Impact of urea fertilization rates on nitrogen losses, productivity and profitability in East African sugarcane plantations

Fertilizer-intensive sugarcane plantations are expanding in sub-Saharan Africa (SSA) amidst increased groundwater pollution and carbon footprint concerns. Yet, the impact of nitrogen (N) levels on N losses, productivity and profitability in these plantations remains unclear. To address this gap, we conducted a completely randomized design experiment in a Ugandan sugarcane plantation using three N fertilization rates (low, standard and high) as treatments. N leaching under the different treatments was determined using the average drainage fluxes across a 1-m-layered profile which we estimated with a suite of pedotransfer functions (PTFs) and leachate N concentrations from suction cup lysimeters. Soil nitrous oxide fluxes were determined using static vented chamber bases and gas chromatography. Partial factor crop productivity was estimated from the average field fresh weight under each treatment and the amount of N fertilizer applied, while the return on investment was determined from the factory price of the field fresh weight and the market price of fertilizers. Our findings indicate that three out of five PTFs effectively estimated soil hydraulic properties at our test site, based on the close match between measured and predicted soil matric potential values. Notably, N leaching at low and standard N rates were comparable but significantly lower than at higher-than-standard N rates. Additionally, we measured comparable soil nitrous oxide emissions and field fresh weight but partial factor productivity and return on investment declined along the fertilizer intensification gradient. In conclusion, the study demonstrates the promising application of certain PTFs in N-leaching modelling in the data-scarce SSA. Furthermore, obtaining comparable field fresh weight with minimal N losses at lower-than-standard N rates presents an opportunity to mitigate groundwater pollution and greenhouse gas emissions. However, the potential impact of the switch from standard to low N rates on soil organic carbon stocks and sugarcane yields warrants further investigation.