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-U100%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 (P0.01); net TOC increment and net TOC loss were different in different moisture contents (P0.01) and nitrogen (P0.05) 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 (P0.05). 【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.