Contribution of Soil Nitrogen Mineralization and Nitrification Pulses to Soil Nitrogen Availability and Nitrate Exports in a Mediterranean Catchment

Abstract:

A good assessment of pulses of microbial nitrogen (N) supply is of paramount importance in helping understand the soil N cycle and catchment N exports in Mediterranean regions. Yet, the real contribution of N mineralization and nitrification pulses to soil N availability and catchment N exports remains unclear because most of the experiments have been performed on single forest ecosystem, while focusing on just few rainfall events. Over a year, we performed biweekly soil incubations to investigate patterns and controls of pulses of net N mineralization (NNM) and nitrification (NN) in three forests (riparian, evergreen oak and beech) coexisting within a Mediterranean headwater catchment. Further, we examined the influence of these pulses on soil N budgets and stream N loads. Within the catchment, riparian soils were hotspots of NNM and NN as they accounted for 30% of measured soil N supply, with median rates being multiple-fold higher than at the oak and beech forests. Pulses of NNM and NN generally occurred in spring immediately after large rainfall events (>20 mm). Moreover, high summer soil temperatures (>16ºC) promoted pulses of microbial activity at the riparian site. Although microbial pulses were restricted in time at all sites, they could contribute between 26-42% of the annual rates of NNM and NN. However, only NN pulses in the riparian site lead to disproportional increases in soil N availability and stream N loads, suggesting that these Mediterranean riparian soils could be a critical source of nitrate to the stream. Our study stresses that intensive monitoring is essential to capture hot moments of soil N processes, and thus to understand the relevance of microbial pulses on soil N biogeochemistry.