Natural attenuation of large anthropogenic nitrate loads in a subtropical stream revealed by δ15N and δ18O

•Agricultural creeks can mitigate downstream nitrogen loads and ecosystem impacts.•Creeks can remove up to ~80% of upstream nitrogen load per km of downstream creek.•Recycled sewage water used on farms contributes to nitrogen loads in nearby creeks.•Spikes in creek nitrogen are likely driven by flushing of soil storages during rainfall.