Data from: Nutrient leaching potential along a time series of forest water reclamation facilities in Northern Idaho

Forest water reclamation is a decades-old practice of repurposing
municipal reclaimed water using land application on forests to filter
nutrients and increase wood production. However, long-term application may
lead to nutrient saturation, leaching, and potential impairment of ground
and surface water quality. We studied long-term effects of reclaimed water
application on nutrient leaching potential in a four-decade time series of
forest water reclamation facilities in northern Idaho. Our approach
compared reclaimed water treated plots with untreated control plots at
each of the forest water reclamation facilities. We measured soil
nitrifier abundance and net nitrification rates and used tension
lysimeters to sample soil matrix water and drain gauges to sample from a
combination of matrix and preferential flow paths. We determined nutrient
leaching as the product of soil water nutrient concentrations and
model-estimated drainage flux. There was more than 450-fold increase in
nitrifier abundance and a 1000-fold increase in net nitrification rates in
effluent plots compared with control plots at long-established facilities,
indicating greater nitrate production with increased cumulative inputs.
Modeled drainage increased 12-120% in treated plots but only during the
wet season in effluent plots at the long-established facilities. There
were no differences in soil water ammonium, phosphate, and dissolved
organic nitrogen concentrations between control and effluent treatments in
tension lysimeter samples. However, concurrent with increased nitrifier
abundance and net nitrification, nitrate concentration below the rooting
zone was 2 to 4-fold higher and nitrate leaching was 4 to 10-fold higher
in effluent treated plots, particularly at facilities that have been in
operation for over three decades. Thus, net nitrification and nitrifier
abundance assays are likely indicators of nitrate leaching potential.
Inorganic nutrient concentrations in drain gauge samples were 2 to 11-fold
higher than lysimeter samples, suggesting nutrient losses occurred
predominantly through preferential flow paths. Nitrate was vulnerable to
leaching during the wet season under saturated flow conditions. Although
nitrogen saturation is a concern that should be mitigated at the
long-established facilities, these forest water reclamation facilities are
able to decrease average soil water nitrate concentrations to less than 2
mg L-1, so that nitrogen and phosphorous are effectively filtered to below
safe water standards.