Relative Gas Diffusivity Research Articles

Study of soil heterotrophic respiration as a function of soil moisture under different land covers

The relationship between soil heterotrophic respiration (Rh) and soil moisture has been often studied using disturbed soil samples and simple gravimetric and volumetric soil moisture indicators. The objective of this study was to investigate the relationship between Rh and soil moisture in terms of water-filled porosity (WFP), matric potential (Ψm), and relative soil gas diffusivity (Dp/Do) using undisturbed soil cores obtained under different land covers. Soil CO2 efflux, WFP, and Ψm were measured in undisturbed soil samples (250 cm3) collected in the 0–5 cm soil layer (without any vegetation or living roots) under laboratory conditions by combining a CO2 gas analyzer, a scale, and precision mini-tensiometers. For each site and land cover, we also measured soil chemical properties, soil physical properties, and soil microbial composition using phospholipid fatty acid analysis. Grassland soils had the largest total microbial biomass (6275 ng g−1), followed by soils from riparian (5327 ng g−1), and cropland (2745 ng g−1) sites. Bacteria were the dominant group representing 46% (SD = 5%) of the total microbial biomass across all sites and land covers. Maximum Rh was 1.88 (SD = 0.40) μmol CO2 m−2 s−1 in grassland, 1.64 (SD = 0.82) μmol CO2 m−2 s−1 in riparian, and 0.94 (SD = 0.56) μmol CO2 m−2 s−1 in cropland soils. Considering all land cover and soil types, our observations revealed that peak Rh occurred at mean WFP = 0.81 ,Ψm = −6 kPa, and Dp/Do = 0.003. Thus, we recommend avoiding the traditional field capacity definition of −33 kPa for representing peak microbial activity. Water-filled porosity was a more consistent predictor of Rh than Ψm or Dp/Do across soils with contrasting organic matter content, total microbial biomass, soil texture, and soil structure.

Computed tomography scanning revealed macropore-controlled N2O emissions under long-term tillage and cover cropping practices

Microscale alterations in soil physical characteristics resulting from long-term soil health practices can contribute to changes in soil nitrous oxide (N2O) emissions. In this study, we investigated soil N2O emissions in relation to pore characteristics influencing soil gas diffusivity under long-term tillage and cover cropping practices. Intact soil cores from tillage (conventional tillage, Conv. T versus no tillage, NT) and cover crop (hairy vetch, HV versus no cover crop, NC) treatments were used for N2O measurements and computed tomography (CT) scanning. Using X-ray CT technique with a resolution of 59 μm, pore structure parameters including macroporosity, number of macropores, anisotropy, fractal dimension, tortuosity, and connectivity were determined. The results showed that Conv. T and HV emitted significantly higher N2O than NT and NC, respectively. A similar trend was observed for macroporosity, Conv. T soils had 27.4 % higher CT-derived macroporosity than the NT soils and HV increased macroporosity by 31.1 % over the NC treatment. The number of macropores and fractal dimension were significantly higher whereas degree of anisotropy was significantly lower under HV compared to NC. In the upper 3 cm of soil, HV had a connected porosity, whereas the pores were disconnected and isolated in NC. These CT-derived properties; however, were not impacted by tillage treatments. N2O emissions were positively and significantly correlated to relative soil gas diffusivity, CT-derived macroporosity, number of macropores, and fractal dimension. Our results demonstrated that soil macroporosity and relative gas diffusivity could lead to improved understanding and predictability of N2O emissions under high soil moisture conditions.

Multifaceted impacts of moss-dominated biocrusts on dryland soils: From soil pore structure to aeration and water infiltration

Biocrusts are a crucial living cover of dryland soils that alter near-surface soil structure and properties. To date, it is still unclear how biocrusts affect soil pore structure, and subsequently alter gas and water transport processes. In this study, we investigated aeolian sand and loess soil colonized with moss-dominated biocrusts and in bare state of the Chinese Loess Plateau. X-ray computed tomography (CT) was used to quantify and visualize the pore characteristics for all considered scenarios. In addition, the soil air permeabilities and relative gas diffusivities were measured together with infiltration rates. CT imaging and reconstruction showed 102–107%, 56–87%, and 72–203% higher macroporosities, surface area densities, and mean pore volumes, respectively, for soils colonized with biocrusts when compared to their bare counterparts. The biocrusts also increased the ratio of connected and isolated porosities as well as pore node densities, while decreasing Euler numbers and tortuosity when compared to the bare soils. This suggests that the pore networks in biocrust-colonized soils exhibit higher continuity and less tortuosity. In addition, the biocrusts increased air permeability by 277–301% and relative gas diffusivity by 47–59%, which is attributable to more air-filled pores and better pore connectivity. However, when compared with bare soils, the biocrusts reduced steady state infiltration rates by 41–50% and the 60 min cumulative infiltration amount by 33–54%. The decrease in infiltrability is attributable to the higher fine particle content and the enhanced water holding capacity of biocrusts. In summary, the biocrusts improved soil pore structure and soil aeration, and reduced infiltrability when compared to bare soils. This implies that biocrusts play a multifaceted and complex role in the transport of gases and water, and as a consequence significantly impact near-surface hydrological and biochemical processes, such as soil evaporation and respiration in drylands.

Comparison of Hydraulic and Aeration Properties of Peat Substrates Used to Produce Containerized White Spruce Seedlings (1+0) in Forest Nurseries

The physical properties of peat substrates from eight tree nurseries were characterized to determine bulk density, air-filled porosity, saturated hydraulic conductivity, pore effectiveness, relative gas diffusivity and chemical properties. There were significant variations among nurseries both in growth of white spruce [Picea glauca (Moench) Voss] seedlings (1+0) and substrate properties. Shoot dry mass and root collar diameter were negatively correlated with air-filled porosity and saturated hydraulic conductivity, whereas root dry mass was positively correlated with bulk density. Seedling growth increased with increasing substrate bulk density up to ~0.11 g cm−3, above which value conditions may become limiting to plant performance. Our results suggest that there was no growth limitation due to restricted aeration (Ds D0−1 > 0.005 m2 s−1 m−2 s for all substrates except one) and that over-aeration reduced seedling growth under dry irrigation management.

Insights about biocrust effects on soil gas transport and aeration in drylands: Permeability, diffusivity, and their connection to hydraulic conductivity

Soil gas transport properties and aeration are essential influencing factors of soil chemical and biochemical reactions, especially the respiration processes of plant roots and microorganisms. As ecosystem engineers in drylands, biocrusts may greatly alter surface soil gas transport properties and aeration, but their influences remain unclear and understudied. In a semiarid dryland ecosystem on the Chinese Loess Plateau, the air permeability of bare soil and three types of biocrusts (cyanobacterial, cyanobacterial-moss mixed, and moss crusts) were measured in-situ with naturally changing soil water content. Undisturbed soil samples were also taken for each treatment, and their air permeability and relative gas diffusivity at air-dried and near-saturated conditions, as well as saturated hydraulic conductivity, were determined in the laboratory. We further analyzed the relationship between saturated hydraulic conductivity and air permeability or relative gas diffusivity and calculated the gas and permeability continuity (specific air permeability and specific gas diffusivity) and tortuosity indexes at air-dry and near-saturation. Our results showed that biocrusts were associated with a significantly higher air permeability in contrast to bare soil, and the in situ mean air permeability ranked in order of moss crusts > mixed crusts > cyanobacterial crusts > bare soil. At air-dried condition, biocrusts increased surface soil air permeability by 26% and relative gas diffusivity by 46% as compared with bare soil through decreased surface soil bulk density. In contrast, at near saturation biocrusts decreased air permeability and relative gas diffusivity by 33% and 20%, respectively, which was attributed to pore-clogging by extracellular polymeric substances of biocrusts (3.37 vs. 2.27 mg g−1). Accordingly, at air-dried condition biocrusts increased specific air permeability by 11% and specific gas diffusivity by 30% but decreased tortuosity by 7%. In contrast, they decreased specific air permeability by 76% and specific gas diffusivity by 39% but increased tortuosity by 28% at near-saturated condition. Additionally, the saturated hydraulic conductivity of cyanobacterial, cyanobacterial-moss mixed, and moss crusts were respectively 77%, 69%, and 61% lower than that of bare soil. We found that the relationship between saturated hydraulic conductivity and air permeability or relative gas diffusivity of biocrusts at air-dried condition can be well fitted with a positive linear log–log model (R2 > 0.95). In general, the improved gas transport properties of surface soil caused by different types of biocrusts leads to better soil aeration, which in turn would facilitate microbial activity, soil respiration, and root growth; and impact surface soil water vapor movement induced by evaporation or condensation processes in drylands. Therefore, biocrusts should be carefully considered in further assessments of global dryland hydrology and carbon or nitrogen cycling under changing climate.

Water‐based spent drilling mud regulates hydrophysical properties of coarse‐textured loessial soil

AbstractAmendment application is one of the effective measures used to remediate structural problems in coarse‐textured soils. However, the application of water‐based spent drilling mud (WBSM) as a potential amendment and its associated impacts on physical and hydraulic soil properties is not well‐studied. This research aimed to investigate the effect of the application of WBSM on basic hydrophysical properties of coarse‐textured loessial soil sampled from the Loess Plateau, China. Application of WBSM to repacked loessial soil at four application rates (0, 25.0, 50.0, 75.0 g kg−1, dry basis), denoted as control (H0), low (HDL), medium (HDM) and high (HDH), respectively, was used to investigate changes in soil bulk density (ρb), soil total porosity (ƒ), saturated hydraulic conductivity (Ks), relative gas diffusivity (Dp/Do), soil‐water retention and soil evaporation characteristics. The results showed that the soil ρb in HDL and HDM decreased by 4.0% and 5.5%, respectively, as compared to H0 (p < 0.05). Measured Ks changed to 36.7% (HWL), 12.2% (HWM) and 4.4% (HWH) as compared to H0 (5.56 cm d−1). The application of WBSM further decreased the percentage of soil pore sizes >30 μm, but increased the proportion of medium pore sizes (≥30–50 μm), micro pore sizes (≥0.3–5 μm) and extremely micro pore sizes (<0.3 μm), which was attributed to the clogging effect of the fine and clay particles contained in WBSM. Decline in Dp/Do, however, was observed with increasing WBSM, likely due to the increased pore tortuosity in fine‐grained soil. The van Genuchten soil‐water retention model parameters differed largely across the WBSM application rates, with increased soil available water‐holding capacity by 50.9%, 65.9% and 64.1% in HDL, HDM and HWH, respectively, as compared to H0. However, WBSM application also increased soil available water‐holding capacity and reduced evaporative water loss, which has useful implications for the management of coarse‐textured soils with a distinct water deficit in fragile ecosystems. Widespread use of WBSM to amend coarse‐textured soils requires further studies to evaluate the effects of WBSM amendments in field trials and its environmental consequences.Highlights Water‐based spent drilling mud (WBSM) altered the texture of loessial soil. WBSM increased soil total porosity and changed pore size distribution. WBSM affected the characteristics of soil water and gas transport. The soil water availability was improved by WBSM application.

Nitrogen Addition Affects Nitrous Oxide Emissions of Rainfed Lucerne Grassland.

Nitrous oxide (N2O) is a potent greenhouse gas. Assessing the N2O emission from lucerne grasslands with nitrogen addition will aid in estimating the annual N2O emissions of such agriculture areas, particularly following summer rainfall events in light of precipitation variation associated with global change. Here, we measured soil N2O emissions, soil temperature and water content of lucerne grasslands with four levels of nitrogen addition over 25 days, which included 10 rainfall events. Results showed that nitrogen addition was observed to increase soil NO3−-N content, but not significantly improve dry matter yield, height or leaf area index. Nitrogen addition and rainfall significantly affected N2O emissions, while the response of N2O emissions to increasing nitrogen input was not linear. Relative soil gas diffusivity (Dp/Do) and water-filled pore space (WFPS) were good indicators of N2O diurnal dynamics, and Dp/Do was able to explain slightly more of the variation in N2O emissions than WFPS. Collectively, nitrogen addition did not affect lucerne dry matter yield in a short term, while it induced soil N2O emissions when rainfall events alter soil water content, and Dp/Do could be a better proxy for predicting N2O emissions in rainfed lucerne grasslands.

In situ nitrous oxide and dinitrogen fluxes from a grazed pasture soil following cow urine application at two nitrogen rates

Cattle grazing of pastures deposits urine onto the pasture soil at high nitrogen (N) rates that exceed the pasture's immediate N demands, increasing the risk of N loss. Nitrous oxide (N2O), a greenhouse gas, and dinitrogen (N2) are lost from the cattle urine patches. There is limited information on the in situ loss of N2 from grazed-pasture systems which is needed for understanding pasture soil N dynamics and balances. The 15N flux method was used to determine N2 and N2O fluxes over time following synthetic urine-15N application at either 400 or 800 kg N ha−1 to a grazed perennial pasture soil. Results showed that daily N2O fluxes were higher under 800 kg N ha−1 than under 400 kg N ha−1, but there was no significant difference in N2 fluxes. Cumulative N2O emissions from soil with 400 kg N ha−1 and 800 kg N ha−1 applied represented 0.16 ± 0.08% and 0.43 ± 0.08% of deposited N, respectively, while emitted N2 accounted for 32.1 ± 4.1% and 14.4 ± 1.7%, respectively, over 95 days after urine application. Codenitrification and denitrification co-occurred, with denitrification accounting for 97.9 to 98.5% of total N2 production. Recovery of urine-15N in pasture decreased with increasing N rate with 14.7 ± 0.5% and 9.9 ± 0.8% recovered at 400 and 800 kg N ha−1, respectively after 95 days. The N2O/(N2 + N2O) product ratio was generally higher during periods of nitrification of urine-N (the first month after urine application) but with no clear relationship to other measured variables. Contrary to our hypothesis, an elevated urine-N rate did not enhance N2 loss. This is speculated to be due to enhanced ammonia volatilisation and transfer of N as nitrate, to deeper soil layers. Soil relative gas diffusivity indicated that high N2 fluxes resulted from entrapped N2 diffusing from the draining soil.

Wetting and drainage cycles in two New Zealand soil types: Effects on relative gas diffusivity and N2O emissions

Nitrous oxide (N2O) is a potent greenhouse gas generated in agricultural soils by microbial processes that vary according to soil redox. Soil oxygen (O2) supply and demand strongly influence soil redox. Migration of O2 into the soil primarily occurs via gas diffusion, expressed as relative gas diffusivity (Dp/Do), and is influenced by soil structure (air-filled porosity and tortuosity of pores) and soil water content. Soil N2O emissions have been shown to increase at low values of Dp/Do but detailed studies examining the relationship between Dp/Do and soil N2O emissions remain limited, with relatively few soil types examined, and no studies of repeated wetting-drainage cycles. Thus, the objectives of this study were to examine how successive wetting-drainage cycles affected both Dp/Do dynamics and associated N2O emissions in two New Zealand soils; a pallic silt loam and an allophanic loam, with the latter also having a higher organic matter content. Soil cores, repacked to varying density, were wetted up with 15N enriched NO3− solution and placed on tension tables where they underwent two consecutive 12-day wetting-drainage cycles from saturation to field capacity (0 to −10 kPa). Over time measurements were made of N2O, N2, inorganic-N and soluble carbon, while Dp/Do was modelled using soil physical characteristics. For both soils each wetting-drainage cycle induced N2O fluxes but with 5-fold lower fluxes in the allophanic soil. Greater aggregation and sand content in the allophanic soil generated higher porosity and Dp/Do values that were almost always greater than recognized anaerobic limits. Thus, wetting-induced N2O fluxes observed in the allophanic soil during early drainage were concluded to result from anaerobic or hypoxic pathways of N2O production potentially within the intra-aggregate zone. While wetting-drainage events induce N2O emissions by altering Dp/Do and the soil aeration status, the draining of soils, especially soils high in organic matter, may enhance O2 demand generating anaerobic zones conducive to denitrification. Further detailed studies examining the interaction between soil structure and soil organic matter content and their effect on N2O emissions under wetting-drainage events, with measures of soil O2, are needed.

Relationships among water and gas transport properties in soils from natural and induced clay gradients

AbstractClay content in soil potentially has strong influence on soil structure and thereby controls soil physical, hydraulic, and gas transport properties. Only few studies have investigated the effect of clay content on saturated hydraulic conductivity (Ks) and gas transport parameters such relative gas diffusivity (Dp/D0) and air permeability (Ka) under varying conditions of clay contents. In this study, two clay gradients were investigated: a natural clay gradient and an induced (clay mineral attapulgite‐controlled) clay gradient. We determined the effect of the two clay gradients on soil air and water parameters and identified the potential relationships between these parameters and other physical properties. The results indicated that all Dp/D0, Ka, and Ks decreased with increasing clay content. The differences among the various treatments were dependent on the soil pore characteristics (air‐filled porosity, ε; pore continuity, POA; pore tortuosity, τ; and pore size distribution). Under the natural clay gradient, the increased clay content resulted in 52.8% more micropores and 35.2% fewer macropores. The slope of τ vs. clay content under the induced clay gradient was found to be higher than that under the natural gradient. Statistically meaningful correlations could be found between Ks and Ka (|r| = .904) and also between Ks and Dp/D0 (|r| = .810), whereas Dp/D0 was evidenced to be better correlated with Ks regardless of the type of clay gradient (P < .01). The results thus provide a valuable experimental and numerical insight to develop useful correlations among Ks, Ka, and Dp/D0 under differing clay gradients.

Soil–air phase characteristics: Response to texture, density, and land use in Greenland and Denmark

AbstractSoil aeration is a key parameter for sustainable and productive agriculture. The intensification of agricultural activity in Greenland involves land use (LU) and LU change, affecting the soil–air phase. The combined effects of natural compaction (bulk density, ρb), texture (texture uniformity index; TUI), and LU on the soil–air phase of subarctic soils are not well known. This study aims to identify and compare the main drivers for air‐filled porosity (ε) and soil‐structure changes within and across sites in Greenland and Denmark. We analyzed comprehensive data sets of ε, relative gas diffusivity Dp/Do), and air‐permeability (ka) measured on intact soil samples from South Greenland (pasture) and Denmark (cultivated, urban, and forest). The mechanical robustness of the air phase was evaluated by linear models of ε as a function of ρb (H‐model). The ratio of ka to Dp/Do served as a soil‐structure index (Ω); the latter significantly correlated to TUI. The Greenlandic pasture soils did not show signs of well‐developed soil structure (low Ω‐values), whereas low H‐values suggested the soils were mechanically robust compared to similar‐textured cultivated soils. The soil–air characteristic curve (ε vs. pF) was parameterized, and the moisture control parameter was accurately predicted by TUI and LU (R2 = .95). Overall, the ρb was found to control the air‐phase functions within a field. However, considering changes in ε‐levels across different fields, texture, LU, and other environmental factors became statistically more relevant than ρb. A modeled response surface for changes in ε with soil conditions may, in perspective, be useful for better‐predicting gas transport in soil, both within and across fields.

No Tillage Improved Soil Pore Space Indices under Cover Crop and Crop Rotation

Assessment of the effects of crop management practices on soil physical properties is largely limited to soil moisture content, air content or bulk density, which can take considerable time to change. However, soil pore space indices evolve rapidly and could quickly detect changes in soil properties resulting from crop management practices, but they are not often measured. The objective of this study was to investigate how soil pore space indices—relative gas diffusion coefficient (Ds/Do) and pore tortuosity factor (τ)—are affected by tillage system (TL), cover crop (CC) and crop rotation (CR). A study was conducted on silt loam soil at Freeman farm, Lincoln University of Missouri during the 2011 to 2013 growing seasons. The experiment design was a randomized complete block with two tillage systems (no tillage or no-till vs conventional tillage), two cover crops (no rye vs cereal rye (Secale cereale L.)) and four crop rotations (continuous corn (Zea mays L.), continuous soybean (Glycine max L.), corn–soybean and soybean–corn successions). All the treatments were replicated three times for a total of 48 experimental units. Soils were collected from two sampling depths (SD), 0–10 and 10–20 cm, in each treatment and soil physical properties, including bulk density (BD), air-filled porosity (AFP, fa) and total pore space (TPS, Φ), were calculated. Gas diffusivity models following AFP and/or TPS were used to predict Ds/Do and τ values. Results showed that, overall, Ds/Do was significantly increased in no-tilled plots planted to cereal rye in 2012 (p = 0.001) and in 2013 (p = 0.05). No-tilled continuous corn, followed by continuous soybean and no-tilled soybean–corn rotations had the highest Ds/Do values, respectively. In magnitude, Ds/Do was also increased in no-till plots at the lower depth (10–20 cm). No-tilled plots planted with cereal rye significantly reduced τ in 2012 (p = 0.001) and in 2013 (p = 0.05). Finally, at the upper depth (0–10 cm), the no-tilled corn–soybean rotation and the tilled soybean–corn rotation had the lowest τ. However, at the lower depth (10–20 cm), the four crop rotations were not significantly different in their τ values. These results can be useful to quickly assess the changes in soil physical properties because of crop management practices and make necessary changes to enhance agricultural resilience.

Soil pore characteristics and gas transport properties of a no-tillage system in a subtropical climate

No-tillage system (NT) has been widely adopted around the world to restore the quality of soil structure in previously tilled soils, especially in tropical and subtropical climates. Our objective was to assess the legacy of chiseling in a NT in a subtropical climate by means of gas transport and pore morphology. The experiment was conducted in a split-plot design with two factors and four replications. The main plot was divided into a part under 18 years of no-tillage (18 NT) and a part with 5 years of no-tillage after receiving 13 years of chiseling tillage down to 25 cm depth (5 NT). The subplots were soybean and maize crops. Sixteen soil samples of 567 cm3 (large cores) were sampled from the 4–12 cm depth and scanned to detect pore diameter > 120 μm. Then, small subsamples of 100 cm3 (small cores) were extracted from the large ones to detect pore diameter > 74 μm using X-ray computed tomography (CT). A range of CT-derived soil pore characteristics were quantified for both sample sizes. In the small samples, air permeability (ka) and relative gas diffusivity (Ds/Do) were determined at −60, −100, and −300 hPa matric potentials. Classical laboratory results showed no significant treatment effects on bulk density (Bd), total porosity (TP), macroporosity (εa>30), ka and Ds/Do. There was no significant treatment effect on CT-macroporosity, although 5 NT tended to be higher than 18 NT. CT images showed a significant tillage effect on CT-maximum connectivity of macropores, where for 5 NT was higher (0.75) than in 18 NT (0.65) on the large samples. Therefore, a slight residual effect from chiseling was detected after 5 years of no-tillage. TP and εa>30 decreased and Bd increased significantly after subsampling. Ds/Do and ka were significantly correlated to CT-derived pore characteristics. This suggests that CT image analysis has high capacity for predicting gas transport.

Effects of long-term tillage systems on soil physical quality and crop yield in a Brazilian Ferralsol

Long-term soil tillage trials can provide important knowledge about sustainable changes in soil physical quality and crop yield. This study evaluated soil physical quality indicators under different long-term tillage systems and examined the relationships between quality indicators and crop yield. The study was carried out on a Rhodic Ferralsol with three tillage systems established in 1989: conventional tillage (CT), strategic tillage (ST), and no-tillage (NT). All treatments had long-term crop rotation. The soil parameters evaluated were total organic carbon (TOC), bulk density (ρs), macro and microporosity (Mac and Mic), relative gas diffusivity (D/D0), pore tortuosity (τ), relative field capacity (RFC), structural stability index (SSI), least-limiting water range (LLWR), and degree of compactness (DC, taking as reference the soil bulk density in which LLWR = 0). Soybean and maize yields in two consecutive summer seasons were measured. Conventional and strategic tillage provided higher ρs in the 0.15–0.30 m layer depth, leading to higher DC in CT. Using soil bulk density at LLWR = 0 as reference proved useful to assess soil DC and plant response. No-tillage provided lower DC in the 0–0.15 m (86 %) and 0.15–0.30 m (78 %) layers than CT (91 % and 94 %, respectively). The maize yield had a negative linear relationship to DC, with the lower values at DC > 87 %. All tillage systems affected D/D0, even at similar porosity values. The better soil physical quality under NT provided 1211 kg ha−1 higher maize yield compared with CT. The differences in soybean yield between treatments were not significant, but NT provided 381 kg ha−1 more than CT. These findings indicate that NT is the best system studied. Our results strongly suggest that ST does not improve physical properties of soils under NT with crop rotation, and that a diversified crop rotation in NT was efficient to avoid soil physical degradation.

Effects of bio-subsoilers on subsoil pore-system functionality: Case study with intact soil columns

Field traffic-induced subsoil compaction poses an escalating problem in relation to key soil functions because of the use of increasingly heavier machinery. Natural recovery from subsoil compaction is slow and therefore there is a need for measures such as bio-subsoilers to stimulate the recovery. This study thus investigated the influence of potential bio-subsoilers on pore functionality and morphology of a sandy loam soil, with an emphasis on the transition between the topsoil and compacted subsoil and on the compacted subsoil per se, in an intact soil column experiment. The control crop was spring barley, and potential bio-subsoilers included chicory, lucerne, fodder radish, tall fescue and kernza. The intact soil columns (Ø = 0.20 m, h = 0.50 m) were X-ray CT-scanned prior to and two years after crop establishment. Undisturbed subsamples (246 cm3) were collected one and two years after crop establishment in five replicates at 0.10, 0.25 and 0.35-m depth from the soil columns. The laboratory measurements and derived indices showed that after one-year non-restrictive air-filled porosity (εa = 0.13 m3 m−3) and relative gas diffusivity (Ds/Do = 0.0059) was reached by lucerne in the Transition layer (0.25 m). In the second year, lucerne also showed a significantly higher εa (0.09 m3 m−3), and a non-significantly higher Ds/Do (0.0029) compared to barley in the Compacted layer (0.30 m). Pore geometry indices derived from the relationship between εa, Ds/Do and air permeability indicated the formation of less tortuous and more connected small pores (at −100 hPa matric potential) throughout the Transition and Compacted layers by chicory, lucerne and tall fescue. A significantly higher CT-number of pores < 1 mm3 was found for lucerne and a higher CT-volume of pores < 1.5 mm in diameter for lucerne and chicory in the Compacted layer. Root number density was significantly higher for chicory, lucerne and tall fescue compared to barley from the first year, and also for kernza from the second year. Lucerne had the greatest subsoil compaction mitigation potential in our experiment in intact columns. Further studies in real field situations are needed.

Gas diffusion characteristics of agricultural soils from South Greenland

AbstractThe Arctic is warming at twice the global average, which may impact agricultural production in Greenland. Therefore knowledge of the functional properties of Greenlandic soil resources is necessary. The relative soil gas diffusivity [the soil gas diffusion coefficient (Dp)–free‐air diffusion coefficient (Do) ratio] is the chief parameter controlling gas transport in soils. Predictions of Dp/Do are needed to estimate root zone aeration and terrestrial greenhouse gas fluxes. We used existing models to analyze the Dp/Do of soils from Greenlandic fields and a pore connectivity index (Cip) to infer their degree of structural development and identify the main parameters controlling Dp/Do. In total, 201 × 3 intact 100‐cm3 soil samples were sampled across six fields with clay and organic C contents of 0.016 to 0.089 and 0.016 to 0.105 kg kg−1, respectively. The Dp/Do was measured with the one‐chamber nonsteady‐state method at soil water potentials between –10 and –1,000 cm H2O. Accurate determination of total porosity (Φ) was ensured by calibrating a particle density model on 129 samples. The soils exhibited a less developed structure and highly tortuous pore networks, resulting in low Dp/Do as a function of air‐filled porosity (ε). Density‐corrected models with air saturation (ε/Φ) reduced the RMSE. Furthermore, Cip at –1,000 cm H2O soil water potential increased linearly with dry bulk density (ρb), suggesting that ρb is a key controller of Dp/Do, which is important for planning cultivation practices for Southern Greenlandic soils. Lastly, we found that an air saturation &gt;35% is required for adequate soil aeration.

Effects of soil erosion and land use patterns on the characteristics of soil water and gas transport in the black soil region

Soil erosion is one of the main causes for the degradation of black soil in Northeast China. Understanding the differences of soil water and air transport under different land use patterns can provide scientific basis for efficient utilization and protection of soil and water resources in the black soil area. In this study, we examined the in-situ soil saturated hydraulic conductivity, air permeability and relative gas diffusion of 0-5 cm soil layer in three typical land use patterns (cropland, woodland, abandoned land) in the Northeast black soil area and explored the impacts of soil erosion and land use pattern on water and gas transport. Results showed that there were significant differences in soil water and air transport between different eroded croplands and between different land use patterns. Soil bulk density of serious erosion cropland was higher than that of other lands, and that of ungraded cropland was significantly lower than other lands. Compared with the ungraded cropland, soil bulk density in the light erosion cropland, the moderate erosion cropland and the serious erosion cropland increased by 12.7%, 17.6% and 39.2%, saturated hydraulic conductivity decreased by 84.4%, 53.7% and 12.7%, air permeability decreased by 94.6%, 64.4% and 14.0%, and relative gas diffusion decreased by 91.3%, 82.6% and 4.3%, respectively. The saturated hydraulic conductivity, air permeability and relative gas diffusion of pine forest decreased by 86.5%, 83.0% and 91.3% respectively compared with that of ungraded cropland, saturated hydraulic conductivity, air permeability and relative gas diffusion of sea-buckthorn forest decreased by 51.7%, 45.6% and 82.6%, and saturated hydraulic conductivity, air permeability and relative gas diffusion of abandoned land decreased by 16.2%, 1.4% and 73.9% respectively compared with that of ungraded cropland. In addition, the measured result of soil air permeability and relative gas diffusion could be used to estimate saturated soil hydraulic conductivity. Soil water and gas transport characteristics were significantly affected by soil erosion and land use pattern.

Soil type, bulk density and drainage effects on relative gas diffusivity and N2O emissions

Nitrous oxide (N2O), a greenhouse gas, contributes to stratospheric ozone depletion. Agricultural fertiliser use and animal excreta dominate anthropogenic N2O emissions. Soil relative gas diffusivity (Dp/Do) has been used to predict the likelihood of soil N2O emissions, but limited information exists about how soil N2O emissions vary with soil type in relation to Dp/Do. It was hypothesised that, regardless of soil type, the N2O emissions would peak at the previously reported Dp/Do value of 0.006. Four pasture soils, sieved and repacked to three different bulk densities, were held at nine different soil matric potentials between near saturation and field capacity. Soil nitrate and dissolved organic matter concentrations were adequate for denitrification at all soil matric potentials. Increasing soil bulk density and soil matric potential caused Dp/Do to decline. As Dp/Do declined to a value of 0.006, the N2O fluxes increased, peaking at Dp/Do ≤ 0.006. This study shows that the elevation of N2O fluxes as a Dp/Do threshold of 0.006 is approached, holds across soil types. However, the variability in the magnitude of the N2O flux as Dp/Do declines is not explained by Dp/Do and is likely to be dependent on factors affecting the N2O:(N2O + N2) ratio.

Soil structure characteristics, functional properties and consistency limits response to corn cob biochar particle size and application rates in a 36-month pot experiment

The particle size (PS) and application rate (RA) of biochar influence soil–biochar interaction with concomitant effects on soil chemical and physical properties. However, limited studies exist that quantify how biochar PS and RA, acting solely or together affect soil structure characteristics and functional properties. This study investigated the effect of PS and RA of corn cob biochar on soil water retention (SWR) and aeration indicators such as relative gas diffusivity, structural characteristics such as pore size distribution and pore organisation, and consistency limits of a tropical sandy clay loam soil. A pot experiment was conducted using two biochar PSs (&amp;lt;2 and 2–4 mm) and four biochar RAs of 0, 20, 40 and 80 Mg ha−1. Both intact soil cores (100 cm3) and disturbed bulk soil samples were collected after 36 months of amendment for measurements of soil chemical and hydraulic properties, and consistency limits. Biochar application of 80 Mg ha−1 significantly (P &amp;lt; 0.05) increased soil organic carbon and SWR. However, plant available water was neither affected by biochar PS nor RA. Further, biochar PS and RA acting solely or together did not significantly modify air permeability, consistency limits and estimated soil pore continuity, organisation and tortuosity. Further studies are needed to confirm whether the overall lack of significant changes in these soil properties after 36 months of amending with corn cob biochar also pertains to other biochar feedstock and soil types. Such information is useful in selecting suitable biochar amendments that improve soil conditions for crop growth.

A Note on the Forchheimer Approach for Estimating Soil Air Permeability

Important soil pore characteristics may be revealed from air permeability data. Recent research has quantified significant bias in estimates of the true Darcian permeability when frequently reported pneumatic pressure differences to drive the convective flow are used. An alternative to measurement at infinitesimal pressure differences is the Forchheimer approach, including a polynomial regression of corresponding values of the superficial air velocity (v) and the pressure gradient (G) applied. However, in situations with Darcian flow at low pressure gradients, this procedure may theoretically give an overestimation of the Darcian permeability. We constructed sample plastic cores (∼3.5 cm high) with different numbers (n = 1–19) of drilled tubes (holes of 1, 2, 4.5, and 5.8 mm diameter). Gas diffusivity was measured with a transient‐state method. Air permeability was measured at four pneumatic pressure differences (0.5, 1, 2, and 5 hPa), and the Darcian permeability was estimated with the Forchheimer approach. The ratio of apparent permeability at 5 hPa pneumatic pressure difference to the Forchheimer‐estimated Darcian permeability was significantly lower than unity (0.06–0.52) for all test samples. For all 1‐ and 2‐mm hole samples and for all four levels of G, the Reynolds number indicated nonturbulent flow, whereas turbulence was predicted for samples with 4.5‐ and 5.8‐mm diameters. A model combining relative gas diffusivity, air permeability, and the space available for gas transport indicated that the Forchheimer estimates of Darcian permeability were correct in situations with nonturbulent flow, whereas erroneous estimates were generated for the larger tubes with turbulence.Core Ideas We tested the Forchheimer approach for simple samples with straight tube pores. For nonturbulent flow conditions, the Forchheimer approach provides accurate results. Air flow at 5 hPa pressure difference implies significant underestimation of permeability.