Pruning and green manure from Alnus acuminata improve soil health, and potato and wheat productivity in Rwanda

Alnus acuminata Kunth. (alnus) has been widely promoted under climate change mitigation and adaptation programs in the East African highlands. However, information is lacking on the impact of its pruning management and green manure (GM) application on soil health and the productivity of associated crops. Using a large suit of soil quality indicators, crop growth and yield measurements, this study identified appropriate combinations of alnus pruning levels and GM application for sustainable intensification of potato ( Solanum tuberosum ) and wheat ( Triticum aestivum ) production. This study compared six treatments, namely, (1) sole crops of potato or wheat (control), (2) sole crop + GM, (3) 75% pruning, (4) 75% pruning + GM, (5) 90% pruning and (6) 90% pruning + GM. The best treatments were the 75% pruning + GM and 90% pruning + GM, which significantly differed from the control in soil pH, soil organic carbon (SOC), total nitrogen (TN), total phosphorus (P), exchangeable calcium (Ca) contents, the N:P and C:P stoichiometric ratios and the soil structural stability index (SSI). These treatments increased soil pH by 5–6%, SOC by 74–82%, TN by 58–68%, exchangeable Ca by 76–81%, total P by 17–21%, the N:P ratio by 40%, C:P ratio by 43–51%, the SSI by 71–79%, potato tuber yields by 17–21% and wheat yields by 21–32% over the control. Therefore, it is concluded that pruning and application of alnus green manure have the potential to significantly improve soil health and crop productivity on degraded soils in the study area.