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Description
(1) Modern food production largely rely on a linear nutrient flow, with nutrients entering the field as bought fertilisers, or potentially as farmyard manure, but with little return of nutrients from the society. Returning organic waste products is a way of closing nutrient cycles between agriculture and society, contributing to a circular economy where resources are reused (Rosemarin et al., 2020), and maintain or increase soil organic matter content compared to using mineral fertilisers (Kätterer et al., 2011). Moreover, in organic farming, crop nutrition largely depends on manure, and low manure accessibility has been argued to contribute to the low adaption to organic farming in regions where crop production is dominating (Nordin, 2021). The aim of this study was to assess the current availability of the organic waste products biogas digestate, mainly produced from food waste and farmyard manure, sewage sludge (although currently not allowed in organic farming), and other organic nutrient sources. Furthermore, we aimed at comparing the nutrient of these materials to crop nutrient demands of today as well as a future scenario with more land cultivated with organic farming management methods, using Sweden as a case study.
(2) Crop production conditions vary greatly in Sweden, a country that stretch from 55°N to 69°N. It has fertile plains in the south, but also much land that is difficult to cultivate with annual crops and where forestry and animal husbandry dominate. The change in latitude also cause differences in cropping season characteristics. To account for this Sweden is divided into eight production areas and the study uses these production areas when estimating crop nutrient demand and availability. Data on land cultivated with different crops and their yields, and production and nutrient content of biogas digestate, manure, sewage sludge and other waste product-based fertilisers produced in Sweden have been obtained from national statistics and from communication with biogas digestate facilities, a sewage sludge certification body and the Swedish Environmental Protection Agency.
(3) The availability of organic waste products from society (biogas digestate and sewage sludge) suitable for use in agriculture, and their potential to cover regional crop nutrient demand, vary between production areas, with higher availability in areas with much crop production and a larger population size, i.e. the southern part of the country. The availability of farmyard manure is unsurprisingly lower in areas with a large proportion of specialised crop production farms. The nutritional quality of biogas digestate compared to farmyard manure is similar for total nitrogen, on average somewhat higher for mineral nitrogen concentrations (kg/t wet weight), and lower for phosphorous and potassium concentrations. Total nitrogen and phosphorous concentration of sewage sludge is substantially higher than those both in biogas digestate and farmyard manure, linked to higher dry matter content. The data are currently being evaluated, scenarios defined and nutrient demands for these different scenarios calculated.
(4) Although biogas digestion of manure can increase its nutritional value (Cavalli et al., 2016) low nutrient concentrations per wet weight and bulkiness of these products are disadvantages, as well as for sewage sludge. There are methods to be used to dehydrate these products or extract the nutrients from the solid or liquid phase and hence increase their fertiliser value (Monfet et al., 2018). However, this comes at a cost of reduced contribution to soil organic matter and soil fertility. Therefore, these methods are more interesting in areas where there is less crop production and hence longer distances to arable fields and these products could instead be sold to regions with more production.
References
Cavalli et al. (2016). Nitrogen fertilizer replacement value of undigestated liquid cattle manure and digestates. European Journal of Agronomy, 73, 34-41.
Kätterer et al. (2011). Roots contribute more to refractory soil organic matter than aboveground crop residues, as revealed by a long-term field experiment. Agriculture Ecosystems and Environment, 141(1-2), 184-192.
Monfet et al. (2018). Nutrient removal and recovery from digestate: a review of the technology. Biofuels, 9(2), 247-262.
Nordin (2021). Is a shortage of manure a constraint to organic farming? Working paper 2021:1. AgriFood Economics Center. Lund, Sweden.
Rosemarin et al. (2020) Circular nutrient solutions for agriculture and wastewater – a review of technologies and practices. Current Opinion in Environmental Sustainability, 45, 78–91.
| Keywords | biofertilisers; circular economy; nutrient recycling; organic fertilisers |
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