Temperature responses of net nitrogen mineralization and nitrification in conifer forest soils incubated under standard laboratory conditions

Variation between soils in the response to temperature of net nitrogen (N) mineralization and nitrification was investigated in laboratory incubations. The soils, which were sampled from coniferous forests of different regional climates of western Europe, were mixed with 15N-labeled wheat straw and incubated at 4, 10, 16, 23 and 30 °C and constant moisture conditions. The accumulation in soil of the extractable NH4+–N and NO3−–N was followed for 550 days. First-order rate equations were fitted to data to estimate net mineralization rate constants and the size of the mineralizable part of straw and native soil N (Nmax). This pool was found to be related to temperature indicating an influence of thermal conditions on the degree of microbial exploitation of organic N sources. The Q10 estimates for the rate constants of mineral N accumulation decreased with increasing temperature but the relationship was less intense for straw than for native soil N results. Optimum temperature for nitrification showed a good correlation with the geographic origin of the soils and it was lower than the highest incubation temperature (30 °C) for the soil of the colder site.