Designing and Evaluating Complex Cover Crop Mixtures

Objective 1

This research compliments and builds upon a three-year experiment established in fall 2014 at the Eastern Shore Agricultural Research and Extension Center (Painter, VA) evaluating cover crop mixtures in no-till corn rotations. The cover crop experiment is a randomized complete block design with twelve different treatments with cover crop mixtures and is replicated four times. Cover crop treatments contain varying amounts of species mixes, from a 9-species “kitchen sink” mix to a “purposeful diversity” 3-species mixes. In rotation after the cover crops, no-till corn was planted during summer of year 3. Four key species—rye, vetch, crimson clover and tillage radish were selected due to their popularity as cover crop species and their contributions to different supporting services (Clark, 2007). There are eight treatments evaluated: one no-till control without cover crops, four monocultures (rye, vetch, radish and clover), two three-species mixtures and one high diversity, nine-species mixture.  A full description of the cover crop mixtures, including species and seeding rates, is shown in table 1.

Table 1. Description of cover crop treatments and mixtures.

Five of the seven cover crop treatments were grown in the current experiment, but three treatments (two monocultures and one mixture) were added to the experiment so that the four cover crop species being examined appeared in a range of proportions. Ecosystem services measurements during the cash crop year of the rotation include corn yield, weed suppression and soil water conservation to reduce irrigation needs. The amount of weed suppression was assessed with total weed counts in the corn crop. Volumetric soil moisture was monitored continuously for the two years of the study in two replications in all 6 plots. The sensors were placed at 15, 30, 45 and 60 cm to capture changes in soil moisture above and below a hard pan located between 30 and 45 cm. Monitoring the hard pan allowed monitoring of deep rooted radish on hard pans. Soil samples were taken to measure inorganic soil nitrogen and carbon concentrations.

Ecosystem services being measured in cover crops included cover crop biomass, plant nitrogen uptake and biological fixation, and improved soil quality through decreased compaction. Above ground biomass was sampled immediately before cover crop termination from four one foot quads randomly placed across the plots. The biomass was separated by plant species and dried individually and weighed. The individual dried species weights were totaled for treatment for dry biomass weight. The nitrogen content of cover crop biomass and soil inorganic N concentration at cover crop termination was measured. Nitrogen accumulation in the cover crop biomass was assumed to be from either biological nitrogen fixation by legumes or nitrogen scavenging by grains and brassicas. The soil nitrogen concentration was measured again before sidedress to assess how much N was added by the cover crop. Compaction was measured with a digital penetrometer after corn planting, when the soil was at field capacity.

Objective 2

A method similar to the one used Jopke et al (2015) was used to look at ecosystem interactions. All possible pairs of ecosystem services measured in objective 1 were correlational and graphed with bagplots using the alpack package in R. Bagplots are similar to box plots as they show the median of the data and where 50% of the data lies, but uses polygons instead of squares. The visualization of the data using the polygon shows if the data follows a non-linear trend, resulting in a weak correlation (Jopke et al., 2015). From bagplots, interactions can be categorized as synergistic, a trade-off or neutral interactions. The total number of trade-offs and synergies for each cover crop mixture and monoculture was tallied to create a ranking to find the cover crop mixture with the greatest number of ecosystem services. This investigative approach highlights potential interactions between ecosystem services, but cannot quantify the level of ecosystem service being provided or lost.

Objective 3

Multiple linear regression was used to estimate significance of cover crop species to ecosystem services. Cover crop data sets were compiled and contained levels of ecosystem services for each monoculture and mixture as the various responses and the proportion of the four cover crop species. Cover crop proportions were expressed as weights (biomass dry weight) or percentage of mixture (species counts prior to termination) because some ecosystem services, not all ecosystem services provided by cover crops are well correlated with above ground biomass (Smith et al., 2014). Model building will be used to select the best model from the full model containing all possible species and species interaction terms. The analysis was repeated using cover crop functional groups—legumes, small grains, and tap-roots—instead of individual plant species to see if the ecosystem service provided is species specific.

The interpretation of the regression analysis can be used to design new cover crop mixtures. Species that have a significant and positive slope should be kept in the mixture because they provide benefits to the mixture, but negative or insignificant regressions could be excluded. In addition, some interactions may be significant and indicate either synergies (mutualistic interaction) or trade-offs (competition) between plant species.