Phosphorus addition impacts on soil nitrogen dynamics in a subtropical plantation

Human activities have globally increased atmospheric nitrogen (N) deposition, which has enhanced the risk of ecosystem N losses. Phosphorus (P), as a macroelement required for life, is closely linked to the biogeochemical cycle of N. Therefore, quantifying how soil N cycle responds to different P supply levels is important. Here we examined the responses of soil N dynamics to altered P supply using a P addition experiment (+0, +25, +50, +100 kg P ha⁻¹ yr⁻¹) in an evergreen broadleaf mixed plantation in subtropical China. We found that P addition led to a more open soil N cycle in the forest ecosystem. The primary source of N₂O emissions in the study plots was fungal denitrification, which accounted for 41%-52% of the total N₂O emissions, based on δ¹⁸O-N₂O, δ¹⁵N^α-N₂O, δ¹⁵N^bulk-N₂O and SP measurements. Nitrogen loss by gas or water and N assimilation by plants were found to be coupled processes at +25 kg P ha⁻¹ yr⁻¹ addition level. The δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ values in runoff and leaching water from different depths were all depleted from −10‰ to +0‰ in the wet season. This result indicates that soil N has a short residence time and rapid NO₃⁻-N loss in the forest ecosystem, and with fewer nitrogen conversions according to the isotope fractionation theory. These observed varied responses of soil N transformation, gaseous loss, and liquid loss to different P supply levels provide new insights into our understanding of N-P relationships in broadleaf forest plantations.