Soil organic matter content controls gross nitrogen dynamics and N2O production in riparian and upland boreal soil

We investigated the pathways of gross soil nitrogen (N) transformations and nitrous oxide (N2O) production with N-15 enrichment techniques in a boreal forest landscape by comparing organic (riparian) and mineral (upland) soil within two catchments in northern Sweden. The values of all soil properties evaluated for the riparin and upland zones were statistically different (P<0.05). The rates of gross N transformation were larger in the riparian than in the upland soil (P<0.05), which can be explained by the larger soil organic matter (SOM) content that provides energy and mineral N as a substrate for other processes. The riparian soil at one site shows a decoupling of nitrification from mineralization; the largest gross mineralization occurred in the soil at this site, but gross nitrification was relatively small. This was probably because of the low pH (2.70.1), which inhibits the activity of autotrophic nitrifiers. Oxidation of organic N was the main source of N2O in the soil at all sites, probably because of low soil pH and large organic carbon content, which favours heterotrophic nitrification. The results of our study confirm that organic matter is the main regulating factor for gross N mineralization and nitrification; the latter are markedly different in the organic-rich riparian soil and the upland soil in the boreal forest landscape.