Copper availability governs nitrous oxide accumulation in wetland soils and stream sediments

Denitrification is microbially-mediated through enzymes containing metal cofactors. Laboratory studies of pure cultures have highlighted that the availability of Cu, required for the multicopper enzyme nitrous oxide reductase, can limit N2O reduction. However, in natural aquatic systems, such as wetlands and hyporheic zones in stream beds, the role of Cu in controlling denitrification remains incompletely understood. In this study, we collected soils and sediments from three natural environments -- riparian wetlands, marsh wetlands, and a stream -- to investigate their nitrogen species transformation activity at background Cu levels and different supplemented Cu loadings. All of the systems displayed low solid-phase associated Cu (40 - 280 nmol g-1), which made them appropriate sites for evaluating the effect of limited Cu availability on denitrification. In laboratory incubation experiments, high concentrations of N2O accumulated in all microcosms lacking Cu amendment except for one stream sediment sample. With Cu added to provide dissolved concentrations at trace levels (10-300 nM), reduction of N2O to N2 in the wetland soils and stream sediments was enhanced. A kinetic model could account for the trends in nitrogen species by combining the reactions for microbial reduction of NO3- to NO2-/N2O/N2 and abiotic reduction of NO2- to N2. The model revealed that the rate of N2O to N2 conversion increased significantly in the presence of Cu. For riparian wetland soils and stream sediments, the kinetic model also suggested that overall denitrification is driven by abiotic reduction of NO2- in the presence of inorganic electron donors. This study demonstrated that natural aquatic systems containing Cu at concentrations less than or equal to crustal abundances may display incomplete reduction of N2O to N2 that would cause N2O accumulation and release to the atmosphere.

CHANGE OF PHYSICAL CHEMICAL PARAMETERS OF WETLAND SOILS IMPACTED BY AGRICULTURE AND MINING (MG)

Áreas úmidas localizadas na região oeste de Minas Gerais vem sendo convertidas em terras agrícolas e de mineração de argila. O objetivo desse artigo foi avaliar as diferenças na concentração e distribuição de macro, micronutrientes e outros parâmetros físicos em solos de áreas úmidas usados para produção agrícola desde 1970 ou pela mineração, comparando com áreas úmidas naturais ou não impactadas, durante a estação seca. Para isso, amostras de solo foram coletadas em diferentes profundidades em 6 pontos. Por meio de trabalhos de campo, foi possível avaliar as mudanças morfológicas do solo e coletar amostras para realizar análises físico-químicas. As análises granulométricas e a química forneceram informações para melhor compreender a extensão dos impactos ambientais em decorrência de atividades econômicas desenvolvidas na área. Dentre os pontos analisados, notou-se mudanças morfológicas nas áreas de cultivo agrícola e principalmente na área de mineração, onde ocorre erosão laminar. A análise granulométrica mostrou mudança textural nos locais de cultivo agrícola, enquanto que os demais pontos foram classificados como textura argilosa ou muito argilosa. A análise química mostrou que as áreas de cultivo agrícola apresentaram teoreselevados de macro e micronutrientes nas camadas superficiais e a saturação por bases foi muito baixa em praticamente todos os pontos estudados. O local da Mineração se mostrou como o mais degradado e menos propício ao crescimento vegetal.

Improved Wetland Soil Organic Carbon Stocks of the Conterminous U.S. Through Data Harmonization

Wetland soil stocks are important global repositories of carbon (C) but are difficult to quantify and model due to varying sampling protocols, and geomorphic/spatio-temporal discontinuity. Merging scales of soil-survey spatial extents with wetland-specific point-based data offers an explicit, empirical and updatable improvement for regional and continental scale soil C stock assessments. Agency-collected and community-contributed soil datasets were compared for representativeness and bias, with the goal of producing a harmonized national map of wetland soil C stocks with error quantification for wetland areas of the conterminous United States (CONUS) identified by the USGS National Landcover Change Dataset. This allowed an empirical predictive model of SOC density to be applied across the entire CONUS using relational %OC distribution alone. A broken-stick quantile-regression model identified %OC with its relatively high analytical confidence as a key predictor of SOC density in soil segments; soils <6% OC (hereafter, mineral wetland soils, 85% of the dataset) had a strong linear relationship of %OC to SOC density (RMSE = 0.0059, ~4% mean RMSE) and soils >6% OC (organic wetland soils, 15% of the dataset) had virtually no predictive relationship of %OC to SOC density (RMSE = 0.0348 g C cm−3, ~56% mean RMSE). Disaggregation by vegetation type or region did not alter the breakpoint significantly (6% OC) and did not improve model accuracies for inland and tidal wetlands. Similarly, SOC stocks in tidal wetlands were related to %OC, but without a mappable product for disaggregation to improve accuracy by soil class, region or depth. Our layered harmonized CONUS wetland soil maps revised wetland SOC stock estimates downward by 24% (9.5 vs. 12.5Pg C) with the overestimation being entirely an issue of inland organic wetland soils (35% lower than SSURGO-derived SOC stocks). Further, SSURGO underestimated soil carbon stocks at depth, as modeled wetland SOC stocks for organic-rich soils showed significant preservation downcore in the NWCA dataset (<3% loss between 0 and 30 cm and 30 and 100 cm depths) in contrast to mineral-rich soils (37% downcore stock loss). Future CONUS wetland soil C assessments will benefit from focused attention on improved organic wetland soil measurements, land history, and spatial representativeness.