Root system size determines plant performance following short-term soil nutrient pulses

Abstract

Heterogeneity in soil resources is common in natural communities, in both space and time. In contrast to substantial research into plant responses to spatial nutrient heterogeneity, little is known about the factors that influence plants responses to temporally heterogeneous nutrient supplies. We examine (1) the effects of nutrient pulse length on plant biomass allocation patterns, (2) whether a plant’s root system prior to a nutrient pulse influences the benefit a plant can gain from that pulse, and (3) how competition and nutrient pulses interact to influence plant biomass. To address these issues, we used a pot experiment with varying initial conditions (nutrient levels and competition) designed to produce a range of root system sizes in the grass Poa pratensis. We then subjected the plants to short-term (3 and 10 days long) nutrient pulses where the same total nutrients were delivered at varying rates per unit time (intensity). Plant biomass and tissue nitrogen concentration were lowest when pulses were short relative to either long pulses or continuous supply. Plants with larger root systems at the start of the nutrient pulse gained more benefit than smaller plants because final plant size relative to initial root system size decreased as pulse intensity (nutrients delivered per unit time) increased. This study has two important implications. First, since larger plants benefit disproportionately more from a nutrient pulse than smaller plants, pre-pulse conditions such as the competitive environment and resource supply are likely key determinants of a plant’s ability to capture nutrients from a pulse. Second, the importance of pre-pulse size provides a belowground mechanism to enhance the size variability among plants in a community.