Effects Of Litterfall On The Dynamics Of Soil Nitrogen And Phosphorus In Subalpine Forests

Nitrogen and phosphorus are the most important limited nutrients for plant growth in terrestrial ecosystem.Plant litter can transport the two nutrients to soil through its decomposition,as a key pathway of nutrients input,thus affecting soil nitrogen and phosphorus dynamics and their avaliblity in forest ecosystem.In addition,litter is important source of soil nitrogen and phosphorus,which can directly increase the soil nitrogen and phosphorus content and contribute to the accumulation and turnover of soil nutrients.On the other hand,the input of easily degradable components and available nutrients in fresh litter can stimulate the growth and breeding of soil microorganisms or improve the enzyme activity,thus promote the existing soil nitrogen and phosphorus mineralization.More importantly,in the natural forest ecosystems,litter production often with differences in the quality and quantity due to the rhythm thus has varying effects on the soil nitrogen and phosphorus dynamics,and this effect may vary in different forests and seasons.Meanwhile,the seasonal freez-thaw cycles,as the main factor that affecting the decomposition and soil ecological processes in the alpine or subalpine regions,would makes the relationship between litter and soil nitrogen and phosphorus nutrients complicated.Therefore,in this study,based on our previous findings and invistigated,we conducted a field experiment using the cultivation method in the typical coniferous forest,broad-leaved forest and mixed forest in western Sichuan.Combined with the temperature and humidity changes at different critical periods of seasonal freezing and thawing,the effects of litter rythmic input on soil nitrogen and phosphorus content,nitrogen and phosphorus mineralization rate and the stoichiometric ratio of soil nitrogen and phosphorus were evaluated from the years of 2017 to 2019.The main results are:(1)Litter input significantly increased the contents of soil dissolved organic nitrogen,microbial biomass nitrogen,ammonium nitrogen,nitrate nitrogen and the total inorganic nitrogen,but had no significant effect on the contents of soil total nitrogen and total organic nitrogen.Meanwhile,the significant increase of soil labile nitrogen content mainly occurred in winter and the early growing season,and the soil dissolved organic nitrogen showed significant difference only when the accumulated litter input occurred.Moreover,during the two-year rhythmic input of litter,soil nitrogen components did not increase with the increasing litter input,but showed obvious seasonal dynamics.In addition,although the relationship between different soil nitrogen components and environmental factors is difference,the numbers of soil freeze-thaw cycles,soil mean temperature,litter carbon to nitrogen ratio,litter nitrogen content,soil water content and litter production were the significant factors that affecting the dynamics of soil nitrogen content in the subalpine forests,and the explanation rate of which ordered as decline.(2)The effect of litter input on soil nitrogen mineralization varies with forest types.In general,litter input significantly increased the rates of soil net ammonification,net nitrification and net nitrogen mineralization in the three forests,and mainly occurred in winter and the early growing season,but in the first winter season,soil net nitrogen mineralization rates were significantly reduced under the litter removal treatment in the broad-leaved forest.Meanwhile,during the two-year rhythmic input of litter,soil net ammonification rate increased as the incubation day preceded.However,the rates of soil net nitrification and nitrogen mineralization vary greatly in different incubation days.Moreover,litter production,litter nitrogen content,numbers of freeze-thaw cycles,soil water content and soil temperature can direct or indirectly affect soil net nitrogen mineralization rate,which indicated that the quantity and quality of litter input and soil environmental factors integrate regulated the soil nitrogen cycle in the subalpine forests.(3)Litter input had no significant effect on soil total phosphorus,total organic phosphorus and availability phosphorus content.However,during the growth season,the accumulated litter input significantly decreased the content of soil total inorganic phosphorus in the coniferous forest and the content of soil dissolved organic phosphorus in mixed forest,while the effects on the two kinds of forest type in winter significantly increased the soil microbial biomass phosphorus and content of soil dissolved organic phosphorus,respectively.Meanwhile,during the two-year litter rhythmic input,soil different phosphorus components did not increase with the incubation day increasing,but vary greatly as incubation day proceeded.In addition,although the relationship between different soil nitrogen components and environmental factors is difference,the litter carbon to phosphorus ratio,numbers of soil freeze-thaw cycles,litter phosphorus content,litter production and litter phosphorus element return were significant factors affecting the soil phosphorus content dynamics in subalpine forests,and the explanation rate of which ordered as decline.However,(4)Accumulated litter input significantly increased the soil gross phosphorus mineralization rate in broad-leaved forest,but had no significant effects in the coniferous and mixed forests.However,characteristic of the litter input during critical periods had no significant effect on the soil gross phosphorus mineralization rate in the three forests.Meanwhile,during the two-year litter rhythmic input,soil gross phosphorus mineralization rate decreased with the increasing incubation days,while the total organic phosphorus content decreased and increased in the early and later incubation stage,respectively.Moreover,litter production,litter phosphorus content,soil mean temperature,numbers of soil freeze-thaw cycles and soil water content can direct or indirectly change soil gross phosphorus mineralization rate,which showed the quantity and quality of litter input and soil environmental factors can integrate regulate the soil phosphorus cycle in the subalpine forests.(5)Litter input had no significant effect on the ratio of soil total nitrogen to total phosphorus and total organic nitrogen to total organic phosphorus,but the accumulated litter input significantly increased the ratio of soil dissolved organic nitrogen to dissolved organic phosphorus in mixed forest,and the ratio of soil microbial biomass nitrogen to microbial biomass phosphorus in coniferous and mixed forests.Moreover,the seasonal litter input had no significant effect on the stoichiometric ratio of nitrogen to phosphorus.In addition,although the relationship between the stoichiometric ratios of different soil nitrogen and phosphorus components and environmental factors is difference,soil mean temperature,litter nitrogen to phosphorus ratio,litter carbon to nitrogen ratio,litter carbon to phosphorus ratio,litter nitrogen content,numbers of soil freeze-thaw cycles,litter phosphorus content,litter production,litter nitrogen element return,litter phosphrous element return and soil water content were significant factors affecting the soil nitrogen and phosphorus patterns in subalpine forests,and explanation rate of which ordered as decline.In a conclusion,although the content and proportion of soil total nitrogen and phosphorus showed little response to the two-year litter rhythmic input,it can promote soil nitrogen mineralization and improve soil nitrogen availability by increasing soil dissolved organic nitrogen and microbial biomass nitrogen content during the winter and the early growing season.Although litter input also promoted the soil phosphorus mineralization,but only the soil microbial phosphorus in the coniferous forest and soil dissolved organic phosphorus in mixed forest has increased;the soil phosphorus availability among the three kind of forest types did not change significantly,this may be related to the rapid absorption of mineral phosphorus by soil and utilization by plants.Meanwhile,long-term litter input is more favorable to increase the relative of soil available nitrogen,which in turn alleviating nitrogen limiting on plant growth,especially in the coniferous and mixed forest.Moreover,the relationship between the litter input and dynamics of soil nitrogen and phosphorus was significantly regulated by soil temperature and freezing-thawing cycle,which indicated that changes in the pattern of the environmental factors driven by climate warming will profoundly affect the soil nitrogen and phosphorus cycle in the subalpine forests.These results revealed the effects of litter rhythmical input on changes of soil nitrogen and phosphorus,and provide a scientific basis for further understanding of nitrogen and phosphorus nutrient cycle in subalpine forest ecosystem.