Disturbance Of Soil Structure Research Articles

Soil disturbance evaluation of soft clay based on stress-normalized small-strain stiffness

Soil disturbance includes the change of stress state and the damage of soil structure. The field testing indices reflect the combined effect of both changes and it is difficult to identify the soil structure disturbance directly from these indices. In the present study, the small-strain shear modulus is used to characterize soil structure disturbance by normalizing the effective stress and void ratio based on Hardin equation. The procedure for evaluating soil sampling disturbance in the field and the further disturbance during the subsequent consolidation process in laboratory test is proposed, and then validated by a case study of soft clay ground. Downhole seismic testing in the field, portable piezoelectric bender elements for the drilled sample and bender elements in triaxial apparatus for the consolidated sample were used to monitor the shear wave velocity of the soil from intact to disturbed and even remolded states. It is found that soil sampling disturbance degree by conventional thin-wall sampler is about 30% according to the proposed procedure, which is slightly higher than that from the modified volume compression method proposed by Hong and Onitsuka (1998). And the additional soil disturbance induced by consolidation in laboratory could reach about 50% when the consolidation pressure is far beyond the structural yield stress, and it follows the plastic volumetric strain quite well.

Using microdialysis with a deuterium oxide tracer to estimate water exchange, water content and active surface area of the probe

Microdialysis is a useful tool for measuring in situ fluxes of soil compounds with minimal disturbance of soil structure and function. Fluxes of sampled compounds are commonly calculated per unit of membrane surface area, assuming that the entire membrane surface is capable of exchange – which is unlikely given varying soil moisture and the occlusion of membrane pores by the soil solid phase. We present a method to quantify the degree of connectivity of the microdialysis probe membrane to the surrounding soil by means of water exchange between a microdialysis perfusate and soil solution using deuterium (2H2O; equilibrated to DHO) as an internal standard. We applied the method to a range of probe membrane surface areas and soil moisture conditions to generate empirical models that estimate membrane surface area active in exchange. Our results suggest that even in a saturated sandy soil, active membrane surface areas reach only 40.3% of the probe surface area, perhaps due to occlusion by soil particles. However, when accounting for volumetric water content of the soil, active surface areas approached 80–90% of the area likely in contact with water, indicating that sampling efficiency of water-filled pores may still be high, particularly at slow flow rates. Furthermore, our method enables assessment of local soil water content around the probe. Models estimating soil water content were applied to field measurements of DHO exchange in three soil horizons (Organic, B1, B2) at two boreal sites, and in situ estimates were similar to those from conventional soil moisture methods when models were calibrated with the same soil type. We present DHO exchange as a powerful method for improving microdialysis flux interpretations in future studies, and for exploring small-scale water variability in relatively undisturbed soils.

Soil Burn Severities Evaluation Using Micromorphology and Morphometry Traits After a Prescribed Burn in a Managed Forest

Prescribed burn is a tool that must imply low soil burn severity (SBS) levels; however, a wide range of soil impacts have been demonstrated because of the influence of very variable factors. The effects on biological, physical, and chemical soil properties are well reported in numerous studies; nonetheless, there are still questions about the effect of prescribed burns on soils at the micro-scale. As a result, an analysis of the link between micromorphological features and SBS does not currently exist. Thus, the main aim of the present study is to perform a micro-scale evaluation for complementing the SBS visual examination after prescribed burning in a managed pine forest in western Mexico. Morphometry and micromorphology analyses of mineral soil revealed that at low SBS levels, only the soil structure in the first centimeter is affected by prescribed burns. While at high SBS, the prescribed burn affected the first 2 cm, showing soil structure disturbance, ash filling porous, and soil aggregates getting reddish. Therefore, immediate actions have to be made by land managers after applying prescribed burns before the first rain to prevent post-fire surface soil erosion, particularly in bare soil patches where the burned aggregates are more susceptible to rain splash and runoff.

Geophysical and numerical stability analysis of landslide incident

Landslide is known for its precarious impact on environment, resources and human life. Recently, landslide has occurred in Lalisa village, Jimma Zone, Ethiopia which harshly caused damage to life and property. The incident resulted in perilous damage of about 27 ha of accessible land. This study hence particularly aimed at investigating the root cause of the incident and analyzing safety of the sliding slope so that the applicable remedial actions can be proposed. Geophysical analysis without soil structure disturbance was adopted to investigate the vertical soil profile, morphological stratification, location and alignment of discontinuity planes. Stability analysis by using Limit Equilibrium method was carried out for both normal and worst conditions to rate safety of the failing slope. Lithology of the site is characterized by highly weathered and fractured rock units exhibiting a significant variability over a little horizontal distance and depth. The stratigraphy also constitutes loose soil near the surface and saturated layer ranging from depth of 10 m to 25 m. The slope failure occurred at the site is of deep by its type that origin of its slip plane extends up to a depth of 12 m from the surface. Furthermore, factor of safety of the slope along the failed zone fell below 1.5 with the maximum value of 1.303 for the normal condition. The conducted investigation also indicated that the detachment and propagation of the sliding mass develops much faster with rise in soil moisture content whereas it categorically remains mild during dry seasons. Hence, the driving agent for the occurrence and propagation of the landslide incident was rainfall infiltration and the existence of weak saturated zone at the stated depth.

Experimental Study on Dynamic Shear Modulus and Damping Ratio of Loess Considering Disturbance

Loess is a special kind of Quaternary sediment. The sub-stable skeleton formed by fine grains and salt crystals makes the loess with great risk of catastrophic changes under dynamic action. In order to study the influence of the natural structure of loess on its dynamic properties, the test was conducted on the loess at the near-fault site of the Lishan piedmont fault to analyze the influence of soil structure disturbance on the dynamic properties of loess. The test adopts dynamic triaxial test system. The results show that the soil structure disturbance has a large effect on the dynamic shear modulus of the loess, while it has little effect on the damping ratio. Under low confining pressure, the maximum dynamic shear modulus of original loess is smaller than that of remodeled loess. Under the high confining pressure, the original loess is larger than the remodeled loess. When the confining pressure is 400 kPa, the maximum dynamic shear modulus of the original loess is 93.28 MPa, which is 53.8% larger than that of the remodeled loess. When the dynamic strain reaches 0.15%, the difference between the damping ratio of the remodeled loess and the original loess is less than 0.04, and it can concluded that the soil structure disturbance has little effect on the loess in the Lishan piedmont fault.

Soil Column Experimental Study on the Effect of Soil Structure Disturbance on Water Chemistry.

The changes in soil/rock structure caused by engineering disturbance or earthquakes could affect water chemistry by increasing the reaction surface, enhancing the oxidation condition, or exposing soluble rocks. However, the details of the mechanisms of the disturbance of soil/rock are little known. Based on the soil column experiment, this study analyzed the concentrations of sulfate (SO4), sulfur, and oxygen isotopic composition of SO4 (δ34S-SO4 and δ18O-SO4) in effluent water. The water-rock interaction mechanisms in the disturbed soil and the contribution of this interaction to the SO4 in groundwater were studied. The results suggest that the concentration of SO4 in the first effluent water sample can reach up to 97 mg/L, much higher than that in natural groundwater (6.8 mg/L). The isotopic composition of SO4 further suggested that SO4 in the first effluent water sample was mainly derived from the dissolution of SO4-containing evaporites. The proportion was estimated to be 93%. SO4-containing evaporites accounted for 23% of the SO4 content in all effluent water samples during the experiment. The disturbance of soil structure led to the exposure and dissolution of SO4-containing evaporites, which were initially insoluble under natural conditions. This study is essential to the clarification of the water-rock interaction mechanisms following the changes in soil/rock structures.

Influence of land use on changes of sediment budget components: western Iran case study

Understanding the processes of displacement and redistribution of soil particles caused by different erosion types is considered an efficient sediment management tool in various scales from the field plot to the watershed. The rainfed agriculture practices on high gradient slopes are one of the main sources of accelerated soil erosion due to the disturbance of soil structure by tillage processes. Therefore, the present study aimed to investigate the synergistic effect of rainfed agriculture practices and slope gradient on soil erosion using the activity concentrations of 137Cs fallout in Khamsan experimental watershed, western Iran. The diffusion and migration model was used for undisturbed soils and mass balance equation type II was used for agricultural lands. The distribution map of soil erosion and deposition of the watershed was prepared using 137Cs method and the interactions between rainfed agriculture and slope gradient (as the most effective geomorphological factor) on erosion rate and sediment redistribution were evaluated. The results indicated that the highest rates of soil erosion in rainfed agriculture (11.46 t ha-1 year-1) and rangeland (1.27 t ha-1 year-1) were recorded in the slope gradient classes of 20%-30% and >60%, respectively. In addition, the maximum sedimentation in rainfed agriculture (5.78 t ha-1 year-1) and rangeland (0.89 t ha-1 year-1) occurred in the slope classes of 30%-60% and in >60%, respectively.

Undrained Shear Behavior of Samples Disturbance of Unsaturated Expansive Clay Soils

Most Expansive Clay soils are sensitive to the disturbance in their internal structure as a result of the extraction of samples, because of the destruction in the natural soil structure. so a change in the properties of the expansive soil happens. This matter must be more precisely defined in the design of the foundations of engineering facilities. This paper presents a laboratory investigation of the influence of simulated sample disturbance on Shear Strength Parameters i.e. (internal friction angle and soil cohesion) by varying their plasticity and method of remolding samples using the direct shear test. Comparisons between simulations and experimental results showed that the behavior of the partially saturated expansive clay soil influenced by the method of remolding samples and is achieved through the relationship between stress and deformation. The disturbance of soil structure due to low shear strength parameters for soil, as the amount of change of the disturbance samples due to the decrease in values of both soil cohesion and the internal friction angle. In addition to that, the decrease in low plasticity soil PI (6%), is higher than the high plasticity soil (PI = 32%) due to the existence of Montmorillonite and kaolinite minerals in high plasticity soil which have high water absorption and cohesion, that preserving the shape of the soil sample.

ҒЎЗА ЭКИНИНИ ЕТИШТИРИШНИНГ ЯНГИ ТЕХНОЛОГИЯСИ

This article provides information on the number of tillages, the passage of aggregates, the disturbance of soil structure and soil layer,the maintenance of soil fertility, the prevention of water and wind erosion, and the reduction of the cost of cotton production

Effect of soil structure disturbance on the shear strength of black volcanic ash soil

The effect of soil disturbance on the shear strength of black volcanic ash soil was investigated using a constant volume direct shear apparatus. Disturbance of soil structure was considered as the pore size distribution which obtained from the soil-water characteristic curve (SWCC). The disturbed sample was used as a representation of soil structure disturbance due to earthquake shakes. A series of cyclic tests were conducted under unsaturated and saturated samples. It was found that the undisturbed samples exhibit a unimodal pore structure, and the disturbed samples indicate to a bimodal pore structure. Since the pore structure of the disturbed sample is unstable, the degradation index value is higher than that of the undisturbed sample and increases with the increasing number of cycles. In other words, the cyclic normalized vertical stress of disturbed samples degrades faster. Furthermore, the degradation index value in the normallyconsolidated samples was found to be larger than the overconsolidated. It might be attributed to increasing of the pore water pressure during shearing. Where in the over-consolidated samples is lower than normally-consolidated. On the other hand, the normalized shear stress of unsaturated samples, it is slightly larger due to the suction forces in the total strength of soils.

Effect of pasture improvement managements on physical properties and water content dynamics of a volcanic ash soil in southern Chile

The pastures in Chile are sustained on volcanic ash soils covering an area of 1,340,000 ha. Since 44% of these pastures are degraded, different strategies to improve these prairies have been implemented. This study examined the impact of the different pasture improvement managements (PIMs) on soil physical properties, water content dynamics and pasture productivity of a volcanic ash soil under sheep grazing. The experiment was established on a Duric Hapludand and considered four types of PIMs (fertilised-naturalised pasture, cultivated pasture, direct-drilled pasture, diverse-direct drilled pasture) including the initial situation (non-fertilised naturalised pasture). The effect of PIMs and grazing events on bulk density (BD), plant available water (PAW), air permeability (ka), saturated hydraulic conductivity (Ks) and pre-compression stress (Pc) in the topsoil (0–10 cm) was determined taking undisturbed soil samples, whereas the penetration resistance (PR), field air permeability (kl field) and herbage mass production were measured in the field. The volumetric water content, matrix potential and soil temperature were continuously registered at different depths. The fertilisation of degraded naturalised pastures, without soil structure disturbance, improved the pasture yield (140%), reaching values comparable to those improved with conventional systems. In the short term, the volume of macropores does not change significantly as a function PIMs. However, tilled soils presented less connected pores compared to the non-cultivated PIMs. The conservation of soil structure plays an important role in water accessibility by plants, so that fertilised-naturalised pastures were able to absorb water to depths of up to 60 cm. Compared to the improved pastures, the degraded non-fertilised pasture presented the lowest above ground herbage biomass as well as negative effects on soil physical properties (e.g. Pc increased by 57% and lower physical resilience) after grazing events.

Extraction mechanism of lead from shooting range soil by ferric salts

The conventional soil washing process uses inorganic acids to extract metal from contaminated soils; however, soil structure disturbance is a negative by-product of the process using the acids. To minimize the adverse effects of soil washing, the use of alternative extractants is needed. Ferric chloride has a low pKa value, and anionic chloride ligand can enhance the extraction of toxic cationic metals from soils. In this study, a soil washing process using ferric chloride and ferric nitrate was applied to a Pb-contaminated model and field shooting range soils, and the outcome was compared to the use of conventional inorganic acids. The extraction of Pb from both soils was mostly dependent on the equilibrium solution pH regardless of the extractants used, and chloride could not significantly enhance that. Ferric iron enhanced the extraction of Pb compared to the inorganic acid treatment because ferric could generate continuously hydrogen ions by the equilibrium reaction between ferric ions and ferric hydroxide. Ferric chloride may be an attractive alternative extractant for the soil washing of Pb-contaminated soils. However, although Pb concentration in soil was significantly decreased by washing process, soil enzyme activity and plant growth was inhibited by severe soil acidification. Additionally, ferric salt-based washing showed similar seed germination test using tomato (Solanum lycopersicum) with HCl-based washing. Therefore, ferric salt based soil washing could remove more Pb from the shooting range soil with similar quality of soil compared to inorganic-based washing.

Technical Note: Disturbance of soil structure can lead to release of entrapped methane in glacier forefield soils

Abstract. Investigations of sources and sinks of atmospheric CH4 are needed to understand the global CH4 cycle and climate-change mitigation options. Glaciated environments might play a critical role due to potential feedbacks with global glacial meltdown. In an emerging glacier forefield, an ecological shift occurs from an anoxic, potentially methanogenic subglacial sediment to an oxic proglacial soil, in which soil-microbial consumption of atmospheric CH4 is initiated. The development of this change in CH4 turnover can be quantified by soil-gas profile analysis. We found evidence for CH4 entrapped in glacier forefield soils when comparing two methods for the collection of soil-gas samples: a modified steel rod (SR) designed for one-time sampling and rapid screening (samples collected ∼1 min after hammering the SR into the soil), and a novel multilevel sampler (MLS) for repetitive sampling through a previously installed access tube (samples collected weeks after access-tube installation). In glacier forefields on siliceous bedrock, sub-atmospheric CH4 concentrations were observed with both methods. Conversely, elevated soil-CH4 concentrations were observed in calcareous glacier forefields, but only in samples collected with the SR, while MLS samples all showed sub-atmospheric CH4 concentrations. Time-series of SR soil-gas sampling (additional samples collected 2, 3, 5, and 7 min after hammering) confirmed the transient nature of the elevated soil-CH4 concentrations, which were decreasing from ∼100 μL L−1 towards background levels within minutes. This hints towards the existence of entrapped CH4 in calcareous glacier forefield soil that can be released when sampling soil-gas with the SR. Laboratory experiments with miniature soil cores collected from two glacier forefields confirmed CH4 entrapment in these soils. Treatment by sonication and acidification resulted in a massive release of CH4 from calcareous cores (on average 0.3–1.8 μg CH4 (g d.w.)−1) (d.w. – dry weight); release from siliceous cores was 1–2 orders of magnitude lower (0.02–0.03 μg CH4 (g d.w.)−1). Clearly, some form of CH4 entrapment exists in calcareous glacier forefield soils, and to a much lesser extent in siliceous glacier forefield soils. Its nature and origin remain unclear and will be subject of future investigations.

Influence of mechanical loading on static and dynamic CO2 efflux on differently textured and managed Luvisols

Mechanical disturbance of soil structure is commonly related to altered physical changes in pore systems, which control CO2 effluxes e.g. by changes in gas transport properties and in microbial activity. Soil compaction mostly leads to reduced CO2 fluxes. In contrast, structured soils can also release physically entrapped CO2 or give access to protected carbon sources inside aggregates due to aggregate breakdown by disruptive forces. In this study it was investigated how far arable soil management affects structure- and compaction-related CO2-releases using incubation experiments and CO2 gas analysis under standard matric potentials (−6kPa). CO2 efflux was analyzed before, during and after mechanical loading using the alkali trap method (static efflux) and a gas flow compaction device (GaFloCoD, dynamic efflux). Intact soil cores (236 and 471cm3) were collected from a Stagnic Luvisol with loamy sand (conservation and conventional tillage systems) and a Haplic Luvisol with clayey silt (under different fodder crops) from the topsoil (10–15cm) and subsoil (35–45cm). Mechanical stability was reflected by the pre-compression stress value (Pc) and by the tensile strength of aggregates (12–20mm). Changes in pore systems were described by air conductivity as well as air capacity and total porosity. While CO2-releases varied highly during the compaction process (GaFloCoD) for different stress magnitudes, soil depths and management systems, basal respiration rates were generally reduced after mechanical loading by almost half of the initial rates irrespective of soil management. For both methods (dynamic and static efflux) restriction in gas transport functionality was proved to have major influence on inhibition of CO2 efflux due to mechanical loading. GaFloCoD experiments demonstrated that decreases in CO2 efflux were linked to structural degradation of pore systems by exceeding internal soil strength (Pc). Otherwise, re-equilibrating matric potentials to −6kPa and re-incubating offset inhibition of soil respiration suggest a re-enhancement of microbial activity. At this state, physical influences were apparently overlapped by biological effects due to higher energy supply to microbes, which could be offered by spatial distribution changes of microorganisms and organic substrates within a given soil structure. This implies the susceptibility of physical protection mechanism for carbon by disruption of soil structure. In future, special focus should be given on a clear distinction between physical and microbiological effects controlling CO2 fluxes in structured soils.

포화도 및 교란에 따른 동래, 인제, 연기지역 불포화 화강풍화토의 강도변화 특성

본 연구에서는 직접전단시험을 이용하여 동래, 인제 및 연기 3개 지역의 교란 및 비교란 화강풍화토의 전단강도에 대한 포화도(또는 함수비) 및 교란 영향을 조사하였다. 0에서 100% 범위의 5개의 서로 다른 포화도를 가지는 교란 및 비교란 시편에 대하여 여러 번의 직접전단시험이 수행되었다. 전단시험 결과로부터 흙구조 교란에 따른 화강풍화토의 점착력 감소비(CRR)는 45-82% 범위로 크나, 마찰각 감소비(FRR)는 6-11% 범위로서 다소 작다는 것을 알 수 있다. 또한 화강풍화토의 겉보기 점착력과 내부마찰각은 포화도가 증가함에 따라 감소한다. 실험결과로부터 동래, 인제 및 연기 지역의 불포화 화강풍화토의 전단강도는 강우침투로 인한 함수비 증가 혹은 흙구조의 교란에 의해서 감소한다는 것을 알 수 있다. This study investigates the effects of degree of saturation (or water content) and disturbance on the strength reduction for both disturbed and undisturbed samples of weathered granite soil using direct shear test. Several series of direct shear tests were carried out on undisturbed or disturbed samples with 5 different degrees of saturation ranging from 0 to 100%. The direct test results showed that the values of cohesion reduction ratio (CRR) of weathered granite soil were in the range of 45 - 82 %, while the values of friction angle reduction ratio (FRR) in the range of 6 - 11%. Apparent cohesion and friction angle of weathered granite soils also decreased with an increase in degree of saturation. The results indicated that shear strength of unsaturated weathered granite soil at Dongrae, Inje and Yeonki sites tends to reduce due to an increase of water content induced from rainfall infiltration and/or due to disturbance of soil structure.

Suitability of Different Cover Crop Mixtures and Seedlings for a New Tree Row Management in an Organic Orchard

In organic orchards disadvantages within the tree row weed management include tree damages, soil structure disturbance and humus erosion due to bare soil.

Effect of soil structure disturbance on erosion and phosphorus losses from Finnish clay soil

No-till (NT) has proved to be an effective method to reduce erosion and losses of particulate phosphorus (PP) from fields to watercourses. However, the accumulation of P in the uppermost soil layer and the increased leaching risk of dissolved reactive phosphorus (DRP) in surface runoff reduce the environmental benefits of NT. The objective of this study was to determine whether the concentration of DRP in percolates could be decreased by mixing surface soil (0–5 cm) and deeper soil layers (5–20 cm) and how this affects erosion. We also tested the impact of high ionic strength on erosion from undisturbed and disturbed soil columns. Soil samples were collected from conventionally tilled (CT) and non-tilled (for 5 years) plots from a clay field (Vertic Cambisol) at 0–2.5, 2.5–10 and 10–20 cm depths. Moreover, undisturbed and disturbed soil columns representing the same plots were saturated with water in the laboratory and allowed to drain. The physico-chemical properties of the percolates were analysed to unravel the impact of cultivation methods and disruption of the aggregate structure. To see the effect of high ionic strength on detachment of soil particles, the soil columns were leached with ammonium acetate solution. The low pH of NT surface soil had enhanced the sorption of P and easily-soluble P had accumulated in the uppermost soil layer. Surprisingly, this enrichment did not increase the DRP concentration in the percolates when water percolated through the 0–5 or 0–20 cm soil layers. Disruption of the aggregate structure increased the turbidity and concentrations of suspended solids and PP in the percolates. In the NT samples, this action increased the concentration of DRP relatively more than in the CT samples. When the disturbed soil columns were leached with a solution of high ionic strength, the turbidity of the eluates was almost as low as before the breakage. To prevent erosion and the leaching of PP and DRP, we have to ensure an even water infiltration by improving the soil structure and by avoiding the disruption of stabilized aggregates. Ploughing the surface layer of NT soil can be recommended only if erosion from the field can be kept under control.

Initial Changes in Refilled Lysimeters Built with Metal Polluted Topsoil and Acidic or Calcareous Subsoils as Indicated by Changes in Drainage Water Composition

Soil translocation for recultivation of soil removed from construction sites and for the preparation of refilled lysimeters inevitably involves disturbance of soil structure, and, if intermediate storage is included, also drying and rewetting of the soil. We report on an experiment with model forest ecosystems, where uncontaminated forest subsoils were covered with non-contaminated or freshly heavy metal (mainly Zn and Cu) contaminated topsoil in large lysimeters. Monitoring of the chemical composition of the drainage water revealed two distinct soil conditioning phases. During an initial phase of about a year strongly elevated nitrate and sulfate concentrations occurred that were attributed to a mineralisation flush caused by the increased accessability of mineralisable nitrogen and sulfur in destroyed aggregates. These effects were significantly larger in lysimeters with calcareous subsoil than in those with acidic subsoil. The second phase was characterised by a gradual decrease in dissolved organic carbon and sulfate concentrations, in particular in the acidic subsoil. This decrease may be attributed to the depletion of pools made accessible during aggregate destruction or the formation of new aggregates. These chemical changes had only little effects on the concentrations of copper and zinc in the drainage water. Based on our results, it can be concluded that large refilled lysimeters can be used for many purposes without risk of compromised results, if a conditioning phase of about 1 year with sufficiently moist soil conditions is respected. Nevertheless, gradual changes in soil chemical characteristics still occur after this initial phase. Implications for the recultivation of sites using relocated soils are discussed.

Role of large‐scale soil structure in organic carbon turnover: Evidence from California grassland soils

We characterized the effect of large‐scale (>20 mm) soil physical structure on the age and recalcitrance of soil organic carbon (SOC) in upper (A) and lower (B) horizons of grassland soils from California's Central Valley. The radiocarbon content of SOC from surfaces and interiors of large‐scale soil structural units (“peds”) was measured in order to characterize the spatial distribution of soil C pools with distinct residence times. The radiocarbon content of CO2 released following sieving was used to identify the C that is readily respired upon physical disturbance of soil structure. We found the longest SOC residence times in the interiors of peds from subsurface B horizons, where limited bioturbation leads to stable large‐scale structure. The radiocarbon value of this interior SOC (Δ14C = −555‰) indicates that this pool has been protected from decomposition for thousands of years. Similarly ancient C (Δ14C = −596‰) was released upon physical disruption of subsurface B horizons from a similar soil, indicating that this SOC was old, but chemically labile. With cultivation, the C released upon physical disruption of B horizons was even older (Δ14C = −812‰) than in the uncultivated soil. In uncultivated A horizons, which are subject to continuous bioturbation, large‐scale structure resulted in contrasting SOC pools only in the surface horizon, where “bomb” C effects are strong. A horizon incubations also suggested effects of smaller‐scale structure. Loss of the labile SOC that is physically protected by large‐scale structure contributes to the rapid reduction in natural soil C inventories following cultivation.

Short-term effects of tillage on mineralization of nitrogen and carbon in soil

Tillage is known to decrease soil organic nitrogen (N) and carbon (C) pools with negative consequences for soil quality. This decrease is thought partly to be caused by exposure of protected organic matter to microbial degradation by the disturbance of soil structure. Little is known, however, about the short-term effects of tillage on mineralization of N and C, and microbial activity. We studied the short-term effects of two types of tillage (conventional plough- and a non-inverting-tillage) on mineralization and microbial N and C pools in a sandy loam under organic plough-tillage management. The release of active and protected (inactive) N by tillage was further studied in the laboratory by use of 15N labelling of the active pool of soil N followed by simulation of tillage by sieving through a 2 mm sieve. Results showed that the two types of tillage as well as the simulation of tillage had very few effects on mineralization and microbial pools. The simulation of tillage caused, however, a small release of N from a pool which was otherwise protected against microbial degradation. The use of soil crushing for disruption of larger macroaggregates (>425 μm) and chloroform fumigation for perturbation of the microbial biomass increased the release from both active and protected N pools. The relative contribution from the protected N pool was, however, similar in the three treatments (22–27%), thus the pools subjected to mineralization were characterised by similar degree of protection. On the basis of isotopic composition the pools of N mineralised were indistinguishable. This suggests that the released N originated from the same pool, that is the soil microbial biomass. The study points to the microbial pool as the main source of labile N which may be released by tillage, and thus to its importance for sustained soil fertility in agricultural systems.