深松轮耕模式对黄土旱塬春玉米土壤理化性质和作物产量的影响

秸秆覆盖深松耕能保护土壤并调节土壤结构,但长期连续深松并不能获得持续增产与培肥土壤效果。为实现持续培肥增产的目的,于2007—2016年在黄土旱塬区连作春玉米田进行了深松轮耕长期定位试验,设置免耕/翻耕/深松(NT/CT/ST)与深松/翻耕(ST/CT)轮耕处理,并以连续深松(ST)为对照,对比分析不同耕作模式对土壤理化性质和玉米产量的影响。结果表明:与ST相比,NT/CT/ST处理机械稳定性团聚体(DR_0.25)和水稳定性团聚体(WR_0.25)较ST分别显著增加9.2%和21.9%,团聚体平均重量直径(MWD)和几何平均直径(GMD)显著增加;ST/CT处理WR_0.25显著增加了11.9%。在0～20 cm土层,NT/CT/ST和ST/CT处理土壤容重较ST分别显著降低7.0%和11.5%,土壤孔隙度分别增加8.4%和13.9%;在20～40 cm土层,ST/CT处理土壤容重增加了6.9%,土壤孔隙度降低了5.7%。NT/CT/ST比ST显著增加了土壤全氮和有机质含量,但降低了土壤全磷和全钾含量。NT/CT/ST处理玉米籽粒产量多年平均值较ST/CT和ST分别提高4.8%和10.2%。DR_0.25、WR_0.25、土壤全氮、有机碳含量均与产量呈显著正相关,说明深松轮耕模式提高了DR_0.25、WR_0.25、土壤全氮、有机碳含量,有利于玉米籽粒产量提高。综合考虑土壤肥力和玉米产量,NT/CT/ST处理有利于培肥土壤,提高土壤结构稳定性,并且可获得较高的玉米产量,为推荐轮耕模式。     

典型黑土耕作区土壤结构对季节性冻融的响应

以东北典型黑土耕作区土壤为研究对象,通过对一季冻融交替后土壤团聚体、土壤楔入阻力、容重、孔隙度、饱和度、广义土壤结构指数等指标的测定和分析,对比研究了季节性冻融对黑土耕作土壤结构特征的影响.结果表明:季节性冻融加剧了黑土耕作区土壤风干团聚体的分散,但显著降低了水稳性团聚体的破坏率(P0.05),表现出促进其团聚的作用;季节性冻融后黑土土壤楔入阻力降低了15.45%;容重趋于一致,在1.10～1.11g.cm-3之间;固相比例持续增加,总孔隙度与毛管孔隙度均不同程度降低;土壤饱和度总体增加了13.06%,削弱了土壤潜在贮水能力.季节性冻融改善了耕作区的土壤结构,使之更适于耕作.虽然季节性冻融增强了土壤团聚体的抗蚀性,却削弱了土壤的抗冲性,增加了黑土水蚀发生的风险.     

干旱荒漠区煤矸石山覆土区土壤水分物理性质的空间异质性

基于网格取样(20 m×20 m),采用经典统计学和地统计学相结合的方法,研究了干旱荒漠区煤矸石山表层(0~5 cm)土壤水分物理性质的空间异质性和分布格局。结果表明:研究区土壤容重、毛管孔隙度、毛管最大持水量、总孔隙度、饱和含水量表现为弱变异,土壤含水率表现为中等变异。除土壤容重的最佳拟合模型为高斯模型外,其余指标的最佳拟合模型均为指数模型;土壤容重和含水率的块基比[C₀/(C₀+C)]较小,空间自相关性强烈;土壤毛管孔隙度、毛管最大持水量、总孔隙度和土壤饱和含水量则表现为中等的空间自相关性;土壤容重与毛管孔隙度、毛管最大持水量、总孔隙度、饱和含水量呈极显著负相关,土壤含水率与其余指标没有显著的相关关系;土壤毛管孔隙度、毛管最大持水量、总孔隙度及饱和含水量两两之间呈极显著正相关关系;Kringing等值线图表明,土壤毛管孔隙度、毛管最大持水量、总孔隙度和饱和含水量分布类似,高值集中在坡中部及下部左侧,土壤容重则与之相反,土壤含水率的大小主要受坡位的影响,由坡上向坡下增大。建议干旱荒漠区煤矸石山覆土区在人工植被恢复时应采取整地措施,疏松根区土壤;在植被恢复初期,应适当提高坡上部的灌水量,以改善煤矸石山覆土区土壤水分状况,为植被恢复营造均一、良好的土壤水分物理条件。     

Root effects on soil water and hydraulic properties

Plants can affect soil moisture and the soil hydraulic properties both directly by root water uptake and indirectly by modifying the soil structure. Furthermore, water in plant roots is mostly neglected when studying soil hydraulic properties. In this contribution, we analyze effects of the moisture content inside roots as compared to bulk soil moisture contents and speculate on implications of non-capillary-bound root water for determination of soil moisture and calibration of soil hydraulic properties. In a field crop of maize (Zea mays) of 75 cm row spacing, we sampled the total soil volumes of 0.7 m × 0.4 m and 0.3 m deep plots at the time of tasseling. For each of the 84 soil cubes of 10 cm edge length, root mass and length as well as moisture content and soil bulk density were determined. Roots were separated in 3 size classes for which a mean root porosity of 0.82 was obtained from the relation between root dry mass density and root bulk density using pycnometers. The spatially distributed fractions of root water contents were compared with those of the water in capillary pores of the soil matrix. Water inside roots was mostly below 2–5% of total soil water content; however, locally near the plant rows it was up to 20%. The results suggest that soil moisture in roots should be separately considered. Upon drying, the relation between the soil and root water may change towards water remaining in roots. Relations depend especially on soil water retention properties, growth stages, and root distributions. Gravimetric soil water content measurement could be misleading and TDR probes providing an integrated signal are difficult to interpret. Root effects should be more intensively studied for improved field soil water balance calculations. Presented at the International Conference on Bioclimatology and Natural Hazards, Pol’ana nad Detvou, Slovakia, 17–20 September 2007.     

轮耕对关中一年两熟区土壤物理性状和冬小麦根系生长的影响

针对关中地区土壤连续单一耕作存在的主要问题,进行了土壤轮耕效应研究。2009年至2012年在关中一年两熟区采用连续4a旋耕(RT)、翻耕-免耕-翻耕-免耕(PNT)和深松-免耕-深松-免耕(SNT)3种耕作处理,对土壤容重、紧实度及小麦根系生长进行了研究。结果表明,与试验前相比,夏玉米收获后(2013年10月)两种轮耕处理显著(P0.05)降低了0—10、10—20 cm土壤容重,旋耕处理在0—10 cm处差异不显著,而10—20 cm土壤容重显著增大;与旋耕处理相比,两种轮耕处理0—10、10—20 cm土壤容重在第4季冬小麦整个生育期内变异系数较小,土壤紧实度较低,且改善效果在冬小麦生育中后期10—20 cm土层体现更为显著;旋耕处理0—10、10—20 cm土壤紧实度与含水量均呈显著负相关,相关系数分别为-0.89、-0.85,两种轮耕处理相关性不显著;0—40 cm土层根重密度和根系活力表现为:两种轮耕处理连年旋耕。可见,长期旋耕后进行轮耕(免耕与翻耕、深松)有利于改善土壤物理状况,促进作物根系生长。     

Variations of soil structure and microbial population in a compost amended soil

Changes in soil structure and in microbial population were recorded in a long term field experiment over the growing season of maize (June–November). Determinations were made on samples from plots which had received, for two years, the following treatments: mineral fertilizers, farmyard manure and three rates of compost. Seasonal variations were observed for the stability of soil aggregates, total porosity, pore size distribution, mycorrhizal infection and aerobic cellulolytic microorganisms. The stability of the soil aggregates changed in a similar way to that found for both mycorrhizal infection and the number of aerobic cellulolytic microorganisms. Physical characteristics were not affected in any instance by the organic dressings and microbiological populations were generally influenced only by the higher doses of compost.     

川南坡地不同退耕模式对土壤团粒结构分形特征的影响

运用分形模型,研究了川南坡地及其退耕成慈竹林、杂交竹林、桤木+慈竹混交林和弃耕地5年后土壤团粒结构分形维数,探讨了分形维数与土壤理化性质之间的关系.结果表明：退耕后,不同退耕模式样地＞0.25 mm的土壤团聚体和水稳性团聚体含量均显著增加,团粒结构分形维数介于1.377～2.826,为慈竹林＜杂交竹林＜桤木+慈竹混交林＜弃耕地＜农耕地,并随＞0.25 mm的土壤团聚体及水稳性团聚体含量的增加而降低;土壤自然含水量、毛管孔隙、有机质、全氮、碱解氮、全磷和全钾含量增加,而土壤容重、非毛管孔隙和通气度降低.退耕后的慈竹林、杂交竹林、桤木+慈竹混交林和弃耕地的土壤团粒结构分形维数与土壤理化性质相关性较好.农耕地退耕对增加＞0.25 mm的土壤团聚体及水稳性团聚体含量和提高土壤结构稳定性具有较好的作用;土壤团粒结构分形维数可以作为坡地退耕后土壤肥力变化的理想指标,在研究区坡地退耕种植慈竹具有较好的培肥改土效益.     

Effect of soil compaction on root growth and uptake of phosphorus

Summary Zea mays L. andLolium rigidum Gaud. were grown for 18 and 33 days respectively in pots containing three layers of soil each weighing 1 kg. The top and bottom layers were 100 mm deep and they had a bulk density of 1200 kg m^–3, while the central layer of soil was compacted to one of 12 bulk densities between 1200 and 1750 kg m^–3. The soil was labelled with³²P and³³P so that the contribution of the different layers of soil to the phosphorus content of the plant tops could be determined. Soil water potential was maintained between –20 and –100 kPa.Total dry weight of the plant tops and total root length were slightly affected by compaction of the soil, but root distribution was greatly altered. Compaction decreased root length in the compacted soil but increased root length in the overlying soil. Where bulk density was 1550 kg m^–3, root length in the compacted soil was about 0.5 of the maximum. At that density, the penetrometer resistance of the soil was 1.25 and 5.0 MPa and air porosity was 0.05 and 0.14 at water potentials of –20 and –100 kPa respectively, and daytime oxygen concentrations in the soil atmosphere at time of harvest were about 0.1 m³m^–3. Roots failed to grow completely through the compacted layer of soil at bulk densities 1550 kg m^–3. No differences were detected in the abilities of the two species to penetrate compacted soil.Ryegrass absorbed about twice as much phosphorus from uncompacted soil per unit length of root as did maize. Uptake of phosphorus from each layer of soil was related to the length of root in that layer, but differences in uptake between layers existed. Phosphorus uptake per unit length of root was higher from compacted than from uncompacted soil, particularly in the case of ryegrass at bulk densities of 1300–1500 kg m^–3.     

Effect of crop and soil management practices on soil compatibility in maize and groundnut plots in a Paleustult in Southeastern Nigeria

Frequent occurrences of soil compaction damage resulting from high raindrop impact energy, and from human and animal trafficking during field operations pose a problem to farmers around the tropics. We studied the effect of some crop and soil management practices (manure, mulch, NPK applications, tillage and crop type) on some soil compactibility indices (dry bulk density, cone index, total soil porosity, gravimetric soil water content) in a Typic Paleustult in southeastern Nigeria. The study was carried out for three consecutive planting seasons using two tillage systems and four other soil management practices (poultry droppings + NPK, mulch + NPK, NPK alone and no amendment). These were laid out as split-plot in a RCB design replicated three times and using maize (Zea mays L.) and groundnut (Arachis hypogea) as test crops. Results indicate that the different soil management techniques adopted influenced dry bulk density, penetration resistance, total soil porosity and gravimetric soil water content at 44 and 66 days after planting (DAP) whereas only gravimetric soil water content was affected at 90 DAP. The dry bulk density of tilled maize and groundnut plots increased significantly (P<0.05) by between 2 and 14% relative to no-till plots at 44 and 66 DAP. In both maize and groundnut plots, dry bulk density decreased significantly (P<0.05) in plots amended with poultry droppings +NPK relative to the control plots by 3–10% at 44 and 66 DAP. Tilled maize and groundnut plots had 37–45% lower (P< 0.05) penetration resistance than their corresponding no-till plots at both 44 and 66 DAP. Penetration resistance measurements were lower by 16.5–25% in plots amended with poultry droppings + NPK relative to unamended plots at 44 and 66 DAP. Cumulative (1996, 1997, 1998) data indicate that gravimetric soil water content in maize and groundnut plots generally increased significantly (P<0.05) in no-till plots relative to tilled plots by 18–27% at both 44 and 66 DAP. Plots amended with poultry droppings + NPK had between 24 and 111% increase (P<0.05) in soil gravimetric soil water content at both 44 and 66 DAP. Results are indicative that all soil compactibility indices measured were not affected at 90 DAP except for soil gravimetric soil water content in 1996 and 1998. Results from this work demonstrate that some crop and soil management practices could be used to reduce soil compactibility problems thus increasing productivity of such soils.     

Effects of legumes on soil physical quality in a maize crop

The effect of intercropped legumes and three N fertilizer rates in a continuous maize (Zea mays L.) cropping system on the physical properties of two soils were investigated for three years. The legumes, being a mixture of alfalfa, clover and hairy vetch, had a significant cumulative effect on some physical properties of both soil. The lowest stability and smallest mean weight diameter of soil aggregates were associated with monoculture maize plots. Aggregate size and stability were not affected by N fertilization at any of the rates of 0, 70, and 140 kg ha^-1 in intercropped plots, except that aggregate stability was actually reduced by N fertilization in one soil, the Ste. Rosalie clay. In maize plots in both soils, stability and size of soil aggregates were significantly increased with increased added N. Intercropped legumes significantly decreased dry bulk density and soil penetration resistance. Added N had no measurable influence on these compaction factors. Soil water properties were not significantly affected by either intercropping or N fertilization. Positive effects noted on soil aggregation and other physical properties in intercropped plots are the result of enhanced root activity, or incorporation of legumes as green manure, or both. Improvement of soil structure in maize plots associated with increasing N application was the result of increased maize-root residues.     

The effect of soil warming on bulk soil vs. rhizosphere respiration

There has been considerable debate on whether root/rhizosphere respiration or bulk soil respiration is more sensitive to long-term temperature changes. We investigated the response of belowground respiration to soil warming by 3 °C above ambient in bare soil plots and plots planted with wheat and maize. Initially, belowground respiration responded more to the soil warming in bare soil plots than in planted plots. However, as the growing season progressed, a greater soil-warming response developed in the planted plots as the contribution of root/rhizosphere respiration to belowground respiration declined. A negative correlation was observed between the contribution of root/rhizosphere respiration to total belowground respiration and the magnitude of the soil-warming response indicating that bulk soil respiration is more temperature sensitive than root/rhizosphere respiration. The dependence of root/rhizosphere respiration on substrate provision from photosynthesis is the most probable explanation for the observed lower temperature sensitivity of root/rhizosphere respiration. At harvest in late September, final crop biomass did not differ between the two soil temperature treatments in either the maize or wheat plots. Postharvest, flux measurements during the winter months indicated that the response of belowground respiration to the soil-warming treatment increased in magnitude (response equated to a Q ₁₀ value of 5.7 compared with ∼2.3 during the growing season). However, it appeared that this response was partly caused by a strong indirect effect of soil warming. When measurements were made at a common temperature, belowground respiration remained higher in the warmed subplots suggesting soil warming had maintained a more active microbial community through the winter months. It is proposed that any changes in winter temperatures, resulting from global warming, could alter the sink strength of terrestrial ecosystems considerably.     

Effects of soil compaction on the microbial populations of melon and maize rhizoplane

Effects of soil compaction on the microbial populations of melon and maize rhizoplane were investigated in quantity and quality. The numbers of culturable bacteria and fluorescent pseudomonads on the rhizoplane were higher when plants were grown in more compacted soil and the relative increase was larger in fluorescent pseudomonads. Total bacterial counts, however, did not appear to be affected by soil compaction, resulting in the increase in the culturable bacteria among total counts in more compacted soil. The determination of extracellular enzymatic properties (pectinase, -glucosidase, -glucosidase and -galactosidase) of each 100 isolates from bulk soil and root samples suggested that the microbial populations on the rhizoplane, especially when plants were grown in highly, compacted soil, were composed of high ratios of bacteria with abilities to utilize root exudates efficiently. The microbial community structure estimated from the colony forming curves of bulk soil and root samples suggested that the microbial populations on the rhizoplane, especially when plants were grown in compacted soil, were likely to be composed of more r-strategists which were defined as those who formed colonies within 2 days.     

Wetting and drying cycles in the maize rhizosphere under controlled conditions. Mechanics of the root-adhering soil

Mechanical properties of the topsoil (sandy Podsol and silty Luvisol, FAO) adhering to maize (Zea mays L.) roots and its bulk soil counterpart were studied as a function of soil texture and final soil water suction at harvest, with three soil water suction values of approximately 30, 50 and 60 kPa. Two scales of observation were also selected: the whole soil:root system and the root-adhering soil aggregates. Three methods were used to characterize the stability of the soil:root system: mechanical shaking in air, and dispersion by low-power ultrasonication, with or without preliminary immersion of the soil:root system in water. Soil disruption kinetics, which were fitted with first-order kinetics equations, were analyzed and discussed. For example, silty soil ultrasonication kinetics, without preliminary water-immersion, could be divided into two parts: the first faster part, which was characterized by a mean rate K value of 6.8–7.2 mJ^-1, is attributed to soil slaking, whereas the second slower part, which was characterized by a mean rate K value of 1.5–1.6 mJ^-1, was attributed to the rupture of the `firmly root-adhering soil' from the roots. A clear plant effect was observed for both aggregate tensile strength and friability, with higher aggregate strength for the root-adhering silty soil (450–500 kPa) than for its bulk silty soil counterpart (410–420 kPa), and lower friability (coefficient of variation of the aggregate strength) for the root-adhering silty soil (e.g. 67% at a soil water suction value of 30 kPa) than for its bulk silty soil counterpart (e.g. 49% at asoil water suction value of 30 kPa). These effects were attributed to root exudation, which was significantly higher for the driest silty topsoil than for the wetter ones. In conclusion, the mechanical properties of the silty topsoil adhering to the maize roots are attributed to both physical and biological interactions occurring in the maize rhizosphere. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同土壤水分条件下容重对玉米生长的影响

用玉米作为实验材料。进行分根实验研究不同土壤水分条件下容重对玉米生长的影响,种子根平分在装有塿土的分隔的白铁皮桶中,土壤容重分4种处理：低容重(两边容重都为1．20g·cm^-3)、中容重(两边容重都为1．33g·cm^-3)、高容重(两边容重都为1．45g·cm^-3)和混合容重(一边为1．20g·cm^-3,另一边为1．45g·cm^-3),土壤水分控制在高基质势(-0．17MPa)和低基质势(-0．86MPa)两个水平,结果表明,当植株生长在紧实土壤或土壤基质势从-0．17MPa降到-0．86MPa时。根长、根干重和地上部干重都显著降低,并且地上部干重的降幅更大,紧实土壤使根长降低的同时还使根的直径增大,无论是容重增大还是土壤水分含量降低所引起的高土壤阻力都使叶片扩展速度降低和植株变小,生长在紧实土壤中的植株变小不仅是因为叶片扩展速度降低,同时是成熟叶片叶面积缩小的结果。然而,当植株生长在混合容重土壤中时,处在低容重土壤中的根系生长得到加强,补偿甚至超补偿高容重土壤中根系生长的不足,整个植株的生长状况与低容重土壤中生长的植株接近。     

不同土地利用方式对黄河三角洲土壤物理特性的影响

黄河三角洲是我国成土最快的河口三角洲之一,探索其土地利用过程中不同土地利用方式对土壤物理性质的影响,对该区土壤肥力保持和土地资源的持续利用具有重要意义。选择黄河三角洲棉田、麦田、苇地、碱蓬地和裸地等5种不同的土地利用方式,通过野外调查与室内分析,研究不同土地利用方式下土壤主要物理特性的变异特征及影响因素。结果显示,与裸地土壤相比,有植被土地利用方式土壤容重降低,土壤孔隙度、团聚体水稳性、饱和含水量与毛管含水量也有相应的提高;土壤有机碳和速效氮、有效磷含量均有显著增加,土壤总盐分含量呈显著降低趋势。在所研究土壤中,土壤物理性质依麦田-棉田-苇地-碱蓬地-裸地的次序从最佳向最差过渡。逐步回归分析和相关分析表明土壤容重、团聚体平均重量直径和毛管孔隙度是土壤毛管含水量的主要影响因子,团聚体水稳性主要由大于0.25 mm水稳性团聚体含量和毛管孔隙度决定,土壤总盐分含量影响土壤饱和导水率;大于0.25 mm水稳性团聚体含量分别与土壤有机碳含量(r=0.8323)、速效氮含量(r=0.7558)和有效磷含量(r=0.9049)具有正相关关系。因此,黄河三角洲地区土地利用应以增加有机质的投入,提高土壤水稳性团聚体形成为基础,促进土壤良好结构形成。这些结果为该区土壤肥力提高和土壤资源可持续利用提供参考依据。     

耕层调控与有机肥处理下麦田土壤和小麦冠层结构特性及其相互关系

本研究基于5年的耕作定位试验,设置深耕(DT)、深耕有机肥(DTF)、浅耕(ST)、浅耕有机肥(STF)、免耕(NT)和免耕有机肥(NTF)处理,以期通过改良耕层土壤结构,优化小麦冠层结构特性.结果表明: 同一耕作处理下,增施有机肥可降低土壤容重、提高土壤孔隙度,提高20～40 cm土层2～5和0.25～2 mm粒级土壤团聚体含量,降低>5 mm粒级团聚体含量、>0.25 mm粒级团聚体的平均质量直径(MWD)和几何平均直径(GMD).与其他处理相比,NTF处理改善了0～20 cm土层土壤容重、增加土壤孔隙度;DTF处理降低了40～60 cm土壤容重和>0.25 mm粒级机械团聚体的稳定性,增加了土壤透气性.花后各时期,有机肥处理的叶片角度指数降低,叶面积指数(LAI)和旗叶净光合速率(P_n)提高.STF处理的角度指数最低,DTF处理的P_n最高,显著大于其他处理.通径分析表明,自变量容重、孔隙度、>0.25 mm粒级团聚体的数量(R_0.25)和MWD对因变量角度指数、LAI和P_n的直接通径系数均达到极显著水平.0～20 cm土层,MWD值增大有利于P_n和LAI的提高;20～40 cm土层,土壤容重在一定范围内的增加可优化叶夹角,提高冠层透光率;40～60 cm土层,高的土壤容重和低的孔隙度限制了LAI和P_n的增加.综上,豫中补灌区增施有机肥下的深耕或浅耕处理有利于改良土壤结构、增加土壤通透性,优化冠层结构,提高冠层受光率、叶面积指数和光合速率.     

Soil aggregate sequestration of cover crop root and shoot-derived nitrogen

Cover crop roots and shoots release carbon (C) and nitrogen (N) compounds in situ during their decomposition. Depending upon the season, these C and N compounds may be sequestered, the C may be respired or the N may be leached below the root zone. A field study was established to identify the contributions of cover crop root and shoot N to different regions within aggregates in the A_p horizon of a Kalamazoo loam soil. Fall-planted rye plants (Secale cerealeL.) were labeled the next May with foliar applications of solutions containing 99% atom (¹⁵NH₄)₂SO₄. Isotopic enrichment of soil aggregates ranging from 2.0 to 4.0, 4.0–6.3 and 6.3–9.5 mm across was determined following plant residue applications. Concentric layers of aggregates were removed from each aggregate by newly designed meso soil aggregate erosion (SAE) chambers. Non-uniform distributions of total N and recently derived rye N in soil macroaggregates, across time, suggested that the formations and functions of macroaggregates are very dynamics processes and soil aggregates influence where N is deposited. Early in the season, more ¹⁵N migrated to the interior regions of the smallest aggregates, 2–4 mm across, but it was limited to only surfaces and transitional regions of the larger aggregates, 6.3–9.3 mm across. Exterior layers of aggregates between 6.0 and 9.5 mm retained 1.6% of the N_{derived from roots} in July 1999, which was three times more than their interior regions. This was slightly greater than the % N_{derived from shoot.} One month later, as the maize root absorption of N increased rapidly, % N_{derived from roots} and % N_{derived from shoot} were nearly equal in exterior layers and interior regions of soil aggregates. This equilibrium distribution may have been from either greater diffusion of N within the aggregates and/or maize root removal form aggregate exteriors. Results supported that most of roots grew preferentially around surfaces of soil aggregates rather than through aggregates. Cover crop roots contributed as much N as cover crop shoots to the total soil N pool. Subsequent crops use N from the most easily accessible zones of soil structure, which are surfaces of larger soil aggregates. Therefore maintaining active plant roots and aggregated soil structure in the soil enhances N sequestration and maximize soil N availability. These studies suggest that the rapid and perhaps bulk flow of soil N solutions may bypass many of the central regions of soil aggregates, resulting in greater leaching losses.     

Subsoil gravel horizon and maize root growth

Summary A series of factoral experiments involving the effects of gravel concentration in the subsoil horizon, the effect of inter-gravel bulk density, the depth of the surface soil above the gravel horizon and the interaction between soil moisture regime and gravel concentration, on growth and development of maize roots were conducted in the greenhouse. Increasing sub soil gravel concentration decreased the total porosity and the available waterholding capacity. Though the root growth of maize seedlings was adversely affected by gravel concentration above 20 per cent, lower concentration of gravels had a beneficial effect on roots. The root development and the leaf concentration of N, P and K improved with increasing depth of surface soil above the gravel horizon. The symptoms of mechanical impedance, such as thickening of root tips and profuse branching behind the tip were commonly observed on the root axis penetrating the gravelly horizon. re]19750916     

Microbiological and chemical properties of soil associated with macropores at different depths in a red-duplex soil in NSW Australia

Some agricultural soils in South Eastern Australia with duplex profiles have subsoils with high bulk density, which may limit root penetration, water uptake and crop yield. In these soils, a large proportion (up to 80%) of plant roots maybe preferentially located within the macropores or in the soil within 1–10 mm of the macropores, a zone defined as the macropore sheath (MPS). The chemical and microbiological properties of MPS soil manually dissected from a 1–3 mm wide region surrounding the macropores was compared with that of adjacent bulk soil (>10 mm from macropores) at 4 soil depths (0–20 cm, 20–40 cm, 40–60 cm and 60–80 cm). Compared to the bulk soil, the MPS soil had higher organic C, total N, bicarbonate-extractable P, Ca⁺, Cu, Fe and Mn and supported higher populations of bacteria, fungi, actinomycetes, Pseudomonas spp., Bacillus spp., cellulolytic bacteria, cellulolytic fungi, nitrifying bacteria and the root pathogen Pythium.In addition, analysis of carbon substrate utilization patterns showed the microbial community associated with the MPS soil to have higher metabolic activity and greater functional diversity than the microbial community associated with the bulk soil at all soil depths. Phospholipid fatty acids associated with bacteria and fungi were also shown to be present in higher relative amounts in the MPS soil compared to the bulk soil. Whilst populations of microbial functional groups in the MPS and the bulk soil declined with increasing soil depth, the differentiation between the two soils in microbiological properties occurred at all soil depths. Soil aggregates (< 0.5 mm diameter) associated with plant roots located within macropores were found to support a microbial community that was quantitatively and functionally different to that in the MPS soil and the bulk soil at all soil depths. The microbial community associated with these soil aggregates thus represented a third recognizable environment for plant roots and microorganisms in the subsoil.     

陕西榆林春玉米高产田土壤理化性状及根系分布

调查分析了陕西榆林2块19500 kg·hm^-2以上超高产春玉米田的产量构成、干物质分配和0～100 cm土层根系分布及土壤理化性状指标.结果表明：其种植密度为105000～123000株·hm^-2、成穗率97.7%～102.2%、千粒重320 g以上,果穗干物质积累量占整株干物质积累量的60.2%～65.5%.0～100 cm土壤平均容重为1.28～1.33 g·cm^-3,层间（每层20 cm）土壤容重、孔隙度和田间持水量均呈“M”型变化.玉米根系主要分布在0～60 cm,0～20 cm土层根系量占根系总量的64.8%～72.1%,20～60 cm土层根系量占根系总量的23.30%～28.17%.根系分布与土壤理化性状关系密切,0～20 cm土层玉米的根系量与土壤有机质、全氮和全磷含量呈显著正相关,20～60 cm土层根系量与土壤容重和田间持水量显著相关.因此,选择通透性和保水保肥能力良好的土壤,实行宽窄行双株密植栽培是获得玉米高产的关键.