Nitrogen and phosphorus enrichment effects on CO2 and methane fluxes from an upland ecosystem.

Reactive nitrogen (N) deposition can affect many ecosystem processes, particularly in oligotrophic habitats, and is expected to affect soil C storage potential through increases in microbial decomposition rate as a consequence of greater N availability. Increased N availability may also result in changes in the principal limitations on ecosystem productivity. Phosphorus (P) limitation may constrain productivity in instances of high N deposition, yet ecosystem responses to P availability are poorly understood. This study investigated CO2 and CH4 flux responses to N and P enrichment using both short- (1year) and long-term (16year) nutrient addition experiments. We hypothesised that the addition of either N or P will increase CO2 and CH4 fluxes, since both plant production and microbial activity are likely to increase with alleviation from nutrient limitation. This study demonstrated the modification of C fluxes from N and P enrichment, with differing results subject to the duration of nutrient addition. On average, relative to control, the addition of N alone inhibited CO2 flux in the short-term (-9%) but considerably increased CO2 emissions in the long-term (+35%), reduced CH4 uptake in the short term (-90%) and reduced CH4 emission in the long term (-94%). Phosphorus addition increased CO2 and CH4 emission in the short term (+20% and +184% respectively), with diminishing effect into the long term, suggesting microbial communities at these sites are P limited. Whilst a full C exchange budget was not examined in the experiment, the potential for soil C storage loss with long-term nutrient enrichment is demonstrated and indicates that P addition, where P is a limiting factor, may have an adverse influence on upland soil C content.