Substantial losses of nitrogen (N) and phosphorus (P) to the environment occur during food production. These emissions of reactive N (Nr) and P have adverse effects on the environment. The life cycle emissions of Nr and P due to resource consumption can be quantified using N and P footprints. In this study, a common framework developed for the purpose of making comparisons was used to examine the food N and P footprints of China, India, and Japan from 1961 to 2013. The footprints increased significantly in China after 1976 (5.4–19.3 kg-N capita−1 yr−1 and 1.20–4.77 kg-P capita−1 yr−1 in 1976–2013) with the higher consumption of meat and vegetables. In India, an increase in milk and vegetable consumption resulted in a gradual increase in the footprints since 1976 (8.5–11.4 kg-N capita−1 yr−1, 0.99–1.6 kg-P capita−1 yr−1 in 1976–2013). In Japan, the footprints increased until 1993 (12.2–28.3 kg-N capita−1 yr−1, 2.59–8.43 kg-P capita−1 yr−1 in 1961–1993) before declining (21.8 kg-N capita−1 yr−1, 6.05 kg-P capita−1 yr−1 in 2013), with a constant increase in meat consumption, a decrease in cereals, and improvements in nutrient use efficiency. The N footprint tends to be more sensitive to the consumption of meat, milk, oil crops, fish, and seafood, and the P footprint tends to be more sensitive to vegetables. By analysing the Asian giants, the key food items to target to reduce the footprints are identified. If the per-capita average footprints in high and middle income countries were the same as that in Japan in 1993, the global food N and P footprints would increase by factors of 1.18–1.89 by 2030. The use of these results with other advances in agriculture practices has the potential to improve nutrient use efficiency and to promote more efficiently-produced food.
Abstract Methanol metabolism can play an important role in marine carbon cycling. We made contemporaneous measurements of methanol concentration and consumption rates in the northwest Pacific Ocean to constrain the pathways and dynamics of methanol cycling. Methanol was detected in relatively low concentrations (<12–391 nM), likely due to rapid biological turnover. Rates of methanol oxidation to CO 2 (0.9–130.5 nmol L −1 day −1 ) were much higher than those of assimilation into biomass (0.09–6.8 nmol L −1 day −1 ), suggesting that >89.7% of methanol was utilized as an energy source. Surface water acted as a net methanol sink at most sites, with an average flux of 9 μmol L −1 day −1 . Atmospheric deposition accounted for 22.7% of microbial methanol consumption in the mixed layer, illustrating that the atmosphere is less important than internal processes for driving methanol cycling in these pelagic waters.
Rates of microbial processes are fundamental to understanding the significance of microbial impacts on environmental chemical cycling. However, it is often difficult to quantify rates or to link processes to specific taxa or individual cells, especially in environments where there are few cultured representatives with known physiology. Here, we describe the use of the redox-enzyme-sensitive molecular probe RedoxSensor™ Green to measure rates of anaerobic electron transfer physiology (i.e., sulfate reduction and methanogenesis) in individual cells and link those measurements to genomic sequencing of the same single cells. We used this method to investigate microbial activity in hot, anoxic, low-biomass (~10
Abstract Aerobic methane oxidation (MOx) mediated by methanotrophs is a crucial mechanism in controlling methane emissions from the surface ocean to the atmosphere. Coastal waters dominate global oceanic methane emissions, but the dynamics, controls and roles of MOx remain largely unconstrained in the marginal seas around China. Here, we conducted a variety of biogeochemical analyses to investigate the controls of methane cycling and the dynamics of methanotrophic activity in the East China Sea and Yellow Sea. Methane was supersaturated in the surface seawater and the concentrations ranged from 2.8 to 19.8 nM. The distribution of methane was regulated by the sources and sinks, which were influenced largely by hydrological and biogeochemical factors. Methane was turned over rapidly with high rates ( k : 5 × 10 −4 –0.04 d −1 ), indicating the enzymatic capability of methanotrophic biomass to metabolize methane. Rates of MOx varied significantly between sites (1 × 10 −3 –0.60 nM d −1 ) and relatively high MOx rates were observed in shallow waters. MOx exhibited the Michaelis‐Menten kinetics with the V max of 0.30 nM d −1 and a K m of 78.3 nM. Methanotrophic activity was impacted by environmental factors such as methane availability, nutrient levels, bacterial production and temperature. Nutrient addition experiments demonstrated that phosphate elevated MOx rates, while the activity was largely inhibited by ammonium probably due to competitive inhibition of the methane monooxygenase by ammonia. Comparing the depth‐integrated MOx rates with the air‐sea fluxes at selected sites showed that methane consumed through microbial oxidation accounted for up to 78.1% of the total methane loss (=sum of MOx rates and air‐sea flux), highlighting the role of MOx as a microbial filter for methane emissions.
The sulfur isotopic composition of marine pyrite (δ34Spyr) is one of the major geochemical tools to reconstruct global changes in Earth's surface environment. Storm-driven variations in depositional environments, however, can play key roles in modifying the δ34Spyr signal. Here we present δ34Spyr values in a complete Holocene muddy storm deposit on the East China coastal plain, which is identified by comprehensive analyses of sediment components, organic matter composition, bulk organic carbon isotopic ratios, carbon and oxygen isotopic ratios of carbonates and trace element concentrations. We find that positive (+5.8‰ to +15.8‰) and negative (−9.0‰ to −19.7‰) δ34Spyr values were preserved in two successive intervals of the muddy storm deposit (ca. 62 cm long), corresponding to the high-energy storm peak phase (HESPP) and the waning-energy late storm phase (WELSP), respectively. We propose that the 34S enrichment in pyrite from the HESPP is most likely due to a combination of storm reworking of sediments and high sedimentation rates, which involves various physical and chemical processes leading to the accumulation of 34S-enriched pyrite, such as oxidation of near-surface sulfides, reduced exchange of sulfate between sediment pore fluids and the overlying water column, and/or input of excess reactive iron minerals. In contrast, lower sedimentation rates and limited sedimentary remobilization during the WELSP allow the isotopic signal of early-formed 32S-enriched pyrite to be preserved. The striking shift in δ34Spyr values, therefore, reflects a dramatic change in the local depositional environment between the HESPP and WELSP, suggesting that non-steady-state deposition induced by storm activity facilitates the formation of isotopically "heavy" pyrite. This sharp shift can be further generalized as a characteristic feature of δ34Spyr values in storm deposits. Our work highlights the critical impact of weather- and climate-event-driven depositional variations on δ34Spyr values, adding to the growing evidence emphasizing the local environmental and diagenetic controls on these records.
Abstract Over‐estimation of summer precipitation over the Tibetan Plateau (TP) is a well‐known and persistent problem in most climate models. This study demonstrates the impact of a Gaussian Probability Density Function cloud fraction scheme on rainfall simulations using the Weather Research and Forecasting model. It is found that this scheme in both 0.1° and 0.05° resolutions significantly reduces the wet bias through both local feedbacks and large‐scale dynamic process. Specifically, increased cloud water/ice content with this scheme reduces surface shortwave radiation, and consequently surface heat fluxes and evapotranspiration. This, in turn, dampens the large‐scale thermal effect of the TP and weakens the exaggerated monsoon circulation and low‐level moisture convergence. It is this large‐scale dynamic process that contributes the most (∼70%) to the wet bias reduction. Although this paper presents a modeling study, it highlights the cloud radiative feedback to the large‐scale dynamics and precipitation over the TP.
Abstract Methane in oil reservoirs originates mostly from thermogenic sources, yet secondary microbial methane production from petroleum biodegradation is known to be pervasive. The conventional approach for identifying this secondary microbial methane commonly relies on geochemical characteristics of other gas molecules such as the carbon isotopic composition of carbon dioxide and propane. This information is sometimes obscured by isotopic variations in source material and may not be available in certain geological reservoirs. To better constrain the presence of secondary microbial methane, we studied the clumped isotopologue compositions of methane in terrestrial Azerbaijanian mud volcanoes, which support the occurrence of secondary microbial gas. Here, a deficit in Δ12CH2D2 of thermogenic methane occurs due to different δD of hydrogen sources that contribute to the formation of methane molecules (i.e., combinatorial effect). The Δ12CH2D2 is expected to move toward equilibrium as thermal maturity increases. More importantly, both Δ13CH3D and Δ12CH2D2 values of methane approach low-temperature thermodynamic equilibrium in most gases, suggesting that the original thermogenic methane has been altered by newly formed microbial methane in addition to isotope exchange among methane molecules catalyzed by the methyl-coenzyme M reductase enzyme. We conclude that methane clumped isotopes provide a unique proxy for identifying secondary microbial methane and understanding the exact evolution stages for natural gases.
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