Changes of soil respiration of paddy fields with water-saving irrigation and its influencing factors analysis

Abstract: The temporal and spatial variation of soil respiration of farmland has great influence on regional and global carbon cycle. Irrigation is an important factor to soil respiration in farmland. Existing study on the effect of irrigation on farmland soil respiration mainly focused on the dry farmland, but less research focused on paddy fields. Relationships between soil respiration and water depth and short-term drying of paddy fields with flooding irrigation were analyzed in existing studies. In order to reveal the difference of soil respiration in paddy fields with water-saving irrigation and traditional irrigation, the diurnal variation of soil respiration, the impact of alternation of wetting and drying on soil respiration, and the influencing factors of soil respiration for paddy fields were analyzed based on lysimeter experiment. The results showed that diurnal variations of soil respiration of paddy fields with water-saving irrigation and normal irrigation were similar at the period when the status difference of soil moisture was small between the two irrigation modes. Diurnal variations of soil respiration of paddy field with water-saving irrigation showed wider range, and the fluxes and rangeability of soil respiration of paddy field with water-saving irrigation were larger than those for traditional irrigation when the status difference of soil moisture was large. The mean value of soil respiration of diurnal variation during the whole growth period for water-saving irrigation was 1.47 times that for traditional irrigation. But during early tillering stage, jointing-booting stage and milk stage in which water managements were obviously different, the mean values of soil respiration diurnal variations for water-saving irrigation were 3.85, 1.42 and 3.74 times those for traditional irrigation. This results could contribute to frequent alternation of wetting and drying of paddy fields with controlled irrigation. The processes of soil rewatering and soil moisture depletion both had a certain effect on soil respiration of paddy fields. Peaks of soil respiration appeared at the critical point of soil moisture depletion. In the critical point of soil rewatering, the increase of soil respiration rate was not obvious if the rewatering depth was thicker, and the increase was significant if the rewatering depth was thinner. And small peaks of soil respiration appeared when soil water content decreased close to the lower limit of soil water content of paddy field with controlled irrigation during the irrigation interval. Soil temperature and soil moisture had great impact on soil respiration of paddy field with water-saving irrigation. Good exponential dependence was showed between soil respiration flux and soil temperature in the depth of 5 cm in paddy fields, and the correlation reached significant level for paddy field with water-saving irrigation (P<0.05). Management mode without water layer for controlled irrigation led the soil moisture conditions to be similar to dry farmland, so the relationship between soil respiration and soil temperature of paddy fields with controlled irrigation was more consistent with the existing results in dry farmland and forest. Water layer was kept in field surface for a long time under traditional irrigation, which led the soil into strict anaerobic conditions, and the effects of soil temperature on soil respiration was relatively weaken. The soil volumetric moisture content of 43% was the critical point for soil respiration of paddy field with water-saving irrigation when the soil volumetric moisture content ranged from 35% to 55%. Soil respiration flux in paddy field with water-saving irrigation increased with the enhancement of soil moisture content when the soil volumetric water content was lower than the critical value (P<0.05). And soil respiration flux in paddy field with water-saving irrigation decreased with the increase of soil moisture content when the soil volumetric water content was larger than the critical value (P<0.05). This conclusion was consistent with the existing research results. The appropriate soil volumetric moisture content was about 43%, which could create a suitable environment for respirations of rice roots, soil microbes and so on, and increase soil respiration rate. The results of this paper can support for more comprehensively evaluating the ecological environment effect of rice water-saving irrigation. At the same time, it can be helpful to accurately evaluate the carbon source/sink features of paddy field ecosystem.