Nitrification Control by Plants and Preference for Ammonium versus Nitrate: Positive Feedbacks Increase Productivity but Undermine Resilience

Abstract

Some plants, via their action on microorganisms, control soil nitrification, i.e. the transformation of ammonium into nitrate. We model how the co-variation between plant control of nitrification and preference for ammonium vs. nitrate impacts ecosystem properties such as productivity, nitrogen (N) losses and overall resilience. We show that the control of nitrification can maximize productivity by minimizing total inorganic N losses. We initially predicted that plants with an ammonium preference should achieve the highest biomass when inhibiting nitrification; and conversely that plants preferring nitrate should achieve the highest biomass by stimulating nitrification. With a parametrization derived from the Lamto savanna (Ivory Coast), we find that productivity is maximal for plants that slightly prefer ammonium and inhibit nitrification.Such situations however lead to strong positive feedbacks that can cause abrupt shifts from a highly to a lowly productive ecosystem. The comparison with other parameter sets (Pawnee short-grass prairie (USA), intensively cultivated field, and a hypothetical parameter set in which ammonium is highly volatilized and nitrate inputs are high) shows that strategies yielding the highest biomass may be counter-intuitive (i.e. preferring nitrate but inhibiting nitrification). We argue that the level of control yielding the highest productivity depends on ecosystem properties (quantity of N deposition, leaching rates and baseline nitrification rates), not only preference.Finally, while contrasting N preferences offer, as expected, the possibility of coexistence through niche partitioning, we stress how control of nitrification can be framed as a niche construction process that adds an additional dimension to coexistence conditions.