Bringing the Benefits of Legume Cover Crops to Northern Midwest Climates

Bringing the Benefits of Legume Cover Crops to Northern Midwest Climates

Summary

SARE_Figures and Captions_Grossman_Y2_2016

Objectives/Performance Targets

Learning outcome 1: Growers will learn if application of nitrogen fixing rhizobia inoculant is necessary with selected cover crop ecotypes. We will characterize nodule occupants among the cover crop varieties in Y1. Mature plants will then be evaluated for nodulation, total biomass N, nitrogen fixed, and rhizobia occupancy.

Learning outcome 2:  Growers will learn basic soil ecological principles to help them manage legume cover crops for optimal function and performance. Blending farmer needs with available data on cover crop biology and soil science, we will develop two hands-on workshops including evidence-based responses to grower knowledge needs. Workshops will be taught as part of grower conferences in the region (i.e. MOSES, Sustainable Farming Association of Minnesota Annual Conference) in the final year of the project.

Action outcome 1:  At least 25 farmers will plan to use a legume cover crop in their farming operation in the three years following project termination. To conduct the formative evaluation, surveys will be provided to workshop attendees and undergraduate course attendees that ask about the degree to which these offerings were useful, met their needs, and what additional questions they still have. Additionally, in the months following the workshops, attendees will again be surveyed and asked to rate their knowledge about utilizing cover crops and the degree to which they are utilizing (farmers) or recommending (those who work with farmers) such practices. Those farmers who identify having most successfully implemented these approaches will be interviewed by phone to understand how and why they were able to engage in these practices. (i.e., what were drivers of their success).

Action outcome 2: At least 60 students will be exposed to examples of legume cover crop use. To meet this action outcome we will develop 4-8 media-rich online case studies showcasing exemplary growers who use cover crops in the North Central SARE region, and highlighting both their successes and challenges in using cover crops. Dr. Grossman will be leading a course in biological principles for use in organic farm management as part of the University of Minnesota’s recently developed Food Systems major, and the development of the case studies dovetails beautifully with this new and innovative program. The case studies will be developed as a course assignment for Dr. Grossman’s class in Y1 and Y2 of the grant, with students conducting all interviews, taking video footage, editing, and developing study questions to be part of the case-study package.

Accomplishments/Milestones

• Following a second year of field experiments at two research station locations, data on rhizobia, and nitrogen fixation was again collected in Spring of 2016.
• Planted a third year of field experiments in Fall 2016, with data to be collected late Spring of 2017.
• Additional data was taken to better understand the contribution of cover crops to labile soil organic matter pools.
• Demonstrations were planted in fall 2015 at Phil Batalden’s 350-acre organic grain farm in Lamberton, MN and Rodrigo Cala’s 40-acre organic vegetable farm in Turtle Lake, WI.  We consulted with both farms regarding species mixes and specific cover cropping challenges in their operations.
• Presented workshop to a group of 25 individuals at Phil Baltaden’s organic row crop farm near Lamberton, MN on April 27, 2016.
• Six media-rich videos were created by undergraduate students at the University of Minnesota via interviews with exemplary farmers using cover crops in the Upper Midwest region.

Impacts and Contributions/Outcomes

Year 2 (2015-2016):

The study design was modified slightly in Year 2 to account for observable differences in survival and crop success.  Treatments in 2016 include: Vetch 1 (Albert Lea Seed variety, MN 2014 #23), Vetch 2 (Buckwheat Growers variety, MN 2014 #25), Red clover (Albert Lea 2014), Vetch 2 + rye, Rye alone, and a bare ground control. Poorly performing vetch variety #3 (Welter Seed) was eliminated due to poor biomass production.  Similarly, Austrian winter pea was eliminated as it performed poorly in Y1. We also included a rye/vetch mix (rye and vetch variety 2) to better reflect farmer practices.  

As well, we have become increasingly interested in the potential contributions of legume cover cropping systems to increase soil organic matter (SOM) in agricultural soils of the upper Midwest. Emerging understandings of SOM development pay particular attention to the role of low molecular weight inputs and microbial activity as a potential driver of SOM stabilization.  This is also a component that greatly expands our understanding of cover crop role in nitrogen transformations.  Therefore, in Y2 we included a bare ground plot that served as a control for SOM studies.  This was not necessary in the first year of the experiment, as a non-leguminous rye plot could be used to control for nitrogen fixation contributions from legume treatments.  Treatments will be the same during the third 2017 field season and both sites were planted in late August 2016, as described below under ‘Fall planting’.

Carbon and Nitrogen Cycling Y2: Cover crops performed well compared to poor establishment and low biomass production in Y1.  Cover crop performance was similar at both sites (Fig. 1 and 2).  This is surprising due to the difference in temperature and soil type between the sites. Both V1 and V2 produced over 1,600 kg/ha, whereas the rye/vetch mix produced over 2500 kg/ha.  While this is an improvement, this is still low when compared to other hairy vetch trials in northern climates.  For example, Teasdale et al. (2004) achieved biomass ranging from 150 – 430 g/m2 in New York, whereas the vetch varieties in our experiment yielded about 160 g/m2.  Clearly, there is much agronomic and breeding work to be done to integrate winter legume cover crops into annual rotations in the upper Midwest.  

Potentially mineralizable nitrogen (PMN) is the amount of nitrogen converted from organic to mineral forms during a 7-day, anaerobic, laboratory incubation.  This is an important proxy for determining the capacity of the soil to provide nitrogen in forms available to plants, especially when employing organic inputs. Preliminarily, we observed increases in PMN from pre-termination to post-termination at one of our two sites (Figs. 3, 4). However, in both locations extractable N increased from pre-tillage to post-tillage (Figs. 5, 6).   At both sites, treatments V1 and V2 appear to have higher amounts of extractable N in comparison to clover,  no-CC control (Lamberton) or rye (Lamberton).

Permanganate oxidizable carbon (POX-C) is an assay used to measure forms of labile, active carbon in the soil.   POX-C is sensitive, and is expected to change relatively quickly in response to management.  We measured POX-C at three different time points during the experiment: fall before cover crop planting, spring immediately prior to termination, and two weeks after termination.  Total POX-C at Lamberton remained stable during all three time points.  Across both sites, there appear to be no treatment differences, despite large differences in cover crop biomass quantities (figs. 7, 8).  We will be performing further assays to determine microbial biomass (quantity of microbes in the soil) and particulate organic matter (light carbon fraction) before and after cover crop termination to provide further information regarding the impact of legume cover crops on soil health.   

Nitrogen Contribution: Percent nitrogen was determined via elemental analysis and extrapolated to biomass production in order to determine the total nitrogen contributions of legume and non-legume cover crops.  Crops in Y2 contributed significantly more plant tissue N as compared to Y1 due to increased biomass production.  In Lamberton, both vetch varieties and the V2/rye biculture contributed between 72-80 kg N ha-1.  In Grand Rapids, V2 outperformed V1 by almost 20 kg N ha-1 and the biculture by more than 30 kg N ha-1, contributing a total of 71.3 kg N ha-1.  In Lamberton, rye contributed 52.8 kg N ha-1, significantly more than clover, while in Grand Rapids, the two treatments were not significantly different (30.0 kg N ha-1 and 34.6  kg N ha-1, respectively).  However, increased C:N ratio for rye may immobilize available soil nitrogen, negating the effect of this increased N.  Further, while the N produced in Y2 was greater than Y1, it is not enough to cover the demands of most field and horticultural crops, some of which require up to 225 kg N ha-1 (i.e. field corn), or in the case of sweet corn, approximately 100 kg N ha-1.

The natural abundance method was used to determine nitrogen derived from the atmosphere (NDfA), which refers to the percentage of legume plant tissue nitrogen that is derived from N2 gas diffused into soil air pores and fixed by rhizobia.  The natural abundance method is based on the isotopic fractionation of 15N and 14N between a nitrogen-fixing legume and a non-fixing reference crop (average of rye treatment and plot weeds).  Because nitrogen fixation is energetically costly to a plant, NDfA is expected to be lower in high N soils and higher in low N soils.  We hypothesized that legumes grown in Lamberton would therefore have a lower NDfA than legumes grown in Grand Rapids, due to higher available N in Lamberton.  In Lamberton, NDfA was highest in the V2 mix at 67.9%.  This is consistent with other findings that suggest grasses to be effective N scavengers that create a nitrogen-scarce root zone (Ranells and Wagger 1997).  NdfA of V1 and clo was 60.7% and 60.4%, respectively, with V2 producing the lowest NDfA at 52.8%.  Data from Y2 in Grand Rapids is in analysis with preliminary results suggesting V2 in biculture producing 76.2% NDfA and clo producing 41.0% NDfA.

Nodulation: Nodulation efficiency is known to be a powerful determinant of biological nitrogen fixation in legume species. Generally, clover had the greatest average nodule number ranging between 36-192 and 80-152 nodules per plant in Lamberton and Grand Rapids, respectively (Fig. 11). Mean nodule number per plant was not affected by vetch species in either field site. While inoculation with efficient rhizobia is often recommended for legume planting, results showed that inoculation did not have any effect on mean nodule number in any treatment except V1 in Grand Rapids, where the mean nodule number ranged between 53-78 and 29-64 nodule per plant when inoculated (WIN) and un-inoculated (NIN), respectively. The similar positive inoculation effect was also found in mean nodule dry weight (nodule mass) in V1 and V2 in Lamberton as well as V1 and mix in Grand Rapids (Fig. 12). While clover showed the greatest mean nodule number, nodule mass was the lowest among the treatments.

On-farm demonstrations: Demonstrations were planted in fall 2015 at Phil Batalden’s 350-acre organic grain farm in Lamberton, MN and Rodrigo Cala’s 40-acre organic vegetable farm in Turtle Lake, WI.  We consulted with both farms regarding species mixes and specific cover cropping challenges in their operations.

Fall Planting: Due to low crop survival in Y1, the study was repeated for a third year (Y3) of data collection at the same two research station locations. This will allow better generalizations about legume performance across multiple site-years.

Cover crop video case study development: Undergraduates at the University of Minnesota developed six 3-minute educational videos highlighting exemplary farmer management of cover crops. The effort was a major class project embedded within HORT 3131 Student Organic Farm: Planning, Marketing and Production. Students received training on design and creation of educational films via workshops presented by library staff with expertise in video production. In April, 2016 students travelled to distant locations across Minnesota to shoot footage and interview farmers, and then assembled videos following the interview. Our intention was to include farmers from across the Upper Midwest SARE region. Travel limitations of students ultimately made this impossible, yet some farms were located in rural regions located 3-4 hours north of our University campus, exposing students to a different (colder!) growing environment and challenges facing farmers in this region.  Videos were then shown to our class and other department faculty, and were promoted via our departmental list-serve for future use in other courses.

Outreach and workshops: Julie Grossman conducted a workshop on cover crop legumes to over 200 farmers at Southern SAWG in January, 2016 and collected data via surveys about knowledge increase resulting from the workshop. Using the Brinkerhoff Success Case Method, subset of growers offering their contact information will be contacted in 2017 to determine if they are using the information learned during the workshop.

A field day held at the Batalden Farm in Lamberton, MN in April 2016 was attended by 20 growers, NRCS, and extension staff, despite inclement weather. The Batalden Farm field day showcased cover crop demonstration plots mirroring our experiment station plots, as well as specific legume-non legume mixes of interest to Phil Batalden.  These included vetch-buckwheat and vetch-oat mixes, as well as a gradient of planting densities of these cover crops. During the field day, Batalden presented on his cropping rotation, cover cropping philosophy, and particular advice since his transition to organic production a decade earlier.  PI Grossman explained legume N fixation, assessing nodulation, and estimating N credits to subsequent crops based on aboveground biomass sampling.  Lastly, the group discussed support for cover cropping with NRCS and Extension, as well as a broader discussion regarding new directions for cover cropping with field day participants.

Two community outreach days were hosted at the Research and Outreach Centers where the research was conducted (Organic Field Day, Lamberton on July 6, 2016; and Visitors’ Day, Grand Rapids on August 24, 2016).  Over 50 community members participated at the Lamberton field day and over 100 community members participated at the Grand Rapids field day.  Community members were given an overview of the research project at the field location and given time for discussion and questions.

Presentations and Outreach

Grossman, J. Improving nitrogen management in organic systems through legume cover crop integration, Department of Natural Resources Seminar Series, University of Illinois, Oct, 14, 2016.

Perrone, S. Winter annual cover cropping research plots in the Upper Midwest. NCROC Visitors Day, Aug 24, 2016, Grand Rapids, MN (100+ participants).

Perrone, S. Winter annual cover cropping research plots in the Upper Midwest. SWROC Organic Field Day, July 6, 2016, Lamberton, MN (50+ participants).

Grossman, J. Seehaver, S., Sooksa-nguan, T. Role of rhizobia inoculation in promoting effective nitrogen fixation in legume green manures in the United States using hairy vetch, crimson clover and winter pea. ICARDA International Conference on Pulses for Drylands, April 18-20, 2016, Marrakesh, Morocco.

Perrone, S. Nitrogen contributions from winter annual cover crops in the Upper Midwest. Presentation at Monsanto Visitors Day, Feb 25, 2016, University of Minnesota, St. Paul, MN.

Perrone, S., Liebman, A., Sooksa-nguan, T., Grossman, J. Nutrient cycling in winter annual cover cropping systems of the Upper Midwest. Presented at Forever Green Seminar, April 29, 2016, University of Minnesota, St. Paul, MN.

Perrone, S., Liebman, A., Sooksa-nguan, T., Gutknecht, J., Grossman, J. Nitrogen contributions from winter annual cover crops in the Upper Midwest. Poster presentation at ASA-CSSA-SSSA Annual Meeting, Nov 8, 2016, Phoenix, AZ.

Collaborators: