温带典型草原土壤总有机碳及溶解性有机碳对模拟氮沉降的响应

2008年5月～2011年10月,以内蒙古温带典型草原为研究对象,利用小区模拟控制试验,设置对照0 g·(m2·a)-1]、低氮5 g·(m2·a)-1]、中氮10 g·(m2·a)-1]、高氮20 g·(m2·a)-1]这4个净氮输入量处理,模拟研究了大气氮沉降水平变化对土壤总有机碳(TOC)以及溶解性有机碳(DOC)含量、年际变化及其垂直分布格局的影响,并分析了两者之间的关系.结果表明,除个别年份外,土壤TOC与DOC含量均随土壤深度增加而递减,4 a的连续施氮并未改变土壤TOC与DOC的垂直分布规律,但施氮降低了土壤TOC的垂直变异,增加了土壤DOC的垂直变异;施氮4 a对于草地表层0～20 cm土壤TOC与DOC含量的变化并未表现出显著影响(P>0.05).不同氮输入水平处理0～20 cm土层有机碳密度在3.9～5.6 kg·m-2之间变动,试验前两年不同氮处理0～20 cm土壤有机碳密度均低于对照或与对照接近,试验后两年,施氮对土壤总有机碳密度逐渐呈现出一定的促进作用,但与对照的差异仍不显著(P>0.05);不同处理0～20 cm土层DOC/TOC约为0.32%～1.09%,氮输入增加普遍降低了DOC在整个TOC中所占的比例;草地土壤TOC与DOC的变化呈显著正相关(P<0.01).不同处理草地土壤DOC随时间的变异均远大于TOC,与TOC相比,草地土壤DOC的变化更为迅速,是研究草地土壤碳库对氮沉降变化响应的重要敏感性指标.

Reponses of Soil Total Organic Carbon and Dissolved Organic Carbon to Simulated Nitrogen Deposition in Temperate Typical Steppe in Inner Mongolia, China

Based on a field manipulative nitrogen (N) addition experiment, the effects of atmospheric N deposition level change on the contents, inter-annual variation and profile distribution of soil total organic carbon (TOC) and dissolved organic carbon (DOC) were investigated from May, 2008 to October, 2011 in a temperate typical steppe in Inner Mongolia of China, and the relationship between TOC and DOC was also discussed. The treatments in the manipulative experiment included N additions at rates of 0, 5, 10, and 20 g·(m²·a)^-1, representing the control (CK), low N (LN), medium N (MN), and high N (HN) treatment, respectively. The results indicated that the concentrations of soil TOC and DOC decreased progressively with soil depth in all cases except for the DOC at 10-20 cm depth in individual years. The increase of N input in typical steppe did not change the vertical distribution of soil TOC and DOC, but reduced the vertical variation of TOC and increased the vertical variation of DOC in the surface soil horizon. In addition, the contents of soil TOC and DOC at 0-10 cm and 10-20 cm soil layers changed insignificantly after the continuous increase in anthropogenic N input for four years. The soil organic C density of 0-20 cm soil layer for different N treatment levels varied between 3.9 kg·m^-2 and 5.6 kg·m^-2, and the soil organic C densities of fertilized treatments in the first two years were similar to or slightly lower than those of CK, while in the following two years, the increase in N deposition gradually played a positive role in increasing soil organic C density, but the differences in soil TOC and DOC contents between CK and fertilized plots were not significant (P>0.05). The ratio of soil DOC to TOC (DOC/TOC) varied from 0.32% to 1.09%. The increase in N deposition generally lowered the proportion of DOC in soil TOC, which was conducive to the accumulation of soil organic C. The change of soil DOC was positively correlated with that of TOC (P<0.01). The temporal variations of soil DOC in different N treatments were all far greater than those of TOC, and the soil DOC was the important sensitive indicator for predicting and evaluating the response of soil C pool to the change in atmospheric N deposition in the temperate grassland ecosystem.