Fine root dynamics and trace gas fluxes in two lowland tropical forest soils

Fine root dynamics have the potential to contribute significantly to ecosystem‐scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 (±35) and 153 (±27) g m^?2 yr^?1 for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k=?0.96 year^?1) than in the sandy loam soil (k=?0.61 year^?1), leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N₂O) emissions were significantly greater in the clay soil (13±1 ng N cm^?2 h^?1) than in the sandy loam (1.4±0.2 ng N cm^?2 h^?1). Root mortality and decay following trenching doubled rates of N₂O emissions in the clay and tripled them in sandy loam over a 1‐year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264–380 g C m^?2 yr^?1, accounting for 24% to 35% of the total soil CO₂ efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521±206 g C m² yr^?1) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land‐use change can contribute significantly to increased rates of nitrogen trace gas emissions.     

Resource allocation in the submerged plant Vallisneria natans related to sediment type, rather than water-column nutrients

1. Phenotypic plasticity in resource allocation by Vallisneria natans was investigated in a greenhouse experiment, using three types of sediment [sandy loam, clay, and a 50 : 50 (by volume) mixture of the two sediments] and two levels of water‐column nutrient. The clay was collected from a highly eutrophic lake in Jiangsu Province, China, and the N and P concentrations applied in nutrient media were at the upper limits observed in most lakes of China. 2. Growth and biomass allocation were significantly affected by sediment type, rather than water‐column nutrients. Plant growth in clay and the mixture were similar, and 2.4–3.4 times higher than that in sandy loam. Compared with the plants grown in clay or the mixed sediments, the plants grown in sandy loam allocated relatively more biomass to root (11–17% versus 7–8% of total biomass), and relatively less to leaf (76–82% versus 86–87% of total biomass). Plastic variations in root area were induced by sediment type alone (P < 0.05), whereas the impacts of sediment type and water‐column nutrients on leaf area were insignificant (P > 0.05). 3. Plant N and P concentrations were significantly affected by both sediment type and water‐column nutrients. Increased nutrient availability in the water column enhanced plant N concentration by 3.5–20.2%, and plant P concentration by 19.1–25.8%. 4. Biomass accumulation and plant nutrient concentration in plants grown in different sediment types and water‐column nutrients indicate that sediment type had more significant impacts on growth and N and P concentrations of V. natans than did water‐column nutrients. Changes in phenotype are a functional response to nutrient availability in sediment, rather than to water‐column nutrients.     

Fungal occurrence,disease incidence and severity,and yield of maize symptomatic for seedling disease in Mississippi

A study was conducted in Mississippi from 1995 to 1997 comparing soil rhizosphere fungi isolated from Pioneer 3167 hybrid maize (Zea mays L.) planted on Brooksville silty clay and Memphis silt loam soils. Maize seedlings were collected over four sampling dates from conventional and no-tillage plots. Eleven fungal genera consisting of nineteen species were isolated from these plants; Trichoderma spp. were most frequently isolated, followed by Fusarium spp. The highest disease incidence occurred in tilled plots of the latest planting date on Brooksville silty clay when samples were collected 17 days after planting. Root disease was most severe in 1996 from seedlings planted on the last planting date in tilled plots sampled 17 days after planting. Yields were significantly (P ≤ 0.05) higher on Brooksville silty clay soil than on Memphis silt loam in both 1995 and 1996. Yields were highest from no-tillage plots and from maize planted on the earliest date. There was a significant correlation between incidence of root infection and disease severity. There was no correlation between the incidence of root infection and yield or between disease severity and yield at either location. This revised version was published online in June 2006 with corrections to the Cover Date.     

Microbial production and uptake of nitric oxide in soil

Abstract Fluxes of NO from three different soils have been studied by a flow-through system in the laboratory as a function of gas flow rate, of NO mixing ratio, and of incubation conditions. The dependence of net NO fluxes on gas flow rates and on NO mixing ratios could be described by a simple model of simultaneous NO production and NO uptake. By using this model, rates of gross NO production, rate constants of NO uptake, and NO compensation mixing ratios could be determined as function of the soil type and the incubation condition. Gross NO production rates were one to two orders of magnitude larger under anaerobic than under aerobic conditions. NO uptake rate constants, on the other hand, were only 5–8 times larger so that the compensation mixing ratios of NO were in a range of about 1600–2200 ppbv under anaerobic and of about 50–600 ppbv under aerobic conditions. The different soils exhibited similar NO uptake rate constants, but the gross NO production rate and compensation mixing ratio was significantly higher in an acidic (pH 4.7) sandy clay loam than in other less acidic soils. Experiments with autoclaved soil samples showed that both NO production and NO uptake was mainly due to microbial metabolism.     

Nitrogen oxide gas emissions from temperate forest soils receiving long-term nitrogen inputs

From spring 2000 through fall 2001, we measured nitric oxide (NO) and nitrous oxide (N₂O) fluxes in two temperate forest sites in Massachusetts, USA that have been treated since 1988 with different levels of nitrogen (N) to simulate elevated rates of atmospheric N deposition. Plots within a pine stand that were treated with either 50 or 150 kg N ha^?1 yr^?1 above background displayed consistently elevated NO fluxes (100–200 µg N m^?2 h^?1) compared to control plots, while only the higher N treatment plot within a mixed hardwood stand displayed similarly elevated NO fluxes. Annual NO emissions estimated from monthly sampling accounted for 3.0–3.7% of N inputs to the high‐N plots and 8.3% of inputs to the Pine low‐N plot. Nitrous oxide fluxes in the N‐treated plots were generally < 10% of NO fluxes. Net nitrification rates (NRs) and NO production rates measured in the laboratory displayed patterns that were consistent with field NO fluxes. Total N oxide gas flux was positively correlated with contemporaneous measurements of NR and concentration. Acetylene inhibited both nitrification and NO production, indicating that autotrophic nitrification was responsible for the elevated NO production. Soil pH was negatively correlated with N deposition rate. Low levels (3–11 µg N kg^?1) of nitrite () were detected in mineral soils from both sites. Kinetic models describing NO production as a function of the protonated form of (nitrous acid [HNO₂]) adequately described the mineral soil data. The results indicate that atmospheric deposition may generate losses of gaseous NO from forest soils by promoting nitrification, and that the response may vary significantly between forest types under similar climatic regimes. The lowering of pH resulting from nitrification and/or directly from deposition may also play a role by promoting reactions involving HNO₂.     

Impact of Rotylenchulus reniformis on Cotton Yield as Affected by Soil Texture and Irrigation

The effects of soil type, irrigation, and population density of Rotylenchulus reniformis on cotton were evaluated in a two-year microplot experiment. Six soil types, Fuquay sand, Norfolk sandy loam, Portsmouth loamy sand, Muck, Cecil sandy loam, and Cecil sandy clay, were arranged in randomized complete blocks with five replications. Each block had numerous plots previously inoculated with R. reniformis and two or more noninoculated microplots per soil type, one half of which were irrigated in each replicate for a total of 240 plots. Greatest cotton lint yields were achieved in the Muck, Norfolk sandy loam, and Portsmouth loamy sand soils. Cotton yield in the Portsmouth loamy sand did not differ from the Muck soil which averaged the greatest lint yield per plot of all soil types. Cotton yield was negatively related to R. reniformis PI (initial population density) in all soil types except for the Cecil sandy clay which had the highest clay content. Supplemental irrigation increased yields in the higher yielding Muck, Norfolk sandy loam, and Portsmouth loamy sand soils compared to the lower yielding Cecil sandy clay, Cecil sandy loam, and Fuquay sand soils. The Portsmouth sandy loam was among the highest yielding soils, and also supported the greatest R. reniformis population density. Cotton lint yield was affected more by R. reniformis Pi with irrigation in the Portsmouth loamy sand soil with a greater influence of Pi on lint yield in irrigated plots than other soils. A significant first degree PI × irrigation interaction for this soil type confirms this observation.     

Development and use of a short-term nutrient-absorption technique for evaluating soil magnesium status

Summary A greenhouse investigation was undertaken to study and evaluate the use of a short-term nutrient-absorption technique for evaluating soil magnesium status. Barley (Hordeum vulgare), variety Arivat, was used as the test plant. The investigation included four experiments with the following objectives: (1) to determine the need for base applications of nitrogen and phosphorus in a soil-magnesium study using a short-term nutrient absorption technique; (2) to study the effect of base applications of N and P on Mg-uptake by plants under three time periods of root-soil contact; (3) to study the effect of increasing soil moisture from 75 to 100 per cent of the soil moisture equivalent on the plant uptake of Mg; and (4) to evaluate the short-term nutrient-absorption technique in determining the magnesium status in six different soils: Gila silt loam, Tours silty clay loam, Cajon clay loam, La Palma fine sandy loam, Yavapai sandy clay loam, and Casa Grande loam. Magnesium was applied in the form of MgSO₄ and Sul-PO-Mag.Plant growth, potassium, calcium, and magnesium uptake were increased by the base application of nitrogen and phosphorus using a 7-day period of root-soil contact.Plant growth was not affected by soil moisture level. Potassium and calcium concentrations in the plant were decreased with increasing soil moisture, but the total plant uptake of these nutrients was not affected. Total plant uptake and concentration of magnesium were increased by increasing soil moisture level.The results obtained in this study agree with previous observations that soil response to Mg does not depend upon the amount of exchangeable magnesium in the soil.Published as Arizona Agr. Expt. Station Technical Publication No.848.     

Effects of temperature and fertilization on nitrogen cycling and community composition of an urban lawn

We examined the influence of temperature and management practices on the nitrogen (N) cycling of turfgrass, the largest irrigated crop in the United States. We measured nitrous oxide (N₂O) fluxes, and plant and soil N content and isotopic composition with a manipulative experiment of temperature and fertilizer application. Infrared lamps were used to increase surface temperature by 3.5±1.3 °C on average and control and heated plots were split into high and low fertilizer treatments. The N₂O fluxes increased following fertilizer application and were also directly related to soil moisture. There was a positive effect of warming on N₂O fluxes. Soils in the heated plots were enriched in nitrogen isotope ratio ( δ ¹⁵N) relative to control plots, consistent with greater gaseous losses of N. For all treatments, C₄ plant C/N ratio was negatively correlated with plant δ ¹⁵N, suggesting that low leaf N was associated with the use of isotopically depleted N sources such as mineralized organic matter. A significant and unexpected result was a large, rapid increase in the proportion of C₄ plants in the heated plots relative to control plots, as measured by the carbon isotope ratio ( δ ¹³C) of total harvested aboveground biomass. The C₄ plant biomass was dominated by crabgrass, a common weed in C₃ fescue lawns. Our results suggest that an increase in temperature caused by climate change as well as the urban heat island effect may result in increases in N₂O emissions from fertilized urban lawns. In addition, warming may exacerbate weed invasions, which may require more intensive management, e.g. herbicide application, to manage species composition.     

Willow water uptake and shoot extension growth in response to nutrient and moisture on a clay landfill cap soil

Extension growth of willow (Salix viminalis L.) and changes in soil water were measured in lysimeters containing clay and sandy loam soils with different amendment and watering treatments. No water uptake was found below 0.3 m in the nutritionally poor unamended clay; amendment with organic matter to 0.4 m depth resulted in water extraction down to 0.5 m depth whereas in the sandy loam, there was greater extraction from all depths down to 0.6 m. With water stress, wilting of plants occurred when the volumetric soil water content at 0.1 m was about 31% in the clay and 22% in the sandy loam. Compared with shoots on plants in the amended clay, those in the unamended treatment showed reduced extension growth, little increase in stem basal area (SBA) and a small shoot leaf area, resulting from a reduced number of leaves shoot⁻¹ and a small average area leaf⁻¹. Water stress also reduced shoot extension growth, SBA gain and the leaf area on extension growth. Shoot growth rates were significantly correlated with air temperature and base temperatures between 2.0 and 7.6 °C were indicated for the different treatments. These studies have helped to explain some of the large treatment effects described previously on biomass production and plant leaf area.     

Physical and biological controls on trace gas fluxes in semi-arid urban ephemeral waterways

Rapid increases in human population and land transformation in arid and semi-arid regions are altering water, carbon (C) and nitrogen (N) cycles, yet little is known about how urban ephemeral stream channels in these regions affect biogeochemistry and trace gas fluxes. To address these knowledge gaps, we measured carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) before and after soil wetting in 16 ephemeral stream channels that vary in soil texture and organic matter in Tucson, AZ. Fluxes of CO₂ and N₂O immediately following wetting were among the highest ever published (up to 1,588 mg C m^?2 h^?1 and 3,121 μg N m^?2 h^?1). Mean post-wetting CO₂ and N₂O fluxes were significantly higher in the loam and sandy loam channels (286 and 194 mg C m^?2 h^?1; 168 and 187 μg N m^?2 h^?1) than in the sand channels (45 mg C m^?2 h^?1 and 7 μg N m^?2 h^?1). Factor analyses show that the effect of soil moisture, soil C and soil N on trace gas fluxes varied with soil texture. In the coarser sandy sites, trace gas fluxes were primarily controlled by soil moisture via physical displacement of soil gases and by organic soil C and N limitations on biotic processes. In the finer sandy loam sites trace gas fluxes and N-processing were primarily limited by soil moisture, soil organic C and soil N resources. In the loam sites, finer soil texture and higher soil organic C and N enhance soil moisture retention allowing for more biologically favorable antecedent conditions. Variable redox states appeared to develop in the finer textured soils resulting in wide ranging trace gas flux rates following wetting. These findings indicate that urban ephemeral channels are biogeochemical hotspots that can have a profound impact on urban C and N biogeochemical cycling pathways and subsequently alter the quality of localized water resources.     

Effects of soil compaction and mechanical damage at harvest on growth and biomass production of short rotation coppice willow

The effects of soil compaction and mechanical damage to stools at harvesting on the growth and biomass production of short rotation coppice (SRC) of willow (Salix viminalis L.) were monitored on clay loam (CL) and sandy loam (SL) soils. Moderate compaction, more typical of current harvesting situations did not reduce biomass yields significantly. Even heavy compaction only reduced stem biomass production by about 12% overall; effects were statistically significant only in the first year of the experiment on sandy loam. Heavy compaction increased soil strength and bulk density down to 0.4 m depth and reduced soil available water and root growth locally. Soil loosening treatments designed to alleviate the effects of heavy compaction did not markedly improve the growth of willow on compacted plots. Hence the focus fell on harvesting. Extensive mechanical damage to stools caused a 9% and 21% reduction in stem dry mass on the clay loam and sandy loam soils as a result of fewer stems being produced. The particularly severe effect on the sandy loam soil probably resulted from a combination of dry conditions in the year of treatment, root damage and soil compaction under stools and might have been aggravated by the young age of the plants (1 year) at the time of treatment.     

Fine root dynamics in relation to nutrients in oligotrophic Bornean rain forest soils

The fine roots of plants are key structures enabling soil resource acquisition, yet our understanding of their dynamics and the factors governing them is still underdeveloped, especially in tropical forests. We evaluated whether Bornean tree communities on soils with contrasting resource availability display different soil resource uptake strategies, based on their fine root properties and dynamics, and related responses of fine roots to the availability of multiple nutrients. Using root cores and ingrowth cores, we quantified variation in community-level fine root properties (biomass, length, and area) and their growth rates, biomass turnover rate, and specific root length (SRL) between clay and sandy loam soils, on which tree community composition differs dramatically. We found that standing fine root biomass and biomass, length, and area growth were higher in sandy loam, the soil type that is better-drained, coarser-textured, and less fertile for most nutrients. In clay SRL was significantly greater, and turnover tended to be faster, than in sandy loam. Across both soils, greater supplies of K⁺, NH₄ ⁺, and PO₄ ^3? were associated with greater standing biomass and growth rates of fine roots, suggesting foraging for these nutrients. Our data support the hypothesis that the sandy loam tree community achieves fine root absorptive area through faster growth and greater investment on a mass basis, whereas trees on clay achieve a similar standing absorptive area through slower growth of less-dense fine root tissues. Furthermore, our results suggest colimitation by multiple nutrients, which may enhance tree species coexistence through increased dimensionality of soil-resource niches.     

Growth and root distribution of Vallisneria natans in heterogeneous sediment environments

Plant growth, biomass allocation and root distribution were investigated in the submerged macrophyte Vallisneria natans growing in heterogeneous sediments. Experimentally heterogeneous sediment environments were constructed by randomly placing 4 cm of clay or sandy loam into the top (0–4 cm) or bottom (4–8 cm) layer within an experimental tray, providing two homogeneous and two heterogeneous treatments. Biomass accumulation was significantly affected by the experimental treatments: higher in the homogeneous sediment of clay (32 mg per plant) and the two heterogeneous treatments (about 27 mg per plant), but lower in the homogeneous sediment of sandy loam (15 mg per plant). Root: shoot ratio was also different among the four treatments. Compared with the treatments of clay in the top layer, plants allocated more biomass to roots at the treatments of sandy loam in the top layer. Heterogeneous sediments significantly affected root distribution pattern. Compared with the treatments of sandy loam in the bottom layer, root number (7–8 versus 13–14) and total root length (3.6–4.0 cm versus 29.5–40.0 cm) in the bottom layer were significantly higher in the treatments with clay in the bottom layer. These results indicate that both sediment structure and nutrient availability influence growth and root system distribution of V. natans.     

Laboratory and field evaluation of broiler litter nitrogen mineralization

Two studies were conducted for this research. First, a laboratory incubation to quantify broiler litter N mineralization with the following treatments: two soil moisture regimes, constant at 60% water fill pore space (WFPS) and fluctuating (60-30% WFPS), three soil types, Brooksville silty clay loam, Ruston sandy loam from Mississippi, and Catlin silt loam from Illinois. Second, a field incubation study to quantify broiler litter N mineralization using similar soils and litter application rates as the laboratory incubation. Broiler litter was applied at an equivalent rate of 350 kg total N ha(-1) for both studies except for control treatments. Subsamples were taken at different timing for both experiments for NO3-N and NH4-N determinations. In the laboratory experiment, soil moisture regimes had no significant impact on litter-derived inorganic N. Total litter-derived inorganic N across all treatments increased from 23 mg kg(-1) at time 0, to 159 mg kg(-1) at 93 d after litter application. Significant differences were observed among the soil types. Net litter-derived inorganic N was greater for Brooksville followed by Ruston and Catlin soils. For both studies and all soils, NH4-N content decreased while NO3-N content increased indicating a rapid nitrification of the mineralized litter N. Litter mineralization in the field study followed the same trend as the laboratory study but resulted in much lower net inorganic N, presumably due to environmental conditions such as precipitation and temperature, which may have resulted in more denitrification and immobilization of mineralized litter N. Litter-derived inorganic N from the field study was greater for Ruston than Brooksville. Due to no impact by soil moisture regimes, additional studies are warranted in order to develop predictive relationships to quantify broiler litter N availability.     

Effect of long-term application of animal manure on physical properties of three soils

Surface soil samples to 15 cm depth were taken from replicated plots in an ongoing long-term field experiment involving application of animal manure on three soils in Virginia. The sampled plots had received either no manure or the equivalent of 289,000 kg ha^–1 of manure as dry weight. The manure was applied annually at the beginning of each spring for 15 years from 1978 through 1992. The plots were cropped similarly since 1978. Soil textures were a fine sandy loam at Holland in the Atlantic Coastal Plain region, a silt loam at Blacksburg in the Appalachian region, and a clay loam at Orange in the Piedmont region of Virginia. The following measurements were made on subsamples: liquid and plastic limits, wet aggregate stability, aggregate size distribution, dispersible clay percentage, water retention at 0. 03, 0.1, 0.3, 0.5, 1.0, and 1.5 MPa tension, and modulus of rupture of moulded briquettes at a water content corresponding to 0.1 MPa tension. Organic matter content by the Walkley-Black method was significantly higher in the manure-treated soils at all three locations. Increases were 3% for the sandy loam and 25% for the silt loam and clay loam. From these values it was estimated that at least 95% of the total applied manure had been degraded over the 15 years. Results showed that the liquid and plastic limits for all three soils were higher (p<0.05) for the manure-treated samples. However, the differences in the limits were only 2 to 3%. The modulus of rupture values were lowered by addition of the animal manure. Decreases (p<0.05) occurred for the silt loam and clay loam samples. The wet aggregate stability increased and the dispersible clay decreased in the manure-treated soils. Increases (p<0.05) in wet aggregate stability occurred for the sandy loam and silt loam samples. Decreases (p<0.05) in dispersible clay were measured for the sandy loam and clay loam samples. Water retention was consistently, but only slightly, increased by manure addition. The increases, in the order of sample texture, were clay loam > sandy loam silt loam. Increases tended to be higher at the lower values of tension. Manure addition consistently increased the weight percentages of aggregates passing a given mesh size. Increases, in order of sample texture, were silt loam > clay loam > sandy loam. In their entirety, these results show that the manure produced measurable changes in the soil physical properties. The magnitude of the changes, in most cases, were small and depended on the soil texture. Given the high total amount of manure applied, the results indicate that manure-induced physical changes in the soil were small and evidently did not accumulate over time. Rapid microbial degradation of the manure could be responsible for the lack of marked changes in the soil physical properties.     

不同冬小麦品种在3种质地土壤中氮代谢特征及利用效率分析

为明确不同土壤质地条件下不同品种冬小麦(Triticum aestivum)的氮代谢和利用特征, 筛选与土壤质地相适宜的高产和氮高效利用的优质小麦品种, 采用大田试验的方法, 在同一生态类型区砂土、壤土和黏土3种质地土壤上, 以当地生产上大面积应用的强筋小麦‘郑麦366’ (‘ZM366’)和中筋小麦‘矮抗58’ (‘AK58’)、‘周麦22’ (‘ZM22’)为材料, 系统地研究了土壤质地对不同冬小麦品种主要生育时期叶片氨同化关键酶谷氨酰胺合成酶(GS)活性、游离氨基酸含量、花前和花后不同器官氮素积累和分配、氮素再分配等氮代谢过程及产量、品质和氮素利用效率等的影响。结果表明: 在这3种土壤质地上, 不同品种冬小麦旗叶GS活性和游离氨基酸含量均呈倒“V”型变化特征。各品种小麦旗叶GS活性、游离氨基酸含量大小及达到最大值的时期不一样, 砂土条件下峰值早于壤土10天左右出现, 且在5月22日已检测不到GS活性和游离氨基酸含量。花前和花后小麦地上部及各器官氮积累量(NA)、氮再分配量(NR)、成熟期籽粒产量和氮素当季利用率(NUE)均以壤土上为最高。氮素转运率(NRE)、花前再分配氮素对籽粒氮素的贡献率(NRC)、氮素生理效率(NPE)、氮收获指数(NHI)以砂土上为最高。其中, 砂土上NRC达82.46%-95.84%, 是花后的7倍左右; 壤土和黏土条件下花后吸收的氮素在籽粒氮素的积累中占有较大的比例, 贡献率分别为36.6%和29.2%。同一土壤质地上3个品种比较, 在砂土上, GS活性、游离氨基酸含量、籽粒产量、蛋白质含量及NUE和NPE以‘郑麦366’最高, 而壤土上以‘矮抗58’最高, 黏土上则以‘周麦22’最高。因此, 在生产上应培育和选择与土壤质地相适应的小麦品种, 砂土地种植‘郑麦366’, 壤土条件下种植‘矮抗58’, 黏土条件下种植‘周麦22’, 可以在获得较高产量和品质的同时, 提高氮素利用效率。     

Soil types with different texture affects development of Rhizoctonia root rot of wheat seedlings

The effect of different soil textures, sandy (97.5% sand, 1.6% silt, 0.9% clay), loamy sand (77% sand, 11% silt, 12% clay) and a sandy clay loam (69% sand, 7% silt, 24% clay), on root rot of wheat caused by Rhizoctonia solani Kühn Anastomosis Group (AG) 8 was studied under glasshouse conditions. The reduction in root and shoot biomass following inoculation with AG-8 was greater in sand than in loamy sand or sandy clay loam. Dry root weight of wheat in the sand, loamy sand and sandy clay loam soils infested with AG-8 was 91%, 55% and 28% less than in control uninfested soils. There was greater moisture retention in the loamy sand and sandy clay loam soils as compared to the sand in the upper 10–20 cm. Root penetration resistance was greater in loamy sand and sandy clay loam than in sand. Root growth in the uninfested soil column was faster in the sand than in the loamy sand and sandy clay loam soils, the roots in the sandy soil being thinner than in the other two soils. Radial spread of the pathogen in these soils in seedling trays was twice as fast in the sand in comparison to the loamy sand which in turn was more than twice that in the sandy clay loam soil. There was no evidence that differences among soils in pathogenicity or soil spread of the pathogen was related to their nutrient status. This behaviour may be related to the severity of the disease in fields with sandy soils as compared to those with loam or clay soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Prediction of nitrogen responses of corn by soil nitrogen mineralization indicators

Soil nitrogen mineralization potential (N min) has to be spatially quantified to enable farmers to vary N fertilizer rates, optimize crop yields, and minimize N transfer from soils to the environment. The study objectives were to assess the spatial variability in soil N min potential based on clay and organic matter (OM) contents and the impact of grouping soils using these criteria on corn grain (Zea mays L.) yield, N uptake response curves to N fertilizer, and soil residual N. Four indicators were used: OM content and three equations involving OM and clay content. The study was conducted on a 15-ha field near Montreal, Quebec, Canada. In the spring 2000, soil samples (n = 150) were collected on a 30- x 30-m grid and six rates of N fertilizer (0 to 250 kg N ha(-1)) were applied. Kriged maps of particle size showed areas of clay, clay loam, and fine sandy loam soils. The N min indicators were spatially structured but soil nitrate (NO3-) was not. The N fertilizer rate to reach maximum grain yield (N max), as estimated by a quadratic model, varied among textural classes and Nmin indicators, and ranged from 159 to 250 kg N ha(-1). The proportion of variability (R2) and the standard error of the estimate (SE) varied among textural groups and N min indicators. The R2 ranged from 0.53 to 0.91 and the SE from 0.13 to 1.62. Corn grain N uptake was significantly affected by N fertilizer and the pattern of response differed with soil texture. For the 50 kg N ha(-1) rate, the apparent N min potential (ANM) was significantly larger in the clay loam (122 kg ha(-1)) than in the fine sandy loam (80 kg ha(-1)) or clay (64 kg ha(-1)) soils. The fall soil residual N was not affected by N fertlizer inputs. Textural classes can be used to predict N max. The N min indicators may also assist the variable rate N fertilizer inputs for corn production.     

EFFICACY OF AN IMMUNO-MAGNTIC SEPARATION SYSTEM FOR RECOVERING CRYPTOSPORIDIUM OOCYSTS FROM SOILS

Studies were conducted to determine the efficacy of the commercially available immuno-magnetic system by Dynal^TM to recover C. parvum oocysts from silty clay, sandy clay loam and clay soils. Each soil type was spiked with known numbers of oocysts and their recovery using percoll-sucrose gradient centrifugation in combination with immuno-magnetic separation system was evaluated. The recoveries varied significantly. The silty clay loam soil had the highest recovery ranging between 91% and 26%, while the sandy clay loam had the lowest recovery ranging between 30% and 2%. The results indicate that though the Dynal^TM IMS system is capable of recovering oocysts from soils, the recovery efficiencies can vary significantly.     

Effect of Soil Texture on the Distribution and Infectivity of Neoaplectana carpocapsae (Nematoda: Steinernematidae)

The vertical migration of N. carpocapsae infective juveniles applied to the soil surface or introduced 14 cm below the soil surface was studied in four different soil types (pure silica sand, coarse sandy loam, silty clay loam, and clay). The percentage of juveniles able to migrate and infect wax moth pupae placed in the soil decreased as the percentage of clay and silt increased. Most nematodes placed on the soil surface remained within 2 cm of the surface, but some penetrated to a depth of 10 cm in pure silica sand and coarse sandy loam to infect pupae. Some pupae at the same depth were also infected with nematodes in silty clay loam soil. In pure silica sand and coarse sandy loam, nematodes introduced 14 cm below the soil surface were able to infect wax moth pupae located between 4 and 24 cm. Movement was least in clay soil and limited in silty clay loam. Nematodes showed a tendency to disperse upwards from the point of application. In all cases the number of migrating nematodes was greatest when wax moth pupae were present.