Effects of Nitrogen and Phosphorus Additions on Soil N 2 O Emissions and CH 4 Uptake in a Phosphorus-Limited Subtropical Chinese Fir Plantation.

Increased nitrogen (N) inputs in subtropical forest ecosystems were widely reported. Extra N additions were reported to cause nutrient imbalance and phosphorus (P) limitation in many tropical and subtropical forests, and further result in changes in soil nitrous oxide (N₂O) and methane (CH₄) fluxes. Here, we conducted experiments with N (high N addition: 15 g N/m², HN), P (low: 5 g P/m², LP; high: 15 g P/m², HP) and their interactive (HNLP and HNHP) treatments to investigate how N and P additions affected CH₄ and N₂O exchanges in an N-rich Chinese fir plantation (Cunninghamia lanceolata), and further explored the underlying mechanisms through the structural equation model (SEM) analysis. The results indicated that N addition alone (HN) significantly (p < 0.05) increased the soil N₂O emissions by 30.15% and 80.47% over annual and 4-month periods, mainly owing to the elevated NH₄⁺-N content. P addition alone (LP and HP) did not significantly affect the soil N₂O emissions as compared with the control. The SEM analysis indicated that increased N₂O emissions under N addition were primarily explained by the increase in available N and contributed more to the stimulated NH₄⁺-N contents. N and P interactive additions slightly (not significant) stimulated the N₂O emissions as compared with that under the N addition alone treatment. High-dose P addition significantly increased the soil CH₄ uptake by 15.80% and 16.23% under the HP and HNHP treatments, respectively, while N addition alone and low P addition (LP and HNLP) did not significantly affect CH₄ uptake as compared with the control. The increased water-soluble organic carbon and microbial biomass carbon explained the increased CH₄ uptake under high P addition. The fertilization effects on N₂O emissions and CH₄ uptake mainly occurred in the first 4 months and diminished after that. Our results suggested that the direction, magnitude and timing of the N and P addition effects on N₂O emissions and CH₄ uptake would depend on the soil nutrient status and plant–microbial competition for N and P in subtropical forests. [ABSTRACT FROM AUTHOR]