Effects Of Biochar Amendment On Soil Organic Carbon Fractions And Aggregates Carbon Under Simulated Nitrogen Deposition

Forest soil carbon pool occupies an integral position in the carbon cycle and carbon balance of global terrestrial ecosystems,which contains highly heterogeneous soil organic carbon（SOC）fractions.Different SOC fractions are an important reference for studying soil carbon cycle,as well as for improving soil fertility.Meanwhile,nearly 90%of SOC in surface soils is stored in soil aggregates,aggregates are closely related to SOC and play a key role in soil carbon cycle.The continuously increasing atmospheric nitrogen（N）deposition has led to a series of ecological and environmental problems,affecting the carbon cycle of forest ecosystem.Biochar（BC）,as a soil amendment,plays a significant role in increasing soil carbon pool and alleviating the negative effects of high N deposition on forest ecosystem,as well as in improving soil structure.At present,there are few reports on the effects of BC amendment on SOC fractions of Cunninghamia lanceolata plantations under simulated N deposition.Meanwhile,the effects of the interaction of simulated N deposition and BC amendment on aggregates distribution and aggregates carbon are still unclear.Here,we took Cunninghamia lanceolata seedlings as the research object and conducted the pot experiments.Three BC application rates(0,12,36 t·ha^-1;B₀,B₁and B₂,respectively)were set under three N deposition levels(0,40,80 kg N·ha^-1·yr^-1;N₀,N₁,and N₂,respectively)for a total of nine treatments（N₀B₀,N₀B₁,N₀B₂,N₁B₀,N₁B₁,N₁B₂,N₂B₀,N₂B₁,N₂B₂）.The responses of SOC fractions and aggregates carbon with different fractions to simulated N deposition and exogenous carbon input were studied.The main results are as follows:（1）BC amendment significantly increased the contents of SOC,recalcitrant pool of carbon（RPC）,and dissolved organic carbon（DOC）by 40.1%～99.2%,104.0%～267.8%and 75.3%～194.7%under different levels of N deposition,respectively.Meanwhile,the proportion of labile C from acid hydrolysis（LPC）in SOC was decreased significantly by 28.8%～53.3%and the proportion of RPC in SOC was increased significantly by 45.7%～84.6%in the treatments of BC amendment combined with different levels of N deposition.The effects were the most significant when higher-rate of BC was applied alone.LPC was significantly reduced only under the higher BC amendment combined with simulated N deposition.The higher rate of simulated N deposition alone significantly decreased soil microbial biomass carbon（MBC）content and the proportion of MBC in SOC,but had no significant effect on the DOC content and the proportion of DOC in SOC.However,the interaction of simulated N deposition and higher rate of BC significantly increased the proportion of DOC in SOC.Soil p H and available nutrients（AP,AK）were the main influencing factors of SOC fractions.Our results indicated that the interaction between simulated N deposition and BC amendment has the greatest impact on RPC fraction,which is conducive to improving the stability of SOC fractions and increasing soil carbon pool in the short term.（2）In this study,the aggregates of>250μm size were found to be the most abundant class in the distribution of soil particle size.The content of>250μm fraction was increased and the contents of 53～250μm and<53μm fractions were reduced in the treatments of BC amendment alone and simulated N deposition alone.The effects were the most significant when lower-rate N deposition was applied alone.No significant differences were found in the distribution of aggregate fractions under the BC amendment combined with simulated N deposition,but the trends were opposite to that of BC alone.Correlation analysis suggested that plant biomass was significantly positively correlated with the content of>250μm fraction,and were significantly negatively correlated with the contents of 53～250μm and<53μm fractions.Therefore,plant biomass is an important factor affecting the distribution of soil aggregates.（3）The carbon content of aggregates was mainly distributed in the>250μm fraction,with the proportion ranging from 84.4%to 92.1%.The carbon content of>250μm fraction was significantly increased and the carbon contents of 53～250μm and<53μm fractions were significantly decreased under simulated lower-rate N deposition alone.While the increase of carbon contents of>250μm aggregates was inhibited under higher-rate nitrogen deposition.Compared with simulated N deposition alone,the carbon contents of aggregates with different particle sizes were increased more under the BC amendment combined with simulated N deposition,with>250μm aggregates having the largest carbon increment.Soil p H,AP and AK were positively correlated with carbon content of aggregates.We found that lower-rate N deposition is conducive to the accumulation of soil carbon pool,and BC amendment achieves the physical protection of soil organic matter mainly by increasing the proportion of carbon storage in the>250μm aggregate fraction.（4）No significant differences were found in theδ¹³C values of bulk soil and aggregate fractions in the treatments of simulated N deposition alone,but BC amendment significantly increased theδ¹³C values of bulk soil and aggregate fractions under different levels of N deposition,indicating that BC amendment significantly increased the contents of new carbon in the bulk soil and aggregates.Meanwhile,theδ¹³C values of aggregates showed a linear correlation of the carbon content of aggregates,both of which increased with the increase of BC amendment,which further confirmed that the input of exogenous new carbon was the main reason for the increase of carbon content of soil aggregates.Theδ¹³C values of aggregates increased with the decrease of particle size of aggregates,and the contribution rate of new carbon followed the order:（<53μm aggregates）>（53～250μm aggregates）>（>250μm aggregates）,which indicated the response of the carbon content in the<53μm fraction was the most sensitive to BC amendment.The increased new carbon was also mainly distributed in the>250μm fraction,accounting for 79.5%～83.3%.（5）BC amendment significantly stimulated decomposition of the native SOC in the53～250μm and<53μm fractions under different levels of N deposition.The contents of the native SOC in the bulk soil and>250μm aggregate fraction also significantly decreased in the treatments of BC amendment alone.The interaction of simulated N deposition and BC amendment significantly stimulated decomposition of the native SOC in the bulk soil.The interaction of higher-rate N deposition and lower-rate BC amendment significantly stimulated decomposition of the native SOC in the>250μm fraction.The results showed that BC amendment stimulated the decomposition of the native SOC of aggregates.