Location: Northwest Irrigation and Soils Research
Title: Delayed sample filtration and storage effects on dissolved nutrients measured in agricultural runoff Author
Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 14, 2012
Publication Date: N/A
Interpretive Summary: It is commonly held that sediment-laden runoff waters sampled to determine dissolved nutrient concentrations must be filtered in the field, immediately after collection. This requirement increases the cost of water quality sampling and limits the use of automatic sampling devices in environmental studies. This investigation explored the effect of delaying the filtration of water samples on measured dissolved nutrient concentrations. The research determined that water samples collected in the field may be stored at 4 deg. C for up to 10 days before being filtered, without significantly changing the measured dissolved concentrations of nitrate- and ammonium-nitrogen or phosphate. The work also demonstrated how dissolved nutrient concentrations in sediment-laden runoff water leaving the field edge can change as the water volume transits downstream through the surface drainage network. This suggested that the development and use of an incubation-type measurement protocol could lead to more accurate estimates of dissolved nutrient concentrations in aquatic environments downstream from agricultural fields. This information can decrease costs associated with water quality sampling and lead to more accurate predictions of farm management practices on aquatic ecosystems.
Technical Abstract: Standard water quality analysis methods recommend that sediment-laden runoff waters sampled to determine dissolved nutrient concentrations be filtered immediately after collection. Few research studies have examined the influence of delayed filtration on sample stability or nutrient loss assessments. Twenty eight runoff water volumes were collected from 3 irrigation furrows during a 12-h irrigation set. Four subsamples of each volume were obtained; 2 were filtered (45 µm) in the field and the other 2 were filtered 10 days later, with or without boric acid treatment (1 mL saturated H3BO3 solution per 100 mL sample). All samples were refrigerated at 4 deg. C. Dissolved reactive P (DRP), NO3-N, and NH4-N concentrations were measured in all filtered samples 10 and 107 days after collection. Samples filtered in the field and those with a 10-day delayed filtration had similar dissolved DRP, NO3-N, and NH4-N concentrations, whether or not boric acid was added. After 107 days storage: 1) measured NO3-N concentrations in delayed filtration samples were 3.7-times greater than that in field-filtered samples, regardless of boric acid addition; and 2) boric acid had stabilized DRP and NH4-N concentrations relative to field-filtered samples. When integrated across the entire irrigation, the mean NO3-N and DRP furrow stream concentration and runoff mass losses computed from runoff water samples were similar for field filtered and delayed-filtration/no-boric-acid treatments. However, when these same parameters were computed using delayed-filtration, extended-storage (107-d) sample results, both NO3-N (4.7x) and DRP (1.9x) were greater in magnitude. The field-filtered or 10-d delayed-filtration without boric acid treatments provided the best dissolved nutrient measurements for comparing agricultural management effects at the field edge; however, results suggest that an incubation-type test of field-edge runoff water may provide a more accurate estimate of field management effects on downstream dissolved nutrient loads and aquatic ecosystems.