You can trap nitrogen, but can you make it work for next spring’s crop?

WOOSTER, Ohio – Nitrogen prices have increased significantly over the last five years, and unfortunately it appears as if they will continue to increase at least slightly this coming year, according to Robert Mullen, Ohio State University soil fertility specialist.
Trapping it. The increased cost for this input has led many to find alternative ways to keep more nitrogen around, including the use of fall cover crops to “trap” or “catch” nitrogen for subsequent crops.
So, does this practice work? Can spring corn nitrogen rates be decreased if utilizing fall cover crops in continuous corn production systems?
Research suggests that fall cover crops (specifically rye) can help mitigate nitrogen loss from agricultural fields, said Mullen.
Little help. Research conducted in Minnesota revealed a decrease in nitrate loss, measured at the tile discharge, of 13 percent when rye was planted as a cover crop after corn and prior to soybean.
But, this decrease in nitrogen loss in the fall does not necessarily translate into subsequent nitrogen release for next year’s crop, said Mullen.
Over eight site-years in Washington State, use of a non-leguminous cover crop (rye or ryegrass) did not decrease the need for nitrogen fertilization with the exception of one year.
Thus, subsequent fertilizer rates could not be decreased for the next corn crop.
Use of vetch as a cover crop did slightly decrease the amount of nitrogen needed to optimize yield.
Study. In this study, all cover crops were planted in late September/early October and allowed to grow all spring until late April.
Data collected in Wisconsin show that in two out of three years, the nitrogen rate for optimum production was decreased (approximately 18 pounds per acre) if a cover crop (rye, oat, ore triticale) was grown on a sandy soil.
The response observed in this study was attributed to a rotational effect, rather than nitrogen release, the subsequent spring.
This was hypothesized because the decrease in nitrogen fertilizer was observed with or without the above-ground biomass being present in the spring.
Timing. So, why is there not a consistent decrease in the optimum nitrogen rate if nitrogen that is susceptible to fall/winter loss is captured?
It all comes down to timing, said Mullen.
Even though nitrogen is taken up by a growing plant in the fall, it will not necessarily be released at the optimum time the next spring so it is not susceptible to loss.
If spring planting is delayed and soil conditions are warm enough for mineralization, early spring rains may move the nitrogen below the rooting zone down to the tile prior to crop emergence.
Another reason is poor growth of the cover crop in the fall and winter. If the cover crop does not accumulate much biomass and subsequent nitrogen, intuitively there will be little nitrogen benefit for the next crop.
Hit and miss. Under certain conditions, this practice may provide some benefit, but enough is not known to accurately quantify the benefit.