Environmental Effects of Agricultural Practices Active

In the Minnesota River Basin, retirement of agricultural land has been actively pursued at the state and federal level and includes removal of tile drains and planting of retired riparian corridors with native grasses and is supported under various programs such as the Conservation Reserve Program (CRP), Conservation Reserve Enhancement Program (CREP), and Reinvest in Minnesota Program (RIM). U.S. Geological Survey scientists observed that suspended sediment as well as nitrite plus nitrate and total nitrogen (N) decreased with increasing percentages of retired land in a basin. In addition, although biological indicators varied in their response to percentage of retired land, most, including algal biovolume, abundance of tolerant invertebrate and fish species, and fish index of biotic integrity (IBI) scores improved as a function of the amount of retired land within 50 to 100 meters of the stream. In contrast, total phosphorus (P) concentrations showed no clear link to percentage of retired land. This observed inconsistent relation between various water quality and biological indicators led to questions of how the distribution and history of retired agricultural land impacted ecological outcomes.

A project was initiated in the Fall of 2011 that builds on the initial findings of Christensen and others (2009) to further address the linkages between riparian buffer extent, age, and continuity, and stream water and biotic quality, with a specific emphasis on sediment and phosphorus.

Features

• Small basin study of land management effects on water quality, in-stream nutrient retention, and organism health
• Distribution gradient of retired riparian corridor
• Incorporates a long-term water-quality site
• Scientists from multiple USGS Science Centers
• Interaction with local, State, and Federal cooperators including Minnesota Board of Soil and Water Resources, Hawk Creek Watershed Project, Renville County Soil and Water Conservation District, and NRCS District Office

Background

Sediments are considered among the most significant agricultural pollutants because sediment runoff physically degrades aquatic habitat, and carries excess nutrients, especially phosphorus.  Riparian buffers, including those retired from agricultural production, are a key agricultural practice used to control runoff of sediment and associated pollutants. Edge-of-field studies have shown that these buffers:

• Reduce streambank erosion
• Dissipate stream energy
• Trap sediment and associated pollutants
• Enhance terrestrial and aquatic wildlife habitat

However, the link between riparian buffers and ecological response is less clear, due to issues of:

• Discontinuities of riparian buffers along the length of a stream
• Vegetation management on retired agricultural land
• The addition of tile-drains to fields that pass through the retired agricultural land in the riparian zone
• Changes in streambank erosion and sediment storage in retired areas
• The effect of in-ditch vegetation on sediment and nutrient retention and support of biodiversity

This project builds on our initial findings to further address the linkages between riparian buffer extent, age, and continuity, and stream water and biotic quality, with a specific emphasis on sediment and phosphorus. The approach:

• Defines a framework for study design that is transferrable to other physiographic and agricultural settings
• Demonstrates how detailed understanding of processes can lead to improved indicators of retired riparian corridor effectiveness
• Produces new understanding of the role of retired riparian corridors in improvement of water and stream biota quality. In addition, this project proposes a biotic response as a critical component of assessing the effects of retired lands.
• Improves understanding of the effect of within ditch vegetation on sediment and nutrient retention, and the relation to biodiversity and stream organism health

This project demonstrates an integrated USGS approach to watershed-scale studies, involves USGS scientists from several Science Centers and National Programs, and is conducted in cooperation with local, state and federal agencies. This project addresses the Midwest Area Environmental Effects of Agricultural Practices Initiative Themes of Watershed Resilience, Ecosystem Services, and Wildlife and Human Health, and addresses the Energy theme by addressing age of retirement and continuity of retired lands.

Research Approach

From the previously-studied Minnesota River basins (Christensen and others, 2009) a series of subbasins were selected to isolate the issues of hydrogeology and buffer characteristics. In order to focus on the impact of buffer character, sites will be selected that involve primarily cropland agriculture. Sub-basins will be chosen to provide a range of (1) time of retirement and (2) hydrogeologic conditions, at the same time controlling for other issues that might complicate the perceived response.  Selected basins will be evaluated in terms of:

• Extent and duration of agricultural land retirement
• Hydrogeology
• Artificial drainage
• Agricultural practice
• Relation of streams to buffers, tile-drains, and agricultural fields

Data collection and sampling design includes:

Ecosystem Quality

• Water quality samples (Stream water, Groundwater, Tile-drained water)
• Water quality components (Nutrients, Suspended sediment concentration), including real-time water-quality monitoring
• Biological sampling and lipid analysis of algae, insects, and fish
• Organism health
• Food sources
• Stalbe isotopic (C & N) analysis of fish and insects
• Food source
• Food chain length

Sediment Characterization

• Sediment samples from multiple flow events
• Collection of sediment source samples
• Samples to be analyzed by USGS labs (Central Mineral and Environmental Resources Lab, Organic Geochemistry Research Lab, Kentucky Sediment Lab, Colorado Plateau Stable Isotope Lab, Upper Midwest Environmental Sciences Center (UMESC) Lipid Lab, National Water Quality Lab)

Photo credits: 

U.S. Geological field crew collects sample in a tributary of the Minnesota River Basin, 2008 (photo by Victoria Christensen)  

 U.S. Geological Survey field crew sampling invertebrates at South Branch Rush River, MN, 2006  (photo by: Victoria Christensen, U.S. Geological Survey)