Lower soil carbon stocks in exotic vs. native grasslands are driven by carbonate losses

Authors: WILSEY, BRIAN - Iowa State University; XU, XIA - Iowa State University; Polley, Herbert; HOFMOCKEL, KIRSTEN - Pacific Northwest National Laboratory; HALL, STEVEN - Iowa State University

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2020
Publication Date: 3/5/2020
Citation: Wilsey, B., Xu, X., Polley, H.W., Hofmockel, K., Hall, S.J. 2020. Lower soil carbon stocks in exotic vs. native grasslands are driven by carbonate losses. Ecology. 101(7):e03039. https://doi.org/10.1002/ecy.3039.
DOI: https://doi.org/10.1002/ecy.3039

Interpretive Summary: The invasion and spread of non-native or exotic plant species has converted many native grasslands to exotic (non-native) dominated systems. This shift to increased exotic abundance has altered several ecosystem processes and services, including grassland productivity. The extent to which the expansion of exotics has influenced belowground processes, such as soil carbon (C) stocks, remains unclear however. Soil C stocks include both organic and inorganic components and are approximately double the size of the atmospheric pool. Should exotic invasion either reduce soil C storage rates or increase soil C losses, invasion would accelerate the ongoing increase in atmospheric CO2 that is linked to climate change. We measured change in both organic and infrequently-studied inorganic soil C stocks beneath experimental mixtures of either all exotic or all native perennial plant species in central Texas. Soil C was measured in 10 cm increments to 100 cm depth beneath experimental mixtures following 8 years of growth. We found that total soil C (organic + inorganic) content was significantly higher under native than exotic species mixtures. Native vs exotic differences in soil C increased with depth. Native and exotic plots differed in soil C because they differed in inorganic (carbonate) rather than organic C fractions. Soil carbonate was ~7% greater (250 g m-2) under native than exotic plantings. Our results indicate that the inorganic soil carbonate fraction can respond to differences in plant species over timescales of years, rather than centuries to millennia as typically assumed. Results imply that significant losses of inorganic soil C and a resulting acceleration of the ongoing increase in atmospheric CO2 concentration can be avoided by conserving native grasslands and replacing exotic plant species with native species, particularly in areas in which soil carbonate concentrations are high.

Technical Abstract: Global change includes invasion by non-native plant species and altered precipitation, and these factors may affect carbon storage. We measured soil carbon changes in mixtures of all exotic or all native plant species under two levels of summer drought stress. After eight years, soils were sampled in 10 cm increments to 100 cm depth to determine if soil C differed among treatments in deeper soils. Total soil C (organic + inorganic) content was significantly higher under native than exotic plantings, and differences increased with depth. Surprisingly, changes in C were due to carbonate and not organic C fractions, where carbonate was ~ 250 g m-2 greater under native than exotic plantings. Our results indicate that the carbonate fraction is an active pool and can respond to differences in plant species traits over timescales of years. Significant losses of inorganic C might be avoided by conserving native grasslands in sub-humid ecosystems.