The growing demand for sustainable construction materials has prompted interest in reusing dredged sediments as an alternative to traditional raw materials in dike construction. Before they can be reused, sediments must undergo the lengthy ripening process, during which they are transformed into stable soil. Accelerating this transformation could significantly improve the feasibility of sediment reuse. This study investigates whether biochar amendment can enhance the biophysicochemical ripening of dredged sediments, focusing on the influence of biochar application rate and particle size.

Dredged material from the port of Hamburg, Germany, that was dewatered and processed at the METHA plant, was mixed with biochar produced by Bio Energy Netherlands from the gasification of wood waste at 800-1000°C for 90-120 minutes. The mixtures contained biochar with varying application rates (2%, 4%, 6%) and particle sizes (<2 mm, 2-5 mm, >5 mm). Over the course of 15 weeks of field ripening, the sediment-biochar mixtures were exposed to natural weather conditions and turned weekly. Biochar amendment introduced additional porosity which increased water holding capacity by 33-72% compared to the control after 15 weeks of ripening, resulting in values of 24-72% DW. The oven-dried COLE, ranged from of 2.2 to 5.4% which represents a decrease of up to 54% relative to the unamended sediments. This improvement can be attributed to the non-plastic behavior of biochar and explains the decreasing shrinkage observed with an increasing application rate. Increasing particle size was correlated to decreasing shrinkage (p <0.05) which could be due to the interrupting effect of coarse biochar particles on tensile load propagation in the rods. A qualitative assessment of the structure development of the experimental variants suggests an acceleration of structure formation with higher biochar application rate and larger particle size when combined with weekly turning. This resulted in a faster breakdown of the dense and platy METHA material into smaller and more aerated aggregates. Overall, the physical ripening of the dredged material was improved with the addition of biochar at increasing application rates and particle size, which promoted a faster stabilization of sediment aggregates and enhanced physical properties beneficial for construction applications.

The occurrence of sulfur oxidation, the main chemical ripening reaction, was evidenced by a loss in the total sulfur content of samples and an increasing electrical conductivity during dry periods. The pH was expected to decrease as a result of the release of protons from this reaction, however this was not observed. Instead, increasing biochar application rates was correlated to a higher pH (p <0.05) and was evidence of the material's buffering capacity which can be attributed to its high functional group and mineral content. The total sulfur content reduced on average by 5% and 22% in the amended samples and the control, and this smaller decrease compared to the control could be explained either by a slower chemical ripening in amended sediments or by measurement limitations. Furthermore, the evolution of electrical conductivity over the 15 weeks of field ripening evidenced the accumulation of chemical reaction products in dry periods.

The influence of biochar on sediment physical and chemical properties, including the increased pore structure, water holding capacity, aeration and buffering capacity, all contributed to creating conditions favorable to microbial activity. A priming effect of biochar application could be observed in the first six weeks of ripening, with high respiration rates, high decomposition rates, and decreasing stabilization of organic matter. In this period, total organic carbon content decreased on average by 30\% in amended samples, compared to only 6% in the control. At the same time, nitrogen content decreased on average by 13% in the samples with biochar, further confirming the high microbial activity. This was followed by a period of decreasing microbial activity until the end of the experiment, which was marked by 14-32% lower respiratory carbon release of the amended samples compared to the control, decreasing decomposition rates and increasing stabilization of organic matter. Thus, biochar application accelerated the decomposition of labile carbon and enhanced the biological stabilization of organic matter in sediments.

These findings suggest that biochar amendment can significantly improve sediment ripening processes and can result in a material with properties desirable for dike construction.

The Limpopo Lipadi Reserve has the difficult task of restoring its natural ecosystems and protecting them from future challenges. The increasing occurrence of droughts due to climate change and the historical use of this land for cattle farming contribute to concerns about the future availability of water for animals, vegetation and staff, as well as the overall health of the soil. By establishing a water balance and investigating soil health, conclusions could be drawn about the current state of the Reserve’s soil and water resources and recommendations made for future research. The parameters of the water balance were defined by combining the literature and the results of field experiments. A climate change model was applied to the water balance to assess how the Reserve will be affected by changes in precipitation and temperatures. Soil sampling was also undertaken at four characteristic sites in the Reserve to assess the impact of bush clearing on soil health and aquifer recharge through changes in physical, biological and hydraulic properties.
The results of the water balance and the different simulated scenarios show that: 1) the aquifers can currently be accounted as reliable when considered as a total available resource for the entire area of the Reserve; 2) when the bush clearing scenario was simulated, it was found that doubling the amount of clearing has a minor impact and only when 50 % of the reserve is cleared the impact becomes significant; 3) due to climate change and its impact on ecosystems, it was found that there will be an intensification of the hydrological cycle (wetter, hotter summer) with an increased seasonality. However, the results of this scenario indicated that there will be no drastic changes in the main pattern of water dynamics in the next 25 years and therefore no immediate threat to the available groundwater storage.
In carrying out the soil characterisation tests, it was noted that 8 different soil types were being studied, which would certainly include a wider range of values for soil properties. However, looking at the effects of bush clearing and considering the different types of soil, the results showed that there was indeed an outcome in the treated areas. For most of the studied sites, it was consistently found that bulk density had increased in the cleared areas, while porosity levels, soil moisture and organic matter decomposition rate had decreased. It was also discovered that as a side effect of bush clearing, insects such as termites were present, which played a role in some of the soil processes. Furthermore, no clear relationship with clearance status could be observed for hydraulic conductivity. These results were then used in a multi-criteria analysis to assess the health of the soils studied. This assessment showed that, overall and for the specific purpose of the research undertaken, all the soils analysed could be classified as ’healthy’ to sustain the current environmental practices of the Reserve, even after clearing was performed.
Although the results presented in this report take into account the current status of the Reserve, it is noted that there may be differences when different time frames are considered. The results provide valuable insights based on the highlights found and, based on these, recommendations that will impact the future environmental management and land use practices of the Reserve are provided.
Further analysis is recommended to gain a complete understanding of the possible effects of bush clearing on water dynamics and to compare the results presented in this research. It must also be
borne in mind that there may be discrepancies in the results obtained due to lack of equipment and time constraints.