Long-term vs. first-time cover crop use: Can cover crops enhance nitrogen availability to corn and organic nitrogen and carbon storage at the same time?

Objectives:

The project established long-term vs. first-time vs. no cover cropping to:

• Quantify nitrogen (N) availability to grain corn by using the ¹⁵N tracer technique.
• Enhance understanding of the mechanism of N availability under cover cropping by delineating the profile distribution of soil organic N and carbon (C) within soil organic matter (SOM) fractions (i.e., particulate organic matter (POM) and mineral-associated organic matter (MAOM)).
• Evaluate economics of cover cropping (i.e., partial profit margins: cost of cover crop seeds and planting, revenue from crop yield, soil organic N and C storage).

Impacts:

• The estimation of N availability to grain corn by comparing long-term vs. first-time vs. no cover cropping will help farmers to adjust (i.e., reduce) the application rate of mineral N without crop yield loss. The improvement of N use efficiency with time since the introduction of cover crops also can reduce N loss from croplands to groundwater (e.g., nitrate leaching) and the atmosphere (e.g., nitrous oxide emissions).
• The analysis of soil organic N and C across SOM fractions (i.e., POM and MAOM) within soil profile will help scientists understand N availability and soil organic C storage response to cover cropping with time. Especially, it provides an opportunity to predict the capacity of long-term cover crops to mitigate climate change by soil organic C sequestration.
• The comparison of N availability, soil organic C store, and economical returns among long-term vs. first-time vs. no cover cropping will improve farmers, consumers, and the government confidence in the adoption of cover crops.

Project Overview:

Cover crops have been widely touted to increase soil organic nitrogen (N) and carbon (C) storage. However, these benefits are expected to occur after long-term use, while additional costs of planting cover crops are immediate. This is the main barrier to cover crop adoption. To reduce this risk, we comprehensively investigated the economic and environmental performance (e.g., N availability and soil organic C storage) of long-term and first-time and no cover cropping in the same fields. In 2007, Dr. Van Eerd established a cover cropping experiment in a vegetable-grain rotation at Ridgetown and repeated it in 2008. In this experiment, cover crop plots had about 17% greater soil health score in 2015/2016, and >30% greater corn yield in 2020/2021, compared to no cover crop plots. We hypothesized that N availability to the following crop was enhanced, due to greater SOM (i.e., greater C and N storage) under long-term cover cropping. The significant question for grain farmers is “How much N fertilizer I can save from long-term cover cropping?”

The proposed research tested the hypothesis and provided insight into the farmers’ question above. We quantified N use efficiency by grain corn and identified possible mechanisms of N availability enhancement by comparing long-term vs. first-time vs. no cover cropping. We sacrificed part of the no cover crop plots where selected cover crops were planted for the first time. However, as it is a challenge to accurately calculate the amount of plant available N from cover crops to the following crop, ¹⁵N tracer was employed to follow the ¹⁵N-enriched fertilizer applied to cover crops after emergence and transferred to the grain corn. The distribution of N and C within SOM fractions was also explored to determine soil organic N and C stores and their relationship with N availability. Additionally, the economic impacts of grain corn with and without cover crops were analyzed to assess the economic returns from cover crops with time (i.e., 15-years vs. 1-year).

Results:

Core project results are summarized in a fact sheet link following this results summary.

Soil mineral N content: Both first-time and long-term cover cropping lowered soil mineral N compared to the no cover control in the fall, thus mitigating the risk of N losses over the winter. The following spring (early May), surface soil mineral N content (6” depth) followed the order of long-term (16 kg N ha^-1) > short-term (10 kg N ha^-1) ≥ no cover cropping (6 kg N ha^-1), suggesting that long-term cover cropping would improve N availability to the corn crop by transferring N from cover crop decomposition as well as from greater soil organic matter. At corn harvest, plots with long-term cover cropping had greater soil mineral N, suggesting contribution from soil organic matter.

¹⁵N recovery: In the fall cover crop and soil to 90 cm depth, total recoveries of applied ¹⁵N were not significantly different between long-term and first-time cover cropping, but both were about 2 times greater than no cover cropping. Therefore, the use of cover crops (even one) is effective at reducing N losses during the non-growing season. In corn and soil, regardless of treatment, the majority of applied ¹⁵N (i.e., 5 times more) was recovered in soil rather than in the whole corn plant. This indicated that most cover crop-derived organic N did not convert into mineral N by corn harvest.

Soil POM and MAOM: To better understand mechanisms of nitrogen availability and carbon storage in soil, particulate and mineral-associated organic matter (POM and MAOM) were measured. The plant-derived POM fraction largely consists of relatively undecomposed plant residues with a fast-cycling rate (i.e., years), while the microbial-derived MAOM fraction is mainly from microbial necromass and soluble plant inputs with a low-cycling rate (i.e., decades to centuries). Both POM and MAOM fractions may contain heterogeneous labile compounds, therefore both would be the sources of bioavailable N inputs.

At the sandy loam project site, POM was a larger N supplier to corn than MAOM. This is likely because the POM fraction stores most soil N that is more easily mineralized, and its larger proportion of total soil mass (>85%) in sand-dominated soil texture compensates for its lower N concentrations (5 times less), compared to the MAOM fraction. No positive correlation was identified between spring-sampled MAOM content and corn yield, suggesting a POM-dominated pathway.

Grain yield was greater by 27.4 bu/ac with long-term cover cropping than the no cover crop controls (P=0.0045; n=16 and 8, respectively), and first-time cover cropping was intermediate. However, it is very important to note that corn was grown without additional fertilizer N in order to quantify nitrogen derived from the soil (i.e., cover crop and soil organic matter mineralization), so these yield differences will not be expected in commercial cropping systems that apply fertilizer N. Grain yield was the least variable with long-term cover cropping, which points to resiliency of the system. Differences between specific cover crops were not great enough to recommend one cover crop over another (it was also outside the scope of the study).

Economic gains. Revenue gains from long-term cover cropping were greater by $178/ac than the no-cover crop (P=0.0099) based on the sale of corn grain and fertilizer nitrogen savings. While these values are less than the cost of planting the cover crop multiple times, further analysis is needed to look at partial profit margins of all years, and it is important to note that this project was carried out in a processing vegetable-grain rotation with a high frequency of cover crops (grown 11 times in 15 years), so results cannot be automatically extrapolated to a standard corn-soybean-wheat grain cropping system. In all 15 years, there was no statistical yield penalty to cover cropping, except for one instance (winter cereal rye before spring wheat).

Project Results Fact Sheet:

Comparing long-term vs. first-time vs. no cover cropping to understand grain corn yields and nitrogen uptake

External Funding Partners:

Matching funding was provided by the Fresh Vegetable Growers of Ontario and the Ontario Agri-Food Innovation Alliance, a collaboration between the government of Ontario and the University of Guelph.

Project Related Publications:

Peng, Y., & Van Eerd, L. L. 2024. Surface soil sampling underestimates soil carbon and nitrogen storage of long-term cover cropping. Geoderma Regional. 39, e00885.

Peng, Y., Rieke, E. L., Chahal, I., Norris, C. E., Janovicek, K., Mitchell, J. P., Roozeboom, K. L.,  Hayden, Z. D.,  Strock, J. S., Machado, S., Sykes, V.R., Deen, B.,  Bañuelos Tavarez, O., Gamble, A.V., Scow, K.M., Brainard, D.C., Millar, N., Johnson, G.A., Schindelbeck, R.R., Kurtz, K.S.M., van Es, H., Kumar, S., and Van Eerd, L.L. 2023. Maximizing soil organic carbon stocks under cover cropping: insights from long-term agricultural experiments in North America. Agriculture, Ecosystems & Environment. 356: 108599.