Eighteen-year nitrogen addition does not increase plant phosphorus demand in a nitrogen-saturated tropical forest

Abstract

Nitrogen (N) deposition usually increases plant tissue N concentrations and thus phosphorus (P) demand in young and/or N-limited forests, but the N deposition effect on plant P demand has rarely been assessed in N-saturated forests. Impacts of 18-year external N additions (Control: 0, Low N: 50, Moderate N:100 and High N: 150 kg N ha−1 year−1) on leaf P of four plant life-forms (tree, shrub, herb and liana), P fractions of bulk and rhizosphere soils were examined in a N-saturated mature tropical forest in southern China. Leaf N, P and N: P ratios of all plant life-forms remained stable under three N additions. Among soil P fractions, moderate labile organic P increased by 25%–33% across three N additions; and soil total P was increased by 11.76% under Low N, and 8.87% under High N, compared with the control. The PLS-PM results showed that path coefficient of microbial community to available P significantly increased and of inorganic P to available P significantly decreased under N additions than control. N additions improved soil P availability through microbe-mediated P transformation: Low N significantly increased soil microbial taxonomic diversity, and a higher microbial diversity could enlarge the sources of nutrient acquisition and stimulate decomposition of recalcitrant organic matters; while High N significantly decreased soil microbial taxonomic diversity, the remaining microorganisms that were screened by N-rich environments had the characteristics of resisting the N addition effects and maintained efficient P acquisition. Synthesis. Our findings provide a novel line of evidence that long-term N deposition did not increase plant P demand in a N-saturated mature tropical forest. The underlying mechanism is that plants did not increase N uptakes therefore nor increase P uptakes (a stable leaf N: P stoichiometry) in an already N-saturated ecosystem. Different N addition rates regulated soil P transformation via microbial community transition. These findings help improve the understanding of plant P acquisition and modelling of biogeochemical N–P cycling and vegetation productivity in N-rich forest ecosystems, particularly considering the fact that chronic N deposition may likely lead to soil N richness and even saturation of many forests in the future.