Exogenous nutrient addition is known to increase aboveground productivity in grasslands by alleviating soil nutrient limitation. However, less is known about nutrient effects on belowground plant dynamics. Understanding nutrient-induced changes in plant allocation is important for soil carbon (C) cycling. Roots tend to contain more recalcitrant C and decompose at slower rates than leaves, and roots are thought to be strong drivers of stable soil organic matter by promoting physio-chemical protection and interacting with metal ions. In response to the addition of limiting nutrients, we generally expect a reduction in the fraction of total biomass as roots, with lower total root stock and belowground net primary productivity (BNPP) and higher root turnover. To test these expectations, we measured above and belowground plant dynamics in nine grassland Nutrient Network sites – a set of standardized experiments that each include nitrogen (N; 10 g m2 yr-1) and phosphorous (P; 10 g m2 yr-1) addition treatments in a factorial design. At each site we determined the aboveground net primary productivity (ANPP), root mass fraction (RMF; fraction of total biomass as roots), BNPP, root stocks, and root turnover.
Results/Conclusions
Across all sites, N and P addition each lowered RMF and increased root turnover. Nitrogen addition decreased total root stock and there was marginal evidence that P addition increased BNPP. Experimental N addition interacted with ambient N deposition such that turnover was highest at the elevated experimental N level and high N deposition. Aboveground, both N and P addition increased biomass. No N*P interactions were observed in any of the response variables. In response to nutrient addition, plants increase allocation to aboveground NPP whereas BNPP remains constant. However, the roots that plants do produce turnover more rapidly with added nutrients, such that standing stocks are reduced. Increases in root turnover are thought to coincide with decreases in root structural C, meaning roots in nutrient amended plots may be less recalcitrant and decompose faster. Despite a lack of strong evidence for differences in BNPP between nutrient-amended and control plots, increased turnover is a possible mechanism of lowering root contribution to stable soil organic matter.