Effects Of Nitrogen Addition On Soil Phosphorus Leaching In A Subalpine Forest

Phosphorus（P）is a key nutrient element limiting the productivity of terrestrial ecosystems and plays an important role in maintaining ecosystem stability and carbon sink capacity.Leaching is an important way of soil P loss.Once soil P is lost,it will be difficult to restore,which will inevitably lead to the loss of ecosystem services and functions.The increase of atmospheric nitrogen（N）deposition will profoundly affect the biogeochemical cycle of P in terrestrial ecosystems.By changing the soil nutrient stoichiometry and causing soil acidification,it will alter the biological utilization strategy of P,process of P adsorption/desorption and precipitation/dissolution,and thus affecting soil P leaching.Therefore,exploring the responses and mechanisms of soil P leaching under the increased atmospheric N deposition is of great significance for maintaining the health and stability of terrestrial ecosystems.Based on the simulated atmospheric N deposition experimental platform（established in 2014）in the National Field Scientific Observatory of Gongga Mountain Forest Ecosystem,this study conducted different levels of N addition(0,8 and 40 kg N ha^-1 yr^-1,respectively,as control,low N and high N)experiments at an altitude of 3000m in the Abies fabri forest.Leachates of two soil layers（organic layer and organic layer+mineral layer）were continuously collected in situ（from 2020 to 2022）.The concentrations of total phosphorus（TP）and its fractions（dissolved inorganic phosphorus（DIP）and dissolved organic phosphorus（DOP））and their colloidal forms（fine colloids（particle size<220 nm）and medium colloids（particle size of 220-450nm））in the leachates were analyzed to explore the effects of N addition on soil P leaching.Meanwhile,the parallel N addition plots(0,8,16 and 40 kg N ha^-1 yr^-1,respectively,as control,low N,medium N and high N)were set up in the study area.Soil samples collected from the parallel N addition plots were used for the P adsorption/desorption experiments and the analysis of physicochemical properties.Based on this,we explored the variations of soil P adsorption/desorption capacity and its influencing factors,and elucidated the effects and mechanisms of N addition on soil P leaching in the subalpine forest.The main results are as follows:（1）Short-term N addition increased soil P mobility.Under the different levels of N addition,the contents of transferable P in the organic and mineral layers varied between-11%to 36%and-12%to 43%,respectively.The mobility of P was related to the levels of the N addition,and the contents of mobile P in the organic layer was increased significantly under the medium N treatment.Nitrogen addition induced soil acidification.Soil pH in organic and mineral layers decreased under low and medium N treatments,but there was no significant difference when compared with the control treatment.The contents of amorphous aluminum and iron in organic layer did not change significantly regardless of N addition level,and the contents of amorphous iron in mineral layer increased significantly under medium N treatment.（2）The N addition reduced soil P leaching,which was attributed to the soil P fractions and the adsorption/desorption of P.Under the N addition,the mobility of P in organic layer increased significantly,and the adsorption capacity of P in two soil layers increased.This resulted in no significant change in the concentrations of P in the organic layer leachates,and the concentrations of P in the organic+mineral layer leachates decreased significantly.Regardless of soil layers,the concentrations of fine and medium colloidal P in soil leachates did not change significantly under the N addition,and fine colloidal P was the main fraction of P lost.The duration of the N addition may be an important factor affecting the concentrations of fractions of P in soil leachates.Among three years,the concentrations of TP and DIP in soil leaching solution in 2020showed inconsistent varied patterns under the N addition when compared with the other two years.Biological uptake and utilization significantly reduced the concentrations of P in the soil leachates.During the growing season（from June to August）,the concentrations of TP and DIP in the soil leachates were significantly lower than those in the non-growing season under the N addition.In addition,the leaching amount of P under the low N treatment was significantly higher than the control and high N treatment.This may be due to the different canopy interception and topographic factors,rather than the effects of N addition.（3）The N addition increased the P adsorption capacity of the soil.Under the low,medium and high N treatments,the maximum adsorption capacity of P in organic layer increased by 13%,28%and 9%,respectively.The maximum adsorption capacity of P in mineral layer increased by 13%,33%and 36%,respectively.Under the N addition,the enrichment of amorphous aluminum in soils increased the adsorption capacity of P.Due to the different competitive ability of adsorption sites,the adsorption of DIP caused the desorption of DOP in the soils,and the desorption amount was positively correlated with the adsorption amount.The adsorption/desorption of P by the soil was the key mechanism affecting the concentrations of fractions of P in the soil leachates（especially in the mineral layer）.The increase in the soil adsorption capacity of P under the N addition resulted in the decrease of DIP leaching,and the adsorption of DIP caused the desorption of DOP in the soils,which led to the increase in the DOP leaching.In summary,the N addition reduced soil P leaching in the subalpine forest of Gongga Mountain,and the soil P fractions and adsorption/desorption of P are the main factors affecting P leaching.The results of this study are helpful to improve the understanding of the responses and mechanisms of soil P leaching to the N addition,and also provide a theoretical basis for the prediction model of soil multi-nutrient balance supply under the increase of atmospheric N deposition in the future.