Litter Input Manipulations Impact Soil Organic Carbon And Nitrogen In Different Fractions In Platycladus Orientalis(Linn.) Franco In Danjiangkou Reservoir Area

Global change is altering the net productivity of forest ecosystems and affecting the carbon exchange between vegetation and soil,yet it remains largely unknown that how soil carbon pool in subtropical forest response to the change of litter input.In this study,we conducted the Detritus Input and Removal Treatments experiment?CK,normal input;NL,no aboveground litter;NR,no root;NRNL,no root and no aboveground litter;DL,double aboveground litter?to explore the impact of long-term regulation of quality and quantity of litter input on soil organic carbon?SOC?dynamics in the coniferous plantation?Platycladus orientalis?Linn.?Franco?in the Danjiangkou reservoir area.We examined both the carbon?C?and nitrogen?N?pools with C and N stable isotopes in the aggregates fractions and density fractions under litter input manipulations for three years.The main conclusions are as follows:?1?Impacts of litter input manipulations on distribution and SOC content of aggregatesLitter input manipulations significantly affected the SOC content of aggregates in the first and third year.The portion of macroaggregate?>250?m?was larger among different litter input treatments,accounting for 52.7%-94.6%of soil weight.The portion of macroaggregate?>2000?m?in the second year in CK was lowest among three years,and hence the mean weight diameter?MWD?of aggregates in CK showed similar trend.The SOC content of microaggregate?<53?m?in NR and NRNL treatments were significantly lower than that in the CK after one year's treatment,as well as the SOC content in microaggregate?<53?m?and macroaggregate?2000-250?m?in NRNL treatment were significantly lower than that in the CK in the third year,indicating that the lack of new C input from root reduced the SOC content of aggregates in different aggregate sizes.SOC content of microaggregate?<53?m?in the first year,macroaggregate?2000-250?m?in the second year and microaggregate?<250?m?in the third year in the DL treatment were lower than that in the CK,supporting that priming effect existed and lasted for a long time.?2?Impacts of litter input manipulations on C and N dynamics of different density fractionsWeight of light and heavy fractions in different aggregates didn't significantly change among litter input treatments,however the SOC content of light and heavy fraction,as well as?¹⁵N value of heavy fractions were susceptible to litter input manipulations and aggregates sizes.New carbon input in NR treatment and DL treatment did not change or increase the SOC content of aggregates in light fraction,yet reduced the SOC content of heavy fraction.The SOC content,total N content and C:N ratios of light fraction exceeded these of heavy fraction because light fraction was composed mostly by plant residue.However the total N content and C:N ratios of light and heavy fractions were not influenced by litter input treatments,indicating that the contributions of litter and root input to soil carbon pool were restricted.The?¹³C value and?¹⁵N value of light fraction were not affected by aggregates sizes and litter input treatments.The?¹³C value of heavy fraction was higher in NR treatment.Nevertheless,the?¹⁵N value of<53?m microaggregate was positive in heavy fraction,whereas the?¹⁵N value of other three aggregates was negative,possibly due to much more microbial decomposition process leading to the accumulation of heavy isotope and thus leading to the?¹⁵N enrichment.Simultaneously the?¹⁵N value of>2000?m macroaggregate was higher than2000-250?m aggragate and 250-53?m aggregate in heavy fraction,but the total N content of>2000?m macroaggregate was lower than those two in light fraction.This may be that microbe preferentially used accessible nitrogen-containing compounds in macroaggregates because of microaggregates being sealed and protected in macroaggregates.Totally,new carbon input can lead to decomposition of more old carbon,which prove the existence of“priming effect”.However protection of microaggregates from macroaggregates in space can decrease the depletion of nitrogen in microaggregates.