农田沙漠化过程中土壤有机碳和氮的衰减及其机理研究

Losses of Soil Organic Carbon and Nitrogen and Their Mechanisms in the Desertification Process of Sandy Farmlands in Horqin Sandy Land

测定了科尔沁沙地不同沙化程度农田 0～ 15cm耕层土壤粒级分布、有机碳 (SOC)和全氮 (N)含量 ,并应用土壤粒级物理分组方法对农田土壤各粒级组分 (2～ 0 .1、0 .1～ 0 .0 5、<0 .0 5mm)中SOC和氮的含量、富集系数、分配比例进行了研究 ,以揭示沙漠化过程中碳、氮衰减的机理。结果表明 ,沙地农田土壤物理稳定性指数大都低于5 % ,是土壤易发生风蚀沙化的内因。沙漠化过程中SOC和氮衰减的机理 ,一方面是与粘粉粒和部分极细沙结合的稳定态的有机碳和氮直接被吹蚀 ,粘粉粒吹蚀 1% ,SOC和全氮含量分别下降 0 .16 9g·kg-1和 0 .0 2 15g·kg-1;另一方面是与沙粒结合的颗粒有机碳 (ParticulateOrganicCarbon ,POC)形成量减少 ,沙漠化程度每增加 1级 ,POC、全氮含量分别下降 0 .4 3g·kg-1和 0 .0 5 9g·kg-1。

Organic carbon (OC) and total nitrogen (N) concentrations from bulk soils and soil particle size fractions in farmlands with different extents of desertification (Potential, Light, Medium, Severe, and Most severe desertified farmlands) were examined to quantitatively elucidate the losses of carbon and nitrogen and its mechanisms in the desertification process. Particle size fractions (2～0.1, 0.1～0.05, <0.05 mm) were obtained by granulometric wet sieving from 30 sandy soils (0～15 cm depth) with different extents of desertification. It was shown that soil physical stability index (St) in most severe desertified farmlands was 5%～7% and St in other farmlands was less than 5%, which was contributed to very low soil organic matter content. This was the intrinsic cause that sandy farmlands in Korqin sandy land was subject to risk of desertification. Desertification resulted in considerable losses of SOC and N. Regression analysis indicated that SOC and N content reduced by 0.169 g·kg -1 and 0.0215 g· kg -1 , respectively, with one percent loss of soil silt and clay content. Losses of SOC and N were mostly the removal of fine particle size fractions (silt and clay, and a less extent very fine sand) from the farmlands by wind erosion, which were enrich in organic matter and nutrients, as well as the depletion of organic C and N associated with coarse particles (>0.05 mm) in desertification process. The concentrations of C and N associated with sand (2～0.1 and 0.1～0.05 mm) significantly decreased with the increase of desertified extent. Silt and clay associated C and N concentrations, however, were less changed, and in contrast, were higher in soils under most severe desertified land than in soils under potential and severe desertified land. The percentage of distribution of sand (>0.05 mm) associated C and N significantly increased with the increase of desertified extent, suggesting that stability of SOC decreased in the desertification process.