Cover crop cocktails: Harnessing diversity to enhance nitrogen retention in agroecosystems

Cover crop cocktails: Harnessing diversity to enhance nitrogen retention in agroecosystems

Summary

Nitrogen (N) management is among the most significant challenges of contemporary agriculture. Provision of sufficient N is essential to meeting growing demands for food, fiber, and energy. At the same time, prevention of N losses is critical to reducing negative environmental impacts of agriculture such as water pollution, greenhouse gas emissions, and biodiversity loss. One strategy to enhance the ecosystem services of N provision and N retention is the use of multi-species cover crop mixtures. Mixtures have the potential to not only increase aboveground cover crop biomass, which is assumed to be a determinant of N provision and retention, but also promote these ecosystem services simultaneously. In August 2011 we initiated an experiment assessing N provision and N retention in eight cover crop monocultures, seven 4-species mixtures, one 8-species mix, and a no cover control. Among cover crops planted in August 2011 and terminated in May 2012, we found that mean aboveground biomass (AGB) produced by mixtures was significantly greater than monocultures (monoculture = 3356+/-500 kg/ha, 4-species = 5120+/-445 kg/ha; 8-species = 7143+/-947 kg/ha) and that the quantity of N in AGB was also significantly higher in mixtures (monoculture = 101+/-15 kg N/ha, 4-species = 155+/-15 kg N/ha; 8-species = 256+/-40 kgN/ha). Biomass production, however, does not appear to be the key determinant of ecosystem services. Winter hardy N-scavengers (canola and cereal rye) in monoculture and mixtures containing these species provided the highest levels of N retention, measured as rate of nitrate flux through the soil profile from November 2011 to May 2012. The highest levels of N provision occurred in monocultures and 4-species mixtures that contain winter hardy N-fixers (red clover and hairy vetch), as indicated by soil mineral N in early July. The use of cover crop mixtures did promote multi-functionality, simultaneously supporting N provisioning and N retention in a 4-species mix containing winter hardy N-scavengers and N-fixers and in the 8-species mix (see figure 1). We are currently repeating the experiment, having planted cover crops in August 2012 that will be terminated in May 2013 prior to corn planting.

• Figure 1

Objectives/Performance Targets

For all objectives below, “year 1” refers to cover crops planted in August 2011 and terminated in May 2012, the associated corn crop is from 2012; “year 2” refers to cover crops planted in August 2012 that will be terminated in May 2013, the associated corn crop will be in 2013.
Objective 1: To quantify the effect of cover crop mixtures on the quantity of N in aboveground cover crop biomass. Progress to date: Aboveground biomass was collected in October 2011 and May 2012 from year 1 cover crops. Biomass from year 2 cover crops was collected in October 2012. Samples from October 2011 have been analyzed for N content; samples from May 2012 will be analyzed in January 2013 and those from October 2012 in February 2013. A preliminary estimate of N in total accumulated AGB (the sum of biomass sampled in fall 2011 and spring 2012) in year 1 was made using N concentrations determined for October 2011 samples. Mixtures of 4 and 8 species have significantly more N than monocultures, though the range of variability is high within monocultures (figure 2).
Objective 2: To assess the effects of cover crop mixtures on microbial N uptake. Progress to date: Soils from year 1 were collected for biological analysis in November 2011 and May 2012; soils from year 2 were collected for biological analysis in November 2012. Soil microbial biomass N (SMB-N) has been determined from soil extracts from year 1 and will be completed on extracts from year 2 by April 2013. SMB-N was not significantly different between treatments in May 2012 (figure 3). Microbial activity on all sampling dates was assessed through a community level physiological profile, the statistical analysis of which is in process.
Objective 3: To measure the effects of cover crop mixtures on nitrate leaching during the cover crop season. Progress to date: In year 1, anion resin membranes were placed in the field in September 2011 and collected in November to measure potentially leachable nitrate in the fall; membranes were extracted and analyzed for nitrate. In November, membranes were replaced with anion resin bags that were removed in May 2012. Bags were extracted and analyzed for nitrate content. In year 2, 4 anion resin bags were buried in each plot in September 2012. Two bags from each field were collected in November 2012; the two remaining bags will be collected in May 2013. Bags removed in November will be extracted and analyzed for nitrate content by April 2013. Fluxes in monocultures and mixtures containing the winter hardy N-scavengers canola and cereal rye were significantly lower than fluxes under the no cover crop control, an indication of greater N retention associated with these species (figure 4).
Objective 4: To determine the effect of cover crop mixtures on nitrogen supply to a subsequent cash crop. Progress to date: In year 1, corn was planted in May 2012 and harvested for grain in November 2012. In year 2, corn will be planted in May 2013 and harvested in November 2013. Grain will be analyzed for N content to provide an indicator of N provisioning. A second indicator of N provisioning is the quantity of soil mineral N in early July. The timing of this sampling coincides with the time at which growers would sample to determine if they will apply supplemental N through a side-dress operation (referred to as the pre-side-dress nitrate test). In year 1, the highest mineral N levels were observed in winter hardy N-fixing monocultures (red clover and hairy vetch) and mixtures containing these species (figure 5).
Objective 5: To increase understanding of the mechanisms through which diversity influences N retention and supply in order to assist farmers in choosing species for cover crop mixtures. Progress to date: This objective will be met through synthesis of all data collected during the course of this two year project.

See attached summary of completed project activities.

• Completed project activities
• Figures 2-5

Accomplishments/Milestones

All field activities planned for August 2011 through December 2012 were completed as proposed and there have been no changes to the proposed plan of work. Based on outcomes from year 1, we moved the planting date for year 2 up one week in an effort to provide a longer window of time for four of our six winter kill species that showed poor establishment in 2011 (sorghum sudangrass, sunn hemp, foxtail millet, and soybean). In year 2 we also added a second 8-species mix comprised of eight N-scavengers (oats, forage radish, sorghum sudangrass, foxtail millet, canola, cereal rye, barley, and ryegrass). We anticipate that we will again sample cover crops and soils in May 2013 prior to cover crop termination and plant corn in late May for November 2013 harvest.
With regard to laboratory analyses, all plant and soil analyses from year 1 have been completed except measurement of aboveground biomass N concentration from May 2012. Analysis of plant and soil samples collected to date in year 2 will be completed in winter 2013. In addition to the proposed analysis, I recently received a grant to conduct metagenomic analysis of soil samples from May 2012 to evaluate microbial community composition under the seventeen cover crop treatments using pyrosequencing.

Impacts and Contributions/Outcomes

Farmers throughout the Northeast have demonstrated interest in cover crops and cover crop mixtures to enhance agricultural productivity and sustainability. One of the goals of my project is to provide farmers and other agricultural professionals with research-based information on the management and benefits of cover crop mixtures. To this end, I have led more than 150 farmers, students, and agricultural professionals on tours of my research plots to date. One tour was part of the Cover Crop Innovations Field Day supported by the Northeast SARE Pennsylvania State Program (http://extension.psu.edu/cover-crops/events/oct27-field-day) as reported in my 2011 annual report. In June 2012 I led a hands-on workshop entitled “Cover Crop Mixtures for Corn Success” with PA-SARE coordinator Charlie White as part of the Strategies for Soil Health and Nutrient Conservation Research Tour. Using data from two field experiments, including this NE-SARE project, we led participants through activities to assess N provision in corn and N retention under various cover crop regimes. Ninety-five percent of participants surveyed reported having some or a great deal of knowledge following the workshop, compared to 73% reporting to have this level of familiarity before attending.

Collaborators: