Tillage intensity and plant rhizosphere selection shape bacterial-archaeal assemblage diversity and nitrogen cycling genes

Abstract

In agriculture, adoption of reduced tillage practices is a widespread adaptation to global change. The cessation of plowing reduces erosion, slows soil organic matter oxidation, and promotes soil carbon accrual, but it can also result in the development of potential N2O spots from denitrification activity. In this study, we hypothesized that 16S rRNA-based composition of bacterial-archaeal assemblages would differ in agricultural soils subjected for forty years to a range of disturbance intensities, with annual moldboard plowing (MP) being the most intensive. No-till planting (NT) represented tillage management with the least amount of disturbance, while chisel-disking (CD), a type of conservation tillage, was intermediate. All long-term tillage plots had been planted with the same crops grown in a three-year crop rotation (corn-soybean-small grain+cover crop), and both bulk and rhizosphere soils were analyzed from the corn and soybean years. We also evaluated denitrification gene markers by quantitative PCR at multiple points (three growth stages of corn and soybean). Tillage intensity, soil compartment (bulk or rhizosphere), crop year, growth stage, and interactions all exerted effects on community diversity and composition. Compared to MP and CD, NT soils had lower abundances of denitrification genes, higher abundances of nitrate ammonification genes, and higher abundances of taxa at the family level associated with the inorganic N cycle processes of archaeal nitrification and anammox. Soybean rhizospheres exerted stronger selection on community composition and diversity relative to corn rhizospheres. Interactions between crop year, management, and soil compartment had differential impacts on N gene abundances related to denitrification and nitrate ammonification. Opportunities for managing hot spots or hot moments for N losses from agricultural soils may be discernible through improved understanding of tillage intensity effects, although weather and crop type are also important factors influencing how tillage influences microbial assemblages and N use.