Cover crops have been rapidly adopted in Delaware, with cereal rye being a popular option for soybean production. The benefits of a rye mulch is weed suppression and soil moisture conservation, but may also cause increased pest presence and disrupt the release of nitrogen (N) to cash crops. While soybeans may not be as affected by the N cycle as corn, the mineralization and release of N in rye may also provide supplemental N to the plant mid-season. These fields may also include corn fodder from the previous cropping year, which will continue to breakdown through the soybean growing season, providing some carbon the soil surface. What is not currently known is now soybean populations and row spacing may affect the decomposition of residues on the soil surface. Earlier canopy may preserve soil moisture, allowing for increased residue decomposition, or may increase evapotranspiration reducing overall soil moisture. This study will take the first steps in measuring decomposition of residues under soybean planting densities.

1) Plant full season soybeans into a rye cover crop at five populations and two row spacings.
2) Use decomposition bags to measure breakdown of corn fodder and rye biomass under different soybean management.

Soybeans will be planted at the Carvel Research and Education Center in Georgetown, DE into a rye cover crop. The cover crop will be terminated two weeks prior to planting, like most field practices. Soybeans will be planted at five densities (80, 100, 120, 140, 160 thousand seeds per acre) and two row spacings (15 and 30 inch) and irrigated throughout the season. Ten subsamples across a gradient will be compared to drone imagery to estimate total field rye biomass. Terminated rye biomass will be collected from outside the plot boundaries and corn fodder will be collected from fields at the research center. Biomass will be separated into decomposition bags for each plot (30 rye and 30 corn fodder), weighed, and placed back into the planted plots in the center of a row. Three subsamples of each will be dried and saved to determine the initial carbon (C), N, and moisture content of the biomass. At the end of the season decomposition bags will be collected from the plots, dried, weighed, and analyzed for C, N, and the biomass loss. Yields will be collected with a plot combine in the late fall. Data will be analyzed in SAS as a randomized complete block design structured by a factorial including biomass loss, changes in C and N, as well as yield. Yield will also be correlated to various predictors from the study.

Updated July 23, 2022:
Cover crops were mapped by drone imagery prior to burndown in April 2022. Additionally, cover crop biomass was sampled from 15 locations using 0.5 m2 quadrats, dried in an oven, and weighed for overall biomass. This data will be matched to the drone map of cover crops to be correlated to all research plots.

Soybeans were planted from 80-160 thousand seeds per acre in both 15 and 30 inch rows. Additional 15 inch rows were planted to correct for potential tractor tire compaction. Previously collected corn fodder and rye cover crop biomass were placed in mesh bags in each of the plots. Plots have been sprayed for weeds and regularly irrigated to keep soil at a 0.25 VWC. Drone flights over the plots have collected imagery by-weekly for post-season comparisons. A moisture sensor was added to the 120k seeding rate plots in one 15 and one 30 inch row spacing to track soil temperature and moisture next to the mesh decomposition bags.

Updated December 26, 2022:
All litter bags were collected prior to harvest in October 2022 and placed in a drier room. Bags were weighed for their post-harvest dry biomass, ground, and sent to the University of Delaware Soil Testing Lab for C:N ratio analyses. We expect results for the C:N ratio to be completed early in 2023. The moisture sensor placed into selected plots was also removed right before harvest and final drone flights were performed as the soybean crop senescenced. Yields were obtained from all plots in October 2022 using a plot combine to harvest the center two rows.

Updated April 6, 2023:
Project completed and final report is attached.

View uploaded report

Compared to the companion study on population and row spacing performed at our irrigation research farm in 2022, no differences in yield were observed. Some of this can be related to issues at planting, which included planter by rye biomass as well as potential seed germination. The drone derived NDVI values show that the errors mostly reside in the 15-inch 150,000 and 180,000 seeding treatments.

While yields and biomass decomposition could not be related to population and row spacing, and therefore canopy cover, some differences did arise related to overall soybean yields.

The 15-inch row spacing mostly caused decreases in corn C/N ratios by the end of the season, which may have been
more consistent had the correct canopy cover occurred. That rye breakdown was greater in higher yielding plots, where yields were also related to higher NDVI (canopy cover) in August could still support the original hypothesis. Still, the study needs better controls on actual growth by treatments to observe if these are related.

Additionally, there is a relationship with corn fodder decomposition when the rye that is present. Where more C was remaining in the rye biomass, the corn fodder decomposition was higher. This study cannot show the mechanism that drove this result, which could be related to biological activity that preferred corn decomposition over rye, or could point to an interaction of fresh rye biomass with corn fodder. The bags are not conducive to larger detritovores which could have also helped with fodder breakdown. As rye decomposition leads to lower C/N ratios, and is related to higher soybean yields, corn fodder appears to prefer the opposite environment, at least during the period of this study. A repeat of this study in 2023 may help further elucidate the relationship.