Impact of perennial forage plants on microbial carbon use efficiency in weathered Ethiopian soils

Abstract. Aims: Ethiopian soils are highly weathered and in urgent need of remediation. Increasing the soil carbon (C) pool by perennials potentially remediates soil degradation while sequestering atmospheric CO₂. The soil microbial loop is thought to play an important role in soil C sequestration as it contributes significantly to the stabilization of plant litter C. The amount of stabilized litter C has been hypothesized to depend on microbial carbon use efficiency (mCUE), which is sensitive to soil nutrients and plant inputs. Here, we investigate the effect of two perennial forage crops on mCUE in four weathered Ethiopian soils.

Methods: We cultivated the grass Urochloa cv. Cayman as single stand and in mixture with the legume Desmodium intortum in a fully factorial greenhouse experiment with four distinct farmers’ soils from the Ethiopian Rift Valley (Amhara and Sidama). Microbial CUE was assessed after 104 days of plant growth using ¹⁸O-H₂O incorporation.

Results: Microbial CUE varied between the four soils, with Amhara soils showing lower mCUE than Sidama soils. Overall, mCUE was little affected by plant growth but increased significantly with plant dry weight (DW) in one of the two Sidama soils. Mass specific growth rates remained small in Amhara soils, indicating that factors other than plant C input inhibited microbial growth. Plant composition had no effect in Amhara soils while the Urochloa × Desmodium mixture tended to have lower mCUE in Sidama soils.

Conclusions: Perennials may improve mCUE in weathered soils, but this stimulation strongly depends on soil nutrient availability and suggests that there are critical nitrogen (N) and phosphorous (P) thresholds below under which mCUE does not respond to growing plants. This implies that highly degraded soils must be ameliorated before using perennials to stimulate nutrient cycling and restore soil health.