Long-term belowground effects of grassland management: the key role of liming.

The functioning of human-managed grassland ecosystems strongly depends on how common management practices will affect grassland "belowground compartment" including soil biogeochemistry and plant roots. Key questions remain about how animal grazing, liming (e.g., the addition of CaCO3 to soils), and nutrient fertilization might affect, in the long-term, soil nutrient cycling and multiple root traits. Here we focus on a mesotrophic grassland located in Berkshire, UK, where contrasting levels of rabbit grazing, liming, and different inorganic fertilizers have been applied since 1991. We ask how (1) soil nitrogen (N) availability and cycling, (2) total root mass, (3) root mass decomposition, and (4) arbuscular mycorrhizal fungal (AMF) root colonization might respond to 22 years of very different management. We found that liming strongly affected total root mass, root decomposition, root AMF colonization as well as soil N availability and cycling and that these effects were mainly driven by liming-induced increases in soil pH. Increases in soil pH were associated with significant (1) decreases in root mass, (2) increases in root mass decomposability and in the mineralization of N in decomposing root detritus, and (3) increases in AMF infection. Soil pH was also significantly related to greater N availability (i.e., soil NO3 levels) and to lower δ15 N natural abundance, which suggests more efficient N uptake by plants in limed soils as we found in our study. The application of multiple nutrients (N, P, K, Mg) also reduced total root mass, while N-only fertilization was associated with greater AMF infection. Surprisingly the long-term impact of grazing was generally weak and not significant on most plant and soil parameters. Despite soil pH affecting most belowground variables, changes in soil pH were not associated with any change in soil C and N stocks. Because liming can improve nutrient cycling (and benefits soil pH and grass yields) without negatively affecting soil C sequestration, we suggest that regular liming applications may provide management solutions for increasing the long-term sustainability of permanent grassland.