Impact of soil inoculation on crop residue breakdown and carbon and nitrogen cycling in organically and conventionally managed agricultural soils

Organic agriculture relies on organic fertilizers and amendments to provide nutrients to plants and will therefore depend on decomposer communities to release nutrients from these organic inputs. However, after conversion of conventional to organic agriculture it may take up to decades before decomposer communities become adapted to the new resource inputs. The aim of the present study is to investigate if the functional capacity of soil communities for decomposing recalcitrant crop residue types can be enhanced by inoculating soil communities from organically into conventionally managed soils. We used a microcosm incubation experiment to test how soil inoculation, agricultural management history, and crop residue type affect carbon and nitrogen cycling with crop residue addition. We collected soil samples from 5 pairs of conventional and nearby organic fields and set up a reciprocal inoculation experiment under controlled lab conditions. We inoculated soil from each conventional field with soil from the paired organic field and vice versa. To each soil mix, five types of crop residues were added: a cover crop mixture, carrot leaves (Daucus carota), alfalfa (Medicago sativa), hay (Lolium perenne), and straw (Triticum aestivum). There was one control treatment without any addition. Soils were incubated for 34 days and we measured mass loss of the crop residues from litter bags, cumulative soil respiration, cumulative potential plant available nutrients, permanganate oxidizable carbon (POXC), and substrate-induced respiration (SIR). Initial soil abiotic conditions (soil organic matter content, pH, C:N ratio, plant available nutrients), soil microbial biomass and soil bacterial and fungal community composition were also determined. We did not find clear effects of inoculation on mass loss and cumulative respiration. Instead, effects of crop residue type on all parameters were substantial. Crop residues with higher C:N ratios generally had lower mass loss and cumulative respiration, and resulted in lower nitrogen availability but higher POXC contents. Organic management enhanced cumulative respiration. There was little overlap in bacterial and fungal ASVs between the organic and conventional soils within each pair, resulting in a potential increase in diversity as a result of soil inoculation. We conclude that decomposition of crop residues declined with their recalcitrance, and that soils from organically managed fields did not increase the capacity of the soil community to decompose recalcitrant residues. Further studies are needed to determine whether compositional differences between soils from organic and conventional fields are a response to farming practices or whether management also has functional implications for soil fertility.