Effects Of Nitrogen Addition And Mowing On Phosphorus Components In Soil Aggregates Of Songnen Grassland And Their Mechanisms

Soil is the main source of phosphorus for plants,and its phosphorus pool includes two major parts:inorganic phosphorus and organic phosphorus.Inorganic phosphorus,as an available form of plant absorption and utilization,has higher mobility,but less abundant content in soil than organic phosphorus.The availability of phosphorus in soil depends on the transformation of inorganic phosphorus components and the mineralization of organic phosphorus,which are mainly driven by factors such as soil physicochemical properties,soil microorganisms,and soil enzyme activity.At the same time,the availability,concentration,and form of phosphorus in soil are also regulated by environmental changes and the size of soil aggregates.Therefore,understanding the response patterns and potential mechanisms of organic phosphorus,inorganic phosphorus components,and phosphorus availability to environmental resource changes（especially nitrogen deposition and mowing）at soil aggregate scale is crucial for formulating scientific grassland management policies and sustainable development.This study was based on a long-term field nitrogen addition and mowing experimental platform in the Songnen Meadow Grassland,to measure large macroaggregates（>2 mm）,small macroaggregates（0.25～2 mm）,microaggregates（<0.25 mm）soil phosphorus components,soil functional microbial communities,gene copy numbers,soil phosphatase activity,and soil physicochemical properties.The aim of the study was to investigate how the different soil aggregate scales,mowing,nitrogen addition,and their interactions influenced the distribution of soil phosphorus components?What were the scale supply sources of available phosphorus required for plant growth,as well as the transformation and mineralization of various phosphorus components?What were the contribution and regulatory mechanisms of soil microbial community structure and phosphatase activity to the mineralization and transformation of soil phosphorus components?Finally,we revealed the response mechanism of available phosphorus supply to plant under nitrogen deposition and mowing in Songnen grassland soil.The main results obtained in this study were as follows:（1）The variation range of soil resin phosphorus（Resin-P）content was 20.00±1.49～51.67±1.52 mg kg^-1,with the highest content at the level of microaggregates.In mown treatment,nitrogen addition reduced the content of Resin-P,and N20(20 g N m^-2 yr^-1)reached20.42±1.28 mg kg^-1,a decrease of 46.43%at the microaggregate level;In unmown treatment,nitrogen addition increased the content of Resin-P,reaching 51.67±1.52 mg kg^-1 under N20treatment,an increase of 21.02%at the microaggregate level.（2）The content of sodium bicarbonate and organic phosphorus（Na HCO₃-Po）in soil decreased with the increase of nitrogen application rate in mown treatment and an increasing trend in unmown treatment.Also,the inorganic phosphorus content of soil sodium bicarbonate（Na HCO₃-Pi）decreased with the increase of nitrogen application rate.However,nitrogen addition increased the content of sodium hydroxide and organic phosphorus（Na OH-Po）in the soil.At the level of small aggregates,the content of sodium hydroxide inorganic phosphorus（Na OH-Pi）in the soil increased along nitrogen addiiotn rate under unnown,while decreased as nitrogen addition under mown.The variation range of soil hydrochloric acid phosphorus components was 80.56±5.18～140.94±4.33 mg kg^-1,with the highest content and proportion in the soil.（3）The variation range of soil alkaline phosphatase activity（ALP）was 265.00±23.27～726.07±15.92 nmol h^-1 g^-1.At both large and small aggregate scales,nitrogen addition increased ALP;At the microaggregate scale,nitrogen addition decreased ALP under mown,while nitrogen addition increased ALP under unmown.The variation range of copy number of phoD functional genes in the soil was 5.03±0.04～5.70±0.05（taking log10）.In unmown treatments,nitrogen addition increased the copy number of soil phoD functional genes;However,in mown treatment,soil phoD functional genes decreased with the increase of nitrogen addition rate.The change in soil organic carbon（TOC）content was consistent with the change in copy number of phoD functional genes.（4）We found that nitrogen addition and mowing exacerbated the phosphorus limitation in grassland ecosystems.In unmown treatments,soil available phosphorus was supplemented by the micraggregate scale to a certain extent with the increase of nitrogen addition.At the scale of small aggregates,we found that Resin-P was closely related to various phosphorus components.By redundancy analysis,we also found that soil ammonium nitrogen content,TOC,ALP,and phoD gene copy numbers in mown treatment were key factors to affect phosphorus turnover.In conclusion,nitrogen addition leads to phosphorus limitation in Songnen grassland,while mowing treatment exacerbates phosphorus limitation.the conversion of inorganic phosphorus components in Songnen Grassland was the primary source of soil available phosphorus conversion;when the transformation of inorganic phosphorus components could not meet the phosphorus demand of plants,soil microorganisms secreted more alkaline phosphatase by increasing the copy number of the phoD gene to promote the mineralization of organic phosphorus components to supply the phosphorus demand of plants.At different aggregate levels,the microaggregate scale was the first absorption scale of available phosphorus for plants.In addition,the small aggregate scale was the first supply scale of available phosphorus for plants,but the large aggregate scale had effects on the turnover of phosphorus components though the effect was not significant.This study deepens the understanding of the effects of nitrogen addition,mowing,and their interactions on the characteristics,changes,and relative content of soil phosphorus components at different aggregate scales.Our results revealed the response of organic phosphorus mineralization,inorganic phosphorus transformation and soil phosphorus availability to nitrogen addition and mowing,which provided scientific basis for predicting the changes of grassland ecosystem productivity under the scenario of global change and artificial grassland management.