Legume cover cropping and nitrogen fertilization influence soil prokaryotes and increase carbon content in dryland wheat systems

Authors: DOMNARIU, HORIA - Oregon State University; Reardon, Catherine - Kate; Manning, Viola; Gollany, Hero; Trippe, Kristin

Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2024
Publication Date: 3/11/2024
Citation: Domnariu, H., Reardon, C.L., Manning, V., Gollany, H.T., Trippe, K.M. 2024. Legume cover cropping and nitrogen fertilization influence soil prokaryotes and increase carbon content in dryland wheat systems. Agriculture, Ecosystems and Environment. 367. Article 108959. https://doi.org/10.1016/j.agee.2024.108959.
DOI: https://doi.org/10.1016/j.agee.2024.108959

Interpretive Summary: Traditionally, in the inland Pacific Northwest and other semi-arid areas of the United States, most farmers practice a winter-wheat – fallow rotation in which one crop is grown every other year. This rotation benefits crop establishment, reduces the risk of crop failure, and helps obtain an economically viable wheat yield by increasing soil water storage. At the same time, wheat-fallow rotations have negative ecological consequences stemming from leaving bare soil and reducing carbon inputs. As the ecological and economic benefits of cover cropping have become realized in other cropping systems, interest in replacing fallow periods with cover crops in dryland wheat production is increasing. However, there is an insufficient understanding of how such diversification of the rotation impacts soil microbes and soil chemistry. In this study, we investigated how replacing fallow periods with winter pea (Pisum sativa L.) coupled with nitrogen (N) fertilization impacts the diversity and activity of microbial communities and other essential soil properties such as soil carbon (C). Soils from a long-term field experiment (Pendleton, OR, USA) were sampled in 2021 at two depths (0-5 cm and 5-10 cm) and characterized for bacterial diversity and function and for soil chemical properties. Our results showed that microbial biomass and enzyme activity were greater in the wheat-pea rotation compared to wheat-fallow, but that fertilization did not influence these properties. Surprisingly, replacing fallow with pea generally decreased bacterial diversity. Our statistical analysis indicated that replacing fallow with pea influenced soil properties, which in turn exerted influence on the microbial community suggesting that above and belowground biodiversity are indirectly coupled in these cropping systems.

Technical Abstract: Simple rotation systems as wheat (Triticum aestivum L.)-fallow practiced in semi-arid regions are currently being transformed by replacing fallow with winter pea (Pisum sativum L.). However, there is insufficient understanding of how such diversification of plant functional groups impacts the biological and chemical soil properties which drive ecosystem services. Here we investigated how diversifying rotation system coupled with nitrogen (N) fertilization impacts the diversity and activity of microbial communities and other essential soil properties such as soil carbon (C). Soils from a long-term field experiment (Pendleton, OR, USA) were sampled in 2021 at two depths (0-5 cm and 5-10 cm) and characterized for prokaryote community composition and diversity (16S amplicon sequencing), activities of enzymes involved in C, N, P and S cycles and soil chemical properties. Our results showed that microbial biomass carbon (MBC) and enzyme activity were greater in the wheat-pea rotation compared to wheat-fallow, while fertilization was not a significant factor for these properties. Enzyme activities normalized by MBC showed fewer differences suggesting that the impact is mostly due to differences in MBC. Replacing fallow with pea decreased alpha diversity metrics at both depths and for both the highest and the lowest level of taxonomic hierarchy, although the effects upon phyla richness were not statistically significant. Nitrogen fertilizer decreased Shannon index and Shannon evenness especially in the 5-10 cm depth. While communities at the amplicon sequence variant (ASV) level were clearly separated by rotation system, they were less separated by fertilization which distinguished communities only in the wheat-fallow system. Redundancy analysis (RA) followed by variance partitioning analysis (VPA) indicated that soil properties explained most of the variation in the community structure at phylum level with rotation and fertilization having no unique contribution. This suggests that the effects of rotation and fertilization are mediated through the changes these practices induce upon other soil properties. Overall, our results show that changing wheat-fallow into wheat-pea cover crop systems increase some soil health indicators while decreasing diversity which suggests that no direct coupling exist between above and belowground biodiversity.