Restoring degraded savannas: a case study of soil carbon and nutrient dynamics in Kenya's Maasai Mara.

Savanna ecosystems cover about 50% of Africa, forming a spectrum from grasslands with scattered trees to bush thickets. These ecosystems are shaped by water availability, seasonal rainfall patterns, grazing by wild and domestic animals, and soil and landform features that influence nutrient and water distribution. The Greater Maasai Mara Ecosystem (Mara) in Kenya is a prime example of such an ecosystem, rich in biodiversity but increasingly degraded by invasive species expansion, climate change and agricultural activities. Degradation processes such as reduced vegetation cover due to overgrazing from livestock herds, leads to soil exposure, increased soil erosion and decreased organic matter inputs, and subsequent impacts on nutrient cycling and biotic activity.

In response to soil degradation in the Mara, afforestation using native plant communities has been proposed as a restoration strategy. Research has focused on identifying optimal tree species for soil restoration through experimental treatments. Three 50 m x 50 m treatment plots were established: Savanna Mimic, reflecting native woodland patterns; Diversity-Rich, with 30 tree species; and Themed Species Assortments, focusing on bioculturally relevant species. A fourth control plot outside the fenced area represented degraded conditions.

Soil samples collected from the plots revealed significant insights. The control plot exhibited the lowest soil organic carbon (SOC) stock, about 40% lower than the treatments. This decline in SOC was linked to overgrazing, which limits plant growth and rhizodeposition, reducing carbon inputs to the soil. The control area also had the highest Carbon Quality Index (CQI = 0.64), indicating highly resistant organic matter, and a high dsDNA:SOC ratio (4.20 mg g^–1), suggesting microbial communities are under stress due to limited organic carbon.

The Diversity-Rich plot emerged as the most effective restoration strategy, promoting SOC accumulation with higher carbon quality (CQI = 0.57). This treatment avoided nitrogen (N) and phosphorus (P) limitations, with the highest ratios of microbial C:N, N:P and C:P acquiring enzymes and available P concentration (18.7 mg kg^–1). Enhanced nutrient cycling in the Diversity-Rich treatment highlights the role of plant diversity in restoring degraded soils.

Overall, the study demonstrates that overgrazing significantly depletes SOC and disrupts ecosystem functions. High-diversity planting schemes offer the most promising outcomes for soil restoration, enhancing carbon storage, nutrient availability, and microbial activity, thus contributing to the resilience of savanna ecosystems. However, we are aware that further monitoring activities should be carried out to have a more reliable picture about the effect of the applied restoration on the investigated soil properties.