Combined application of organic and mineral fertilizers has been proposed as a measure for sustainable yield intensification and mitigation of greenhouse gas (GHG) emissions. However, fertilizer effects strongly depend on the soil type and still no precise information is available for Nitisols in Ethiopia. The study evaluated effects of different ratios of biowaste compost and mineral fertilizers (consisting of nitrogen (N), phosphorus (P), and sulphur (S)) on maize (Zea mays L. Bako-hybrid) yields in a two-year field trial. Soil samples from each treatment of the field trial were used to estimate emissions of nitrous oxide (N₂O), carbon dioxide (CO₂), methane (CH₄), and microbial activity in a 28-day incubation experiment with two moisture levels (40% and 75% water-filled pore space, WFPS). The application of fertilizers corresponded to a N supply of about 100 kg ha⁻¹, whereby the pure application of mineral fertilizers (100 min) was gradually replaced by compost. Maize yields were increased by 12 to 18% (p < 0.05) in the combined treatments of compost and mineral fertilizers compared to the 100 min treatment. The cumulative emissions of N₂O and CO₂ but not CH₄ were affected by the fertilizer treatments and soil moisture levels (p < 0.05). At 75% WFPS, the N₂O emissions in the 100 min treatment was with 16.3 g ha⁻¹ more than twice as high as the treatment with 100% compost (6.4 g ha⁻¹) and also considerably higher than in the 50% compost treatment (9.4 g ha⁻¹). The results suggest that a compost application accounting for 40 to 70% of the N supply in the fertilizer combinations can be suitable to increase maize yields as well as to mitigate GHG emissions from Nitisols in Southwestern Ethiopia.

The fraction of absorbed photosynthetic active radiation (FAPAR) is an essential climate variable for assessing the productivity of ecosystems. Satellite remote sensing provides spatially distributed FAPAR products, but their accurate and efficient validation is challenging in forest environments. As the FAPAR is linked to the canopy structure, it may be approximated by the fractional vegetation cover (FCOVER) under the assumption that incoming radiation is either absorbed or passed through gaps in the canopy. With FCOVER being easier to retrieve, FAPAR validation activities could benefit from a priori information on FCOVER. Spatially distributed FCOVER is available from satellite remote sensing or can be retrieved from imagery of Unmanned Aerial Vehicles (UAVs) at a centimetric resolution. We investigated remote sensing-derived FCOVER as a proxy for in situ FAPAR in a dense mixed-coniferous forest, considering both absolute values and spatiotemporal variability. Therefore, direct FAPAR measurements, acquired with a Wireless Sensor Network, were related to FCOVER derived from UAV and Sentinel-2 (S2) imagery at different seasons. The results indicated that spatially aggregated UAV-derived FCOVER was close (RMSE = 0.02) to in situ FAPAR during the peak vegetation period when the canopy was almost closed. The S2 FCOVER product underestimated both the in situ FAPAR and UAV-derived FCOVER (RMSE > 0.3), which we attributed to the generic nature of the retrieval algorithm and the coarser resolution of the product. We concluded that UAV-derived FCOVER may be used as a proxy for direct FAPAR measurements in dense canopies. As another key finding, the spatial variability of the FCOVER consistently surpassed that of the in situ FAPAR, which was also well-reflected in the S2 FAPAR and FCOVER products. We recommend integrating this experimental finding as consistency criteria in the context of ECV quality assessments. To facilitate the FAPAR sampling activities, we further suggest assessing the spatial variability of UAV-derived FCOVER to benchmark sampling sizes for in situ FAPAR measurements. Finally, our study contributes to refining the FAPAR sampling protocols needed for the validation and improvement of FAPAR estimates in forest environments.

Proper fertilizer management and applications could effectively reduce global warming potential (GWP) and improving soil fertility under cereal production. However, the effect of soil fertilizer management practices on GWP and soil fertility is less understood in the agricultural soils of Ethiopia. The present study evaluated the effects of fertilizer application strategy on GWP, and soil fertility in a Nitisol. Both field and laboratory incubation experiments were conducted with the same treatments. Seven treatments (Cont: no input; 100 min: 100% mineral fertilizer, 80 min: 80% mineral fertilizer + 1.4 t ha⁻¹ compost; 60 min: 60% mineral fertilizer + 2.8 t ha⁻¹ compost; 50 min: 50% mineral fertilizer + 3.5 t ha⁻¹ compost; 30 min: 30% mineral fertilizer + 4.9 t ha⁻¹ compost, and 100 comp: 100% compost) with four replications were applied on maize crop (Zea mays L. Bako hybrid 661) for two consecutive growing seasons. The laboratory incubation experiment was also carried out with two moisture levels (40% and 75% of water-filled pore space) to simulate the seasonal rainfall pattern. GWP was calculated by summing up the quantified gas emissions of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄). The laboratory result for GWP shown that soil fertilized with mineral fertilizer alone was significantly (p < 0.05) increased by 27–34% of the average GWP compared to combined fertilizer treatments in the soil at a moisture level of 75%. From the field experiment, most plant nutrients were significantly increased in combined fertilizer treatments compared to sole mineral fertilizer application. For example, carbon, calcium, magnesium and potassium were increased by 26.2–39.8%, 73.2–168.8%, 146.6–251.5% and 47–99% respectively in combined fertilizer compared to 100 min treatment. The study revealed that combining 30 or 50 kg N ha⁻¹ of mineral fertilizer with biowaste compost (4.9 or 3.5 t ha⁻¹) would be a suitable combination to mitigate the GWP and improve soil quality in smallholder farming systems, due to a slow release of N during decomposition into the soil compared to mineral fertilizer alone. However, to evaluate GWP under the field conditions, future investigations are recommended.