Effects of different forms of nitrogen addition on N₂O emissions from the soil of Moso bamboo (Phyllostachys edulis) forest

Objectives Nitrous oxide (N₂O) emission is one of the main ways of nitrogen (N) loss in subtropical region, and the effects of different forms of nitrogen compounds on soil N₂O emission were investigated in this study.

Methods A laboratory incubation experiment was conducted using soil from Moso bamboo (Phyllostachys edulis) forest. The N fertilizer treatments were addition of KNO₃, NH₄NO₃, and NH₄Cl with equal mole of N input, KCl was added for comparison of K⁺ and Cl⁻, and no N addition was set as the control (CK). The soil N₂O emission rate, NH₄⁺-N, NO₃⁻-N, soluble organic carbon (DOC) and water-soluble N (WSN) concentrations were measured after 0.5 h, 1 day (d), 3 d, 5 d, 7 d, 14 d, 28 d and 60 d of incubation. On the 14th day of incubation, the gene abundance of ammonia oxidizing bacteria (AOB), ammonia oxidizing archaea (AOA), nirS, nirK, nosZⅠ and nosZⅡ were detected by fluorescence quantitative PCR.

Results The N fertilizers and KCl addition significantly increased soil DOC concentration, while NH₄NO₃ and NH₄Cl addition significantly increased soil WSN concentration and decreased soil pH after 60 days of incubation. Both N and KCl additions increased soil AOA, AOB, and nirK gene abundance, but decreased nosZⅠ and nosZⅡgene abundance. The N₂O emission in all the treatments reached peaks at the 14th day of incubation. Compared with CK, KNO₃, NH₄NO₃, NH₄Cl, and KCl increased cumulative N₂O emissions by 524.3%, 771.1%, 652.7%, and 98.6%, respectively. The N₂O emission rates were significantly and positively correlated with NO₃^–, WSN, and nirK gene abundance and negatively correlated with pH and nosZⅠ and nosZⅡ gene abundance.

Conclusions The addition of NH₄⁺-N caused higher soil N₂O emission than NO₃^–-N in Moso bamboo forest. The simultaneous input of NH₄⁺-N and NO₃^–-N, like NH₄NO₃, promoted higher emission than their individual inputs, but less than that of the sum of KNO₃ and NH₄Cl.