Porewater sampling for NH4+ with Rhizon Soil Moisture Samplers (SMS): potential artifacts induced by NH4+ sorption

Biogeochemical studies done to elucidate sediment-water transfer of solutes and benthic reaction rates in limnic and marine ecosystems often rely on the study of porewater distribution and temporal dynamics of target solutes and subsequent diagenetic modeling. Hydrophilic poly-ether-sulfone (PES) membranes are used increasingly often to sample soil and sediment porewater because they are versatile and easily adapted to field studies in benthic biogeochemistry. Nevertheless, possible interference with components of filtered water by membrane surface properties impedes accurate measurement of concentrations of solutes in filtrates. We identified and described NH4+ sorption onto PES membranes. We quantified and constrained sorption characteristics of these samplers because of the potential influence of PES membranes on NH4+ filtrate composition. Maximum adsorption capacity was associated with low operational temperatures and reduced filtrate salinity. NH4+ partition coefficients (K*) > 8 x 10(-2) cm were measured at 5 degrees C and 0 parts per thousand salinity. However, K* followed an exponential decay curve with salinity and a negative exponential decay with temperature. NH4+ desorption from the membranes was rapid, but affected the 1st mL of filtered sample after removal of internal volume. We put forward recommendations for improved practice to minimize the influence of membrane adsorption capacity on measurement of NH4+ composition of porewater samples when using PES membranes.