Chemical and biochemical properties in a silty loam soil under conventional and organic management

To improve soil fertility, efforts need to be made to increase soil organic matter content. Conventional farming practice generally leads to a reduction of soil organic matter. This study compared inorganic and organic fertilisers in a crop rotation system over two cultivation cycles: first crop broad bean (Vicia faba L.) and second crop mixed cropped melon-water melon (Cucumis melo-Citrullus vulgaris) under semi-arid conditions. Total organic carbon (TOC), Kjeldahl-N, available-P, microbial biomass C (Cmic), and N (Nmic), soil respiration and enzymatic activities (protease, urease, and alkaline phosphatase) were determined in soils between the fourth and sixth year of management comparison. The metabolic quotient (qCO₂), the Cmic/Nmic ratio, and the Cmic/TOC ratio were also calculated. Organic management resulted in significant increases in TOC and Kjeldahl-N, available-P, soil respiration, microbial biomass, and enzymatic activities compared with those found under conventional management. Crop yield was greater from organic than conventional fertilizer. The qCO₂ showed a progressive increase for both treatments during the study, although qCO₂ was greater with conventional than organic fertilizer. In both treatments, an increase in the Cmic/Nmic ratio from first to second crop cycle was observed, indicating a change in the microbial populations. Biochemical properties were positively correlated (p < 0.01) with TOC and nutrient content. These results indicated that organic management positively affected soil organic matter content, thus improving soil quality and productivity.     

Microbial reaction in activity, biomass, and community structure after long-term continuous mixing of a grassland soil

Long-term continuous mixing at 40% water holding capacity (WHC) or as slurry at 400% WHC should result in increased soil organic matter decomposition rates in comparison to a control treatment at 40% WHC, but may have strong impacts on soil microbial indices for activity, biomass, and community structure. The amount of extractable inorganic N (NO₃-N+NH₄-N) accumulated in the soil solution after 40 weeks of incubation at 25 °C was 3% of total N in the control treatment and 4% in the two continuous mixing treatments. However, in the treatment mixing at 40% WHC, this 33% increase compared to the control treatment might be explained solely by the decrease in microbial biomass N. In the control treatment, microbial indices decreased in the order microbial biomass C (−10%), microbial biomass N (−40%), ergosterol (−45%) and ATP (−60%). In the treatment mixing at 40% WHC, all four microbial biomass indices were significantly lower than the respective index in the control treatment. This was especially true for microbial biomass N. In the treatment mixing as slurry, only the contents of microbial biomass C and ATP were significantly lower in comparison to the control treatment. The correspondence analysis ordination biplot of the phospholipid fatty acid (PLFA) profiles showed distinct clusters for the three treatments at the end of the incubation. The strongest relative decline of 64% was observed for the fungi-specific PLFA 18:3ω6 in the treatment mixing as slurry in comparison to the control treatment. The content of total bacterial PLFA decreased only by 23%. The differences between the control treatment and the treatment mixing at 40% WHC were less apparent. Fungi represent on average 21% of total microbial biomass C at the end of the incubation if the ergosterol content is recalculated into fungal biomass C. In accordance with this percentage, 22% of the group-specific PLFA could be attributed to fungi.     

Long-term effects of tillage and fallow-frequency on soil quality attributes in a clay soil in semiarid southwestern saskatchewan

Reduced tillage management is being adopted at an accelerated rate on the Canadian prairies. This may influence soil quality and productivity. A study conducted on a clay soil (Udic Haplustert) in southwestern Saskatchewan, Canada, to determine the effects of fallow frequency [fallow-wheat (F-W) vs. continuous wheat (Cont W)] and tillage [no-tillage (NT) vs. conventional (CT) or minimum tillage (MT)] on yields of spring wheat (Triticum aestivum L.), was sampled after 3, 7 and 11 years to assess changes in selected soil quality attributes. Tillage had no effect on amount of crop residues returned to the land, but the tilled systems had significantly (P<0.05) lower total organic C and N in the 0–7.5 cm soil depth, though not in the 7.5–15 cm depth. Further, these differences were observed after only 3 years and persisted for the entire 11 years of the study. For example, in the 0–7.5 cm depth, organic C in F-W (MT) after 3 years was 10 480 kg ha⁻¹ and in F-W (NT) 13 380 kg ha⁻¹, while in Cont W (CT) and Cont W (NT) corresponding values were 11 310 and 13 400 kg ha⁻¹, respectively. After 11 years, values for F-W (MT) and F-W (NT) were 11 440 and 14 960 kg ha⁻¹, respectively, and for Cont W (CT) and Cont W (NT), 12 970 and 16 140 kg ha⁻¹, respectively. In contrast to total organic matter, two of the more labile soil quality attributes [i.e., C mineralization (C_min) and N mineralization (N_min)] did not respond to fallow frequency until after 7 years and only in the 0–7.5 cm depth. Microbial biomass (MB) and the ratio of C_min to MB [specific respiratory activity (SRA)], two attributes also regarded as labile, were not influenced by the treatments even after 11 years. After 11 years, only C_min and N_min among the labile soil quality attributes responded to the treatments. Surprisingly, the labile attributes were no more sensitive to the treatments than was total organic C or N. More research is required to determine why responses in this soil differed from those reported elsewhere.     

Film fully-mulched ridge-furrow cropping affects soil biochemical properties and maize nutrient uptake in a rainfed semi-arid environment

Abstract

This study evaluated the effects of plastic mulched ridge-furrow cropping on soil biochemical properties and maize (Zea mays L.) nutrient uptake in a semi-arid environment. Three treatments were evaluated from 2008 to 2010: no mulch (narrow ridges with crop seeded next to ridges), half mulch (as per no mulch, except narrow ridges were mulched), and full mulch (alternate narrow and wide ridges, all mulched with maize seeded in furrows). Compared to the no mulch treatment, full mulch increased maize grain yield by 50% in 2008 and 25% in 2010, but reduced yield by 21% in 2009 after low precipitation in early growth. Half mulch had a similar grain yield to no mulch in the three cropping years, suggesting half mulch is not an effective pattern for maize cropping in the area. Mulch treatments increased aboveground nitrogen (N) uptake by 21?34% and phosphorus (P) uptake by 21?42% in 2008, and by 16?32% and 14?29%, respectively, in 2010; but in 2009 mulching did not affect N uptake and decreased P uptake. Soil microbial biomass and activities of urease, β-glucosidase and phosphatase at the 0?15 cm depth were generally higher during vegetative growth but lower during reproductive growth under mulch treatments than no mulch. Mulching treatments increased carbon (C) loss of buried maize residues (marginally by 5?9%), and decreased light soil organic C (15?27%) and carbohydrate C (12?23%) concentrations and mineralizable C and N (8?36%) at harvest in the 0?20 cm depth compared with no mulch, indicating that mulching promotes mineralization and nutrient release in soil during cropping seasons. As a result of these biological changes, mineral N concentration under mulch was markedly increased after sowing in upper soil layers compared with no mulch. Therefore, our results suggest that mulched cropping stimulated soil microbial activity and N availability, and thus contributed to increasing maize grain yield and nutrient uptake compared with no mulch.     

Small-scale spatial variability of selected soil biological properties

A strategy for sampling soil from intact monolith lysimeters was established based on measurements of spatial heterogeneity within the lysimeter area. This was part of an ongoing study to determine relationships between soil microbial diversity and nutrient loss by leaching. The sampling protocol had to allow collection of soil on a regular basis (as opposed to destructive sampling) and ensure high spatial independence of subsamples. On each of the two sites (one developed under organic crop management and the other under conventional crop management), ten 15 cm soil cores (sampling points) were taken from three areas (replicates) of 50 cm diameter (lysimeter surface area) and separately analysed for biotic (microbial biomass carbon and nitrogen; arginine deaminase activity) and abiotic (total carbon and nitrogen) soil properties. The data were tested for variability, expressed as coefficient of variance (biotic and abiotic), and spatial heterogeneity using geostatistics (biotic properties). The biotic soil properties showed significant differences among sampling points, whereas the abiotic parameters were useful in differentiating on a larger scale, i.e. between sites. For all soil properties tested, the differences among the replicates were smaller than those between the sites or among points indicating that, in the main experiment, all treatments can be sampled following the same pattern. Geostatistical analysis and fitting of an exponential model showed that a spatial structure exists in the biotic soil properties and that the samples are independent beyond separation distances of 25-30 cm. A revised sampling pattern consisting of 11 samples per lysimeter is described.     

Earthworm dynamics and soil physical properties in the first three years of no-till management

Earthworms are often referred to as ecosystem engineers due to their ability to alter the soil environment. Since earthworms influence a wide range of critical chemical and physical soil properties it is important to understand how their populations are impacted by soil management. Earthworms were sampled during the spring and summer of 2001, 2002, and 2003 from conventional tillage (CT) and no-till (NT) plots established in 2000. Although there was a strong trend for higher earthworm density in NT plots in 2001 (p = 0.08) and 2002 (p = 0.19), statistically significant differences were not detected between tillage treatments until 2003 (p = 0.04) when mean earthworm density was 37.7 individuals m⁻² in CT and 149.9 individuals m⁻² in NT during spring and 17.1 individuals m⁻² in CT and 58.4 individuals m⁻² in NT in summer. A high mortality rate between spring and summer, combined with greater cocoon production under NT suggests that the earthworm population turns over rapidly in NT plots. Data also suggest that adverse soil environmental conditions will limit earthworm density in these dryland agroecosystems. Despite significantly higher earthworm density after three years of NT management, soil bulk density, saturated hydraulic conductivity, and aggregate stability of the 0.5- to 1-mm size fraction were not different between the two tillage treatments. The apparent lack of impact of reduced disturbance and increased earthworm density on soil physical properties may be due to the short time this soil has been under NT management, limited seasonal earthworm activity due to environmental conditions, or differences in the scale at which soil physical properties have been affected after three years of NT management and the scale at which our measurements were made.     

Effects of diatomite on soil physical properties

Organic and inorganic soil amendments are commonly added to soil for improving its physical and chemical characteristics which promote plant growth. Although many inorganic amendments are extensively used for this purpose, diatomite (DE) is not commonly used. This study was conducted to determine effects of diatomite applications (10, 20, and 30% v/v) on physical characteristics of soils with different textures (Sandy Loam, Loam, and Clay), under laboratory conditions. The results indicated that diatomite application protects large aggregate (> 6.4 mm) formation in clay-textured soils, however it reduced the mean weight diameter in sand-textured soil. 30% diatomite reduced mean weight diameter in sand-textured soils from 1.74 to 1.49 mm. Diatomite applications significantly increased aggregate stability of all the experimental soils in all aggregate size fractions. In overall, aggregate stability increased from 28.04% to 45.70% with the application rate of 30%. Diatomite application also significantly increased soil moisture content at field capacity in SL textured soil. 30% diatomite increased field capacity in sand-textured soil in the percent of 43.78 as compared with control. Therefore it is suggested that diatomite may be considered as a soil amendment agent for improving soil physical characteristics. However, its effectiveness in enhancing soil properties depends on initial soil factors and texture. Moreover, since its protective effect against large aggregate (> 6.4 mm) formation and reducing effect on soil penetration resistance in clay textured soils, diatomite might be an alternative soil amendment agent in soil tillage practices and seedling.     

Immobilization and mineralization of nitrogen in a saline and alkaline soil during microbial use of sugarcane filter cake amended with glucose

A 42-day incubation was conducted to study the effect of glucose and ammonium addition adjusted to a C/N ratio of 12.5 on sugarcane filter cake decomposition and on the release of inorganic N from microbial residues formed initially. The CO₂ evolved increased in comparison with the non-amended control from 35% of the added C with pure +5 mg g⁻¹ soil filter cake amendment to 41% with +5 mg g⁻¹ soil filter cake +2.5 mg g⁻¹ soil glucose amendment to 48% with 5 mg g⁻¹ soil filter cake +5 mg g⁻¹ soil glucose amendment. The different amendments increased microbial biomass C and microbial biomass N within 6 h and such an increase persisted. The fungal cell-membrane component ergosterol initially showed a disproportionate increase in relation to microbial biomass C, which completely disappeared by the end of the incubation. The cellulase activity showed a 5-fold increase after filter cake addition, which was not further increased by the additional glucose amendment. The cellulase activity showed an exponential decline to values around 4% of the initial value in all treatments. The amount of inorganic N immobilized from day 0 to day 14 increased with increasing amount of C added, in contrast to the control treatment. After day 14, the immobilized N was re-mineralized at rates between 1.3 and 1.5 μg N g⁻¹ soil d⁻¹ in the treatments being more than twice as high as in the control treatment. This means that the re-mineralization rate is independent of the actual size of the microbial residues pool and also independent of the size of the soil microbial biomass.     

Modelling mineralization of plant residues on soil: effect of physical protection

A mechanistic dynamic model (Verberne et al. 1990) was used to simulate mineralization of white-clover materials in a loam (25% clay) and a sandy loam soil (5% clay). I tested the model‘s ability to simulate the observed temporal patterns and to take account of altered physical protection as affected by soil compaction or spatial residue distribution. With default parameter values, the model greatly overestimated net N mineralization. The model was very sensitive to changes in the C/N ratio of the microbial biomass. Reducing this value from 8.0 to 6.0 improved the model performance. Nevertheless, initial N mineralization was appreciably overestimated. Two hypotheses may explain the discrepancies: (1) the C/N ratio of the microbial biomass is initially low (3–4) and gradually increases because of a succession from bacterial- to fungal-dominated biomass (H ₁); (2) the C/N ratio of the substrates first attacked by microorganisms, i.e. water-soluble components such as sugars and free amino acids, is higher than the average value (6.0) assumed for the readily decomposable fraction (H ₂). Conceptually, this fraction originally included N-containing polymers (proteins and nucleic acids), which in large part are water insoluble and probably attacked somewhat later than the monomers. Modification of the model, either by implementing a dynamic C/N ratio of the biomass and the effect of faunal grazing or by increasing the C/N ratio of the easily decomposable fraction, improved the model performance substantially. The two hypotheses need to be tested experimentally. The model adequately simulated measured effects of spatial residue distribution and soil compaction on N mineralization after adjustment or parameter values regulating physical protection of microbial biomass and metabolites. Moreover, there was a good agreement between simulated and measured microbial biomass N in the two soils. Received: 9 December 1996     

不同碳氮比有机肥对有机农业土壤微生物生物量的影响

有机肥能提高土壤微生物活性, 改善土壤品质。碳氮比是影响有机肥肥效的重要因素。本试验以无肥处理为对照(CK), 设置4个有机肥碳氮比处理(20︰1、15︰1、10︰1、5︰1), 在温室中进行茄子盆栽试验, 定期采集土壤样品, 用熏蒸提取法测定土壤微生物生物量碳(SMBC)、氮(SMBN), 研究等氮条件下不同碳氮比有机肥料对土壤生物活性的影响。试验结果表明, 不同碳氮比的有机肥均能提高土壤的SMBC和SMBN含量, 具体表现为SMBC: 20︰1>10︰1≈15︰1>5︰1>CK, SMBN: 15︰1>10︰1>20︰1>5︰1>CK。SMBC/SMBN的比率反映土壤氮素生物活性, 其值越低, 生物活性越大, 氮素损失越少, 本试验SMBC/SMBN表现为: 15︰1<10︰1<20︰1≈5︰1相似文献   

Changes in soil physical properties with depth in a conventionally tilled soil after no-tillage

A no-tillage system was imposed on a structurally degraded fine-textured soil (Humic Gleysol) that had been under continuous corn with moldboard tillage for more than 20 years. After 3 years of no-tillage, several soil structural properties were compared with the conventional tillage treatment to assess whether the soil structure had improved.

No significant difference (P<0.05) was found between tillage treatments for the saturated hydraulic conductivity, porosity and penetration resistance in the surface 5 cm. Measurements of soil penetration resistance and in situ saturated hydraulic conductivity (K_wp) using the well permeameter method were sensitive to structural changes that had occurred at 5–20 cm depth. The K_wp at this depth was significantly greater in the moldboard treatment than in the no-tillage treatment. Resistance measurements indicated significantly greater soil strengths at 10–20 cm under no-tillage. Aggregate stabilities were assessed by wet sieving twice during the growing season. No-tillage resulted in larger soil aggregates, especially at the surface, compared with the moldboard tillage.

These data suggest that degraded soils with low structural stability may initially suffer further deterioration with the elimination of tillage, owing to the loss or reduction of mechanically formed pores.     

不同培肥措施下种植制度及撂荒对土壤微生物量碳氮的影响

以黄土高原南部半湿润易旱区已进行17年的田间定位试验为研究对象,研究了不同培肥措施(不施肥、施用氮磷钾及氮磷钾与有机肥配合施用)下两种种植制度(一年1熟及一年两熟)和撂荒对土壤微生物量碳、氮(SMBC、SMBN)及可溶性有机碳、氮(SOC、SON)等含量的影响.结果表明,与一年1熟的小麦一休闲种植制度相比,一年两熟小麦一玉米轮作提高了0～10 cm土层SMBC、SMBN、有机碳(TOC)、全氮(TN)和土壤SOC、SON的含量,而对10～20 cm土层上述测定指标影响不大.与不施肥(CK)或单施化肥处理(NPK)下小麦-休闲和小麦-玉米轮作方式相比,撂荒处理显著提高了0～10 cm土层各测定指标的含量.不同培肥措施相比,氮磷钾配施有机肥显著提高了0～10 cm、10～20 cm土层SMBC、SMBN含量;NPK处理0～10 cm土层SMBN含量显著增加,10～20 cm土层SMBN和0～10 cm、10～20 cm土层SMBC含量增加但未达显著水平.不同培肥措施和种植制度对SMBC/TOC和SMBN/TN的比例无明显影响.     

Seasonality of soil biological properties in a poplar plantation growing under elevated atmospheric CO2

Microorganisms are the regulators of decomposition processes occurring in soil, they also constitute a labile fraction of potentially available N. Microbial mineralization and nutrient cycling could be affected through altered plant inputs at elevated CO_2. An understanding of microbial biomass and microbial activity in response to belowground processes induced by elevated CO₂ is thus crucial in order to predict the long-term response of ecosystems to climatic changes. Microbial biomass, microbial respiration, inorganic N, extractable P and six enzymatic activities related to C, N, P and S cycling (β-glucosidase, cellulase, chitinase, protease, acid phosphatase and arylsulphatase) were investigated in soils of a poplar plantation exposed to elevated CO_2. Clones of Populus alba, Populus nigra and Populus x euramericana were grown in six 314 m² plots treated either with atmospheric (control) or enriched (550 μmol mol⁻¹ CO₂) CO₂ concentration with FACE technology (free-air CO₂ enrichment). Chemical and biochemical parameters were monitored throughout a year in soil samples collected at five sampling dates starting from Autumn 2000 to Autumn 2001.

The aim of the present work was: (1) to determine if CO₂ enrichment induces modifications to soil microbial pool size and metabolism, (2) to test how the seasonal fluctuations of soil biochemical properties and CO₂ level interact, (3) to evaluate if microbial nutrient acquisition activity is changed under elevated CO₂.

CO₂ enrichment significantly affected soil nutrient content and three enzyme activities: acid phosphatase, chitinase and arylsulphatase, indicators of nutrient acquisition activity. Microbial biomass increased by a 16% under elevated CO₂. All soil biochemical properties were significantly affected by the temporal variability and the interaction between time and CO₂ level significantly influenced β-glucosidase activity and microbial respiration. Data on arylsulphatase and chitinase activity suggest a possible shift of microbial population in favour of fungi induced by the FACE treatment.     

Quantifying seedbed condition using soil physical properties

Soil physical condition following tillage influences crop yield, but the desired condition cannot be adequately evaluated with current techniques. This study was conducted to determine a soil condition index (SCI) that could be used to select the type of implement needed to achieve an optimal seedbed with minimum energy input. Effects of bulk density, moisture content, and penetration resistance resulting from three tillage systems (no-till, chisel plow and moldboard plow), on the growth of corn (Zea mays L.) were studied. The experiment was conducted in Boone County, Ames, IA, on soils that are mostly Aquic Hapludolls, Typic Haplaquolls and Typic Hapludolls with slopes ranging from 0 to 5%. The results are from the 2000 season, which had normal weather conditions and yield levels for the Iowa state. The average corn grain yield at this site was 9.36 Mg/ha. At the V2 corn growth stage, the average dry biomass was 1.34 g per plant. The soil physical properties were normalized with respect to reference values and combined via multiple regression analysis against corn biomass at V2 stage into the SCI. Mean SCI values for the no-till, chisel and moldboard plow treatments were 0.86, 0.76, and 0.73, respectively, all with a standard error of 0.0127. The lower the SCI, the more optimum the soil physical conditions. An analysis of variance showed significant differences among mean SCI for each treatment (p-value=0.001). The use of the SCI could improve the tillage decision-making process in environments similar the one studied.     

Changes in 15N abundance and amounts of biologically active soil nitrogen

 Estimation of the capacity of soils to supply N for crop growth requires estimates of the complex interactions among organic and inorganic N components as a function of soil properties. Identification and measurement of active soil N forms could help to quantify estimates of N supply to crops. Isotopic dilution during incubation of soils with added ¹⁵NH₄ ⁺ compounds could identify active N components. Dilution of ¹⁵N in KCl extracts of mineral and total N, non-exchangeable NH4₄ ⁺, and N in K₂SO₄ extracts of fumigated and non-fumigated soil was measured during 7-week incubation. Samples from four soils varying in clay content from 60 to 710 g kg^–1 were used. A constant level of ¹⁵N enrichment within KCl and K₂SO₄ extracted components was found at the end of the incubation period. Total N, microbial biomass C and non-exchangeable NH₄ ⁺ contents of the soils were positively related to the clay contents. The mineralized N was positively related to the silt plus clay contents. The active soil N (ASN) contained 28–36% mineral N, 29–44% microbial biomass N, 0.3–5% non-exchangeable NH₄ ⁺ with approximately one third of the ASN unidentified. Assuming that absolute amounts of active N are related to N availability, increasing clay content was related to increased N reserve for crop production but a slower turnover. Received: 7 July 1998     

Cultivation effects on biochemical properties, C storage and N natural abundance in the 0–5 cm layer of an acidic soil from temperate humid zone

Changes in some soil chemical, including ¹⁵N values, and biochemical properties (microbial C, FDA hydrolysis, glucosidase and urease activities) due to two tillage systems, conventional tillage (CT) and no-tillage (NT), were evaluated in an acid soil from temperate humid zone (NW of Spain) and compared with values obtained for a reference forest soil. The results showed that in the surface layer (0–5 cm depth) tillage tended to increase soil pH and to decrease organic matter levels and microbial biomass and activity values. The data also indicated that 8 years of NT, compared to CT, resulted in greater organic matter content and increased microbial biomass and activity, the changes being more pronounced for the microbial properties. Adoption of NT resulted in an increase of soil C storage of 1.24 Mg C ha⁻¹ year⁻¹ with regard to CT. The suitability of ¹⁵N as a potential tracer of land-use in this acid soil was also confirmed.     

Effects of increasing periods under intensive arable vegetable production on biological, chemical and physical indices of soil quality

 The effects on soil condition of increasing periods under intensive cultivation for vegetable production on a Typic Haplohumult were compared with those of pastoral management using soil biological, physical and chemical indices of soil quality. The majority of the soils studied had reasonably high pH, exchangeable cation and extractable P levels reflecting the high fertilizer rates applied to dairy pasture and more particularly vegetable-producing soils. Soil organic C (C_org) content under long-term pasture (>60 years) was in the range of 55 g C kg^–1 to 65 g C kg^–1. With increasing periods under vegetable production soil organic matter declined until a new equilibrium level was attained at about 15–20 g C kg^–1 after 60–80 years. The loss of soil organic matter resulted in a linear decline in microbial biomass C (C_mic) and basal respiratory rate. The microbial quotient (C_mic/C_org) decreased from 2.3% to 1.1% as soil organic matter content declined from 65 g C kg^–1 to 15 g C kg^–1 but the microbial metabolic quotient (basal respiration/C_mic ratio) remained unaffected. With decreasing soil organic matter content, the decline in arginine ammonification rate, fluorescein diacetate hydrolytic activity, earthworm numbers, soil aggregate stability and total clod porosity was curvilinear and little affected until soil organic C content fell below about 45 g C kg^–1. Soils with an organic C content above 45 g C kg^–1 had been under pasture for at least 30 years. At the same C_org content, soil biological activity and soil physical conditions were markedly improved when soils were under grass rather than vegetables. It was concluded that for soils under continuous vegetable production, practices that add organic residues to the soil should be promoted and that extending routine soil testing procedures to include key physical and biological properties will be an important future step in promoting sustainable management practices in the area. Received: 18 November 1997     

Complexed organic matter controls soil physical properties

It is shown that, for mineral soils, it is not the total amount of organic carbon (or organic matter) that controls soil physical behaviour but the amount of complexed organic carbon (COC). We assume that this complex is formed by the association of unit mass (i.e. 1 g) of organic carbon with n grams of clay. Analysis of data from two French and two Polish databases shows that, for these soils, n = 10. A consequence of this is that in soils with small contents of organic carbon (OC), such as arable soils, COC is proportional to OC. However, in soils with large contents of organic carbon, such as pasture soils, COC is proportional to the clay content. This explains why we find that soil bulk density is significantly correlated with OC in French arable soils but with the clay content in French pasture soils. The use of COC instead of OC enables the arable and pasture soils to be considered on the same scale.

Water retention data were fitted to a double-exponential equation which allows both the matrix and structural porosities to be estimated. It is shown that in soils with low contents of organic carbon, the carbon content is positively correlated with the matrix porosity. In contrast, in soils with high contents of organic carbon, the matrix porosity is constant at its maximum value and the structural porosity is not significantly correlated with either the total organic carbon or the non-complexed organic carbon (NCOC). It is suggested that the complexed organic carbon can be considered as being sequestered. The soil clay content can similarly be partitioned between clay that is complexed with organic carbon and clay that is not complexed. It is shown that non-complexed clay is more easily dispersed in water than clay that is complexed with organic carbon. These findings indicate how improved pedo-transfer functions for the prediction of soil physical properties may be produced. Such functions need to use the values of complexed and non-complexed organic carbon and clay which must be determined by algorithms. The values produced by the algorithms may then be used in the improved pedo-transfer functions.     

Response of soil physical properties to tillage and residue management on two soils in a cool temperate environment

In view of their potential benefits, reduced or no tillage (NT) systems are being advocated worldwide. Concerns about impairment of some soil conditions, however, cast doubt on their unqualified acceptance. We evaluated the effects of 6 years of tillage and residue management on bulk density, penetration resistance, aggregation and infiltration rate of a Black Chernozem at Innisfail (loam, 65 g kg⁻¹ organic matter, Udic Boroll) and a Gray Luvisol at Rimbey (loam, 31 g kg⁻¹ organic matter, Boralf) cropped to monoculture spring barley (Hordeum vulgare L.) in a cool temperate climate in Alberta, Canada. Tillage systems were no tillage and tillage with rototilling (T), and two residue levels were straw removed (−S) and straw retained (+S). Bulk density (BD) of the 0–7.5 and 7.5–15 cm depths was significantly greater under NT (1.13–1.58 Mg m⁻³) than under T (0.99–1.41 Mg m⁻³) in both soils, irrespective of residue management. In both soils, penetration resistance (PR) was greater under NT than under T to 15 cm depth. Residue retention significantly reduced PR of the 0–10 cm soil in NT, but not in T. In the 0–5 cm depth of the Black Chernozem, the >2 mm fraction of dry aggregates was highest under NT + S (72%), and lowest under T − S (50%). The wind-erodible fraction (dry aggregates <1 mm size) was smallest (18%) under NT + S and largest (39%) under T − S. Soil aggregation benefited more from NT than from residue retention. Proportion of wind-erodible aggregates was generally greater in the Gray Luvisol than in the Black Chernozem. In the Black Chernozem, steady-state infiltration rate (IR) was significantly lower (33%) under NT than under T. Residue retention improved IR in both NT and T. In the Gray Luvisol, IR was not significantly affected by tillage and residue management. Despite firmer soil, NT and residue retention are recommended to improve aggregation in the cool temperate region of Western Canada.