1 Institute of Soil Research, Department of Forest and Soil Sciences, University of Natural Resources and Life Sciences (BOKU), 1190 Vienna, Austria
2 Biogas Systems GmbH, 7111 Parndorf, Austria
3 Department of Microbiology and Ecosystem Science, University of Vienna, 1090 Vienna, Austria
4 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), 8903 Birmensdorf, Switzerland
5 Department of Geosciences and Natural Resource Management, University of Copenhagen, 1958 Frederiksberg C, Denmark
6 Division of Radiation Protection, Department Radiation Protection and Radiochemistry, Austrian Agency for Health and Food Safety (AGES), 1220 Vienna, Austria
Nitrogen (N) inputs from atmospheric deposition can increase soil organic carbon (SOC) storage in northern temperate and boreal forests, thereby mitigating the adverse effects of anthropogenic CO2 emissions on global climate. Direct evidence of N-induced SOC sequestration from decadal, low-dose N-addition (e.g., <50 kg N ha–1 y–1) experiments, however, is scarce worldwide and virtually absent for European temperate forests. Here we quantified belowground C and N pools of two coniferous forests in Switzerland and Denmark which have been subject to low-dose N addition treatments for approximately two decades. The addition of 22 kg N ha–1 y–1 stimulated tree productivity and decreased soil pH as well as exchangeable calcium at the Swiss site. At the Danish site, the addition of 35 kg N ha–1 y–1 reduced tree vigor, increased fine root biomass (FRB) and led to an accumulation of N in various belowground pools. Across sites, N addition increased SOC pools in the moderately decomposed organic horizons but decreased SOC pools in mineral topsoil. As a result, N addition led to vertical shifts in SOC distribution but did not affect SOC storage within 30 cm depth. Our results imply that a N-induced re-allocation of SOC from older, mineral associated pools to younger, unprotected pools may lead to a greater vulnerability of SOC in temperate forest soils which could positively feedback on carbon-induced climate change.
Keywords: carbon sequestration, nitrogen deposition, fine roots, nitrogen saturation, exchangeable cations, soil carbon, soil pH, soil respiration, Norway spruce