Alleviation of Nutrient Co-limitation Increases Grassland Biomass Production, But Not Carbon Storage

Intensification of agriculture and industry over the past two centuries has increased the amount of biologically limiting nutrients entering ecosystems. In areas such as grasslands, where growth is primarily constrained by one or more nutrients, increasing anthropogenic nutrient input has the potential to restructure plant communities and alter ecosystem carbon pools and cycling. Using a 13-year nutrient-manipulation experiment, we show that plant biomass increased with addition of nitrogen alone but even more strongly when nitrogen was combined with phosphorus or potassium and micronutrients, providing evidence for serial co-limitation of biomass. In contrast, plant functional group composition responded only when all nutrients were added together. The effects of nutrients on soil organic matter were mediated through changes in soil pH and base cation concentrations, and there was no direct effect of nitrogen alone, or in combination with other nutrients, on carbon fluxes or soil organic matter. Our work demonstrates how co-limitation may shift through time and may depend the effect of fertilization on soil pH and base cation concentration. Our experiment reveals that nitrogen added alone for more than 13 years does not affect plant community composition at our study site, overturning a long-standing interpretation of past work.