Constraints of protein depolymerization on soil nitrogen availability along a latitudinal transect in Western Siberia

Birgit Wild1, Jörg Schnecker1, Anna Knoltsch1, Mounir Takriti1, Maria Mooshammer1, Norman Gentsch2, Robert Mikutta2, Ricardo Alves3, Antje Gittel4, Nikolay Lashchinskiy5, Andreas Richter1
1 Department of Microbiology and Ecosystem Science, University of Vienna
2 Institute of Soil Science, Leibniz University Hannover
3 Department of Ecogenomics and Systems Biology, University of Vienna
4 Department of Bioscience, Center for Geomicrobiology, Aarhus
5 Central Siberian Botanical Garden, Siberian Branch of Russian Academy of Sciences

O 4.8 in Below ground turnover of C and nutrients in forest soils

14.07.2014, 16:15-16:35, H17

Soil N availability is constrained by the breakdown of proteins to oligo-peptides and amino acids that fuel plant and microbial growth, as well as other N transformations such as N mineralization (ammonification) and nitrification. Protein depolymerization rates have rarely been measured, and possible controls on protein depolymerization, as well as the connection to other N transformations are unknown. We investigated N cycling in seven ecosystems along a 1,500 km latitudinal transect in Western Siberia (67°N-54°N), including tundra, boreal forest (taiga), forest steppe, and temperate steppe. At each site, we sampled organic topsoil, mineral topsoil and mineral subsoil horizons, and applied a set of 15N pool dilution assays to determine gross rates of protein depolymerization, N mineralization and nitrification. The investigated ecosystems showed a large variability in N transformation rates, with highest rates of both protein depolymerization and N mineralization in middle and southern taiga. Differences between ecosystems were exceeded by differences between soil horizons: Rates of all measured N transformations decreased with soil depth following the decrease in organic matter content and microbial biomass. When normalized to N supply by protein depolymerization, N mineralization was higher in mineral than in organic soil horizons. Our findings thus suggest lower N limitation of microorganisms in mineral than in organic soil horizons that facilitated a higher allocation of N to mineralization. Correspondingly, we found a higher contribution of mineral N forms (ammonium and nitrate) to the dissolved N pool in deeper soil horizons. We propose that in deep soil horizons, microbial N demand was constrained by low quantity and quality of C. Since microbial C and N demand control a large range of microbial processes, including rates and patterns of organic matter decomposition, and the partitioning of C and N between sequestration and mineralization, we suggest fundamental changes in microbial functions with soil depth, and emphasize the importance of deep soil horizons for understanding ecosystem C and N cycling.

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last modified 2014-04-03