Recycling of organic resources into agriculture has the potential to greatly increase nutrient use efficiency and improve soil carbon balance, but improper management can have adverse effects on the environment. Agriculture therefore faces large challenges to increase yields while decreasing these emissions to the environment. In this paper, we review (i) the availability and composition of organic resources, (ii) their agronomic value and risk of emissions, (iii) potential measures to reduce their emissions, and (iv) future challenges to support farmers and policy makers. The total amount of organic resource applied to soil amounted on average 41 kg nitrogen per ha agricultural land, 9 kg phosphorus per ha, and 456 kg carbon per ha in EU-27 + UK in 2017. Solid pig and cattle manures and cattle slurry are the most used organic resources. The availability of new organic resources from food processing, sewage sludge, municipal bio-wastes, and upcoming manure treatment techniques as fertilizer or soil conditioner is expected to strongly increase over the coming decade. Insight is needed into the composition of organic resources, the plant-availability of nutrients, the degradability of organic matter and the presence of contaminants. Measurement techniques become available to characterize soils, manures, crops, and emissions to the environment. However, the interpretation, and integration of data, and recommendations to farmers and policymakers using large amounts of data is expected to become more and more challenging. Many measures are available to improve nutrient and carbon management and to reduce emissions, including proper application, technological measures and structural changes in agriculture. For many measures, there is a risk of trade-offs that could lead to pollution swapping at different scales. We should focus on finding synergies between measures and no-regret management choices to develop effective mitigation strategies. The main future challenge for managing organic resources in agriculture is the development of an integrated nutrient management approach, including (i) the characterization of organic resources, their agronomic value and their environmental risks, (ii) knowledge of potential synergies and trade-offs between management measures, and (iii) implementation of this knowledge into decision support tools, models and legislation to support farmers and policy makers.

CONTEXT: Current agricultural systems in the Netherlands and other parts of Europe depend on large quantities of nutrients from imported feed and artificial fertilizers. This may result in unwanted nutrient accumulation and losses and negative environmental impacts. One way of making these systems more sustainable is to balance livestock numbers with the local feed supply and nutrient requirements for crop production. However, there is currently limited quantitative- and spatially-explicit insight into the effects of reduced imports on livestock numbers and nutrient balances. OBJECTIVE: The aim of this study is to quantify the number of livestock that can be sustained when minimizing nitrogen (N) imports of feed and artificial fertilizers into Dutch agriculture. METHODS: We developed a spatially-explicit mathematical programming model to optimize livestock numbers in different scenarios of allowed N imports. The model takes into account current flows of nutrients, related to nutrient supply from livestock manure, feed and fodder, and nutrient requirements in agricultural production. RESULTS AND CONCLUSIONS: Minimizing N feed imports and artificial N fertilizer use under current land use resulted in an overall reduction of livestock density of 57.4% compared to the current livestock density based on 2020 activity data. In addition, it led to an increase in artificial N fertilizer inputs of 20.5% to maintain the N requirements of agricultural land. In contrast, focusing on minimizing artificial fertilizer use led to a reduction in livestock density of 25.8% compared to the current livestock levels. Depending on the scenario, we found strong regional variation in nutrient balances and livestock numbers. For instance, while currently the application rates of artificial fertilizers and livestock manure are relatively constant in space, this was no longer the case when N import in the form of feed was minimized. In that case, the crop-dominated areas showed a deficit in livestock manure and a substantial increase of artificial fertilizers. SIGNIFICANCE: The presented model allows for assessing spatially explicit impacts of reduced N imports of feed and artificial fertilizers on livestock density. Using the Netherlands as case study, we show that reducing feed imports is a more effective leverage point to reduce N surplus and external N inputs into agriculture as compared to when reducing artificial fertilizer use. Our findings can inform the development of area-specific strategies that are aimed at reconnecting livestock and agricultural land.

Recovering nutrients from organic materials to reduce artificial fertilizer inputs requires the implementation of processing technologies and can involve considerable logistics and transportation costs. Reducing such costs by directly applying organic materials to agricultural land can contribute to pollution due to potential contaminants and unbalanced nutrient ratios. Assessing the cost of increased recycling requires a spatially explicit approach because availability of organic materials, nutrient demand and agro-ecosystem properties vary spatially. A multi-objective model was developed to estimate the trade-offs between costs of nutrient recovery and improvements in nutrient distribution for a case study area in The Netherlands. The evaluated recovery processes included solid-liquid separation followed by reverse osmosis to recover nutrients from pig manure which was compared to a conventional process via hygienisation and export. Results indicate that, even in a nutrient saturated area, replacement potential of artificial nitrogen (N) and phosphorus (P) fertilizers through locally reclaimed nutrients is limited to about 17% N and 55% P. A cost optimum was found when about 48% of the initial pig manure quantities were processed via nutrient recovery and directed to land. Increasing manure processing for nutrient recovery led to a redistribution of nutrients and trace metals (zinc (Zn) and copper (Cu)), resulting in more localized concentration. Zn and Cu were enriched by about 8% and 2%, respectively, when maximizing nutrient recovery. Our generic model offers a methodology to assess the trade-offs between increased recycling and associated spatial effects to facilitate sustainable recycling infrastructures for achieving more circular agriculture.

Widespread adoption of improved cropland management measures is advocated to increase soil organic carbon (SOC) levels, thereby improving soil fertility and mitigating climate change. However, spatially explicit insight on management impacts is limited, which is crucial for region-specific and climate-smart practices. To overcome these limitations, we combined global meta-analytical results on improved management practices on SOC sequestration with spatially explicit data on current management practices and potential areas for the adoption of these measures. We included (a) fertilization practices, i.e., use of organic fertilizer compared to inorganic fertilizer or no fertilizer, (b) soil tillage practices, i.e., no-tillage relative to high or intermediate intensity tillage, and (c) crop management practices, i.e., use of cover crops and enhanced crop residue incorporation. We show that the estimated global C sequestration potential varies between 0.44 and 0.68 Gt C yr-1 , assuming maximum complementarity among all measures taken. A more realistic estimate, not assuming maximum complementarity, is from 0.28 to 0.43 Gt C yr-1 , being on the lower end of the current range of 0.1-2 Gt C yr-1 found in the literature. One reason for the lower estimate is the limited availability of manure that has not yet been recycled. Another reason is the limited area for the adoption of improved measures, considering their current application and application limitations. We found large regional differences in carbon sequestration potential due to differences in yield gaps, SOC levels, and current practices applied. The highest potential is found in regions with low crop production, low initial SOC levels, and in regions where livestock manure and crop residues are only partially recycled. Supporting previous findings, we highlight that to encourage both soil fertility and SOC sequestration, it is best to focus on agricultural soils with large yield gaps and/or where SOC values are below levels that may limit crop production.