Amelioration of a degraded Oxic Paleustalf by leguminous and natural fallows

Abstract. The restorative ability of herbaceous ( Psophocarpus palustris, Pueraria phaseoloides ) and woody ( Leucaena leucocephala, Senna siamea, Acacia leptocarpa, Acacia auriculiformis ) legume species and of natural regrowth was studied on an eroded and compacted Oxic Paleustalf in southwestern Nigeria. Compared to the control treatment that was continuously cropped for 15 years, four years of fallowing significantly improved test crop yields. However, fallowing with the above species did not substantially improve soil properties, particularly soil bulk density. A longer fallow period may be needed to amend soil physical conditions of this degraded Alfisol. Soil chemical properties were greatly improved following land clearing and plant biomass burning in 1993. However, the residual effect of burning on soil fertility was insignificant in the second cropping year. Among the fallow species, P. palustris and natural fallow showed the best residual effect on test crop performance. Despite the high biomass and nutrient yields of S. siamea and A. auriculiformis , test crop yields on these plots were low due to the border effects from the uncleared and fallowed subplots.     

水旱轮作地区土壤长期休闲与耕种的肥力效应

长期定位试验水旱轮作地区土壤长期休闲和耕种对土壤肥力的影响研究结果表明，长期休闲土壤有机质、全N含量显著高于连续种植作物的土壤，其差异随土壤深度的增加而减小；休闲地15cm以上土层土壤N素矿化势高于耕种地；耕种土壤连续施入猪粪、作物秸秆等有机肥可保持与休闲土壤相当或远高于休闲土壤的有效磷水平，单施化肥或不施肥料的耕种土壤有效磷均低于休闲土壤；休闲和耕种对15-30cm土层土壤缓效钾含量无影响响，15cm以上土层土壤缓效钾含量休闲高于耕种；休闲土壤速效钾含量在整个耕作层（30cm）均高于耕种土壤；休闲土壤PH值略低于耕种土壤。     

Field hydraulic properties of an Alfisol under various fallow systems in southwestern Nigeria

Abstract. The effects of various fallow management systems and cropping intensities on water infiltration were measured on an Alfisol at Ibadan in southwestern Nigeria. The objective was to determine the influence of the land use systems (a combination of crop–fallow sequences and intercropping types) on soil hydraulic properties obtained by disc permeameter and double-ring infiltration measurements. The experiment was established in 1989 as a split-plot design with four replications. The main plots were natural fallow, planted Pueraria phaseoloides and planted Leucaena leucocephala . The subplots were 1 year of maize/cassava intercrop followed by 3-year fallow (25% cropping intensity), or 2-year fallow (33% cropping intensity), or 1-year fallow (50% cropping intensity), or no fallow period (100% cropping intensity). Water infiltration rates and sorptivities were measured under saturated and unsaturated flow. Irrespective of land use, infiltration rates at the soil surface (121–324 cm h⁻¹) were greater than those measured at 30 cm depth (55–144 cm h⁻¹). This indicated that fewer large pores were present below 30 cm depth compared with 0–30 cm depth. Despite some temporal variation, sorptivities with the highest mean value of 93.5 cm h^−½ increased as the cropping intensity decreased, suggesting a more continuous macropore system under less intensive land use systems. This was most likely due to continuous biopores created by perennial vegetation under long fallow systems. Intercropped maize and cassava yields also increased as cropping intensity decreased. The weak relationship between crop yields and hydraulic conductivity/infiltration rates suggests that the rates were not limiting.     

Long‐term effects of cropped vs. fallow and fertilizer amendments on soil organic matter I. Organic carbon

A long‐term field experiment, conducted since 1962 in Gumpenstein (Austria) on a Dystric Cambisol, was used for the present investigation. We combined a physical fractionation procedure with the determination of natural abundance of ¹³C and FT‐IR spectroscopy to study the influence of fertilizer amendments (organic manure and mineral fertilizers) and management practices (fallow vs. cropped) on changes in organic carbon (OC) associated with different particle‐size fractions. The OC content in bulk soil decreased or was not affected by slurry+straw, PK, and NPK treatments in both fallow and cropped plots after 28 and 38 yr of treatment. However, OC in plots receiving organic manures increased depending on the quality of the organic manures applied. The ranking among the different treatments under both fallow and cropped plots was: animal manure (liquid) > animal manure (solid) > cattle slurry = slurry+straw = PK = NPK. Results showed that the two types of management practices, fallow (non‐tilled) vs. cropped (tilled) had effects on OC concentrations. Comparing the OC contribution of particle‐size fractions to the total OC amount revealed the following ranking: silt > clay > fine sand > coarse sand except in the plots receiving solid or liquid animal manure. Size fractions within treatments showed larger variations of ¹³C abundances than bulk samples between treatments. The natural abundances of ¹³C increased especially in cropped (and tilled) plots. It was shown by cluster analysis that FT‐IR spectra differentiated between the different treatments originating from different land management practices. The present study revealed that below‐ground C deposition by agricultural plants can hardly compensate the C losses due to tillage.     

Compression characteristics of a clay soil as influenced by crops and sampling dates

This study was undertaken to determine the influence of previous crop sequences on soil compression behaviour. Soil compression tests were performed on undisturbed soil samples of a Kamouraska clay which had been cropped continuously for 4 years to barley, corn, alfalfa, or timothy. Samples were taken from the surface soil in the non-trafficked zone, five times during the season. Cropping treatments had small but significant effects on soil compression characteristics. Soils on which crops were grown presented lower bulk density under standard compression (300 kPa) and higher pre-compression pressure values than the soil left unplanted (fallow). These effects were attributed to water content at time of sampling which was lower in the cropped than in the bare soils. Seasonal variations of these two virgin compression parameters were important and were related to soil moisture. Conversely, the soil compression index (slope of the virgin compression line) was not affected by crops or water content and seems to be a characteristic dependent on soil constituents.     

Chemical and microbial properties of semiarid tropical soils of short-term fallows in Burkina Faso,West Africa

In the soudano–sahelian zone of Burkina Faso, the short-term fallow effect on the soil chemical and microbial properties was evaluated. In four farm experiments, two types of fallows were compared with cultivated fields: a natural vegetation fallow and a fallow enriched with Andropogon gayanus. After 5 to 7 years of experiments, soil chemical and microbial characteristics were determined in laboratory for 0–10 cm soil depth. Soil organic carbon (+64%), nitrogen (+35%), microbial biomass (+76%), basal respiration (+141%), and β-glucosidase activity (+86%) were significantly higher in fallows plots than in cultivated fields. The metabolic quotient was not significantly different on fallows compared to the cropped plots. Also, no significant difference was highlighted between natural vegetation fallows and the A. gayanus-enriched one.     

Runoff and soil erosion under rainfall simulation of Andisols from the Ecuadorian Páramo: effect of tillage and burning

In northern Ecuador, soils of high altitude grasslands (páramos) are mainly non-allophanic Andisols developed on Holocenic volcanic ash. These soils have a high water retention capacity and are the “water tank” of central Ecuador. To assess the effect of land use (burning and tillage) on soil hydrodynamic properties, rainfall simulation was conducted at two different sites. At Pichincha near Quito, the simulation was conducted on a recent volcanic ash soil comparing natural, tilled and burned plots. At El Angel, the simulation was conducted on a mature non-allophanic Andisol comparing natural, recently tilled and formerly cultivated plots.On natural plots, the infiltration rate was very high and sediment loss very low. Results for infiltration rate and runoff indicated that land use change on páramos increased runoff flow and reduced saturated hydraulic conductivity. Superficial reorganisation of the soil surfaces occurred on tilled plots at both sites. This crusting process was fast and resulted in surfaces with very low conductivity at Pichincha. The same processes seemed to be slower at El Angel. The soil surface of recent Andisols at Pichincha was prone to crusting whereas the mature Andisol, at El Angel, with a lower bulk density, was compacted when the kinetic energy of raindrops was high. Water repellency occurred after burning at Pichincha and following long natural air drying after tillage in the non-allophanic A horizon at El Angel. Water repellency combined with the low bulk density of soil aggregates explain the intensity of sediment losses in the abandoned soils after cultivation (Bare fallow plots). Erosion occurred in these areas through floating hydrophobic and stable aggregates.     

Wheat and barley straw decomposition under field conditions: Effect of soil type and plant cover on weight loss,nitrogen and potassium content

The decomposition of wheat and barley straw buried in a coarse sand and a sandy loam soil was followed under field conditions using chopped straw enclosed in mesh bags. The straw was buried at 10 cm depth in early autumn. Plots were either kept fallow or covered in plants during the summer and autumn following straw burial. Samples were recovered on 10 dates over 15 months and were analyzed for weight loss, C, N and K content. Results obtained were corrected for organic matter and nutrients introduced to the straw by soil entering the mesh bags. Soil temperatures and soil moisture contents were determined.The plant cover reduced soil temperatures, soil moisture contents and straw weight loss rates during the summer. The overall weight loss pattern was similar for the two straw types. In both soils, the straw showed an initial weight loss of 30% during the first month after burial. After 6 months (spring), the straw had lost about 50% of its weight. The weight loss then proceeded faster in fallow than in the planted plots. Seasonal changes in weight loss rates were relatively small. The absolute weight loss rates, determined by a simple linear regression model, ranged from 0.10% day^?1in planted plots of the coarse sand to 0.17% day^?1 in uncovered plots of the sandy loam. For planted plots, weight losses after 15 months of exposure were 67% in the coarse sand and 78% in the sandy loam. Corresponding values for fallowed plots were 75 and 92%, respectively.Initial straw N contents differed, but overall N dynamics was less influenced by straw type than by soil type. In the sandy loam, net immobilization began just after straw burial and reached a maximum in the spring, where 3 mg N g^?1 initial straw was immobilized. The N release was more rapid in fallowed than in planted plots. In the coarse sand, N was initially leached from the straw, and the immobilization of N was less extensive than in the sandy loam. Further, the release of N from the straw was grossly similar in fallowed and planted plots. For all treatments, the net release of N from the straw began when the C-to-N ratio was between 28 and 35.Most of the straw K content was leached during the first month after straw burial. Subsequent straw K content was not affected by initial K content or plant cover.     

Comparison of DTPA and resin extractable soil Zn to plant Zn uptake

Abstract

Extraction of soil zinc with routine chemical extractants does not always reflect differences in Zn availability as detected by plant uptake. This study was undertaken to explore and compare the use of an ion exchange resin and diethylenetriaminepentaacetic acid (DTPA) for extracting soil Zn as related to plant Zn uptake. Beans were grown in 1989 following differential cropping with corn and beans or fallow in 1988 on a Portneuf silt loam near Kimberly, Idaho. Two Zn fertilizer treatments were imposed across previous cropping treatments. A batch method for determining resin extractable soil Zn was established.

Both plant Zn concentration and Zn uptake by beans in 1989 were significantly higher in Zn fertilized than unfertilized treatments regardless of previous crop; and higher in plots previously cropped with corn than beans or fallow, regardless of Zn treatment. DTPA and resin extractable soil Zn were significantly higher in Zn fertilized plots compared to unfertilized plots but did not differ between previous cropping treatments. Resin and DTPA extractable soil Zn concentrations were positively correlated. Resin extracted soil Zn correlated better with plant Zn concentration and Zn uptake throughout the growing season than DTPA extracted soil Zn, particularly in plots that had been fallowed or previously cropped with corn. Resin may be extracting labile soil Zn not extracted with DTPA and, therefore, be better simulating plant uptake. Both extraction methods correlated better with Zn uptake when evaluated within cropping treatments, emphasizing the need to consider previous crop when calibrating soil tests.     

Vulnerability of Bavarian silty loam soil to compaction under heavy wheel traffic: impacts of tillage method and soil water content

Soil compaction caused by traffic of heavy vehicles and machinery has become a problem of world-wide concern. The aims of this study were to evaluate and compare the changes in bulk density, soil strength, porosity, saturated hydraulic conductivity and air permeability during sugar beet (Beta vulgaris L.) harvesting on a typical Bavarian soil (Regosol) as well as to assess the most appropriate variable factors that fit with the effective controlling of subsequent compaction. The field experiments, measurements and laboratory testing were carried out in Freising, Germany. Two tillage systems (conventional plough tillage and reduced chisel tillage) were used in the experiments. The soil water contents were adjusted to 0.17 g g⁻¹ (w₁), 0.27 g g⁻¹ (w₂) and 0.35 g g⁻¹ (w₃).Taking the increase in bulk density, the decrease in air permeability and reduction of wide coarse pore size porosity (−6 kPa) into account, it seems that CT (ploughing to a depth of 0.25 m followed by two passes of rotary harrow to a depth 0.05 m) of plots were compacted to a depth of at least 0.25 m and at most 0.40 m in high soil water (w₃) conditions. The trends were similar for “CT w₁” (low soil water content) plots. However, it seems that “CT w₁” plots were less affected than “CT w₃” plots with regard to bulk density increases under partial load. In contrast, diminishments of wide coarse pores (−6 kPa) and narrow (tight) coarse pores (−30 kPa) were significantly higher in “CT w₁” plots down to 0.4 m. Among CT plots, the best physical properties were obtained at medium soil water (w₂) content. No significant increase in bulk density and no significant decrease in coarse pore size porosity and total porosity below 0.2 m were observed at medium soil water content. The soil water content seemed to be the most decisive factor.It is likely that, CS (chiselling to a depth of 0.13 m followed by two passes of rotary harrow to a depth 0.05 m) plots were less affected by traffic treatments than CT plots. Considering the proportion of coarse pore size porosity (structural porosity) and total porosity, no compaction effects below 0.3 m were found. Medium soil water content (w₂) provides better soil conditions after traffic with regard to wide coarse pore size porosity (−6 kPa), air permeability (at 6 and 30 kPa water suction), total porosity and bulk density. Proportion of wide coarse pores, air permeability and bulk density seems to be suitable parameters to detect soil compaction under the conditions tested.     

Effects of tillage practices combined with mucuna fallow on soil erosion and water dynamics on Ishigaki Island,Japan

Abstract

This study was conducted on a sloping field at the Japan International Research Center for Agricultural Sciences, Okinawa Subtropical Station, Ishigaki Island, Okinawa Prefecture, Japan, to evaluate the effects of zero tillage farming combined with mucuna fallow as a cover crop on soil erosion and water dynamics. Two fallow systems (natural and mucuna) in combination with two soil tillage treatments (zero tillage and conventional tillage) were imposed on three sloping fields (2.0°, 3.5° and 5.0°). A sorghum crop (Sorghum bicolor (L) moench) was planted after the soil tillage treatment. Soil loss for zero tillage farming combined with mucuna fallow was equivalent to only 3% of that for the conventional tillage farming with natural fallow. Runoff water was also reduced by between 74% and 77% when compared with the conventional tillage system with natural fallow. These results indicate that zero tillage with mucuna fallow is a very effective measure for the control of soil erosion and water runoff. Moreover, this farming system improved water infiltration during both the fallow and the sorghum cropping periods. For the zero tillage plot, water loss as deep percolation increased 1.6-fold compared with that for the conventional farming under heavy rainfall conditions. It is expected that under less rainfall areas or seasons, the effects of zero tillage farming combined with the mucuna cover crop may be more pronounced on water runoff control and, therefore, may greatly improve soil water conditions.     

Fallow management practices in Guatemala's Western Highlands: social drivers and biophysical impacts

Land pressures and environmental degradation are driving forces behind shortened fallow periods in the tropics, often resulting in reduced crop yields and increased migration from rural areas. This paper describes contemporary fallow practices in the Western Highlands of Guatemala based on interdisciplinary data collected using participatory rural appraisal and qualitative research methods in combination with a quantitative evaluation of the impacts of fallow management decisions on soil fertility. Case studies of two communities in San Marcos department illustrate contemporary and traditional land use practices. Currently, over 70 per cent of families engage in a variety of fallow management practices, with combined cropping‐fallow cycles within a field averaging 3–6 years. Despite the reduction in length of fallow cycles, new fallow practices in the study area appear to improve some aspects of soil fertility while also providing fodder and fuelwood. Calcium and magnesium concentrations in fallow soil were twice that of cropped plots, indicating that weathering reactions and atmospheric deposition during fallow periods are able to restore base cation fertility that is taken up by potato crops during cropping cycles. Soil in cropped plots, however, showed 25 per cent higher soil organic matter and five times higher nitrate concentrations than soil in fallow plots, which resulted from additions of compost and inorganic fertilizer to cropped plots. Nevertheless, the ¹³C/¹²C isotopic ratio of soil organic carbon indicated that as soil organic matter content decreases in cropped plots, the remaining carbon is increasingly degraded. Potential improvements in fallow management practices proposed by farmers and researchers are also presented. Copyright © 2007 John Wiley & Sons, Ltd.     

不同耕作深度对红壤坡耕地耕层土壤特性的影响

红壤坡耕地不同耕作深度对耕层质量和作物产量具有重要影响。以江西红壤坡耕地示范区耕层为研究对象,从土壤属性角度,对红壤坡耕地不同耕作深度处理下垂直深度土壤水分、容重、孔隙度、土壤紧实度、土壤抗剪强度、土壤有机质、有效磷和速效钾等进行分析。结果表明:(1)不同耕作深度对土壤孔隙度、饱和含水量和田间持水量的影响为免耕翻耕20 cm翻耕10 cm常规耕作翻耕30 cm,对容重的影响为翻耕30 cm常规耕作翻耕10 cm免耕翻耕20 cm;与常规耕作比较,翻耕30 cm使土壤饱和含水量、田间持水量和土壤孔隙度分别提高了18.17%,12.67%,5.94%,土壤容重降低6.90%。(2)不同耕作深度下土壤紧实度表现为翻耕30 cm翻耕10 cm翻耕20 cm免耕常规耕作,土壤抗剪强度表现为翻耕30 cm常规耕作翻耕10 cm免耕翻耕20 cm;与常规耕作对照,翻耕30 cm使土壤紧实度和抗剪强度分别降低27.07%和24.82%。(3)土壤有机质含量以翻耕20 cm处理下最高(13.48 g/kg),免耕处理含量最低(9.39 g/kg),土壤速效养分主要集中分布在0-20 cm土层,但20-40 cm土层中翻耕处理较免耕处理有不同程度的增加,以翻耕20 cm和常规耕作表现显著。(4)主成分分析结果表明,翻耕30 cm处理对红壤坡耕地土壤的综合改善效果最好。研究结果可为红壤坡耕地耕层土壤改善和合理耕层构建提供技术参考。     

Physical environment near the surface of plowed and no-tilled soils

Mechanical tillage is a dynamic soil process that influences the physical environment near the surface, thus affecting biological processes in the soil. Water content, bulk density, air permeability, hydraulic conductivity and organic carbon were compared for moldboard plowed and non-tilled conditions at five locations from east-central U.S.A. to the Great Plains. Soils were Blount, Maury, Nicollet, Webster and Crete-Butler cropped to continuous corn (Zea mays L.), and Alliance and Duroc cropped to wheat/fallow (Triticum aestivum L.). Major differences in soil physical characteristics between tillage practices were largely confined to the top 75 mm of soil. The volumetric water and organic carbon (C) contents of the 0–150-mm layer at time of sampling ranged from 8 to 66% and 12 to 75% higher, respectively, in no-tilled than in plowed soils. Bulk density was greater and total porosity in the surface layer was as much as 10% less for no-tilled than for plowed treatments. Because of generally higher water contents and/or lower porosity, the water-filled pore space (WFPS) in the 0–150-mm layer of no-tilled soils was 6–28% higher than that of plowed soils. Air permeability in the surface layer of no-tilled soil was less than for plowed, but there were no differences due to tillage in the 75–150-mm depth. Physical soil characteristics influence the soil water regime, and thereby affect rate of biological reactions in the soil. At many points in the soil drying cycle, water added to a no-tilled soil usually creates a less aerobic environment compared to adding the same amount of water to plowed counterpart.     

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。     

Long‐term effects of cropped vs. fallow and fertilizer amendments on soil organic matter II. Nitrogen

The present study evaluated the effect of fertilizer amendments (organic manure and mineral fertilizers), management practices (fallow and untilled vs. cropped and tilled) on changes of N in bulk soil and N associated with different particle‐size fractions. The long‐term field experiment was conducted since 1962 in Gumpenstein, Austria, on a Dystric Cambisol. The N content of the topsoils changed distinctively during 28 and 38 yr of treatments under both fallow and cropped management practices. Highest increase in total N content was found in animal‐manure (liquid)‐treated plots. The remaining ranking was: animal manure (solid) > cattle slurry > half cattle slurry + straw = PK = NPK. Quite short N‐half‐life values of around 2 yr were found for the cattle‐slurry application, while animal manure exhibited longer N‐half‐lives of around 8 yr. Crop removal of N and mineralization losses in cropped plots obviously were higher than N losses from the bare soil plots lacking a plant cover to keep N in the system. This was confirmed by a consistent shift in the natural ¹⁵N abundances. Comparing the mean N contribution of particle‐size fractions to the total N amounts revealed the following ranking after 28 and 38 yr of different treatments: silt > clay > fine sand > coarse sand, with small exceptions. Particle‐size separates showed more significant responses to changes in the N dynamics of the system due to the various treatments than the bulk soil and can be regarded as the better indicators in this respect.     

Winter legumes as a nitrogen source for no-tillage cotton

Winter legumes can serve dual purposes in no-tillage cropping systems. They can provide a no-tillage mulch, and supply a considerable quantity of N for thesummer crops. Cotton (Gossypium hirsutum L.) was no-tillage planted into crimson clover (Trifolium incarnatum L.), common vetch (Vicia sativa L.), and fallowed soil for two years to determine the effects of winter legume mulches on growth, yield, and N fertilizer requirements. The legumes were allowed to mature and reseed prior to planting cotton. The winter legumes produced no measurable changes in soil organic matter, N, or bulk density, but water infiltration was more rapid in the legume plots than in the fallowed soil. In the fallow system, 34 kg ha^?1 N fertilizer was required for near maximum yields. In the clover plots, yields without N fertilizer were higher than when N (34 and 68 kg ha^?1) was applied. In the vetch plots, cotton yields were highest without N fertilizer the first year, but yields were increased with 34 kg ha^?1 N the second year because of a poor vetch seed crop and a subsequently poor legume stand. In the clover plots, a 20–30% cotton seedling mortality occurred in one year, but this stand reduction apparently did not affect cotton yields. Winter legume mulches can provide the N needs for no-tillage cotton without causing an excessive and detrimental quantity of N in sandy soils naturally low in soil N (0.04%). Unless the reseeding legume systems are maintained for at least 3 years, the legumes do not, however, provide an economical N source for cotton when N fertilizer requirementsare low (34 kg ha^?1 in this study). A possible disadvantage of the system for reseeding legumes is that cotton planting is delayed 4–6 weeks beyond the normal planting date, which can reduce yields in some years.     

Soil C and N changes on conservation reserve program lands in the Central Great Plains

The Conservation Reserve Program (CRP) was initiated to reduce water and wind erosion on marginal, highly erodible croplands by removing them from production and planting permanent, soil-conserving vegetation such as grass. We conducted a field study at two sites in Wyoming, USA, in order to quantify changes in soil C and N of marginal croplands seeded to grass, and of native rangeland plowed and cropped to wheat–fallow. Field plots were established on a sandy loam site and a clay loam site on wheat–fallow cropland that had been in production for 60+ years and on adjacent native rangeland. In 1993, 6 years after the study was initiated, the surface soil was sampled in 2.5 cm depth increments, while the subsurface soil was composited as one depth increment. All soil samples were analyzed for total organic C and N, and potential net mineralized C and N. After 60+ years of cultivation, surface soils at both study sites were 18–26% lower (by mass) in total organic C and N than in the A horizons of adjacent native range. Six years after plowing and converting native rangeland to cropland (three wheat–fallow cycles), both total and potential net mineralized C and N in the surface soil had decreased and NO₃–N at all depths had increased to levels found after 60+ years of cultivation. We estimate that mixing of the surface and subsurface soil with tillage accounted for 40–60% of the decrease in surface soil C and N in long-term cultivated fields; in the short-term cultivated fields, mixing with tillage may have accounted for 60–75% of the decrease in C, and 30–60% of the decrease in N. These results emphasize the need to evaluate C and N in the entire soil solum, rather than in just the surface soil, if actual losses of C and N due to cultivation are to be distinguished from vertical redistribution. Five years after reestablishing grass on the sandy loam soil, both total and potential net mineralized C and N in the surface soil had increased to levels equal to or greater than those observed in the A horizon of the native range. On the clay loam soil, however, significant increases in total organic C were observed only in the surface 2.5 cm of N-fertilized grass plots, while total organic N had not significantly increased from levels observed in the long-term cultivated fields.     

FALLOW VERSUS WHEAT CROPPING OF UNAMENDEDAND MANURE-AMENDED SOILS RELATED TO MYCORRHIZAL COLONIZATION,YIELD, AND PLANT NUTRITION OF DRY BEAN AND SWEET CORN

Previous studies reported manure application to eroded Portneuf silt loam soil (Coarse-silty, mixed, superactive, Durinodic Xeric Haplocalcid) improved dry bean (Phaseolus vulgaris L., cv. Viva) yield to levels of topsoil. These yield increases only correlated with whole-plant zinc (Zn) concentration and soil organic matter. This might be related to enhanced arbuscular mycorrhizal (AM) colonization stimulated by manure application. A greenhouse study with dry bean suggested a relationship between manure application, increased AM colonization, and whole-plant Zn uptake, while field studies with wheat (Triticum aestivum L.) and sweet corn (Zea mays L.) did not. To clarify the apparent contradiction of manure application on AM relationships, the present field study with dry bean and sweet corn was conducted in subsoils on the same experimental site established in 1991 and used in previous studies. The existing rotation also allowed the study of the effects of previously fallowed versus wheat cropped subsoils on yield, AM colonization and nutrition of dry bean and sweet corn. Average mycorrhizal root colonization in dry bean was greater on unamended than on manure-amended soils but was not related to increases in yield, Zn concentration, or Zn uptake. Average colonization of sweet corn roots was generally greater in unamended than manure-amended soils, but yields were greater in manure-amended soils. Colonization of sweet corn roots measured over time was consistently greater in subsoils previously cropped to wheat than fallowed, but yields were similar. Previous wheat-cropping resulted in leveling off of colonization beginning 7 July (second sampling) in dry bean, while previous fallow resulted in continuously increasing colonization throughout the five sampling periods. Bean yields were greater on subsoils previously cropped than fallowed; thus yields were generally not related to AM colonization. Results of our study confirm other field results where AM colonization was greater in unamended than manure-amended soils and in cropped than in fallowed soils. Any yield increases observed were not closely related to AM colonization.     

Soil management alters seedling emergence and subsequent autumn growth and yield in dryland winter wheat–fallow systems in the Central Great Plains on a clay loam soil

Stand establishment and subsequent autumn development and growth are important determinants of winter wheat (Triticum aestivum L.) yield. Soil management practices change soil properties and conditions, which alter seedling emergence, crop development and growth. Pre-plant soil management practices were studied for 6 years in a wheat–fallow rotation in eastern Colorado, USA, to isolate the impacts of pre-plant tillage (PT) and residue level on winter wheat seedling emergence and autumn development and growth. A split plot design was used with PT, using a moldboard plow that incorporated surface residue, and with no-tillage (NT). The tillage systems represented the main plots and three residue levels within each tillage treatment as subplots: no residue (0R), normal residue (1R) and twice-normal residue (2R). Residue amount had little effect on emergence or autumn growth and development. PT resulted in soil water loss from the plow zone. NT plots had more favorable soil water levels in the seeding zone which resulted in faster, more uniform and greater seedling emergence in 4 out of the 6 years. This is especially critical for stand establishment in years with low rainfall after planting. Soil or air temperature did not account for differences among treatments. Earlier and greater seedling emergence in NT treatments resulted in greater autumn development and growth. Shoot biomass, tiller density and leaf numbers were greater in NT, and again residue amount had little effect. At spring green-up, NT treatments had greater soil water in the profile. Grain yield was always equal or greater in NT than in PT, and positively correlated with earlier/greater seedling emergence and autumn growth. NT will enhance soil protection and likely increase snow catch, reduce evaporation and benefit yield in semiarid eastern Colorado.