Abstract The gram‐positive soil bacterium, Arthrobacter nicotianae , uses multiple organic acid functional groups to adsorb lanthanides onto its cell surface. At relevant soil pH conditions of 4.0–6.0, many of these functional groups are de‐protonated and available for cation sorption and metal immobilization. However, among the plethora of naturally occurring site types, A. nicotianae is shown to possess high‐affinity amide and phosphate sites that disproportionately affect lanthanide adsorption to the cell wall. We quantify neodymium (Nd)‐selective site types, reporting an amide‐Nd stability constant of log 10 K = 6.41 ± 0.23 that is comparable to sorption via phosphate‐based moieties. These sites are two to three orders of magnitude more selective for Nd than the adsorption of divalent metals to ubiquitous carboxyl‐based moieties. This implies the importance of lanthanide biosorption in the context of metal transport in subsurface systems despite trace concentrations of lanthanides found in the natural environment.
Abstract Deserts comprise a large portion of the Earth's land area, yet their role in the fluxes and cycles of greenhouse gases is poorly known and their likely response to climate change largely unexplored. We report a reconnaissance investigation of the concentrations and fluxes of CO 2 , CH 4 , and N 2 O along two elevation (climate) gradients in the southwestern United States. In‐soil concentrations of CO 2 increased with elevation (up to 5,000 ppm). Concentrations of CH 4 declined with depth in all soils (to less than 1 ppm), but the rates of decrease with depth increased with elevation. In contrast, concentrations and depth trends of N 2 O varied erratically. Soils were net CO 2 sources (0 to >1,500 kg CO 2 ·ha −1 ·year −1 ), and net CH 4 sinks (0.2 to >3 kg CH 4· ha −1 ·year −1 ). The small and variable N 2 O fluxes were inconsistent with the trends in soil N δ 15 N values, which decreased by 5‰ to 6‰ over about 1,000 m of elevation. The high soil N δ 15 N values (up to nearly 17‰ at the lowest elevation) indicate that there is a soil N loss mechanism that is highly depleted in 15 N, and gaseous losses—either NH 3 or N 2 O/N 2 —are suspected of driving these values. In summary, there appears to be a strong climate control on both soil CO 2 and CH 4 concentrations and to a lesser degree on calculated fluxes. The soil N trace gas concentrations indicate that deserts can be either small sources or sinks of N 2 O and that there may be significant consumption of arid soil N 2 O.
Abstract The stable N and O isotope composition of soil and soil‐respired N 2 O is increasingly measured, yet a solid theoretical framework for interpreting the data remains to be developed. Here, the physical processes that affect soil N 2 O and its isotopes are embedded in a diffusion/reaction model. Numerical experiments are compared to data to demonstrate how various soil processes influence depth profiles and surface fluxes of soil N 2 O, δ 15 N N2O , and δ 18 O N2O . Model predictions and data suggest that the isotope composition of the net N 2 O soil flux, in soils that have N 2 O consumption, is a function of the net flux rate, and the isotope differences between the atmosphere and the biological source. Asymptotically large negative or positive δ 15 N flux and δ 18 O flux values occur as the net soil N 2 O flux approaches zero from positive or negative flux rates, respectively. This implies that the isotopic imprint of soil fluxes on the global atmospheric N 2 O pool is more variable than previously suggested. Additionally, the observed isotope values in static flux chambers are possibly complicated by the fact that consumption fluxes increase as the concentration in the chambers increases. This work reveals that even simple chamber flux measurements may possess isotope effects imparted by consumption during the chamber measurement and suggests ways to experimentally test this possibility. Additionally, simple methods to estimate depth‐dependent net production/consumption and its isotope effects are suggested. However, understanding the gross rates of the production and consumption of soil N 2 O remains an elusive goal.
