Per Schjønning

Turnover of organic matter in differently textured soils: I. Physical characteristics of structurally disturbed and intact soils

Abstract Soil type effects on organic matter turnover are most often ascribed directly to differences in soil clay content. Since soil texture determines the physical characteristics of soil, aggregation and water holding capacity may be more relevant to address in the search for controls of organic matter turnover. Most studies of microbial processes in soils are based on structurally disturbed soil, where the abiotic conditions for the microbial activity may be quite different from those in intact soils. In this study, basic physical characteristics were determined for structurally disturbed and intact soil samples from differently textured soils. Bulk soil was retrieved from 0–20 cm depth at six locations along a textural gradient in an arable field on Weichselian morainic deposits in Denmark. The samples (NA1 to NA6) ranged in clay from 11 to 45% and in silt from 7 to 15%. Clay and silt-sized organomineral complexes were isolated from NA2 soil by ultrasonic dispersion and sedimentation in water. The clay and silt fractions were added individually and in varying proportions to NA1 soil, providing three clay-amended (CL2, CL4 and CL6) and three silt-amended (SI2, SI4 and SI6) soils. All 12 soils were crushed in air dry state to 100 μm). Air diffusivity and permeability measurements showed disturbed soils to have a less continuous and more tortuous pore system than undisturbed reference samples. Water-filled pore space at a critical level of air diffusion potential was significantly higher for undisturbed than for disturbed samples, especially in soils high in clay. Drop cone measurements showed disturbed soils to be structurally weaker than undisturbed ones. Intact and structurally disturbed soils were found to differ significantly in physical properties even after 17 months of soil structure regeneration. Water-filled pore space seems to reflect the potential of available water and aeration status to regulate aerobic microbial activity of structurally disturbed soil, but not of intact field soil.

Turnover of organic matter in differently textured soils. II. Microbial activity as influenced by soil water regimes

To evaluate the effect of soil texture and soil water content on decomposition of organic carbon (OC), turnover of partially stabilized 14C-labelled ryegrass residues was studied at four matric potentials in twelve differently textured soils of similar origin and cropping history. Six soils were from a naturally occurring clay gradient and had 11, 16, 21, 31, 37 and 45% clay (termed NA1 to NA6). Three clay-amended soils (CL2, CL4, CL6) and three silt-amended soils (SI2, SI4, SI6) were prepared by adding clay or silt sized organomineral complexes extracted from the NA2 soil to a portion of the NA1 soil. After 14C-labelled ryegrass had decomposed for eight months under field-like conditions, soil cores were sampled, adjusted to four matric potentials (-30, -100, -500 and -1500 hPa) and incubated at 20°C for 15 weeks. The content of native soil organic carbon (SOC) in the NA soils was not related to texture. The SOC content increased with clay and silt in the CL and SI soils because of OC contained in the applied size separates. The relative CO2-evolution from CL and SI soils was lower than from the texturally corresponding NA soils, indicating a slower turnover of C supplied with the clay separate than of bulk OC. Differences in the decomposability of native SOC and residues of 14C-ryegrass were better explained by soil moisture parameters than by soil textural composition. Within each set of soils, evolution of CO2 from native SOC was highly correlated with the volumetric water content. The same was true for 14CO2-evolution, but correlations were significantly improved when 14CO2 was related to water retained in soil pores with diameters >0.2 m. This indicated that the water available for the turnover of residues from ryegrass and of native SOC was retained in different fractions of the pore volume. Our study suggested that water was the main factor in controlling turnover of SOC. Texture effects were indirect and expressed through soil structure which in turn defined the soil pore system and thus the ability of the soils to retain water of different availability to the decomposer organisms.

Modeling diffusion and reaction in soils : IX. The Buckingham-Burdine-Campbell equation for gas diffusivity in undisturbed soil

Accurate description of gas diffusivity (ratio of gas diffusion coefficients in soil and free air, D s /D 0 ) in undisturbed soils is a prerequisite for predicting in situ transport and fate of volatile organic chemicals and greenhouse gases. Reference point gas diffusivities (R p ) in completely dry soil were estimated for 20 undisturbed soils by assuming a power function relation between gas diffusivity and air-filled porosity (e). Among the classical gas diffusivity models, the Buckingham (1904) expression, equal to the soil total porosity squared, best described R p . Inasmuch as our previous works (Parts III, VII, VIII) implied a soil-type dependency of D s /D 0 (e) in undisturbed soils, the Buckingham R p expression was inserted in two soil- type-dependent D s /D 0 (e) models. One D s /D 0 (e) model is a function of pore-size distribution (the Campbell water retention parameter used in a modified Burdine capillary tube model), and the other is a calibrated, empirical function of soil texture (silt + sand fraction). Both the Buckingham-Burdine-Campbell (BBC) and the Buckingham/soil texture-based D s /D 0 (e) models described well the observed soil type effects on gas diffusivity and gave improved predictions compared with soil type independent models when tested against an independent data set for six undisturbed surface soils (11-46% clay). This study emphasizes that simple but soil-type-dependent power function D s /D 0 (e) models can adequately describe and predict gas diffusivity in undisturbed soil. We recommend the new BBC model as basis for modeling gas transport and reactions in undisturbed soil systems.

Gas Permeability in Undisturbed Soils: Measurements and Predictive Models

Accurate prediction of changes in the gas permeability during variable soil-moisture conditions is a prerequisite for improved simulation and design of soil-venting systems for removal of volatile organic chemicals in polluted soils. Air permeability, k, as a function of soil air-filled porosity,

Water-Dispersible Colloids: Effects of Measurement Method, Clay Content, Initial Soil Matric Potential, and Wetting Rate

The fraction of clay that disperses in water, water-dispersible clay (WDC), is recognized as an important property with respect to predicting soil erosion and colloid leaching. Using six mineralogically similar soils with 12, 18, 24, 28, 37, and 43% clay, we studied the influence of soil clay content, initial matric potential (IMP; ψ = −2.5, −100, and −15500 hPa), and wetting rate on WDC released in response to infiltration of low–ionic strength rainwater, using a low-energy input measurement of WDC (LE-WDC). These results were referenced by WDC obtained by a conventional, high-energy input measurement based on air-dried soil (HE-WDC). The energy input in the dispersion procedure significantly affected the release of WDC. The amount of HE-WDC increased with clay content, while the amount of LE-WDC decreased with increasing clay content. The decrease in LE-WDC was explained by an increase in cohesive strength, reflected by the increase in water-stable aggregates (≥4 mm). A strong dependency of IMP on LE-WDC was observed, with maximum release of LE-WDC from soils that were at −2.5 hPa before measurement. Decreasing soil matric potential in the period before measurement reduced LE-WDC and also reduced the dependency of soil clay content, with soils incubated at −15500 hPa releasing a low amount of LE-WDC independent of clay content. The content of particulate organic C (POC) in the LE-WDC decreased with increasing clay content, and increased after drying to −15500 hPa. Colloid dispersibility changed as a function of time and moisture status, with the main changes occurring during or immediately after adjustment of the moisture content. Increasing the wetting rate resulted in a doubling of the amount of LE-WDC released from the initially dry soil (−15500 hPa), while no effect of wetting rate was observed at higher initial matric potentials.

Predicting saturated hydraulic conductivity from air permeability: Application in stochastic water infiltration modeling

Several relationships exist for predicting unsaturated hydraulic conductivity K(ψ) from saturated hydraulic conductivity Ks and the soil-water retention curve. These relationships are convenient for modeling of field scale system sensitivity to spatial variability in K(ψ) . It is, however, faster and simpler to measure air permeability ka at ψ = −100 cm H2O, than Ks. This study explores the existence of a general prediction relationship between ka, measured at −100 cm H2O, and Ks. Comparative analyses between ka-Ks relationships for nine Danish and Norwegian soils, six different soil treatments, and three horizons validated the establishment of a soil type, soil treatment, and depth/horizon independent log-log linear ka-Ks relationship. The general ka-Ks relationship is based on data from a total of 1614 undisturbed, 100-cm3 core samples and displays general prediction accuracy better than ±0.7 orders of magnitude. The accuracy and usefulness of the general relationship was evaluated through stochastic analyses of field scale infiltration and ponding during a rainstorm event. These analyses showed possible prediction bias associated with the general ka-Ks relationship, but also revealed that sampling uncertainty associated with estimation of field scale variability in Ks from a limited number of samples could easily be larger than the possible prediction bias.

Spatial and temporal effects of direct drilling on soil structure in the seedling environment

Despite more than 30 years of research and practical experience the interest in shallow tillage and especially direct drilling has remained low in Scandinavia. Excessive compaction of the topsoil layer is one of the major problems encountered when adapting shallow tillage and direct drilling in particular. The purpose of this study was to evaluate temporal and spatial effects of two different direct drilling techniques on bulk density and penetration resistance in the near seed environment. A sandy loam growing small grain cereals was followed during the first 3 years after conversion from conventional tillage to direct drilling to reveal short-term changes in soil structure. A field experiment with four blocks was conducted in 1999–2001 where a conventional mouldboard ploughing–harrowing system (PL) was compared with direct drilling performed by either a chisel coulter drill (DD-C) or a single disc drill (DD-D). Effects on density and penetration resistance were measured in the field after first, second and third year of crop establishment (T1, T2 and T3). Bulk density was determined at 0–100 mm depth using a dual probe gamma-ray transmission system. Penetration resistance was recorded in the field at 0–150 mm depth. At T2 column samples (diameter: 180 mm, height: 200 mm) were taken with the seed row through the centre. Penetration resistance was determined in these samples in a 10 mm×10 mm grid using a micropenetrometer (3 mm cone base diameter) at 0 to approximately 150 mm depth. Two samples from each treatment were analysed by a medical CT-scanner to determine spatial differences in bulk density. Irrespective of coulter type direct drilling gave a fast compaction of the arable layer below seeding depth when shifting from mouldboard ploughing to direct drilling. Soil strength was substantially higher already in the first year of direct drilling (i.e., maximum 0.4 and 1.2 MPa, for PL and DD-D/DD-C, respectively). Critical high penetration resistance (>2.0 MPa) and bulk density levels (>1.5 g cm-3) were reached at T2 and remained at the same level at T3. The DD-C direct drill produced a more favourable soil environment for crop establishment than the DD-D drill. A layer of approximately 40 mm loose granular soil above seeding depth and no indication of a direct compaction effect was found for the DD-C treatment. In contrast, the field as well as the laboratory results indicated a direct compacting effect for the DD-D drill. Despite the lack of direct compaction effect from the DD-C drill itself, evidence suggest that periodic non-inversion soil loosening of the lower part of the arable layer is needed on direct drilled sandy loam soil in a moist and cool climate.

MODELING DIFFUSION AND REACTION IN SOILS: X. A UNIFYING MODEL FOR SOLUTE AND GAS DIFFUSIVITY IN UNSATURATED SOIL

Diffusion processes in the soil water and air phases often govern transport and fate of nutrients, pesticides, and toxic chemicals in the vadose zone. This final paper in a 10-part series on diffusion-reaction processes in soils concerns the development of a unifying model platform for predicting solute and gas diffusion coefficients as functions of fluid-phase (water or air) content and pore-size distribution in unsaturated soils. We find that the Buckingham (1904) expression predicts solute diffusivities in water-saturated porous media more accurately than other classical expressions and, extended with a pore-size distribution-based term, yields a new and accurate model for solute diffusivity in unsaturated soil. The same was shown for gas diffusivity in undisturbed soil in Part IX of this series. Thus, the predictive diffusivity models can be rewritten in a common form with two model parameters that vary between solute and gas diffusivity and, in the case of gas diffusivity, also between undisturbed and repacked soil. It is suggested that the two parameters in this unified diffusivity model (UDM) represent porous media (solids-induced) tortuosity (T) and water-induced fluid phase disconnectivity (W), respectively, with W increasing with clay content for solute diffusion but being constant (repacked soil) or decreasing (undisturbed soil) for gas diffusion. Tested against data for 77 soils, the UDM model was markedly more accurate than commonly used soil-type independent models, with 35–50% (gas diffusivity) and 75% (solute diffusivity) reduction in root mean square error of prediction. The use of the new UDM to predict effective diffusion of sorbing chemicals in the soil water and air phases is illustrated. The UDM concept enables a new definition of the relative diffusion coefficient in soil, i.e. relative to the diffusion coefficient in a fluid-saturated porous media instead of in free water or air. This provides new possibilities for analyzing tortuosity phenomena in the soil water and air phases and their effects on diffusive and convective transport parameters in unsaturated soil.

Subsoil compaction caused by heavy sugarbeet harvesters in southern Sweden; II. soil displacement during wheeling and model computations of compaction

Traffic with high wheel loads in combination with high inflation pressure implies a risk for subsoil compaction, but effects will depend on the soil strength. Soil displacement during traffic with a heavy sugarbeet harvester (total load approximately 35 Mg on two axles) was determined at 0.3, 0.5 and 0.7 m depths during harvesting in the autumn. Measurements were made on one occasion on a clay loam (Eutric Cambisol) and a sand (Haplic Arenosol), and at different water contents on a sandy clay loam (Eutric Cambisol). Soil mechanical properties (precompression stress and shear strength) were determined for each traffic occasion. Field measurements were also compared with model computations of soil compaction, based on calculation of soil stresses and on the mechanical properties measured. On the sandy clay loam in the driest condition, displacement occurred only at 0.3 m depth, while it was registered down to 0.7 m depth in the wettest condition, when soil moisture was around field capacity. On the clay loam and the sand there was displacement down to 0.5 and 0.7 m depth, respectively. Model predictions of compaction correlated well with the depth to which displacement was measured in the field. One important task in subsoil compaction research is to define methods to determine soil mechanical properties that are easy to use and that still make it possible to predict compaction. The results clearly demonstrate that heavy sugarbeet harvesters may cause compaction to more than 0.5 m depth during normal field conditions in the autumn, with soil water content as the most decisive factor.

Air and water permeability in differently textured soils at two measurement scales

Air permeability can be used to describe the structure of the soil but may also be used to predict saturated hydraulic conductivity. This raises the question of whether the two parameters exhibit the same degree of scale dependency. In this study the scale dependency of water permeability (saturated hydraulic conductivity, Kw) and air permeability (ka, at a matric water potential of −50 cm H2O) was tested at four different sites (three horizons at each site), by using two measurement scales (100 cm3 and 6280 cm3). No clear effect of scale on variability was observed. Air and water permeability displayed higher variabilities for two structured loamy soils compared with two sandy soils. For the more structured soils, the variability between measurements was lower for air compared with water permeability. Both air and water permeabilities were higher at the large scale compared with the small scale, but this scale-dependent difference was less pronounced in sandy soils, suggesting a smaller representative elementary volume. For three of the four soils, a highly correlated relationship between Kw and ka on both small and large soil samples was observed. For the fourth soil, water retention data revealed that the samples were not sufficiently drained at −50 cm H2O to validate a comparison between the two parameters. Predictive Kw (ka) relations for the remaining three soils at the two scales compared favorably with a general Kw (ka) relation proposed by Loll et al. (1999). This study supports the use of a general predictive relation between ka near field capacity (at around −50 to −100 cm H2O) and Kw, but caution should be taken if the soil has a large content of pores that will drain at or close to a matric water potential of −50 cm H2O.