A link between plant diversity, elevated CO₂ and soil nitrate

Abstract

Interactive effects of reductions in plant species diversity and increases in atmospheric CO₂ were investigated in a long-term study in nutrient-poor calcareous grassland. Throughout the experiment, soil nitrate was persistently increased at low plant species diversity, and CO₂ enrichment reduced soil [NO₃ ^–] at all levels of plant species diversity. In our study, soil [NO₃ ^–] was unrelated to root length density, microbial biomass N, community legume contents, and experimental plant communities differed only little in total N pools. However, potential nitrification revealed exactly the same treatment effects as soil [NO₃ ^–], providing circumstantial evidence that nitrification rates drove the observed changes in [NO₃ ^–]. One possible explanation for plant diversity effects on nitrification lies in spatial and temporal interspecific differences in plant N uptake, which would more often allow accumulation of NH₄ ⁺ in part of the soil profile at low diversity than in more species-rich plant communities. Consequently, nitrification rates and soil [NO₃ ^–] would increase. Elevated CO₂ increased soil water contents, which may have improved NO₃ ^– diffusion to the root surface thereby reducing soil [NO₃ ^–]. Higher soil moisture at elevated CO₂ might also reduce nitrification rates due to less aerobic conditions. The accordance of the diversity effect on soil [NO₃ ^–] with previous experiments suggests that increased soil [NO₃ ^–] at low species diversity is a fairly general phenomenon, although the mechanisms causing high [NO₃ ^–] may vary. In contrast, experimental evidence for effects of CO₂ enrichment on soil [NO₃ ^–] is ambiguous, and the antagonistic interaction of plant species reductions and elevated CO₂ we have observed is thus probably less universal.