Investigating the response of soil nitrogen cycling to grass invasion

In heathlands, high mineral N input causes replacement of Calluna vulgaris, the dominant plant, by fast-growing grasses such as Molinia caerulea. The vegetation shift signifies altered litter quality from low- to high-quality litter due to differences in lignin content. Litter quality usually affects decomposition processes, which can, in turn, alter nutrient cycling. Therefore, the change in plant dominance in this ecosystem possibly alters soil carbon and nutrient cycles, and consequently, ecosystem services (e.g. biodiversity conservation, groundwater recharge, …). We hypothesise that, because of its higher litter quality, nutrient turnover becomes faster with grass encroachment. We tested this hypothesis in a field set-up consisting of 14 plots presenting a gradient of increasing grass dominance (from 0% to 100%). We measured nine soil parameters and assessed possible associations between grass dominance and the soil parameters using multivariate analysis and linear mixed models. We found that grass dominance significantly impacted net N mineralisation and the root biomass. Our results showed very low net N mineralisation rates (0.09 ± 0.04 mg N (kg soil)⁻¹ day⁻¹) and relative nitrification rates (1.99 ± 0.62%). At high grass levels, acid phosphatase activity was significantly lower than at lower grass percentages. These results show that grass encroachment has a minimal impact on heathland soil biochemistry at this point. Still, we consider that it may take many years to translate a change in litter quality and dynamics into a change in soil functioning.