| The priming effect of soil organic carbon decomposition plays an important role in terrestrial carbon cycle.However,the effect of nitrogen deposition on priming effect and its microbial mechanism are still unclear in subtropical forests with nitrogen deposition.Based on the simulation nitrogen deposition experiment platform of subtropical evergreen broad-leaved forest with continuous nitrogen application for 9 years,soil samples with different nitrogen application gradients（Control-CT,Low nitrogen-LN and High nitrogen-HN）were collected to investigate the effects of long-term nitrogen deposition on soil physical and chemical properties and soil microbial communities.A 29-day indoor incubation experiment was carried out by adding 13C-labeled glucose,cellulose（simulated root exudates and litter）and inorganic phosphorus（P）.The CO2efflux was monitored and determined priming effect during the incubation.At the end of incubation,we conducted destructive sampling and measurements of microbial activity.The aim was to clarify whether the effect of nitrogen deposition on priming effect was regulated by fresh organic carbon quality and phosphorus limitation（i.e.,the interaction of nitrogen deposition,fresh organic carbon quality and phosphorus availability）,and to reveal the potential microbial mechanism.The results show that:Long-term nitrogen deposition significantly changed soil physical and chemical properties and microbial communities structure of subtropical evergreen broad-leaved forest.Nitrogen deposition significantly reduced soil pH and total phosphorus（TP）content,which means increasing soil acidification and phosphorus limitation.Nitrogen deposition reduced soil C:N but increased N:P,indicating that nitrogen deposition affected soil nutrient balance.Nitrogen deposition reduced microbial biomass carbon（MBC）,nitrogen（MBN）,phosphorus（MBP）and total phospholipid fatty acids（PLFAs）to varying degrees.Redundant analysis showed that pH and TP were the main environmental factors affecting microbial community structure.In addition,nitrogen application increased soilβ-glucosidase（BG）and acid phosphatase（ACP）activities,indicating that nitrogen deposition increased microbial demand for carbon and phosphorus,which increased microbial carbon and phosphorus limitation.The results of incubation showed that the effect of nitrogen deposition on priming effect was regulated by the phosphorus availability,that is,there was a significant interaction between nitrogen deposition and phosphorus addition.In the treatment without phosphorus addition,nitrogen deposition significantly reduced the priming effect compared with CT.However,there was no significant difference between the priming effect of nitrogen addition and CT in treatment phosphorus addition,indicating that P addition only improved the priming effect of nitrogen deposition.In addition,the interaction between phosphorus addition and fresh organic carbon also had a significant effect on the priming effect:in the treatment without phosphorus addition,the glucose-induced priming effect was significantly higher than cellulose,and the glucose-induced priming effect was positive,while the cellulose-induced priming effect was negative,indicating that the soil priming effect was affected by the quality of fresh organic carbon.Phosphorus addition significantly improved the priming effect induced by glucose and cellulose,especially made the priming direction induced by cellulose change from negative to positive,which also increased the priming effect induced by glucose by14.18%.The changes of soil available nutrients,microbial biomass and soil enzyme activities were analyzed after incubation to reveal the mechanism of priming effect.The results showed that mineral nitrogen andβ-1,4-N-acetyl-glucosaminidase（NAG）did not change with the nitrogen deposition gradient in the without phosphorus addition treatment,indicating the reduction of priming effect by nitrogen deposition treatment did not conform to the nitrogen mining hypothesis.Phosphorus addition significantly increased MBP and MBC of nitrogen addition treatment,but had no significant effect on CT treatment,indicating that phosphorus addition alleviated phosphorus limitation in nitrogen treatment,which means priming effect under long-term nitrogen deposition was consistent with stoichiometric decomposition hypothesis.In addition,we also found that MBP and MBC in glucose treatment were significantly higher than cellulose,indicating that glucose has higher microbial availability.The addition of phosphorus increased MBP in glucose and cellulose treatment,but only decreased ACP in cellulose treatment,indicating that the treatment of addition of cellulose had stronger phosphorus limitation than glucose.Therefore,phosphorus addition changed the direction of cellulose-induced priming effect.Finally,considering the effect of carbon,nitrogen and phosphorus addition on the priming effect,soil available phosphorus content is the main factor affecting the priming effect,and has a significant positive correlation with the priming effect.In summary,our results showed that the soil priming effect（especially cellulose-induced priming effect）of subtropical evergreen broad-leaved forest under long-term nitrogen deposition was limited by phosphorus,which was consistent with the stoichiometric decomposition hypothesis.The results are of great significance for predicting soil carbon sequestration in subtropical forests of China under the background of nitrogen deposition. |
