Responses of soil faunal food webs to pesticide seed treatments

Responses of soil faunal food webs to pesticide seed treatments

Summary

Crop seeds coated with pesticides are commonly used in conventional agriculture to reduce pest pressures prior to seed germination and during early crop establishment. Pesticide seed treatments, which are commonly comprised of both insecticides and fungicides, are intended to directly affect pest populations in the soil food web by suppressing them via deterrents or toxins. These practices may also have unintended effects on soil food web composition and, subsequently, agriculturally important ecosystem services including decomposition and nutrient cycling. Because soil fauna occupy multiple trophic levels and contribute to these agriculturally important ecosystem services, we sought to examine both the direct and indirect effects pesticide seed treatments have on soil food web diversity, structure, and function via a two-year field experiment.

We established a field experiment at Penn State University’s Russell Larson Agricultural Research Center in Rocks Springs, PA in May of 2013.  The treatments were maize containing a pesticide seed treatment (Syngenta’s Cruiser Maxx® 250) and a control (maize without pesticide seed treatment). The same glyphosate resistant maize genotype was used in both treatments.  Treatments were assigned to 20 ft x 10 ft plots in a completely randomized design and replicated five times. Litter decomposition bags were placed in line with the crop after emergence, and sub-samples were removed at three time points during the growing season to monitor decomposition rates in the treatments. We observed no differences in decomposition rates between treatments. To quantify nitrogen turnover, we buried ion exchange resin strips in the rows and left them there for two weeks (repeated three times during the summer). We are still processing these samples. Soil fauna that colonized the litter decomposition bags and inhabiting the soils below the litter bags were extracted throughout the summer using Tullegren-Berelese funnels. The collected fauna are currently being identified, organized and prepared for isotopic analysis. The fauna samples will help us to understand how both the composition and structure of these soil food web communities are influenced by pesticide seed treatments. As this project continues, we hope to learn more about how the use of pesticide seed treatments affects soil food web communities. This information can then be used to inform our assessment of how and when pesticide seed treatments should be used. 

Objectives/Performance Targets

A comprehensive understanding of the effects pesticide seed treatments have on in situ soil food webs and the soil processes they control will inform our agricultural management practices so that both the potential benefits and limitations are accounted for prior to implementation. By meeting our objectives, we aim to assess the influence pesticide seed treatments have on soil food web composition, structure and function, including litter decomposition and nutrient cycling.

Our specific objectives are to:
1. Determine if pesticide seed treatments reduce soil food web diversity and complexity, measured by species richness and abundance, via direct and indirect pathways;
2. Determine whether pesticide treated seeds foster soil food webs with less diversified consumer-resource links;
3. Ascertain if the loss of diversity and simplified consumer-resource links reduces the food web’s capacity to perform agriculturally important ecosystem services including decomposition and nitrogen cycling.

In order to meet our overall objectives we:

1. Established the pesticide seed treatment experiment with maize in May and monitored leaf chlorophyll content and plant height throughout the growing season.
2. Measured rates of litter decomposition and nitrogen cycling.
3. Collected all soil fauna that colonized the decomposition litter bags as well as the fauna inhabiting the soil underneath each bag in each treatment three times during the growing season.

Over the next year, we plan to:
A. Establish pesticide treated and untreated soybeans (identical genotype) in the same plots used in 2013 to determine if there are cumulative effects on the soil food web community from the use of the pesticide seed treatments as part of a maize-soybean rotation.
B. Evaluate the effect of pesticide seed treatments on the composition and structure of the soil food web community.
C. Assess the year-to-year effect of pesticide seed treatments on the ability of the food web to perform agriculturally important ecosystem services. 

Accomplishments/Milestones

The field for this experiment was prepared with an S-tine cultivator and disked in early May 2013. Maize was planted on May 16th in a completely randomized design with 20 ft x 10 ft plots (six rows per plot) with six row buffers as initially proposed. After the seedlings were established, litter decomposition bags filled with nine grams of dry rye biomass were secured to the soil in line with the crop rows (May 31). The first set of ion exchange resin strips were placed June 5th at a 5-15 cm depth (June 5). Throughout the summer the ion exchange resin strips were replaced every two weeks to avoid strip saturation which would interfere with future calculations of NH₄⁺ and NO₃^– turnover rates.  Plant growth, leaf chlorophyll content, and soil moisture were monitored throughout the summer.

Decomposition bags and soil cores were collected from the field on July 2, July 31, and October 8. All fauna colonizing the litter bags and near-by soil samples were extracted using Tullegren-Berlese funnels.  The remaining rye litter was incinerated in a muffle furnace to determine the amount of mass remaining in each bag. We are currently in the process of identifying, sorting, and preparing the soil fauna for future isotopic analyses. We anticipate finishing these activities prior to the 2014 field season.

Upon Atwood’s return to New Hampshire in August, she was approached by an undergraduate student who was interested in conducting an independent research project under her advisement. The student’s interests in agroecosystems and soil fauna and timeliness of Atwood’s final soil collection (mid-October) enabled them to create a study within the framework of the pre-existing pesticide seed treatment experiment. The student’s project will explore the effect pesticide seed treatments have at the end of the growing season on the soil food web community inhabiting bulk soil without surface litter. To date, this undergraduate student has developed his own protocols, processed his soil samples, identified soil fauna, and researched the known effects of pesticide seed treatments. Atwood meets with him each week to discuss the project and work through fauna identifications together. 

Impacts and Contributions/Outcomes

To date this research has already impacted the undergraduate student that Atwood mentors by providing a platform for him to hone his scientific skill set. This spring semester, we will expand the impact of our research by sharing our findings and research with UNH undergraduates via lectures and laboratories in Smith’s Agroecology course. We have begun to plan these activities so that they facilitate not only a discussion regarding soil food web communities, but also how various agricultural management practices can affect the agroecosystem.  

We will be presenting the results from this summer’s research at several meetings in 2014, including the Annual Meeting of the Ecological Society of America in Sacramento, CA, as well as at the University of New Hampshire Graduate Student conference in March. Atwood and a colleague, Cynthia Kallenbach, recently proposed an organized oral session entitledRhizosphere Interactions: An Exploration of Patterns Across Systems for the 2014 Annual Meeting of the Ecological Society of America. The goal of this session is to integrate observations on soil biological interactions across ecosystems that are exposed to climate or land-use change to better identify unifying principles underlying rhizosphere community dynamics. The pesticide seed treatment experiment funded by NE-SARE was part of the impetus behind this session idea. Finally, once the data are fully analyzed, we also intend to present the results of this work at several farmer-focused conferences and meetings. 

Collaborators: