ALBERT

Description: Bioavailable organic carbon in aquifer-recharge waters and sediments can fuel microbial reactions with implications for groundwater quality. A previous incubation experiment showed that sedimentary organic carbon (SOC) mobilized off sandy sediment collected from an arsenic-contaminated and methanogenic aquifer in Bangladesh was bioavailable; it was fermented into methane. We used high-resolution mass spectrometry to molecularly characterize this mobilized SOC, reference its composition against dissolved organic carbon (DOC) in aquifer recharge water, track compositional changes during incubation, and advance understanding of how composition relates to bioavailability in anaerobic conditions.

Description: Aims: Hydro-biogeochemical processes in the rhizosphere regulate nutrient and water availability, and thus ecosystem productivity. We hypothesized that two such processes often neglected in rhizosphere models - diel plant water use and competitive cation exchange - could interact to enhance availability of K+ and NH4+, both high-demand nutrients. Methods: A rhizosphere model with competitive cation exchange was used to investigate how diel plant water use (i.e., daytime transpiration coupled with no nighttime water use, with nighttime root water release, and with nighttime transpiration) affects competitive ion interactions and availability of K+ and NH4+. Results: Competitive cation exchange enabled low-demand cations that accumulate against roots (Ca2+, Mg2+, Na+) to desorb NH4+ and K+ from soil, generating non-monotonic dissolved concentration profiles (i.e. 'hotspots' 0.1-1 cm from the root). Cation accumulation and competitive desorption increased with net root water uptake. Daytime transpiration rate controlled diel variation in NH4+ and K+ aqueous mass, nighttime water use controlled spatial locations of 'hotspots', and day-to-night differences in water use controlled diel differences in 'hotspot' concentrations. Conclusions: Diel plant water use and competitive cation exchange enhanced NH4+ and K+ availability and influenced rhizosphere concentration dynamics. Demonstrated responses have implications for understanding rhizosphere nutrient cycling and plant nutrient uptake.

Description: Angle Lake and Lake Killarney (western Washington State, USA) were sampled monthly or bi-monthly on a total of 29 dates between July 2016 and December 2017. Water column temperature and dissolved oxygen profiles were measured using a multi-parameter water quality probe (In-Situ smarTROLL MP) at approximately the deepest point in each lake. A peristaltic pump with acid-washed tubing was used to collect filtered (0.45 µm Geotech cartridge filter) water samples into acid-washed polypropylene bottles that were acidified with trace metal grade nitric acid (1% v/v). Phytoplankton samples were collected by net tow (20-153 micron). Sediment trap samples were collected at mid-depth (~7 m and ~2 m from bottom in Angle Lake and Lake Killarney, respectively) and near bottom (~1 m above bottom) in both lakes. Phytoplankton tow samples and sediment trap samples were filtered onto 5 µm polycarbonate membrane filters then subjected to a microwave-assisted (CEM MARS 5) total digestion protocol (modified EPA method 3015). Aqueous arsenic concentrations in water samples, plankton digestion solutions, and sediment trap digestion solution were determined by ICP-MS on an Agilent 7900 at the University of Washington Tacoma. In-situ sediment and porewater sampling was conducted in Lake Killarney in July 2016, Aug. 2017, and Dec. 2017 and in Angle Lake in Sept. 2016, Aug, 2017, and Dec. 2017. Porewaters were collected using passive diffusive samplers (peepers) using sample vials (LDPE, 5 mL) filled with a de-oxygenated reverse tracer solution (200 µM KBr). Sediment cores were collected by in situ freezing, then sectioned under a N2 atmosphere. The solid phase of arsenic in sediments was investigated using a 3-step sequential extraction protocol: a phosphate-exchangeable fraction (PE) targeting strong adsorbed arsenic, a citrate-bicarbonate-dithionite (CBD) treatment targeting arsenic associated with iron oxides, and a microwave-assisted total nitric digestion targeting residual arsenic (R). Aqueous arsenic concentrations in porewater samples and sediment extraction solutions were determined by ICP-MS; bromide concentrations in peeper samples were measured using an ion selective electrode. A temperature manipulation experiment was performed on sediment cores were collected from Lake Killarney and Angle Lake in January 2018. Duplicate cores from each lake were incubated at 10°C and 20°C and the concentration of arsenic in the overlying water was monitored at 7 timepoints over 48 days; aqueous arsenic concentrations were determined by ICP-MS.

Description: Dissolved oxygen measurements and water samples for dissolved arsenic were taken from a boat at approximately the deepest point in each lake in June 2019. Dissolved oxygen was measured every half meter using an In-Situ smarTROLL MP equipped with an optical Rugged Dissolved Oxygen (RDO) sensor. Water samples were collected using a peristaltic pump, filtered (0.45 µm Geotech cartridge filter), and then acidified with 1% HNO3 (v/v) in the laboratory. Concentrations of total arsenic in water samples were determined by inductively-coupled plasma mass spectrometry (ICP-MS) on an Agilent 7900 at the University of Washington Tacoma.

Description: Between 2 and 6 littoral sediment samples were collected in 2018 and 2019 from nearshore (1-2 m depth) in each lake using a dredge (Wildco Petite Ponar Stainless Steel Grab) and dried at 60 °C overnight and homogenized using a porcelain mortar and pestle. Oven-dried sediment samples were digested using a microwave-assisted (CEM MARS 5) total digestion protocol (modified EPA method 3015a). Concentrations of total arsenic in digested sediment samples were determined by inductively-coupled plasma mass spectrometry (ICP-MS) on an Agilent 7900 at the University of Washington Tacoma.