Effect of perennial forage species on soil microbial nutrient cycling in Ethiopian leys

Ethiopian agriculture is under severe pressure due to erosion and degradation of arable soils. These problems are exacerbated by high livestock numbers in small-holder farming, leading to intense grazing on limited communal pastures and on crop residues. Introducing leys of perennial forage species (grasses and legumes) into soils predominately used for cereal cropping could help restore degraded soils while simultaneously providing high quality feed for livestock. To optimize perennial species selection for different soils, we studied microbial nutrient cycling responses to perennial plant inputs in six contrasting soils in Ethiopia. Two grasses, Urochloa hybrid Cayman and Megathyrsus maximus, and two legumes, Desmodium intortum and Stylosanthes guianensis, were sown in varying mixtures at three field sites in two different regions. To assess how soil microbial nutrient stoichiometry and nutrient demand changed with plant cover, we measured soil exoenzyme activity, soil microbial biomass, C, N and P stoichiometry and nitrification potentials before and after the 1.5-year field experiments. Changes in microbial nutrient limitation in response to species ratios were estimated by a combined Vector and Threshold Element Ratio model. We found variable responses for the different soils, with the largest differences between the two regions. Across all fields we saw that P-limitation of microbes decreased with increasing ratios of legumes in the perennial mixtures. We conclude that increased legume incorporation reduces P-limitation and positively affects nutrient cycling in Ethiopian soils.