PNNL

Abstract

Increased nutrient inputs due to anthropogenic activity are expected to enhance primary productivity across terrestrial ecosystems, but allocation above- and belowground will have differing effects on soil carbon (C) storage. Given that roots are major contributors to soil C storage, understanding belowground net primary productivity (BNPP) and biomass responses to changes in nutrient availability is essential to predicting carbon-climate feedbacks in the context of interacting global environmental changes. To address this gap, we measured how a decade of nitrogen (N) and phosphorus (P) fertilization influenced aboveground and belowground biomass and productivity at nine grassland sites spanning the continental United States, which are part of a globally distributed nutrient addition experiment (The Nutrient Network). To assess how fertilization, site conditions, and their interactions influenced these responses, we measured aboveground biomass, root production, and root biomass, and quantified effects of climate and soil factors and plant diversity. Fertilization effects were strong aboveground, with both N and P addition stimulating aboveground biomass at nearly all sites. While P addition increased root production, belowground responses to fertilization were weaker than aboveground, ranging from positive to negative across sites. Atmospheric N deposition was as an important predictor of root properties and the response of roots to fertilization. Atmospheric N deposition was positively related to root turnover rates, and this relationship was amplified with N addition. Nitrogen addition increased root biomass at sites with low N deposition but decreased it at sites with high N deposition. Overall, these results suggest that the effects of nutrient supply on belowground plant properties are context dependent, particularly with regard to background N supply rates, and therefore site factors must be considered when predicting how grassland ecosystems will respond to increased nutrient loading from anthropogenic activity.