| Nitrogen (N) availability in tropical and subtropical ecosystems is rapidly increasing as a result of industrialization and associated increases in anthropogenic N deposition from agricultural and industrial emissions, and will have an important impact on global carbon cycle because of the continuous increases of N deposition. Also, the global changes besides N deposition have been shown can greatly change the amount of organic carbon inputs to soil, which could cause substantial effects on belowground carbon (C) pool and soil respiration (Rs). The effects of N deposition on Rs from forest soils range from positive to negative depending on forest type, N-status of the soil, and the rate of N deposition. In forest ecosystems where biological processes are limited by N supply, N additions either stimulate Rs or have no significant effect, whereas in "N-saturated" forest ecosystems, N additions decrease Rs. However, most studies of the consequences of enhanced N deposition on Rshave been performed in temperate forests. There are very few studies of Rs responses to N deposition in subtropical and tropical forests. However, the effects of increasing N availability and the amount of various of different carbon input from soil organic matter (SOM) on Rs in tropical and subtropical forests are poorly understand. We therefore conducted a N fertilization experiment and treatment of changes in carbon input in a subtropical, N-limited Schima superba forest of Tiantong National Park in eastern China to better understand the short-term response of Rs to N additions. Besides the control (0kg N ha-1yr-1), two levels of N additions were used:low-N (50kg N ha-1yr-1) and high-N (100kg N ha-1yr-1). Soil temperature, soil moisture, and Rs were measured in each treatment, as well as fine root biomass, SOM, microbial biomass, and N concentration in fine root. The main results of this study are as follows:(1) The mean Rs were2.52±0.31、3.24±0.32'3.08±0.28μmol CO2m-2s-1in control, low-N treatment and high-N treatment, respectively. Short-term addition of N lead to a significantly increase in Rs compared to the control, yet the magnitude of this increase was dependent on the amount of N added, with Rs being greatest in the low-N treatment. Low-N treatment significantly increased the Rs, but high-N treatment did not change Rs. N treatments did not change the seasonal pattern of Rs, and the Rs was significantly higher in wet season that it in dry season. Low-N treatment significantly increased Rs in wet season, however, N treatment had no significant effect on Rs in dry season. Soil temperature can explain the differences of Rs by89.53、65.51and73.18%in control, low-N and high-N treatment, respectively. The Q10was2.42,2.02and2.00in in control, low-N and high-N treatment, respectively, and N treatments decreased the Q10value. There were significantly exponential relationships between Rs and soil temperature among different N treatments, and there were no significant relationships between Rs and soil moisture among different N treatments. So, soil temperatue was the good predictor for determining the differences of Rs.(2) There were significant effect of litter manipulation and interactive effect of litter manipulation and N treatments on soil moisture. Litter removal significant decreased Rs, and the contribution ratio of litter removal was-25.32%in control of N treatment. However, litter doubling did not significantly increase Rs by the smiliar ratio. Litter removal had no significant effect on Rs, but litter doubling significantlu increased Rs among different N treatments. The Rs of litter removal and doubling had significantly difference between wet and dry seasons under control and low-N treatment. Low-N and high-N treatments had significant effects on Rs of litter removal, and high-N treatment had diginificant effect on Rs of litter doubling within wet or dry season.(3) N treatments extended the decreasing effects on contribution ratio of Rs under litter removal and litter doubling treatments with increasing N treatment levels. N+litter removal treatments gradually significantly increased the contribution ratio of Rs in litter removal with increasing N treatment levels, and this significant effect mainly occur in wet season. Low-N+litter doubling treatments significantly increased the contribution ratio of Rs with increasing N treatment levels, and this significant effect mainly occur in dry season. However, high-N+litter doubling treatments significantly decreased the contribution ratio of Rs with increasing N treatment levels, and this significant effect mainly occur in wet season. There were significant relationship between soil temperature and Rs under litter removal and doubling among different N treatments.(4) Root exclusion had no significant effect on soil temperature, but had significant effect on soil moisture. Root exclusion did not change the seasonal variability of soil temperature among different N treatments. N treatments had no significant effect on soil temperatue and soil moisture of root exclusion, and no significant differences of soil temperature and soil moisture in root exclusion between wet and dry season among different N treatments. Root exclusion significantly decreased Rs within each N treatment, and the similar situation also had been found in wet season. The contribution ratio of root exclusion was-11.28%under N control treatment.N treatments extended the decreasing effects on contribution ratio of Rs under root exclusion treatments with increasing N treatment levels, and this significant effect in dry season was much higher than in wet season. N+root exclusion treatments gradually significantly increased the contribution ratio of Rs increasing N treatment levels, and this significant effect mainly occur in dry season. N treatment alleviated the contribution ratio decreasing of Rs under root exclusion treatment. N treatment decreased the temperature sensitive coefficient of soil respiration under root exclusion. There were significant relationship between soil temperature and Rs under root exclusion among different N treatments, but no significant relationship between soil moisture and Rs. (5) On the whole, N treatments have significantly increased the respiration from root and litter, but decreased the soil microbial respiration. The significant responses of three Rs components to different N treatments mainly occurred in wet season, which is similar to significant differences during the whole study period. The Rs of different components were higher in dry season compared with that in wet season, also the differences in dry season were not significant. The significant responses of contribution ratio from root respiration to low-N treatment mainly occurred in wet season, and the significant responses of contribution ratio from litter respiration to low-N treatment mainly occurred in both wet season and dry season. Also, the significant responses of contribution ratio from soil microbial respiration to low-N treatment mainly occurred in dry season. The contribution ratio differences from root respiration and soil microbial respiration to low-N treatment were significant between wet and dry seasons.Our results suggest that the significantly increasing response of Rs to short-term simulated N deposition in subtropical Schima superba forest in eastern China may vary depending on the status of the rate of N deposition and seasonal variability. How Rs respond to N deposition in subtropical Schima superba forest under the background of future high N deposition is worthy further study. |
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