Spatial analysis of saturated hydraulic conductivity in a soil with macropores

Saturated hydraulic conductivity (K_S) is an important soil hydraulic parameter for it establishes a limit on the rate of water and solute transmission through soil. However, its determination in the laboratory has been shown to be much influenced by column size. We evaluated the spatial variability of laboratory K_S measurements using three different column sizes: firstly, sixty 5.1 cm long columns of 5 cm diameter were used (type-I), next, thirty 20 cm long and 20 cm diameter columns were considered (type-II), and finally, thirty columns 100 cm long and of 30 cm diameter (type-III) were studied. All columns were collected along a transect in a sandy loam soil with macropores. Estimates of macroporosity at three depths (2.5, 12.5, and 16.5 cm) for twenty-four of the type-II columns were calculated from stained dye patterns obtained during ponded infiltration. The geometric mean of K_S decreased with increasing column size, i.e., from 2.24, 1.68 to 0.56 cm/h for type-I, -II, and -III columns, respectively. The coefficient of variation (CV) based on a log-normal distribution showed a similar trend: 619% for type-I, 217% for type-II, and 105% for type-III. Type-II and type-III columns were large enough to encompass a representative elementary volume (REV). The percentage of dye-staining (macropore cross-sectional area) decreased from 3% at 2.5 cm to 1.7% and 1.6% at 12.5 and 16.5 cm, respectively. Percentage of depth-averaged macropore area was moderately variable with CV = 51%. A geostatistical analysis revealed that a weak spatial structure existed for type-I K_S measurements whereas type-II and type-III columns displayed better spatial correlation with a range of approximately 14 m and 11 m, respectively. Spatial correlation was also observed for depth-averaged macropore area with a range of 12 m. The cross-semivariogram calculated between type-II K_S values and depth-averaged macropore area obtained from the same columns indicated positive spatial cross-correlation for all lags.     

Soil spatial variability along transects

In this study, soil properties were measured along two parallel 100-m transects separated by 60 cm. Soil samples were taken at the 30 and 60 cm depths at 1 m intervals along each transect, for a total of 400 samples. Soil-water-characteristic curves, determined in the laboratory on these samples, were fitted to van Genuchten's five-parameter model which was simplified by assuming that the residual water content was negligible. Particle size measurements from each soil sample were fitted to a three-parameter model. In addition, saturated hydraulic conductivity K_S and bulk density were measured. Several spatial correlations beyond lag 0 were found to be statistically significant for the van Genuchten model parameter n, for lnK_s and for B₁, which is an empirical parameter in the particle size distribution model. The cross-correlation functions also contain significant values beyond lag 0 for n versus lnK_s, n versus B₁ and lnK_s versus B₁. The parameter B₁ was positively correlated with lnK_s and n. Most of those properties, especially B₁ and n, revealed a strong periodic behaviour with a main cycle of 100 and 50 m at the 30-cm and 60-cm depths, respectively. Removing these cycles resulted in short lags with a spatial dependence of only 1 m. A state-space analysis was employed for predicting saturated hydraulic conductivity. The best result was obtained using a four-dimensional model containing Θ_s, n, lnK_s and bulk density.     

秸秆还田对滨海粘质盐土物理性状和棉花产量的影响

为了研究秸秆还田对滨海粘质盐土物理性状的改良效果,在黄河三角洲滨海粘质盐化潮土区进行田间试验,设置了秸秆用量为0、1.5、3.0、4.5 t/hm24个不同处理,探讨不同秸秆还田量对土壤容重、孔隙状况、饱和导水率、土壤盐分含量和分布以及棉花产量产生的影响。研究结果表明:在0~20 cm的秸秆耕翻层,随秸秆还田量的增加,土壤容重降低,土壤总孔隙度增加;与对照处理相比,3个秸秆处理的土壤通气孔隙比例分别增加2.70%、9.52%和12.16%(P0.05),土壤饱和导水率提高56.28%、100.93%和161.4%(P0.01),0~20 cm表层土壤含盐量分别下降了28.08%、50.75%和40.71%(P0.01)。秸秆粉碎还田会明显改善土壤通气状况和土壤水分入渗性能,有效抑制表层土壤盐分积累,对滨海粘质盐土改良具有重要作用,是盐碱土改良的重要途径。但是,要注意控制秸秆还田量,以3.0 t/hm2的秸秆还田量为宜。     

Chemical changes in a saline-sodic soil after gypsum application and cropping

Reclamation is needed on three million ha of slowly permeable saline-sodic soils in the Indus Plain of Pakistan. Previous studies measured an increased field-saturated hydraulic conductivity (K_fs) in the soil under study with cropping and gypsum application. This field experiment was conducted on a low permeability, saline-sodic soil (a fine-loamy, mixed thermic Typic Natrustalf) to compare the leaching of sodium and soluble salts and changes in chemical properties after various treatments. Treatments were: (i) perennial alfalfa (Madicago sativa L.), (ii) a rotation of sesbania [Sesbania bispinosa (Jacq.)W.F. Wright]-wheat (Triticum aestivum L.)-sesbania, (iii) incorporated wheat straw at 7.5 Mg ha⁻¹ and (iv) a fallow control. These four treatments were each combined with and without 25 Mg ha⁻¹ of gypsum and open-ditch drainage. Electrical conductivity (EC), pH, Na⁺, Ca²⁺, Mg²⁺ and Cl⁻ of the soil in the saturated paste extract under each treatment were measured in each 20 cm increment to 120 cm after 6 month and 1 yr. Gypsum application increased the soluble Na⁺ in the top 20 cm soil. Poor internal drainage of the soil caused the exchanged Na⁺ to remain in the soil solution. However, one year after the treatments, the crop rotation with gypsum significantly decreased SAR, EC, pH and Cl⁻ in the top 20 cm of soil. Alfalfa decreased these same parameters when compared to fallow in the top 80 cm of soil in gypsum-treated plots. The open-ditch drainage was not helpful in reclamation of this soil. In general, for surface soil improvement, a combination of added gypsum plus crop rotation was the best. For improvement of the deeper soil profile, gypsum plus alfalfa was the most effective of the treatments used.     

保护性耕作对潮土物理性质的影响

为了研究保护性耕作对土壤物理性质的影响,以黄淮海平原典型潮土区小麦—玉米轮作系统的长期定位试验为基础,探讨不同耕作方式与秸秆还田处理下土壤容重、饱和导水率、紧实度、含水量的变化。结果表明,0~20 cm土层,土壤容重、土壤紧实度随着翻耕频率的减少而增加,每两年翻耕一次(CNTW2T)和每四年翻耕一次(CNTW4T)的土壤紧实度比长期免耕分别降低了40%、17%,土壤紧实度与土层深度呈显著正相关;保护性耕作改变土壤导水性能,随着免耕年限的增加,土壤饱和导水率显著降低;少免耕土壤含水量比传统翻耕高0.77%~3.01%。秸秆覆盖能显著降低土壤紧实度,改良土壤压实状况,提高土壤饱和导水率,减少土壤水分蒸发,从而改善土壤保水能力。免耕有利于土壤蓄水保墒,但长期免耕易导致土壤压实,不利于作物生长,少耕(CNTW2T、CNTW4T)既可改善土壤导水、保水性能,又可减少土壤压实,因此在黄淮海平原实施少耕与秸秆还田相结合的保护性耕作有利于改良土壤物理性质,提高土壤保水能力。     

Effect of Ten Successive Rollings on Soil Physical Properties and Yield of Groundnut (var. TMV 3) in a Very Permeable Sandy Soil in Southern India

Trial in a farmer's holding, which was sandy containing 80-90 per cent of fine and coarse sand fraction, was conducted to evaluate the effect of ten successive rollings by 400 kg stone roller (1 meter width) on the soil physical properties and yield of groundnut (var. TMV 3). By rolling, the bulk density was increased from 1.50 g/cc to 1.63 g/cc in the 20-30 cm layer; Hydraulic conductivity was reduced from 36.7 cm/h to 17.3 cm/ h in the same subsoil layer (53 per cent over control) which resulted in higher moisture retension capacity of the soil, the increase being 51.5 per cent over control in the 30-40 cm layer. The final dry pod yield increase was 19 per cent over control. The effect of compaction was seen at the post-harvest soils also, in that the increase in bulk density, decrease in hydraulic conductivity were observed. The variations were more effective at 10-20 cm laver.     

条带深松对不同密度玉米群体根系空间分布的调节效应

为探究条带深松耕作(SS)对密植玉米群体根系空间分布与容纳量的调节效应,本试验设置3个种植密度(低密：4.50万株 hm^-2、中密：6.75万株 hm^-2、高密：9.00万株 hm^-2),以土壤免耕(NT)为对照,利用小立方原位根土取样器,通过“3D monolith”根系空间取样方法,比较研究玉米个体与群体根系的空间分布对种植密度与土壤耕作方式的响应。结果表明,单株根长受种植密度影响显著,在0~50 cm土层中(每10 cm为一土层),高密种植的单株根长较低密种植减少110.31、43.18、15.73、10.49和17.45 m;在高密种植条件下,与土壤免耕比,条带深松耕作增加20~30 cm、30~40 cm、40~50 cm土层中的单株根长13.32%、19.80%、47.20%;单株根干重与单株根长的变化一致。种植密度对群体总根长的影响不显著,却显著影响群体根系的空间分布。与低密种植比,高密种植的植株中心根长密度在0~10 cm、10~20 cm土层中分别降低3.82 cm cm^-3、0.62 cm cm^-3,但植株之间的根长密度在0~10 cm、10~20 cm、20~30 cm、30~40 cm土层中分别增加1.13 cm cm^-3、0.18 cm cm^-3、0.06 cm cm^-3、0.05 cm cm^-3;在高密种植条件下对土壤进行条带深松耕作,与土壤免耕比,植株中心的根长密度在0~10 cm土层中降低16.10%,在10~20 cm、20~30 cm土层中却分别增加47.45%和13.37%,植株之间的根长密度在20~30 cm、30~40 cm、40~50 cm土层中分别增加50.26%、30.72%和106.15%;条带深松耕作显著提高密植玉米群体下层根系的容纳量。高密条件下条带深松耕作增加了群体根干重、深层根系量、植株间根系分布及根表面积,进而增加了地上部群体叶面积指数及地上部干重,最终促进产量显著提高。说明密植群体通过条带深松耕作改善了群体的根系空间分布,减弱了上层根系的拥挤,通过增加深层土壤根系量及植株之间根系量增加了群体根系容纳量,发挥了密植群体根系功能,实现了密植群体的高产。     

Das Penetrationsvermögen von Wurzeln unterschiedlicher Roggen- und Triticalegenotypen in Abhangigkeit von der Bodenlagerungsdichte

The penetration capability of different rye and triticale varieties in dependence on the soil bulk density By means of a laboratory test the primary roots of 9 rye and 4 triticale varieties were tested for their capability of penetrating overcompactions. The roots served as “natural probes” and as a mark of genotypic differences. The test was carried out in the stages 1.50, 1.65 and 1.80 g/cm³ of soil bulk density. The soil substrate (faint loamy sand) consisted of 5.4 % of clay, 13.8 % of silt, 80.8 % of sand and 0.6 % of total C_t. The tested corn roots respond to compacted soil structures with considerable differences in the rootability. These level differences are not only specific marks for species but also for varieties within the species. Though most of the tested corn varieties respond with restricted root growth in compacted soil, the results cause the expectation that single varieties have a good adaptability to overcompacted soil and can tolerate it better than other ones. At the specific bulk density limit of faint loamy sand (1.50 g/cm³) all tested varieties have an intensive root growth. The rootability of triticale varieties is clearly reduced at the bulk density of 1.65 g/cm³; the roots of rye varieties, however, grow without essential restrictions in this range. Against all expectations the root growth of single rye varieties is similarly intensive as in loose soil at the bulk density of 1.80 g/cm³. Other rye varieties respond very sensitively and with considerable restrictions in root growth in this range. Since a characteristic increase of roots' diameter is to be observed with increasing bulk density, no differences in the root dry matter, caused by bulk density, can be found out. The applied experimental method permits a first selection of plant varieties, which could be interesting in future in view of the mark “Penetration performanc” of primary roots.     

A comparison of near-saturated hydraulic properties measured in small cores and large monoliths in a clay soil

The effect of measurement scale on hydraulic properties close to saturation was investigated in a clay soil. Results from measurements on undisturbed “standard” small soil core samples were compared with results from three large intact soil monolith samples which were assumed to reflect natural soil hydraulic behaviour. An intermediate sample size, which was used to characterize soil water retention relations (θ(ψ)) in each layer in the intact monoliths, was obtained by cutting the intact monoliths in three layers. The small cores were subsequently sampled from the cut monoliths so that all measurements were made on the same soil material. Measurements of θ(ψ) and saturated hydraulic conductivity (K_s) were compared, as well as the exponential relationships between K_s and macroporosity (ϕ_ma), the latter derived from the θ(ψ) data for small cores and from specific yields for intact monoliths. Between soil water pressure heads of −60 cm and −15 cm the small core data and the cut monolith data showed similar θ(ψ) relations, whereas they diverged closer to saturation. When comparing K_s and ϕ_ma, small core data were correlated with the intact monolith data although the small cores had smaller values. Thus, the “standard” small cores can satisfactorily predict the investigated soil hydraulic properties for a natural soil at pressure heads equal or less than −15 cm, but they may be in error, especially in soils with macropores, in the pressure head range −15 cm to saturation, i.e. in the largest macropores.     

Evaporation from soil in relation to residue rate,mixing depth,soil texture and evaporativity

The role of crop residues as a surface mulch on evaporation has been widely studied. But information on evaporation and its reduction by crop residues mixed in surface soil to different depths particularly in relation to soil texture and evaporativity (E_o) is lacking. We studied the effect of four rates of paddy straw, viz. 0, 2, 4 and 8 Mg ha⁻¹ used as mulch and mixed in top soil layer to two depths (2 and 5 cm) under two evaporativities (E_o's) viz. 2.0 ± 0.5 and 8.7 ± 1.5 mm day⁻¹ in silty clay loam and sandy loam soil columns of 0.95 m length and 0.1 m diameter. Cumulative evaporation was predicted from water transmission properties of the soil and E_o as influenced by these variables. The otherwise short-lived benefit of evaporation reduction with mulch per se, which peaked after a few days, plateaued when residue was mixed with soil at peak reduction, and as a result the benefit was prolonged. The maximum reduction achieved was more and sustained for a longer period in finer textured soil, and a higher rate of mulch mixed to a greater depth. Mixing of residue in the surface soil layer not only reduced evaporation but also resulted in higher water content in the near surface soil after drying.     

不同改良材料对粘质盐土物理性状和棉花产量的影响

为了研究不同类型的改良材料对滨海粘质盐土物理性状的改良效果,在黄河三角洲滨海粘质盐土区进行田间试验,设置了泥沙用量为20、40 t/hm~2,3种硅藻泥用量为1.5、3.0、4.5 t/hm~2和对照试验,探讨不同改良材料与用量对土壤容重、孔隙状况、饱和导水率、土壤水盐分布以及对棉花生长和产量的影响。研究结果表明:在0~20 cm表层土壤中,不同处理的土壤容重和总孔隙度无显著差异;与对照相比,3个硅藻泥处理的大孔隙数量比分别降低0.51%、2.53%和4.75%,土壤饱和导水率降低1.18%、15.88%、22.65%,土壤含盐量降低62.76%、72.01%和75.76%(P0.05),棉花产量降低2.8%、5.0%、11.1%;2个泥沙处理的大孔隙数量比分别增加了17.75%和37.09%,土壤饱和导水率增加17.94%、60.29%(P0.01),土壤含盐量降低23.17%、25.08%(P0.05),棉花产量增加11.1%、16.7%。引黄泥沙有效改善土壤质地和孔隙状况,增加土壤饱和导水率,适宜改良粘质盐土的物理性状。硅藻泥不适用粘质盐土物理性状的改良。因此,改良粘质盐土的关键是选择增加土壤大孔隙数量、比例,提高土壤饱和导水率的材料。     

Effect of Different Methods and Timings of Stand Establishment on Performance of Rainfed Lowland Rice Under 0–50 cm Water Depth

About one-third of the total rice is grown under rainfed lowland conditions, mostly m south and south-east Asia. Crop productivity in this ecosystem can be improved by adopting suitable management practices as drainage of excess water is not feasible in the catchment and coastal areas. Field experiments were conducted using a long-duration (165 days), photosensitive, semi-tall (150 cm) rice cultivar Utkalprabha established through direct sowing or transplanting on different dates under 0–50 cm water depth at Cuttack, India, during 1989–91. Direct sowing was done in lines in dry soil from 10 May onward using 400 seeds/m² and continued at 10 day intervals until June. Transplanting was done after accumulation of water in the field from July until 15 August with seedlings raised in nursery seed-beds with or without fertilizer application (100 kg N and 8.7 kg P and 16.7 kg K/ha) and tillers removed from the direct-sown crop. Seedling emergence varied significantly from 127–212/m, irrespective of sowing date and was dependent on rains received after sowing. However, the early sown crops in spite of poor germination, performed well due to better establishment and tiller production before water rose to higher depths in the field. There was a decreasing trend in grain yield, particularly when the sowing was delayed beyond end of May. The loss in yield with delayed sowing in June was due to poor crop stand which could not be compensated for by applying 50 % more seed (600/m²) and N fertilizer (60 kg N/ha). Removal of some of the tillers (100–130/m²) from crops sown on 30 May with 600 seeds/m² for planting on an equivalent plot area did not cause any adverse effect on the performance of mother crop. The anticipated shortfall in yield due to lower panicles/m² with clonal tiller separation was compensated for by the resulting increase in panicle weight. The performance of transplanted crops depended greatly on the water depth at or soon after planting. In 1990, planting on 15 July in 30 cm water depth helped in relatively better establishment and grain yield at par with sowing on 10 May. However, in 1991, when there was a sudden and rapid increase in water level to higher depths (50 cm) immediately after planting, the early planted crops produced only a negligible yield (0–1.2 t/ha). Highest yield was obtained from the crop planted with clonal tillers followed by that raised with fertilized and unfertilized nursery seedlings. Clonal tillers were taller (90 cm) and had more dry weight (1.78 g) compared with nursery seedlings (50–80 cm and 0.25–0.91 g). Therefore, the clonally propagated crop established well and acclimatized faster in the similar flooded environment, resulting in significantly higher grain yield particularly under late planted conditions. The results suggested early sowing by the end of May and transplanting with clonal tillers uprooted from the direct-sown crops for higher productivity of rice under uncontrolled excess water conditions.     

Thermal characterization of a tropical rice soil in relation to puddling,flood-water depth and percolation rate

Effects of puddling, flood-water depth and percolation rate on the thermal properties of a tropical rice soil were studied under field and glass-house conditions. Puddling increased volumetric heat capacity (C_V), but decreased thermal conductivity (Kt), thermal diffusivity (Dt) and damping depth (D) compared to a nonpuddled soil. A percolation rate of 40 mm d⁻¹ showed higher Kt, Dt and D than zero percolation. The values of Kt, Dt and D were highest with 50 mm and lowest with 10 mm flood- water depth. The C_v did not change with different flood-water depths or percolation rates. Consequently, puddling, 40 mm d⁻¹ percolation and 50–100 mm submergence kept the maximum temperature of surface 100–150 mm soil at relatively low level under tropical conditions. These treatments also buffered soil against extreme diurnal temperature fluctuations. The time lag between maximum solar radiation and maximum temperature of submerged soil varied, on an average, between 1.2 hours at soil surface and 11.6 hours at 300 mm soil depth; which is more than that in an upland soil.     

滴灌条件下不同盐水平对棉花根系分布的影响

 通过盆栽试验研究膜下滴灌土壤盐分分布对棉花根系生长的影响。结果表明：受滴灌水分运移的影响,在0～30 cm土层形成脱盐区,土壤盐分明显下降,养分含量也很低,30 cm以下是积盐区。土壤盐分对棉花根系分布影响显著,0.32 dS·m^-1、1.12 dS·m^-1和1.90 dS·m^-1处理总根长分别为26.35 m、49.54 m和34.40 m,但在土层表现不同。在0～30 cm脱盐区内,1.12 dS·m^-1和1.90 dS·m^-1处理的根长密度明显高于0.32 dS·m^-1处理,而在40～50 cm却低于0.32 dS·m^-1处理。研究认为,在盐胁迫条件下,棉花根系分布进行适应性变化,产生补偿效应,通过显著的增加脱盐区（0～30 cm土层）根系数量,来获得更多的水分、养分保证棉花生长的需要。     

种棉改良对粘质盐土物理性状及水盐动态的影响

为了研究棉花种植对滨海粘质盐土物理性状及水盐动态的影响,笔者在黄河三角洲滨海粘质盐化潮土区进行田间试验,设置了连续种棉改良3年和盐碱荒地2种不同处理,探讨种棉改良对土壤容重、饱和导水率和不同条件下土壤水盐动态的影响。结果表明:与盐碱荒地相比,种棉改良后0~20 cm土层的土壤容重降低了15%(P0.01),土壤饱和导水率增加了89.23%(P0.05)。在春季蒸发条件下,0~20、20~40、40~60、60~80、80~100 cm 5个土层中,种棉改良的土壤含盐量比盐碱荒地分别降低了52.07%、55.17%、55.23%、53.78%和47.43%。种棉改良还增加了少量降雨对土壤盐分的淋溶,有效抑制表层土壤盐分积累,使棉花生育期内的土壤耕作层含盐量低于3 g/kg,保证了棉花不同时期的正常生长,是盐碱荒地改良和高效利用的有效途径之一。     

Effects of land reclamation practices on physical properties of an acid,infertile Oxisol

Large areas of soils in tropical rainforests have been damaged by careless mechanical land clearing procedures. The objective of this study was to evaluate the effect on soil physical properties of various management practices to reclaim an acid, infertile Oxisol cleared by bulldozer. Crops would not grow on these soils due to low pH, poor fertility or poor physical properties. Properties in the 0- to 25-cm depth of the Typic Haplorthox soil, measured at the beginning of the study, were: 74% clay, bulk density 1.09 Mg m⁻³, pH 3.66, ECEC 8.9 cmol_c 1⁻¹, and available P 0.4 mg 1⁻¹. Land reclamation treatments included all combinations of six tillage practices and three fertility levels. Tillage practices were: hand hoeing to a depth of 15 cm; hoeing plus applying mulch; hoeing and incorporation of green manure; deep forking, i.e., hand digging to 30 cm with a digging fork; deep forking in alternate 20-cm wide strips; and roto-tilling to the 10-cm depth. Fertility levels were:

• 1.1. no fertilizer or lime added;
• 2.2. addition of lime to maintain an Al saturation near 40% and fertilizer to maintain nutrients just above their critical levels;
• 3.3. lime to neutralize all exchangeable A1 and fertilizer to satisfy the P fixation capacity of the soil and to maintain nutrients at optimum levels.

Soil physical properties monitored periodically while growing five consecutive crops were bulk density, mechanical impedance, and infiltration rate. Macroporosity was evaluated after growing the fifth crop. Deep forking and forking in strips decreased bulk density and mechanical impedance and increased macroporosity and infiltration rate. Deep tillage by forking resulted in the driest soil 10 days after rainfall and the rice crop on this treatment experienced temporary wilting. Increasing the fertility level increased root activity which in turn increased mechanical impedance (the soil was drier) and water infiltration rate. We conclude that the soil physical properties of this soil were damaged very little by the land clearing operation and that the inability of the soil to grow crops was due to other factors.     

Physical Properties Affecting Water Entry in some Humid Tropical Soil

Field and laboratory experiments were conducted to study the effect of physical properties on water entry in three soils, differing appreciably in texture. The results indicate that the saturated hydraulic conductivity (K) of lateritic soil was manifold higher than the alluvial soil. In lateritic soil the value of K was 10⁻¹ cm/hr and it ranged from 10⁻² to 10⁻³ cm/hr in alluvial soil. In general, the conductivity decreases in the sub surface layers and may be attributed high bulk density resulting from compact blocky structure of the soil. The infiltration rate of lateritic soil was found to be 4 times more as compared to the alluvial soils.     

Relations between pore-space and hydraulic properties in compacted beds of silty-loam aggregates

The aim of this study was to better understand features of the hydrodynamics of aggregated soil from experiments conducted on repacked soil samples. In the laboratory, aggregates 2 to 3 mm in diameter were prepared using a silty-loam material. These aggregates were wet to a water content close to their air-entry point value, and compacted to a predetermined bulk density. Two sets of samples were studied, with mean bulk density ranging between 1.0 and 1.6 Mg·m⁻³. Aggregates were extracted from samples of a first set, and their bulk density and water retention curve were determined. Water retention curve and unsaturated hydraulic conductivity of samples of a second set were determined using an instantaneous profile method. Experimental results were interpreted with respect to a functional pore-space classification. The water retention properties of the aggregates extracted from the samples were different, although differences in porosity between such aggregates were sometime too small to be measured. This was explained by a narrowing in the constrictions of intra-aggregate pores induced by compaction, even when this compaction was small. Discontinuities in the shape of the hydraulic properties of a loosely-compacted bed of aggregates were found. This was consistent with an independent desaturation of the inter- and intra-aggregate pores. However, unsaturated hydraulic conductivity of the loosely-compacted sample was affected by a small amount of water, which formed menisci between the aggregates close to the air entry point value of the aggregates. Soil hydraulic conductivity increased with the degree of compaction for a range of massbased water contents dryer than the water content at the air-entry point value of the aggregates. The influence of the contact surface between the aggregates on the unsaturated hydraulic conductivity of the samples was quantified, by assuming that water flows only through the aggregates. These experimental results are related to aspects of soil hydrodynamics, which are difficult to study in situ and difficult to predict with the existing models for the soil hydraulic properties.     

Hydraulic conductivity measurements of the spatial variability of a loamy soil

The spatial variability of the parameters K_fs and α of the exponential form of the hydraulic conductivity given by K(ψ)=K_fs exp (αψ) where ψ is the water pressure head in the soil has been examined on a bare agricultural soil, by performing surface infiltration measurements with the Guelph Pressure Infiltromer. Thirty two measurements were made at each node of a 4 m × 8 m grid. Both the early-time transient and steady-state flow rates, for three hydraulic heads (6, 16 and 25.5 cm) imposed at the soil surface have been analyzed. It is shown that the best estimates of the field-saturated hydraulic conductivity, K_fs and of the alpha parameter (α) are given by calculations based on the steady-state regime simultaneously applied to the three heads. The results indicate that both, K_fs and α are better described by a lognormal than a normal distribution. They also show that the parameters are autocorrelated up to about 25 m and to 20 m respectively and negatively spatially correlated together within a distance of 24 m. These findings tend to invalidate some assumptions classically used in the stochastic models of unsaturated water flow in porous materials.     

棉花秸秆还田对土壤微生物数量及酶活性的影响

为研究棉花秸秆还田对土壤微生物数量及酶活性的影响,于山东农业大学棉花科研基地德州市抬头寺经济开发区试验田进行试验,设棉花秸秆还田与未还田2个处理,研究连续4年棉花秸秆还田对0~60 cm土层土壤微生物数量及酶活性的影响。结果表明:棉花秸秆还田能够显著增加0~20 cm,20~40 cm,40~60 cm土层周年土壤微生物平均总数量,分别比未还田增加了19.87%,20.07%,56.15%;其中周年土壤细菌和真菌平均数量分别比未还田增加了20.91%、26.38%(0~20),20.59%、31.18%(20~40)和56.85%、32.30%(40~60),均达显著差异水平;周年土壤放线菌平均数量分别比未还田增加了4.29%,11.62%,54.00%,其中在20~40 cm和40~60 cm土层提高效果显著。棉花秸秆还田有利于提高土壤脲酶活性,0~20 cm,20~40 cm,40~60 cm土层土壤脲酶活性分别比未还田提高4.27%,13.43%和24.03%,其中对20~40 cm全部取样时期(9月除外)和除7月外40~60 cm土层其他取样时期的提高效果达显著差异水平;秸秆还田使土壤蔗糖酶活性在0~20 cm全部取样时期,除5月外20~40 cm土层其他取样时期,除8月外40~60 cm土层其他取样时期均得到了显著提高,分别比未还田提高了27.08%,46.96%,57.59%;除7,8月外,0~20 cm土层其他取样时期土壤过氧化氢酶活性在棉花秸秆还田条件下得到了显著提高,比未还田提高了8.73%,但除5,6月20~40 cm土层外,秸秆还田对20~40 cm和40~60 cm土层土壤过氧化氢酶活性的提高效果未达显著差异水平。以上结果说明持续棉花秸秆还田有利于保持和改善土壤的生物学特性。