Effects Of Soil Properties And Soil Water Contents In The Winter Crop Season On CH₄ Flux During Rice Growth Period And Its Mechanisms

CH₄ is one of the main greenhouse gases, with importance second only to CO₂. The exploration of controlling factors, development of mitigation strategies and quantitative estimation of CH₄ emission from rice fields are important issues and should be addressed by soil and environment scientists. Clarifying the mechanisms of spatial variability of CH₄ emissions from rice fields is the key to reasonably estimate the total amount of rice-originated CH₄ emission on regional, national and global scales, ascertain the very kind of rice fields with the highest urgency to reduce its CH₄ emission, and figure out more mitigation options. Rice soil differed greatly in moisture and thermal condition and soil water content in the non-rice growth season (NRGS) were selected to investigate their effects on CH₄ production, oxidation and emission by pot and incubation experiments. Fifteen paddy soils were collected nation-widely for the incubation experiment to investigate the effects of soil properties on CH₄ production and oxidation.Of 15 soil samples, two paddy soils collected from rice fields in Wuxi and Yingtan, respectively, were treated by five water content levels ranging from air-dryness to flooding during NRGS. Soil samples were collected before ricetransplanting to determine soil properties and CH4 production potential. CH4 oxidation ability was measured after rice transplanting. A close-chamber method was used to measure CH4 fluxes from pot rice soils.CH4 production potentials of 15 paddy soils differed greatly, with the total CH4 production over the whole incubation period varied pronounced in ranges of 1.18-1176.16 jj-g-g'soil (anaerobic incubation, 158 days) and 0.41-136.30 jig-g" 'soil (aerobic incubation, 132 days). CH4 oxidation capacities of 15 paddy soils also differed to some extent. The maximum CH4 oxidation rate and oxidation activity of different soils ranged from 0.22 to 4.85 |xg g'soil h"1 and from 0.55 to 1.52xl0"3 g" 'soil h"1, respectively.There existed significant and positive correlation between CH4 production and the contents of soil organic carbon, labile carbon and total nitrogen. However, CH4 production was not significantly affected by the other soil properties, such as particle sizes, soil pH, soil cation exchange capacity and active Fe and Mn contents. Thus soil organic matter content is the most important soil property to affect CH4 production. Integration of the results of all the available similar experiments showed that the larger the spatial scale represented by the soil samples, the less soil properties correlated with CH4 production, and the less significance a individual soil property correlated with CH4 production. We found no simple significant correlation between CH4 oxidation capacities and all the above-mentioned soil properties. These results demonstrated that there might be a more important factor than soil properties to affect CH4 production and oxidation on large spatial scale.Soil water content in NRGS significantly influenced CH4 production, emission and soil redox potential (Eh) during the following rice-growing season. The mean CH4 fluxes and CH4 production rates during rice growth period increased and soilEh decreased significantly with the increase of soil water content in NRGS, except the case of air-dryness water condition. The anaerobic CH4 production rates were significantly higher than aerobic CH4 production rates mainly due to CH4 oxidation under aerobic condition. CH4 oxidation capacities of soils measured after rice transplanting generally increased with the increase of soil water contents in NRGS. Even if incubated under the optimum soil water condition, soils air-dried in NRGS still showed greatly inhibited CH4 oxidation potentials.Soil water content in NRGS considerably affected soil organic C, active C, total N and active Fe and Mn contents before rice transplanting and temporal variation pattern of CH4 production rate, flux and soil redox potential (Eh) during the rice-growing period. The higher soil water content in the NRGS, the higher soil organic C, active C, total N contents (except air-dryness water condition), the lower soil active Fe and Mn contents (no exception) before rice transplanting, the quicker soil Eh declined, and the earlier CH4 production and emission initiated after rice transplanting (except air-dryness water condition). Thus it might be water history induced change of soil oxidative and reductive capacity (represented here by organic C, Fe, and Mn contents) that affected soil reduction rate, and then CH4 production and emission during the rice growing period.Though the seasonal mean CH4 flux was significantly correlated with the seasonal mean soil Eh, the seasonal variation of CH4 fluxes was not always significantly correlated with soil Eh. For the treatment flooded in NRGS, there was no significant correlation between CH4 flux and soil Eh, but there was significant correlation between CH4 flux and soil temperature during rice growth season. In contrast, for the other four treatments, it was soil Eh, not soil temperature that was related significantly to the temporal variation of CH4 emissions. The statisticalsignificance correlated between CH4 flux and soil Eh decreased with increase of soil water content in NRGS, except air-dryness water condition. The soil Eh in the treatment flooded in NRGS was lower enough for CH4 production during whole rice growing period, thus it was not a dominated factor controlling CH4 emission and was not related significantly to CH4 emission from the treatment.There were significant correlation between CH4 fluxes and CH4 production rates and soil properties, such as soil organic C, active C, total N and active Fe and Mn contents determined before rice transplanting. However, CH4 fluxes were significantly correlated with soil properties only when the data from the tested two soils were regressed separately, while correlation of CH4 flux and production rate offered a good linear fit to all the data of 2 soils with a single line. This suggests CH4 production potential is a much better indicator of CH4 emission from rice fields than soil properties, such as organic C contents.Both CH4 production and oxidation potentials were stimulated by higher soil water content in the preceding NRGS. Though CH4 emission from rice soil also increased significantly as CH4 production rate with the increase of soil water content in NRGS, it varied in a much lower range (3.7-22.6 mg m'2 h"1) than CH4 production (0.33-4.39 ug g'soil h"1), which might be mainly due to the increasing buffering effect of CH4 oxidation of soil with higher historic water content. The ability of soil self-adjusting CH4 oxidation capacity would be important to mitigate CH4 emissions from rice fields.The water content of air-dried soil is extremely low so that the decomposition of soil organic matter during NRGS is substantially inhibited due to the inactivity of soil microorganisms. Therefore the organic C content of air-dried soil was an exception to the regularity between organic C content before rice transplanting andsoil water content in NRGS. This, together with the extremely low CH4 oxidation capability and air-dry effect with unclear mechanisms might be the main reasons why CH4 flux, CH4 production rate, soil Eh and their temporal variations of air-dried soil didn't follow the above-mentioned regularities.The mechanism of spatial variability of CH4 emissions from rice fields was suggested. We chose air-dry water condition as one of the treatments for the purpose to achieve a largest water-content-change range. However, due to precipitation or irrigation during NRGS, it is nearly impossible for air-dried soil to exist under field conditions in rice growing region during the whole season. Thus, all the exceptions in case of air-dryness water condition in this thesis could only count theoretically. A positively significant relationship might actually exist between soil water content in the NRGS and CH4 production and emission within rice growth season without exception. This suggests that a regional variation of CH4 emissions from rice fields on large scale could be attributed at least partially to soil moisture variation in the NRGS. Though CH4 emission was also significantly affected by soil properties measured before rice transplanting, only the data of the same soil can be fitted by linear correlation, and even the changes of soil properties were at least partially caused by different water content in NRGS, which reminds us of the dominant importance of soil water history in controlling CH4 emission during rice cultivation. We found CH4 production was only significantly affected by soil organic matter content in the incubation experiment with soil properties varied greatly, but the statistic significance was lower than that in similar experiments with smaller sampling scale. No significant correlation was observed between CH4 production and any soil properties, including soil organic C content, in similar experiment using soil samples collected on even larger spatial scale.There was no significant correlation of CH4 oxidation capacity and any soil properties in CH4 oxidation experiment with 15 paddy soils. All these results suggested that soil water history might be a more important factor than soil property in controlling the spatial variability of CH4 emissions from rice fields.Almost all the previous studies on and mitigation options for CH4 emissions from rice fields focused on the rice growth periods. Our results demonstrate that water management in NRGS also plays an important role in regulating CH4 emission, production and oxidation within the following rice growing period and should be taken into account for estimating the amount and figuring out more mitigation options of CH4 emission from rice fields. Since the highest CH4 emission was from the flooded treatment, special attention should be paid to the annually flooded rice field that is a kind of rice field of China's characteristic.