A framework to parameterise nutrient management options and their impacts on maize yield, nutrient losses and long term soil fertility

Climate Smart Agriculture (CSA) encompasses all practices that mitigate climate change and at the same time enhance farmers’ livelihoods and adaptation to changing climate. Nutrient management is an important tool for farmers to increase yields. Efficient use of nutrients is necessary to limit greenhouse gas (GHG) emissions and costs of external inputs. To enable an overall assessment of the opportunities that nutrient management presents for CSA, nutrient cycles and dynamics need to be thoroughly understood. This report describes a framework to parametrise the impacts of nutrient inputs on the nutrient balance, crop yield and soil fertility, both in the short and the long term. Separating short term versus long term processes and internal crop processes versus crop-soil interactions results in four sets of equations that are mutually consistent. These can be viewed as alternative approaches to quantify nutrient requirements and associated impacts, depending on purpose of a study. Set 1 expresses nutrient uptake requirement as a function of target yield, both in short and long term. Set 2 expresses input requirements as function of target uptake, soil nutrient supply and fertiliser recovery in the short term. Set 3 is similar in structure and purpose to Set 2, but includes a long term feedback of nutrient input on fertiliser recovery via soil fertility. Set 4 is a reduction of Set 3, under a specific boundary condition imposing ‘efficient management’ in a simplified manner. The full sequence of relating a target yield to input requirement (or vice versa) is expressed by combinations of these sets: Sets 1 and 2 (short term), Sets 1 and 3 (long term equilibrium, generic), and Sets 1 and 4 (long term equilibrium, efficient management). The processes expressed by the sets of equations are governed by parameters that depend on management options (e.g. 4Rs: right source, rate, timing, placement), and have a direct bearing on overall nutrient use efficiency as expressed in agronomic N use efficiency (ANE, kg extra grain yield per kg N input), and associated impacts. The framework presented in this report is applied in other components of the Crop Nutrient Gap Project (Bringing CSA practices to scale), notably in (i) presenting spatial maps of nutrient requirements for different levels of yield gap closure in sub-Saharan Africa (www.yieldgap.org), (ii) assessing GHG emissions for contrasting intensification scenarios; and (iii) assessing trade-offs between food security, farm income and climate change mitigation.