The role of tephra covers on soil moisture conservation at Haleakala's crater (Maui, Hawai'i)

The influence of tephra covers on soil water was studied in Haleakala (Maui, Hawai'i) during two summers; eight sites with tephra layers and silverswords (Argyroxiphium sandwicense DC.) were sampled at 2415–2755 m. At each site, eight paired-sample sets were obtained in bare soils and under adjacent tephra, at three depths. Tephra were sharply separated from underlying soils and showed prominent vertical stratification. Tephra clast size-distribution was assessed by photosieving and on interstitial-gravel samples; stones included 45.6% cobbles, 29.4% pebbles, and 25% blocks.Moisture content increased with depth in both positions, but soils below tephra had more water at all depths than exposed areas. Surface soils beneath tephra contained 83% more water than bare ground. Soils at 5–10 cm had  106% greater moisture under rocks, but only  70% at 10–15 cm. Differences between plots were statistically significant ( p < 0.001) for surface soils, but less pronounced for subsoils. Soils above 2650 m had greater water content than at lower elevations, and moisture disparity between sample pairs increased with altitude.All soils were coarse, with  20% gravel and  94% sand; most fine material (≤ 0.063 mm) was silt, as clay content was negligible. Organic-matter percentage was low (1.65%). Bulk density and porosity were associated with moisture variation both in tephra-insulated and bare soils; 80% of field moisture was statistically (p < 0.001) accounted for by pore space. Air and soil temperatures were recorded at three sites during  one-week periods prior to moisture sampling. Tephra substantially decreased soil maxima and daily thermal amplitude in underlying soils, but did not noticeably affect nightly minima. Thin (5–6 cm) tephra layers were nearly as effective as thicker (9–15 cm) deposits in depressing soil maxima. Possible water-conservation mechanisms under tephra include: decreased evaporation due to ground shielding and lower maxima; reduced capillary flow; greater infiltration depth; nocturnal dew condensation; and fog interception by blocks.     

Strip-tillage effect on seedbed soil temperature and other soil physical properties

The no-tillage system is perceived as having lower soil temperatures, wetter soil conditions, and greater surface penetration resistance compared with conventional and other conservation tillage systems. Concerns associated with the effect of the no-tillage system on certain soil physical properties (i.e. soil temperature, moisture, and compaction) prompted this study to evaluate the effect of an alternative tillage system, strip-tillage, on these physical properties, compared with chisel plow and no-tillage systems. The study was conducted on two Iowa State University research and demonstration farms in 2001 and 2002. One site was at the Marsden Farm near Ames, where the soils were Nicollet loam (Aquic Hapludolls) and Webster silty clay loam (Typic Haplaquolls). The second site was at the Northeast Research and Demonstration Farm near Nashua, where the soils were Kenyon loam (Typic Hapludolls) and Floyd loam (Aquic Hapludolls).Soil temperature increased in the top 5 cm under strip-tillage (1.2–1.4 °C) over no-tillage and it remained close to the chisel plow soil temperature. This increase in soil temperature contributed to an improvement in plant emergence rate index (ERI) under strip-tillage compared with no-tillage. The results show no significant differences in soil moisture status between the three tillage systems, although the strip-tillage soil profile has slightly greater moisture content than chisel plow. Moisture content through the soil profile particularly at the lower depths under all tillage treatments was greater than the plant available water (PAW). However, the changes in soil moisture storage were much greater with strip-tillage and chisel plow than no-tillage from post-emergence to preharvest at 0–30 and 0–120 cm. It was observed also that most change in soil moisture storage occurred between post-emergence and tasseling. Penetration resistance was similar for both strip-tillage and no-tillage, but commonly greater than chisel plow. In general, the findings show that strip-tillage can contribute effectively to improve plant emergence, similar to chisel plowing and conserve soil moisture effectively compared with no-tillage.     

Vertical distribution of soil mites (Acari) in conventional and no-tillage agricultural systems

Summary Soil mite abundance was measured at four depths (0–5, 6.5–11.5, 13–18, and 19.5–24.5 cm) in agricultural plots under no-tillage or conventional tillage in Clarke County, Georgia, USA. The vertical distribution of mites was not significantly different between the two tillage systems: Most mites were found in the top 0–5 cm zone. This was the zone where greater moisture content occurred, and (in other studies) was the zone of maximum root biomass and microbial activity. Among mite suborders, only the Prostigmata were found in any abundance below 5 cm. Mite populations declined dramatically on occasions when the soil moisture exceeded field capacity, but did not appear to migrate vertically.Dedicated to the late Prof. Dr. W. Kühnelt     

免耕对黑土春夏季节温度和水分的影响

通过田间定位试验,研究免耕与常规耕作对东北黑土区玉米和大豆生长早期土壤温度和水分的影响。研究结果表明:播种前,由于免耕与常规耕作(秋翻)覆盖率和含水量不同,免耕处理的玉米和大豆小区土壤的白天5cm地温均低于常规耕作处理,夜间差异不大;相同深度的玉米和大豆秋翻处理土壤日平均温度分别比免耕高0. 7℃和0. 5℃;随土壤深度的增加,土壤温度的差异逐渐减小。播种后,除了下午免耕5cm地温略低于秋翻外,下午至夜间免耕的10cm和15cm地温,均略高于秋翻的土壤温度。这是由于免耕下土壤水分增高引起的土壤热容量加大,从而缓解夜间降温和寒流影响,减缓土壤温度下降的结果。播种前,免耕处理的玉米和大豆地土壤水分分别比秋翻处理高2. 4%和1. 8%。播种后的一个月期间,免耕大豆土壤含水量比秋翻高2. 3%。初步的研究结果表明,免耕可以在一定程度上缓解春季黑土墒情不好的问题,这对保证出苗和幼苗的健康生长非常重要。     

Activity and biomass of soil microorganisms at different depths

We measured microbial biomass C and soil organic C in soils from one grassland and two arable sites at depths of between 0 and 90 cm. The microbial biomass C content decreased from a maximum of 1147 (0–10 cm layer) to 24 g g^-1 soil (70–90 cm layer) at the grassland site, from 178 (acidic site) and 264 g g^-1 soil (neutral site) at 10–20 cm to values of between 13 and 12 g g^-1 soil (70–90 cm layer) at the two arable sites. No significant depth gradient was observed within the plough layer (0–30 cm depth) for biomass C and soil organic C contents. In general, the microbial biomass C to soil organic C ratio decreased with depth from a maximum of between 1.4 and 2.6% to a minimum of between 0.5 and 0.7% at 70–90 cm in the three soils. Over a 24-week incubation period at 25°C, we examined the survival of microbial biomass in our three soils at depths of between 0 and 90 cm without external substrate. At the end of the incubation experiment, the contents of microbial biomass C at 0–30 cm were significantly lower than the initial values. At depths of between 30 and 90 cm, the microbial biomass C content showed no significant decline in any of the four soils and remained constant up to the end of the experiment. On average, 5.8% of soil organic C was mineralized at 0–30 cm in the three soils and 4.8% at 30–90 cm. Generally, the metabolic quotient qCO₂ values increased with depth and were especially large at 70–90 cm in depth.     

Spatial pattern and heterogeneity of soil properties in sand dunes under grazing and restoration in Horqin Sandy Land, Northern China

Applying a combination of classical and geostatistical methods, we identified soil properties and their spatial variation in a 5-year grazed sand dune (GSD5) and a 20-year recovered sand dune (RSD20) in Horqin Sandy Land, northern China. The paper assesses the effect of grazing, topography and vegetation restoration on spatial heterogeneity of soil properties. The results showed that soil organic carbon, total nitrogen, very fine sand (0.1–0.05 mm) content and their coefficients of variation were lower in GSD5 than in RSD20, while soil water contents (0–20 cm and 20–40 cm depths) were higher in GSD5 than in RSD20. Geostatistical analysis revealed that the spatial structured variance accounted for the largest proportion of total sample variance in soil properties at the measured scale under grazing and restoration. The spatial autocorrelation ranges were 66.30 m for soil organic carbon and 50.80 m for total nitrogen in GSD5 less than those in RSD20 (70.00 m and 76.10 m, respectively), while the spatial autocorrelation ranges of soil particle size fractions and soil water contents in RSD20 were less than those in GSD5. Kriging-interpolated maps also showed that the heterogeneity of soil organic carbon and total nitrogen and their degree of patch fragmentation were higher in GSD5 than in RSD20. These results suggested that continuous grazing resulted in an increase in spatial variability of soil nutrient and a decrease in spatial variability of soil particle size fractions and soil water content. Soil organic carbon and total nitrogen of sand dunes are associated closely with soil particle size fractions, relative height of sampling site and vegetation cover. Spatial patterns of soil properties are most strongly related to grazing, topography and plant-induced heterogeneity in sand dune ecosystems prone to wind erosion.     

Nitrogen mineralization in relation to site history and soil properties for a range of Australian forest soils

Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha^-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha^-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.     

Infiltration patterns into two soils under conventional and conservation tillage: influence of the spatial distribution of plant root structures and soil animal activity

The characteristics and properties of the soil macropore system may cause different infiltration behavior under different tillage practices. To evaluate the effect of a specific tillage system on infiltration and percolation with particular regard to the influence of crop structure and soil animal activity dye tracer irrigation experiments were conducted in a silty (Luvisol) as well as in a sandy loam soil (Podzolluvisol). The spatial distribution of water flow paths was experimentally examined at four square areas of 0.49 m², under conservation and conventional tillage. Natural rainstorms were simulated by irrigating the plots with 2.8×10⁻³ M methylene blue solutions. For both soils the root crowns of the agricultural crop, wormcasts and stained soil sections as well as macroscopic conduits were traced on plastic sheets. The investigated soil depths were 0, 5, 10 and 20 cm for the both soils. For the Luvisol, the 30, 40, 50, 80 and 120 cm depths were also studied.For the Luvisol, the conservation tillage plot revealed pronounced vertical connectivity and continuity of the macropore network (maximum depth of stained pores=120 cm), while at the conventional tillage plot, continuous macropores were observed to soil depths of 50 cm, but mainly restricted to the ploughed topsoil (0–30 cm soil depth).For the Podzolluvisol, at the conservation tillage site extensive mulch residues prevented water transport beneath 5 cm soil depth. In contrast, at the conventionally tilled site stained water reached a depth of 20 cm. For all investigated plots on both soil types, the location of the root crowns of agricultural crop and of wormcasts was not related to percolation patterns.The results suggest that conservation tillage on silty soils under agricultural landuse could induce an increased water retention capacity reducing the significance of fast runoff components.     

Effects of animal manure and mineral fertilizer on the total carbon and nitrogen contents of soil size fractions

Summary Soil was sampled in autumn 1984 in the 132 field (sandy loam soil) of the Askov long-term experiments (started in 1894) and fractionated according to particle size using ultrasonic dispersion and sedimentation in water. The unmanured plot and plots given equivalent amounts of N (1923–1984 annual average, 121 kg N/ha) in either animal manure or mineral fertilizer were sampled to a depth of 15 cm, fractionated and analysed for C and N. Mineral fertilizer and animal manure increased the C and N content of whole soil, clay (<2 m) and silt (2–20 m) size fractions relative to unmanured samples, while the C content of the sand size fractions (fine sand 1, 20–63 m; fine sand 2, 63–200 m; coarse sand, 200–2000 m) was less affected. Clay contained 58% and 65°70 of the soil C and N, respectively. Corresponding values for silt were 30% and 26%, while sand accounted for 10% of the soil C. Fertilization did not influence this distribution pattern. The C : N ratio of the silt organic matter (14.3) was higher and that of clay (10.6) lower than whole-soil C:N ratios (12.0). Fertilization did not influence clay and silt C : N ratios. Animal manure caused similar relative increases in the organic matter content of clay and silt size fractions (36%). In contrast, mineral fertilizer only increased the organic matter content of silt by 21% and that of clay by 14%.     

Effect of deep ploughing on the water status of highly and less compacted soils for coconut (Cocos nucifera L.) production in Sri Lanka

Soil compaction limits soil water availability which adversely affects coconut production in Sri Lanka. Field experiments were conducted in coconut (Cocos nucifera L.) plantations with highly and less compacted soils in the intermediate climatic zone of Sri Lanka. Soil physical properties of sixteen major soil series planted with coconut were evaluated to select the most suitable soil series to investigate the effect of deep ploughing on soil water conservation. Soil compaction and soil water retention with respect to deep ploughing were monitored during the dry and rainy seasons using cone penetrometer and neutron scattering techniques, respectively. Evaluation of soil physical properties showed that the range of mean values of bulk density (BD) and soil penetration resistance (SPR) in the surface soil (0–10 cm depth) of major soil series in coconut lands was from 1.38 ± 0.02 to 1.57 ± 0.07 g/cm³ and 55 ± 10 to 315 ± 16.4 N/cm² respectively. The total available water fraction increased with clay content of soil as a result of high micropores. However, due to soil compaction, ability of soils to conserve water and to remain aerated was low for those series. Deep ploughing during the rainy and dry periods in highly compacted soils (BD > 1.5 g/cm³ and SPR > 250 N/cm²) greatly increased conserved soil water in the profile, while in less compacted soils (BD < 1.5 g/cm³ and SPR < 250 N/cm²) conserved water content was adversely affected. Soil water retention in bare soils of both highly and less compacted soil series was higher than that of live grass-covered soil. Amount of water conserved in ploughed Andigama series with respect to bare soils and grass-covered treatments during the severe dry period was 10.4 and 16.9 cm/m, while water storage reduction in the same treatments with ploughed Madampe series was 6.55 and 5.45 cm/m respectively. In addition, deep ploughing even in the effective root zone with live grass-covered highly compacted soils around coconut tree was favorable for soil water retention compared to that of live grass-covered less compacted soils.     

Movement of the protozoan pathogen Cryptosporidium parvum through three contrasting soil types

The potential for transfer of the protozoan pathogen Cryptosporidium parvum through soil to land drains and, subsequently, water courses following the application of livestock waste to land was monitored in the laboratory using simulated rainfall and intact soil cores. Following irrigation over a 21-day period, Cryptosporidium parvum oocysts applied to the surface of soil cores (initial inoculum concentration 1×10⁸ oocysts core^–1) were detected, albeit in low numbers, in the leachates from clay loam and silty loam soils but not in that from a loamy sand soil. Variations in leaching patterns were recorded between replicate cores. At the end of the study soil cores were destructively sampled to establish the location of oocysts remaining within the soil. Distribution within cores was similar in all three soil types. The majority (72.8+-5.2%) of oocysts were found in the top 2 cm of soil, with numbers decreasing with increasing depth to 13.2±2.8%, 8.39±1.4%, and 5.36±1.4% at depths of 10, 20, and 30 cm, respectively.     

Physical properties of tsunami-affected soils in Aceh, Indonesia: 2½ years after the tsunami

On 26 December 2004, a tsunami caused extensive loss of life, damaged property and degraded agricultural land in the province of Aceh, Indonesia. While some of the associated soil chemical changes have been documented, information on soil physical properties is sparse. The objective of this study was to quantify physical properties of some tsunami-affected upland agricultural soils in Aceh, Indonesia. Soil was sampled approximately 21/2 years after the tsunami, from the 0–0.1 m, 0.1–0.3 m and 0.3–0.5 m depths in four sites in the villages of Kling Cot Aroun in Aceh Besar sub-district, Kuta Kruen in Aceh Utara sub-district, Udjong Blang Mesjid in Bireuen sub-district and Meue in Pidie Jaya sub-district on the east coast of Aceh. These sites were located within 1 km from the sea at elevations ranging from 0 to 5 m ASL. The soils were Ultisols except for Meue, which was an Entisol. Soil properties measured were bulk density, structural stability and particle size distribution. Soil water retention, pore-size distribution and saturated hydraulic conductivity were estimated by inserting the values of bulk density, clay, sand and silt contents into pedotransfer functions from the literature. The analyses conducted during this study did not permit us to ascertain what proportion of the soil particles were of tsunami-origin. Nonetheless, deposition of finer-textured material may have occurred in two of the sites. In comparison with the greyish-white, coarse textured soil in the rest of the profile, a finer-textured yellow horizon was present in the lower slopes of the Udjong Blang Mesjid site. At Meue, clay and silt contents were higher in the surface 0.3 m than in the 0.3–0.5 m depth, although a distinct horizon was absent. Particle size distribution in all sites was dominated by the sand fraction, although clay and silt contents were relatively high (20–30 g 100 g^− 1) at Kuta Kruen. Among the sand fractions, fine sand (0.02–0.25 mm) was highest at Kling Cot Aroun, Kuta Kruen and in the “yellow horizon” at Udjong Blang Mesjid, making them more prone to hardsetting and compaction after intensive tillage. Soil compaction was present in all sites with that in the “yellow horizon” at Udjong Blang Mesjid being highest. The relatively low porosity in this layer may be beneficial, as it is likely to reduce the high rates of water drainage and nutrient leaching in this sandy soil. The more compacted soils were characterised by higher numbers of micropores (r, pore radius < 4.3 μm), lower water retention at saturation, smaller numbers of macropores (r > 14.3 μm), lower hydraulic conductivity and intensive gleying, indicating frequent waterlogging. The soils in all depths from Kling Cot Aroun and the “yellow horizon” at Udjong Blang Mesjid were very dispersive, that at Meue moderately dispersive in the 0.3–0.5 m depth but stable in the 0–0.1 m depth, and at Kuta Kruen very stable in all depths. Soil physical degradation was a feature of the soils examined, and its amelioration will be the key to improving and sustaining crop yields in these soils. Possible management interventions include organic amendments such as compost or manure, and minimum tillage options such permanent beds or zero tillage with retention of crop residues as in situ mulch together with suitable cover crops.     

Recovery of enchytraeid populations after severe drought events

The potential impact of changes in precipitation patterns associated with climate changes was investigated in Enchytraeidae (Oligochaeta) in a Danish heathland. The amount of precipitation was manipulated during spring and summer in an experimental field site in order to reveal effects of three different drought regimes: weak drought (WD), medium drought (MD) and high drought (HD). Enchytraeids were sampled every six to eight weeks (0–9 cm depth) for more than the eight months and soil water potential (SWP) and soil water content (SWC) was measured on a regular basis for five months.The enchytraeid communities were generally reduced due to a natural drought spell. The HD treatment significantly reduced the moisture level of the soil further with SWP below −15 bar (5 and 10 cm depth) and SWC around 5% (v/v) for more than two months. As a result almost no enchytraeids were found in HD plots after two months with high drought stress. Nevertheless, the HD-treated enchytraeid communities recovered within two months, as there was no significant difference in biomass and density of the different treatments at that time. During periods with extreme low SWP enchytraeids were practically absent in the top soil (0–3 cm), but a few animals were found in 3–6 cm. During this period SWP was around −15 bar even in 20 cm depth, indicating that active stages could not have survived. Thus, we suggest that the species present must be dependent on a drought tolerant stage, as vertical migration could not have supported the observed recovery.SWP and SWC were both significantly correlated with the total density and biomass of enchytraeids. However, density was better correlated with the SWP and SWC compared to biomass, which can be due to hatching of cocoons and increased fragmentation rate.     

Microbial biomass,N mineralization,and the activities of various enzymes in relation to nitrate leaching and root distribution in a slurry-amended grassland

High rates of cattle slurry application induce NO _inf3 ^sup- leaching from grassland soils. Therefore, field and lysimeter trials were conducted at Gumpenstein (Austria) to determine the residual effect of various rates of cattle slurry on microbial biomass, N mineralization, activities of soil enzymes, root densities, and N leaching in a grassland soil profile (Orthic Luvisol, sandy silt, pH 6.6). The cattle slurry applications corresponded to rates of 0, 96, 240, and 480 kg N ha^-1. N leaching was estimated in the lysimeter trial from 1981 to 1991. At a depth of 0.50 m, N leaching was elevated in the plot with the highest slurry application. In October 1991, deeper soil layers (0–10, 10–20, 20–30, 30–40, and 40–50 cm) from control and slurry-amended plots (480 kg N ha^-1) were investigated. Soil biological properties decreased with soil depth. N mineralization, nitrification, and enzymes involved in N cycling (protease, deaminase, and urease) were enhanced significantly (P<0.05) at all soil depths of the slurry-amended grassland. High rates of cattle slurry application reduced the weight of root dry matter and changed the root distribution in the different soil layers. In the slurry-amended plots the roots were mainly located in the topsoil (0–10 cm). As a result of this study, low root densities and high N mineralization rates are held to be the main reasons for NO _inf3 ^sup- leaching after heavy slurry applications on grassland.     

Effect of the initial soil moisture content on the spatial distribution of the water retention

The initial soil moisture content affects the water flow and solute transport through the vadose zone, but researchers are in disagreement about the extent and nature of its effects. Better understanding of the initial moisture effect on the water movement will help to prevent groundwater contamination and increase crop production by improving the efficiency of water use in irrigation practice. Therefore, in this study, the effect of the initial moisture content on the spatial distribution of the water retention was investigated in the field. A total of 4 cm of water was applied to duplicate plots with each of three initial moisture conditions within 2 h using a rainfall simulator. Following the application and a 2-h redistribution period, 100 soil samples were taken from different depths of each plot using a grid sampling system to be analyzed for their gravimetric water content in the laboratory. Statistical and geostatistical analyses were performed to analyze the spatial structure of the collected data. The results showed that the preferential flow was more evident in the case of the dry initial soil water content than for the two wetter initial conditions. Both the classical and geostatistical analyses supported that the overall water retention was uniformly distributed throughout the profile except at 20–30 cm, where the coefficient of variation and the percent nugget to total semivariance ratio were high, indicating some degree of preferential flow through large pores (macropores). These results suggest that similar studies should be conducted on different field soils under more different initial moisture conditions so that the effect of the macropores on the water flow and chemical transport can be better understood. Published in Russian in Pochvovedenie, 2008, No. 10, pp. 1241–1249.     

Influence of soil water on stress–strain behaviour of a compacting soil in semi-arid Kenya

Depending on the top and subsoil textures, semi-arid soils exhibit cohesive and frictional properties that are associated with the relatively high soil strength, bulk density and penetration resistance. The objective of this study was to gain the knowledge of mechanical properties of the compacting chromic luvisols in order to improve the design of tillage tools. Therefore, we applied critical state soil mechanics to study the stress–strain behaviour of the luvisols using triaxial tests under laboratory conditions. Field investigations involved random collection of undisturbed soil samples which were subjected to triaxial testing first by isotropic consolidation and compression and then triaxial shearing. Plots of deviatoric stress against axial strain were made to determine the soil shear strengths at the critical states over different soil water levels and the two soil depths of 0–20 cm for the plough and 20–40 cm for the hard pan layers, respectively. An exponential model used to fit the deviatoric stress–axial strain test data accurately predicted the trends. Soil water significantly influenced the shear strength, cohesion (c′) and internal angle of friction (′) and hence the mechanical behaviour of the luvisols. The regression equations developed showed that c′ and ′ have quadratic relationships with soil water. The very high clay bonding strength in the subsoil (hard pan) layer resulted in high shear strength, bulk density and penetration resistance values for this soil layer. The increase in shear strength with decreasing water content affected the deviatoric stress–axial strain relationships between the upper and lower plastic limits of the sandy soil. Thus, as the soil dried, the soil ceased to behave in the plastic (ductile flow) manner and thus began to break apart and crumble. The crumbling was indicative of brittle failure. The transition stage from an increase to a decrease in c′ and ′ values with soil water occurred in the soil water content range of 6–10%. Knowledge of stress–strain behaviour of compacting soils is of practical significance in the design of appropriate tillage tools for the specific soil type.     

Cropping system effects on carbohydrate content and water-stable aggregates in a calcareous soil of Central Iran

The effect of humus, readily decomposable organic matter, and carbohydrates of a nonspecific nature on the formation of water-stable aggregates in field rotation members of a calcareous soil in central Iran, Isfahan (fine loamy mixed thermic typic haplargid) has been studied. The study was carried out at Lavark experimental farm in plots receiving 0 (T1), 25 (T2), 50 (T3) and 100 (T4) Mg/ha of manure for 5 years successively with a cropping rotation of wheat (Triticum aestivum L.)–corn (Zea mays L.) every year and plots under similar chemical fertilizer management but with three different cropping rotations (T5, T6 and T7) that has been the prevalent cropping systems. Three replications of soil sample in each treatment and at the depths of 0–5 and 5–15 cm were used to measure organic carbon (OC), hot-water-soluble carbohydrate, dilute acid hydrolysable carbohydrate, cold-water-soluble carbohydrate and mean weight diameter of water stable aggregates. The highest amount of carbohydrate (700 mg/kg) and aggregate stability (0.8 mm) were obtained in plots with 100 Mg/ha manure (T4). The amount of carbohydrate extracted from soil samples decreased in the order of hot water, dilute acid and cold water extracts.Aggregate stability had a better correlation with hot water (r = 0.74**) and dilute acid-soluble carbohydrate (r = 0.73**) than organic carbon (r = 0.62**) content of soil. This indicates that the carbohydrate extracted by hot water and dilute acid may be a suitable indicator for showing soil quality, particularly in relation to soil aggregation.     

Denitrification under different cultivated plants: effects of soil moisture tension,nitrate concentration,and photosynthetic activity

Summary Plant effects on the denitrification rate were investigated in pot experiments at different soil moisture tensions and nitrate concentrations. Nitrate concentrations and the soil moisture tension were regulated immediately before each measurement. The effects of the plants on denitrification rates were dependent on the soil moisture tension. At a low soil moisture tension (–7 cm H₂O), there was a 10-fold increase in the denitrification rate (planted versus unplanted soil). At a medium moisture tension (–30 cm H₂O) the plants had practically no effect, and at the highest tension (–60 cm H₂O) the effect was slightly negative. Large differences in denitrification rates under different plant species were observed. At a low soil moisture tension, the average denitrification rate (g N kg^–1 soil h^–1) was 39–42 under small grains (barley, wheat, and oats), 47–82 under the grasses (cocksfoot, meadow grass, meadow fescue, and timothy) and 18 under red clover. The differences between the monocots were attributable to differences in plant growth rates, rather than to any specific difference in stimulation or inhibition of denitrification, since the variations in photosynthetic activity fairly well predicted the differences in denitrification rates under different monocots. Clover, however, gave much lower denitrification rates than those predicted by the photosynthetic activity.     

A comparison between seasonal changes in soil water storage and penetration resistance under conventional and conservation tillage systems in Aragón

Low and extremely variable precipitations limit dryland crop production in the semi-arid areas of Aragón (NE Spain). These areas are also affected by high annual rates of topsoil losses by both wind and water erosion. A long-term experiment to determine the feasibility of conservation tillage in the main winter barley production areas of Aragón was initiated in 1989 at four locations, three on loam to silt loam soils (Xerollic Calciorthid) and one on a silty clay loam (Fluventic Ustochrept), receiving between 300 and 600 mm of average annual rainfall. In this study, we compared, under both continuous cropping and cereal-fallow rotation, the effects of conventional tillage (mouldboard plough) and two conservation tillage systems, reduced tillage (chisel plough) and no-tillage, on soil water content and penetration resistance during the first two growing seasons. Whereas reduced and conventionally tilled treatments generally had similar soil water content during the experimental period, the effects of no-tillage were inconsistent. No-tilled plots had from 26% less to 17% more stored soil water (0–80 cm) than conventional tilled plots at the beginning of the growing season. In contrast to the conventional and reduced tillage treatments, penetration resistances were between 2 and 4 MPa after sowing in most of the plough layer (0–40 cm) under no-tillage at all sites. Fallow efficiencies in moisture storage in the cereal-fallow rotation, when compared with the continuous cropping system, ranged from −8.7 to 12%. The highest efficiencies were recorded when the rainfall in the months close to primary tillage exceeded 100 mm. Since this event is very unlikely, long fallowing (9–10 months) appears to be an inefficient practice for water conservation under both conventional and conservation management. Our results suggest that, up to now, only reduced tillage could replace conventional tillage without adverse effects on soil water content and penetration resistance in the dryland cereal-growing areas of Aragón.     

The spatial distribution of soil microbial biomass in a permanent row crop

The effects of soil texture (silt loam or sandy loam) and cultivation practice (green manure) on the size and spatial distribution of the microbial biomass and its metabolic quotient were investigated in soils planted with a permanent row crop of hops (Humulus lupulus). The soil both between and in the plant rows was sampled at three different depths (0–10, 10–20, and 20–30 cm). The silt loam had a higher overall microbial biomass C concentration (260 g g^-1) than the sandy loam (185 g g^-1), whereas the sandy loam had a higher (3.1 g CO₂-C mg^-1 microbial Ch^-1) metabolic quotient than the silt loam (2.6 g CO₂-C mg^-1 microbial C h^-1), on average over depth (0–30 cm) and over all treatments. There was a sharp decrease in the microbial biomass with increasing depth for all plots. However, this was more pronounced in the silt loam than in the sandy loam. There was no distinct influence of sampling depth on the metabolic quotient. The microbial biomass was considerably higher in the rows than between the rows, especially in the silt loam plots. There was no significant difference between plots without green manure and plots with green manure for either the microbial biomass or the metabolic quotient.