| Ecosystem carbon cycle, nitrogen cycle, and water cycle process is the most important and the fundamental of three kinds of substance circulation, and also is the most important carrier of energy transfer, nutrient cycling, and water movement of ecosystem. Forest soil system is an important part of terrestrial ecosystems; doing research on the soil carbon-nitrogen-water coupling cycling are vital to understanding how forest ecosystem play an important role in energy conversion, nutrient cycle, water cycle, and ecological functions, which to provide the scientific basis for the global scale of carbon-nitrogen-water cycle control and greenhouse gas source or sink management, and also improving ecosystem management and safeguard the ecological security which is urgent need. The forest in Beijing Mountain is the important green barrier, which play an important role in higher living standards, ensuring economic development, and improve the ecological environment. But lacking of comprehensive and unified understanding of the forest soil in Beijing Mountain play an important role in the mechanism of regulating in carbon-nitrogen-water coupling cycling and greenhouse gas source or sink management. Therefore, this study base on the three representative forests stands of Pinus tabulaeformis L., Platycladus orientalis L., and Quercus variabilis L., the aim of this study was to systematically measuring the factors process of soil carbon cycle, nitrogen cycle, and water cycle, and the various environmental factors, and also to determine the effect of the environmental factors on soil carbon cycle, nitrogen cycle, and water cycle. We carried out the indoor and outdoor experiment for the aims. First of all, analysis the spatio-temporal variability of soil carbon cycle, and nitrogen cycle, and water cycle base on the long-term observation. Secondly, determined the contribution of litter and root growth on the soil carbon cycle, nitrogen cycle, and water cycle base on the treatment of litter removal and cutting roots. And then, comprehensive analysis the effect of the biotic and abiotic factors on the soil respiration rate, soil gross nitrification, and soil denitrification. Last but not least, reveal the coupling relationship between-soil carbon, soil nitrogen, and soil water. The main conclusions were as follows:(1) We found that the spatio-temporal variability of soil carbon, soil nitrogen, and soil water cycle were different in forest ecological systems. The soil carbon and nitrogen pool, soil repitation rate, soil gross nitrification, and soil denitrification in top soil layer were higher than lower soil layer, and they were showing the trend of peaky curve with high value in summer and low value in winter and spring, but the total nitrogen, nitrate nitrogen, and ammonium nitrogen were not found the same trends. Moreover, the soil volumetric water content was low level value and stable in the spring and winter time, while in the summer time, we found that a pulse wave variation in soil volumetric water content. Additionally, we also found that the variation characteristic of soil evaporation variation was not obvious with has high value in summer time, and the infiltration capacity of coniferous forest was larger than broad-leaved forest.(2) The litter significantly affected the process of soil carbon cycle, nitrogen cycle, and water cycle. The soil carbon and soil nitrogen pool, soil respiration rate, soil gross nitrification rate, and soil denitrification rate were has lower value after litter removal treatment in comparison with natural treatment. Additionally, the average contribution rate of litter on soil carbon cycle was from 21.35%to 40.16%, and the average contribution rate of litter on soil nitrogen cycle was from 14.73% to 51.85%. Moreover, we found that a significant impact on the soil volumetric water content in 5cm soil layer after litter removal, but did not found in 15,25cm soil layer. In addition, the daily average value of soil evaporation in forests stands were increased after litter removal, which was increased from 33.61% to 42.24%, and simultaneously enlarged the initial infiltration rate in corresponding forests stands was from 1.47% to 14.11%, but did not found obvious change in the steady infiltration rate.(3) The root growth significantly affected the process of soil carbon cycle, nitrogen cycle, and water cycle. In addition, the soil carbon and soil nitrogen pool, soil respiration rate, soil gross nitrification rate, and soil denitrification rate were has lower value after cutting roots treatment in comparison with natural treatment. Additionally, the average contribution rate of root growth on soil carbon cycle was from 19.54%to 54.29%, and the average contribution rate of root growth on soil nitrogen cycle was from 14.65% to 65.44%. Moreover, soil volumetric water content, soil evaporation, and the steady infiltration rate were not found obvious effect after cutting roots.(4) The soil respiration rate, soil gross nitrification rate, and soil denitrification rate were affecting by raining. Moderate to heavy rains, which rainfall at 20-30 mm largely promoted soil respiration rate, soil gross nitrification rate, and soil denitrification rate, but not in light rain. Additionally, the rate soil respiration, soil gross nitrification, denitrification increased with the increasing of rainfall in relative terms. Moreover, we also found that the soil volumetric water content was significantly increased when the rainfall was greater than 12.00 mm, and gradually decreased with increasing soil depth.(5) We found that the critical point of inhibition soil respiration rate, soil gross nitrification rate, and soil denitrification rate under different soil water content conducted by simulation experiments in laboratory. The soil respiration rate, soil gross nitrification rate, and soil denitrification rate were increased with increasing soil water content, beyond a certain point it caused its decreased. Moreover, the critical point of water content of inhibition of soil respiration rate in forests stands of Pinus tabulaeformis L., Platycladus orientalis L., and Quercus variabilis L. were 19.97%,16.65%, and 16-.90%, respectively, and were 25.36%,21.79%, and 19.56%, respectively, for soil gross nitrification rate, and were for 26.37%,20.02%,20.51%, respectively, for soil denitrification rate.(6) The correlation analysis showed that soil respiration rate, soil gross nitrification rate, and soil denitrification rate were significantly related with soil temperature and meteorological factors, but not in soil volumetric water content and leaf area index (LAI). Moreover, the comprehensive analyzing result showed that biotic and abiotic can collectively account for the change of soil respiration rate, soil gross nitrification rate, and soil denitrification rate were 65.9%～89.1%,71.3%～93.5%, and 63.3%～87.9%, respectively. In addition, the biotic and abiotic factors of soil temperature, photosynthetically active radiation (PAR), and solar radiation were significantly affected soil respiration rate, soil gross nitrification rate, and soil denitrification rate.(7) The LAI values of evergreen species of Pinus tabulaeformis L., and Platycladus orientalis L., were maintained relatively stable values at 2.43, and 2.71, respectively. The soil respiration rate, soil gross nitrification rate, and soil denitrification rate were not changing by LAI values. While that did not the same trends in deciduous broad-leaved species of Quercus variabilis L., we found that the soil respiration rate, soil gross nitrification rate, and soil denitrification rate were increased with increasing LAI, which based on the seasonal variability of LAI in Quercus variabilis L.(8) We found that the strongly coupling relationship of soil carbon and nitrogen, and the nonliner coupling relationship in soil carbon and water, and soil nitrogen and water. The coupling relationship of different forms of soil C/N ratio and soil volumetric water content was relatively complex, which showed carbon-nitrogen-water coupling relationship. |
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