The effects of cover crops on soil arthropod communities in the Inland Pacific Northwest

Progress report for GW20-217

Project Type: Graduate Student
Funds awarded in 2020: $24,993.00
Projected End Date: 10/31/2022
Grant Recipient: University of Idaho
Region: Western
State: Idaho
Major Professor:
Dr. Sanford Eigenbrode
University of Idaho
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Project Information

Summary:

To improve farming sustainability and resiliency, growers in the Inland Pacific Northwest (IPNW) are transitioning from traditional cereal production systems by diversifying crops and rotations. Diversification with cover crops benefits soil health and increases agroecosystem resiliency. However, adoption of this alternative strategy is novel in the IPNW and it is unknown how aspirational crops and cropping systems impact belowground soil arthropod communities. Conserving soil arthropod biodiversity and function is critical to soil health, decomposition, nutrient cycling, and pest control and promotes plant productivity. This project will compare belowground arthropod community composition and functionality in soils under cover crops with soils from less intensive and less diverse “business-as-usual” practices on three, representative working farms over two-years. To complement the on-farm surveys, a replicated small-plot study will examine how cover crop taxonomic and functional diversity drives soil arthropod community dynamics. Finally, to tie soil arthropod community composition to function, manipulative field experiments will examine associations between arthropod biodiversity and residue decomposition. This study will be the first in the region to examine rigorously how cover crop diversity impacts the biodiversity of belowground arthropod communities and their contributions to soil processes. Results will be disseminated to regional farmers through outreach at cereal schools, field days, and extension literature, and to the scientific community via peer-reviewed publications and presentations at professional meetings. Outcomes will be more informed adoption and improved management of cover crops as a component of sustainable farming practices and improved understanding of arthropods in agricultural soils.

Project Objectives:

1. Determine how cover crops currently used by some producers in the IPNW affect soil faunal diversity. Working on three cooperating producers’ farms we will assess the effect of cover crops vs. BAU farming without cover crops on belowground arthropod biodiversity over two years. We will test the hypothesis that soil arthropod biodiversity is greater under cover crops than in the absence of cover crops.

2. Determine whether cover crops with greater plant species diversity have greater belowground arthropod biodiversity. In small-plot studies with collaborators at an NRCS site, we will assess the effects of experimentally manipulated cover crop diversity on the abundance and diversity of belowground arthropods. We will test the hypothesis that soil arthropod diversity increases in response to increased cover crop diversity.

3. Measure the association between soil arthropod diversity and a key soil ecosystem function, crop residue decomposition. On soils with differing belowground arthropod diversity as assessed in Objective 2 we will measure decomposition rates of a standardized cover crop residue. We will test the hypothesis that rates of residue decomposition increase with soil arthropod community diversity. In year 2 of the study, we will validate results on producer farms to provide them with information on arthropod-mediated residue decomposition rates in their fields.

4. Assess IPNW soil health using arthropods as bioindicators. The QBS-ar index will be used evaluate soils in cover crops and BAU farming and among treatments in the small plot study to demonstrate the applicability of this novel tool for assessing soil health in the IPNW.

Cooperators

Click linked name(s) to expand
  • Alison Detjens - Producer
  • Dane Elmquist (Researcher)
  • Wayne Jensen - Producer
  • Frank Wolf - Producer

Research

Materials and methods:

Growers in our study region are increasingly interested in rotational diversification with alternative crops and intensification strategies such as cover cropping. This project addresses how cover crop use and cover crop diversity affect soil arthropod biodiversity and functioning relative to the “business-as-usual” (BAU) rotations that reflect past and current conventional management strategies. The typical BAU rotation for farms in our study region consists of a spring legume-winter wheat-spring grain rotation. Growers in the region are shifting away from BAU rotations by diversifying their rotations with cover crops (e.g., spring legume-winter wheat-cover crop). By focusing on soil arthropods, which are typically understudied in the context of cover crops and agricultural diversification, this project is making unique contributions to research on cover crops and how they affect soil health in Inland Pacific Northwest (IPNW) agroecosystems.

Throughout 2019-2020, we have investigated how the taxonomic and functional diversity of cover crop mixes affects soil arthropod communities (Obj. 2 of our proposal). This research is being conducted on small-scale replicated cover crop plots (1.6×2.4 m2) that were established in Pullman, WA in 2019 to initiate a multiyear study. The cover crops we are investigating include flax, sunflower, spring pea, and sweet clover. Cover crop treatments include each species planted individually, a mixture of all species, and a fallow control. Treatments (n=6) are replicated 3 times for a total of 18 plots. Soil arthropod communities are collected using soil cores (2L) at a depth of 0-10 cm. At sampling, soil temperature and volumetric water content from each sample are measured and a soil sample from each core is taken to measure soil pH. Soil arthropods are extracted from soil cores using a Berlese funnel system, collected in a preservative, and are characterized and counted under a stereomicroscope. The effects of cover crop treatment on community metrics, including taxon richness, functional group richness, abundance, evenness, and Shannon’s Diversity are being compared.

Information on how monoculture cover crops affect soil arthropod biodiversity and function relative to polyculture cover crops will enable producers to select cover crops that augment desired arthropod-mediated ecosystem services. Support from WSARE has enabled us to continue this evaluation for a third year (2021). The historic current drought conditions of 2021 resulted in weak cover crop establishment, but we will proceed with sampling these plots for soil arthropods to examine legacy effects of the prior two years of cover crop treatments. Information from the small plot research will be provided to producers to help them understand the effects of different cover crop composition on soil biota and soil health and will help inform cover crop mix selection. 

In fall 2021 through summer 2022, we will evaluate the legacy effects (i.e., arthropod biodiversity) of each cover crop treatment on winter wheat, a primary cash crop in the IPNW, which will be planted over the small plot experiment this fall. Evaluating cover crop legacy effects on winter wheat is relevant to farmers who will be using cover crops in rotation with cereals. This change in our original design will improve the applicability of our results. By the conclusion of the project, we will have 3 years of data on soil arthropod communities from 6 cover crop treatments and 1 year of data assessing the legacy effects of each cover crop treatment on soil arthropod communities in the following winter wheat crop. Furthermore, to link diversity and function, a litterbag decomposition experiment will also be conducted in fall 2021-summer 2022. Cover crop termination leaves crop residue on fields, and soil arthropods play a substantive role in decomposition, but their overall contribution to organic matter breakdown has not been studied in the context of cover crops in the IPNW.

To measure the contribution of soil arthropods to litter decomposition, two litterbag mesh sizes will be used: mesh size 4.0 mm allows both microbes and soil arthropods to access the litter, and mesh size 0.4 mm excludes soil arthropods. 18 litterbags (10×10 cm) filled with 3.5 g of standardized wheat residue will be placed at the soil-litter interface in each of the small-plot treatments (18 plots for a total of 324 bags; 162 of each mesh size). Litterbags will be collected after 50, 100, and 150 days to assess litter decomposition dynamics. Six litterbags (3 of each mesh size) will be collected from each plot during each collection. After collection, subsamples (0.5g) of litter from each litterbag will be stored at -80 ̊C to assess microbial biomass via phospholipid fatty acid (PLFA) analysis. The remaining litter will be placed in a Berlese funnel for soil arthropod extraction, then dried and weighed to determine litter mass loss for each mesh size and treatment. Samples from each treatment, collection date, and litterbag mesh size will be analyzed for total carbon and nitrogen at 0, 50, 100, and 150 days.

To complement the small-plot research, WSARE support is enabling us to conduct research on producer-cooperator farms that are experimenting with cover crops in 2021 and 2022. Our goal is to determine how cover crops affect belowground arthropod biodiversity relative to traditional BAU rotational crops on working farms. Two of three cooperator farms are large-scale commercial operations, and one is a smaller-scale organic for-profit farm. Working on these cooperator farms in 2021 (Table 1), we are sampling soils in cover crop fields and BAU fields at 3 time points during the growing season: at the onset of planting, during the growing-period, and post-termination. Samples of the soil arthropod community are collected using soil cores obtained at two randomly selected sites along each of three randomly oriented 50m transects in each field (25m transects at the organic farm). Arthropod communities are extracted, characterized, and analyzed as previously described. Producer-cooperators will each be provided with a yearly summary of the soil arthropod communities in their fields. Additionally, we will utilize the Soil Biological Quality index (QBS-ar) to characterize the effects of different rotational crops (i.e., BAU or cover crops) on soil health. The QBS-ar is an index that weights soil arthropod taxa based on their adaptation to the soil habitat, assuming that the number of arthropod groups morphologically adapted to the soil is higher in healthier soils. The index is sensitive to land use change and short-term variations in management practices, making it ideal for our producer-cooperators to assess how their management decisions influence soil health. We will work with our producer-cooperators to understand the utility of the QBS-ar index as a tool for producers to measure soil health in IPNW agroecosystems.

Table 1. Producer-cooperator farms being sampled in 2021. 

Farm   Crops Sampled
Commercial Farm 1   BAU Cover Crop
  Field Pair 1 Spring wheat Spring pea, spring wheat, mustard, turnip, clover
  Field Pair 2 Chickpea Flax, turnip, chickpea
  Field Pair 3 Chickpea Spring pea, spring oat, turnip, radish, triticale
Commercial Farm 2      
  Field Pair 1 Spring cereal Multi-species mix
  Field Pair 2 Spring legume Multi-species mix
Organic Farm      
  Field Pair 1 Potatoes Spring pea, oats
  Field Pair 2 TBD Fall cover crop (TBD)

 

 

Research results and discussion:

Results

Field sampling and data analysis for 2021 are underway. Initial results from the small-plot studies in 2019-2020 indicate that cover crop species and diversity affect soil arthropod diversity with possible implications for soil and crop health. We observed that the Shannon diversity of the soil arthropod community was higher in all cover crop treatments compared to fallow (Fig. 1). We also observed that polyculture cover crops had the highest diversity of predatory soil arthropods compared to all other treatments (Fig. 2).

 

graph displaying community diversity by cover crop
Figure 1. Effects of cover crop treatment on soil arthropod diversity (mean values of the Shannon diversity index). Different letters between treatments indicate significant differences (p<0.05) based on ANOVA. Polyculture=mix of all species.
graph displaying predator diversity by cover crop
Figure 2. Effects of cover crop treatment on predatory soil arthropod diversity (mean values of the Shannon diversity index). Different letters between treatments indicate significant differences (p<0.05) based on ANOVA. Polyculture=mix of all species.

 

Discussion

While higher soil biodiversity is known to increase ecosystem functionality and resilience, cover crops that augment the diversity of specific functional groups might be desired to target specific needs of agricultural producers, such as pest control or improved nutrient cycling. Augmenting predator communities improves control of soil pests and can regulate soil microbial communities through top-down control of soil arthropods that feed on microbes. Compared to conventional farming practices, like fallow, cover crops can increase soil arthropod biodiversity, potentially improving the delivery of ecosystem services in agroecosystems. Cover crops that augment desirable functional groups, like predators, can potentially be used to engineer agroecosystems ecologically to benefit producers. Results from this research so far indicate that cover crops, in addition to agronomic benefits they may provide, can simultaneously promote diversity in beneficial soil arthropod communities, enhancing ecosystem services and agroecosystem sustainability. These soil arthropod communities and their responses to cover crops have never been documented in the IPNW prior to this project.

 

Participation Summary
3 Farmers participating in research

Educational & Outreach Activities

2 Curricula, factsheets or educational tools
1 Webinars / talks / presentations
2 Workshop field days

Participation Summary:

49 Farmers
5 Ag professionals participated
Education/outreach description:

We have participated in several outreach activities centered around agricultural diversification in 2021. Information on cover crops, soil arthropods, and preliminary project results were shared with producers and professionals at the Genesee Area Crop Tour in Latah Co., ID and the Bonners Ferry Field Day in Boundary Co., ID. Factsheets that included project results and take-home messages complemented the field day presentations and were distributed to all attendees. Field Day Handout_2021

Additionally, information about cover crops, soil arthropods, and the objectives/summary of this project are posted on the University of Idaho Soil Stewards Farm featured research webpage. This information will be updated throughout the course of the project. The first update will summarize 2021 results. 

Finally, project objectives and preliminary results were presented and discussed during the 2021 University of Idaho Entomology, Plant Pathology and Nematology departmental seminar series in a talk titled, “The ecology of soil arthropods in agroecosystems: Effects of agricultural diversification on community structure, function, and interactions”.

There are several upcoming outreach activities where this project will be featured. Elmquist will deliver an oral presentation that reports and discusses project results titled, “Effects of cover crops on the structure and function of soil arthropod communities” at the 2021 Entomological Society of America national meeting (Oct 31-Nov 4 Denver, CO).

Results and take-aways from this project will be presented to producers and professionals in a workshop session at the 2021 Washington State University Wheat Academy (Dec. 15-16) in a session titled, “Macrofauna & Soil Health: Getting to know our below-ground partners in PNW wheat systems”. The workshop will include a breakout period where attendees will observe live soil arthropods. A similar workshop session that focused on soil arthropods and soil health in 2019 sparked the interest of several producers who wanted to know how these organisms impact soil health. We will be prepared this year to answer such questions more substantively based on the results of this WSARE funded project.

Finally, results from this project will be reported to the Landscapes in Transition (USDA-NIFA award #2017-68002-26819) project leadership and the stakeholder advisory committee, as well as presented at the Landscapes in Transition annual meeting in 2022 and included on the Landscapes in Transition website (https://www.pnwlit.org/home).

Project Outcomes

2 Farmers reporting change in knowledge, attitudes, skills and/or awareness
1 Farmers intend/plan to change their practice(s)
Did this project contribute to a larger project?:
Yes
2 New working collaborations
Project outcomes:

Achieving long-term agricultural sustainability goals will require preserving agroecosystem biodiversity and the ecosystem services that it provides. Although this principle is generally understood, the contributions of soil arthropods are underappreciated, and management of these taxa is not currently a part of stewardship. We are developing a robust understanding of how cover crops affect the biodiversity of belowground arthropods. Our study is evaluating how cover crops can influence soil arthropod biodiversity and the ecosystem services they provide, like decomposition. We are conveying this information to IPNW producers to inform their decisions about cover crop mix selection and management. We are incorporating our findings into outreach presentations and, eventually, extension materials aimed at producers and other stakeholders. This research is contributing to a regional cover crop knowledge base to promote the adoption of cover crops and improve agroecosystem resiliency, thus benefiting the overall sustainability and well-being of rural communities in the region. Information from this research will not only inform soil management practices but will also fill knowledge gaps about the complex interactions between plants, soil arthropods, and their associated ecosystem services. These results are also applicable to other regions looking to utilize cover crops.

 

Knowledge Gained:

Producers in the IPNW are adopting new practices to improve agricultural sustainability. Nonetheless, cover crop adoption has been slow in the region. Through our interactions with producers as part of this project, we have become more knowledgeable about the constraints faced by farmers and what is motivating them to understand and implement cover cropping on their farms, even by conducting their own on-farm trials. This interest and enthusiasm for alternative agricultural practices, including cover crops, is growing with potential to guide impactful ongoing research partnerships between producers and scientists like those affiliated with land grant universities including the University of Idaho. 

We have also learned that growers in our region, as across the USA, are interested in how farming practices impact the biological parameters of their soil, specifically microbes and arthropods. This understanding can help us refine our research and outreach approaches by focusing specifically on how farming practices affect the abundance and function of soil organisms. A clear need is improved delineation of the functions of certain soil organisms and their contributions to desirable agricultural outcomes (e.g., pest control or litter decomposition). In addition, improving how we communicate with producers and stakeholders about soil biology may increase producer awareness of the system-wide benefits of sustainable agriculture that go beyond economics.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.