控制条件下水肥耦合对黄泥田还田秸秆腐解及土壤碳转化的影响

【目的】研究秸秆还田后不同水温和肥剂管理措施下土壤碳素转化特征。【方法】以华中双季稻区低产水稻土黄泥田为供试材料,模拟早稻和晚稻秸秆还田的田间环境,在实验室控制条件下,开展了两种温度环境中(15℃、35℃)不同水分(40%和100%最大田间持水量,即40%WHC、100%WHC)、配施氮肥类型(尿素、猪粪即U、M)、以及促腐菌剂添加对秸秆腐解效果及其过程中土壤碳素转化影响的研究。对水稻秸秆腐解过程中土壤CO2释放量、以及土壤可溶性有机碳(DOC)和总有机碳(TOC)含量在105天培养周期内变化特征进行动态监测分析。【结果】两种温度环境中整个培养周期内,各处理的CO2释放速率和释放总量通常表现为100%WHC-M100%WHC-U40%WHC-M40%WHC-U,即猪粪优于尿素的规律,而不论配施何种氮肥都存在100%WHC40%WHC(P0.01)的现象,同时40%WHC条件下辅施菌剂可显著提升CO2释放量;与此相反,两种温度环境下DOC含量都表现为40%WHC-M40%WHC-U100%WHC-M100%WHC-U(后两者差异小),即40%WHC条件下DOC含量显著高于100%WHC(P0.05),且配施猪粪处理优于配施尿素处理,但这两种氮肥处理间差异随培养时间延长而减小;以CO2-C释放量计算0 7 d、0 28 d、0 105 d内物料分解率,结果表明,35℃时100%WHC-U的处理中物料分解最快,15℃时40%WHC-M的处理中物料分解最慢。与之对应,105 d内TOC含量和净增量则在35℃时100%WHC-U的处理中最小(P0.01),而在15℃时40%WHC-M的处理中最大(P0.01);TOC的净增量和净损失量在相同温度条件下,尤其试验前期不同水分(P0.01)、氮素(P0.05)间均存在显著差异,且促腐菌剂添加普遍减小TOC含量;培养周期内所有处理的CO2释放速率与DOC含量间存在显著相关(P0.05)。【结论】水分状况对碳素的转化存在极大影响,其次是氮肥类型,且氮肥的影响作用随秸秆还田时间的延长而减弱;高湿条件更利于促进秸秆腐解,但导致土壤DOC含量较低,TOC的固持量也较少,而配施猪粪则可促进土壤DOC含量的提升及TOC的固持;促腐菌剂添加可促进秸秆腐解,但由于40%WHC条件下显著激发了CO2的释放而不利于土壤固碳。因此在华中低产黄泥田双季轮作稻区,早稻还田时由于气温高周期短,建议保持100%WHC、辅施适量尿素、并配合添加秸秆腐解菌剂,侧重秸秆快腐;而晚稻还田时气温低周期长,建议保持40%WHC并辅施缓效猪粪,侧重土壤固碳。

Effect of soil moisture and nitrogen fertilizer on the decomposition of straw returned to field and the transformation of carbon under controlled conditions

【Objectives】 Tostudy the effect of soil moisture and nitrogen fertilizer on the decomposition of straw returned to field and the transformation of soil carbon (C).【Methods】We investigated impacts of moisture content [40% vs. 100% of water holding capacity (WHC)], N fertilizer [urea(U) vs. pig manure(M)], and supplementary addition of decay-facilitating microbial inocula on the decomposition of straw residues and soil C transformation using a low yield yellow-paddy soil in double rice cropping area, central China. A controlled laboratory experiment was conducted in two temperature (15℃ vs. 35℃) regimes with a total incubation period of 105 days. During the period, CO₂, dissolved organic carbon (DOC) and total organic carbon (TOC) were monitored periodically. 【Results】 In general, under both temperature regimes, the CO₂ release rate and cumulative CO₂ emission were in a order of 100%WHC-M > 100%WHC-U > 40%WHC-M > 40%WHC-U. Those treatments added with manure were higher than with urea, those treatments under 100%WHC were always higher than those under 40%WHC (P< 0.01), regardless of N fertilizer, in which microbial inoculation significantly improved CO₂ emissions. On the contrary, DOC contents of treatments under two temperatures followed a sequence of 40%WHC-M > 40%WHC-U>100%WHC-M > 100%WHC-U, namely DOC contents detected under condition of 40%WHC were significantly higher than 100%WHC (P< 0.05), and treatments with manure had higher DOC contents than those with urea but accompanied by a decreasing difference between them. Material decomposition ratio during the 0-7 d, 0-28 d and 0-105 d, which were calculated by CO₂ release, showed the maximum value in 35℃-100%WHC-U while the minimum value in 15℃-40%WHC-M. Additionally, TOC contents and net TOC increment were the least in 35℃-100%WHC-U (P< 0.01) while the largest in 15℃-40%WHC-M (P<0.01); net TOC increment and net TOC loss were different in different moisture contents (P<0.01) and nitrogen (P<0.05) when incubated under the same temperature, in particular the early stage, and microbial inocula generally reduce TOC content; CO₂ release rates were significantly correlated to DOC contents in all treatments (P<0.05). 【Conclusions】The water content showed the greatest effect on C transformation, followed by N fertilizer, of which the effect was reduced over time of straw returning; high humidity was more conducive for facilitating the decomposition of straw residues as compared with low humidity, and also caused relatively lower DOC content and TOC immobilization. Application of pig manure could enhance the content of DOC and the immobilization of TOC in soils. Moreover, supplementary addition of decay-facilitating microbial inocula generally accelerated the decomposition of straw residues, but this might be a drawback for C sequestration if at 40%WHC since the addition stimulated a significant release of CO₂. Therefore, the coupling management of keeping 100%WHC incorporated with application of urea and microbial inocula was suggested as the best management practice (BMP) during the field-returning period of early season rice straw, with emphasis to stimulate the decomposition of straw residues as soon as possible since the temperature was usually high and the time provided for decomposition before the sowing of late rice was usually short. The coupling management of keeping 40%WHC incorporated with application of pig manure was suggested as the BMP during the field-returning period of late season rice straw, with emphasis to promote soil C sequestration since the temperature was usually low and the time provided for decomposition before the sowing of next early season rice was usually enough.