Effects of nitrogen and phosphorus additions on nitrous oxide emission in a nitrogen-rich and two nitrogen-limited tropical forests

Abstract. Nitrogen (N) deposition is generally considered to increase soil nitrous oxide (N₂O) emission in N-rich forests. In many tropical forests, however, elevated N deposition has caused soil N enrichment and further phosphorus (P) deficiency, and the interaction of N and P to control soil N₂O emission remains poorly understood, particularly in forests with different soil N status. In this study, we examined the effects of N and P additions on soil N₂O emission in an N-rich old-growth forest and two N-limited younger forests (a mixed and a pine forest) in southern China to test the following hypotheses: (1) soil N₂O emission is the highest in old-growth forest due to the N-rich soil; (2) N addition increases N₂O emission more in the old-growth forest than in the two younger forests; (3) P addition decreases N₂O emission more in the old-growth forest than in the two younger forests; and (4) P addition alleviates the stimulation of N₂O emission by N addition. The following four treatments were established in each forest: Control, N addition (150 kg N ha⁻¹ yr⁻¹), P addition (150 kg P ha⁻¹ yr⁻¹), and NP addition (150 kg N ha⁻¹ yr⁻¹ plus 150 kg P ha⁻¹ yr⁻¹). From February 2007 to October 2009, monthly quantification of soil N₂O emission was performed using static chamber and gas chromatography techniques. Mean N₂O emission was shown to be significantly higher in the old-growth forest (13.9 ± 0.7 µg N₂O-N m⁻² h⁻¹) than in the mixed (9.9 ± 0.4 µg N₂O-N m⁻² h⁻¹) or pine (10.8 ± 0.5 µg N₂O-N m⁻² h⁻¹) forests, with no significant difference between the latter two. N addition significantly increased N₂O emission in the old-growth forest but not in the two younger forests. However, both P and NP addition had no significant effect on N₂O emission in all three forests, suggesting that P addition alleviated the stimulation of N₂O emission by N addition in the old-growth forest. Although P fertilization may alleviate the stimulated effects of atmospheric N deposition on N₂O emission in N-rich forests, this effect may only occur under high N deposition and/or long-term P addition, and we suggest future investigations to definitively assess this management strategy and the importance of P in regulating N cycles from regional to global scales.