Nitrogen niche partitioning between tropical legumes and grasses conditionally weakens under elevated CO2

Plant 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 agro‐ecosystems, such as pastures, that occurs when multiple species together are more productive than the component species alone. Importantly, species interactions that can result in over‐yielding may shift in response to rising CO2 concentrations and changes in resource availability, and the consequences these shifts have on production is uncertain 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 above‐ground biomass for these two‐species mixtures. We found that N niche partitioning in species‐level use of soil N versus BNF drove species interactions in mixtures. Importantly partitioning and overyielding were generally reduced 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 species. This study demonstrates that rising atmospheric CO2 may 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 may have consequences for grassland composition that are not yet considered in larger‐scale projections for impacts of climate change and species distributions. Read the free Plain Language Summary for this article on the Journal blog. 摘要 当环境条件变化时，植物群落生物多样性可以部分通过种间互作的资源补偿效应和竞争效应转换来维持。种间互作也推动了包括生产力在内的生态系统功能，并促进超产效应—这是一种在诸如牧场的农作环境中优先考虑的生态系统服务，即多物种混播产量高于单播。重要的是，随着CO2浓度的升高和资源可利用性的变化，可能导致超产物种的种间互作发生转变，而这些变化对产量的影响尚不清楚，尤其是在热带混播草原中。 我们通过控制温室中的CO2浓度水平，研究了两种热带禾本科牧草和豆科牧草在单播和混播中的相对产量。同时，利用氮稳定同位素区分混播物种地上生物量氮获取中的土壤氮与生物固氮来源比例，以期探究氮素生态位分化和生物促进作用如何实现超产效应。 我们发现，混播后豆科和禾本科植物的氮素利用来源发生了显著的氮生态位分化，即豆科植物更加依赖豆科固氮，而禾本科植物更加依赖于土壤氮库。更重要的是，氮生态位分化和超产效应均在CO2升高时受到了抑制