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Net N mineralization was studied in three different forest sites (Belgium): a mixed deciduous forest with oak ( Quercus robur L . and Quercus rubra L .) and birch ( Betula pendula Roth) as dominant species, a deciduous stand of silver birch ( Betula pendula) and a coniferous stand of Corsican pine ( Pinus nigra ssp . Laricio). The organic (F + H) layer and mineral soil at different depths (0–10, 10–20 and 20–30 cm) were sampled at three locations in the mixed deciduous forest (GE, GF1, GF2), at one location in the silver birch stand (SB) and one in the Corsican pine stand (CP). All samples were incubated over 10 weeks under controlled temperature and moisture conditions. The net N mineralization rates in the organic and upper mineral layer (0–10 cm) were found to be significantly different from the other layers and accounted for 66–95% of the total mineralization over the first 30 cm. Net N mineralization rates in the organic layer ranged from 4.2 to 27.3 mg N m–2 day–1. Net N mineralization and nitrification rates were positively correlated. For the mineral soil, net N mineralization rates decreased with depth and the upper 10 cm showed significantly higher rates, ranging from 8.9 to 33.5 mg N m–2 day–1. The rates of the 10–20 cm and 20–30 cm sublayers were similar, ranging from 1.2 to 7.4 mg N m–2 day–1. The net N mineralization rates for the total mineral layer (0–30 cm) ranged from 17.4 mg N m–2 day–1 (SB) to 36.1 mg N m–2 day–1 (CP). Both from PCA and multiple regression analysis, we could conclude that net N mineralization rates were closely related to the initial mineral N content (Ninitial). Furthermore, significant correlations were observed between the net N mineralization rate, the total carbon (TC) and NH4 +-N content for the mineral layers and between net N mineralization rate, total nitrogen (TN), hemicellulose content and C/N for the organic layers. [ABSTRACT FROM AUTHOR]

Abstract: Twenty-two long-term measurements of direct N2O emissions from soils in an intensive agricultural area were used for the validation of the process-based DNDC model (version 8.3P). Model simulations were evaluated for temporal patterns of N2O, NH4 +, NO3 − and water-filled pore space (WFPS) and total N2O emissions. Several soil and crop input parameter adjustments to the model were evaluated but only the recalculation of the WFPS at wilting point and at field capacity, using pedotransfer functions, resulted in a clear improvement of the simulated variables (WFPS in all cases, N2O in some cases). Therefore, only this adjustment was made to DNDC 8.3P. This change, however, resulted for some cases (both cropland and grassland) in retardation of nitrate leaching and to a lesser extent of NH4 + to the deeper soil layers. The goodness of fit of the simulated temporal pattern of N2O varied considerably between sites. The total simulated N2O emissions from cropland showed a good agreement with the measurements, although there was a systematic overestimation of 7.4kgN2O-Nha−1. Grassland soils, in contrast, gave a low agreement between total simulated and measured N2O losses. On the basis of all measured data a regional emission factor of 3.16 with a 95% confidence interval of −0.89 to 7.21 could be calculated. DNDC simulations resulted in an emission factor of 6.49 with a 95% confidence interval of 4.04–8.93. The overall outcome of the N2O emission measurements and DNDC simulations were compared with several empirical regression models, which may be applicable for a temperate climate system. All of the tested regression models showed reliable results up to a N2O emission of 10kgN2O-Nha−1. Higher emissions, however, were systematically underestimated. Though DNDC both under- and overestimated specific sites, the general agreement, over the whole range between measurements and simulations of total N2O losses (simulations=0.82×meas.+6.2), was better than for the different regression models. [Copyright &y& Elsevier]