| Due to global warming, the global precipitation pattern have abnormally changed. The substantial increase in precipitation is one reflection of the abnormal changes in the pattern of precipitation. The frequency, range and duration of precipitation increase in recent decades have increased. Forest ecosystem is the largest terrestrial ecosystem, and soil carbon cycle, soil nitrogen cycle and soil water cycle are the three most important processes in forest ecosystem. The increased precipitation will alter the environmental factors of soil to further affect litter decomposition, soil carbon and nitrogen, soil respiration, SOC mineralization and other key ecological processes, which is bound to have a significant impact on the forest ecosystem structure and function as well as C-N-water relation. Shimian county Sichuan Province is located in dry river valley area which is the middle reaches of the Dadu River, with a distinct transition from dry season to wet season in the whole year. Since the last century 50's to 70's and 90's to now, precipitation in this region is on the rise. Especially in the early 21 century, precipitation was significantly increased, compared with the last century 90's precipitation, the current precipitation increased by as much as 48%. Pinus yunnanensis is the main afforestation tree species in this region.The study based on average annual precipitation of dry river valley area which is the middle reaches of the Dadu River(Shimian, Yaan, Sichuan), and then we had carried on the increase precipitation test to Pinus yunnanensis plantation from Jun,2013 to May,2015. Four treatments were set, ie, CK(natural state), A1 (increasing precipitation 10%,80 mm·m-2·a-1), A2(increasing precipitation 20%,160 mm·m-2·a-1) and A3(increasing precipitation 30%,240 mm·m-2·a-1). Based on the effects of increased precipitation on litter decomposition, soil carbon and nitrogen, soil respiration and SOC mineralization in Pinus yunnanensis plantation, the preliminary responses of soil C-N-water relation to increased precipitation was discussed in the study. The main results are as follows.(1)Litter decomposition, soil respiration and SOC mineralization in Pinus yunnanensis plantation had obvious seasonal variation, with the fast in wet season and the slow in dry season. At the same time, soil temperature, soil moisture, soil total N, SMBC and SMBN also had obvious seasonal variation, with the high in wet season and the low in dry season, but soil NO3--N and soil NH4+-N with the low in wet season and the high in dry season. There was no obvious seasonal variation in STOC.(2)Increasing precipitation promoted litter decomposition of Pinus yunnanensis plantation, and the effect of promotion was A1>A2>A3. Certainly increasing precipitation also promoted lignin degradation, with A3>A2>A1, but cellulose degradation was promoted in Al, and it was inhibited in A2 and A3. There was no obvious effect about nutrient release patterns with increasing precipitation. Increasing precipitation promoted the release of C, N, P, and K elements, but Al promoted the release of Ca and Mg, A2 and A3 inhibited the release of Ca and Mg. C/N, C/P and Lignin/N ratio of litter leaf increased with the decomposition time. Increasing precipitation increased the ratio of C/N and C/P, but Lignin/N ratio reduced in Al, and increased in A2 and A3(3)STOC content of Pinus yunnanensis plantation was increased, but this effect did not increase with increasing precipitation. However soil total N, soil NO3--N and soil NH4+-N content was reduced by increasing precipitation. In wet season, A1 increased SMBC content and had no effect to SMBN, A2 increased SMBC and SMBN content, but A3 reduced SMBC and SMBN content. In dry season, SMBC content increased with increasing precipitation, and SMBN content reduced only in A3.(4)SOC mineralization in Pinus yunnanensis plantation increased, but this effect did not increase with increasing precipitation. Correlation analysis showed that, there was no significant relationship between SOC mineralization and STOC, and significant relationship between SOC mineralization and SMBC. Increasing precipitation did not significantly alter relationship between SOC mineralization and STOC, but A2 and A3 reduced relationship between SOC mineralization and SMBC.(5)Precipitation increase treatment significantly increased soil respitation(Rs) and CO2 flux in A2, significantly decreased Rs and CO2 flux in A3, and did not significantly affectted Rs and CO2 flux in A1. There was no effect on soil temperature and soil moisture with increasing precipitation in wet season, but in dry season, increasing precipitation increased soil moisture, and had no effect to soil temperature. There was high significantly relationship index between Rs and soil temperature, increasing precipitation did not significantly alter their relationship. Soil respiration Q10 value calculated from soil temperature was 2.13(CK) s 2.11(A1)?2.11(A2) and 2.53(A3) under precipitation increase treatments, and A3 was significantly higher than CK(P<0.05). There was high significantly relationship binomial models between Rs and soil moisture, after lowering the effect of temperature, there was no significantly relationship between them, and increasing precipitation did not significantly alter their relationship. Compared with soil temperature, effect of soil moisture on soil respiration was relatively small. There was high significantly relationship binary linear regression models between Rs and soil temperature, soil moisture, and compared to single factor analysis, their variance explained significantly increased, as well as increasing precipitation decreased their relationship. There was high significantly relationship between Rs and SMBC, Rs and SMBN. Under precipitation increase treatments, the relationship between Rs and SMBC was enhanced in Al and A2, and was reduced in A3, but the relationship between Rs and SMBN was enhanced only in Al.(6)Study on relationship of soil carbon and soil water in Pinus yunnanensis plantation. there was only significantly relationship between soil water and Rs. Increasing precipitation did not significantly alter relationship of soil carbon and soil water. Study on relationship of soil nitrogen and soil water, there was only significantly relationship between soil water and soil NO3--N, soil NH4+-N, SMBN. Under precipitation increase treatments, relationship between soil water and soil NH4+-N was reduced in Al and A2, and relationship between soil water and SMBN was reduced in Al and A3. Study on relationship of soil carbon and soil nitrogen. there was no significantly relationship between STOC and soil nitrogen, and increasing precipitation did not significantly alter their relationship. There was significantly relationship between SMBC and soil total N, soil NO3--N, soil NH4+-N. Under precipitation increase treatments, relationship between SMBC and soil total N was reduced, relationship between SMBC and soil NO3--N was reduced in A2 and A3, relationship between SMBC and soil NH4+-N, SMBN was increased in A1 and A2, and relationship between SMBC and soil NH4+-N was reduced in A3. There was significantly relationship between Rs and soil nitrogen. Relationship between Rs and soil total N, soil NO3--N was reduced with increasing precipitation, relationship between Rs and soil NH4+-N was reduced in A3, and relationship between Rs and SMBN was increased in A1. There was significantly relationship between SOC mineralization and soil total N, and overall increasing precipitation reduced their relationship.Study on relationship of soil carbon, soil nitrogen and soil water. There was no significantly relationship between most soil water and soil C/N. and overall increasing precipitation did not significantly alter their relationship. Only between soil water and Rs/soil total N, soil water and SOC mineralization/SMBN had significantly relationship, and increasing precipitation reduced their relationship. Also there was no significantly relationship between soil water and SMBC/SMBN, but their relationship was increased in A2. In general, the response of soil C-N-water relationship to increasing precipitation is complicated. To deeper understand the effect of increasing precipitation on soil C-N-water relationship, further research will be needed to study the mechanism. |
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