Influence Of Nitrogen And Phosphorus Addition On Soil Organic Carbon Decomposition And Available Nutrients In Subtropical Forests

Forests are an important part of terrestrial ecosystems and their soils store about 73%of the world’s organic carbon.Soil organic carbon decomposition is an important process that regulates soil organic carbon stocks,and small changes in its rate can have profound effects on atmospheric CO₂ concentrations and the global carbon cycle.It is not clear how nitrogen and phosphorus additions affect soil organic carbon decomposition in forest soils in the northern subtropics zone,especially in 20-40 cm soil layer.Therefore,in this study,four typical stand types:Chinese fir forest（Cunninghamia lanceolata）,Pinus taiwanensis forest（Pinus taiwanensis）,pecan forest（Carya cathayensi）and chestnut forest（Castanea seguinii）in the Dabie mountains of the north subtropical transition zone were used to collect soils from 0-10 cm,10-20 cm and 20-40 cm and to determine the changes of soil organic carbon decomposition rate and fast-acting nutrients by using indoor simulated incubation and nitrogen and phosphorus nutrient addition.The effects of different levels of N and P additions on soil organic carbon decomposition and available nutrients in different soil layers of different stand types were investigated,and the mechanisms of N and P additions on soil organic carbon decomposition were elucidated in the context of soil physicochemical properties,enzyme activities,and microbial communities.The main findings obtained from this study are as follows.（1）There were significant differences（P<0.05）between stand types and soil layers on soil cumulative released carbon through 30 days of indoor simulated incubation.In general,soil cumulative released carbon in coniferous and broadleaved forests largely decreased as the soil layer deepened.In 0-10 cm soil layer,soil cumulative released carbon from broadleaved forests was significantly greater than that from coniferous forests,while in 20-40 cm soil layers,soil cumulative released carbon from coniferous forests was significantly greater than that from broadleaved forests.（2）Influence of nitrogen addition on soil cumulative released carbon varied between stand types and soil layers.Nitrogen addition only had a significant effect（P<0.05）on the soil cumulative released carbon from 0-10 cm and 10-20 cm soil layers of Chinese fir forest,but not on soils of Pinus taiwanensis forest,pecan forest,and chestnut forest.In the 0-10 cm and 20-40 cm soil layers of the Chinese fir forest,soil cumulative released carbon was significantly higher（P<0.05）under the N25(25 mg N·kg^-1 dry soil)and N50(50 mg N·kg^-1 dry soil)treatments than the control,while soil cumulative released carbon under the N100(100 mg N·kg^-1 dry soil)and N150(150 mg N·kg^-1 dry soil)treatments were significantly higher（P<0.05）.The cumulative released carbon was not significantly different from that of the CK control.（3）Nitrogen addition significantly increased soil mineral nitrogen content（P<0.05）,but soil mineral nitrogen content did not increase linearly with increasing nitrogen addition,but there was a threshold effect.As the amount of nitrogen added increased,the soil mineral nitrogen content tended to increase and then decrease.The addition of low to medium amounts of nitrogen（N25,N50,N100）increased soil mineral nitrogen content,but the addition of high amounts of nitrogen（N150）decreased soil mineral nitrogen content.The intensity of the effect of N addition on mineral nitrogen was related to stand type and soil depth.（4）Phosphorus addition and simultaneous addition of nitrogen and phosphorus had no significant effect（P>0.05）on soil cumulative released carbon in any of the four stand types.However,compared to the control group,phosphorus addition tended to increase soil cumulative released carbon in the 20-40 cm soil layer of Chinese fir forest and pecan forest,and to decrease soil cumulative released carbon in the 0-10 cm soil layer of chestnut forest and pecan forest.The simultaneous addition of nitrogen and phosphorus tended to increase the cumulative released carbon from 20-40 cm soil layer in chestnut forest and pecan forest,but decreased the cumulative released carbon from 0-10 cm soil layer in chestnut forest,probably because the short-term nitrogen and phosphorus nutrient addition did not have a significant effect.The simultaneous addition of nitrogen and phosphorus did not change the differences in soil carbon release between stand types and soil layers.In contrast,there was a significant interaction between nitrogen and phosphorus additions on mineral nitrogen content（P<0.05）,and simultaneous nitrogen and phosphorus additions also resulted in significantly higher levels of mineral nitrogen in soils in the 0-10 cm and 10-20 cm soil layers of Chinese fir forest,Pinus taiwanensis forest and chestnut forest than with the same level of nitrogen addition.Effects of nitrogen fertilization was facilitated by the application of phosphorus.These results indicate that the addition of low to medium amounts of nitrogen（N25,N50,N100）increased the mineral nitrogen content of the soil,but that the addition of high amounts of nitrogen（N150）decreased the mineral nitrogen content of the soil and that the application of phosphorus fertilizer promoted the effect of nitrogen fertilization.It was also found that the cumulative carbon release from coniferous and broadleaved forest soils largely decreased gradually with deepening of the soil layers.The results of this paper provide further guidance for the management of forest fertilization and management,and provide a useful theoretical basis for predicting forest soil carbon pools and improving forest soil carbon sink capacity in subtropical regions of China.