龙山县植烟土壤 pH时空变异与影响因子研究

为研究龙山县植烟土壤 pH时空变异特征与其影响因子,分别于 2000与 2015年在龙山烟区采集土壤样品 191和 300个进行土壤 pH与各指标的测定,利用描述性统计、地统计学、相关分析、多重比较、逐步回归与等样本平滑回归对该地区土壤 pH的时空变异与其影响因子进行了研究。结果表明:从描述统计来看 15年间龙山县植烟土壤 pH均值由 6.32变为6.03,降幅为4.64%,极低(<5)的样本增加了19.05%,偏低(5.0～5.5)的样本增加了 1.1%,适宜(5.5～ 7.0)的样本降低了 19.45%;从时空变化来看,pH在 5.0～ 6.0范围的区域大幅增加,6.0～ 7.0范围的区域大幅减少,总体呈酸化趋势,但有小部分区域 pH呈碱化趋势,有两级分化现象。由相关分析与逐步回归分析得出土壤交换性钙、交换性镁为土壤 pH酸化的外在控制因子,有效硫与碱解氮为土壤酸化的外在致酸因子,土壤粘粒为土壤酸化的内在致酸因子,海拔也对 pH有重要影响。     

Changes in soil solution composition and pH in urine-affected areas of pasture

Changes in soil solution composition and concentrations of exchangeable cations and mineral N in undisturbed cores of pasture soil were investigated in two experiments following applications of sheep urine to the cores. The major cations applied in the urine were K⁺ and Na⁺, and the major anions were HCO₃^? and Cl^?. Addition of urine increased concentrations of exchangeable K⁺, Na⁺ and NH₄⁺ and measured ionic strength of the soil solution throughout the surface 15 cm of soil, demonstrating that the urine moved through the core by macropore flow immediately following addition. Immediately following urine application the ionic strength in soil solution in the surface 2.5 cm of soil increased from 4–6 MM to 24–41 mM. Hydrolysis of urine-urea was extremely rapid, and in less than 1 d high concentrations of NH₄⁺-N (i.e. 270–370 mg N kg^?1) had accumulated in the surface 0–2.5 cm of the urine patch, and soil pH had risen by over one unit. Nitrification then proceeded and, after approximately 15 d, NO₃^? became the dominant form of mineral N present. During nitrification, soil pH declined and the ionic strength of the soil solution increased substantially with NO₃^? becoming the dominant anion present in solution. There were concomitant increases in the concentrations of Ca²⁺ and, to a lesser extent, Mg²⁺ in the soil solution as NO₃^? concentrations increased. After approximately 30 d, concentrations of exchangeable NO₃^? had risen to 250–330 mg N kg^?1, soil solution NO₃^? concentrations had increased to about 80 mmol, dm^?3, and ionic strength in the soil solution had increased to 130–140 mM. These results demonstrate the dominating effect of N transformations in causing large fluctuations in the pH, ionic composition and ionic strength of the soil solution in the urine patch. It was concluded that nutrient availability in the patch was affected directly by nutrient addition in urine, and also probably indirectly through the fluctuations in soil solution pH and ionic strength that occur.     

河南省土壤pH值时空变化特征分析

p H值是土壤的重要化学性质,会对土地生产力产生重要影响。为掌握河南省土壤pH值近年来变化状况,本文采用空间统计方法,基于河南省第二次土壤普查及2015年土壤p H值数据对近30年该省土壤p H值时空变化及其影响因素进行了分析。结果表明:(1)河南省土壤p H值总体上呈下降趋势。全省平均土壤pH值由20世纪80年代的7.62降至2015年的7.17,下降了0.45个单位。p H值为碱性的土壤与中性土壤面积大幅度下降,弱碱性土壤和弱酸性土壤面积增加,增加比例为18.84%和22.42%。(2)第二次土壤普查结果河南省酸性土壤主要分布于南部地区、西部地区,土壤pH值最高的土壤分布于东部地区;2015年酸性土壤主要分布于南部地区,土壤pH值平均值为5.95;土壤pH值最高土壤主要分布于北部地区。(3)信阳中部的水稻土、南阳及驻马店山地丘陵的黄褐土和砂姜黑土上,水稻土、黄褐土和砂姜黑土成为pH值最低的三个土类。(4)广泛分布于河南省西部和北部山区的褐土pH值在各土类中最高,且近30年下降幅度最小;(5)近30年来土壤pH值的变化幅度在降水量小于1250mm时与其呈负相关,与海拔呈正相关。     

Carbon replacement and stability changes in short-term silvo-pastoral experiments in Colombian Amazonia

There is little information on the effects of land use change on soil Carbon stocks in Colombian Amazonia. Such information would be needed to assess the impact of this area on the global C cycle and the sustainability of agricultural systems that are replacing native forest. The aim of this study was to evaluate soil carbon stocks and changes after the clearing of the native forest, the establishment of pastures and the reclamation of the degraded pasture, in Caquetá, Colombia.We compared the contents of Total C, Oxidizable C and Non-Oxidizable (stable) C in four different land use systems, namely Monoculture (Brachiaria grassland), Association (Brachiaria + Arachis pintoi), Forage Bank (a mixture of forage tree species), and Natural Regeneration of the pasture in both a flat area and a sloping one. The Degraded Pasture was the reference.Results showed that in the sloping area all treatments have higher Total Carbon stocks than the Degraded Pasture, while three of the treatments significantly increased the stocks of Non-Oxidizable C.In the flat landscape, only the Association significantly increased Total C stocks. Plowing and fertilization cause significant increases in Oxidizable carbon and decreases in Non-Oxidizable carbon. This effect needs further research, as C stability will influence equilibrium stocks.In the sloping area, improved pastures and fodder bank rapidly increased Total Carbon contents and stocks, with increases as large as10 ton.ha^?1 yr^?1. In the Traditional Fodder Bank, which showed the largest increase, this is partially due to the application of organic manure. Surprisingly, also C stocks under Natural Regeneration were significantly higher than under the original Degraded Grassland. This increase was fully due to Non-Oxidizable Carbon, which is difficult to explain.Stable isotope analysis indicated that under improved grassland, especially Brachiaria monoculture, up to 40% of the original C in the upper 10 cm was replaced in 3.3 years.     

Temporal variation in soil microbial communities in Alpine tundra

Temporal variation in soil microbial communities was studied at a mid-alpine environment in Latnjajaure, northern Sweden, using phospholipid fatty acid (PLFA) analysis. The results show two seasonal shifts in microbial composition. The first shift was associated with snowmelt and mainly related to a decrease in fungal PLFAs, accompanied by an increase in branched 17:0 and methylated PLFAs (biomarkers for Gram-positive- and actinobacteria, respectively), resulting in a decrease in the ratio of fungi-to-bacteria. The second shift occurred across the growing season, and was associated with a switch from shorter to longer PLFAs and an increase in 18:1ω7 (biomarker for Gram-negative bacteria). Vegetation, snow cover dynamics, and N turnover seem to be of minor importance to broad-scale microbial community structure in this area.     

Effects of aggregate size,soil compaction,and bovine urine on N2O emissions from a pasture soil

The dominant N₂O emission source in New Zealand, calculated using the Intergovernmental Panel on Climate Change methodology, is agricultural soils. The largest source of N₂O emissions in New Zealand occurs as a result of excreta deposition onto pasture during grazing. There is a dearth of studies examining the effect of soil compaction and soil aggregate size on N₂O emissions from urine patches in grazed pastures. In this study, we repacked soil cores with four different soil aggregate sizes (<1.0–5.6 mm diameter), applied bovine urine, and then subjected the soil cores to four levels of soil compaction. Fluxes of N₂O were monitored for 37 days after which soil cores were allowed to dry out prior to a rewetting event. There was an interaction between aggregate size and soil compaction with the cumulative loss of N₂O over the first 37 days ranging from 0.3% to 9.6% of the urine-N applied. The highest N₂O emissions occurred from the smallest and most compacted aggregates. Even under the highest levels of compaction the N₂O loss from the large aggregates (4.0–5.6 mm diameter) was <1% of the urine-N applied. Reasons for the observed differences in the N₂O flux from the different-sized aggregates included varying gas diffusivities and higher rates of denitrification in the smallest aggregates, as evidenced by the disappearance of nitrate.     

Effects of the nitrification inhibitor dicyandiamide on soil mineral N, pasture yield, nutrient uptake and pasture quality in a grazed pasture system

Recent lysimeter studies have demonstrated that the nitrification inhibitor, dicyandiamide (DCD), can reduce nitrate (NO) leaching losses from cow urine patches in grazed pasture systems. The objective of this study was to quantify the effects of fine particle suspension (FPS) DCD on soil mineral N components, pasture yield, nutrient uptake and pasture quality under grazed pasture conditions. A field study was conducted on the Lincoln University dairy farm, Canterbury, New Zealand, from 2002 to 2006. FPS DCD was applied to grazed pasture plots at 10 kg ha^?1 in early May in addition to applied cow urine patches at a nitrogen (N) loading rate of 1000 kg N ha^?1, with DCD reapplied in early August. Soil mineral N levels in the urine patches were monitored. Pasture yield, N and cation concentrations and uptake were measured in treatment urine patches and inter‐urine areas of the pasture. Comparisons were made with control plots which did not receive DCD. NO levels under the DCD‐treated urine patches (0–7.5 cm) were in the order of 10 kg N ha^?1 compared with 40–80 kg N ha^?1 under untreated patches, and soil ammonium (NH) levels were consistently higher under the DCD‐treated patches. The DCD significantly and consistently increased pasture yield in both the urine patches, and inter‐urine areas of the pasture in all 4 years of the trial. Mean annual dry matter (DM) yields over 4 years were inter‐urine areas, 10.3; inter‐urine + DCD, 12.4; urine, 12.4 and urine +DCD 16.0 t DM ha^?1, representing an average DM yield increase of 20 and 29% in inter‐urine and urine patch areas, respectively. On a whole paddock basis, the increase in annual DM yield resulting from DCD application was estimated to be 21%. N, calcium (Ca), magnesium (Mg) and potassium (K) concentrations in pasture were unaffected by treatment with DCD; however, total annual uptake of these nutrients by pasture was significantly higher in all years where DCD had been applied. Pasture DM, protein, carbohydrate, metabolizable energy and fibre levels and sward clover content were not affected by treatment with DCD. The results demonstrate the agronomic value of the DCD treatment in addition to the environmental benefits in a grazed pasture system.     

盐渍土二次相变温度变化规律

孔隙溶液中水分的冻结与盐分的结晶是盐渍土产生冻胀和盐胀的前提,确定盐渍土孔隙溶液的相变机理有利于清晰认识冻胀和盐胀在盐渍土低温变形中的贡献。为了深入探究降温过程中盐渍土孔隙溶液的相变规律,该研究通过系列盐渍土的降温试验,研究了不同含水率和含盐量条件下盐渍土二次相变温度的变化规律,并基于多孔介质相变理论建立了可以预测盐渍土二次相变温度的理论模型。结果表明：NaCl盐渍土和Na2SO4盐渍土的二次相变温度分别低于-24 ℃和-1.25 ℃;初始含水率和含盐量不同,盐渍土的二次相变温度也不同,盐渍土二次相变温度与二次相变发生前的液态水含量有关;第一次相变过程中冰晶或者盐晶体析出降低了土体二次相变发生前的液态水含量,使得盐渍土二次相变温度随含水率和含盐量不同而发生变化;盐渍土二次相变温度预测模型充分考虑了盐渍土的相变过程,较好地揭示了盐渍土二次相变温度的变化机理。该研究不仅对深入认识盐渍土孔隙溶液的相变机理具有重要的理论意义,而且可对确定盐渍土在降温过程中的盐胀和冻胀贡献提供有效依据。     

Tillage effects on compaction, earthworms and other soil quality indicators in Hungary

The philosophy toward tillage throughout the last century in Hungary can be characterized as a fight against extreme climatic and economic situations. The ‘Hungarian reasonable tillage’ approach that was promoted by Cserháti at the end of the 1800s was aimed at reducing tillage without increasing the risk of crop failure in arable fields. Recently, new tillage trends and systems have been introduced because of the rise in energy prices and because of the need to cut production costs, conserve soil and water resources and protect the environment. In Hungarian relation, the rationalized plowing, loosening and mulching systems are counted to the new tillage solutions. There are new steps in the sowing methods too, such as seedbed preparation and plant in one pass, till and plant, mulch-till and plant and direct drilling, which are environment capable, throughout improving soil condition and avoiding the environment harms. The applicability of various soil conservation tillage methods is currently being tested in research projects and discussed in workshops throughout the country. In this paper, soil quality problems such as compaction, trends in soil tillage, and factors affecting soil quality or condition as well as improvement and maintenance are summarized. The data show that annual disking and plowing causes subsoil compaction at the depth of tillage within 3 years and that the compacted layer expanded both in surface and deeper layers after the 5th year. Soil quality deterioration by tillage-pans was improved by subsoiling and maintained by direct drilling and planting soil-loosening catch crops. Within a loam and a sandy loam soil there were close correlations between earthworm activity and soil quality. Earthworm numbers increased on undisturbed but noncompacted soils and soils that included stubble residues remaining on the surface, but did not increase on soils that were deteriorated by tillage-pans or left bare by the absence of mulch. Our goal for the new millennium, is to use only enough tillage to create and maintain harmony between soil conservation, soil quality and crop production.     

Temporal and seasonal change in microbial community structure of an undisturbed,disturbed, and carbon-amended pasture soil

Disturbance and change to C inputs can alter microbial community structure and impact ecosystem function. Particularly in temperate regions, seasonal change also has an effect on microbial communities both directly through climate and indirectly through plant function. The temporal change in microbial communities of an undisturbed pasture, disturbed pasture (similar to a single tillage event) and pasture soil amended with two forms of particulate carbon were monitored over eight consecutive seasons after grass was reestablished. The soil microbial community was assessed by a DNA fingerprinting technique (terminal restriction fragment length polymorphism, TRFLP) of bacterial, fungal and archaeal communities, and also from phospholipid fatty acid (PLFA) analysis. The single disturbance had a significant effect on fungal microbial community structure (by TRFLP) and significantly decreased the fungal:bacterial ratio. Though the change was relatively small, it persisted throughout the sampling period. Nitrate was also higher on the disturbed treatment providing evidence for the theory that changes to fungal:bacterial ratios can alter nutrient cycling and retention. Fungal communities were the most altered by the C amendments, and while bacteria were also affected by the C amendments, seasonal change was a greater cause of variation. Correlation to soil and climatic variables explained more of the total variability for PLFA (78% for all treatments) than bacterial (50%), fungal (35%) and archaeal (14%) restriction fragments. Most climate and soil variables explained significant variation for seasonal patterns in the multivariate community structures but measurements of soil moisture were important for all communities while pH was relatively more important for bacteria, temperature for fungi, and soil C:N ratio for archaea. Autumn was particularly distinct from other seasons for bacteria (less so for the fungal community) and although there was seasonal change in pH suggesting pasture management was a factor, the significant correlation of other soil characteristics suggests that plant physiological changes (most probably root exudates) also played a significant role. The large change in the saprotrophic fungal community due to the particulate C addition but minor seasonal change would tend to suggest that the fungal community may be more responsive to changes in litter inputs rather than root exudates while the reverse is true for bacteria.     

Impact of perennial pasture and tillage systems on carbon input and soil quality indicators

Soil degradation associated with tillage is a major problem in Uruguayan agriculture. Either rotation of crops with pastures (ROT) or no-till (NT) cropping have been proposed as alternatives to minimize the impact of agriculture on soil quality. The combined impact on soil properties of ROT and NT has not been evaluated. In this study, we report results of the first 12 years of a long-term experiment established on a clay loam soil in western Uruguay. The objective was to determine the influence of conventional tillage (CT) and NT on systems under continuous cropping (CC, two crops per year) or ROT (3.5-year annual crops/2.5-year pastures). Soil samples taken at the beginning of the experiment in 1994 and in 2004 were analyzed for organic carbon (SOC), total organic carbon (TSOC) and total nitrogen content (STN), and for water-stable aggregation (WAS). Soil loss and erodibility indicators were studied using microrain simulator. With 12 years, the cumulative carbon (C) inputs of aboveground biomass were similar between tillage, but C input in CC was 50% higher than ROT. This difference was explained because 84% of the pastures dry matter was consumed by animals. Nevertheless we estimated a higher below ground biomass in ROT compared to CC systems (24.9 Mg ha⁻¹ vs. 10.9 Mg ha⁻¹). NT presented 7% higher SOC than CT (0–18 cm) with no differences between rotation systems. While all treatments declined in STN during 12 years, ROT had 11% and 58% higher STN and WAS than CC systems, with a large impact of the pasture under CT. Runoff and erosion were minimized under NT in both rotations systems. Thus, including pastures in the rotation, or switching from CT to NT improved soil quality properties. The expected benefit of combining NT and ROT will likely require more years for the cumulative effect to be detectable in both C input and soil properties.     

湘西植烟土壤有效硼时空变异及影响因素研究

为探明湘西植烟土壤有效硼时空变异特征,为湘西烟区硼素营养管理提供科学依据,分别于2000年和2015年在湘西烟区采集土壤样品并测定了有效硼含量,利用经典统计学、地统计学等方法,对该地土壤有效硼及其影响因素进行了研究。结果表明:有效硼含量均值由2000年的0.21 mg/kg增加到2015年0.64 mg/kg,增幅达204.76%;2015年土壤有效硼含量"适宜"和"高"等级的面积大幅增加,分别增加了73.27%和18.39%,而"低"等级则大幅下降,比2000年下降了91.19%。海拔高度、有机质、土地利用方式、轮作方式、地形和土壤母质均对土壤有效硼含量有显著影响,其中海拔高度的解释能力最大,可以解释10.2%的土壤有效硼变异,土壤母质独立解释能力最小,仅解释2.4%的土壤有效硼变异,6项指标累计可解释26.5%的土壤有效硼变异。     

微孔深松耕降低土壤紧实度提高棉花产量与种籽品质

长期传统耕作导致土壤紧实形成犁底层是影响农田土壤质量和作物生长的关键障碍因子之一。为解决这一问题,于2013年4月至2014年5月在山西运城南花农场开展为期1 a的大田试验,对比研究微孔深松耕技术和旋耕机旋耕15~20 cm的传统耕作方法对土壤紧实度以及棉籽品质性状和生长发育的影响。结果表明:微孔深松耕技术较传统耕作方式,棉花苗期犁底层40 cm处土壤紧实度由9 069.70降低到558.80 k Pa,吐絮期犁底层40 cm处的土壤紧实度由8 089.70降低到1 174.20 k Pa,吐絮期0~40 cm土层中微孔深松耕土壤容重最大为1.05 g/cm3,传统耕作最大为1.56 g/cm3;在30 cm土层中,微孔深松耕的总根量比传统耕作方式多187.03%;微孔深松耕处理棉株棉铃的5室铃率较传统耕作增加15.00%,每个棉瓤的种子数平均增加1~2粒;棉籽的籽指、密度、绒长均明显增加,脂肪含量显著降低(P0.05),蛋白质含量显著增加(P0.05),单株铃数比传统耕作增加6.34%,铃质量增加5.75%,皮棉产量增加10.12%。效益分析表明,采用微孔穴深松耕作种植棉花,每公顷净收益增加3 338.00元。该研究揭示了微孔深松耕作可有效打破犁底层,具有疏松土壤紧实度,并提高棉籽品质增加棉花产量,为该项技术应用于生产提供试验依据。     

Temporal variation of indoor air quality in an enclosed swine confinement building

Human health hazards can exist in swine confinement buildings due to poor indoor air quality (IAQ). During this study, airborne dust and ammonia concentrations were monitored within a working farrowing facility as indicators of IAQ. The purposes of this study were to assess the temporal variability of the airborne dust and ammonia levels over both a daily and seasonal basis, and to determine the accuracy of real-time sensors relative to actively sampled data. An ammonia sensor, aerosol photometer, indoor relative humidity sensor, and datalogger containing an indoor temperature sensor were mounted on a board 180 cm above the floor in the center of a room in the facility. Sensor readings were taken once every 4 minutes during animal occupancy (3-week intervals). Measurements of total and respirable dust concentrations by standard method, aerosol size distribution, and ammonia concentrations were taken once per week, in addition to temperature and relative humidity measurements using a thermometer and sling psychrometer, respectively. Samples were taken between September 1999 and August 2000. Diurnal variations in airborne dust revealed an inverse relationship with changes in indoor temperature and, by association, changes in airflow rate. Ammonia levels changed despite relatively stable internal temperatures. This change may be related to both changes in flow rates and in volatility rates. As expected, contaminant concentrations increased during the cold weather months, but these differences were not significantly different from other seasons. However, total dust concentrations were very low (geometric mean = 0.8 mg/m3) throughout the year. Likewise, ammonia concentrations averaged only 3.6 ppm in the well-maintained study site.     

Temporal variation of soil organic matter content and potential determinants in Tibet, China

Understanding temporal variations in soil organic matter (SOM) is important because SOM improves soil, sustains soil productivity, and sequesters carbon. The Tibetan Plateau plays a critical role in climate change at both regional and global scales. Thus, the main objective of this study was to investigate temporal variations in SOM content in Tibetan soils as an important indicator of climate change. SOM content data were obtained from soil surveys conducted in the 1980s and resampled in the 2000s. In this study, SOM content data from the A horizon of the soil profile were compared over time, and a paired t-test was conducted to verify differences between the two datasets. The results reveal that SOM content in the A horizon decreased significantly. To determine the causes of SOM content decreases in the A horizon, soil samples were grouped into ten soil types, four land-use types, and seven physiogeographical units. SOM content in four soil types (felty soils, cambisols in the World Reference Base (WRB) for soil resources; cold calcic soils, cambisols in the WRB; bog soils, gleysols in the WRB; and meadow soils, umbric gleysols/haplic phaeozem in the WRB), pasture, and broad valleys and basins of the south Tibetan zone of montane shrubby steppe decreased significantly in the A horizon. Based on the analysis of some known realities, this change should be attributed to pasture degradation resulting from overgrazing and climate warming.     

Temporal variation in soil carbon in various paddy soil types in a cold temperate continental monsoon climate

Soil organic carbon (SOC) is the most important carbon pool in the terrestrial ecosystem. However, temporal variations in paddy SOC under a temperate continental monsoon climate are poorly understood. Here, we demonstrate that significant SOC variations occur in meadow soil (MS), black soil (BS) and planosol (PS) paddy soils. Several soil samples were collected from different regions where rice was cultivated for 1, 6, 10, 23 and 40 years for MS samples; for 1, 6, 10, 20 and 35 years for BS samples and 1, 5, 10, 15 and 25 years for PS samples. The total organic carbon (TOC) content and humus organic carbon (HOC) content were found to increase as the rice cultivation duration increased, while the mineralizable organic carbon (MOC) content and carbohydrate organic carbon (COC) content exhibited the opposite trend. The relationships between the relative carbon accumulation (Y) in the three soil types and time (X) were consistent with the following models: Y_TOC = 0.9973X^0.0245, Y_HOC = 0.9936X^0.0457, Y_MOC = 1.023X^−0.073, and Y_COC = 1.040X^−0.059, describing the temporal variation in the various forms of organic carbon in paddy soils under a temperate continental monsoon climate. The results of this study provide a reference for soil carbon pool management and fertilization management.     

Temporal variation outweighs effects of biosolids applications in shaping arbuscular mycorrhizal fungi communities on plants grown in pasture and arable soils

Landspreading of biosolids (treated sewage sludge) in agroecosystems is a common waste management practice worldwide. Evidence suggests biosolids may be detrimental to arbuscular mycorrhizal fungi (AMF); however, previous studies focused on arable systems and often unrealistically high biosolids application levels. We investigated the effects of biosolids on AMF communities in grassland and arable agroecosystems, in the context of the natural seasonal dynamics of AMF community composition and diversity. A pasture and arable system under commercial farming management were amended annually with two different types of biosolids, applied at levels meeting current European Union regulations, in a factorial, replicated field-scale plot experiment. AMF root colonisation and community composition were measured in Lolium perenne roots from the pasture and Trifolium repens roots growing in arable soil across the seasons of two years. AMF community compositions were assessed by terminal-restriction fragment length polymorphism analyses. Biosolids had no significant effect on AMF root colonisation or community composition in either agroecosystem. Soil chemical analyses indicated several changes in the top 0–5 cm layer of the pasture soil, including small increases in heavy metal concentrations in biosolids relative to control plots. Temporal AMF dynamics were detected in soils from both agroecosystem indicating that the effect of seasonality outweighed that of biosolids application.     

Tillage systems and soil compaction in Africa

Introduction of mechanized agriculture induces profound changes in soil characteristics. Soil compaction originating from mechanical land clearing, mechanized cultivation, and continuous cropping is aggravated by crusting and hard-setting phenomena of soils, and widespread occurrence of naturally compacted upland soils and subsoil gravel horizons. Natural and anthropogenically induced soil compaction has detrimental effects on growth and yields of a wide range of crops. Furthermore, compaction can persist for a long time if no adequate measures are taken to minimize or alleviate it.

In humid and subhumid regions of Africa, the no-tillage system with crop residue mulch is an important method of controlling soil compaction, followed in significance, by biological and mechanical loosening where motorized land clearing is the causative agent. Biological methods involve cover crops and alley cropping or agroforestry. Where new land areas need to be opened up, land clearing should be done by the slash-and-burn method, so that most of the nutrients in the vegetation are returned to the soil. Where mechanical land clearing is inevitable, forest removal should be done by the use of shear blade, whereby most of the roots and stumps are left in the ground intact, and the forest litter is not removed.

In semi-arid and arid regions of Africa, alleviation of soil compaction can be done by two methods. One method is to use the controlled traffic tillage system. Controlled traffic results in both a loose-rooting zone and a firmed traffic lane, thereby providing good plant growth and trafficability for timely field operations. The second method is to use mechanical loosening techniques, i.e. ploughing by animal traction or tractor power, chiseling, deep ripping, subsoiling, and tied-ride system. The effect of mechanical loosening, however, tends to be of short duration if the ensuing field traffic is not controlled.     

玉米根系在土壤剖面中的分布研究

玉米根系在土壤剖面中的分布是准确量化植被与气候相互作用不可缺少的参数,也是玉米生产科学管理和节水农业发展的重要科学依据.在中国气象科学研究院固城生态环境与农业气象实验站内的大型根系观测系统中,采用地下室玻璃窗观测法和方形整段标本法,观测了"屯玉46号"玉米的根深、根宽、根长和根重,分析了玉米根长、根长密度、根重密度和根系粗度等在土壤剖面中的分布状况.结果表明,玉米根长、根干重均随土壤深度的增加基本呈递减类型.吐丝期0～40 cm土层根长占整层根长51.5%,0～80 cm土层占76.2%,0～120 cm土层占90.5%.乳熟后期其分布趋势与吐丝期相似.玉米根系粗度随着土壤深度增加,在上层呈减少分布型,在下层呈增加分布型.乳熟后期,玉米最大根深可达230 cm,根长总量达8.288 km·m-2,显示出该玉米品种有较发达的根系.通过玻璃窗观测的根深大于远离玻璃窗处的根深.