The impact of cover crops on carbon cycling as part of an agricultural rotation

Stoker, Mandy and Shield, Ian and Stevens, Carly (2023) The impact of cover crops on carbon cycling as part of an agricultural rotation. PhD thesis, Lancaster University.

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Abstract

Sustainably providing food for a growing population whilst maintaining a healthy soil is currently a large challenge for agriculture. Integral to the provision of a healthy soil is the input of carbon. The government’s 25 Year Environment Plan (2018) includes measures to increase carbon in soil, and cover crops have been put forward as a sustainable solution for agricultural. A cover crop is a crop grown in between cash crops, either over winter or as part of a rotation. Typically, cover crops have been used as a green manure, allowing the plant to be turned into the soil to decompose, but their impact on the flow of carbon is not well understood, especially when used alongside other farming practices such as weed eradication using herbicides (glyphosate) and use of plant growth promoters such as microbial inoculants. The overarching aim of this research was to understand the impact of cover crops on soil carbon and the soil microbial community. An in-field experiment was carried out over a three year period. A replicated experiment was used to investigate the impact of cover crops, glyphosate, and a commercial inoculant. The soil’s microbial community and organic matter were analysed. Results showed that no treatments alone, or in combination, increased the soil organic matter above the pre-treatment baseline level. However, there was an increased presence of the phospholipid fatty acid (PLFA) fungal group of biomarkers. An increase in fungi could lead to an increase of stabilized carbon in soil. Inoculants used together with cover crops had a negative impact on microbial activity, and the yield of above ground biomass. Glyphosate showed an increase of gram- negative bacteria nearly 3 years after it was applied. A pot trial using a solution of plant materials from 7 different plant species dosed into either a clay, or sandy soil, showed significant differences across all soil types and within each soil type. Turnips and mustard showed higher levels of microbes associated with high carbon:nitrogen, including fungal and gram-positive functional groups, whereas clover and phacelia (nitrogen fixing legumes), showed higher levels of gram-negative bacterial groups. A survey on the use of cover crops by farmers, the barriers and difficulties encountered, and how farmers source information about this or any new farming techniques showed that of the farmers surveyed, 94 % of respondents used cover crops and many felt that although it was an additional expense, they were beneficial. The main issues arising were lack of knowledge about which crop species to plant, and the timing of sowing ii and destroying the crop. Information gathering is a combination of word of mouth, agronomists, Farmers Weekly and the internet. Although cover crops did not increase soil organic matter, other benefits were apparent. Further work needs to be carried out to get a better understanding of the long term effect of a mix of agricultural chemical and biological interventions over a harvest year. Specifically, an understanding about microbial activity in the rhizosphere of a standing cover crop, and the effect of the cover crop biomass as it is incorporated into the soil over several harvest. This information could assist farmers in deciding cover crop mixes and timings within the whole farm scheme.