Ammonia oxidation and nitrate reduction marker genes are key indicators of nitrogen losses in temperate forest catchments

Abstract

Chronic nitrogen inputs can alleviate N limitation and potentially impose N losses in forests, indicated by soil enrichment in ¹⁵ N over ¹⁴ N. However, the complexity of the nitrogen cycle hinders accurate quantification of N fluxes. Simultaneously, soil ecologists are striving to find meaningful indicators to characterise the "openness" of the nitrogen cycle. We integrate soil δ¹⁵ N with constrained ecosystem N losses and the functional gene potential of the soil microbiome in 14 temperate forest catchments. We show that N losses are associated with soil δ¹⁵ N and that δ¹⁵ N scales with the abundance of soil bacteria. The abundance of the archaeal amoA gene, representing the first step in nitrification (ammonia oxidation to nitrite), followed by the abundance of narG and napA genes, associated with the first step in denitrification (nitrate reduction to nitrite), explains most of the variability in soil δ¹⁵ N. These genes are more informative than the denitrification genes nirS and nirK, which are directly linked to N₂ O production. Nitrite formation thus appears to be the critical step associated with N losses. Furthermore, we show that the genetic potential for ammonia oxidation and nitrate reduction is representative of forest soil ¹⁵ N enrichment and thus indicative of ecosystem N losses.