Long-term nitrogen and phosphorus enrichment alters vegetation species composition and reduces carbon storage in upland soil

Reactive nitrogen (N) deposition can affect ecosystem processes, particularly in oligotrophic upland habitats. Phosphorus (P) addition has been proposed to reduce the effects of N enrichment on N leaching and acidification, since P limitation can reduce biomass production and consequent sequestration of reactive N. However, biodiversity is often reduced in more productive ecosystems and P limitation may protect against this effect. Responses to P availability in instances of high N deposition are poorly understood. This study investigated the ecosystem response to alleviation of P limitation, using a long-term nutrient addition experiment (1996–2012) three years after ceasing N inputs and 15 years after a single P application. Substantial differences were observed in the structure and composition of vegetation species and above-ground vegetation biomass. Vegetation height was greater in the N + P addition treatments (+ 38% cf. control), with increased cryptogam cover (+ 47%), whereas N addition increased graminoid species cover (+ 68%). Vegetation diversity was significantly reduced by the addition of P (− 21%), indicating that P limitation is likely to be an important mechanism that limits biodiversity loss in upland habitats exposed to chronic N deposition. Significant differences in soil C and N contents were also observed between treatments. Relative to control, the addition of N increased soil C (+ 11%) and N (+ 11%) pool sizes, whereas the addition of N and P reduced soil C (− 12%) and N (− 13%) pool sizes. This demonstrated the importance of P availability for upland ecosystem processes, and highlights the long-term effects of P addition on vegetation species composition and C storage. Thus, the addition of P cannot be endorsed as a method for reducing impacts of N deposition.