Speaker
Description
Introduction
In Europe and the UK, farms have typically become highly specialised with little or no integration between crops and livestock enterprises either between or within farms, resulting in an agricultural system that is highly dependent on purchased inputs for the supply of nutrients to both crops and livestock. The consequence of this specialisation has meant there is a risk of soil fertility declining in arable areas. In contrast, in livestock dominated areas, there has been an excessive build-up of nutrients in the soil, increasing the risk of pollution (Watson et al., 2019). Pollution of the environment coupled with greenhouse gas emissions from agriculture has resulted in increasing interest in the concept of the nutrient circularity. Frequently overlooked in assessing nutrient circularity in agricultural systems, is the return of nutrients to the soils which are critical for maintaining soil health and rebuilding natural capital. Co-products, such as crop residues, provide materials that can be manipulated and used in different ways to enhance soil fertility. Here, the effect of co-product use on-farm has been assessed through a rapid evidence assessment (REA). The review focused on assessing the effect of co-product management on soil nutrient status, carbon stocks and nutrient losses.
Methodology
The focus of the REA was to assess the effect of the co-product management on soil nutrients, carbon stocks and nitrous oxide emissions. The REA conducted in SCOPUS on 26/2/2023 was restricted to systematic reviews and meta-analysis, and articles published in English. The co-product management options included in the search term were catch crop, grazed and ungrazed cover crops, waste, manure, residues, composts, digestates and straw. Initially 236 papers were identified of which 20 contained data directly relevant to European climates. Papers that reported the results at a global scale were excluded from the analysis. The data extracted was the effect of the application of the co-product on soil nutrients, carbon stocks or nutrient loss relative to the same management without the addition of the co-product.
Results
The results indicate that there are few meta-analyses or systematic reviews that report the effect of the use of co-products on soil nutrients and nutrient loss from agricultural systems. Composts and treated manures were only reported in one paper each. The co-products of cover crop residues and manure, crop residues were reported in twelve, seven and four papers, respectively. These papers mainly focused on soil carbon and nitrous oxide emissions. Except for crop residues and soil carbon, the number of observations included in each of the mean data points ranges from 362 for nitrous oxide emissions from manure to 966 for nitrous oxide emissions from cover crops. The effect of including cover crops was to increase nitrous oxide emissions by a factor of 2.25 (range 0.64-5.95), more than doubling the initial emissions. This compares with a factor of 1.8 (0.6-3.63) and 1.49 (0.98-2.16) for manures and crop residues, respectively. The comparable values for soil carbon were 1.08 (1.04-1.13), 1.11 (1.05-1.29), 1.14 (1.03-1.24) for cover crops, manures and crop residues respectively.
Discussion
The REA highlighted that there is currently a lack of meta-analyses or systematic reviews that assess the effect of co-product use that is specifically relevant to European agriculture. Although the results from the global meta-analysis may be relevant, it is important to recognise that crops, and pedoclimatic conditions may influence the magnitude of these effects. The results highlight that use of cover crops, manures and crop residues are likely to increase both soil carbon storage and nitrous oxide emissions. Increasing soil carbon is likely to improve the soil structure and improve fertility. However, from a net zero perspective, this needs to be traded off against the increase in nitrous oxide emissions. It is also important to recognise that the papers assessed have focused on soil carbon storage, which may not result in long-term carbon sequestration. This work also highlights the importance of assessing all the flows within the farming systems and identifying opportunities within the entire system to mitigate emissions. The social and economic implications of co-product utilisation also need to be considered.
References
Watson, C. A. et al. (2019). Linking Arable Cropping and Livestock Production for Efficient Recycling of N and P. Agroecosystem Diversity: Reconciling Contemporary Agriculture and Environmental Quality, 169–188.
| Keywords | Co-products; REA; soil carbon, nitrous oxide |
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