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

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. This study 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 organic 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 microbial biomass carbon (MBC), prokaryote community composition and diversity (16 S amplicon sequencing), activities of enzymes involved in C, N, phosphorous (P) and sulfur (S) cycles and soil chemical properties. The results indicated that 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 the 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 the rotation system, they were less separated by fertilization which distinguished communities only in the wheat-fallow system. Redundancy analysis (RDA) followed by variance partitioning analysis (VPA) indicated that soil properties explained most of the variation in the community structure at the 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, the results show that changing wheat-fallow into wheat-pea cover crop systems increases some soil health indicators while decreasing diversity which suggests that no direct coupling exists between above and belowground biodiversity.