Depletion of soil carbon and aggregation after strong warming of a subarctic Andosol under forest and grassland cover

The net loss of soil organic carbon (SOC) from terrestrial ecosystems is a likely consequence of global warming and may affect key soil functions. The strongest changes in temperature are expected to occur at high northern latitudes, with forest and tundra as prevailing land cover types. However, specific soil responses to warming in different ecosystems are currently understudied. In this study, we used a natural geothermal soil warming gradient (0–17.5 ∘C warming intensity) in an Icelandic spruce forest on Andosol to assess changes in the SOC content between 0 and 10 cm (topsoil) and between 20 and 30 cm (subsoil) after 10 years of soil warming. Five different SOC fractions were isolated, and their redistribution and the amount of stable aggregates were assessed to link SOC to changes in the soil structure. The results were compared to an adjacent, previously investigated warmed grassland. Soil warming depleted the SOC content in the forest soil by −2.7 g kg−1 ∘C−1 (−3.6 % ∘C−1) in the topsoil and −1.6 g kg−1 ∘C−1 (−4.5 % ∘C−1) in the subsoil. The distribution of SOC in different fractions was significantly altered, with particulate organic matter and SOC in sand and stable aggregates being relatively depleted and SOC attached to silt and clay being relatively enriched in warmed soils. The major reason for this shift was aggregate breakdown: the topsoil aggregate mass proportion was reduced from 60.7±2.2 % in the unwarmed reference to 28.9±4.6 % in the most warmed soil. Across both depths, the loss of one unit of SOC caused a depletion of 4.5 units of aggregated soil, which strongly affected the bulk density (an R2 value of 0.91 and p<0.001 when correlated with SOC, and an R2 value of 0.51 and p<0.001 when correlated with soil mass in stable aggregates). The proportion of water-extractable carbon increased with decreasing aggregation, which might indicate an indirect protective effect of aggregates larger than 63 µm on SOC. Topsoil changes in the total SOC content and fraction distribution were more pronounced in the forest than in the adjacent warmed grassland soils, due to higher and more labile initial SOC. However, no ecosystem effect was observed on the warming response of the subsoil SOC content and fraction distribution. Thus, whole profile differences across ecosystems might be small. Changes in the soil structure upon warming should be studied more deeply and taken into consideration when interpreting or modelling biotic responses to warming.

Geo-mechanics and Hydraulic Conductivity Study of Claystone in Boyolali, Central Java, Indonesia

Claystone middle Miocene age were found in Wonosegoro sub-district, Boyolali region, Central Java, Indonesia. The purpose of the paper is to examine and discuss the geology and typical behavor of this claystone and its micro-level mechanism. The hydraulic conductivity was assessed using consilidated apparatuses; 150 mm diameter column mound using aggregated sample and 60 mm oedometer mould using slurry sample as a reference. Claystone materials used were treated under various conditions. In long term test under constand vertical stress and hydraulic gradient, the hydraulic conductvity decreases with time although the volume of void volume of the sample increased by swelling. Water contens of the individul aggregated increased by swelling, by which strength of particles decreases with and aggregate breakdown was enchanced. As a result, large void created by large particles could be redused in its size, leading the reduction of hydraulic conductuvity. The hydralic conductivities (K values) obtained from the aggregated sample varied in a broad range compared with those from slurry sample. In the test using the higher percentage of gravel-sized aggregate (up until couarse gravel-sized; retained in 26.5 mm sieve), K values changed from 10-5 to 10-7 m/s under vertical stresses from 5 up to 245 kPa. The test was repeated using smaller percentage of gravel-sized aggregate (up until fine gravel-sized, retained in 4.75 mm sieve) and the observed K values changed from 10-5 to 10-10 m/s. While the K values obtained in the specimen made from surry under same vertical stresses was 10-9 to 10-11 m/s. All of the extruded aggregate samples had higher water content than the initial ones, which suggest the alteration mechanism of soled consolidation phase to more deformable plastic phase, whice enables thesample to decrease the void size. These results conclude that using coarse gravel-sized aggregated, which is a reasonable scenario of a practically feasible aggregate size in a field, may noy produce the aimed hydraulic conductivity by the regulated standard. Therefore, breakdown of the aggrgate size and enchament of swelling are crucial factors for the application of the clay stones as a barrier material.

Impact of Microbial Iron Oxide Reduction on the Transport of Diffusible Tracers and Non-diffusible Nanoparticles in Soils

In situbioremediation to achieve immobilization of toxic metals and radionuclides or detoxification of chlorinated solvents relies on electron donor additions. This practice promotes microbial Fe(III)-oxide mineral reduction that could change soil pore structure, release soil colloids, alter matrix surface properties, and cause the formation of secondary (i.e., reduced) Fe-mineral phases. These processes in turn may impact rates of bioremediation, groundwater quality, and ultimately contaminant fate. Continuous flow columns packed with water-stable soil aggregates high in Fe-oxides were infused with artificial groundwater containing acetate as electron donor and operated for 20 or 60 days inside an anoxic chamber. Soluble Fe(II) and soil colloids were detected in the effluent within one week after initiation of the acetate addition, demonstrating Fe(III)-bioreduction and colloid formation. Br-, 2,6-difluorobenzoate (DFBA), and silica-shelled silver nanoparticles (SSSNP) were selected as diffusible tracer, low-diffusible tracer, and non-diffusible nanoparticles, respectively, to perform transport experiments before and after the active 20-day bioreduction phase, with an aim of assessing the changes in soil structure and surface chemical properties resulting from Fe(III)-bioreduction. The transport of diffusible Br-was not influenced by the Fe(III)-bioreduction as evidenced by identical breakthrough curves before and after the introduction of acetate. Low-diffusible DFBA showed earlier breakthrough and less tailing after the bioreduction, suggesting alterations in flow paths and surface chemical properties of the soils. Similarly, non-diffusible SSSNP exhibited early breakthrough and enhanced transport after the bioreduction phase. Unexpectedly, the bioreduction caused complete retention of SSSNP in the soil columns when the acetate injection was extended from 20 days to 60 days, though no changes were observed for Br-and DFBA during the extended bioreduction period. The large change in the transport of SSSNP was attributed to the enhancement of soil aggregate breakdown and soil colloid release causing mechanical straining of SSSNP and the exposure of iron oxide surfaces previously unavailable within aggregate interiors favorable to the attachment of SSSNP. These results demonstrate that microbial activity can affect soil properties and transport behaviors of diffusivity-varying solutes and colloids in a time dependent fashion, a finding with implication for interpreting the data generated from soil column experiments under continuous flow.HighlightsFe(III)-bioreduction causes time-dependent aggregate breakdown and colloid release.Short-term bioreduction alters soil aggregate surface chemistry and tracer transport.Electron donor amendment enhances transport of nanoparticle tracer.