Global Assessment of Nitrification Inhibition by tropical Grasses – GAIN-GRASS

Savannas and tropical grasslands represent about 25% of terrestrial ecosystems. In humid savannas, primary productivity can be as high as in tropical rain forests, although savannas are extremely constrained by fire, herbivory, rainfall seasonality and nutrient-poor soils. Such paradoxically high productivity may be partly due to the capacity of perennial grasses (Poaceae) to inhibit nitrification (actually its first step: oxidation of ammonia to nitrite), a process called Biological Nitrification Inhibition (BNI). By limiting the production and thus possible losses of nitrate, BNI leads to more nitrogen (N)-conservative ecosystems. So far, it is not known (i) how common such a capacity is among tropical grasses across the globe, and (ii) whether BNI only inhibits bacterial ammonia oxidizers or also archaeal ammonia oxidizers, and how BNI affects nitrifiers at the activity, abundance and diversity levels. Further, (iii) by limiting nitrate production, BNI could also decrease denitrification and N2O emissions, a potent greenhouse gas, but this remains to be studied. Last, (iv) whether BNI abilities of tropical grasses could be used in agroecology approaches and allow a better maintenance of soil N fertility under low fertilizer inputs in tropical agroecosystems remains to be explored.

Our project thus aims at answering the following questions: (1) What is the occurrence of BNI ability among tropical savanna grasses across the globe? Is this ability correlated to particular environmental constraints? (2) What are the mechanisms underlying BNI, i.e. its impacts on soil microbial communities, in particular nitrifier groups, and BNI feedbacks to plants? (3) Can BNI-grasses be used as cover crop to increase agriculture sustainability? (4) What is the impact of BNI on the N budget of tropical (agro)ecosystems?

The project is based on collaborations already initiated through two international consortia (an International project on savannas funded by the Belmont Forum, and the BNI International Consortium) led by two of the project partners. It combines multiple disciplines (savanna ecology, plant physiology, microbial ecology, agroecology and agronomy, biogeochemistry) and several approaches: the in situ and common garden screening of BNI capacities of savanna grasses; the analysis in greenhouse experiments of the mechanisms underlying BNI in terms of plant-soil microorganisms relationship; an agroecological proof-of-concept experiment using perennial BNI-grasses as cover crop; and the modelling of the role of BNI on N dynamics in tropical (agro)ecosystems. Importantly, the complementary skills of project partners and complementary approaches are well integrated within the project.

The expected impacts of the project range from (1) a strong academic impact about the relationships between BNI-grasses, soil microbial communities and N cycling in tropical savannas of the world (i.e. breakthrough concerning the understanding of the environmental drivers favorable to and mechanisms underlying BNI ability) to (2) the assessment of the interest of grasses with high BNI capacities to the design of innovative, N-conservative agricultural systems (breakthrough for agroecology which could have major societal outcomes on a longer term).

The success of the project will be maximized by the fact that (1) it gathers key expertise (the project PI has discovered BNI; the Japanese Partner has discovered the molecular mechanism of BNI; the LEM partner has discovered biological control on denitrification and its molecular mechanism); (2) the project will build on an already established network of savanna scientists who have agreed to provide samples for the worldwide scanning of BNI-grass capabilities; (3) two French teams have been working for 20 years in Ivory Coast, and collaborated with the team from Ivory Coast; and (4) three CGIAR centers are part of the international BNI consortium.