<p style="text-align: center;">by Prof. Hana Santruckova (guest researcher at Dept. of Soil & Environment), Faculty of Science, Department of Ecosystem Biology, University of South Bohemia, ceske Budejovice, Czech Republic</p>
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Mountain ecosystems of the Czech Republic have been exposed to continuously increasing sulphur (S) and nitrogen (N) deposition over more than one century. The S deposition has decreased drastically since 1990s (~87 %), while the decrease of N deposition was lower (~ 50%). Long term N depositions to N-limited ecosystems has often lead to N saturation with resultant decrease of soil C:N ratio and leaching of nitrates, which can be followed by a decrease of DOC leaching. I will clarify the feedback mechanisms leading to the decreased DOC leaching and its consequences.
Organisms need carbon (C) and nutrient resources in a distinctive ratio, which is determined by elemental composition of the body and resource use efficiency. The most limiting resource, together with environmental conditions, determines the organisms’ ability to produce biomass. Soil microorganisms with a lower C:N ratio than plant litter, their main food resource, are obviously N limited in natural ecosystems. In ecosystems that have been exposed to long-term N deposition, the resource stoichiometry (organic C:N ratio) can be substantially changed in favor of N and natural N limitation can be alleviated.
Nitrogen plays two roles in the microbial metabolism; it is an essential nutrient for biomass production (N immobilization) but it also serves in energy metabolism as an electron acceptor in denitrification and electron donor for nitrifying bacteria (N mineralization). The soil processes of N immobilization and N mineralization occur simultaneously and their equilibrium depends on the N availability related to the C availability.
Long-term input of NO3- and SO42- increases acidity of soil solution, which suppresses DOC availability by coagulation of individual molecules and by absorption to Al and Fe oxyhydroxides. It amplifies a decrease of DOC availability already caused by a lower C allocation to the roots.
I am going to discuss that decreased ability of acidified spruce forest soils to retain N is not only due to a shift in microbial N transformations from N immobilization towards N mineralization but also to a decrease of bioavailable DOC in acidifying soils. These changes can be sufficiently strong to switch microbial communities from N limitation to C limitation.