Fertiliser use and soil carbon sequestration: trade-offs and opportunities

Current initiatives to store carbon in soils as a measure to mitigate climate change are gaining momentum. Agriculture plays an important role in soil carbon initiatives, as almost 40% of the world’s soils are currently used as cropland and grassland. Thus, a major research and policy question is how different agricultural management practices affect soil carbon sequestration. This working paper focuses on the impact of mineral fertiliser use on soil carbon sequestration, including synergies with the use of organic inputs (for example crop residues, animal manure) and trade-offs with greenhouse gas (GHG) emissions. Findings from scientific literature show that fertiliser use contributes to soil carbon sequestration in agriculture by increasing biomass production and by improving carbon:nitrogen (C:N) ratios of residues returned to the field. The use of mineral fertiliser can also support the maintenance of carbon stocks in non-agricultural land if improved fertility on agricultural land reduces demand for land conversion. Combining organic inputs with mineral fertiliser seems most promising to sequester carbon in agricultural soils. Increasing nutrient inputs (either organic or mineral fertilisers) may however lead to trade-offs with GHG emissions such as N2O. Improving the agronomic nitrogen use efficiency of nutrient inputs (i.e., additional grain yield per kg N applied) can alleviate this trade-off. While soil carbon sequestration can benefit soil fertility under some conditions and compensate for some GHG emissions related to agriculture (first assessments indicate up to 25% of the emissions related to crop production, depending on region and cropping system), it seems unlikely it can compensate for GHG emissions from other economic sectors. If soil carbon sequestration is a policy objective, priorities should be areas with higher storage potential (wetter and colder climates) and/or regions where synergies with soil fertility and food security are likely to occur (for example farming systems in tropical regions, on sandy soils and/or when cultivating more specialized crops). However, regions with the highest storage potential most likely do not overlap with regions where the largest benefits for soil fertility and food security occur

Fertiliser use and soil carbon sequestration: Key messages for climate change mitigation strategies

Reducing greenhouse gas (GHG) emissions and increasing soil or biomass carbon stocks are the main agricultural pathways to mitigate climate change. Scientific and policy attention has recently turned to evaluating the potential of practices that can increase soil carbon sequestration. Forty percent of the world’s soils are used as cropland and grassland, therefore agricultural policies and practices are critical to maintaining or increasing the global soil carbon pool. This info note explains the current understanding of the impact of mineral fertiliser use on soil carbon sequestration as a mitigation strategy in agriculture. The science and understanding on soil carbon sequestration and mitigation is still emerging, especially in tropical regions. Taking this into consideration, this info note discusses related effects of fertiliser use on climate change mitigation, such as nitrous oxide (N2O) and carbon dioxide (CO2) emissions from nitrogen fertiliser use and production, and the potential effects of mineral fertiliser use on land use change

Developing a running prototype of a bio-economic farm model for a trade-off analysis of different nutrient management options for maize cultivation in East-Africa

Considering projected population trends, food requirements in East Africa will drastically increase in the coming decades (van Ittersum et al., 2016). One way to ensure supply will meet demand is by raising crop yields in the region. In East Africa, agricultural yields still have large potential to increase due to the large gaps between actual and potential yields. A recent study has shown that intensification of agriculture in regions with low current yields (such as in East Africa) is an option to reduce greenhouse gas emissions by avoiding or reducing agricultural land expansion into forests and/or grasslands, thus preserving carbon stocks (Van Loon, Hijbeek, ten Berge and Van Ittersum 2018, in prep). This is however only valid if higher yields are obtained with highly efficient use of fertilisers. For a successful implementation of such climate smart agricultural intensification, improved nutrient management options need to be economically viable for farmers in East Africa. It is however often unclear under which conditions agricultural intensification is beneficial for farmers’ income in sub saharan Africa (Marenya and Barrett, 2009; Place et al., 2003; Sheahan et al., 2013). Besides a number of good agricultural practices (such as improving planting densities and sound crop protection measures), farmers need to apply more nutrients to intensify production. The amounts of additional nutrients required represents the ‘nutrient gap’ between current nutrient applications and the total amount of nutrients removed from fields with increased yields (de Vries et al., 2017). Farmers can use several nutrient management options to close the nutrient gap (e.g. use mineral or organic fertilisers, split application of fertilisers, combine with local or hybrid seeds). The nutrient management option a farmer chooses not only affects his or her nutrient use efficiency (how much of the applied nutrients are recovered by the crop), but also his or her income generation and the contribution to greenhouse gas emissions. Some practices might be most beneficial for farmers’ income, but have a larger contribution to greenhouse gas emissions. Others might have the reversed effect. So far, trade-offs and/or synergies between farmers’ income and greenhouse gas mitigation as a function of nutrient management options have not been systematically assessed. Additionally, it is uncertain how such trade-offs or synergies might evolve over time, in cases where soil carbon and nutrient pools respond over longer time frames to the management exposed. We therefore address the following question: Can certain nutrient management practices be identified which are beneficial for both climate change mitigation and for farmers’ income in East Africa? The aim of this report is to develop a running prototype of a bio-economic model which can be used to assess trade-offs between yields, farmers ‘income and greenhouse gas emissions in function of different nutrient management options, both on the short and the long term. The proposed model will focus on nitrogen (N) as the main limiting nutrient, which is also highly relevant for greenhouse gas emissions (i.e. N2O). The model will be useful for R&D investors, agri-business (including fertiliser companies) and government agencies for ex ante assessment of specific nutrient management options

Spatial frameworks for robust estimation of yield gaps

Food security interventions and policies need reliable estimates of crop production and the scope to enhance production on existing cropland. Here we assess the performance of two widely used ‘top-down’ gridded frameworks (Global Agro-ecological Zones and Agricultural Model Intercomparison and Improvement Project) versus an alternative ‘bottom-up’ approach (Global Yield Gap Atlas). The Global Yield Gap Atlas estimates extra production potential locally for a number of sites representing major breadbaskets and then upscales the results to larger spatial scales. We find that estimates from top-down frameworks are alarmingly unlikely, with estimated potential production being lower than current farm production at some locations. The consequences of using these coarse estimates to predict food security are illustrated by an example for sub-Saharan Africa, where using different approaches would lead to different prognoses about future cereal self-sufficiency. Our study shows that foresight about food security and associated agriculture research priority setting based on yield potential and yield gaps derived from top-down approaches are subject to a high degree of uncertainty and would benefit from incorporating estimates from bottom-up approaches

Minimum emission pathways to triple Africa’s cereal production by 2050

Cereals play a central role in food security in sub-Saharan Africa (SSA), where they account for approximately 50% of caloric intake and total crop area. Cereal demand in the region is projected to nearly triple between 2015 and 2050 due to rapid population growth (van Ittersum et al. 2016). Increases in cereal yields are very slow in most SSA countries and agricultural area expansion is still an important means to keep up with the growing demand, causing losses of forests or grasslands, thereby reducing carbon stocks. At the same time the Paris Conference of the Parties (COP21) Agreement aims to keep global warming below 2 °C or even 1.5 °C by 2100. SSA has already seen a continuous increase in emissions from agriculture-driven deforestation between 1990 and 2015. Yet, intensification, i.e. higher yields per hectare with sufficient and judicious use of inputs, will also lead to higher emissions per unit area because of the required fertiliser use. This info note summarizes results of three recent studies that assessed whether SSA can be self-sufficient in cereals by 2050 under different scenarios of intensification on existing cereal area. For each scenario, yield increases and area expansion to meet cereal demand by 2050 were assessed. Increased demands for fertiliser use and associated GHG emissions were quantified

Efficiency of mineral and organic fertilizers across two continents

To mitigate climate change, greenhouse gas emissions from the agricultural sector need to decrease. In this light, increasing agronomic use efficiency of nitrogen (N) application (i.e., additional grain yield per kg of N applied) is a promising avenue to attain similar yields with less inputs in regions such as Europe (with high N inputs). In contrast, on the African continent, N inputs need to increase to raise yields, which may contribute to improved food security and prevent land use change. In such case, increasing agronomic N use efficiency (N-AE) and simultaneously increasing N inputs can also be a mitigation strategy by decreasing losses to the environment and improving profitability. In both contexts, it is relevant to understand how much N-AE can be increased in a certain location, compared to the current status, and which N source (organic and/or mineral fertilizer) will be most efficient. In this working paper we present ongoing work on N benchmarking from the crop nutrient gap project (full name: Bringing Climate Smart Agriculture practices to scale: assessing their contributions to narrow nutrient and yield gaps). First, we compare current observed N-AE to the values they could potentially reach under optimal agronomic management. For this, we propose a new benchmarking method based on recent insights on the shape of N response curves and introduce the related ‘degree of good agronomy’. Second, we compare the performance of mineral versus organic fertilizers for cereal cultivation on two continents (Europe and sub-Saharan Africa) based on large number of field experiments. Finally, we assess whether and how N-AE of mineral N fertilizer can be improved when combined with organic amendments. Preliminary findings show that the proposed benchmarking method can work but relies on availability of data on soil N supply, potential yield and attainable yields. Currently, this information is sparsely available which might be a barrier for uptake of the method. We show that N supplied by mineral fertilizers is taken up more efficiently than from organic sources, with variation depending on the type of organic amendment. Variation was larger for sites in Africa than Europe, which makes targeted fertilizer strategies less straightforward. Based on European experimental data, we show that organic amendments do not increase the N-AE of mineral fertilizer N application, most likely due to the increased total N availability. In future research, we hope to improve the data requirements for the proposed benchmarking method, assess drivers of variation for nitrogen fertilizer replacement values of organic amendments and disentangle effects of organic amendments on the efficiency of mineral fertilizer N use, while extending our analysis to tropical regions

D1940: Results from the Fertilizer demonstration experiment with maize at IOP Farm in Iringa, Tanzania in 2018

In 2018, an experiment was run at the IOP farm in Tanzania. Four nutrient management treatments were combined factorial with two tillage options. The results show that the lowest maize yield was obtained under conventional tillage without fertilizer application, and the highest with reduced tillage and NPK fertilizer to target 70% of water-limited yield and the addition of micronutrients. A number of field visits was organized, and from the six villages surrounding the farm, hence a total of 120 farmers made at least 4 visits to the farm, one every month between February and June. That brings the number of farmers that learned from the 2018 trial to 480+. In addition, the Tanzania Uhuru Torch made a stop at the Trial, to recognize the importance of the training tool for the farmers. During this festive day a large array of different stakeholders visited the farm, such as village leaders, region and district level leaders and young and older farmers

Results from the fertilizer demonstration experiment with maize at Farm for the Future Tanzania in Iringa, in 2020

In 2020, an experiment was run for the third consecutive season at the Farm for the Future Tanzania Ltd. (FFF), which is part of Ilula Orphan Program’s (IOP) Farm, Ilula, Iringa Region, in Tanzania. The FFF farm is training farmers in 16 villages with a focus on dissemination activities at regional and national levels. The purpose of the experiment is to test and demonstrate crop fertilization strategies that combine high maize yields with high nutrient use efficiency (NUE) and low greenhouse gas (GHG) emissions. Five nutrient management treatments were combined in a full factorial setup with two tillage options. Highest yields were obtained with reduced tillage combined with NPK fertilizer to target 70% of waterlimited yield (Yw) and micro-nutrients (Mg, S, Zn combined), and with half NPK fertilizer and half composted manure. The lowest maize yields were obtained from both the treatment without fertilizer application and the fertilizer treatment with only P and K applied at reduced and conventional tillage. Results showed no significant differences in both agronomic N use efficiency (N-AE, additional grain yield per kg N applied when correcting for the P and K applied) and fertilizer use efficiency (additional grain yield per kg N applied when including yield effects from P and K) between reduced and conventional tillage. N-AE obtained in the experiment of 34.0 kg yield/kg N was much higher compared to the current average N-AE in sub-Saharan Africa of 14.3 kg yield/kg N. When targeting 70% of Yw for maize, this improved N-AE value could result in 58% reduction in GHG emission per hectare (ha) from fertilizer application (direct and indirect emissions). Despite the cancellation of the farmers field days, due to the Covid19 pandemic, ten young farmers still took part in the experimental setup and trial planting

Results from the fertilizer demonstration experiment with maize at Farm for the Future Tanzania in Iringa, in 2021: Final narrative report

In 2021, an experiment was run for the fourth consecutive season at the Farm for the Future Tanzania Ltd (FFF) which is part of Ilula Orphan Program (IOP)’s Farm, Ilula, Iringa Region, in Tanzania. The FFF farm, is training farmers in 16 villages, with a focus on dissemination activities at regional, as well as national level. The purpose of the experiment is to experimentally test and demonstrate low-emission crop fertilisation strategies that combine high maize yields with high nutrient use efficiency and low emissions. Four nutrient management treatments were combined in a full factorial setup with two tillage options. Lowest yields were obtained with no fertilizer addition (control treatment) under conventional tillage and reduced tillage (1-2 t/ha), while the treatments with fertilizer addition consistently showed very high yields (close to 10 t/ha). Root number and root length were larger for reduced tillage compared to conventional tillage at the control treatment, but this difference was not significant. Root penetration resistance was significantly higher at conventional tillage compared to reduced tillage for the control treatment. Around 100 pupils from primary as well as secondary school made two field visits to the trial. Also, a farmers field day was organized, and from the sixteen villages surrounding the farm, a total of about 400 farmers attended the planting session and the field day

Grain legume production in Europe for food, feed and meat-substitution

Partial shifts from animal-based to plant-based proteins in human diets could reduce environmental pressure from food systems and serve human health. Grain legumes can play an important role here. They are one of the few agricultural commodities for which Europe is not nearly self-sufficient. Here, we assessed area expansion and yield increases needed for European self-sufficiency of faba bean, pea and soybean. We show that such production could use substantially less cropland (4–8%) and reduce GHG emissions (7–22% current meat production) when substituting for animal-derived food proteins. We discuss changes required in food and agricultural systems to make grain legumes competitive with cereals for farmers and how their cultivation can help to increase sustainability of European cropping systems.</p