| Soil nitrogen (N) availability is an important factor to limit plant growth and the key factor to influence ecosystem productivity. Most N in soil exists in the form of organic N, and can not be directly absorbed by plants before it was transformed to inorganic N (NH4+-N and NO3--N) by soil microbes. Thinning is considered as an main practice of forestry management, which is generally helpful to improve the stand structure and environmental condition, therefore to increase soil N and nutrient cycling. This study, we selected Pinus massoniana plantations of different thinning intensities-control (3750tree/hm2), low thinning intensity (LTI,2400tree/hm2), middle thinning intensity (MTI,1500tree/hm2) and high thinning intensity (HTI,900tree/hm2)-to study soil N transformation and nutrient cycling using closed-top PVC tube incubation and ion-exchange resin bags method. Our results would provide some valuable information to manage forest plantation in hilly red soil region. The main results are as follows:(1) The underground vegetation investigation showed species richness was higher in MTI than other plantations while Shannon-Wiener diversity index and Pielou evenness index were higher in LTI, which indicated that thinning would improve the plant diversity of Pinus massoniana plantation.(2) Soil NH4+-N concentration and ammonification rate were higher in wild shrubs, followed by plantation without thinning (control), and lower in thinning plantation (P<0.05). Soil NO3--N concentration and nitrification rate showed HTI and MTI> LTI> control> wild shrubs. Clearly, thinning would be helpful to increase soil nitrification and mineralization, but also easily to result in groundwater pollution due to NO3--N leaching.(3) Soil relative ammonium rate showed MTI> HTI> control> LTI> wild shrubs and relative nitrification rate showed MTI> LTI> HTI> Control> wild shrubs, which further indicated that moderate thinning would be helpful to improve mineral N supply, but clear cutting could easily lead to soil N loss and result in groundwater pollution.(4) Soil urease, sucrase, acidic phosphatase, neutra phosphatase and alkaline phosphatase activities showed various responses to thinning intensity. Soil urease activity showed largest difference among four thinning intensities, and was recommended as an effective indicator to express soil quality dynamics in response to thinning.(5) Nitrogen concentration in fresh needle and litterfall showed HTI> Control> LTI. The concentrations of N and P in fresh needle showed an increasing tendency from spring to summer, thus a decreasing tendency from summer to winter, and were highest in summer during the whole growing season. The N/P ratios in fresh needle and litterfall were highest in HTI than other plantations, and the C/N ratio in litterfall was lowest in HTI than other plantations. In contrast, soil net N mineralization was highest in HTI than other plantations, which indicated that soil N mineralization would be a negative relationship with litterfall C/N.In conclusion, the thinning with60%-75%intensities would be helpful to improve soil N transformation and ecosystem nutrient cycling of19years old Pinus massoniana plantation. However, excessive thinning intensity could easily lead to N leaching and environment pollution. Therefore, we would like to recommend middle thinning intensity in forest management. |
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