Assessing nitrogen flows and greenhouse gas emissions in low input cropping systems of lake Victoria basin
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Too little nitrogen (N) is a threat to cropping systems and soil fertility in sub-Saharan Africa (SSA). Nitrogen budgets (NB) and nitrogen use efficiency (NUE) are critical tools for assessing N dynamics in agriculture and have received little or no attention in the region. The study aimed to assess key N performance indicators at field and farm level based on N use scenarios and helps to inform policy on formulating relevant decision and practices to enhance N management for improved crop productivity and environmental sustainability. To achieve this, data were collected from farmers’ fields and clustered into two categories, farmers applying and farmers not applying N fertilizers. Collected data on yield and N fertilizer were used to make projections on the changes of NUE based on scientific and policy recommendations for Sub-Saharan Africa for 2020 (base year), 2025, 2030, and 2050. Scenarios of N use were simulated under field conditions for maize crop in two catchments of the Lake Victoria basin. The scenarios included Business as Usual (BAU, 0 kg N ha-1), 25 % of the Abuja declaration (ADS 12.5 kg N ha-1), 50% of the Abuja declaration (ADS 25 kg N ha-1), and Abuja declaration-Abuja scenario (ADS, 50 kg N ha-1). A laboratory incubation experiment to assess the effects of two N use scenarios (12.5 and 50 kg N ha-1) and liming practices (CaCO3) on three acidic soils types from Lake Victoria basin on greenhouses gases was conducted. Data were analyzed using R programming language version 4.1.0. The results revealed negative N balances in different fields and farms. Similarly, at the farm gate, a deficit of -78.37 kg N ha -1 was observed; an indicator of soil N mining. Significant differences in maize grain yield for both fertilized and unfertilized farms were realized with very low yields of 2.4 t ha-1 (fertilized) and 1.4 tha-1 (unfertilized). The graphical representation of NUE of both maize and rice showed that most farmers were in the zone of soil N mining. Projected results showed that most maize farmers within Lake Victoria region will continue to experience NUE values >90%, low N inputs <50 kg N ha-1) and less than 5 t ha-1 maize crop yield over the years. For rice farmers, Nyando and Nzoia catchments had surpassed the set target of both yield (6 t ha-1) and N input (50 kg N ha-1). The results revealed that increasing N input levels significantly influenced the growth and yield of maize crops. N deficits were observed in all the N use scenarios with a range of -66.6 to -125.7 kg N ha-1 in Nyando and -62.5 to -105.4 kg N ha-1 in Rangwe catchments with 50 % ADS scenario having the highest deficits. The partial N balance observed at Nyando (1.56 - 3.11) and Rangwe (1.10- 4.64) were higher than the optimal values, a sign of insufficient N inputs and possible risk of soil N depletion in all the scenarios. The values of δ13C of CO2 showed that the source of CO2 from soils was negligible and the largest contributor was from application of CaCO3 followed by N fertilization in Ferralsol 1 and 2 while no visible differences observed in the Vertisol. N2O-N fluxes were highly influenced by N fertilization and CaCO3 in Ferralsol 1 and 2 which was 13 (81.73 mg m-2) and 4 (38.80 mg m-2) times higher when compared to unfertilized and unlimed control. The study concludes that Lake Victoria basin faces an extreme depletion of available N resources, the recommended nutrient level; at 50 kg N ha-1 is not sufficient to correct the negative balances, while managing acidity and soil fertility will lead to higher GHG fluxes but increase N availability.