Soil Chemical Variation Along a Four-Decade Time Series of Reclaimed Water Amendments in Northern Idaho Forests

Application of municipal reclaimed water to forests for water reclamation is a pragmatic approach that provides water and nutrients to soil and lowers the liability of reclaimed water disposal, yet little is known about the long-term impacts of reclaimed water amendment on forest soil chemical properties. We hypothesized that reclaimed water constituents will increase plant nutrient availability in soil with the magnitude of response depending on the facility establishment date. We collected samples from three mineral soil depths to 75 cm from treated and control plots at five water reuse facilities that represent a four-decade time series. Depth explained most of the observed variation. Several plant nutrients increased in soil at the different sites in response to reclaimed water treatments, including N, Ca, Fe, S, and B concentration as well as B content, while P was not significantly affected. Increases in cation concentrations positively correlated with pH and salinity. The treatment response was significantly greater at all facilities for total N, B and Na. However, the treatment response only occurred at long-established facilities for NO3-N and Ca concentrations and for Fe and S content. The outcomes of this study are useful for guiding future management of soil at forest water reclamation facilities and for limiting the risk of downstream environmental impacts.