical determinations was carried out using an 8 cm</p><p>diameter hand operated core sampler, and soil was col-</p><p>lected down to the bedrock, with soil depths of 2533 cm</p><p>for converted soils and 3246 cm for cultivated soils. At</p><p>each sampling point of soil profiles, three soil cores were</p><p>collected within an 80-cm range on the contour, and each</p><p>core was segmented into subsample sections at 5 cm depth</p><p>increments from the soil surface to the bedrock, and they</p><p>were combined across subsamples by depth for each</p><p>sampling point. Soil bulk densities (kg m-3) were deter-</p><p>mined for each segment using the oven-dried (at 105 C for24 h) weight and sample volume (Liu 1996). Measured soil</p><p>bulk densities could be used for direct calculations of SOC</p><p>inventories because little gravel was found in the soils</p><p>derived from mudstone. Soil thickness and bulk density</p><p>were used to calculate SOC inventories as the product of</p><p>the concentration, soil bulk density, and soil thickness.</p><p>For the original uncultivated land, soil samples were</p><p>taken from ancient tomb areas where the natural land has</p><p>not been converted for agricultural use. Four replicates of</p><p>soil profile samples were randomly taken from a relatively</p><p>level plot, with the same sampling and analysis methods as</p><p>described above. The sampling depth to the bedrock was</p><p>39 cm for original uncultivated soils (Table 1). The soil</p><p>properties for original uncultivated soils were not thought</p><p>to be influenced by landscape positions that were pre-</p><p>sumably caused by non-erosional effects.</p><p>Soil samples were air-dried, crushed, and passed</p><p>through a 2 mm-mesh sieve to remove coarse fragments.</p><p>Composite soil samples for each 5-cm depth were passed</p><p>through a 0.25 mm-mesh sieve and analyzed for SOC</p><p>concentrations. The SOC concentration was determined by</p><p>wet oxidation with K2Cr2O7, and the measurement of total</p><p>nitrogen (TN) followed the classical Kjeldahl digestion</p><p>method (Liu 1996). Soil particle-size fractions were ana-</p><p>lyzed using the Mastersizer 2000 laser diffraction particle</p><p>Fig. 1 A map showing thelocation of the study area in the</p><p>Sichuan Basin of southwestern</p><p>China</p><p>276 Environmental Management (2014) 53:274283</p><p>123</p></li><li><p>size distribution analyzer. Soil pH was determined using a</p><p>digital pH meter with a glass electrode by mixing 10 ml of</p><p>soil sample with 20 ml of deionized water.</p><p>Statistical Analysis</p><p>Simple linear regression was used to test correlations</p><p>between the SOC concentration/inventory and slope land-</p><p>scape elements (the significance of the regression at</p><p>P \ 0.05). An analysis of variance was performed to detectthe significance of differences between different land-uses</p><p>and between different positions in the landscape using post</p><p>hoc Fisher LSD analysis (P \ 0.05). All statistical analyseswere conducted on the basis of the original data.</p><p>Results</p><p>Characteristics of Soil Depth and SOC Depth</p><p>Distribution at Different Landscape Positions</p><p>On cultivated slopes, there was an increasing trend in the soil</p><p>depth from the upper to lower slope positions. The soil depth</p><p>on the converted land exhibited a similar distribution along</p><p>the slope transects (Table 1). When compared with the</p><p>original uncultivated soils, soil depths for both cultivated</p><p>and converted soils decreased in the upper slope positions,</p><p>and slightly decreased in the middle positions. However,</p><p>both cultivated and converted soils had the same soil layer</p><p>thickness as the original uncultivated soils at the lower slope</p><p>positions. Overall, it was determined that SOC concentra-</p><p>tions in the soil profile decreased with soil depth. The largest</p><p>change in SOC concentrations was observed at the 05 cm</p><p>depth in converted soils, with a mean increase of 59 % (from</p><p>a mean of 8.14 g kg-1 for cultivated soils to 12.95 g kg-1</p><p>for converted soils), while SOC concentrations in layers</p><p>below the 10 cm depth remained constant for converted</p><p>soils (Fig. 2). As a result, SOC concentrations increased</p><p>only in the surface soil within a period of 10 years after the</p><p>conve...</p></li></ul>

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