Abraham Joel

Soil erosion is a major environmental problem driven by land use change and unsustainable agricultural practices, particularly in Rwanda, where half of agricultural land is moderately to severely degraded. This study assessed soil loss in the Akanyaru Sub-catchment using the Revised Universal Soil Loss Equation (RUSLE) integrated with GIS and validated with field measurements from 876 rainfall events recorded on 12 runoff plots representing four land-use types. The model incorporated rainfall erosivity (R), soil erodibility (K), slope length and steepness (LS), cover management (C) for 2024 and 2025, and a constant conservation practice (P) factor. R values ranged from 1100 to 2400 MJ mm ha⁻¹ h⁻¹ yr⁻¹, K from 0.20 to 0.35 t h MJ⁻¹ mm⁻¹, LS from 0 to 256, and C from 0.01 to 0.99. Estimated annual soil loss ranged from 0 to 650 t ha⁻¹ yr⁻¹. From 2024 to 2025, mean and median soil erosion rates decreased by 5.21 and 18.7 t ha⁻¹ yr⁻¹, respectively, and total annual soil loss declined by 812,654 t (11.45%). Model validation showed strong agreement with field observations (R² = 0.97, NSE = 0.95, nRMSE = 26.32%, PBIAS = −12.47%), demonstrating the reliability of the RUSLE–GIS approach for soil erosion assessment and conservation planning.

Identifying the main drivers of soil CO₂ emissions in tropical agroecosystems is essential for balancing productivity and climate mitigation. This study evaluated the effects of crop type, irrigation, phenological stage, fertilization, soil cover condition, and season on total soil respiration in a humid marshland system in Rwanda using a two-season field experiment. Five crops (maize, soybean, common bean, Irish potato, and Brachiaria) were grown under irrigated and rainfed conditions, and total soil CO₂ emissions were measured across 19 sampling campaigns in both crop-covered and adjacent non-vegetated conditions in all plots using the closed static chamber method. Crop type and growth stage were the dominant drivers of soil CO₂ emissions (p < 0.001), while irrigation had no significant direct effect despite increasing yields (p < 0.001). As a result, irrigation reduced yield-scaled CO₂ emissions for beans and Irish potato (p < 0.05). Brachiaria showed higher emissions, particularly during the development stage, but its high biomass led to lower emissions per unit yield. Fertilization significantly increased total soil respiration (p < 0.001), and emissions were higher under crop-covered soil than non-vegetated soil conditions (p < 0.001). Season did not significantly affect soil CO₂ emissions (p = 0.123), and similar emission patterns were observed across the two cropping seasons. Because the measurements represented total soil respiration, the observed differences reflect the combined contributions of autotrophic (root) and heterotrophic (microbial) respiration and do not distinguish between these individual components. These findings indicate that crop traits, plant developmental stage, vegetation cover, and nutrient inputs are the primary factors associated with variation in total soil CO₂ emissions under moisture-sufficient tropical conditions and highlight the importance of biological drivers in regulating carbon dynamics in marshland agroecosystems.

Identifying the main drivers of soil CO₂ emissions in tropical agroecosystems is essential for balancing productivity and climate mitigation. This study evaluated the effects of crop type, irrigation, phenological stage, and fertilization on soil respiration in a humid marshland system in Rwanda using a two-season field experiment. Five crops (maize, soybean, common bean, Irish potato, and Brachiaria) were grown under irrigated and rainfed conditions, and soil CO₂ emissions were measured across 19 sampling campaigns in both crop-covered and adjacent bare soil conditions in all plots. Crop type and growth stage were the dominant drivers of soil CO₂ emissions (p < 0.001), while irrigation had no significant direct effect despite increasing yields (p < 0.001). As a result, irrigation reduced yield-scaled CO₂ emissions for several crops (p < 0.05–0.01). Brachiaria showed higher emissions, particularly during the development stage, but its high bio-mass led to lower emissions per unit yield. Fertilization significantly increased soil respiration (p < 0.001), and emissions were higher under crop-covered soils than bare soils (p < 0.001). These findings indicate that crop traits and nutrient inputs primarily control soil respiration under moisture-sufficient tropical conditions.

Few studies have explored greenhouse gas (GHG) emissions from arable land in sub-Saharan Africa (SSA), and particularly from rice paddy fields, which can be a major source of methane (CH₄) and nitrous oxide (N₂O) emissions. This study examined the effect of drainage on CH₄ and N₂O emissions from rice fields in Rwanda under shallow drainage to 0.6 m, with the drain weir open four times per week, and deep drainage to 1.2 m with the weir open four times or two times per week. CH₄ and N₂O fluxes from the soil surface were measured on nine occasions during rice flowering and ripening, using a closed chamber method. Measured fluxes made only a minor contribution to total GHG emissions from rice fields. However, drainage depth had significant effects on CH₄ emissions, with shallow drainage treatment giving significantly higher emissions (~0.8 kg ha⁻¹ or ~26 kg CO₂-equivalents ha⁻¹) than deep drainage (0.0 kg) over the 44-day measurement period. No treatment effect was observed for N₂O fluxes, which ranged from low uptake to low release, and were generally not significantly different from zero, probably due to low nitrogen (N) availability in soil resulting from low N fertilization rate (in the region). Overall, the results suggest that deep drainage can mitigate CH₄ emissions compared with traditional shallow drainage, while not simultaneously increasing N₂O emissions.

Changes in pollution pressure exerted on the Rocha River in Bolivia from diffuse sources were assessed using potential non-point pollution indexes (PNPI) for 1997 and 2017. PNPI is a simple, low-effort, time- and resource-saving method suitable for data-scarce regions, as it works at catchment level with commonly available geographical data. Land use type (obtained by Landsat imagery classification), runoff (determined by runoff coefficient characterisation) and distance to river network (calculated at perpendicular distance) were each transformed into corresponding indicators to determine their relative importance in generating pollution. Weighted sum, a multi-criteria analysis tool in the GIS environment, was used to combine indicators with weighting values. Different weighting values were assigned to each of the indicators resulting in a set of six equations. The results showed that higher PNPI values corresponded to human settlements with high population density, higher runoff values and shorter distance to river network, while lower PNPI values corresponded to semi-natural land use type, lower runoff coefficient and longer distances to river. PNPI values were positively correlated with measured nitrate and phosphate concentrations at six sub-catchment outlets. The correlation was statistical significant for phosphate in 2017. Maps were produced to identify priority source areas that are more likely to generate pollution, which is important information for future management.

Maintenance of agricultural drainage ditches can be difficult to optimize if farmers have no guidelines on where to target their maintenance efforts. A main concern is whether ditch banks will experience soil erosion or mass movement (failure). In order to help identify sites that are more likely to experience soil erosion and/or mass movement, soil susceptibility to detachment was assessed in this study using a cohesive strength meter (CSM) and measurements of shear strength in unsaturated direct shear tests. The results showed that soil roots play an important role in stabilizing ditch banks against mass movement and in reducing the rate of soil detachment. A positive stabilizing effect was detected by CSM and confirmed by shear strength measurements. The conclusion is that native vegetation should be maintained on ditch banks, instead of being removed during maintenance work as is currently the case.

Drainage management is important in intensification of irrigated paddy rice production. This study assessed the effects of drainage intensity on water and nitrogen use efficiency and rice grain yield in a field experiment conducted during three seasons in Rwanda. The experiment comprised 12 plots with four blocks and three treatments: DS_0.6 (0.6 m deep drain), DD1_1.2 (1.2 m deep drain, control structure open four times per week), and DD2_1.2 (1.2 m deep drain, control structure open two times per week). Outflow was calculated from water balance. Nitrogen (N) content in drainage water was determined weekly. Crop yield and N uptake were determined in grain and straw. In all seasons, grain yield was 61–131% higher, crop N uptake was 24–90% higher, harvest index (HI) was 24–65% higher and water use efficiency (WUE) was 50–150% higher in treatments DD1_1.2 and DD2_1.2 than in DS_0.6. There was a decrease in soil carbon/nitrogen ratio at the end of Seasons 2 and 3. Recirculating straw to fields is thus necessary to replenish SOC for long-term soil fertility. A practical implication of the study is that managed deep drainage systems could enhance water use efficiency and rice grain yield in poorly drained paddy fields.

Increasing pressures caused by human activities pose a major threat to water availability and quality worldwide. Water resources have been declining in many catchments during recent decades. This study investigated patterns of river water quality status in a peri-urban/rural catchment in Bolivia in relation to land use during a 26 year period. Satellite images were used to determine changes in land use. To assess water quality, data in the dry season from former studies (1991–2014), complemented with newly collected data (2017), were analysed using the National Sanitation Foundation-Water Quality Index method and the Implicit Pollution Index method. The highest rates of relative increase in land use area were observed for forest, urban, and peri-urban areas, whereas relative decreases were observed for water infiltration zones, bare soil, shrubland, and grassland areas. The water quality indices revealed clear water quality deterioration over time, and from catchment headwaters to outlet. Statistical analyses revealed a significant relationship between decreasing water quality and urban expansion. These results demonstrate the need for an effective control programme, preferably based on water quality index approaches as in the present study and including continuous monitoring of runoff water, mitigation of pollution, and water quality restoration, in order to achieve proper water management and quality.

Poor maintenance of drainage systems has been identified as one of Swedish agriculture's weak points, as impaired land drainage can lead to major environmental problems. Drain maintenance work is receiving increasing attention in work conducted within the European Union Water Framework Directive. Moreover, the functionality and maintenance of agricultural ditches will need to be improved to cope with predicted higher pressures on water infrastructure in a future climate. In Sweden, ditch maintenance work is mandatory, but must be carried out with great consideration for the environment. There is currently no simple method available in Sweden for assessing the comprehensive status of open drainage ditches and determining whether remedial measures are needed. This study tested an on-site method, Minnesota Agricultural Ditch Research Assessment for Stability, for evaluating the status of drains and the need for improvements. The study also included an evaluation of physical ditch properties that can be assessed visually on-site and the processes they represent. The results showed that it is viable to evaluate ditch properties such as bank stability, erosion and deposition susceptibility visually on-site. However, more intensive surveys are needed to identify the processes affecting bank stability. For assessing how improvement of drain status affects bank stability and hydraulic properties, Hydrologic Engineering Centre's River Analysis System simulations can be a useful tool when complemented with updated cross-section measurements and existing documentation.