Impact of a 5-year winter cover crop rotational system on the molecular diversity of arbuscular mycorrhizal fungi colonizing roots of subsequent soybean

The impact of winter cover crops, specifically wheat (Triticum aestivum L.), red clover (Trifolium pratense L.), and rapeseed (Brassica napus L.) or winter fallow, on community composition of arbuscular mycorrhizal fungi (AMF) in subsequent soybean roots was investigated in a 5-year field trial on andosolic soils in Japan. Soybean roots were sampled at full-flowering and analyzed for AMF communities using a partial LSU rDNA region. Phylogenetic analysis detected 22 AMF phylotypes, including eight Glomus, three Gigaspora, two Scutellospora, three Acaulospora, two Rhizophagus, and one of Funneliformis, Diversispora, Paraglomus, and an unknown glomeromycete in the roots. The 5-year rotation of different winter cover crops or winter fallow did not impact the molecular diversity of AMF communities colonizing the roots of subsequent soybean. In all of the rotations, Glomus and Gigaspora phylotypes were common to soybean roots over the 5-year period. Redundancy analysis (RDA) demonstrated that AMF communities in the roots of subsequent soybean were not significantly different among winter cover crop rotations or fallow. However, AMF communities in soybean roots were clearly influenced by rotation year suggesting that climate or other environmental factors were more important than winter cover cropping system management.     

Diversity and vertical distribution of indigenous arbuscular mycorrhizal fungi under two soybean rotational systems

To evaluate the importance of arbuscular mycorrhizal fungi (AMF) to crop production, it is imperative to move beyond the plow layer to include the full soil profile impacted by plant roots. To illustrate this, we investigated the vertical distribution of AMF biomass and community structure within the top 100 cm of soil in soybean (Glycine max (L.) Merr., cv: Enrei) rotational systems cropped to wheat (Triticuma estivum L. cv: Bandowase) or left fallow using fatty acid methyl ester (FAME) biomarkers and molecular analysis, respectively. AMF biomass, as measured by concentration of C16:1cis11, declined during fallow and with increasing soil depth. Greater than 50 % of the stored AMF biomass was found at depths below 35 cm. Phylogenetic analysis revealed 16 AMF phylotypes, including nine Glomus, two Gigaspora, two Scutellospora, and one each of Diversispora, Paraglomus, and an unknown glomeromycete, at different sampling depths in this study. Cluster analysis based on the number and abundance of each AMF phylotype formed two distinct clusters separating wheat from fallow rotations. There was no distinct relationship with soil depth beyond clustering AMF communities above and below 20 cm under wheat. Redundancy analysis (RDA) and hierarchical cluster analysis demonstrated that AMF communities by soil depth within each rotation were not significantly different. However, AMF communities were clearly influenced by crop rotation, where the distribution of specific AMF phylotypes responded to the presence of the wheat crop.     

Conservation tillage for soybean in the U.S. Southeastern Coastal Plain

Double cropping of soybean has progressed less rapidly in the U.S. Southeastern Coastal Plains than expected by the ample rainfall and long frost-free season. Post-emergence herbicides, the management of plant residues to reduce water use by cover crops, and a no-till planter with a combination subsoiler are the innovations that have facilitated this new production. Full-season soybean (Glycine max L.) was planted following a grazed cover crop of winter rye (Secale cereale L.) or late-season soybean was planted following winter wheat harvest. In both cases, a special planter was used with an integral subsoil shank ahead of the opener. Full-season soybean under conservation tillage produced yields equal to or better than yields in conventional clean tillage. In a dry summer, soybean yields under conservation tillage exceeded conventional tillage because of suppressed early biomass production which conserved stored soil water and favored growth during the reproduction phase of the crop-cycle. Late-season soybean yields behind wheat favored the conservation tillage practice of in-row subsoil-planting into stubble. However, planting in burned-off wheat stubble produced the highest yields in this study. In a dry spring, the cover crop accelerated soil water use which resulted in lower soybean yields under conservation tillage. Comparisons of 76 vs. 97 cm row spacing were inconclusive, but the trend suggests that wider rows conserve water under periods of drought and that the narrower-row configuration favors adequate water regimes.     

Predicting soil erosion in conservation tillage cotton production systems using the revised universal soil loss equation (RUSLE)

Despite being one of the most profitable crops for the southeastern USA, cotton (Gossypium hirsutum L.) is considered to create a greater soil erosion hazard than other annual crops such as corn (Zea mays L.) and soybeans (Glycine max (L.) Merr.). Reduced tillage systems and cover cropping can reduce soil erosion and leaching of nutrients into ground water. The objectives of this study, which was conducted in north Alabama from 1996 to 1998, were to assess the impact of no-till and mulch-till systems with a winter rye (Secale cereale L.) cover crop and poultry litter on soil erosion estimates in cotton plots using the revised universal soil loss equation (RUSLE). Soil erosion estimates in conventional till plots with or without a winter rye cover crop and ammonium nitrate fertilizer were double the 11 t ha⁻¹ yr⁻¹ tolerance level for the Decatur series soils. However, using poultry litter as the N source (100 kg N ha⁻¹) gave soil erosion estimates about 50% below the tolerance level under conventional till. Doubling the N rate through poultry litter to 200 kg N ha⁻¹ under no-till system gave the lowest soil erosion estimate level. No-till and mulch-till gave erosion estimates which were about 50% of the tolerance level with or without cover cropping or N fertilization. This study shows that no-till and mulch-till systems with cover cropping and poultry litter can reduce soil erosion in addition to increasing cotton growth and lint yields, and thus improve sustainability of cotton soils in the southeastern USA.     

The effect of trends in tillage practices on erosion and carbon content of soils in the US corn belt

The EPIC model was used to simulate soil erosion and soil C content at 100 randomly selected sites in the US corn belt. Four management scenarios were run for 100 years: (1) current mix of tillage practices maintained; (2) current trend of conversion to mulch-till and no-till maintained; (3) trend to increased no-till; (4) trend to increased no-till with addition of winter wheat cover crop. As expected, the three alternative scenarios resulted in substantial decreases in soil erosion compared to the current mix of tillage practices. C content of the top 15 cm of soil increased for the alternative scenarios, while remaining approximately constant for the current tillage mix. However, total soil C to a depth of 1 m from the original surface decreased for all scenarios except for the no-till plus winter wheat cover crop scenario. Extrapolated to the entire US corn belt, the model results suggest that, under the current mix of tillage practices, soils used for corn and/or soybean production will lose 3.2 × 10⁶ tons of C per year for the next 100 years. About 21% of this loss will be C transported off-site by soil erosion; an unknown fraction of this C will be released to the atmosphere. For the base trend and increased no-till trend, these soils are projected to lose 2.2 × 10⁶ t-C yr^?1 and 1.0 × 10⁶ t-C yr^?1, respectively. Under the increased no-till plus cover crop scenario, these soils become a small sink of 0.1 × 10⁶ t-C yr^?1. Thus, a shift from current tillage practices to widespread use of no-till plus winter cover could conserve and sequester a total of 3.3 × 10⁶ t-C yr^?1 in the soil for the next 100 years.     

Corn and soybean grain yield responses to soil amendments and cover crop in upland soils

A field study was conducted on upland soils for six years to determine interactive effects of winter wheat (Triticum aestivum L.) cover crop, organic and inorganic soil amendments on grain yields and nutrient utilizations in a no-till corn (Zea mays)-soybean (Glycine max) rotation. Experimental design was a split-plot arrangement with four replicates. Cover crops were the main plots and fertilization treatments used as sub-plot. Fertilization treatments included an unfertilized control, poultry litter, poultry litter (PL) plus flue gas desulfurization (FGD) gypsum and inorganic N fertilizer applied every other year to corn. Corn grain yield and grain N and P uptake were greater with PL than inorganic fertilizer in 2014 and 2016. Addition of FGD gypsum to PL significantly increased corn grain yield by 15% in 2016. Cover crop increased corn and soybean grain yields in a year with less seasonal rainfall possibly by conserving soil moisture.     

The influence of tillage on the structure of rhizosphere and root-associated arbuscular mycorrhizal fungal communities

Soil environmental factors affect the structure of arbuscular mycorrhizal (AM) fungal communities present in soil. However, it is not understood to which degree management practices such as tillage lead to dissimilarities between intra- and extraradical AM fungal communities. This study aims to assess the influence of two different soil management practices (conventional tillage and no-till) on the diversity of AMF communities, both in rhizosphere soil and inside corn roots. We hypothesized that under no-till, roots are colonized as they grow through the undisturbed fungal mycelia left from the previous crop whereas under conventional tillage they are colonized by those propagules that survived disturbance and can re-establish in their new relocated and mixed environment. We predicted that the degree of similarity of AM fungal communities inside versus outside the roots would be greater under no-till than under tillage. Using terminal restriction fragment length polymorphism (T-RFLP) and denaturing gradient gel electrophoresis we observed a different AM fungal community present in roots under no-till than under conventional tillage. Moreover, the communities present in the rhizosphere soil were different than in the roots of the corn plants. These results suggest that soil management does alter the diversity of AM fungal communities associated with corn roots and that plants influence the structure of the AMF community colonizing their roots. Sequencing results indicated that the majority of AMF species found in this agricultural soil was Glomus spp. However, further work is required to determine the extent to which AM fungal genotypic alterations by soil management influences competitive relationships.     

Interrill soil erosion as affected by tillage and residue cover

No-till cropping systems are effective in reducing soil erosion. The objective of this study was to determine whether high infiltration rates and low runoff and soil loss under long-term, no-till conditions in loessial regions of the Midwest US result from both the well-structured, porous condition of the soil and the protective cover of crop residue or primarily from residue cover. Soil loss, runoff, and infiltration were measured using a rainfall simulator on interrill erosion plots with and without residue cover on a conventional and two no-till systems in central Illinois. For both conventional till and no-till conditions, removing surface residue significantly decreased infiltration rates and increased soil loss. Tilling the no-till surface while maintaining an equal surface cover as with the no-till system slightly increased interrill erosion. Removing residue on a no-till system, however, increased soil loss significantly. A no-till soil condition without adequate residue cover will seal, crust, and erode with extremely high soil losses following surface drying.     

Residue cover in wheat systems following dry pea and lentil in the Palouse region of Idaho

Crop management practices are needed that increase crop residue groundcover and reduce soil erosion after winter wheat (Triticum aestivum L.) planting in the Palouse region of northern Idaho and eastern Washington. Trials were conducted in 1997 and 1998 at the University of Idaho Kambitsch Research Farm near Genesee, Idaho, using farm scale equipment to evaluate dry pea (Pisum sativum L. subsp. sativum) and lentil (Lens culinaris Medik) residue production and groundcover across cultivars and tillage intensity. After harvest, legume plot areas were prepared for winter wheat seeding using four main plot tillage systems designed to give progressive levels of tillage intensity: no-till (NT), Ripper–Shooter™ (RS), RS plus one cultivation, and RS plus two cultivations. In 1997, the two dry pea cultivars produced significantly greater residue than the lentil cultivars. In 1998, ‘Pro 2100’ dry pea had significantly higher residue production than ‘Columbian’ pea and ‘Crimson’ lentil cultivars. In 1997, initial residue cover was highest with NT, averaging 74% groundcover across legume cultivars. After winter wheat seeding, residue cover declined for all tillage treatments, but was still highest at 40% residue cover under NT. In 1998, residue cover was lower for all tillage treatments across all cultivars than in 1997, but NT still had the highest initial residue cover. Wheat yield was not affected by tillage or previous crop treatments in either year. This study showed that NT and reduced tillage systems can maintain previous crop residue on the surface for soil conservation and subsequent crop yields.     

Subsequent nitrogen utilisation and soil water distribution as affected by forage radish cover crop and nitrogen fertiliser in a corn silage production system

Forage radish is a unique winter cover crop that is relatively new but becoming rapidly adopted in temperate, humid North America. Little is known about how the use of this cover crop may influence subsequent nitrogen availability, soil water accumulation in the soil profile in corn silage production system. In this present work, the average nitrogen uptake by silage corn increased significantly by 11.6% in cover plots compared with the no-cover control plots. The recovery efficiency and agronomic efficiency of applied nitrogen in silage corn declined in cover plots compared to no-cover plots. The average soil water storage in cover plots was significantly higher than in the control after corn sowing and at the harvest stage. With increasing nitrogen application level, the average corn grain yield increased significantly at 56 and 112 kg N ha^?1 by 13.1% and 39.8%, respectively. Planting a forage radish cover crop can facilitate growth of silage corn and markedly improve total nitrogen uptake of corn. Consideration should be given to nitrogen application rate and also to avoiding excessive nitrogen input in the subsequent crop following a cover crop, thereby truly improving subsequent fertiliser use efficiency.     

宁南山区保护性农业措施对冬小麦农田休闲期土壤水分的影响

[目的]研究宁南山区冬小麦农田休闲期保护性农业措施对土壤水分的影响,为该区降雨资源的高效利用以及保护性农业的可持续发展提供理论依据。[方法]基于不同时期多个保护性农业试验土壤水分数据比较分析。[结果](1)在干旱的情况下,免耕留茬处理能够显著增加土壤表层(0—20cm)含水量。(2)在免耕的情况下,随着秸秆留茬高度的增加,土壤水分呈现增加趋势。(3)在降雨量较大且降雨量具有连续性的情况下,常规耕作处理在土壤表层保蓄了较多的土壤水分(0—20cm),免耕秸秆覆盖处理能够增加土壤20—80cm土壤水分含量。(4)冬小麦休闲期种植豆科作物,降低了土壤水分,其降低幅度与降雨量以及种植密度有关,种植密度越大,对土壤水分的影响越大。[结论]免耕+留茬耕作措施能够提高冬小麦农田休闲期土壤水分含量,覆盖作物降低了休闲期土壤水分含量。     

Soil erosion estimation in conservation tillage systems with poultry litter application using RUSLE 2.0 model

Soil erosion is a major threat to global economic and environmental sustainability. This study evaluated long-term effects of conservation tillage with poultry litter application on soil erosion estimates in cotton (Gossypium hirsutum L.) plots using RUSLE 2.0 computer model. Treatments consisting of no-till, mulch-till, and conventional tillage systems, winter rye (Secale cereale L.) cover cropping and poultry litter, and ammonium nitrate sources of nitrogen were established at the Alabama Agricultural Experiment Station, Belle Mina, AL (34°41′N, 86°52′W), beginning fall 1996. Soil erosion estimates in cotton plots under conventional tillage system with winter rye cover cropping declined by 36% from 8.0 Mg ha⁻¹ year⁻¹ in 1997 to 5.1 Mg ha⁻¹ year⁻¹ in 2004. This result was largely attributed to cumulative effect of surface residue cover which increased by 17%, from 20% in 1997 to 37% in 2004. In conventional tillage without winter rye cover cropping, soil erosion estimates were 11.0 Mg ha⁻¹ year⁻¹ in 1997 and increased to 12.0 Mg ha⁻¹ year⁻¹ in 2004. In no-till system, soil erosion estimates generally remained stable over the study period, averaging 0.5 and 1.3 Mg ha⁻¹ year⁻¹with and without winter rye cover cropping, respectively. This study shows that cover cropping is critical to reduce soil erosion and to increase the sustainability of cotton production in the southeast U.S. Application of N in the form of ammonium nitrate or poultry litter significantly increased cotton canopy cover and surface root biomass, which are desirable attributes for soil erosion reduction in cotton plots.     

不同种植模式背景下栽植生姜对紫色土细菌多样性的影响

为了解栽植生姜对岷江下游紫色丘陵区不同种植模式背景下典型紫色土微生物多样性的影响,采用PCR-DGGE与DNA序列分析方法,研究了紫色丘陵区玉米+红薯间作、大豆单作、生姜连作、水稻-紫云英轮作等4种典型种植模式背景下栽植生姜前后土壤细菌多样性变化特征。PCR-DGGE分子指纹图谱结果表明,同一种植模式下栽种生姜前后DGGE条带数目、位置和亮度差异明显。栽种生姜均降低了各模式下土壤细菌群落丰富度和Shannon-Wiener指数,明显改变了土壤细菌群落结构,其中生姜连作模式背景下土壤细菌丰富度和Shannon-Wiener指数下降幅度最大,玉米+红薯间作模式下降幅度最小,但栽植生姜后4种种植模式之间土壤细菌群落结构相似度明显增加。DGGE特征条带的序列分析表明,栽植生姜后,土壤细菌中的部分绿弯菌门类群消失,部分硝化螺旋菌门类群仅在水稻-紫云英轮作模式中发现、玉米+红薯间作模式中出现了短波单胞菌门的细菌。这些结果为了解栽植生姜对土壤环境的影响以及生姜种植模式的优化提供了一定的科学依据。     

Contrasting grain crop and grassland management effects on soil quality properties for a north-central Missouri claypan soil landscape

Abstract

Crop management has the potential to either enhance or degrade soil quality, which in turn impacts on crop production and the environment. Few studies have investigated how crop management affects soil quality over different landscape positions. The objective of the present study was to investigate how 12 years of annual cropping system (ACS) and conservation reserve program (CRP) practices impacted soil quality indicators at summit, backslope and footslope landscape positions of a claypan soil in north-central Missouri. Claypan soils are particularly poorly drained because of a restrictive high-clay subsoil layer and are vulnerable to high water erosion. Three replicates of four management systems were established in 1991 in a randomized complete block design, with landscape position as a split-block treatment. The management systems were investigated: (1) annual cropping system 1 (ACS1) was a mulch tillage (typically ≥ 30% of soil covered with residue after tillage operations) corn (Zea mays L.)–soybean (Glycine max (L.) Merr.) rotation system, (2) annual cropping system 2 (ACS2) was a no-till corn–soybean rotation system, (3) annual cropping system 3 (ACS3) was a no-till corn–soybean–wheat (Triticum aestivum L.) rotation system, with a cover crop following wheat, (4) CRP was a continuous cool-season grass and legume system. In 2002, soil cores (at depths of 0–7.5, 7.5–15 and 15–30 cm) were collected by landscape position and analyzed for physical, chemical and biological soil quality properties. No interactions were observed between landscape and crop management. Relative to management effects, soil organic carbon (SOC) significantly increased with 12 years of CRP management, but not with the other management systems. At the 0–7.5-cm soil depth in the CRP system, SOC increased over this period by 33% and soil total nitrogen storage increased by 34%. Soil aggregate stability was approximately 40% higher in the no-till management systems (ACS2 and ACS3) than in the tilled system (ACS1). Soil aggregation under CRP management was more than double that of the three grain-cropping systems. Soil bulk density at the shallow sampling depth was greater in ACS3 than in ACS1 and ACS2. In contrast to studies on other soil types, these results indicate only minor changes to claypan soil quality after 12 years of no-till management. The landscape had minor effects on the soil properties. Of note, SOC was significantly lower in the 7.5–15-cm soil depth at the footslope compared with the other landscape positions. We attribute this to wetter and more humid conditions at this position and extended periods of high microbial activity and SOC mineralization. We conclude that claypan soils degraded by historical cropping practices will benefit most from the adoption of CRP or CRP-like management.     

不同栽培模式对黄瓜根际土壤酶活性及细菌群落结构的影响

分别以小麦、燕麦、毛葱、芹菜、白菜与黄瓜伴生或套作,研究了不同栽培模式对黄瓜根际土壤酶活性及细菌群落结构的影响,为连作土壤环境修复提供理论依据。结果表明:小麦/黄瓜、燕麦/黄瓜伴生,毛葱/黄瓜套作显著提高了根际土壤过氧化氢酶活性（P0.05）;芹菜/黄瓜套作和小麦/黄瓜伴生显著提高了根际土壤过氧化物酶活性（P0.05）;芹菜/黄瓜套作显著提高了根际土壤脲酶活性（P0.05）;不同栽培模式均显著提高了各时期根际土壤转化酶活性（P0.05）。PCR-DGGE分析结果显示,不同栽培模式在一定程度上提高了黄瓜根际土壤细菌群落结构多样性。DGGE条带测序显示,黄瓜根际土壤细菌大多与不可培养的细菌种属具有较高的同源性,测序比对推测,主要分属于-变形菌纲（Alphaproteobacteria）、-变形菌纲（Betaproteobacteria）、鞘脂杆菌纲（Sphingobacteria）和芽单胞菌纲（Gemmatimonadetes）四个纲。本研究说明不同栽培模式对土壤酶活性和土壤细菌群落结构均产生一定影响,改变了土壤环境,其中小麦与黄瓜伴生栽培模式效果较好。     

Least limiting water range indicators of soil quality and corn production, eastern Ontario, Canada

The least limiting water range (LLWR) attempts to incorporate crop-limiting values of soil strength, aeration, and water supply to plant roots into one effective parameter (on the basis of soil water content). The LLWR can be a useful indicator of soil quality and soil physical constraints on crop production. This study focused on assessing dynamic cultivation zone LLWR parameters between different cropping/tillage/trafficked clay loam plots at Winchester, Ont., to identify potential management impact on surficial soil physical conditions for contrasting growing seasons. This study also evaluated dynamic cultivation layer LLWR variables as indicators of corn (Zea mays L.) plant establishment and corn yield. The results suggest that no-till soils had lower average air-filled porosities (AFP) and O₂ concentrations than respectively managed tilled plots for both years of study. Potential trafficking effects on aeration properties were most evident in no-till relative to till; preferentially trafficked no-tilled plots had lower AFP and O₂ concentrations than respective non-preferentially trafficked no-till plots for both years of study. Corn establishment and yield variability were principally explained by cumulative differences between daily AFP and aeration threshold values, and the cumulative number of days daily AFP was below an AFP aeration threshold for specific corn growth stage periods. Lower AFP was linked to lower yields and plant establishments. Soil strength, as measured by cone penetration resistance, was important over certain sites, but not as important globally as AFP in predicting crop properties. Overall, conventional tilled soils that were not preferentially trafficked had most favorable aeration properties, and subsequently, greatest corn populations and yields. No-till soils were at greater risk of aeration limiting conditions, especially those in continuous corn and preferentially trafficked.     

菌根对紫色土上间作玉米生长及磷素累积的影响

丛枝菌根真菌(arbuscular mycorrhizal fungi,AMF)在土壤与植物系统的磷素循环中发挥着关键的作用。本文通过盆栽模拟试验研究了不同AMF接种状况[不接种(NM)、接种Glomus mosseae(GM)、接种G.etunicatum(GE)]和玉米/大豆间作体系不同根系分隔方式(不分隔、尼龙网分隔、塑料膜分隔)对间作玉米植株生长及磷素吸收累积的影响。研究结果表明:GM处理下的间作玉米根系侵染率在不同根系分隔方式之间的差异不显著,而GE处理则在塑料膜分隔处理下对玉米的侵染率最高。接种不同AMF对间作玉米促生效果不同,GM和GE处理在不同根系分隔情况下表现出各自的优势,与未接种处理相比,GM处理能使玉米生物量、株高有一定程度增加并在根系不分隔处理下玉米磷吸收较多、生长较好;GE处理能使植株生物量有一定程度增加并在尼龙网分隔处理下的玉米磷吸收较多、生长较好。间作体系不同根系分隔方式对玉米的影响也不同,其中玉米地上部生物量在根系分隔处理下普遍小于不分隔处理,但根系生物量的大小情况则刚好相反。另外,无论何种接种状况,玉米根系磷含量及吸收量均以尼龙网分隔处理显著较高。而根系磷吸收效率则以接种G.mosseae且不分隔根系处理显著高于分隔处理。所有复合处理中,以接种G.etunicatum与尼龙网分隔根系组合处理对间作玉米的生长及磷素累积的促进作用最好,若应用于滇池流域,可望有效控制坡耕地土壤磷素的迁移。     

Potassium and magnesium uptake by wheat and soybean roots as influenced by fertilizer rate

Abstract

Greenhouse‐pot experiments were conducted to compare wheat (Triticum aestivum L.) and soybean [Glycine max (L.) Merrill] in terms of their potassium (K) and magnesium (Mg) uptake. Previously, a field study indicated that various rates of K and Mg fertilization did not produce a significant wheat‐yield response. However, a yield increase with residual K and Mg was measured for the subsequent soybean crop. The 0 to 15 cm layer of Norfolk loamy fine sand (fine loamy, siliceous, thermic Typic Kandiudult) from two different sites was used for the pot experiments. Soil from both sites had a pH of 5.1. Potassium as potassium sulfate (K₂SO₄) was mixed into the soil from the K‐deficient site and Mg as magnesium sulfate (MgSO₄) was mixed into the soil from the Mg‐deficient site. ‘Florida 301’ wheat and ‘Cobb’ soybean were grown in winter and summer, respectively.

Soybean and wheat were similar in K uptake/g of roots on the first and second sampling dates. However, by the third sampling date, K uptake/g of wheat roots was about twice as high as for soybean. Potassium uptake/cm of soybean roots was two to five times that of wheat at each sampling date. Magnesium uptake/g of soybean roots was about four to five times as high as wheat on each sampling date. Similarly, Mg uptake/cm of soybean roots was 10 to 30 times higher than for wheat. Soybean showed higher total K and Mg content than wheat, suggesting that soybean has a higher demand for both K and Mg. The higher demand for K and Mg by soybeans than by wheat suggests that wheat could meet its demand for K and Mg at much lower soil levels than that for soybean. This would also explain a grain‐yield response to K and Mg by soybeans in the previously reported field study, despite a lack of yield response by wheat grown on the same site.     

马铃薯连作栽培对土壤微生物多样性的影响

为阐明马铃薯连作对土壤微生物群落结构和功能多样性的影响,揭示马铃薯连作障碍机制,本试验采用BIOLOG技术结合丛枝菌根真菌(AMF)形态学鉴定方法,就连作0(迎茬)、2 a、4 a、6 a、10 a的马铃薯田块土壤进行研究。结果表明:土壤养分含量随马铃薯连作年限增加有一定程度下降,其中,连作10 a马铃薯根际土壤的全磷、速效磷、速效钾和碱解氮与连作4 a相比分别下降61.32%、26.86%、26.87%和17.24%,但没有明显的养分亏缺和不均衡现象。土壤微生物群落结构发生了较大变化,放线菌、真菌数量显著随连作年限的延长先增加后减少,呈单峰型变化趋势;细菌数量随连作年限的延长呈逐步减少趋势,但差异不显著。连作4~6 a土壤微生物群落依然有较强的功能多样性,培养120 h后,连作6 a较迎茬AWCD值提高3.89%;群落组成中随连作年限的延长以碳水化合物、氨基酸类为碳源的微生物类群代谢能力明显下降,但代谢功能多样性趋于一致。连作马铃薯土壤AM真菌优势种发生改变,迎茬土壤为沙漠球囊霉(Glomus deserticola),连作2 a土壤为扭形球囊霉(Glomus delhiense)和福摩萨球囊霉(Glomus formosanum),连作10 a土壤为球泡球囊霉(Glomus globiferum)。多元分析结果表明,土壤微生物结构与功能多样性、AM真菌多样性变化受土壤p H、全磷含量、放线菌数量、细菌数量及土壤中以碳水化合物、氨基酸类等为碳源基质的微生物类群影响。说明长期连作栽培会影响土壤真菌、放线菌的数量,使真菌群落中AM真菌种的多样性显著下降,优势种发生改变,打破了微生物群落结构与功能平衡,引起土壤微生物群落结构与功能的失调。     

Arbuscular mycorrhizal fungi respond to rhizobial inoculation and cropping systems in farmers' fields in the Guinea savanna

 It has been difficult to explain the rotation effect based solely on N availability in maize-soybean cropping systems in the moist savanna zone of sub-Saharan Africa. Although arbuscular mycorrhizal fungi (AMF) can contribute to plant growth by reducing stresses resulting from other nutrient deficiencies (mainly P) and drought, their role in the maize/soybean rotation cropping systems in the Guinea savanna has not yet been determined. Pot and field experiments were conducted for 2 years using 13 farmers' fields with different cropping histories in two agroecological zones (Zaria, northern Guinea savanna and Zonkwa, southern Guinea savanna) in Nigeria. We quantified the influence of cropping systems and rhizobial inoculation on plant growth, mycorrhizal colonization and diversity of promiscuous soybean and maize grown in rotation. The relationships between these variables and selected soil characteristics in farmers' fields were also examined. Percentage mycorrhizal colonization in promiscuous soybean roots ranged from 7% to 36%, while in maize it varied between 17% and 33%, depending on fields and the previous cropping history. A large variation was also observed for mycorrhizal spores, but these were not correlated with mycorrhizal colonization and did not appear to be influenced by rotation systems. Soybean mycorrhizal colonization was higher (13% increase) in Zonkwa, but not in Zaria, if the preceding crop was maize and not soybean. These differences were related to the soil P concentration, which was positively related to mycorrhizal colonization in Zonkwa but negatively to this parameter in Zaria. The previous crop did not affect mycorrhizal colonization of maize in both locations. Soybean cultivars inoculated with rhizobia had a higher mycorrhizal colonization rate (25%) and more AMF species than maize or uninoculated soybean (19%). Maize grown in plots previously under inoculated soybean also had higher percentage mycorrhizal colonization than when grown after uninoculated soybean and maize. Four AMF genera comprising 29 species were observed at Zaria and Zonkwa. Glomus was the dominant genus (56%) followed by Gigaspora (26%) and Acaulospora (14%). The genus Sclerocystis was the least represented (4%). Received: 28 October 1998