Investigating the potential of winter camelina to mitigate nitrogen immobilization and nitrate leaching as a cover crop for the Upper Midwest

Project formulation

A key challenge integrating cover crops in corn-soybean rotations in the Upper Midwest is the lack of species other than cereal rye or other winter cereals that will reliably overwinter when planted following the harvest of either crop. This hampers the ability to add diversity to the rotation and can result in the need for adaptive management, especially when corn follows rye. Rye's explosive growth after May 1 results in much of the soil nitrogen tied up in its biomass, which is slowly released because of the higher carbon to nitrogen ratio of the maturing plant. Later termination timings because of wet springs or a desire to plant green can then result in early season nitrogen deficiencies for corn. Furthermore, rye and corn are both grasses, so at-planting or delayed terminations can enhance the green bridge effect, increasing the potential for pathogens, especially seedling diseases like Pythium spp. to be passed from the cover crop to the corn, potentially affecting yield.

Winter camelina is a winter-hardy brassica with a similar planting window and survivability to cereal rye. Even after bolting in the early spring, winter camelina maintains a lower carbon to nitrogen ratio and does not pull as much nitrogen out of the soil as cereal rye. As a brassica, it has a reduced likelihood of passing shared pathogens onto the following corn crop. This opens up a pathway for cover crop-curious farmers who may not be as comfortable with the risks posed in managing a cereal rye cover crop in the spring. While these traits looked good on paper it had still not been tested extensively on farm

One of the realms where cereal rye shines as a cover crop is in reducing overwinter nitrate leaching. If winter camelina were to replace rye acres, how would it fare in the nitrate leaching realm? These two questions guided our research approach.

Methods 

In order to evaluate the yield differences between camelina and rye, farmer collaborators interested in the potential of camelina as a cover crop planted strips of both cover crops into a soybean field following harvest in the fall. These strips were at least 500 feet long and replicated 3 times to facilitate harvest with a yield monitor on certain farms--minimum harvest length for yield monitor accuracy is typically 300-500 feet--and encompass in-field variability. Strip width was determined by drill width and the need to accommodate the width of the farmer's corn head for their combine, typically 15 feet wide, but double passes for some farms with larger corn platforms resulted in 30 feet wide strips. An additional camelina drone seeding treatment was used to test if an earlier establishment of camelina during soybean leaf drop would increase biomass in the fall and result in more overall biomass. A no cover control served as a baseline for yield and nitrate leaching measurements. The 4 total treatments: rye (30-50 lbs/acre), camelina-drill (6-8 lbs/acre), camelina-drone (10 lbs/acre), and a no cover control with 3 replications resulted in 12 total strips. In the spring, the cover crop was chemically terminated at the timing desired by the farmer, either prior to or at corn planting. Yields and moisture were recorded in the fall with either a yield monitor or weigh wagon. 

To evaluate the ability of these cover crop treatments to prevent nitrate leaching, a relatively novel and low-cost method of resin lysimeters was deployed. These devices sandwich a mixed bed ion exchange resin that is used in water filtration systems between two pieces of mesh suspended in a short PVC pipe to passively intercept nitrates in the soil water. These lysimeters were placed beneath the root zone of the cover crops (20 inches). To reduce soil disturbance above the lysimeter, a 6 inch diameter earth auger was used at a 45 degree angle to the soil surface to dig a slanted hole. A side pocket was then excavated with a soil knife to ensure the lysimeter was under undisturbed ground at a vertical depth of 20 inches for the lysimeter. The lysimeters were installed just prior to cover crop planting. This placement did disturb the drone seeding, but the angled nature of the access hole placed the lysimeters beneath a significant portion of undisturbed cover crop. The lysimeters were left over winter to measure nitrate leaching after cover crop seeding and removed just prior to corn planting. Following removal, the lysimeters were sent to a Dr. Steven Hall's lab at UW Madison for the resin extraction, and the nitrate load analyzed via a potassium chloride extraction. As an additional method for evaluating soil nitrate loss, composite soil nitrate samples were taken at 1 and 2 foot depths with a soil probe in each strip. Baseline samples were taken in the fall at time of resin installation and in the spring, another sample was taken at time of removal. One final soil sample taken prior to sidedress in mid-June sought to evaluate the speed of cover crop decomposition and nitrogen availability for the corn crop as it begins to substantially increase nitrogen uptake following the V4-6 growth stages. 

Additional sampling included clipping biomass samples at the soil surface within a quadrat of a known area randomly dropped in each strip 4 times. These 4 samples were compiled, weighed fresh in the field, and a subsample sent to the lab to be analyzed for percent dry matter, percent nitrogen, and carbon to nitrogen ratio. These data aided interpretation in being able to show stand variability across strips and farms. When paired with the soil sample and lysimeter nitrogen data, these analyses provide data  for a partial nitrogen budget, accounting for baseline nitrogen levels, loss, uptake, and soil nitrogen remaining at corn planting.