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- (1 year) and
long-term (16 year) 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