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Conservative And Non-conservative Tracer Studies Toward Measuring Effect Of Agricultural Land Use On Riparian Buffer Function

Congress: 2015
Author(s): Bonnie Robertson, Sheng Li, Sanjayan Satchithanantham, Xing Zisheng, Josee Owen
Agriculture and Agri-Food Canada1

Keyword(s): Special session 8: Agricultural research,
AbstractRobertson B1, Li S1, Satchithanantham S2, Xing Z1, and Owen J1

Correspondence author: bonnie.robertson@agr.gc.ca, 1-506-460-4335; 1Agriculture and Agri-Food Canada Potato Research Centre, 850 Lincoln Road, Fredericton, New Brunswick, Canada; 2Agriculture and Agri-Food Canada Central Research Station, 101 Route 100, Unit 100, Morden, Manitoba, Canada

Introduction: In recent years, riparian areas are increasingly seen as nutrient sinks for agricultural runoff. While riparian areas undeniably take up nutrients, growing evidence suggests limitations to this functionality. Little study has been reported on whether land use adjacent to the riparian area has a measurable effect on in-stream nutrient transport. While environmental regulations in Eastern Canada typically require a 30-m buffer be maintained between agricultural land and water courses, agricultural practices are still carried out in some places right up the water's edge, with detrimental effects to native riparian vegetation. On the other hand, there exist stream reaches with non-disturbed buffer zone much wider than 30 m. To quantify the effect of land use on in-stream nutrient transportation, a series of conservative and non-conservative tracer studies are being undertaken within a small model watershed in the province of New Brunswick in Canada. Information obtained from these studies will enhance our understanding of the functionality of riparian zones under different land use regimes and can be used to develop decision support tools and more targeted conservation efforts. This presentation discusses the first phase of the study, which was to conduct an initial tracer experiment to determine whether land use has an impact on in-stream nutrient cycling.

Methods/Materials: The Black Brook Watershed (BBW) in New Brunswick, Canada, is a 1450 hectare model watershed in the heart of the province's potato belt that has been studied for more than 20 years to provide quantitative data on the effects of agricultural practices on soil and water quality. A stream reach of about 300 m long was selected for the tracer release experiment. The stream reach was divided by a water monitoring station (V-notch weir) into two sections. The lower section has a buffer zone of about 30 m and beyond which are cultivated fields. The upper section is mostly forested, with a > 100 m wide un-disturbed buffer zone. A mixture of conservative (chloride, Cl) and biologically active non-conservative ions (nitrate (NO3) and phosphate (PO4)) was used as tracers. The tracer solution was made of sodium chloride (NaCl), potassium nitrate (KNO3), and mono potassium phosphate (KH2PO4). Targeted concentrations needed above the ambient levels are 50 mg/L, 20 ppm and 0.5 ppm respectively for NaCl, NO3- and PO43-. The tracer release experiment was conducted at the lower reach first. Tracer solution was added into the stream at a constant rate for 10 seconds. Measurement of electrical conductivity and temperature of the runoff water was carried out using conductivity data loggers placed at five locations of 20-30 m intervals along the flow path, and logging measurements every 30 seconds. Water samples were taken using an ISCO autosampler at the first and last locations with dataloggers. At the first water sampling location (approximately 10 m downstream the release location), samples were taken immediately after the release and then every three minutes until the conductivity went back to ambient level. A handheld conductivity meter was used for estimating the arrival of the tracer at downstream sampling locations so that the sampling could be done in a timely manner. Similar setup of data monitoring and water sampling were applied for the upper reach. An additional tracer release experiment was conducted on a 60 m long road side ditch, which was used to represent a condition of non-forested stream or a field edge channel. All water samples were packed with ice for temporary storage and transported to the Soil Hydrology Laboratory for analyses of NO3- and PO43-.

Results and Discussion: Results of the tracer release experiment showed that different land use in the buffer zone differed in nutrient uptake rate. Although there appear to be little differences in flow rate and speed, the upper reach with wider buffer zone showed stronger ability in nutrient uptake. This may be attributed to the reduced disturbance in the upper reach which preserved the natural development of stream morphology and vegetation diversity, and in turn, facilitates higher uptake of nutrients. Compared to the road-side ditch, the in-stream release results showed minimal nutrient uptake and storage, while the road-side ditch release showed a greater uptake by the riparian vegetation, although the flow was much faster, probably due to the fact the ditch is fully grassed and the flow is temporary. This experiment was conducted once during the growing season. It remains to be seen whether there are differences in the uptake capacity during active versus non-active growing stages of the plants, as well as soil conditions at various temperatures. In addition, this experiment only looked at a grassed waterway as an edge-of-field treatment. Future studies should examine various vegetation types to determine if nutrient uptake can be optimized.

Conclusion: The data collected provided insight into nutrient uptake and storage both within the stream and by a vegetated buffer during a time when temperatures are conducive to active nutrient cycling. The vegetated buffer uptakes nutrients before they are released to the stream. A vegetated buffer between agricultural activities and a waterbody reduces the impact of nutrients on surface water.

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