Comparison of soil N dynamics between dry dipterocarp forest and dry evergreen forest in Northeastern Thailand

We conducted a comparative study of nitrogen (N) dynamics in soil by examining a dry dipterocarp forest (DDF) and a dry evergreen forest (DEF), under similar climatic conditions, in northeastern Thailand. The soil N dynamics were described by measuring seasonal N mineralization and nitrification rates, soil microbial biomass, and the size of inorganic N pools and extractable organic N and C pools. We estimated the fluxes and production of inorganic N in soil using the ion exchange resin core and buried bag methods. Carbon and N accumulations were higher in the DEF than in DDF; DEF had likely been protected from fire for a longer time than DDF. We also found temporal and vertical differences in soil N dynamics between DDF and DEF. The greatest inorganic N flux in the soil occurred at the onset of the wet season in DDF, whereas inorganic N was produced throughout the wet season in DEF. N uptake occurred in the uppermost soil horizon in DDF, whereas N uptake in DEF occurred in the deeper soil horizon. The mean residence time of inorganic N was greater in DEF than in DDF. These results suggest that N cycles were faster in DDF than in DEF. During long-term incubation experiments, the dominant inorganic N form changed from NH4+ to NO3-, coinciding with declining extractable organic C (EOC) concentrations at both sites. Shortages of EOC occurred earlier in DDF than in DEF. This finding indicates that EOC may be a useful index of C availability for soil N dynamics.