COS 276-5 - Biological Nitrification Inhibition (BNI) by forest tree species: do vertical changes in nitrifier community along the soil profile matter to predict nitrates leaching from soil?

Abstract: Plants have developed diverse strategies to effectively uptake the scarce nitrogen (N) resource in soils with low soil pH and low N availability, typical for most forests. Some plant species can directly influence nitrification in soil through a process known as biological nitrification inhibition (BNI), by releasing specific compounds to the soil via plant litter decomposition or root exudation that inhibit nitrification pathways. BNI has been mainly observed for grasslands and agro-ecosystems, but rarely in forest ecosystems. Indeed, evidence for the main nitrifier(s) group(s) targeted and the implication for nitrates percolation along the soil profile at the ecosystem scale are scarce to date.

Here we evaluated (i) how different temperate forest tree species can biologically control soil nitrifier activity and abundance, (ii) which is(are) the main nitrifier group(s) targeted by BNI, and (iii) whether and how BNI effects on nitrifiers matter to predict water percolation and N leaching along the soil profile. We hypothesize that BNI could mainly occur in superficial soil horizons as most BNI compounds produced by herbaceous plants are weakly mobile in soil.

We evaluated the effects of BNI in 24 plots of 5 tree monocultures (beech, spruce, Corsican pine, Douglas fir and Nordmann fir) plus the native forest in a French long-term experimental site on the activity and abundance of 6 main nitrifier groups (AOB, AOA, Nitrospira, Nitrobacter and Comammox from clades A and B) in upper (0-15 cm) and deeper (15-30, 30-45 and 45-60 cm) soil horizons. Further, we coupled microbial data to water percolation and N leaching along the soil profile. Soil solutions were collected down to 60 cm depth with ceramic tension-cup lysimeters every four weeks during > 20 years.

We showed that potential nitrification were 10- to 1000-fold lower for spruce, Nordmann fir and the native forest than for beech, Corsican pine and Douglas fir, and that the abundances of nxrA-Nitrobacter amoA-AOA genes, were strongly and positively correlated with nitrification, with Nitrobacter spanning a broader range (50-1000-fold) across tree stands than AOA. Finally, we showed Nitrobacter abundance in the superficial – but not in the deeper – soil horizons was the best predictor of nitrates leaching along the soil profile over a year-period.

In conclusion, our results provide first evidence for BNI in forest ecosystems, and represent an important breakthrough for understanding the role of the nitrifier community as driver of nitrates leaching along the soil profile at the ecosystem scale.