Belowground Biomass Response to Nutrient Enrichment Depends on Light Limitation Across Globally Distributed Grasslands
2019
Anthropogenic activities are increasing
nutrientinputs to ecosystems worldwide, with consequences for global carbon and
nutrient cycles. Recent meta-analyses show that aboveground primary production is often co-limited by multiple
nutrients; however, little is known about how root production responds to changes in
nutrientavailability. At twenty-nine grassland sites on four continents, we quantified shallow root biomass responses to nitrogen (N), phosphorus (P) and potassium plus micronutrient enrichment and compared below- and aboveground responses. We hypothesized that optimal allocation theory would predict context dependence in root biomass responses to
nutrientenrichment, given variation among sites in the resources limiting to plant growth (specifically light versus
nutrients). Consistent with the predictions of optimal allocation theory, the proportion of total biomass belowground declined with N or P addition, due to increased biomass aboveground (for N and P) and decreased biomass belowground (N, particularly in sites with low canopy light penetration). Absolute root biomass increased with N addition where light was abundant at the soil surface, but declined in sites where the grassland
canopy intercepteda large proportion of incoming light. These results demonstrate that belowground responses to changes in resource supply can differ strongly from aboveground responses, which could significantly modify predictions of future rates of
nutrient cyclingand carbon sequestration. Our results also highlight how optimal allocation theory developed for individual plants may help predict belowground biomass responses to
nutrientenrichment at the ecosystem scale across wide climatic and
environmental gradients.
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