Converting acidic forests to managed plantations reduces soil nitrogen loss by inhibiting autotrophic nitrification while inducing nitrate immobilization in the tropics

Soil gross nitrogen (N) transformation rates are highly sensitive to land use change. However, understanding the effect of land use change on internal N cycling patterns and its underlying mechanisms in tropical soils remains elusive. Here, four typical land uses including forest (> 400 years), eucalyptus (15 years), rubber (35 years), and paddy field (40 years) plantations in tropical region of China were investigated. The technique of ¹⁵N tracing was used to quantify soil gross N transformation rates. We also measured soil biochemical properties as well as carbon (C) and N fractions to evaluate the controls on any changes in soil N cycling processes. We found that converting natural tropical forests to managed ecosystems shifts the soil N dynamics from nitrate-dominated N forms towards ammonium-dominated N forms, suggesting that managed ecosystems becoming conservative (i.e., lower ratio of autotrophic nitrification (O_NH4) to ammonium immobilization (I_NH4) and nitrous oxide (N₂O) emissions and higher nitrate immobilization) than the natural tropical forest. The higher tendency of N loss (i.e., higher O_NH4/I_NH4 and N₂O emissions) of the natural tropical forest was mainly due to the higher concentrations of soil total N and hydrolysable ammonium N and microbial biomass, which stimulated O_NH4. Lower microbial biomass, hydrolysable ammonium N, particulate organic C, and gross N mineralization, however, significantly decreased O_NH4 in managed ecosystems. Our study also showed a pivotal role of soil C and N fractions in controlling soil heterotrophic nitrification, which enhanced significantly with decreasing amino sugar N, amino acid N, dissolved organic C, easily oxidizable organic C, and light fraction organic C. Our findings highlighted the pivotal role of soil C and N fractions in regulating soil N cycling under future land use changes.