Balancing the supply and demand of nitrogen (N) in crop production is challenging. In most cases, application of N fertilizer is needed to ensure that adequate N is available to corn. Nitrogen that is not taken up by plants can be lost from cropping systems as nitrate through leaching or runoff. 

Cover crops can help to manage nutrients within cropping systems. One function of cover crops is to capture available soil N by storing it in their tissues. The period between corn harvest and winter is limited by cool air temperatures and short days, making cover crop establishment following corn harvest a challenge. Interseeding cover crops into corn can increase the amount of growing degree days and solar radiation available for cover crop establishment and growth prior to winter. However, this strategy may raise concerns about competition of cover crops with corn. 

Cover crop study

To evaluate the effect of interseeded cover crops on N supply to corn in a corn-soybean rotation with spring strip tillage, cover crops were evaluated at Grand Rapids, Lamberton, and Waseca, MN from 2016 to 2018. Cover crops were broadcast into corn at the dent stage. Cover crop treatments included cereal rye (CR) and annual ryegrass (AR) alone, in two-way mixtures with crimson clover (CC), and in three-way mixtures with forage radish (FR). Cereal rye was the only winter-hardy cover crop, and its regrowth in the spring was terminated with herbicide before strip tillage. Annual ryegrass, CC, and FR winter-kill, so they did not regrow in the spring. 

Nitrogen fertilization for corn included a broadcast application of urea at 65 lb N/acre at planting, followed by 60 lb N/acre of urea ammonium nitrate solution that was injected into the soil at the six leaf collar stage. We collected weather data, soil samples from the 0-16 inch depth in the fall after corn harvest and after the spring thaw, weekly soil solution during the spring and fall from ceramic suction cups buried at a depth of 40 inches from plots with CR, CRCC, CRCCFR, and no cover crop, biomass of cover crops, and corn grain yield. A mean value for nitrate-N in soil solution was calculated seasonally for spring (March-May) and fall (September-November) by averaging the concentrations collected during each period. 

Soil nitrate-N results

Following corn in the spring of 2017, CR reduced soil nitrate-N in the top eight inches of soil compared with no cover crop and plots with a two- and three-way mix of winter-killed cover crops at Lamberton. Also in the spring of 2017, CR reduced soil nitrate-N in the 8-16 inch soil layer compared with no cover crop at Waseca. In the fall of 2017, ARCC in the top eight inches of soil and ARCCFR in the 8-16 inch soil layer reduced soil nitrate-N compared with no cover crop at Lamberton. In the spring of 2018 at Waseca, CR again reduced soil nitrate-N in the 8-16 inch soil layer compared with no cover crop. These results indicate that cover crops scavenged nitrate-N and potentially prevented its loss.

Considered seasonally, cover crops with CR reduced nitrate-N concentration in soil solution in the spring and fall of 2017 and 2018 compared to no cover crop (Table 1). At Grand Rapids, nitrate-N concentration in soil solution with CRCC was less than that with CR in the fall of 2017, and CR and CRCC had lower nitrate-N levels than no cover crop in the fall of 2018. At Lamberton in the spring of 2017, nitrate-N concentration in soil solution with CR, CRCC, and CRCCFR was less than that with no cover crop. Similarly, at Waseca in the spring of 2018, CR reduced nitrate-N in soil solution compared with no cover crop. 

Table 1. Average seasonal nitrate-N concentration in soil solution in corn for cover crop strategies in spring (March-May) and fall (September-November) at Grand Rapids, Lamberton, and Waseca, MN.

N in cover crop biomass and corn grain

Nitrogen accumulated in cover crop biomass did not exceed 24 lb N/acre at any of the three locations in the spring or fall of 2017 or 2018. No differences among cover crop treatments were observed at Grand Rapids at any point during the study. At Lamberton in 2017, the three-way mixes accumulated more N than monocropped cover crops by fall frost. In the fall of 2018 at Waseca, N accumulation in cover crop biomass was greater for ARCCFR than all other strategies. No difference in N accumulation in cover crop biomass was observed in the spring at any location in 2017 or 2018.

Nitrogen content of corn grain did not differ between cover crop strategies and the no cover crop control. However, cover crops reduced nitrate-N concentration in the soil and soil solution in the fall and spring. It is possible that larger reductions in nitrate-N concentration in the soil and soil solution, along with more intense competition between cover crops and corn, could occur with greater cover crop biomass production and N accumulation.