Nitrogen processing following experimental sediment rewetting in isolated pools in an agricultural stream of a semiarid region

Water scarcity represents an important driver of stream microorganisms and N transformations rates. In semiarid regions many agricultural streams rich in NO 3 − undergo periods of flow cessation of severe duration and spatial extent. After drought, rainfalls tend to firstly rewet stream sediments by creating isolated pools that can remain in streambeds until the entire stream is reconnected. Sediment rewetting may lead the reactivation of nutrient processing, yet usually causes release of inorganic N. Thus, information about the role of sediments during this rewetting period on N processing and fluxes downstream is especially valuable when semiarid watercourses are affected by chronic NO 3 − exposure. We conducted a microcosm study to evaluate how a short-term sediment rewetting over a 7 days period in disconnected pools can control denitrification rates and N availability in an agriculturally impacted semiarid stream. We observed a significant stimulation of denitrification in dry sediments ( −2 h −1 ) upon rewetting with a maximum rate of 830 μg N m −2 h −1 after 24 h. In parallel, NO 3 − in dry sediments (1170 mg m −2 ) dropped considerably after 7 days of rewetting (72 mg m −2 ), despite the fact that denitrification was restrained from 24 h thereafter. However, from this time a notably increase of NH 4 + concentration was detected until the end, which suggests that at this point, NO 3 − is transformed into NH 4 + through dissimilatory reduction (DNRA) supported by low Eh (40 mV). If compared with NO 3 − , NH 4 + can be either more easily retained in sediments or assimilated preferably by organisms. We highlight that the diversification of N species provided by isolated pools creation just before flow resumption may progressively improve water quality in NO 3 − -rich streams, which is a key aspect in the face of future agriculture intensification and water scarcity scenario.