Increased nitrogen (N) deposition has been found controversial affecting soil CO₂ emission in terrestrial ecosystems, which leads to serious debate on the efficiency of estimated C sequestration induced by N enrichment. The forms of input N might be responsible for this controversy. This study aims to explore the effects of NH₄ ⁺ (reduced N) and NO₃ ⁻ (oxidized N) on soil CO₂ flux and the underlying microbial mechanisms. An N addition experiment, two N fertilizers (NH₄Cl and NaNO₃) and two rates (40 and 120 kg N ha⁻¹ year⁻¹), was carried out in a slash pine plantation of southern China. Soil-atmospheric CO₂ exchange, soil microbial biomass, and community composition were measured using static chamber-gas chromatography and phospholipid fatty acid (PLFA) analyses in the active growing and nonactive growing seasons, respectively. Low level of NaNO₃ addition significantly increased soil CO₂ flux in the active growing season, whereas other N treatments did not change soil CO₂ flux. High level of NH₄Cl addition significantly reduced soil fungal biomass (fungal PLFA) and changed microbial community composition (ratio of fungal to bacterial (F/B) PLFAs). The positive relationships between the change in soil CO₂ flux and the change in fungal biomass, as well as between the change in soil CO₂ flux and the change in community composition, were observed in the nonactive growing season. The N forms as NO₃ ⁻ or NH₄ ⁺ are important factors affecting C cycles in the subtropical coniferous plantation. These results suggested that the variations of soil CO₂ emission and microbial biomass and community composition in the subtropical plantation depended on the seasons and the levels and forms of N addition.

To date, few studies are conducted to quantify the effects of reduced ammonium (NH₄⁺) and oxidized nitrate (NO₃⁻) on soil CH₄ uptake and N₂O emission in the subtropical forests. In this study, NH₄Cl and NaNO₃ fertilizers were applied at three rates: 0, 40 and 120 kg N ha⁻¹ yr⁻¹. Soil CH₄ and N₂O fluxes were determined twice a week using the static chamber technique and gas chromatography. Soil temperature and moisture were simultaneously measured. Soil dissolved N concentration in 0–20 cm depth was measured weekly to examine the regulation to soil CH₄ and N₂O fluxes. Our results showed that one year of N addition did not affect soil temperature, soil moisture, soil total dissolved N (TDN) and NH₄⁺-N concentrations, but high levels of applied NH₄Cl and NaNO₃ fertilizers significantly increased soil NO₃⁻-N concentration by 124% and 157%, respectively. Nitrogen addition tended to inhibit soil CH₄ uptake, but significantly promoted soil N₂O emission by 403% to 762%. Furthermore, NH₄⁺-N fertilizer application had a stronger inhibition to soil CH₄ uptake and a stronger promotion to soil N₂O emission than NO₃⁻-N application. Also, both soil CH₄ and N₂O fluxes were driven by soil temperature and moisture, but soil inorganic N availability was a key integrator of soil CH₄ uptake and N₂O emission. These results suggest that the subtropical plantation soil sensitively responses to atmospheric N deposition, and inorganic N rather than organic N is the regulator to soil CH₄ uptake and N₂O emission.