Modelling approach to analyse the effects of nitrification inhibition on primary production

Wet tropical savannas have high grass productivity despite the fact that nitrogen is generally limiting for primary production and soil nutrient content is typically very low. Nitrogen recycling, and especially nitrification, is supposed to be a strong determinant of the balance between conservation and loss of nutrients at the ecosystem level. The important primary production observed in wet tropical savannas might be due to a tight nutrient cycling and the fact that some grass species inhibit soil nitrification. Using a general theoretical ecosystem model taking both nitrate and ammonium into account, we investigate analytically, using a four-compartment-differential-equation system the general conditions under which nitrification inhibition enhances primary production. We then estimate the quantitative impact of such a mechanism on the dynamics and budget of nitrogen in a well-documented ecosystem, the Lamto savanna (Ivory Coast). This ecosystem is dominated by the grass Hyparrhenia diplandra, which drastically reduces nitrification in the whole savanna except for a small zone. While this small zone supports a lower grass primary production, nitrification is higher, most likely due to the presence of another genotype of H. diplandra, which has no effect on nitrification processes. Ultimately, we test whether differences in nitrification fluxes can alone explain this variation in primary production. Model analysis shows that nitrification inhibition enhances primary production only if the recycling efficiency - that is, the fraction of nitrogen passing through a compartment that stays inside the ecosystem - of ammonium is higher than the recycling efficiency of nitrate. This condition probably manifests itself in most soils as ammonium is less mobile than nitrate and is not touched by denitrification. It also depends partially on the relative affinity of plants for ammonium or nitrate. The numerical predictions for this model in the Lamto savanna show that variations in nitrification inhibition capacity may explain observed differences in primary production. In conclusion we find that nitrification inhibition is a process which probably enhances ecosystem fertility in a sustainable way, particularly in situations of high nitrate leaching and denitrification fluxes. This mechanism could explain the ecological advantage exhibited by native African grasses over indigenous grasses in South-American pastures.