Soil Organic Carbon Controls on Nitrate Removal in Saturated Riparian Buffers

Issue

A novel approach for reducing nitrate-nitrogen loss is to intercept a field drainage tile where it crosses a riparian buffer and divert a fraction of the flow as shallow groundwater within the buffer. This practice is called a saturated riparian buffer (SRB), and although it is promising, data on practice performance and controlling factors is limited. One likely important factor in performance variance of SRBs identified so far is the availability of labile organic carbon within soils to drive microbial denitrification. However, while thought to be important, little research has been conducted to specifically assess the role of soil organic carbon on nitrate loss rates within SRBs.We predict that soil organic carbon content and quality may play a critical role in determining the effectiveness of SRBs in removing NO3, which may be predictable as a function of vegetation and soil characteristics that can inform effective placement of new SRBs.

Objective

The overarching goal of this project is to advance understanding of the factors controlling nitrate-nitrogen loss within saturated buffers and use the data to refine criteria and guidelines for their proper siting and design. Specific objectives are to:

1. quantify long-term NO3 loss within saturated buffers established under a range of conditions;
2. evaluate the role of soil organic carbon, current and previous vegetation/crop type, and other soil factors in regulating NO3 loss within saturated buffers; and
3. develop site evaluation and design criteria to maximize NO3 removal.

Approach

Research will be conducted within 17 established and monitored saturated buffers. At each location, subsurface drainage flow is intercepted and infiltrated as shallow groundwater at the top of the buffers. Flow and NO3 concentrations in the subsurface drainage will be measured continuously. The fate of the NO3 in the buffers will be followed by sampling the groundwater within the buffers from multiple transects of wells spanning the width of the buffers. Results will be compared looking at assessment of carbon availability, source and turnover dynamics within the soil-water matrix. These results will be correlated with additional characteristics of the buffers' soils and vegetation to determine the amount, sources and turnover times of fast- and slow-cycling soil organic carbon pools. This will enable quantitative assessment of linkages between carbon availability/source and performance of SRBs.