长期秸秆还田水稻土对氮的固持与释放特征及其机制研究

Straw amendment is an important strategy for enhancing soil fertility. The area and intensity of straw returning become larger in China. Studies show that nitrogen cycling would be changed with straw amendments. However, the mechanisms of soil nitrogen adsorption and availability affected by straw return are still unclear. Based on four long-term fertilization experiments which located along the Yangtze River, laboratory incubation experiments, soil nitrogen absorption-release dynamics experiments, and the real-time PCR technology are used in this program. We first analyze dynamics of soil nitrogen release and absorption at the four sites. And we also examine the effects of long-term straw amendments on soil physical, chemical, and biological characteristics under different regions, aiming to find out the main impact factors of soil nitrogen adsorption and release. Then, we use soil samples from Wangcheng site to analyze the response of soil nitrogen absorption and mineralization to different nitrogen application (i.e., straw and chemical nitrogen fertilizer), aiming to find out the effects of on soil nitrogen absorption-release characters and clarify their response mechanisms to soil physical, chemical and microbial properties. Our results will provide scientific basis for chemical nitrogen fertilizer application with straw return and sustainable development of agriculture.

秸秆还田是维持和培育地力的重要措施，我国秸秆还田面积与力度不断扩大。大量研究表明，秸秆还田改变了土壤氮素循环特征及有效性。然而，长期秸秆还田对土壤物理、化学、生物学过程的作用及其对土壤氮固持与释放的影响相关机制尚不明晰。本项目以重庆北碚、湖南望城、江西进贤和江苏望亭4个长期定位试验为基础，以单施化肥处理为对照，采用室内模拟试验比较土壤对硝态氮、铵态氮和酰胺态氮吸附与释放的差异；利用传统土壤物理化学方法结合实时荧光定量PCR技术，研究长期秸秆还田影响土壤物理、化学、生物学特性的区域差异，明确长期秸秆还田影响土壤氮固持与释放的主要因素；以望城试验点土壤为材料，采用室内灭菌与非灭菌培养方式，利用15N同位素示踪技术研究土壤氮矿化与固持特征及其对不同外源氮的响应，深入探讨土壤氮固持与释放的物理-化学-生物学影响机制。研究结果将丰富农田土壤氮循环的基础理论，为长期秸秆还田下氮素养分调控提供理论依据。

秸秆还田是维持和培育地力的重要措施，是保持农业可持续生产的重要手段。研究长期秸秆还田对土壤物理、化学、生物学过程的作用及其对土壤氮固持与释放的影响具有重要意义。本研究发现，与长期不施肥相比，长期单施化肥和秸秆还田配施化肥显著降低了望城试验点和北碚试验点耕层土壤的pH值。长期秸秆还田配施化肥显著提高耕层和亚耕层土壤有机质、全氮、铵态氮、硝态氮、速效磷、速效钾含量，尤其是在双季稻轮作的望城和进贤试验点，秸秆还田提升土壤养分效果显著。各试验点不同处理土壤中优势菌属均为硝化螺旋菌属，长期秸秆还田配施化肥提高了各试验点硝化螺旋菌属比例，主成分分析结果表明稻麦轮作（北碚）与双季稻轮作（望城、进贤）的土壤微生物群落结构差异较大。长期秸秆还田对不同土壤氮的吸附-解吸作用影响不同，对于土壤吸附位点较多且钾离子含量丰富的紫色土而言，长期秸秆还田有利于土壤对氮的吸附；而对于土壤偏酸性的红壤性水稻土而言，长期秸秆还田则可能因为增加了土壤有机质含量而减少了土壤对铵态氮的吸附位点，从而降低了土壤对氮的吸附保持能力。采用长期定位试验点耕层土壤进行灭菌与非灭菌培养试验发现，灭菌条件下的土壤铵态氮含量在培养30~130天期间维持在较高水平，向硝态氮转化过程受阻，其硝态氮、微生物氮和可溶性有机氮含量较低；而非灭菌条件下各处理微生物氮和可溶性有机氮分别于培养20天和30天时达到最大值。长期秸秆还田配施化肥的土壤加入外源氮素对铵态氮、硝态氮、微生物氮和可溶性有机氮的提升效果均显著优于长期单施化肥土壤水平。同位素示踪结果表明，秸秆还田条件下添加15N尿素导致其15N铵态氮含量所占比例降低（16.01~20.85%）。氮的吸附解吸动力学研究发现，在长期定位试验土壤基础上添加不同外源氮能显著提高土壤对铵态氮的吸附能力，以添加秸秆效果最好，秸秆配施尿素次之，对照吸附量和解吸量最低，此外，灭菌培养土壤对铵态氮的吸附、解吸能力显著下降。