Effects Of Biochar Addition On Soil Labile C And N Contents In A Subtropical Evergreen Forest

Biochar is a C-rich product pyrolysing biomass at high temperatures and oxygen-limited conditions,with high porosity and in many case,rich nutrients.As a soil amendment,biochar is widely known to improve soil fertility,promote plant growth and reduce greenhouse gas emissions in agricultural systems.Biochar has also been used in foresty to promote timber production and carbon sequestration in forest soils,but the effects of biochar on soil C pool and nutrient cycle,including those in subtropical forest ecosystems,are still not fully understood.Soil labile organic C and N are the most active fractions of soil organic matter,which are important short-term nutrient reservoirs for plant.In this study,we implemented a field experiment with biochar addition of 0,10,and 30 t ha^-1 and 5 replications in a subtropical forest in Zhejiang,since April,2013.Four years after the addition of biochar,soil samples were collected from the field seasonally?2016/9-2017/9?and the soil physicochemical properties?moisture content,pH?,total carbon?TC?,total nitrogen?TN?,total phosphorus?TP?,labile organic C and N contents as well as inorganic nitrogen?NO₃^--N,NH₄⁺-N?were examined.Labile organic C and N were extracted by two methods including extractable organic C?EOC?and N?EON?in the hot water,and microbial biomass?MBC,MBN?.We also measured the greenhouse gases emissions rate?CO₂,CH₄,N₂O?by laboratory incubation.Moreover,we analyzed the relative growth rate of dominant species in 4 years?2013-2016?.The main results are as follows:1)Biochar addition significantly increased soil moisture content,TC,ratio of C to N,ratio of C to P,and NO₃^--N,while there was no significant effect on other soil properties including soil pH,TN,TP,NH₄⁺-N.2)There were no significant effects of biochar addition on either EOC,EON or MBC,MBN under three biochar addition.Specifically,Compared to the control,EOC increased by 6.7%and 7.6%,EON decreased by 11.6%and 10.4%,MBC increased by59.4%and 44.7%,respectively,in 10 and 30t ha^-11 biochar treatments;MBN decreased by 2.2%in 10t ha^-11 but increased by 7.4%in 30t ha^-1.3)Biochar addition had no significant effect on the relative growth rate of basal diameter,height and volume for the dominant species in plots?Pseudolarix amabilis and Phoebe chekiangensis?.The relative growth rate of volume in Pseudolarix amabilis increased by 2.9%and 1.9%,while Phoebe chekiangensis decreased by 4.7%and 0.7%in 10,30t ha^-1 respectively,compared to the control.4)Biochar addition decreased the average of soil greenhouse gases emission?CO₂,CH₄,N₂O?and equivalent CO₂?CO₂-e??CO₂+CH₄+N₂O?although the effect is not significant statistically.Specifically,biochar addition at a rate of 10,30t ha^-11 reduced CO₂ by 33.9%and 25.9%,CH₄ by 23.2%and 11.6%,N₂O by 29.1%and 26.8%, CO₂-e by 26.5%and 16.0%,respectively,compared to the control.In summary,this study concluded that biochar addition increased soil labile organic C and N contents and reduced greenhouse gases emission,but the difference was no significant among treatments.The large uncertainty may be due to a high precipitation climate?1374.7mm annual average?of the subtropical forest.Such high precipitation during the period of experiment,increased soil moisture and could lead to soil partial anaerobic environment,which may partially offset the promoting effect of biochar addition on soil labile C and N.Our study indicated that biochar addition has a positive effect on the ecological restoration of subtropical forests and mitigation of climate change.