Effects Of Fire And Nitrogen Addition On NET Nitrogen Mineralization Rate In An Ecotone Forest Of Subtropical And Temperate Regions

It has been predicted that fire frequency and atmospheric nitrogen(N)deposition will increase as a result of intensified human activities.As two of important disturbance factors in forest ecosystem,fire and N deposition could have profound impacts on forest ecosystem structure and function.Soil N plays an important role in regulating ecosystem productivity,and the alterations in soil N content or transformation processes can affect ecosystem stability.An experiment with burning and N addition was conducted in a Quercus acutissima-Pinus massoniana mixed forest in the Mountain Xian,Xinyang,Henan Province,Central China since 2014.There were four treatments,each with five replicates:control,burning,N addition,and burning plus N addition.We investigated the effects of burning and N addition on soil physical-chemical properties,microbial biomass,and soil net N mineralization rate and the relationships between them.Across the 2 years,burning significantly increased soil temperature and pH by 1.8 ? and 0.4,respectively.No effect of burning on soil moisture was found.Nitrogen addition significantly decreased soil pH(0.2)and soil moisture(-0.7 V/V%,absolute change),but had no effect on soil temperature.In addition,burning and N addition had interactive effects on soil pH.Across the 2 years,either burning or N addition had no effect on soil organic carbon,soil total N,and soil C/N.However,there were significantly interactive effects of burning and N addition on soil total N and soil C/N ratio in 2016.In 2015,burning marginally increased microbial biomass C by 12.2%and significantly enhanced microbial biomass N by 29.6%,which suggests that more soil nutrient may be stored in microbial biomass to reduce nutrient loss.Burning marginally enhanced microbial biomass C/N by 19.5%.There were no effects of N addition on microbial biomass C,microbial biomass N,or microbial biomass C/N.The positive dependence of microbial biomass N on soil temperature indicates that the increased soil temperature caused by fire can stimulate the growth of microorganisms.Burning significantly decreased soil ammonium and nitrate nitrogen in both of 2015 and 2016.In 2015,burning significantly reduced ammonium and nitrate nitrogen by 29.6%and 30.4%,respectively.In 2016,burning significantly decreased ammonium and nitrate nitrogen by 25.0%and 40.6%,respectively.In 2015,N addition did not affect ammonium nitrogen,but significantly enhanced nitrate nitrogen(45.3%).In 2016,N addition marginally increased ammonium nitrogen by 10.8%and significantly enhanced nitrate nitrogen by 38.9%.The main effects of burning on net N nitrification rate and N mineralization rate varied between 2015 and 2016.In 2015,burning significantly stimulated net N nitrification rate and N mineralization rate by 75.5%and 118.0%,respectively.However,burning did not affect net N nitrification rate and N mineralization rate in 2016.There were no effects of N addition on net N nitrification rate or N mineralization rate in both years.Net N nitrification rate and N mineralization rate showed positive correlations with microbial biomass C and net N nitrification rate was positively correlated with microbial biomass N in 2015.The results of this study indicate that the increased net nitrogen mineralization rate after fire in the short time will ameliorate the nitrogen limitation,which can facilitate the restoration and reconstruction of the burned forest in the ecotone forest of subtropical and temperate regions.