By Zhehan Tang
Figure 1: Winter cereals, including wheat, cereal rye and triticale, are increasingly used as cover crops after corn silage as they can reduce erosion risk, add organic matter to the soil, and capture end of season nitrogen.
Nitrogen (N) is essential for crop production, but crop N needs are difficult to predict due to uncertainty of the weather and inability to accurately predict N supply and loss. As a result, it is not unlikely that we have excess N at the end of the growing season. This excess N can be lost to the environment through leaching (most typical in well-drained soils) or denitrification (more likely in poorly drained soils). Cover crops planted after the harvest of a main crop like corn can take up and sequester some of this end-of-season N (in addition to sequestering carbon) and thus carry some N over to the next year. Winter cereals, including wheat, cereal rye and triticale, are increasingly used as cover crops as they have a greater cold tolerance than many other cover crops. In New York, winter cereal biomass accumulation in the fall, when seeded after corn silage harvest, typically ranges from 0.25 to 1 ton per acre (Figure 1) for a typical N uptake of 20-30 lbs N/acre (Ort et al., 2013).
Estimating N Content of Cover Crops
In the past six years, we have measured fall biomass of winter cereals to determine fall N uptake, in collaboration with farm advisors and farmers throughout New York State. This was done to determine total N uptake and carbon sequestration under New York growing conditions in rotations where the winter cereal was seeded after corn silage harvest. The assessment showed that there is a strong linear relationship between the above ground biomass (shoot only) and total N uptake (roots and shoots combined) of wheat, cereal rye, and triticale in New York (Figure 2). The differences in N content among the three species were very small as long as the total biomass was less than 1.2 tons of dry matter per acre. The relationships between above ground biomass and total N uptake were very consistent among different locations (farm fields), suggesting that the equations in Figure 2 can be used independent of soil type. This allows a farmer or farm advisor to determine the above ground biomass of these winter cereals in his or her field and then estimate total (below and above ground) N in the cover crop.
Figure 2: Relationship between above ground biomass of dry matter (tons/acre) and the total N (lbs/acre) uptake by winter cereal between seeding after corn silage and a killing frost in late fall or early winter.
Step-wise Protocol to Determine N Uptake
Because total N uptake and above ground biomass are so closely related for these winter cereals, we can use the relationship between total N content in winter cereal cover crop and its above ground biomass (shoots) to determine total N uptake in a step-wise approach:
- Create a sampling frame (for example a 24 by 12 inch wooden frame) and precisely measure its length and width in inches.
- Place the sampling frame in a representative location of the field whose biomass and N content you want to estimate. Clip all above ground biomass of the winter cereal that is within the frame, as close to the soil surface as possible, while avoiding soil contamination, and place the sample within a bag large enough for samples from 3 frames. Sample when the plants are dry (avoid sampling when there is dew, rain, or snow).
- Repeat the sampling process at two more representative sites in the field, collecting samples from all three locations in the same bag.
- Measure the total weight of the sample collected in pounds (lbs). Make sure to subtract the bag weight itself so that only plant weight is included in the total.
- If your sample does not have much external moisture, you can assume it has about 18% dry matter, then skip step 6 and continue with step 7. If you want to estimate total biomass and N content more precisely, continue with step 6.
- After carefully mixing the sample in the sampling bag, take a subsample of approximately 0.25 lbs from the harvested biomass, and determine the weight of the subsample in pounds (if the weight of the total sample is 0.25 lbs or less, skip the step of taking a subsample). Dry the subsample with a microwave until it has obtained a stable weight. Use short drying periods of 30-60 seconds and place an 8-oz glass of water in the corner to avoid burning of the sample. More detailed instructions for drying of plant biomass with a microwave can be found at http://extension.psu.edu/publications/i-106. Once the weight has stabilized, measure the final dry weight. The dry weight divided by the initial weight is the percent dry matter. If you have a scale that measures in grams, you can also weigh 100 grams of freshly harvested biomass, dry it following the same procedure, and weigh the dry weight at the end of the drying process; the dry weight will be the dry matter percent.
- Once you have all data collected, download the Cornell University “Fall Nitrogen and Carbon Pools of Winter Cereals” excel spreadsheet calculator from the Nutrient Management Spear Program website ((http://nmsp.cals.cornell.edu/software/calculators.html). Select the species and enter your information for frame length, frame width, number of samples taken, total wet weight, subsample wet and dry weight. The calculator (Figure 3) will report the percent carbon (C) and N, the ratio of both (C:N ratio), estimated biomass (tons/acre dry matter), and total carbon and N pools (lbs/acre). Table 1 shows the N content of different winter cereals.
Figure 3: Cornell University “Fall N and Carbon Pools of Winter Cereals” excel spreadsheet calculator.
In Summary
Cover crops are important for erosion control and soil health. In addition, winter cereals are effective in taking up N in the fall. We developed a calculator to estimate how much N is accumulated by the onset of the winter in winter cereals planted as cover crops (biomass less than 1.2 tons of dry matter per acre; winter cereals seeded after corn silage or soybean harvest only). This calculator converts estimates of the above ground biomass to total N uptake and carbon sequestration by the winter cereal (roots and shoot combined).