Influence Of Warming And Nitrogen Addition On Soil Organic Carbon Decomposition In Four Types Forests

Forest soil organic carbon accounts for about 46%of the global total soil organic carbon.Even a small change in forest soil organic carbon will lead to a huge change in the global carbon cycle and atmospheric CO₂ concentration.Global warming and atmospheric nitrogen deposition affect soil organic carbon decomposition by changing environmental temperature and soil nitrogen availability.At present,the studies on the effects of warming and nitrogen addition on soil organic carbon decomposition mainly focus on the surface soil in different temperature zones,but there are few studies on the deep forest soil in the transition zone from subtropical to temperate zone.In this study,soils with 0-10 cm,10-20 cm and 20-40cm depth in four typical forest forests（i.e.,Cunninghamia lanceolata,Carya cathayensis,Pinus taiwanensis and Castanea seguinii）in Mazongling Forest Farm of Tianma National Nature Reserve,Anhui Province,which is a transition zone from subtropical to temperate zone,were collected to explore the effects of warming and nitrogen addition on soil organic carbon decomposition.All soil samples with N addition(i.e.,0,50 and 100 mg N kg^-1 soil)were incubated for 30 days at the 20℃and 25℃,respectively,and soil organic carbon decomposition was measured.In addition,soil available nutrients,enzyme activities and microbial community structure were measured at the same time to explore how warming and nitrogen addition influence soil organic carbon decomposition.The main results of this study are as follows:（1）The warming significantly promoted the decomposition of forest soil organic carbon.The cumulative release of soil organic carbon decomposition in different soil layers generally showed 0-10 cm>10-20 cm>20-40 cm soil layers.That means the deeper the soil layer depth,the less organic carbon decomposition,which was related to the low content of active organic carbon in deep soil layers.Increasing temperature in the 0-10 cm soil layer increased soil organic carbon decomposition in the 0-10 cm,10-20 cm and 20-40 cm soil layers of the four forests by 23.31%-42.72%,5.06%-31.72%and 1.98%-23.24%,respectively.This indicates that the effect of temperature increase on soil organic carbon decomposition diminished with increasing soil depth.The response of soil organic carbon decomposition to warming did not differ significantly among forest types.（2）The effect of nitrogen addition on soil organic carbon decomposition was related to nitrogen addition,forest type and soil depth.Low nitrogen supplemental level（50 mg N kg-1 soil,N50）significantly promoted soil organic carbon decomposition（P<0.05）,but high nitrogen supplemental level(100 mg N kg^-1 soil,N100)had no effect on soil organic carbon decomposition.In 0-10 cm soil layer,N50 treatment increased soil organic carbon decomposition by 10.11%and 11.54%in C.lanceolata,and P.taiwanensis,respectively,but reduced soil organic carbon decomposition by 21.76%and 10.50%in C.cathayensis and C.seguinii.In 10-20 cm soil layer,N50 treatment only increased the organic carbon decomposition of four kinds of forest soil by 1.63%to 5.60%,but in 20-40 cm soil layer,N50 treatment increased the organic carbon decomposition of four kinds of forest soil by12.21%to 41.23%.（3）Nitrogen addition regulated the effect of nitrogen addition on the temperature sensitivity of soil organic carbon decomposition(Q₁₀),and was related to forest type and soil depth.N50 treatment significantly reduced Q₁₀,while N100 treatment had no effect on soil organic carbon decomposition Q₁₀.In 0-10 cm soil layer,N50 treatment reduced the soil organic carbon decomposition Q₁₀ of C.lanceolata and P.taiwanensis by 47.49%and11.54%,while increased C.cathayensis and C.seguinii.by 16.69%and 40.65%.In the 10-20 cm and 20-40 cm soil layers,N50 treatment reduced the Q₁₀ of the four forests soil organic carbon decomposition,and the reduction range was 11.61-37.58%in the 10-20 cm soil layer and 14.95%-58.95%in the 20-40 cm soil layer.（4）Increasing temperature promoted the decomposition of soil available nutrients,and significantly reduced inorganic nitrogen content by 9.53%-75.89%.However,it increased the total microbial population by 6.22%-45.18%and changed the composition of microbial community.At the same time,it enhanced the activities ofβ-glucogase,chitinase,acid phosphatase and leucine aminopeptidase,which were significantly related to the decomposition of organic carbon.The N50 low nitrogen addition significantly increased soil inorganic nitrogen content by 29.60%-307.25%,enhanced nitrogen availability,and changed soil C/N ratio.At the same time,the total amount of soil microorganisms increased by9.84%-94.53%,the ratio of bacteria to fungi and the ratio of Gram-positive bacteria to Gram-negative bacteria in soil were changed,and the activities of chitinase and acid phosphatase related to the decomposition of organic carbon also increased significantly.In combination with correlation analysis and random forest analysis,it was concluded that chitinase secreted by bacteria was the most important factor affecting soil organic carbon decomposition under warming and nitrogen addition treatment and was positively correlated with soil organic carbon decomposition.The above results showed that increasing temperature promoted the decomposition of soil organic carbon,and the effect of nitrogen addition on the decomposition of organic carbon was related to the level of nitrogen addition.Low nitrogen addition promoted the decomposition of soil organic carbon but reduced the sensitivity of soil organic carbon decomposition temperature.Warming and nitrogen addition regulated soil organic carbon decomposition by influencing soil inorganic nitrogen content and enhancing soil microbial activity and enzyme activity.The four kinds of forests had the same response to warming in different soil layers,and the response intensity decreased with increasing soil depth.The effects of nitrogen addition on the four kinds of forests showed similar rules in the deep soil.This study deepens the understanding of the response of forest soil organic carbon decomposition to warming and nitrogen addition,and provides theoretical support for the management of forest soil carbon pool in the transition zone between subtropical and warm temperate zones in response to global changes.