Carbon dioxide (CO2) efflux from soil represents one of the biggest ecosystem carbon (C) fluxes and high-magnitude pulses caused by rainfall make a substantial contribution to the overall C emissions. It is widely accepted that the drier the soil, the larger the CO2 pulses will be, but this notion has never been tested for water-repellent soils. Soil water repellency (SWR) is a common feature of many soils and is especially prominent after dry periods or fires. An important unanswered question is to what degree SWR affects common assumptions about soil CO2 dynamics. To address this, our study investigates, for the first time, the effect of SWR on the CO2 pulse upon wetting for water-repellent soils from recently burned forest sites. CO2 efflux measurements in response to simulated wetting were conducted both under laboratory and in situ conditions. Experiments were conducted on severely and extremely water-repellent soils, with a wettable scenario simulated by adding a wetting agent to the water. CO2 efflux upon rewetting was significantly lower in the water-repellent scenarios. Under laboratory conditions, CO2 pulse was up to four times lower under the water-repellent scenario as a result of limited wetting, with 70% of applied water draining rapidly via preferential flow paths, leaving much of the soil dry. We suggest that the predominant cause of the lower CO2 pulse in water-repellent soils was the smaller volume of pores in which the CO2 was replaced by infiltrating water, compared to wettable soil. This study shows that SWR should be considered as an important factor when measuring or predicting the CO2 flush upon rewetting of dry soils. Although this study focused mainly on short-term effects of rewetting on CO2 fluxes, the overall implications of SWR on physical changes in soil conditions can be long lasting, with overall larger consequences for C dynamics.

The magnitude of forest fires' impacts on the environment is directly related to the changes induced on soil physical, chemical and biological properties. Using available organic resources to rehabilitate burnt forest soils can help reduce post-fire soil fertility loss, accelerating ecosystem recovery. In the present study, the potential of four soil amendments: a mycotechnosol, a eucalypt residue mulch, dredged sediments from a freshwater lagoon and an organic-mineral biofertilizer, to improve the quality of burnt forest soils in terms of organic matter, carbon, nitrogen and phosphorus contents, was evaluated. Two experiments were set-up, one in a recently burnt eucalypt plantation and another in the laboratory using soils from the same area, to assess the effects of the amendments on soil quality, with both experiments lasting for 7 months. The effects of the amendments on nutrient leaching along the soil profile were also evaluated in the laboratory, to investigate possible negative impacts on groundwater and surface water quality. All amendments increased the organic matter and nutrient contents of burnt soils, confirming their potential for ecosystem rehabilitation. The biofertilizer, however, was found to promote nutrient losses by leaching, largely owing to its high solubility, increasing the risk of contamination of ground and surface waters. Using available organic resources to rehabilitate burnt forests as was done in the present work complies with the idea of a circular economy, being key for the sustainability of forest ecosystems.

Purpose In low lying areas with dense networks of canals for land drainage, sediments accumulate in the waterways and have to be periodically dredged. These adjacent areas are mainly used for farming and agriculture and suffer from high rates of subsidence. The recycling of organic amendments, such as sediments, compost and manure, in agricultural soils can improve plant growth and yield, soil carbon content, and microbial biomass and activity, and have the potential to reverse the process of land subsidence. Materials and methods The effect of mixing bio-waste compost and the solid fraction of swine manure with dredged sediments before dewatering and biochemical ripening was investigated in terms of type and quantity of organic matter, CO₂ production and O₂ consumption, and N, P and S content. The water released during dewatering, the aggregate stability, and the undrained shear strength after ripening were also assessed since these areas have to be assessable by trucks and cattle. Results and discussion For the sediment with compost and manure the transformations in the type of organic matter, CO₂ production and O₂ consumption were larger compared to the individual fractions, indicating a positive priming effect. Most volume lost during ripening can be attributed to the loss of water and not to the loss of organic matter. In addition, the mixtures result in very stable aggregates and showed an undrained shear strength three times higher than measured for the sediments. Conclusions Sediments, compost and manure can be used and applied as beneficial use to reverse the process of land subsidence in low lying areas.