Divergent resource limitations of belowground versus aboveground productivity across grasslands

Water and nutrients are fundamental resources that constrain grassland productivity. Despite significant advances in our understanding of aboveground resource limitations, it remains unresolved whether these insights can be applied to belowground productivity. Here, by combining meta-analyses of 387 nitrogen (N) and 299 phosphorus (P) addition experiments with our long-term experiment in grasslands, we find that, globally, belowground productivity experiences peak N limitation under intermediate precipitation, despite N limitation on aboveground productivity intensifying with increasing precipitation. Moreover, this global pattern of belowground N limitation is mirrored in temporal dynamics. These consistent patterns suggest that belowground N limitation relies strongly and nonlinearly on spatial and temporal water variability. As grassland roots primarily function to capture water, this peak N limitation may reflect a maximal co-limitation of N and water for grassland plants under intermediate water conditions, which is also evidenced by our long-term experiment. Overall, this study highlights the divergent resource limitations of belowground versus aboveground productivity. Future changes in precipitation could significantly modify the patterns of belowground nutrient limitations, ultimately affecting grassland productivity and multifunctionality.