Spatial heterogeneity of nitrification contributes to tree-grass coexistence in West African savannas

1. In savannas, the coexistence between trees and grasses is determined by complex mechanisms based on water partitioning and disturbances. But little is known about the contribution of other resources, such as soil nitrogen (N). In West African savannas, nitrification inhibition by grasses and nitrification stimulation by trees create spatial heterogeneity in nitrification fluxes and N stocks. Savanna trees can also extend part of their roots in the surrounding open area to absorb N. 2. To investigate the role of the spatial heterogeneity of nitrification in tree-grass coexistence, we used a two-patch model that simulates N dynamics between an open patch (without trees) and a tree clump patch (trees with grasses under their canopy). The open patch was characterized by a low nitrification rate, while the tree clump patch was characterized by a high nitrification rate. Both patches were connected through horizontal fluxes due to soil horizontal exploration by tree roots. We tested coexistence for different spatial tree distributions, as they are known to strongly influence savanna dynamics. 3. Our results show that the spatial heterogeneity of nitrification induces spatial partitioning between ammonium (NH4+) and nitrate (NO3-) promoting tree-grass coexistence. As nitrification inhibition by grasses leads to high NH4+ availability in the open, the possibilities of coexistence are optimized when trees have different preferences in the open versus under their canopy. Thus, tree-grass coexistence is observed when grasses prefer NH4+, while trees prefer NH4+ in the open and NO3- under their canopy. 4. Contrary to random tree distribution, tree clumping enhances tree-grass coexistence. Intraspecific aggregation strengthens the effect of spatial heterogeneity, which decreases interspecific competition and favours tree-grass coexistence. On the contrary, increasing the surface explored by tree roots in the open tends to increase tree-grass competition. This enhances the competitive ability of trees for N acquisition and consequently favours tree invasion. 5. Synthesis. This study shows that this new coexistence mechanism based on mineral N partitioning into NH4+ and NO3- can be determinant in the functioning of West African humid savannas. This mechanism likely interacts with mechanisms based on disturbances, but such interactions should be studied using new models.