长期施氮引起的黄土高原旱地土壤不同形态碳变化

【目的】提高土壤碳固持，特别是增加有机碳累积、减少碳损失，对于提高旱地土壤肥力、缓解大气温室效应具有重要意义。黄土高原旱地土壤有机碳含量低，增施氮肥是这一地区重要的作物增产措施，但氮肥投入对土壤碳的影响如何，一直没有报道。【方法】利用黄土高原旱地持续23年的长期定位试验，在每年施磷39 kg P2O5?hm-2条件下，设置0、45、90、135、180 kg N?hm-2 5个氮水平种植冬小麦，在小麦收获期采集0—40 cm不同土层的土壤样品，研究长期施用不同用量的氮肥对旱地土壤总碳、有机碳、轻质有机碳及无机碳的影响，分析不同氮肥用量引起的土壤有机碳、轻质有机碳及无机碳累积量的变化，定量分析氮肥用量对旱地土壤不同形态碳的影响。【结果】随氮肥用量增加，旱地土壤不同土层总碳无显著变化，但0—30 cm土层有机碳含量却随之增加，与不施氮肥相比，增幅可达7%—28%；0—40 cm土层轻质有机碳含量也增加，增幅达31%—106%，但施氮量过高不利于有机碳累积。对不同形态土壤碳累积量与氮肥用量的回归分析表明，施氮量120 kg N?hm-2时，0—30 cm土层有机碳累积量达最高值36.6 Mg；施氮量161 kg N?hm-2时，0—40 cm土层轻质有机碳累积量达最高值2.69 Mg；每千克肥料氮每年可使土壤有机碳增加1.34 kg?hm-2，轻质有机碳增加0.31 kg?hm-2；0—20 cm表层土壤轻质有机碳占有机碳的百分比也随施氮量增加而升高。相反，5—20 cm土层土壤无机碳含量却随氮肥用量增加而显著降低，施氮量180 kg N?hm-2时，无机碳累积量比不施氮减少2.8 Mg，每千克肥料氮每年可使无机碳减少0.67 kg?hm-2。【结论】在黄土高原旱地长期施用不同用量的氮肥虽不显著影响土壤的总碳数量，却显著地改变了旱地土壤碳的组成，即通过增加土壤的轻质有机碳，增加了土壤的有机碳累积量，同时降低了土壤的无机碳累积。因此，合理调控氮肥用量，不仅是旱地作物增产的关键措施，对增加土壤有机碳固持、培肥土壤也有重要意义。同时，施用氮肥引起的土壤无机碳损失不容忽视，其潜在的农业、生态与环境效应需引起大众关注。

Changes of Different Carbon Fractions Caused by Long-Term N Fertilization in Dryland Soil of the Loess Plateau

【Objective】 Increased organic carbon (C) sequestration, especially the accumulation of organic C in soil, and decreased carbon losses from soil are of great importance for improving the dryland soil fertility and mitigating the greenhouse effect. Soils in the Loess Plateau areas are known typically lower in contents of organic C, and increasing application of nitrogen (N) fertilizer is proved to be the important tactical measure to increase crop yields, but information about how the N fertilizer input can affect the soil C is still lacking. 【Method】 A 23 years long-term experiment was conducted with winter wheat as test crop growing at five N rates of 0, 45, 90, 135, 180 kg N?hm-2 per year on a basal fertilization of 39 kg P2O5?hm-2 per year in dryland area of Loess Plateau. Soil samples were collected from 0 to 40 cm soil layers for each plots at harvest of winter wheat, in order to study the effects of long-term application of different nitrogen fertilizer rates on soil total carbon (TC), total organic carbon (TOC), light fraction organic carbon (LFOC) and inorganic carbon (IC), estimate the changes of TOC, LFOC and IC accumulation, and quantify the effects of N fertilizer application rates on different kinds of C in dryland soil. 【Result】 Obtained results showed that, with the increase of N fertilizer rates, the TC showed no significant change, while the TOC contents in 0 to 30 cm soil layers were increased by 7% to 28%, and the LFOC contents in 0-40 cm soil were increased by 31% to 106 %, but over application of N fertilizer showed no advantages on the organic carbon accumulation. Analysis of regression with the accumulation of different soil C fractions to N fertilizer application rates showed that, TOC accumulation in 0 to 30 cm soil layers reached the maximum of 36.6 Mg at 120 kg N?hm-2 and LFOC in 0 to 40 soil layers reached the maximum of 2.69 Mg at 161 kg N?hm-2, with the application of 1.0 kg fertilizer N per year led to an increase of TOC by 1.34 kg?hm-2 and LFOC by 0.31 kg?hm-2. In addition, the ratio of LFOC to TOC in 0 to 20 cm soil layers was found also increased with the increase of N fertilizer rate. In contrast, the IC content decreased significantly with the increase of N fertilizer application rates, especially in the 5 to 20 cm soil layers, where accumulation of IC was found decreased by 2.8 Mg at 180 kg N?hm-2, with the application of 1.0 kg fertilizer N per year led to a decrease of IC by 0.67 kg?hm-2. 【Conclusion】 Long term application of different rates of N fertilizer showed no significant effects on the amount of TC, but the composition of different C fraction was remarkably changed in the dryland soil of the Loess Plateau, with the accumulation of TOC increased due to increase of LFOC sequestration in soil, but the IC accumulation decreased. Therefore, rational application of N fertilizer is not only the key measure to increase the crop yield, but is also of great importance to increase the organic carbon sequestration and then the soil fertility. However, the loss of inorganic carbon from soil caused by application of N fertilizer should not be overlooked, and sufficient attentions and concerns should be paid to the effects from the losses of soil IC on the sustainability of agriculture, ecology and environment in this area.