Effects Of Simulated Nitrogen Deposition And Litter On Carbon And Nitrogen,and Their Mineralization In Forest Soil

Plant litter in the forest ecosystem is primary source of soil organic matter（SOM）,which forms after litter decomposition by microorganisms to affect the material and energy cycle in soil.However,soil SOM compositions are not uniform,and different pools have different responses to changes in external conditions.The stability mechanism of SOM in different functional pools,the input of materials from the decomposition of litters,and the mineralization and transformation of soil mineral-associated components are important processes involved in the carbon and nitrogen cycle.The global anthropogenic activity induced nitrogen deposition may change the key processes of the carbon and nitrogen cycle in forest ecosystems.Therefore,it is necessary to study the maintenance and stability of soil components responding to litter and simulated nitrogen deposition,which would help us to understand the long-term mechanisms of carbon and nitrogen in the soil different components,and to provide scientific reference for maintaining the balance of the forest ecosystem under nitrogen deposition.In this study,broad-leaved forest（Castanopsis faramii forest）and coniferous forest（Cunninghamia lcunninghamia）were selected as experimental plots in 2009,and three levels of nitrogen（NH₄NO₃）were considered:control group(0 kg/（hm²?a）,low nitrogen group（75 kg/（hm²?a））,and high nitrogen group(150kg/(hm^2?a)).Litter disposal methods include the removal of litter and preservation of litter.Field sampled soil was used to measure carbon and nitrogen-related indexes,and the effects of nitrogen addition and litter on soil carbon and nitrogen status were analyzed from biological and abiotic perspectives.The soil samples of different treatments were physically wet-sieved and graded to obtain>250μm,250-53μm and<53μm.Three fractions and bulk soil were then incubated for 63 days to explore the differences of carbon and nitrogen mineralization in different soil fractions.The results found that:1)After the experiment with simulated nitrogen deposition and litter effects for 12years,nitrogen deposition reduced soil total carbon in coniferous forest,which was mainly found in the treatment with the removal of litter.LPI-C,RP-C and HCl-C decreases can provide evidence for a total C decrease.The results of the enzyme data showed that the activities ofβ-glucosidase（βG）and cellulose hydrolase（CBH）were much more affected by nitrogen deposition than those of oxidase,which indicated that the effect of nitrogen addition was mainly based on the utilization of activated carbon by microorganisms.Compared with the removal of litter,the RP-C of coniferous forest soil was significantly increased by retaining litter,indicating that the role of litter carbon in soil organic carbon accumulation was mainly related to the increase of recalcitrate carbon.Under HN treatment,the presence of litter promotes the rise ofβG and oxidase,indicating that more carbon is needed to meet requirements under higher levels of nitrogen addition,and the MBC/MBN ratio is the highest,which increases the utilization of soil recalcitrate carbon.The litter removal treatment in broad-leaved forest had higher oxidase activity,nitrogen addition had no significant effect on MBC/MBN ratio.The higher soluble carbon in the broad-leaved forest soil was beneficial for improving the carbon utilization efficiency of microorganisms.Therefore,it was shown that nitrogen addition had no significant effect on the activities ofβ-glucosidase（βG）and cellulose hydrolase（CBH）,which explained that it had no significant impact on soil carbon in broad-leaved forests.After litter removal in coniferous forest soil,nitrogen addition decreased total nitrogen content,which may be related to the decrease in LPI-N and acid-hydrolyzed ammonium nitrogen.Under the same nitrogen content,the presence of litter significantly increased the total nitrogen content,indicating that litter plays a vital role in soil nitrogen retention.Nitrogen addition increases soil nitrogen availability and may also affect different enzymes.The results showed that LN treatment reduced LAP in coniferous forests under litter removal,but HN treatment had the opposite effect on NAG under the condition of retained litter.On the other hand,nitrogen addition decreased soil MBN,possibly related to the decrease of soil carbon.The increase in carbon demand promotes the mineralization of organic nitrogen to obtain carbon,reducing soil nitrogen.Due to the relative increase of carbon input in the case of retaining litter,the decomposition of soil nitrogen by microorganisms was reduced.In the case of litter removal,compared with CK,LN treatment increased the content of inorganic nitrogen,and HN treatment increased the content of ammonium nitrogen and hydrolyzed amino acid,while did not reduce the recalcitrate nitrogen pool.The study found that nitrogen addition increased soil NAG and decreased LAP in the broad-leaved forest,which may be the main reason why nitrogen addition had no significant effect on the total nitrogen of broad-leaved forest.Compared with the removal of litter,the presence of litter reduces the activity of soil LAP,which is possible in turn to inhibit the hydrolysis of organic nitrogen,thereby increasing the total nitrogen in the soil.2)The organic carbon mineralization of aggregate is similar to bulk soil.The changing trend of the emission rate of<53μm fraction was most similar to bulk soil.Overall,nitrogen addition reduces cumulative CO₂emissions.In coniferous forest,in the case of litter removal,compared with CK,HN treatment significantly reduced the CO₂emission of the bulk soil,showing consistent performance in the components.LN treatment increased,mainly in the increase of<53μm fraction CO₂emissions,which may be related to the increase of HCl-C content in<53μm fraction had a certain relationship.In broad-leaved forest,under the removal of litter,nitrogen addition had a significant difference in CO₂emission of 250-53μm fraction,which was CK>LN>HN processing.This may be because the soil DOC content in nitrogen treatment was significantly higher than that in CK treatment at 250-53μm fraction.However,in the case of litter retention,nitrogen addition showed an opposite trend.The change of bulk soil may be related to the decrease of LPI-C and hydrolytic enzyme activity（βG,CBH）.In the case of litter removal,LN treatment significantly increased the total nitrogen mineralization rate and net nitrification rate of the coniferous forest.The contribution of<53μm fraction was the largest,and the effects on the ammonification and nitrification of this component were opposite.HN treatment reduced the rate of soil net ammonia,which was related to the difference of different fractions affected by nitrogen addition.The higher soluble organic nitrogen in the<53μm fraction may be the main reason for maintaining the higher ammonification rate,while nitrogen addition did not improve the net nitrification rate,which should be related to the low nitrifying microbial activity of this fraction.After litter retention,it was found that LN treatment significantly reduced the total nitrogen mineralization rate and net nitrification rate of the bulk soil compared with CK treatment.This finding indicated that litter inhibited soil mineralization under low nitrogen addition treatments.In the broad-leaved forest,nitrogen addition significantly increased net nitrogen mineralization rate in the bulk soil.The effect of retained litter on soil nitrogen in the broad-leaved forest was much more greatly than that in coniferous forest,and the differences in the performance of each component were more noticeable.This is mainly related to the reduction in soluble organic nitrogen and acid hydrolyzed amino acids of total soil and>250μm.In conclusion,the responses of soil carbon and nitrogen to long-term nitrogen deposition and litter input in different subtropical forest types were inconsistent,which was related to the inconsistency of mineralization rates caused by the microbial activities,and the different responses of soil fractions.The results of this study can provide certain scientific references for soil fertility maintenance and understory management of subtropical forest ecosystems.