Silver nanoparticles (AgNPs) are largely discharged into sewers and mostly accumulated in the sediments and sludge. The toxicity of AgNPs to environmental microorganisms has attracted great attention. However, the effect of AgNPs on anaerobic ammonium-oxidizing (anammox) granules remains unknown. Here we present the underlying promotion mechanism of AgNPs on anammox granules from a morphological and molecular biology perspective. Our results demonstrate a positive effect of AgNPs on the proliferation of anammox bacteria. AgNPs resulted in a change in the three-dimensional structure of anammox granules and led to larger pore size and higher porosity. In addition, the diffusion capacity of the substrate and metal ions was enhanced. Furthermore, the expression of anammox-related enzymes, such as nitrite oxidoreductase (NirS), hydrazine dehydrogenase (Hdh), and hydrazine synthase (HZS), was upregulated. Therefore, the growth rate and the nitrogen removal performance of the anammox granules were improved. Our findings clarify the underlying mechanism of AgNPs on anammox granules and provide a promising method for the treatment of AgNPs-rich wastewater.

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Summary Pore structure is sensitive to land management, fertilization and tillage. The response of pore structure to long‐term fertilizer application is crucial for understanding the mechanisms of various soil processes involved. In this study, the effect of chemical fertilizer and organic manure on microscale pore structure and the pore network in soil was quantified by synchrotron‐based X‐ray microcomputed tomography (SR‐mCT) and a pore network model. Macroaggregates of 5–7 mm in diameter were collected from a paddy soil (Typic Haplustalf) with a long‐term fertilization experiment established in 1996 under an annual rice–wheat crop rotation. The treatments were control (CK), chemical fertilizers (NPK), rice straw (RS), pig manure (PM), chemical fertilizer plus straw (NPK + RS) and chemical fertilizer plus pig manure (NPK + PM). Results showed that the application of NPK greatly reduced the C/I ratio (ratio of connected porosity and isolated porosity) of the macroaggregates compared with all the other treatments. The pore network model revealed that application of NPK reduced the complexity of the pore network system. Heatmap analyses showed that NPK increased the isolated porosity of macroaggregates and that straw markedly changed the pore shape by increasing the proportion of elongated pores, whereas the PM increased the connected porosity of macroaggregates. From our results, we believe a ‘good pore structure’ comprised a moderate total porosity and C/I ratio, which can balance the air, water exchange and moisture, and nutrient retention. We considered that total porosity and C/I ratio were the crucial pore variables to evaluate the effect of fertilization on pore structures at the microscale level. Therefore, we proposed that total porosity and the C/I ratio in macroaggregates could be used as effective indicators to evaluate the response of pore structure to fertilization. Application of a balanced approach of chemical fertilizer together with manure (NPK + PM) or straw (NPK + RS) improved pore structure. This suggested that fertilization management should include the integrated use of both mineral fertilizer and organic manure or straw to maintain a suitable pore environment in soil. Highlights Effects of long‐term fertilization on pore structure of macroaggregates of paddy soil. Pore system of macroaggregates investigated by pore network model and cluster analysis. Application of NPK (alone or combined with organic matter) will increase proportion of isolated pores. The C/I ratio has the potential to predict change in pore structure of soil at the aggregate scale.

Purpose Soils have a wide range of pore size distribution (PSD) from nanometer to micrometer scale. The detailed characterization of soil pore structure in a wide pore size range is important for understanding the soil processes. In this study, three different techniques are used to quantitatively describe the soil pore characteristics in a wide pore size range and to evaluate whether different pedogenic processes affect porosity and PSD of the soils. Materials and methods The four different types of soils: black soil (BS, Udic Agriboroll), Shajiang black soil (SBS, Aquic Pelludert), paddy soil (PS, Aeric Endoaqualf), and latosolic red soil (LRS, Typic Kandiudults) were selected to represent the most important soil types in China. A combination technique of nitrogen adsorption isotherm (NAI), mercury intrusion porosimetry (MIP), and synchrotron-radiation-based Xray computed microtomography (SR-mCT) was used to describe the pore structure characteristics including the porosity, PSD, and pore geometry in the soils. Results and discussion The NAI method revealed larger differences in the Brunauer-Emmett-Teller (BET) surface area and 0.002-0.15 μm pore volume for the studied soils. The latosolic red soil (LRS) has the largest volume of 0.002-0.15 μm pore, while BS has the smallest. The PSD determined by MIP exhibited that BS and paddy soil (PS) had multimodal peaks, indicating the existence of a more heterogeneous pore system. The PSD from LRS and SBS exhibited a single, sharply defined peak at a pore diameter from 0.02 to 0.04 and 0.01 to 0.06 μm, respectively. The volume of the >3.7 μm pores determined by the SR-mCT method was in the order of BS (18.2 %) > PS (10.6 %) > LRS (7.2 %) > SBS (5.4 %). The pore shape measured by SR-mCT has an obvious difference in pore system for different soils. Regular pores were more frequent in BS than in SBS, PS, and LRS. Conversely, lower percentage of irregular pores was found in the BS.Conclusions The combination of NAI, MIP, and SR-mCT techniques can quantify the porosity and PSD of soils over a wide range of pores. For the overlapping pore region, the pore volume determined by the MIP and SR-mCT agreed well. Our results indicated that pedogenic processes can greatly influence the soil pore structure both in terms of PSD and pore shape.

Pore characteristics and chemical composition of soil aggregates from two contrasting soil types (Fe‐rich Alisol and SOM‐rich Phaeozem) were determined with synchrotron radiation X‐ray micro‐computed tomography (SR‐mCT) and scanning electron microscopy coupled with an energy‐dispersive X‐ray spectrometer (SEM–EDS). Data fusion to combine 3‐D SR‐mCT images and 2‐D SEM–EDS data was used to establish the relation between pore structure and cementing substances within the soil aggregates. The SR‐mCT analysis showed that Phaeozem aggregates had greater total measurable porosity and were more anisotropic (DA) than Alisol aggregates. Soil aggregates of 5–10, 2–5 and 0.5–2 mm diameter were measured and found to have a similar pore‐size distribution (PSD) across a range of pore sizes for the Alisol, but not for the Phaeozem. The SEM–EDS mapping revealed that Fe‐rich areas in Alisol aggregates were connected, whereas the carbon‐rich areas in Phaeozem aggregates were isolated. Wavelet analysis suggested that the Alisol aggregates might have been formed by global interactions among microaggregates and iron oxides, whereas the Phaeozem aggregates were formed by localized interactions among microaggregates and carbon, the main cementing agent. The heatmap analysis showed that iron oxide not only acts as a binding agent in Alisol aggregates, but also affects the pore size in addition to the pore shapes, whereas carbon functions only as a binding agent and has little effect on pore structure of the Phaeozem aggregates. The combination of SR‐mCT and SEM–EDS techniques provides a powerful tool for quantifying the pore characteristics of soil aggregates and investigating the interactions between the pore structure and cementing substances within the aggregates. Highlights The effects of different cementing substances on the pore structure of Fe‐rich and SOM‐rich soil aggregates. A multi‐scale analysis and data fusion method were applied to data from synchrotron imaging and SEM. Iron oxides affected only the pore structure of the Fe‐rich soil aggregates. The data fusion method has potential for understanding the key features of soil at the aggregate scale.

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