Anthropogenic activities have drastically altered the global nitrogen cycle, leading to a substantial increase in atmospheric N₂O concentrations. While synthetic fertilizer use is a major contributor, manure management also plays a crucial role. Africa possesses a large portion of the global livestock population, with most animals in semi-arid and arid regions. These livestock are typically kept in temporary enclosures or bomas overnight. Manure isn't widely used as fertilizer, accumulating in bomas which are subsequently abandoned. The study investigates the hypothesis that these abandoned bomas serve as significant hotspots for N₂O emissions. This research is important because it addresses the significant knowledge gap concerning the contribution of abandoned livestock enclosures to atmospheric N₂O emissions, a crucial greenhouse gas, and may lead to better strategies for mitigation and sustainable land management in Sub-Saharan Africa.

Previous studies have highlighted the role of abandoned bomas as soil fertility hotspots, impacting landscape diversity and ecosystem dynamics. These bomas, rich in carbon, nitrogen, and phosphorus, support increased plant biomass. However, their potential as N₂O emission hotspots remained unexplored. Existing literature demonstrates a positive correlation between N₂O emissions and soil nitrogen availability across various systems and climates. This study builds upon these existing findings by directly investigating N₂O emissions from abandoned bomas in a Sub-Saharan African context, a region with a unique combination of pastoralist practices and environmental conditions that are not well represented in existing literature regarding N2O emissions.

This study measured N₂O fluxes from 46 abandoned bomas and 22 adjacent savanna sites in Kenya using the fast-box chamber method. Measurements were taken across seven locations, spanning a boma age gradient (0.1–40 years). Soil temperature, gravimetric water content, and soil C and N content were also assessed. To account for potential biases, diurnal patterns and short-term rainfall effects on N₂O fluxes were investigated through experiments simulating rainfall events. Data analysis included comparing N₂O fluxes between bomas and savanna sites, analyzing the relationship between N₂O fluxes and boma age, and exploring the influence of various environmental factors (soil moisture, temperature, organic layer thickness, vegetation cover). Statistical analyses like Welch's t-test and multiple regression models were employed. Finally, to extrapolate the findings to a continental scale, a spatial upscaling approach was developed incorporating various parameters like livestock numbers, boma area per livestock unit, boma use duration, and manure extraction rates. Monte Carlo simulations were used to assess the uncertainty in the upscaling process.

Abandoned bomas consistently exhibited significantly higher N₂O fluxes than adjacent savanna sites, maintaining this elevated level for at least four decades. N₂O fluxes showed an exponential decline with increasing time since abandonment, correlated with decreasing soil N content. Simulated rainfall events significantly increased N₂O fluxes in bomas but not in savanna sites. Diurnal variations revealed higher afternoon fluxes compared to morning. Upscaling estimations indicate that abandoned bomas in semi-arid and arid SSA release approximately 50.2 Gg N₂O annually (25–75th percentile: 29.5–70.6 Gg N₂O yr⁻¹), contributing ~5% of total anthropogenic N₂O emissions for the continent. This is considerably higher than estimates from manure management in existing databases.

The findings underscore the substantial contribution of abandoned bomas to Africa's N₂O emissions, a source previously underestimated. The significant difference in N₂O emissions between bomas and savannas highlights the long-lasting impact of livestock enclosure practices on soil N cycling. The exponential decline in N₂O fluxes with boma age indicates a progressive decrease in nitrogen availability in the soil. The rainfall simulation experiment shows that the N2O pulse from bomas after rainfall needs to be considered in future work. The upscaling exercise provides crucial evidence to support the argument for the inclusion of abandoned bomas in future emission inventories, potentially refining current estimates of Africa’s contribution to global N₂O emissions. These findings have implications for land management strategies and call for improved manure management practices in pastoralist systems.

This study reveals a significant and previously unaccounted-for source of N₂O emissions in Sub-Saharan Africa: abandoned livestock bomas. These contribute substantially to the continent's total anthropogenic N₂O emissions. The results highlight the need for refined methodologies for estimating N₂O emissions from pastoralist systems, incorporating the contribution of abandoned bomas. Further research should explore indirect N₂O emissions from bomas via hydrological and atmospheric pathways. Simple yet effective manure management strategies are suggested to mitigate N₂O emissions and enhance rangeland productivity.

The study focuses solely on abandoned bomas in Kenya, and the extrapolation to the entire SSA relies on various assumptions regarding livestock numbers, boma area, boma use duration, and manure extraction rates, introducing uncertainties into the upscaling process. The study primarily focuses on direct N₂O emissions, neglecting indirect emissions through leaching and volatilization, which may further contribute to the overall N₂O budget. While diurnal variation in N₂O emissions was investigated, it was limited to one site, and more extensive studies are needed for comprehensive understanding of temporal variations. Finally, this study considers mainly abandoned bomas, neglecting the direct N2O emissions from active bomas.