Temperature Sensitivities Of Soil Organic Matter And Rice Straw Decomposition And Their Responses To Moistute In Rice Paddy Soil In Northeastern Japan

Global warming is a great issue for all human beings and there is no doubt that rice paddy ecosystem plays a vital role in global warming. Various practices to increase carbon ?C? sequestration in paddy ecosystem and reduce methane ?CH₄?emission caused by rice straw ?RS? return have caused extensive concern. Therefore,experiments with combination of aerobic and anaerobic incubations were carried out in a rice paddy soil with and without ¹³C and 15N labelled RS addition by modeling soil temperature and moisture during off-rice and rice growth seasons in Yamagata Prefecture, Northeastern Japan. The main objectives of this study were to 1)investigate and compare the responses of soil organic matter ?SOM? and RS decomposition to soil temperature and moisture, 2) compare the decomposition rates of RS derived C calculated by SOC content, cumulative productions of carbon oxide?CO₂? and CH₄, and ?¹³C value during aerobic and anaerobic incubations, 3) analyze the reliability of denitrification rate of RS derived nitrogen ?N? calculated by ?15N value during the anaerobic incubation. The major results of this study were presented as follows:1. In aerobic incubation, soil temperature and moisture significantly promoted CO₂ emission in paddy soil after 24-week aerobic incubation and their positive interactions were also significant ?P<0.01?. SOC content in paddy soil slowly decreased with incubation time, but ?¹³C value in paddy soil slowly increased with incubation time. Soil temperature significantly promoted the decrease of SOC content and the increase of ?¹³C value. However, soil moisture had no great impact on ?¹³C value throughout the 24-week aerobic incubation but had significantly effects on SOC content only after 12 and 24 weeks of aerobic incubation. During the subsequent anaerobic incubation, CH₄ and CO₂ productions in paddy soil under all treatments significantly decreased with the increase of soil temperature in previously aerobic incubation ?P<0.05?. This result could be attributed to the fast and substantial depletion of easily decomposed substrates caused by increasing soil temperature during aerobic incubation of paddy soil. Soil moisture during previously aerobic incubation still significantly affected CO₂ production during anaerobic incubation but had no great impact on CH₄ production probably due to the great accumulation of nitrate-nitrogen ?NO₃^--N? after aerobic incubation. ±5 ? was used to model the freeze-thaw process in Yamagata paddy field during off-rice season. Its significant effect on CO₂ and CH₄ productions was not found in this study.2. SOC aerobic decomposition in RS-added soil was significantly promoted by soil temperature and moisture ?P<0.05?. RS addition greatly promoted CH₄ production during anaerobic incubation under high temperature and submerged conditions, indicating that RS was a great source of CH₄ emission in paddy soil.Except temperature increase from ±5 ? to 5 0C at 60% WFPS ?L5/±5?,temperature sensitivity coefficient ?Q10? of SOC decomposition calculated by RS-derived CO₂ production was higher than those at temperature increase from 15? to 25?. This result indicated that RS-derived C decomposition at low temperatures ?±5? and 5??was more sensitive to those at high temperatures ?15? and 25??. Both CO₂ and CH₄ productions during anaerobic incubation significantly decreased with soil temperature in previous aerobic incubation. The reasons could be ascribed to that a lot of undecomposed RS at low temperatures under aerobic incubation provided substantial C source and energy for methanogenesis during subsequent anaerobic incubation under 30 ? and submerged condtion. Decreasing Eh induced by submersion during subsequent anaerobic incubation was also favorable to methanogenesis. This result implied that increasing soil temperature and moisture to some extent in Yamagata paddy field after RS return during the off-rice season could reduce RS-derived CH₄ production in following rice growth season. Total decomposition rates of RS ?aerobic and anaerobic incubation? derived C calculated by SOC content, CO₂ and CH₄ productions, and 8¹³C value varied from 45.0% to 64.2%,from 29.8% to 48.1% and from 25.7% to 34.4%, respectively. Since the responses of CO₂ and CH₄ productions to soil temperature and moisture were more sensitive to those of SOC content and ?¹³C value, decomposition rate of RS calculated by CO₂ and CH₄ productions might have the highest accuracy in this study.3. There were no significant differences in total nitrogen ?TN? content in paddy soil under all soil temperature and moisture treatments after 24-week aerobic incubation. Ammonium-nitrogen ?NH4⁺-N? concentration in control paddy soil significantly decreased with the increase of soil temperature and incubation time.However,NO₃^--N concentration increased with the increase of soil temperature and incubation time. It indicated that soil temperature significantly promoted N mineralization in paddy soil ?P<0.05?. In this study, there was no significant effect of moisture on NH4⁺-N and NO₃^--N concentrations. Moreover, the effect of soil moisture on N mineralization in paddy soil was not as obvious as soil temperature. The probable reasons could be due to sufficient air and shallow soil layer ?<1 cm? in this study resulting in no inhibition of Oxygen diffusion into soil under high moisture condition. During anaerobic incubation, net organic N mineralization productions?NH4⁺-N production? at ±5 ? and 5 ? were much higher than those at ±5 ? and 5?, which could be probably attributed to the promoted microbial immobilization of mineral N or nitrification caused by high soil temperatures during previously aerobic incubation. N mineralization at 100% WFPS was lower than at 60% WFPS,suggesting that high moisture during aerobic incubation could inhibit N mineralization in paddy soil during subsequent anaerobic incubation.4. Soil temperature and moisture had no significant impacts on both TN content and 815N value in 15N labelled RS-added paddy soil. Also, TN content and 815N did not obviously changed with incubation time. NH4⁺-N concentration decreased with soil temperature, but increased with soil moisture. Both NO₃^--N and NH4⁺-N concentrations in RS-added soil samples in all treatments were lower than those in control paddy soil samples. This result indicated that RS addition accelerated microbial immobilization of mineral N derived from initial soil. During the subsequent anaerobic incubation,NH4⁺-N production significantly increased with previous soil temperature and moisture treatments in aerobic incubation. These results indicated that the increase of soil temperature and moisture during aerobic incubation was not only beneficial to microbial immobilization of soil mineral N but also promoted N mineralization in subsequent anaerobic incubation under high temperature and submerged condition. The fractionation of 15N and ¹⁴N caused by ammonification could occur during drying of control and RS-added soil samples after anaerobic incubation resulted in the overestimate of the N loss caused by denitrification. More specific reason should be further studied.In summary, our study revealed that soil temperature and moisture not only promoted aerobic decomposition of SOM and RS but also affected their anaerobic decomposition. The effect of soil moisture on SOM and RS decompositions was not as obvious as soil temperature. Increasing soil temperature via appropriate practice in paddy soil in Yamagata Prefecture during off-rice season could effectively reduce RS-induced CH₄ emission during following rice growth season. Our study preliminarily revealed the responses of SOM and RS to soil temperature and moisture which is helpful for local government to reduce RS-induced CH₄ emission by optimizing filed management in off-rice season.