Cover crops are increasingly being included in lists of best management practices for water quality, but their adoption among non-organic farmers has been slow. One approach to increasing the use of cover crops is to provide evidence that farmers can use them while either breaking even on costs or even gaining a cost benefit. A potential cost saving from cover crops is from a nitrogen credit—nitrogen from cover crop residues could offset nitrogen needed from fertilizer, lowering this input cost (and gaining environmental benefits at the same time). Measuring nitrogen credits from cover crops requires research on the decomposition of their residues under the following crop. Because organic matter decomposition rates vary widely based on soil type, temperature, moisture, background nutrient levels, and microbial community composition, as well as on the biochemical composition of the plant residue, decomposition must be measured locally in order to make reliable recommendations to local farmers. This study will measure the decomposition rates and nutrient release timing of two common cover crops (cereal rye and field pennycress) under two nitrogen rates (0 lbs/acre and MRNT 150 lbs/acre) in a soybean/corn rotation in Rosemount and Waseca, Minnesota. This data will be used to develop evidence-based Minnesota Agricultural Extension cover crop recommendations (including optimal nitrogen rates and optimal timing of cover crop termination for nutrient release).
Concrete outcomes from this research include:
- A Minnesota Agricultural Extension publication regarding the nitrogen credit from select cover crops (in lbs N/acre) in a corn/soybean rotation, including recommendations on date of cover crop termination, and optimal nitrogen rates that will be available to farmers.
- A journal article in a peer-reviewed journal.
- A poster or oral presentation at a relevant conference.
Initially, we planned to use cereal rye and medium red clover as our two cover crop species. However, because of poor winter survival and spring regrowth of the clover, there was insufficient biomass. Therefore, we elected to substitute field pennycress, which was part of the companion study conducted on the same plots, and which is also physiologically distinct from cereal rye. Additionally, less agronomic research has been conducted on pennycress than red clover, so this research will potentially have a larger scientific impact with pennycress than red clover.
The study was conducted at two locations in Minnesota – Rosemount and Waseca research and outreach centers.
On 4/28/17 and 5/3/17 two (0.5 meter)2 quadrats of rye and pennycress biomass were collected from plots at the Waseca and Rosemount research locations (respectively). At the same time, biomass was collected from the edges of plots and placed in large mesh bags. The plant material from the quadrats was dried in 140° C ovens, weighed, ground, and submitted to the University of Minnesota’s Research Analytical Laboratory (RAL) for analysis of total carbon and nitrogen content. A sub-sample of the biomass collected from plot edges was weighed, dried at 140° C, ground, submitted to RAL for combustion and ICP analysis of macro and micro-nutrients, and reserved for analysis in the UMN forages lab for lignin and hemicellulose determination.
Within several days of biomass collection, fields were sprayed with glyphosate. Fields were disked on 5/8/17 and chisel plowed 5/11/17 (Waseca) and on approximately 5/24/17 (Rosemount), and corn was planted on 5/12/17 (Waseca) and 5/31/17 (Rosemount). In order to replicate the field conditions as closely as possible, the remaining biomass was spread between two layers of mesh bags on racks near a large door allowing access to fresh air and breezes. It was allowed to air dry. On 5/3/17 (Waseca) and 5/5/17 (Rosemount), the biomass was roughly chopped into approximately 4-6 inch pieces. The remaining biomass was stuffed into 1mm mesh bags approximately 10 inches x 10 inches, each weighing 20.00 +/- 0.05 g (for rye) and 45.00 +/- 0.05g (for pennycress). A portion of the biomass was weighed, dried at 140° C, and weighed again to determine moisture content.
On 5/15/17 (Waseca) and 5/31/17 (Rosemount), mesh bags were buried. Three bags of rye and pennycress were reserved from each site to determine moisture content at time of burial and to analyze for any nitrogen lost to volatilization while air drying. In each location, bags were buried in plots that had rye and pennycress treatments, and which received either 0 lb N/ac or 150 lb N/ac (as EDTA coated urea on 5/15/17 (Waseca) and 5/31 (Waseca)), with three replications. In each plot (for a total of twelve per location), six bags were buried at a depth of six inches, with small field flags attached to mark the location, in row four of each six row plot. One bag from each plot was retrieved on the following dates:
May 30, June 12, July 3, July 31, August 23, October 13
June 12, June 26, July 18, August 8, August 31, October 26
In the field, the majority of excess soil and infiltrating plant roots were removed from the mesh bags. They were placed in labeled paper bags and dried in 140° C ovens. Bags were then stored at room temperature until February, when we began processing the samples. Samples were again placed in 140° C ovens to remove all moisture. In order to remove soil from the plant material, mesh bags are then rinsed with warm water from a kitchen sink style sprayer. This removes most of the remaining soil from the samples. Mesh bags are then re-dried at 140° C, then weighed, ground, and if enough plant material is remaining analyzed for macro and micro-nutrient content at RAL. Mesh bag processing is expected to be completed by April 1, 2018.