| Phosphorus?P?is an essential element for plant growth,which usually includes inorganic P and organic P.Most soil organic P need mineralization into water soluble free inorganic P that can be absorbed by plant.Composition of organic P in the soil is relatively complex.The effectiveness of the various forms of P is the dynamic change of soil organic P and the conversion between each composition,which mainly controlled by soil microorganisms,soil enzyme activity,and soil physical and chemical properties such as nitrogen availability.Nitrogen deposition is an important part of global change,which can impact soil P cycle by changing the aforementioned organic P convertion regulating factors.In this study,a long term nutrient addition platform with six nitrogen addition gradients in a meadow grassland was used to study the impacts of N addition on organic P composition and mineralization rate.The organic P component and phosphorus mineralization rate were determined by the in situ culture method;other parameters including soil phosphatase activity,soil physicochemical properties etc.were also measured.We intend to solve the following scientific questions:1)whether the addition of nitrogen will cause changes in the organic P component,and if so,what are the underlying mechanisms of the effects?2)under the condition of nutrient changes,what is the relationship between the organic phosphate mineralization rate and the change of organic P components?How do soil organic P bank change?The ultimate goal was to unravel the responses of soil organic P and soil P mineralization to nitrogen addition in Songnen grassland.The main results of this study are as follows:?1?Nitrogen addition affects soil physical and chemical properties.With the increase of nitrogen addition level,soil pH value decreased significantly,and available nitrogen content and microbial nitrogen content increased significantly.We also detected a decrease in soil temperatureand microbial P content,and an increase in soil moisture with increasing N addition,but the effects were not statistically significant.Nitrogen addition significantly increased the content of inorganic P in soil and significantly reduced the percentage of soil organic P in total P;?2?The addition of nitrogen leads to the change of organophosphorus composition,and there is a certain seasonal change.The effect of nitrogen addition was most obvious in July,which increased the content of labile organic P?LOP?by22.19%,and the content of moderately labile organic P?MLOP?and nonlabile organic P?NOP?by 29.22%and 42.23%,respectively.In July,the NOP content decreased from 39.02±4.02 mg kg-11 to 22.54±2.91 mg kg-1,the MLOP content decreased from 257.35±39.86 mg kg-11 to 182.16±27.85 mg kg-1,and the LOP content increased from 26.65±2.32 mg kg-11 to 34.25±2.42 mg kg-11 with the addition of nitrogen;?3?Nitrogen addition promoted the mineralization of organic P,and the path analysis showed that the influencing ways included increasing the available N content in soil and promoting the microbial activity.The activity of alkaline phosphatase was positively correlated with the content of LOP?P=0.001?,and negatively correlated with the content of MLOP?P=0.006?and NOP?P<0.001?.Alkaline phosphatase activity was positively correlated with soil organic phosphate mineralization rate?P=0.02?.The results showed that phosphatase plays an important role in promoting the mineralization of organic P and the transformation of its components,which can promote the transformation of P resources in soil from stable phosphorus components to more available P components,and alleviate the shortage of P resources caused by the nitrogen addition;?4?Under the condition of the nitrogen addition,available P resources of grassland ecosystem are scarce,which stimulated microorganisms to produce more alkaline phosphatase,promotes the conversion of soil organophosphorus components from NOP to MLOP and then to LOP,thus accelerating the mineralization of soil organic P.This study explored the change and the influence of nitrogen addition to the soil organic phosphorus component characteristics and their relative content,and analyzed change rate of phosphate and soil organic phosphorus availability on its response.The results provided scientific basis for the prediction of the situation of grassland ecosystem productivity changes under the scenarios of global changes,especially increase in N deposition rate.The research of nitrogen addition impacts on components and mineralization of soil organic phosphorus furthered the understanding of the grass nutrition mechanisms,which are useful for the management utilization and sustainable development of grassland ecosystem. |
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