稻田微氧层和还原层土壤有机碳矿化对氮素添加的响应

田间条件下，淹水稻田由于上覆水中溶解氧的扩散作用，使其表层土壤存在约1 cm厚的微氧层，这个特殊层次中碳氮转化的特征尚未明晰.以亚热带典型稻田土壤为对象，采用100 d室内模拟培养试验，结合¹³C稳定同位素示踪和磷脂脂肪酸(PLFA)分析技术，研究稻田土壤微氧层(0～1 cm)和还原层(1～5 cm)外源新鲜有机碳(¹³C-水稻秸秆)和原有土壤有机碳矿化对氮肥施用(NH₄)₂SO₄]的响应规律及其微生物过程.结果表明，氮素添加使土壤总CO₂和¹³C-CO₂累积排放量分别提高11.4%和12.3%;培养结束时，氮素添加下还原层比微氧层土壤总有机碳含量和¹³C回收率分别降低2.4%和9.2%.培养前期(5 d),氮素添加提高还原层微生物总PLFAs,且细菌和真菌PLFAs响应一致，但对微氧层微生物丰度无显著影响；氮素添加对微氧层和还原层总¹³C-PLFAs丰度均无显著影响...

Response of Organic Carbon Mineralization to Nitrogen Addition in Micro-aerobic and Anaerobic Layers of Paddy Soil

In field conditions, a micro-aerobic layer with 1 cm thickness exists on the surface layer of paddy soil owing to the diffusion of dissolved oxygen via flooding water. However, the particularity of carbon and nitrogen transformation in this specific soil layer is not clear. A typical subtropical paddy soil was collected and incubated with¹³C-labelled rice straw for 100 days. The responses of exogenous fresh organic carbon(¹³C-rice straw) and original soil organic carbon mineralization to nitrogen fertilizer addition(NH₄)₂SO₄]in the micro-aerobic layer(0-1 cm) and anaerobic layer(1-5 cm) of paddy soil and their microbial processes were analyzed based on the analysis of ¹³C incorporation into phospholipid fatty acid(¹³C-PLFAs). Nitrogen addition promoted the total CO₂ and ¹³C-CO₂ emission from paddy soil by 11.4% and 12.3%, respectively. At the end of incubation, with the addition of nitrogen, the total soil organic carbon (SOC) and¹³C-recovery rate from rice straw in the anaerobic layer were 2.4% and 9.2% lower than those in the corresponding micro-aerobic layer, respectively. At the early stage(5 days), nitrogen addition increased the total microbial PLFAs in the anaerobic layer with a consistent response of bacterial and fungal PLFAs. However, there was no significant effect from nitrogen on microbial abundance in the micro-aerobic layer. Nitrogen addition had no significant impact on the abundance of total ¹³C-PLFAs in the micro-aerobic and anaerobic layers, but the abundance of ¹³C-PLFAs for bacteria and fungi in the micro-aerobic layer was decreased dramatically. At the late stage(100 days), the effect of nitrogen addition on microbial PLFAs was consistent with that at the early stage. The abundances of total, bacterial, and fungal ¹³C-PLFAs were remarkably increased in the anaerobic layer. However, the abundance of ¹³C-PLFAs in the micro-aerobic layer showed no significant response to nitrogen addition. During the incubation, the content of NH₄⁺-N in the anaerobic soil layer was higher than that in the micro-aerobic soil layer. This indicates that nitrogen addition increased microbial activity in the anaerobic soil layer caused by the higher NH₄⁺-N concentration, as majority of microorganisms preferred to use NH₄⁺-N. Consequently, the microbial utilization and decomposition of organic carbon in the anaerobic soil layer were accelerated. By contrast, richer available N existed in the form of NO₃^--N in the micro-aerobic soil layer owing to the ammoxidation process. Thus, the shortage of NO₃^--N preference microorganisms in the paddy soil environment prohibited the microbial metabolism of organic carbon in the micro-aerobic layer. As a whole, nitrogen fertilization enhanced organic carbon loss via microbial mineralization in paddy soil with a weaker effect in the micro-aerobic layer than that in the anaerobic layer, indicating the limited microbial metabolic activity in the surface micro-aerobic layer could protect the organic carbon stabilization in paddy soil. This study emphasizes the heterogeneity of paddy soil and its significant particularity of carbon and nitrogen transformation in micro-aerobic layers. Consequently, this study has implications for optimizing the forms and method for the application of nitrogen fertilizer in paddy cropping systems.