高[CO2]对稻田土壤氮素周转过程的影响机制

It is well known that elevated CO2 concentrations often increase rice grain yield, with the increase more pronounced under the strong-responsive rice cultivars than under weak-responsive rice cultivars. At present, many studies have focused on the effects of elevated CO2 concentrations on nitrogen (N) metabolism, water use efficiency, photosynthesis and yield formation process of rice, and thus tried to uncover the mechanisms of increasing rice yield in response to elevated CO2. However, little research has been conducted on the effects of elevated CO2 on soil nitrogen cycles in two rice cultivars with distinctly weak and strong responses to elevated CO2. In this project, a 15N tracing incubation experiment combined with 15N tracing model will be employed to investigate the effects of elevated CO2 on soil gross N transformation rates (mineralization, nitrification, and immobilization) under different rice cultivars from a FACE (Free-Air CO2 Enrichment) Project system under a typical rice-wheat rotation system in China. Meanwhile, 15N tracing techniques in combination with selective biomass inhibitors and modern molecular biotechnology will be employed to analyze the related microbial mechanism. The aim is to reveal the effects of elevated CO2 on soil N supply capacity, N retention mechanism, and the related underlying microbial mechanism. The research results can improve our understanding of the mechanism regarding the difference in increasing rice yield under elevated CO2 between two rice cultivars with contrasting performance under elevated CO2. In addition, understanding the effects of elevated CO2 levels on gross N transformation rates in intensively managed agricultural soils can predict the soil N supply capacity and provide the basis for reasonable fertilization schemes, and finally reduce the input of N fertilizer and N-associated pollution to the environment in response to climate change in the future.

高CO2浓度可以提高水稻产量，且高应答水稻比低应答水稻增产更加明显。已有大量研究从水稻自身氮代谢、水分利用效率、光合作用、产量形成过程等方面尝试剖析高大气CO2浓度增产的内在机制。但是，鲜有研究关注高CO2浓度对不同水稻品种种植下稻田土壤氮素循环的影响及其反馈机制。本项目以我国典型稻麦轮作FACE（Free-Air CO2 Enrichment）系统为研究平台，采用15N示踪技术结合数值模型测定大气CO2浓度提高下高、低应答水稻品种的土壤氮素矿化、硝化、微生物同化等初级转化速率，揭示高CO2浓度对高、低应答水稻种植下稻田土壤供氮能力和保氮机制的影响，并解析其内在的微生物学机制，结果将有助于理解大气CO2浓度升高下高、低应答水稻品种增产响应差异的机制，亦可以为预测全球变化形势下土壤的供氮能力，制定合理的施肥措施、最终减少氮输入及与氮相关的污染提供理论依据和技术支持。

本项目以我国典型稻麦轮作FACE系统为研究平台，采用15N示踪技术结合数值模型，重点研究了大气CO2浓度和温度升高1）对稻田土壤供氮能力和保氮机制的影响；2）对稻田土壤N2O产生途径的影响；3）对稻田土壤产甲烷菌的群落结构及其机理的影响。研究工作紧紧围绕项目计划书内容开展，已经达成预期目标。共发表SCI收录论文7篇，培养硕士研究生3名。主要进展如下：1）采用15N成对标记技术结合MCMC数值分析模型分别测定了好氧和淹水条件下不同处理稻田土壤氮素的初级转化速率，发现好氧条件下大气CO2浓度升高可以降低稻田土壤的自养硝化速率，而对矿化和铵态氮同化无影响，这将有利于稻田土壤的氮保持；温度升高提高了稻田土壤的矿化、铵态氮同化和硝化速率，提高了稻田土壤的供氮能力；大气CO2浓度和温度同时升高可以显著降低稻田土壤的矿化速率，而对铵态氮同化速率和硝化速率无显著影响，这将不利于土壤的氮供应。淹水条件下，温度升高或大气CO2浓度升高均能显著提高稻田土壤的矿化-同化周转率，有利于提高土壤的供氮能力。2）利用15N示踪和C2H2抑制技术，发现好氧条件下硝化作用对N2O累积排放量的占比均达55%以上，表明各处理土壤的N2O都主要来自硝化过程；N2O同位素异位体位嗜值法的结果进一步确认，自养硝化作用和细菌主导的反硝化作用是所有处理组稻田土壤N2O产生的主要途径。淹水培养结果发现稻田土壤存在强烈的反硝化作用，且进行的较为彻底，使得该过程产生的N2O绝大部分都被还原为了N2。3）通过对产甲烷菌mcrA功能基因高通量测序，发现不同处理稻田土壤的产甲烷菌在目水平上主要分布于甲烷八叠球菌目、第七产甲烷古菌目、甲烷胞菌目、甲烷杆菌目等；从mcrA基因定量结果可知，温度升高、大气CO2浓度升高对产甲烷菌的丰度没有促进作用；从产甲烷菌mcrA基因进一步探索产甲烷机理，我们发现不同处理稻田土壤土壤均主要以乙酸型产甲烷途径为主。