Cover crop diversity can increase soil carbon, microbial diversity and soil health

Article: Impacts of diverse undersown cover crops on seasonal soil microbial properties

Authors: Shrestha Rashmi et al.

Journal: FEMS Microbiology Ecology

Year: 2025

Modern agriculture characterized by monoculture, seasonal fallow periods, intensive tillage and fertilizer application can reduce soil biodiversity and cause soil organic carbon loss. In contrast, diversified cropping systems that incorporate year-round ground vegetation are suggested to enhance soil biodiversity and functioning including soil organic carbon storage. Soil microorganisms make up a significant portion of soil biodiversity and play crucial role in soil nutrient cycling in agriculture and can contribute to improve soil quality and enhance crop production. Positive relationships between plant diversity, microbial diversity, and ecosystem functioning have been widely observed in experimental grasslands. However, the impact of cover crop species diversification on soil microbial diversity and function in croplands remains underexplored.

Here, the aim was to study how agricultural diversification affects soil microbial biomass and activity in different seasons, and whether these changes resulted in legacy effects on the following year crops. To achieve this, an experimental field of the TWINWIN project was established in May 2019 in Helsinki. The experimental setup included different number (1, 2, 4, or 8) of cover crop species with different function (nitrogen fixing and deep rooting), undersown into barley. In addition, the setup field included plots without vegetation (“bare fallow”), barley monoculture and barley monoculture with herbicide. The soil samples were collected throughout the year to study soil microbial properties (soil microbial biomass, respiration, and their capacity to utilize soil organic matter). To evaluate the legacy effects on the following year barley, soil bacteria and fungi were analysed from soil samples collected in the following spring before cover crop termination. Additionally, the spring soil samples were used to conduct a pot experiment with flax to study how the cover crop diversity influenced the mycorrhizal fungal colonization in roots.

The results showed seasonal changes in microbial biomass carbon. More importantly, the microbial carbon increased, while metabolic quotient decreased, when the vegetation was present (i.e. compared to bare fallow) and with higher cover crop richness. This was due to continuous and increased supply of diverse food resources and habitats. It indicates that cover crop diversity may provide a stable environment with improved soil carbon retention.

Microbial respiration decreased in the presence of legumes, which may be due to increased nitrogen supply by the legumes. The diversity of the fungi overall and the (arbuscular) mycorrhizal fungi increased with the cover crop richness, whereas the presence of legumes helped suppress fungal plant pathogens.

In the pot experiment with flax, the presence of vegetation in general, and the legumes in particular, increased the mycorrhizal fungal colonization of the roots. The results suggest that undersowing diverse cover crops, particularly legumes, can increase the soil microbial diversity and soil health. It benefits both the current and next-year crops. Overall, the study showed that the positive effect of cover crop diversity can be seen in bulk soil microbiota already after one year, and the effect can be stronger with time.

The research was part of the TWINWIN project of the Carbon Action network.