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Sea level rise will change inundation regimes in salt marshes, altering redox dynamics that control nitrification - a potential source of the potent greenhouse gas, nitrous oxide (N2 O) - and denitrification, a major nitrogen (N) loss pathway in coastal ecosystems and both a source and sink of N2 O. Measurements of net N2 O fluxes alone yield little insight into the different effects of redox conditions on N2 O production and consumption. We used in situ measurements of gross N2 O fluxes across a salt marsh elevation gradient to determine how soil N2 O emissions in coastal ecosystems may respond to future sea level rise. Soil redox declined as marsh elevation decreased, with lower soil nitrate and higher ferrous iron in the low marsh compared to the mid and high marshes (P < 0.001 for both). In addition, soil oxygen concentrations were lower in the low and mid-marshes relative to the high marsh (P < 0.001). Net N2 O fluxes differed significantly among marsh zones (P = 0.009), averaging 9.8 ± 5.4 μg N m(-2)  h(-1) , -2.2 ± 0.9 μg N m(-2)  h(-1) , and 0.67 ± 0.57 μg N m(-2)  h(-1) in the low, mid, and high marshes, respectively. Both net N2 O release and uptake were observed in the low and high marshes, but the mid-marsh was consistently a net N2 O sink. Gross N2 O production was highest in the low marsh and lowest in the mid-marsh (P = 0.02), whereas gross N2 O consumption did not differ among marsh zones. Thus, variability in gross N2 O production rates drove the differences in net N2 O flux among marsh zones. Our results suggest that future studies should focus on elucidating controls on the processes producing, rather than consuming, N2 O in salt marshes to improve our predictions of changes in net N2 O fluxes caused by future sea level rise., (© 2015 John Wiley & Sons Ltd.)

In the age of biofuel innovation, bioenergy crop sustainability assessment has determined how candidate systems alter the carbon (C) and nitrogen (N) cycle. These research efforts revealed how perennial crops, such as switchgrass, increase belowground soil organic carbon (SOC) and lose less N than annual crops, like maize. As demand for bioenergy increases, land managers will need to choose whether to invest in food or fuel cropping systems. However, little research has focused on the C and N cycle impacts of reverting purpose‐grown perennial bioenergy crops back to annual cropping systems. We investigated this knowledge gap by measuring C and N pools and fluxes over 2 years following reversion of a mature switchgrass stand to an annual maize rotation. The most striking treatment difference was in ecosystem respiration (ER), with the maize‐converted treatment showing the highest respiration flux of 2,073.63 (± 367.20) g C m−2 year−1 compared to the switchgrass 1,412.70 (± 28.72) g C m−2 year−1 and maize‐control treatments 1,699.16 (± 234.79) g C m−2 year−1. This difference was likely driven by increased heterotrophic respiration of belowground switchgrass necromass in the maize‐converted treatment. Predictions from the DayCent model showed it would take approximately 5 years for SOC dynamics in the converted treatment to return to conditions of the maize‐control treatment. N losses were highest from the maize‐converted treatment when compared to undisturbed switchgrass and maize‐control, particularly during the first conversion year. These results show substantial C and N losses occur within the first 2 years after reversion of switchgrass to maize. Given farmers are likely to rotate between perennial and annual crops in the future to meet market demands, our results indicate that improvements to the land conversion approach are needed to preserve SOC built up by perennial crops to maintain the long‐term ecological sustainability of bioenergy cropping systems. [ABSTRACT FROM AUTHOR]

Microbial transformations of nitrogen (N) largely determine whether N is retained in ecosystems via net primary productivity or lost via gaseous emissions and leaching. The controls on soil N cycling are often studied at single locales, making it difficult to predict N cycling at regional to global scales. We hypothesized that contemporary soil properties exhibit consistent relationships with instantaneous gross N cycling rates across diverse biomes that create a continuum in these properties. We measured ex situ gross N cycling rates and soil properties at 33 study sites representing five biome classifications in California including deserts, grasslands, shrublands, forest, and wetlands. Desert soils had significantly lower total N, organic carbon (C), microbial biomass N, and soil moisture as well as higher pH than all other biomes, whereas forests and wetlands had significantly lower soil nitrate (NO 3 − ) concentrations (P < 0.001 for all). Gross mineralization rates were best predicted by the combination of soil moisture and soil C:N ratios (R 2 = 0.46), which exerted positive and negative controls, respectively. Grasslands exhibited marginally higher gross mineralization than all other biomes, whereas deserts had the lowest rates due to low soil moisture (P = 0.09). Gross nitrification rates were positively correlated to soil NO 3 − concentrations (R 2 = 0.34) and negatively correlated to soil C:N ratios (R 2 = 0.31). The negative relationship between gross nitrification and soil C:N ratios was driven by forest soils, which had significantly higher C:N ratios and lower gross nitrification than all other biomes (P < 0.05). Dissimilatory NO 3 − reduction to NH 4 + (DNRA) occurred in soils from all biomes. The strong positive correlation between DNRA rates and soil NO 3 − (R 2 = 0.41) suggests NO 3 − limitation of DNRA. Predictable patterns in gross N cycling across biomes in California suggest that contemporary soil properties are important drivers of instantaneous soil N cycling rates that integrate over differences in vegetation type, atmospheric N deposition rates, and local climate. [ABSTRACT FROM AUTHOR]

Abstract: The accurate measurement of soil nitrate () is critical for determining rates of nitrogen (N) cycling and potential N losses from ecosystems. Iron (Fe) can interfere with the colorimetric analysis of soil extracts to cause the appearance of anomalously low concentrations. To resolve the interference, imidazole or NH4Cl–DTPA has been recommended to replace NH4Cl–EDTA as the buffer in the analysis. Here we show that phosphate () filtration can completely remove Fe interference whereas any of these buffers alone may not. Regardless of which buffer was used, 5.5–55 mg Fe L−1 ferrous iron (Fe2+) interfered with determination in 0.3 mg N– L−1 2 M KCl solutions. Phosphate filtration removed all detectable Fe2+ in 2 M KCl anaerobic soil slurry extracts with high Fe2+ concentrations (25.9 ± 1.7 mg Fe2+ L−1). With each of the three buffers tested, the measured concentrations in the anaerobic soil slurry extracts were significantly higher with filtration compared to without filtration. After filtration, the measured concentrations were similar across all three buffers, suggesting that concentrations were accurately measured in filtered soil extracts regardless of the buffer used. The Fe:N ratio of Fe interference with determination depended on Fe concentration, concentration, buffer, and cadmium column age, so that the amount of Fe interference that could occur can be difficult to predict. We suggest comparing measured concentrations for unfiltered and filtered soil extracts to determine the potential for Fe interference in colorimetric determination as standard additions may not detect all forms of Fe interference. [Copyright &y& Elsevier]

Microbial metabolism couples elemental reactions, driving biogeochemical cycles. Assimilatory coupling of ele- mental cycles, such as the carbon (C), nitrogen (N), and phosphorus cycles, occurs when these elements are incorporated into biomass or released through its decomposition. In addition, many microbes are capable of dissimilatory coupling, catalyzing energy-releasing reactions linked to transformations in the oxidation state of elements, and releasing the transformed elements to the environment. Different inorganic elements provide varying amounts of energy yield, and the interaction of these processes creates a microbial energy economy. Dissimilatory reactions involving C, N, iron, and sulfur provide particularly important examples where micro- bially mediated oxidation-reduction (redox) transformations affect nutrient availability for net primary pro- duction, greenhouse-gas emissions, levels of contaminants and natural toxic factors, and other ecosystem dynamics. Recent discoveries of previously unrecognized microbial dissimilatory processes are leading to re- evaluation of traditional perceptions of biogeochemical cycles. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to an article related to nitrogen production in soil, published in a previous issue.

Invasive plants can increase soil nitrogen (N) pools and accelerate soil N cycling rates, but their effect on gross N cycling and nitrous oxide (N2O) emissions has rarely been studied. We hypothesized that perennial pepperweed (Lepidium latifolium) invasion would increase rates of N cycling and gaseous N loss, thereby depleting ecosystem N and causing a negative feedback on invasion. We measured a suite of gross N cycling rates and net N2O fluxes in invaded and uninvaded areas of an annual grassland in the Sacramento‐San Joaquin River Delta region of northern California. During the growing season, pepperweed‐invaded soils had lower microbial biomass N, gross N mineralization, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification‐derived net N2O fluxes (P < 0.02 for all). During pepperweed dormancy, gross N mineralization, DNRA, and denitrification‐derived net N2O fluxes were stimulated in pepperweed‐invaded plots, presumably by N‐rich litter inputs and decreased competition between microbes and plants for N (P < 0.04 for all). Soil organic carbon and total N concentrations, which reflect pepperweed effects integrated over longer time scales, were lower in pepperweed‐invaded soils (P < 0.001 and P = 0.04, respectively). Overall, pepperweed invasion had a net negative effect on ecosystem N status, depleting soil total N to potentially cause a negative feedback to invasion in the long term. [ABSTRACT FROM AUTHOR]