Nitrogen niche partitioning between tropical legumes and grasses strengthens under elevated CO2

Abstract

AbstractPlant community biodiversity can be maintained, at least partially, by shifts in species interactions between facilitation and competition for resources as environmental conditions change. These interactions also drive ecosystem functioning, including productivity, and can promote over-yielding-an ecosystem service prioritized in working landscapes that occurs when there is either less competition, more facilitation, or both, between species in a community than within species. Importantly, shifts in species interactions that can result in over-yielding are unclear given rising CO2concentrations, especially in the context of tropical mixed-species grasslands.We examined the relative performance of two species pairs of tropical pasture grasses and legumes growing in monoculture and mixtures in a glasshouse experiment manipulating CO2. We investigated how over-yielding can arise from nitrogen (N) niche partitioning and biotic facilitation using stable isotopes to differentiate soil N from biological N fixation (BNF) within N acquisition into aboveground biomass for these two-species mixtures.We found that N niche partitioning in species-level use of soil N vs. BNF drove species interactions in mixtures. Importantly partitioning, though not necessarily overyielding, was generally enhanced under elevated CO2. However, this finding was mixture-dependent based on biomass of dominant species in mixtures and the strength of selection effects for the dominant mixture species.This study demonstrates that rising atmospheric CO2may alter niche partitioning between co-occurring species, with negative implications for the over-yielding benefits predicted for legume-grass mixtures in working landscapes with tropical species. Furthermore, these changes in inter-species interactions have consequences for shifts in grassland composition that are not yet considered in larger-scale projections for impacts of climate change and species distributions.Graphical abstract (Image by H. Zhang):Among our tropical pasture species we found that grasses (dotted lines) grown in monoculture rely fully on soil nitrogen (N), while legumes (solid lines) grown in monoculture relied approximately equally on soil N and biological nitrogen fixation (BNF) to meet N requirements. When grown with tropical grasses, however, legumes shifted to rely more strongly on BNF, indicative of niche partitioning and decreased competition for soil nutrients with grasses. This separation of niche space was strengthened under elevated CO2conditions, ultimately reducing legume production.