Effects Of Litter And Nitrogen Addition On Carbon And Nitrogen Of Soil Fractions In Castanopsis Faberi Hance Forest

Soil carbon and nitrogen play an important role in forest ecosystem material cycle.The change of soil carbon availability affect the stability of soil organic matter and further affect the structure of soil aggregates.Soil organic nitrogen is the main nitrogen form,and its mineralization is an important pathway in nitrogen cycle.Human activities have significantly increased nitrogen deposition rates,and the excess input of nitrogen impacted the forest ecosystems to a large extent.The organic carbon and nitrogen released from decomposing plant litters play an important role in the formation,stability and transformation of soil carbon and nitrogen.However,soil organic matter is mineral-associated status combined with minerals as different proportions rather than homogeneous.Therefore,the mineralization of soil organic matter can be showed in changes of the transformation and distribution of mineral-associated organic carbon and nitrogen.In addition,most of the studies about the effects of nitrogen deposition have been studied in a whole soil.Furthermore,the response of carbon and nitrogen in different fractions to nitrogen deposition and the role of litter in these processes could provide more information about the impact of nitrogen deposition in soil.Therefore,soil from subtropical evergreen broad-leaved forest was fractioned by physical and chemical methods to study the effects of litter decomposition and,carbon and nitrogen addition on soil mineral-associated nitrogen content and transformation.The results showed that:1)Soil carbon and nitrogen contents in the HUMIN fraction was the highest,accounting for 33.5%of the total carbon and 33.3%of the total nitrogen.The extraction capacity of Na₂B₄O₇ solution for soil organic matter was higher than other extractions and followed by NaOH and Na₄P₂O₇ solution.The soil soluble total carbon,soluble total nitrogen and soluble organic nitrogen extracted by the three reagents accounted for 46.2%,47.9%and 76.5%of the total extractions,respectively.These results suggested that the mineral-associated carbon and nitrogen were maintained mainly by electrostatic and Van der Waals interactions?such as hydrogen bonding?and also by strong bonds with mineral.In addition,nitrogen addition significantly increases the carbon and nitrogen content in Na₂S₂O₄ and HUMIN fractions.Removal of litter reduced the carbon content in Na₂B₄O₇,H₂SO₄,Na₂S₂O₄ and HUMIN fractions,and nitrogen content in NaOH,HF and HUMIN fractions.The nitrogen content in the K₂SO₄ extraction was significantly increased by the interaction of litter and nitrogen addition.The current study demonstrate that the litter and nitrogen addition can affect the soil carbon and nitrogen process by altered carbon and nitrogen concentration of soil fractions with different chemical stability.2)After dry sieving,the proportion of soil aggregates with particle sizes>2000?m,250-2000?m,53-250?m and<53?m accounted for 55%,40%,5%and<1%of the original soil mass,respectively.The carbon and nitrogen contents increased as the soil particle size decreases.Compared with CK,nitrogen addition significantly increased the carbon and nitrogen contents of each fraction of dry sieved soil,with the largest increase in<53?m,while litter treatment had no significant effect.After the wet sieving,the soil C/N decreased with the decrease of soil particle size,and the lowest fraction was in<53?m.Nitrogen addition resulted in a significant increase of soil carbon content in250-2000?m and soil carbon and nitrogen content in 20-250?m,but no significant effect on soil carbon and nitrogen content in<20?m.Compared with the litter remained,litter removed resulted in a significant reduction of soil carbon content in 250-2000?m,while the significant effects in other fractions were not found.These results indicated that nitrogen addition could inhibit the decomposition of soil organic matter and promoted the stability of carbon and nitrogen in small aggregates.Removal of litter resulted in a significant reduction in soil carbon content,mainly in the fraction of 250-2000?m.3)Carbon addition significantly increased the mineral nitrogen concentration and net mineralization rate in>2000?m soil.The addition of nitrogen?alanine,ammonium nitrate?significantly increased the mineral nitrogen concentration and net mineralization rate in each aggregates,and the alanine addition increased the net ammonization rate in each aggregate more than that of ammonium nitrate.The addition of carbon increased the soluble organic nitrogen content in each aggregate by 71.4%-373.6%,increased the mineral nitrogen content in 250-2000?m and>2000?m by 20.8%-52.4%,but decreased the ammonium nitrogen content in<20?m by 15.7%-55.0%.In addition,nitrogen addition significantly increased the mineral nitrogen content in>2000?m soil.These results suggested that during one month of incubation,carbon addition promoted the decomposition of organic nitrogen in<20?m to enhance soluble organic nitrogen,but did not induce the mineralization of soluble organic nitrogen.In contrast,carbon addition significantly induced the nitrogen mineralization in>2000?m.It was found that the effect of nitrogen addition on nitrogen mineralization enhanced with the increase of soil aggregate size.