Evidence that co-existing cadmium and microplastics have an antagonistic effect on greenhouse gas emissions from paddy field soils.

Accumulation of microplastics (MPs) and cadmium (Cd) are ubiquitous in paddy soil. However, the combined effects of MPs and Cd on physiochemical and microbial mechanisms in soils and the attendant implications for greenhouse gas (GHG) emissions, remain largely unknown. Here, we evaluated the influence of polylactic acid (PLA) and polyethylene (PE) MPs on GHG emissions from Cd-contaminated paddy soil using a microcosm experiment under waterlogged and drained conditions. The results showed that PLA significantly increased CH4 and N2O emission fluxes and hence the global warming potential (GWP) of waterlogged soil. Soils treated with MPs+Cd showed significantly reduced GWP compared to those treated only with MPs suggesting that, irrespective of attendant consequences, Cd could alleviate N2O emissions in the presence of MPs. Conversely, the presence of MPs in Cd-contaminated soils tended to alleviate the bioavailability of Cd. Based on a structural equation model analysis, both the MPs-derived dissolved organic matter and the soil bioavailable Cd affected indirectly on soil GHG emissions through their direct influencing on microbial abundance (e.g., Firmicutes, Nitrospirota bacteria). These findings provide new insights into the assessment of GHG emissions and soil/cereal security in response to MPs and Cd coexistence that behaved antagonistically with respect to adverse ecological effects in paddy systems.

Accumulation and partitioning of toxic trace metal(loid)s in phytoliths of wheat grown in a multi-element contaminated soil.

Cropland contamination by toxic trace metal (loid)s (TTMs) has attracted increasing attention due to the serious consequential threat to crop quality and human health. Mitigation of plant TTM stress by silica amendment has been proposed recently. However, the relationship between the siliceous structure of phytoliths and TTMs in plants, and the environmental implications of phytolith-occluded trace metal (loid)s (PhytTMs) remain unclear. This study assessed the accumulation of five metal (loid)s, including lead (Pb), zinc (Zn), cadmium (Cd), copper (Cu) and arsenic (As), in the organic tissues and phytoliths of wheat grown in a mixed-TTM contaminated soil under both lightly and heavily contaminated conditions. The results show that the concentrations of plant TTMs and PhytTMs were significantly (p < 0.05) positively correlated, and higher in heavily contaminated wheats than those in lightly contaminated ones. The bio-enrichment factors between phytoliths and organic tissues were higher for As (1.83), Pb (0.27) and Zn (0.30) than for Cd (0.03) and Cu (0.14), implying that As, Pb and Zn were more readily co-precipitated with silicon (Si) in phytolith structures than Cd and Cu. Network analysis of the relationship between soil and plant elements with PhytTMs showed that severe contamination could impact the homeostasis of elements in plants by altering the translocation of TTMs between soils, plants, and phytoliths. The accumulation of TTMs in phytoliths was affected by the capacity of Si deposition in tissues and chelation of TTMs with silica, which could impact the role of PhytTMs in global biogeochemical TTM cycles.

Synchronous evaporation and aquatic primary production in tropical river networks.

Rivers play an important role in global water and carbon cycling, but there are still large uncertainties concerning evaporation and aquatic photosynthesis. Here we combined measurements of water chemistry, isotopic compositions (i.e., δDw, δ18Ow, δ13CDIC and ▵14CDIC) and geographic characteristics (i.e., river width) to elucidate in-stream hydrological and biogeochemical processes across rivers in Hainan Island, China. The results showed that dissolved inorganic carbon (DIC) in river waters was largely of modern origin, with about 95% from contemporary biogenic sources based on an isotopic mass balance of ▵14CDIC. Significant evaporation and aquatic primary production co-occurred in these tropical rivers with large amounts of water and DIC being rapidly turned over in the water column, altering the water cycle and the carbon balance. High rates of evaporation and aquatic primary production were observed in the headwater segments, with narrow river width but broad available reactive surface area at the air-water interface. The asymmetric aquatic photosynthesis at different river segments caused the spatial heterogeneities of dissolved solutes. The results suggest that the available reactive area at the water-air interface is responsible for synchronous water loss and dissolved carbon evolution in flat tropical rivers. This study provides evidence that intense evaporation and aquatic photosynthesis mainly occurred in headwater segments, which has implications for understanding global carbon cycling.

New insights into mechanisms of sunlight- and dark-mediated high-temperature accelerated diurnal production-degradation of fluorescent DOM in lake waters.

The production of fluorescent dissolved organic matter (FDOM) by phytoplankton and its subsequent degradation, both of which occur constantly under diurnal-day time sunlight and by night time dark-microbial respiration processes in the upper layer of surface waters, influence markedly several biogeochemical processes and functions in aquatic environments and can be feasibly related to global warming (GW). In this work sunlight-mediated high-temperature was shown to accelerate the production of FDOM, but also its complete disappearance over a 24-h diurnal period in July at the highest air and water temperatures (respectively, 41.1 and 33.5 °C), differently from lower temperature months. Extracellular polymeric substances (EPS), an early-state DOM, were produced by phytoplankton in July in the early morning (6:00-9:00), then they were degraded into four FDOM components over midday (10:00-15:00), which was followed by simultaneous production and almost complete degradation of FDOM with reformation of EPS during the night (2:00-6:00). Such transformations occurred simultaneously with the fluctuating production of nutrients, dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and the two isotopes (δ15N and δ18O) of NO3-. It was estimated that complete degradation of FDOM in July was associated with mineralization of approximately 15% of the initial DOC, which showed a nighttime minimum (00:00) in comparison to a maximum at 13:00. FDOM identified by excitation-emission matrix spectroscopy combined with parallel factor analysis consisted of EPS, autochthonous humic-like substances (AHLS) of C- and M-types, a combined form of C- and M-types of AHLS, protein-like substances (PLS), newly-released PLS, tryptophan-like substances, tyrosine-like substances (TYLS), a combined form of TYLS and phenylalanine-like substances (PALS), and their degradation products. Finally, stepwise degradation and production processes are synthesized in a pathway for FDOM components production and their subsequent transformation under different diurnal temperature conditions, which provided a broader paradigm for future impacts on GW-mediated DOM dynamics in lake water.

Silicon Effects on Biomass Carbon and Phytolith-Occluded Carbon in Grasslands Under High-Salinity Conditions.

Changes in climate and land use are causing grasslands to suffer increasingly from abiotic stresses, including soil salinization. Silicon (Si) amendment has been frequently proposed to improve plant resistance to multiple biotic and abiotic stresses and increase ecosystem productivity while controlling the biogeochemical carbon (C) cycle. However, the effects of Si on plant C distribution and accumulation in salt-suffering grasslands are still unclear. In this study, we investigated how salt ions affected major elemental composition in plants and whether Si enhanced biomass C accumulation in grassland species in situ. In samples from the margins of salt lakes, our results showed that the differing distance away from the shore resulted in distinctive phytocoenosis, including halophytes and moderately salt-tolerant grasses, which are closely related to changing soil properties. Different salinity (Na+/K+, ranging from 0.02 to 11.8) in plants caused negative effects on plant C content that decreased from 53.9 to 29.2% with the increase in salinity. Plant Si storage [0.02-2.29 g Si m-2 dry weight (dw)] and plant Si content (0.53 to 2.58%) were positively correlated with bioavailable Si in soils (ranging from 94.4 to 192 mg kg-1). Although C contents in plants and phytoliths were negatively correlated with plant Si content, biomass C accumulation (1.90-83.5 g C m-2 dw) increased due to the increase of Si storage in plants. Plant phytolith-occluded carbon (PhytOC) increased from 0.07 to 0.28‰ of dry mass with the increase of Si content in moderately salt-tolerant grasses. This study demonstrates the potential of Si in mediating plant salinity and C assimilation, providing a reference for potential manipulation of long-term C sequestration via PhytOC production and biomass C accumulation in Si-accumulator dominated grasslands.

Insights into solubility of soil humic substances and their fluorescence characterisation in three characteristic soils.

Soil humic substances (HS) are involved in almost all biogeochemical processes and functions in soils, thus their extraction from aiming to their characterization is very important. However, many factors that influence HS extraction from soil still need further studies. The aim of this work was to assess and quantify comparatively the solubility of soil HS as a function of extraction time, various extractants, solid to liquid ratio and sequential extraction. In this work three different soils, i.e. a forest, a maize and a paddy soil, were examined to assess the solubility of soil HS based on their fluorescence (excitation-emission matrix, EEM) features and changes in nutrient (NO3--N, PO43--P and dissolved Si) contents using multiple extraction approaches (time-dependent, various extractants, solid to liquid ratio, and sequential extraction). Three fluorescent components, i.e. humic acids-like (HA-like), fulvic acids-like (FA-like), and protein-like fluorophores (PLF), were identified by parallel factor (PARAFAC) analysis of EEM spectra of the various soil extracts. The solubility of HS, dissolved organic carbon (DOC) and nutrients were shown to increase with extraction time, except for PLF. The FA-like fraction disappeared completely in KCl extracts of all three soils, suggesting the inefficiency of salt extraction. Conversely, HS and nutrients solubility substantially increased in alkaline extracts, and dissolved Si was correlated significantly with the fluorescent intensities of HA-like and FA-like, thus confirming the well-known typical process of alkaline dissolution of HS bound to phytolith and silicate minerals. The relative solubility of HS and nutrients was higher at lower solid to liquid ratio (1:250-1:100), whereas their maximum yields was achieved at high solid to liquid ratio (1:10) for all three soils. Sequential extraction results showed that the first water extraction step contributed 42-55% of HS, which suggested that a single extraction was insufficient to recover HS. In conclusion, water and alkaline extraction could provide, respectively, the labile and insoluble complexed HS existing in soil.

Abiotic pyrite formation produces a large Fe isotope fractionation.

The iron isotope composition of sedimentary pyrite has been proposed as a potential proxy to trace microbial metabolism and the redox evolution of the oceans. We demonstrate that Fe isotope fractionation accompanies abiotic pyrite formation in the absence of Fe(II) redox change. Combined fractionation factors between Fe(II)(aq), mackinawite, and pyrite permit the generation of pyrite with Fe isotope signatures that nearly encapsulate the full range of sedimentary δ(56)Fe(pyrite) recorded in Archean to modern sediments. We propose that Archean negative Fe isotope excursions reflect partial Fe(II)(aq) utilization during abiotic pyrite formation rather than microbial dissimilatory Fe(III) reduction. Late Proterozoic to modern sediments may reflect greater Fe(II)(aq) utilization and variations in source composition.