Land use in subalpine grasslands affects nitrogen cycling via changes in plant community and soil microbial uptake dynamics
Résumé
1. Nitrogen (N) cycling is a key process determining ecosystem functioning in subalpine grasslands where traditional mowing and manuring are being abandoned. However, the roles of the plant and microbial communities in mediating changes in N availability are still poorly understood. 2. We inoculated 15 subalpine grassland fields with dual-labelled ammonium nitrate (15 NH 4 + , 15 NO 3)) during July 2005 and used pool dilutions over 1 month to calculate inorganic N fluxes into the microbial pool and uptake in plant communities by grasses, forbs and legumes. The effects of current land abandonment were assessed by comparing manured and mown terraces (ancient crop-lands) with other terraces where these practices have ceased, and mown versus unmown unterraced meadows. 3. Rapid cycling of inorganic N and high soil N availability in forb-dominated manured and mown terraces resulted from fast plant N uptake and low microbial C:N ratio. In grass-dominated unmown terraces, N cycling was slower and N retention was greater; microbial N uptake remained similar to that in the other terraces, although a higher C:N ratio suggested a shift towards fungal dominance. 4. In unterraced meadows, pH was low due to reduced mixing of soil with the underlying calcare-ous rock. Soil [NH 4 + ] was high and [NO 3) ] low, but current management had no effect on N pool size, although plant N uptake was greater in the mown than unmown fields. This may be partially explained by high N retention by dominant Festuca paniculata tussocks. The microbial N pool and N uptake were both low and the microbial C:N ratio was high, suggesting that fungi slowed N cycling and reduced the influence of mowing on N turnover. 5. Synthesis. In these marginal long-term grasslands, with low productivity and high biodiversity value, changes in ecosystem function associated with reduced management intensity were mediated through slower N cycling. This response was expressed as more gradual nutrient uptake but greater retention by unmown plant communities, slower microbial uptake and smaller soil N pools. In contrast to more productive ecosystems, such as northwestern European grasslands, reduced management is detrimental to both biodiversity and the maintenance of soil-related ecosystem services. These costs will need to be balanced against potential benefits, such as carbon storage.