Scattered trees in smallholder farms improve soil properties and litter decomposition in humid-agroecosystems in Ethiopia

Low agricultural productivity associated with poor soil fertility management characterizes the sub-Sahara African agriculture. Trees in farmlands are believed to improve soil properties and agricultural productivity, yet smallholders are limited in their choice of agroforestry tree species. Here, we assessed the effect of Cordia africana and Croton macrostachyus trees on soil properties and leaf litter decomposition in parkland agroforestry in Bullen District, Benshangul Gumuz, northwestern Ethiopia. We used a randomized complete block design with a distance from a tree trunk setup to draw soil samples and conduct a litter burial experiment across five farmlands. In each farmland, three mature trees per species were identified, separately, and under each tree, three transects containing three concentric radial distances measured from a tree trunk were established. Using this setup, a total of 45 composite soil samples per the study species were drawn and analyzed for soil pH, total nitrogen, available phosphorus, organic carbon, and cation exchange capacity. Additionally, leaf litter mass losses of the study species were quantified with the litterbags method. The results revealed that all the measured soil variables and litter decomposition were significantly different between distances for both C. africana and C. macrostachyus. A significantly higher nutrient contents and litter decomposition were observed under the tree canopies than outside, and for C. macrostachyus than C. africana. We conclude that trees in farmlands might improve soil properties through litter addition and decomposition. Future studies should disentangle the role of litter addition and microclimate effects of trees in farmlands on soil properties.