Precipitation manipulation experiments may be confounded by water source

Authors: Reinhart, Kurt; Vermeire, Lance

Submitted to: Oikos
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: Problem Climate change experiments routinely irrigate plots with mineralized water. Unfortunately, little is known about how the mineralized water may complicate results and interpretations. Accomplishment We found that irrigating plots with tap water dramatically increased soil sulfate concentrations and to a lesser degree increased iron and calcium. Addition of these minerals appeared to reduce the availability of plant available phosphorus. Contribution Climate change experiments with irrigation treatments should collect and apply rainwater to plots or utilize filtered (RO) water.

Technical Abstract: Climate change experiments routinely include precipitation variability treatments. Water addition treatments are a critical component of these experiments, and the majority use mineralized water in place of rainwater (i.e. demineralized water). Limited attention has been paid to the effects of water mineralization on the results of climate change experiments. Here, we conducted a 6-yr experiment with three spring rainfall variability treatments (rainfed ambient control, -30% of ambient, +30% of ambient). We tested effects of rainfall treatments on plant biomass and six soil properties and interpreted the confounding effects of water impurities. Rainfall treatments affected all response variables. Sulfate was the most common dissolved solid in irrigation water and was 41 times more abundant in irrigated than other plots. Soils of irrigated plots also had elevated iron and pH. Rainfall variability treatments also had a nonlinear (hump-shaped) effect on plant available phosphorus (P). Plant and microbial biomasses are often limited by and positively associated with available P, suggesting the predicted positive linear relationship between plant biomass and P was confounded by additions of mineralized water. In other words, the unexpected nonlinear relationship was likely driven by components of mineralized irrigation water (i.e. calcium, iron) and/or shifts in soil pH that immobilized P. Our results suggest robust climate change experiments should either capture rainwater when possible (or use demineralized water) or consider the confounding effects of mineralized water on plant and soil properties.