Contribution of pulses of soil nitrogen mineralization and nitrification to soil nitrogen availability in three Mediterranean forests

Abstract

Pulses of microbial nitrogen (N) supply often occur during storms inMediterranean regions, but their contribution to soil N availability and catchment N exports is still unknown.We investigated patterns and controls of pulses of net N mineralization (NNM) and nitrification (NN) at three forest sites (riparian, evergreen oak and beech) that coexist within a Mediterranean headwater catchment. In addition, we examined the effect of these pulses on soil N availability and stream N loads. For a year, we measured NNM, NN, precipitation, moisture and temperature within each forest site. Mean NNM and NN rates varied widely among forest sites (NNM=1.35, 0.62 and 0.50 μgNgsoil−1 day−1; NN=1.18, 0.24 and 0.07 μgNgsoil−1 day−1 for riparian, oak and beech, respectively). In general, pulses of NNM and NN occurred in spring, immediately after large rain events (>20 mm). High soil temperatures (>16∘C) promoted microbial pulses in summer at the riparian site, but there were no pulses of NN at the beech site. Although pulses of microbial activity were infrequent, they could account for account for 21–35% of the annual rates of the annual rates of NNM and NN. However, NN pulses only at the riparian site led to disproportional increases in soil N availability and stream N loads. These results suggest that upland Mediterranean forests are sinks of N even after storms, whereas riparian soil can be a critical source of nitrate for the stream. Our study emphasizes the relevance of intensive monitoring in to evaluate the effect of microbial pulses on soil N biogeochemistry in Mediterranean catchments.