Changes in Microbial Nitrogen Dynamics with Soil Depth, and along a Latitudinal Transect in Western Siberia
Tuesday, 15 December 2015: 11:20
2008 (Moscone West)
Plant productivity is often limited by low N availability, and this has been attributed to the slow breakdown of N-containing polymers such as proteins into amino acids that are small enough for uptake. Under such conditions, plants and microorganisms efficiently use the available N for growth, and the microbial release of excess N as ammonium (N mineralization), as well as the transformation of ammonium into nitrate (nitrification) is low. Nitrogen limitation is expected to increase towards high latitudes as conditions become less favourable for decomposition. On the other hand, within an ecosystem, microbial N limitation is expected to decrease with soil depth, following the decrease in the C/N ratio of organic matter. To test these hypotheses, we sampled organic topsoils, mineral topsoils and mineral subsoils from seven ecosystems along a latitudinal transect in Western Siberia, ranging from tundra (67°N) to boreal forest and further to steppe (54°N), and determined gross rates of protein depolymerization, N mineralization and nitrification using 15N pool dilution assays. We found that all rates decreased with depth following the decrease in organic matter content. Related to microbial biomass, however, only protein depolymerization decreased with depth, whereas N mineralization and nitrification significantly increased. This pattern was consistent across the seven ecosystems studied. Furthermore, we did not find indications for a decrease in microbial N limitation from arctic to temperate systems. Our findings thus challenge the perception of ubiquitous N limitation at high latitudes, but suggest a transition from N to C limitation of microorganisms with soil depth. With microbial N immobilization constrained by low C availability, subsoils might harbour an easily available N pool that can contribute to plant N nutrition, but might also promote N losses from the ecosystem, e.g., by nitrate leaching, even in high latitude systems such as tundra and boreal forest.