Effects Of Nitrogen And Litter Addition On Carbon And Nitrogen Of Soil Particulate And Mineral-associated Fractions In Subtropical Forest

Litter decomposition is an important source of soil organic matter,and continuous input of litter plays an important role in the formation,stabilization,and transformation of carbon and nitrogen in forest soils.The differences of the litter decomposition materials at different stages lead to different DOC compositions entering the soil,which further affects the composition of mineral-associated carbon and nitrogen.However,the addition of nitrogen changes the material released during the litter decomposition process,thereby regulating the soil nitrogen cycle through the promotion or inhibition effect on the different soil mineral-associated fractions.Therefore,the effects of different types of litter on the soil nitrogen content during the decomposition process coupling with soil mineral-associated carbon and nitrogen is particularly important for understanding the nitrogen cycle and sustainablely ecological effects,and relationship between litter decomposition and nitrogen retented and stabilized in soil.In this study,the soils of coniferous forests and subtropical broadleaf were selected.After field sampling,the soil and litter were leached and cultured in the experiment.The litter composition changes and its effects on the soil were studied.In addition,the external litter extraction solution,carbon,and nitrogen were added in different soil particulate and mineral-associated fractions got by the physical classification of the bulk soil.Furthermore,the changes and effects of carbon and nitrogen on soil particulate and mineral-associated carbon and nitrogen were studied.The results showed that:1)Whether for coniferous or broad-leaved forest,the carbon and nitrogen nutrients in leaching were mainly released at the early stages of litter decomposition.The presence of litter could reduce the loss of soil nitrogen by leaching,especially for NO₃^--N,which may be related to a large amount carbon input in soil.The DOC from broad-leaved forest litter was utilized more than that from coniferous forest litter after it entering the soil.The different responses of soil NH₄⁺-N in the extracts to litter addition between broad-leaved forest and coniferous forest was probably related to the amount of tannins released from the decomposing litter.Nitrogen addition decrease DOC in litter leaching and had a different impact on soil DOC between two forests.Under nitrogen addition condition,there are more NO₃^--N in leaching of coniferous forest litter at the later period(200 and240 days),while it was in<140 days and>200 days for broad-leaved forest.Nitrogen addition mainly reduced NO₃^--N extracted from soil in the late incubation period between broad-leaved forest and coniferous forest,and had non-significant effect on coniferous and broad-leaved forest.2)During the litter decomposition,soluble sugars,glucose and polyphenols were continuously released,and soluble sugars and glucose released from litter decomposition in coniferous soil were more easily leached than in broad-leaved forest.The amino acids released in litter decomposition had no significant effect on the soil in a short term.Most of the soluble carbon and nitrogen released from litter during the decomposition process were retained in the soil,which plays an important role in returning nutrient in the soil,and leaching conditions accelerated this effect.There were more soluble sugars and polyphenols in broad-leaved forest litter,and more polyphenols in soil were utilized than in coniferous forests.Nitrogen addition induced the decomposition of the sugar in the litter and stimulates the mineralization of soil organic carbon to release more soluble sugar.Nitrogen addition has a smaller effect on the polyphenols content in the broad-leaved forest soil than in coniferous forest soil.The coniferous forest soil has more sensitive response to nitrogen addition and litter than broad-leaved forest.3)Litter carbon and glucose addition can promote nitrogen mineralization and reduce the DON in both coniferous forest and broad-leaved forest,but inhibit DON mineralization of<2?m component.They can only retain NH₄⁺-N in bulk soil and250-2000?m fraction,decrease NO₃^--N in 250-2000?m in broad-leaved forest.Litter carbon inhibited DON mineralization of 20-53?m components in coniferous forests,but broad-leaved forests did the opposite.The positive effect of glucose on the DON mineralization of bulk soil and 250-2000?m components in broad-leaved forest was larger than that in coniferous forest,indicating that the influence of carbon on the mineralization of nitrogen in different soil and different soil components changes.Organic nitrogen promotes the ammonification in the bulk soil and each soil fraction in the coniferous forest,and also promotes the net nitrification in both 250-2000?m fraction and the bulk soil.There was a similar effect of inorganic nitrogen and organic nitrogen addition on soil inorganic nitrogen in broad-leaved forests.Inorganic nitrogen promotes the mineralization of nitrogen in 20-53?m and<2?m fractions,which was different between coniferous forests and broad-leaved forests related to the forms and amount of mineralizable nitrogen in different fractions.DOC was mineralized much in small soil particle sizes fractions,but DON in large particle components.The results indicated that the changes in net ammonification rate of bulk soil was mainly attributed to the increase in 20-53?m and<2?m fractions,and the decrease in 250-2000?m fractions,while the net nitrification rate was in verse.Variation of carbon and nitrogen in soil components could reflect the changes in the bulk soil.