Nick Helland’s central Iowa farm looks much like every other nearby farm on this chilly March day, with corn stubble stretching from a gravel road up over a low hill to the northern horizon.

But look closely, and you can see patches of muddy ground where a few months ago crews buried low-tech systems called bioreactors and streamside buffers that filter fertilizer-borne nitrates from water as it drains from Helland’s field into nearby Big Creek and eventually the Des Moines River.