Genetic and Environmental Controls on Nitrous Oxide Accumulation in Lakes

We studied potential links between environmental factors, nitrous oxide (N(2)O) accumulation, and genetic indicators of nitrite and N(2)O reducing bacteria in 12 boreal lakes. Denitrifying bacteria were investigated by quantifying genes encoding nitrite and N(2)O reductases (nirS/nirK and nosZ, respectively, including the two phylogenetically distinct clades nosZ (I) and nosZ (II)) in lake sediments. Summertime N(2)O accumulation and hypolimnetic nitrate concentrations were positively correlated both at the inter-lake scale and within a depth transect of an individual lake (Lake Vanajavesi). The variability in the individual nirS, nirK, nosZ (I), and nosZ (II) gene abundances was high (up to tenfold) among the lakes, which allowed us to study the expected links between the ecosystem’s nir-vs-nos gene inventories and N(2)O accumulation. Inter-lake variation in N(2)O accumulation was indeed connected to the relative abundance of nitrite versus N(2)O reductase genes, i.e. the (nirS+nirK)/nosZ (I) gene ratio. In addition, the ratios of (nirS+nirK)/nosZ (I) at the inter-lake scale and (nirS+nirK)/nosZ (I+II) within Lake Vanajavesi correlated positively with nitrate availability. The results suggest that ambient nitrate concentration can be an important modulator of the N(2)O accumulation in lake ecosystems, either directly by increasing the overall rate of denitrification or indirectly by controlling the balance of nitrite versus N(2)O reductase carrying organisms.