Optimizing Denitrification in Agricultural Streams. Todd V. Royer & Jennifer L. Tank

Transcription

1 Optimizing Denitrification in Agricultural Streams Todd V. Royer & Jennifer L. Tank

2 Goals Lay a general groundwork for a discussion of how to optimize denitrification in agricultural landscapes Suggest that in agricultural landscapes, the question of how to manage N is really a question of how to manage water Describe the constraints on managing N in agricultural landscapes

3 What is an agricultural landscape? Pasture Tree plantation Intensive row-crop

4 Generally, the largest N application is to row-crops, often in combination with artificial drainage Channelization & Dredging Tile drainage

5 Our Context Benton County, Indiana Champaign County, Illinois Headwater (1 st -3 rd order) Land cover is >80% row-crop, mainly corn & soy beans Landscape is tile-drained, streams are channelized Application to corn is kg N ha -1 yr -1

6 7 km 2 basin 5 km stream 58 tile drain outfalls From: Baker et al. USGS Report

7 Headwater streams have the capacity to reduce N loading to downstream water bodies but that capacity is greatly reduced as water depth increases From: Alexander et al JAWRA

8 Even at base flow, streams do not have infinite capacity to remove N Increasing [N] decreases N removal efficiency LINX sites had either low N or low Q; not representative of streams in row-crop landscapes From: Mulholland et al.

9 To increase the capacity of streams for N removal, one could attempt to increase denitrification rates (as mass of N per unit area) Denitrification is an enzymatic process, so there is a limit (V max ) Nitrate-rich, productive agricultural streams tend to have high denitrification rates may often be near that limit

10 From: Mulholland et al. Approximately 2 orders of magnitude in range

11 Focus is often on denitrification, but should recognize that other processes play a role in N retention From: Mulholland et al.

12 Structural Equation Model, from Mulholland et al L&O

13 There are many factors that have an indirect influence on denitrification To optimize denitrification rates, one must know the limiting factor AND be able to adjust it this is impossible to do for a natural system Thus, we argue that attempting to optimize rates, per se, is not a fruitful approach

14 N removal = Denitrification Rate X Residence Time Residence time is more amenable to management than is denitrification rate In fact, it already is managed

15 Two Examples of Residence Time and N Removal Reservoir: Lake Shelbyville, IL Two-stage Manipulation: Shatto Ditch, IN

16 From: David et al Ecol. Applications

17 From: Goolsby et al. 1999

18 Many ecological problems associated with impounding rivers Creating new reservoirs is not a realistic solution But existing reservoirs potentially could be managed more effectively for N removal

19 Conventional Management Series of drains allow agriculture to occur Channel maintained Unstable in high flows Low nutrient processing and retention Two-Stage Ditch Design Assumes natural channel evolution Retains sediment Increased surface area for nutrient processing TNC/NRCS interest in implementation Graphic: Ohio State University,

20 1 day post-construction: 620m long, ~5m wide/side The effect of a 2-stage ditch is to increase the spatial and temporal extent of denitrification by increasing surface area (~3X during high flows) and water residence time Does not take additional land out of production Does not require routine maintenance

21 Conclusions from Shatto Ditch Two-stage ditch is a potentially effective management strategy to increase N removal via denitrification in agricultural streams Despite increased N removal via DN, % of N load denitrified is small Calls for combination of management strategies of which 2-stage manipulation is but one option

22 Constraints Approaches to optimizing denitrification in agricultural landscapes must work within the constraints of an agricultural production system Constraints exist on all of these factors From: Seitzinger et al Ecol. Applications

23 Fields must be drained and crops must be fertilized to maintain (profitable) agricultural production Riparian buffer strips are ineffective in tile-drained regions Very expensive to take land out of production

24 Conclusions Managing water is key to increasing N removal by in-stream denitrification in agricultural landscapes Optimizing N removal by denitrification will likely require a combination of approaches Decreased N inputs Increased water residence time Strategic restoration of geomorphic features