Environmental context. Soils are the environmental compartment likely to be exposed most to manufactured nanoparticles, but there is no method available at present to assess their retention, which determines potential mobility and bioavailability. Optimisation and application of a method to determine retention values for silver (Ag) and cerium oxide (CeO2) manufactured nanoparticles in soils found in many cases that they differed from the partitioning of their bulk and soluble counterparts. Wider application of this method can assist in comparing the risk of many different manufactured nanoparticles with other contaminants in soil systems and model their relationship to soil properties. Methods to study the retention of manufactured nanoparticles (MNP) are lacking for soils that are likely to be increasingly exposed to MNP. In this study we present, for the first time, a method to determine retention values (Kr) of Ag and CeO2 MNP, that can be ranked among solid–liquid partitioning (Kd) values of bulk (micrometre-sized) forms, soluble salts and other possible contaminants of soils. After method optimisation, suspensions containing 1.24 mg kg–1 Ag as Ag MNP and 1.30 mg kg–1 Ce as CeO2 MNP were added to five soils. More than 7% of Ag MNP occurred as soluble AgI after 24 h and the range of Kr values of Ag MNP (77–2165 L kg–1) and CeO2 MNP (1.1–2828 L kg–1) contrasted with Kd values of soluble AgI, CeIII and CeIV salts and bulk Ag and CeO2 powders in different soils.

Gully erosion is a dominant source of sediment and particulates to the Great Barrier Reef (GBR) World Heritage area. We selected the Bowen catchment, a tributary of the Burdekin Basin, as our area of study; the region is associated with a high density of gully networks. We aimed to use a semi-automated object-based gully networks detection process using a combination of multi-source and multi-scale remote sensing and ground-based data. An advanced approach was employed by integrating geographic object-based image analysis (GEOBIA) with current machine learning (ML) models. These included artificial neural networks (ANN), support vector machines (SVM), and random forests (RF), and an ensemble ML model of stacking to deal with the spatial scaling problem in gully networks detection. Spectral indices such as the normalized difference vegetation index (NDVI) and topographic conditioning factors, such as elevation, slope, aspect, topographic wetness index (TWI), slope length (SL), and curvature, were generated from Sentinel 2A images and the ALOS 12-m digital elevation model (DEM), respectively. For image segmentation, the ESP2 tool was used to obtain three optimal scale factors. On using object pureness index (OPI), object matching index (OMI), and object fitness index (OFI), the accuracy of each scale in image segmentation was evaluated. The scale parameter of 45 with OFI of 0.94, which is a combination of OPI and OMI indices, proved to be the optimal scale parameter for image segmentation. Furthermore, segmented objects based on scale 45 were overlaid with 70% and 30% of a prepared gully inventory map to select the ML models’ training and testing objects, respectively. The quantitative accuracy assessment methods of Precision, Recall, and an F1 measure were used to evaluate the model’s performance. Integration of GEOBIA with the stacking model using a scale of 45 resulted in the highest accuracy in detection of gully networks with an F1 measure value of 0.89. Here, we conclude that the adoption of optimal scale object definition in the GEOBIA and application of the ensemble stacking of ML models resulted in higher accuracy in the detection of gully networks.

Grapevine cane and stalks were considered for pyrolysis at 400 to 700 °C to produce biochar for increasing the water holding capacity of vineyard soil. Feedstocks were pyrolysed using a continuous feed reactor and the resulting biochars characterized in terms of physico-chemical properties, including water retention performance. Hydrophobicity was found in biochar from both feedstocks pyrolysed at 400 °C, but not at higher temperatures. At low soil matric potential, the pyrolysis temperature was the defining variable in determining water retention whereas at higher pressures, the feedstock was the more important variable. Available water content (AWC) of biochar increased with increasing pyrolysis temperatures, with optimal results obtained from grapevine cane at a pyrolysis temperature of 700 °C, which had an AWC 23% higher than a typical clay type soil. Principal component analysis showed variability in water retention of these biochars to be closely associated with the zeta potential, as well as the carbon and ionic content, suggesting that surface charge and hydrophobicity are key properties determining water holding capacity. Pure biochars were superior in water retention performance to typical sandy soils, and so biochar amendment of these soil types may improve water holding (particularly at field capacity). Further study with pot or field trials is recommended to confirm water retention behaviour and assess the feasibility of application under different viticultural scenarios.

Coastal wetland rehabilitation can provide nature-based solutions for climate change mitigation. The high carbon accumulation rate and carbon secured, potentially for several millennia, as soil organic carbon (SOC), is among the reasons. Measuring SOC storage and accrual over time are the main tools to understand rehabilitation success. However, SOC is partitioned among different organic matter fractions with varying physio-chemical properties and stabilities. In this research, we separated different organic matter fractions based on density (free light fraction: f-LF, occluded light fraction: o-LF and heavy fraction: HF) and solubility (dissolved fraction: DF) from soils taken from a wetland under rehabilitation and a pristine mangrove forest in Queensland, Australia. The f-LF and o-LF contain particulate organic carbon (POC_f-LF and POC_o-LF), whereas HF consists of mineral-associated organic carbon (MAOC_HF). Mangroves are superior to wetlands under rehabilitation and terrestrial forests in terms of C storage in each fraction. Soils from both mangroves and wetlands under rehabilitation are dominated by MAOC_HF. However, MAOC_HF from mangrove soils are relatively physio-chemically stable, while wetlands under rehabilitation are leaching aged-SOC (>1000 years) from the HF to DF as dissolved organic carbon (DOC_DF). Therefore, reducing the risk of mobilisation of aged-SOC can be a key to achieve rehabilitation success.

Blue carbon ecosystems, including mangroves, offer nature-based solutions against the impacts of climate change. Degraded coastal agricultural lands that were previously coastal wetlands harbour ancient (>1000 years) soil organic carbon (SOC) pools and can be potential candidates for coastal wetland restoration. We investigated the SOC storage capacity of coastal wetlands under rehabilitation (CWUR) by measuring the SOC stocks of coastal wetlands under different stages of rehabilitation (Phase 1 and Phase 2 sites) and comparing their stocks to those from well-established mature mangroves (reference sites). The location of our study is on Maroochy River floodplain on the Sunshine Coast, Queensland, Australia. In addition, the contribution of different sources to SOC in mature mangroves and CWUR was investigated using Bayesian stable isotope mixing models. Dating of sediments by ²¹⁰Pb and ¹³⁷Cs radionuclides was used to estimate the historical sedimentation and carbon accumulation rates for mature mangroves. CWUR had SOC stocks ranged from 242.6 to 322.2 Mg OC ha^-1 m^-1 whereas the average SOC stocks of mature mangroves were 461.4 Mg OC ha^-1 m^-1. Not only are the carbon stocks in mature mangroves higher than those for CWUR but they are also comparable to those from mangroves in subtropical Australia and estuarine settings. Unexpectedly, CWUR had higher SOC stocks compared to SOC stocks for pasturelands from previous studies. The suspended particulate matter (SPM) supplied by tidal and fluvial processes is a main constituent of SOC pools in both mature mangroves and CWUR that we studied. Furthermore, mangrove roots contributed significantly to peat and organic-rich sediment formation in mature mangroves on Maroochy River floodplain. Presence of remnant SOC (>1000 years) and availability of accommodation space for sediment deposition in CWUR are advantageous for blue carbon restoration.