Temperature sensitivity of C and N mineralization in temperate forest soils at low temperatures

Climate models predict warmer winter in temperate regions, but little is known about the temperature sensitivity of soil carbon (C) and nitrogen (N) mineralization at low temperatures. Here, we assess the temperature sensitivities of gross ammonification, gross nitrification, C and net N mineralization of top soil horizons, under a European beech and a Norway spruce temperate forest. We tested the hypotheses that (1) substrate quality affects the temperature sensitivity of C and N mineralization and (2) that temperature sensitivity of C mineralization is higher than of gross ammonification. Soil incubations were conducted at constant temperatures of -4, -1, +2, +5 and +8 degrees C. Gross ammonification and nitrification were measured by the N-15 pool dilution technique. Temperature sensitivities of C, gross and net N mineralization were calculated using the Arrhenius equation and C mineralization was taken as proxy for substrate quality. Gross ammonification and C mineralization was much larger in the beech than in the spruce soil, while gross nitrification was in the same order of magnitude. Gross ammonification, nitrification and C mineralization almost ceased at -4 degrees C, but already increased at -1 degrees C. Net ammonification in Oi/Oe horizons was low at -4 and -1 degrees C and increased strongly between +2 and +8 degrees C. Net nitrification was low in both soils, but increased in the spruce soil at temperatures >2 degrees C whereas no temperature response occurred in the beech soil. Apparent Q(10) values of gross ammonification and C mineralization in the temperature range of 4 to +8 degrees C were in the range of 3-18. Q(10) were lowest in soil horizons of low substrate quality. The ratio of C mineralization to gross ammonification varied between 0.5 and 2.9, suggesting preferential mineralization of N rich organic substrates or rapid turnover of the N pool in microbial biomass. Rising winter temperatures might have substantial effects on net N mineralization, but effects decrease with soil depth, likely due to decreasing substrate quality of soil organic matter. (C) 2013 Elsevier Ltd. All rights reserved.