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Despite the fundamental importance of soil temperature for Earth's carbon and energy budgets, ecosystem functioning, and agricultural production, studies of climate change impacts on soil processes have mainly relied on air temperatures, assuming they are accurate proxies for soil temperatures. We evaluated changes in soil temperature, moisture, and air temperature predicted over the 21st century from 14 Earth system models. The model ensemble predicted a global mean soil warming of 2.3 ± 0.7 and 4.5 ± 1.1 °C at 100-cm depth by the end of the 21st century for RCPs 4.5 and 8.5, respectively. Soils at 100 cm warmed at almost exactly the same rate as near-surface (~1 cm) soils. Globally, soil warming was slightly slower than air warming above it, and this difference increased over the 21st century. Regionally, soil warming kept pace with air warming in tropical and arid regions but lagged air warming in colder regions. Thus, air warming is not necessarily a good proxy for soil warming in cold regions where snow and ice impede the direct transfer of sensible heat from the atmosphere to soil. Despite this effect, high-latitude soils were still projected to warm faster than elsewhere, albeit at slower rates than surface air above them. When compared with observations, the models were able to capture soil thermal dynamics in most biomes, but some failed to recreate thermal properties in permafrost regions. Particularly in cold regions, using soil warming rather than air warming projections may improve predictions of temperature-sensitive soil processes.

Despite many demonstrated benefits that biochars can have on agricultural soils, there are few examples of profitable biochar utilization on commercial farms. Barriers to profitability include successfully pairing waste streams, production facilities, and farms where biochar is utilized. However, farm-scale biochar systems, which utilize agricultural wastes as feedstocks and produce energy and biochar for on-farm use, may have efficiency advantages over regional, industrial biochar production. Two critical uncertainties for the feasibility of on-farm production are 1) whether a biochar can be produced from on-farm feedstocks with appropriate qualities for soil amendment, and 2) whether on-farm feedstocks are sufficiently abundant to meet on-farm demand. Here we evaluate these issues for a farm-scale gasification system in NE Washington State that produces biochar from grass seed screenings and straw. Field trials to evaluate the biochar as a liming alternative found it was highly effective when broadcast at a rate of at least 18 Mg ha −1 . Biochar outperformed hydrated lime in the first year of the study and improved yields by a factor of 2.88 across both years. Biochar produced from on-farm feedstocks were sufficient to amend 6.3–11.8% of the production area annually, translating to a return interval of 9–16 years. Potential co-production of electrical power far exceeded on-farm demand for operating a seed cleaning mill. We conclude that an on-farm biochar production system is physically feasible for meeting demands for both power and liming amendments. [ABSTRACT FROM AUTHOR]

High-frequency soil CO2 flux data are valuable for providing new insights into the processes of soil CO2 production. A record of hourly soil CO2 fluxes from a semi-arid ponderosa pine stand was spatially and temporally deconstructed in attempts to determine if variation could be explained by logical drivers using (i) CO2 production depths, (ii) relationships and lags between fluxes and soil temperatures, or (iii) the role of canopy assimilation in soil CO2 flux variation. Relationships between temperature and soil fluxes were difficult to establish at the hourly scale because diel cycles of soil fluxes varied seasonally, with the peak of flux rates occurring later in the day as soil water content decreased. Using a simple heat transport/gas diffusion model to estimate the time and depth of CO2 flux production, we determined that the variation in diel soil CO2 flux patterns could not be explained by changes in diffusion rates or production from deeper soil profiles. We tested for the effect of gross ecosystem productivity (GEP) by minimizing soil flux covariance with temperature and moisture using only data from discrete bins of environmental conditions (±1 °C soil temperature at multiple depths, precipitation-free periods and stable soil moisture). Gross ecosystem productivity was identified as a possible driver of variability at the hourly scale during the growing season, with multiple lags between ∼5, 15 and 23 days. Additionally, the chamber-specific lags between GEP and soil CO2 fluxes appeared to relate to combined path length for carbon flow (top of tree to chamber center). In this sparse and heterogeneous forested system, the potential link between CO2 assimilation and soil CO2 flux may be quite variable both temporally and spatially. For model applications, it is important to note that soil CO2 fluxes are influenced by many biophysical factors, which may confound or obscure relationships with logical environmental drivers and act at multiple temporal and spatial scales; therefore, caution is needed when attributing soil CO2 fluxes to covariates like temperature, moisture and GEP. [ABSTRACT FROM PUBLISHER]

• Biochar applied as mulch did not improve soil moisture or germination in pots. • Biochar only improved seedling establishment when mixed into soil in pots. • Modeling showed application method impacted the timing of moisture improvements. Biochar produced from low-value juniper was evaluated for its utility in improving restoration of native bunchgrasses. We hypothesized that biochar could expand the window of favorable conditions for seedling establishment by slowing soil drying. In a greenhouse study, applications of biochar on the soil surface were compared to incorporation of biochar, and improvements in water content, germination, and seedling survival were evaluated. Underlying hydraulics were investigated with measurements of soil and biochar water retention and Hydrus-1D simulations. The greenhouse study showed that applying any depth of biochar on top of soil was not effective at improving soil water content, and furthermore inhibited germination of bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] Á. Löve). In contrast, mixing biochar into soil significantly slowed initial soil drying and increased seedling biomass by 70% compared to unamended soil. However, moisture benefits of biochar incorporation were temporary and, as the soil dried, the treatment with incorporated biochar lost more water than soil that was unamended, because biochar increased unsaturated hydraulic conductivity. Model simulations of the experimental conditions similarly showed that incorporating biochar improved early soil water content, but also suggested that this result was specific to the drainage conditions of pots and may not translate to a deeper soil. Model simulations also supported the idea that biochar applied as a mulch could have an increasing benefit over time by reducing evaporation. This study suggested that both incorporation and mulch application of biochar may benefit seedling establishment, but further investigations in a naturally-draining soil are needed to validate these findings. [ABSTRACT FROM AUTHOR]

• Biochars increased water content at saturation but not at field capacity in 4 soil textures. • Differences in biochar properties did not translate to differences in soil water retention. • Biochar did not have a drought benefit in the first year following incorporation. Few studies have evaluated the ability of biochar to improve soil water content under field conditions. Our objective was to assess the potential drought benefits of biochar across a range of soil textures by measuring indicators of hydraulic capacity and water-retention curves for field-amended soils. Two biochars, conifer wood and wheat straw, were incorporated by rotary tillage at amendment rates of 9−36 Mg ha−1 in four locations. All four soils showed a significant increase (p≤ 0.04) in saturated water content of 0.05–1.7 % volume per Mg ha−1 biochar added, but no increase in water content at field capacity. There were significant (p< 0.01) decreases in volumetric water content at wilting point by 0.09 to 0.8 % per Mg ha−1 of biochar added in all soil textures. This translated to small, significant increases in plant-available water capacity of the same magnitude. Although the conifer wood biochar was less hydrophobic and had 14 % more internal pore space than the wheat straw biochar, both biochars had similar impacts on volumetric soil water content (θ). Simulations of soil drying and recharge using moisture retention curves and the Hydrus-1D model indicated that biochar amendment would generally decrease θ in the loam soil, and would have minimal impact on θ (± 1 %) in the more coarse textured soils. Monitoring of in situ θ in two of these soils showed similar results. Biochar sped up soil drying in the loam following irrigation, but had no impact on θ in the loamy sand. These results suggest limited utility for biochar as a drought adaptation tool in the first year following incorporation into these medium- to coarse-textured soils. [ABSTRACT FROM AUTHOR]