Gasified Grass and Wood Biochars Facilitate Plant Establishment in Acid Mine Soils.

Heavy metals in exposed mine tailings threaten ecosystems that surround thousands of abandoned mines in the United States. Biochars derived from the pyrolysis or gasification of biomass may serve as a valuable soil amendment to revegetate mine sites. We evaluated the ability of two biochars, produced by gasification of either Kentucky bluegrass seed screenings (KB) or mixed conifer wood (CW), to support the growth of plants in mine spoils from the abandoned Formosa and Almeda Mines in Oregon. To evaluate the potential for plant establishment in mine tailings, wheat was grown in tailings amended with biochar at rates ranging from 0 to 9% (w/w). Both KB and CW biochars promoted plant establishment by increasing soil pH, increasing concentrations of macro- and micronutrients, and decreasing the solubility and plant uptake of heavy metals. Formosa tailings required at least 4% biochar and Almeda soil required at least 2% biochar to promote healthy wheat growth. A complimentary experiment in which mine spoils were leached with simulated precipitation indicated that biochar amendment rates ≥4% were sufficient to neutralize the elution pH and reduce concentrations of potentially toxic elements (Zn, Cu, Ni, Al) to levels near or below concern. These findings support the use of gasified biochar amendments to revegetate acid mine soils.

High soil carbon sequestration rates persist several decades in turfgrass systems: A meta-analysis.

Managed turfgrass is a common component of urban landscapes that is expanding under current land use trends. Previous studies have reported high rates of soil carbon sequestration in turfgrass, but no systematic review has summarized these rates nor evaluated how they change as turfgrass ages. Here we conducted a meta-analysis of soil carbon sequestration rates from 63 studies globally, comprised mostly of C3 grass species in the U.S., including 24 chronosequence studies that evaluated carbon changes over 75 years or longer. We showed that turfgrass established within the last ten years had a positive mean soil C sequestration rate of 5.3 Mg CO2 ha-1 yr-1 (95% CI = 3.7-6.2), which is higher than rates reported for several soil conservation practices. Areas converted to turfgrass from forests were an exception, sometimes lost soil carbon, and had a cross-study mean sequestration rate that did not differ from 0. In some locations, soil C accumulated linearly with turfgrass age over several decades, but the major trend was for soil C accumulation rates to decline through time, reaching a cross-study mean sequestration rate that was not different from 0 at 50 years. We show that fitting soil C timeseries with a mechanistically derived function rather than purely empirical functions did not alter these conclusions, nor did employing equivalent soil mass versus fixed-depth carbon stock accounting. We conducted a partial greenhouse gas budget that estimated emissions from mowing, N-fertilizer production, and soil N2O emissions. When N fertilizer was applied, average maintenance emissions offset 32% of C sequestration in recently established turfgrass. Potential emission removals by turfgrass can be maximized with reduced-input management. Management decisions that avoid losing accrued soil C-both when turfgrass is first established and when it is eventually replaced with other land-uses-will also help maximize turfgrass C sequestration potential.

Carbon Sequestration in Turfgrass-Soil Systems.

Plants are key components of the terrestrial ecosystem carbon cycle. Atmospheric CO2 is assimilated through photosynthesis and stored in plant biomass and in the soil. The use of turfgrass is expanding due to the increasing human population and urbanization. In this review, we summarize recent carbon sequestration research in turfgrass and compare turfgrass systems to other plant systems. The soil organic carbon (SOC) stored in turfgrass systems is comparable to that in other natural and agricultural systems. Turfgrass systems are generally carbon-neutral or carbon sinks, with the exception of intensively managed areas, such as golf course greens and athletic fields. Turfgrass used in other areas, such as golf course fairways and roughs, parks, and home lawns, has the potential to contribute to carbon sequestration if proper management practices are implemented. High management inputs can increase the biomass productivity of turfgrass but do not guarantee higher SOC compared to low management inputs. Additionally, choosing the appropriate turfgrass species that are well adapted to the local climate and tolerant to stresses can maximize CO2 assimilation and biomass productivity, although other factors, such as soil respiration, can considerably affect SOC. Future research is needed to document the complete carbon footprint, as well as to identify best management practices and appropriate turfgrass species to enhance carbon sequestration in turfgrass systems.

Soil moisture effects on the carbon isotope composition of soil respiration.

The carbon isotopic composition (delta(13)C) of recently assimilated plant carbon is known to depend on water-stress, caused either by low soil moisture or by low atmospheric humidity. Air humidity has also been shown to correlate with the delta(13)C of soil respiration, which suggests indirectly that recently fixed photosynthates comprise a substantial component of substrates consumed by soil respiration. However, there are other reasons why the delta(13)CO(2) of soil efflux may change with moisture conditions, which have not received as much attention. Using a combination of greenhouse experiments and modeling, we examined whether moisture can cause changes in fractionation associated with (1) non-steady-state soil CO(2) transport, and (2) heterotrophic soil-respired delta(13)CO(2). In a first experiment, we examined the effects of soil moisture on total respired delta(13)CO(2) by growing Douglas fir seedlings under high and low soil moisture conditions. The measured delta(13)C of soil respiration was 4.7 per thousand more enriched in the low-moisture treatment; however, subsequent investigation with an isotopologue-based gas diffusion model suggested that this result was probably influenced by gas transport effects. A second experiment examined the heterotrophic component of soil respiration by incubating plant-free soils, and showed no change in microbial-respired delta(13)CO(2) across a large moisture range. Our results do not rule out the potential influence of recent photosynthates on soil-respired delta(13)CO(2), but they indicate that the expected impacts of photosynthetic discrimination may be similar in direction and magnitude to those from gas transport-related fractionation. Gas transport-related fractionation may operate as an alternative or an additional factor to photosynthetic discrimination to explain moisture-related variation in soil-respired delta(13)CO(2).

High-frequency analysis of the complex linkage between soil CO(2) fluxes, photosynthesis and environmental variables.

High-frequency soil CO(2) flux data are valuable for providing new insights into the processes of soil CO(2) production. A record of hourly soil CO(2) 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) CO(2) production depths, (ii) relationships and lags between fluxes and soil temperatures, or (iii) the role of canopy assimilation in soil CO(2) 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 CO(2) flux production, we determined that the variation in diel soil CO(2) 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 CO(2) 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 CO(2) assimilation and soil CO(2) flux may be quite variable both temporally and spatially. For model applications, it is important to note that soil CO(2) 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 CO(2) fluxes to covariates like temperature, moisture and GEP.