Soil depth and grassland origin cooperatively shape microbial community co-occurrence and function

Authors: UPTON, RACHEAL - Iowa State University; SIELAFF, ALEKSANDRA - Iowa State University; HOFMOCKEL, KIRSTEN - Iowa State University; XU, XIA - Iowa State University; Polley, Herbert; WILSEY, BRIAN - Iowa State University

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2019
Publication Date: 1/1/2020
Citation: Upton, R.N., Sielaff, A.C., Hofmockel, K.S., Xu, X., Polley, H.W., Wilsey, B.J. 2020. Soil depth and grassland origin cooperatively shape microbial community co-occurrence and function. Ecosphere. 11(1):e02973. https://doi.org/10.1002/ecs2.2973.
DOI: https://doi.org/10.1002/ecs2.2973

Interpretive Summary: Soil microbes (fungi, bacteria) decompose dead plant material and release mineral elements such as nitrogen into soil that support plant growth. Microbial composition and functioning, in turn, are influenced by plant root production and rooting depth. Yet, we know little of the composition and functioning of microbes at soil depths greater than a few centimeters or how microbes are influenced by changes in plant communities, including the widespread replacement of native plant communities by communities of non-native (exotic) plant species. We compared microbial community composition and functioning to 1 m depth in soil on which mixtures of native and exotic grassland plant species had been grown for 8 years in central Texas to determine how soil depth and the replacement of native by exotic plants in the Southern Plains of the United States may be affecting soil microbes. Fungal communities differed between native and exotic plant communities to 1 m depth. Both the number and relative abundances of soil fungi and bacteria decreased with depth. Microbial functioning also differed between shallow and deeper soil depths. Activity of carbon cycling enzymes, for example, declined with soil depth. Two major findings emerge in our results. First, soil depth is a major determinant of microbial community structure and functioning in grassland soils. Second, the replacement of native by exotic plant communities has altered fungal communities to 1 m depth with possible feedback effects on the decomposition of dead plant material and nutrient release to plants. Microbial change caused by the native to exotic shift in plant species may influence both the capacity of soils to mitigate the continuing increase in atmospheric carbon dioxide concentration by sequestering carbon and growth of forage for livestock production.

Technical Abstract: Many soils are deep and highly structured, yet soil below 20 cm in depth remains largely unexplored. Exotic plant species are sometimes found to be more shallowly rooted than native species, and plant-soil interactions may change with depth as a result. We studied microbial community composition, alpha and beta diversity, and function between native and exotic grassland plant communities, with and without summer irrigation, at soil depths up to 100 cm. We hypothesized that 1) both fungal and bacterial community composition will change and diversity and enzymatic function will decrease with depth; 2) native plots will have higher microbial enzymatic function than exotic plots and this effect should be most pronounced at shallower soil depths because of the stronger impact of that canopy in surface soils. Microbial community diversity measures decreased with depth and community composition was distinctly different between shallow and deeper soil depths. Deeper soil communities had lower beta diversity than shallow layers. Fungal communities differed between native and exotic plant communities with no interaction with depth. Microbial extracellular enzyme activity for beta-1,4-glucosidase and beta-1,4-xylosidase, carbon cycling enzymes, significantly declined with soil depth, but all other enzymes measured continued to have similar activities at top soil to 1 m deep. Our results indicate that native to exotic plant communities have significantly different microbial communities all the way to lower depths, up to 1 m. Exotic plant species communities’ soil legacy effects remain unknown, however, the disruption of native soil communities at lower soil depths, down to 1 m as demonstrated in this study, could indicated a larger impact on the microbiome than previously appreciated. Additionally, soil depth is a major determinant of microbial community structure and function in prairie soils and distinct microbial communities are found along the soil vertical gradient.