The Effects Of Throughfall Reduction And Phosphorus Addition On Soil Nitrogen Cycling In Tropical Lowland Monsoon Rain Forests

The soil nitrogen cycle process is an important part of the global biogeochemical cycle.Under the background of global climate change,the rainfall pattern has changed drastically.The climate model predicts that extreme weather phenomena such as drought will continue to occur frequently in the future,which can directly or indirectly affect the soil nitrogen cycle of forest ecosystems.The soil nitrogen cycle of tropical forests plays an important role in the global biogeochemical cycle.It has the characteristics of abundance of nitrogen reservoirs and rapid nitrogen cycle turnover.Drought can weaken the nitrogen reservoir of tropical forest ecosystems and affect the productivity of the entire ecosystem.At the same time,tropical forest ecosystems are often limited by phosphorus,and whether drought caused by throughfall reduction and phosphorus addition have an interactive effect on the soil nitrogen cycle remains to be discussed.This study takes the tropical lowland monsoon rain forest in the Ganza Ridge Nature Reserve in Sanya,Hainan as the research object.Relying on the two-factor interactive experimental platform for the throughfall reduction in forest（50%）and phosphorus addition established in 2019,Using a two-factor block design,a total of 3 blocks are set up,and 440m*40m plots are established in each block,including four treatments:control（CK）,throughfall reduction in forest（about 50%）（D）,phosphorus addition（TSP 50kg/ha·yr）（P）and the interaction between throughfall reduction and phosphorus addition in the forest（D*P）.The field resin core in-situ culture method is used to study the effect of throughfall reduction and phosphorus addition on the process of soil nitrogen mineralization and leaching,and the static chamber-gas chromatography method is used to observe the effect of N₂O flux.The aim is to deeply understand the response mechanism of the key processes related to the soil nitrogen cycle in the tropical lowland monsoon rain forest,in order to provide a basis for the tropical lowland monsoon forest to respond to future drought stress and other climate changes.The main results are as follows:（1）During the entire research process,throughfall reduction treatment has no significant effect on the soil temperature at depths of 5cm,15cm and 40cm,but significantly reduced the soil moisture content of each soil layer.The depth of 5cm was reduced from 0.119 to 0.083m³/m³,The depth of 15cm was reduced from 0.090 to 0.055 m³/m³,and the depth of 40cm was reduced from 0.097 to 0.058 m³/m³.Phosphorus addition treatment and interactive treatment of throughfall reduction and phosphorus addition had no significant effect on soil temperature and moisture content at each depth.The soil p H of the throughfall reduction treatment was reduced from 4.83 to 4.58,and the soil microbial biomass nitrogen content was lower in the dry season than in the wet season.（2）During the entire cultivation process,the drought treatment did not have a significant effect on the content of 0～10cm soil inorganic nitrogen（including ammonium nitrogen and nitrate nitrogen）in the dry season or the wet season,but the phosphorus addition treatment significantly reduced the soil nitrate nitrogen in the dry season.The content indicated that the effect of phosphorus addition treatment on nitrogen availability was mainly reflected in the dry season,not the wet season.At the same time,throughfall reduction and phosphorus addition had no interactive effect on the content of soil inorganic nitrogen（including ammonium nitrogen and nitrate nitrogen）.Throughfall reduction treatment significantly reduced the soil net ammonification rate and net nitrogen mineralization rate in both dry and wet seasons,while phosphorus addition treatment had a significant effect on the soil net ammonification rate,net nitrification rate,and net nitrogen mineralization rate in both dry and wet seasons,and there was no interaction effect between throughfall reduction and phosphorus addition on the above indicators.The results showed that the drought caused by the reduction of throughfall can significantly reduce the rate of soil net nitrogen mineralization.The change of soil moisture content was the main factor affecting the soil nitrogen mineralization of the study plot.At the same time,throughfall reduction and phosphorus addition had no interactive effect on the net nitrogen mineralization rate of the soil.（3）Throughfall reduction treatment significantly reduced the leaching concentration of soil inorganic nitrogen（including ammonium nitrogen and nitrate nitrogen）in both dry and wet seasons.Phosphorus addition treatment had an effect on the leaching concentration of soil inorganic nitrogen（including ammonium nitrogen and nitrate nitrogen）in drought.There was no significant impact in the wet season,and the effects of drought and phosphorus addition on the leaching concentration of soil inorganic nitrogen（including ammonium nitrogen and nitrate nitrogen）had no interactive effects,it indicated that the nitrogen leaching process was mainly affected by changes in soil moisture.（4）Throughfall reduction treatment significantly reduced soil N₂O flux in the dry and wet`\the wet season.It showed that after the rainfall decreases,the soil moisture content decreased and the O₂availability increased,which ultimately reduced the soil N₂O emission flux.Phosphorus addition treatment had no significant effect on the soil N₂O flux in the dry and wet seasons.There was no interaction between throughfall reduction and phosphorus addition on soil N₂O flux in the wet season,but there was an interaction effect in the dry season,and the addition of phosphorus under the throughfall reduction treatment promoted the soil N₂O emission flux.