Do savanna trees and grasses interact to determine nitrification?

Abstract

In the humid savanna of Lamto (Ivory Coast) grasses inhibit nitrification, which increases the availability of nitrogen and primary production, while trees stimulate it. Trees and grasses should thus influence soil functioning in an interactive way. We expect intermediate values of nitrification in the soil below grass tussocks inside tree clumps. Answering this question is important to develop “agrosavannery” practices based on the use of savanna trees and grasses in cropping systems. We sampled soil inside tree clumps and outside, both below and between grass tussocks (in total four different situations) and measured basic soil chemical properties, the nitrification and denitrification enzymatic activities, the abundances of the amoA-AOA, amoA-AOB, nirK and nirS genes. There were higher nitrification and AOA gene abundance below tree clumps than outside tree clumps. Denitrification was higher inside tree clumps between grass tussocks than in the three other treatment combinations and the nirS gene was more abundant below grass tussocks. The results show that nitrifying archaea are responsible for nitrification in the Lamto savanna and that they are specifically targeted by nitrification inhibition by grasses and stimulation by trees. Overall, tree impact on nitrification and nitrifying archaea were somehow overriding the inhibition capacities of grasses. This means that the use of both savanna grasses and trees in close vicinity to develop “agrosavannery” cropping systems would likely lead to high nitrification rates and that such systems will only benefit from nitrification inhibition if grasses are grown away from savanna trees.