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.