国际森林土壤研讨会课件N in fores soil (Fuzhou).pptx

Characteristics of N transformation in humid subtropical forest soils in ChinaZucong CaiNanjing Normal Universityzccai@njnu.edu.cnFuzhou, CHINAOctober 25-26, 2015Effects of N addition on C storages in temperate and moist tropical forests (Matson et al., 1999, Biogeochemistry, 46, 67-83)Question 1: How do humid subtropical forest soils retain inorganic N and make N saturated for vegetation growth? Distribution of soil samples collected from temperate and (sub)tropical forest soilsSoil pH in temperate and subtropical forest soilsInorganic N in temperate and subtropical forest soilsInorganic N is dominated by NH4+, which is neither easily leached and runoff due to adsorption on mineral surface and nor easily volatilized due to low soil pH in humid subtropical soils. T-test: P=0.275T-test: p0.001Why is inorganic N dominated by NH4+ in humid subtropical soils rather than NO3- as in temperate soils? Nitrate ratio in total inorganic NTotal inorganic NMethods for measuring gross N transformation ratesLabeled with 15NH4NO3 and NH415NO3, respectivelyIncubated at 25?C and 60%WHC in laboratory or in fieldDetermined NH4+, NO3-, organic N, and N2O emission and their 15N abundance at designed intervals during the incubation Applied the model developed by Muller et al. (2007) to the calculation of gross N transformation ratesConcept model of N transformation in soils Muller, et al., 2007Gross rates of autotrophic, heterotrophic and total nitrification, and nitrate immobilizationIt is strong NO3- immobilization and weak NH4+ nitrification that makes NO3- content low and NH4+ domination in inorganic N in the humid subtropical soils.Gross rates of mineralization and turnover rate of organic N and NH4+ immobilizationHigh mineralization rate of organic N in humid subtropical soils can meet the vegetation demand for N. Turnover rate = total N/mineralization rateSummary of the mechanisms for N retention and supply in humid subtropical soilsLow NO3- content resulted from strong NO3- imm