Abstract. Elevated nitrogen (N) deposition affects soil N
transformations in the N-rich soil of tropical forests. However, the change
in soil functional microorganisms responsible for soil N cycling remains
largely unknown. Here, we investigated the variation in soil inorganic N
content, net N mineralization (Rm), net nitrification (Rn),
inorganic N leaching (Rl), N2O efflux and N-related functional
gene abundance in a tropical forest soil over a 2-year period with four
levels of N addition. The responses of soil net N transformations (in situ Rm
and Rn) and Rl to N additions were negligible during the first
year of N inputs. The Rm, Rn, and Rl increased with the
medium nitrogen (MN) and high nitrogen (HN) treatments relative to the
control treatments in the second year of N additions. Furthermore, the
Rm, Rn, and Rl were higher in the wet season than in the dry
season. The Rm and Rn were mainly associated with the N
addition-induced lower C:N ratio in the dry season but with higher microbial
biomass in the wet season. Throughout the study period, high N additions
increased the annual N2O emissions by 78 %. Overall, N additions
significantly facilitated Rm, Rn, Rl and N2O emission.
In addition, the MN and HN treatments increased the ammonia-oxidizing
archaea (AOA) abundance by 17.3 % and 7.5 %, respectively. Meanwhile,
the HN addition significantly increased the abundance of nirK denitrifiers but
significantly decreased the abundance of ammonia-oxidizing bacteria (AOB)
and nosZ-containing N2O reducers. To some extent, the variation in
functional gene abundance was related to the corresponding N-transformation
processes. Partial least squares path modelling (PLS-PM) indicated that
inorganic N contents had significantly negative direct effects on the
abundances of N-related functional genes in the wet season, implying that
chronic N deposition would have a negative effect on the N-cycling-related
microbes and the function of N transformation. Our results provide evidence
that elevated N deposition may impose consistent stimulatory effects on soil
N-transformation rates but differentiated impacts on related microbial
functional genes. Long-term experimentation or observations are needed to
decipher the interrelations between the rate of soil N-transformation
processes and the abundance or expression of related functional genes.
The simulated N deposition accelerates net N mineralization and nitrification in a tropical forest soil
