Can legumes improve the productivity and resilience of semi-arid Kenyan grasslands via influence on soil processes?

Semi-arid savanna grasslands in Kenya are vital for food production and rural livelihoods, with livestock grazing accounting for more than 90% of household incomes in arid and semi-arid areas.  However grassland soils have become degraded in many areas due to vegetation loss and soil erosion, often caused by overgrazing.  Soil degradation (depletion of organic matter and nutrient levels, disrupted soil biological process, and poor structure) reduces plant productivity and resilience to extreme weather conditions such as drought. Given that drought has been increasing in severity, duration and frequency over recent decades, this has severe implications for food security across sub Saharan Africa. 

Grassland restoration often focusses on re-seeding grasses with high grazing value, but poor soil conditions may hinder successful re-vegetation. Soil processes therefore need to be restored to ensure the long-term sustainability of grazing lands. Legumes, found alongside grasses in natural grasslands, may play a key role in soil processes, particularly nutrient cycling which is likely to be important for semi-arid grassland soils as they are often highly nitrogen limited. However, while there is a significant body of research on legume-soil interactions in temperate grasslands and the tropical grasslands of Asia and Latin America, there has been little research on how the highly weathered soils and semi-arid grasslands of sub Saharan Africa function and may respond to restoration interventions.

In this mesocosm experiment, grasses were grown in native soil with and without legumes, under both droughted and well-watered conditions. This aimed to assess the impact of legumes on grassland productivity via their influence on soil processes, and whether this can mediate the effects of drought stress.

Grass biomass was higher when grasses were grown alongside native legumes than in a grass-only mix although the impact varied between grass species. This was accompanied by higher root growth and nitrogen content of plant tissue. These trends were observed in both well-watered and droughted conditions.  These findings suggest that legumes play an important role in the productivity and drought resilience of grasslands, likely by helping to mitigate nitrogen limitation. Further work is needed to test these findings on a wider range of grass and legume species and improve our understanding of the mechanisms involved.