Native Prairie Soil Research Articles

Soil compaction and arbuscular mycorrhizae affect seedling growth of three grasses

Soil compaction is a limitation to establishment of native forest species on reclaimed surfacemined lands in Appalachia. Previously, non-native forage species such as tall fescue (Schedonorus arundinaceus(Schreb.) Dumort., nom. cons.) have been planted because they easily established on reclaimed mine soil. There is now interest in establishing robust native prairie species to enhance biodiversity and provide greater potential for root activity in the compacted soil. We conducted a 10-week glasshouse study comparing growth of “Pete” eastern gamagrass (Tripsacum dactyloidesL.), “Bison” big bluestem (Andropogon gerardiiVitman), and “Jesup MaxQ” tall fescue at soil bulk densities (BD) of 1.0, 1.3, and 1.5 g·cm-3. We also examined effects of arbuscular-mycorrhizal fungi (AMF) on plant growthin relation to compaction. Sources of AMF were a reclaimed surface coal mine soil and a native tallgrass prairie soil. Shoot and root biomass of tall fescue and big bluestem were reduced at 1.5 BD while eastern gamagrass growth was not affected. Growth ofbig bluestem and eastern gamagrass was greaterwith AMF than without, butsimilar between AMF sources. Tall fescue growthwas not enhanced by AMF. Overall, tall fescue biomass was 3 times greater than eastern gamagrass and 6 times greater than big bluestem when comparing only AMF-colonized grasses. Eastern gamagrass and big bluestem are both slower to establish than tall fescue. Eastern gamagrass appears to be more tolerant of compaction, while big bluestem appears somewhat less tolerant.

Diffuse-Reflectance Mid-infrared Spectral Properties of Soils under Alternative Crop Rotations in a Semi-arid Climate

We carried out mid-infrared (mid-IR) spectral interpretation of soils 0–5 and 5–15 cm deep in selected alternative crop rotations (ACR) treatments and an adjacent native prairie soil. Ashing and spectral subtraction shows that absorbance at 3700–2850 and 1700–1550 cm−1 indicates organic absorbance. Prairie soils, with their greater carbon (C) content, have different spectral properties from the cropped soils. Prairie soils have greater absorbance at the 2950–2870 cm−1 and the 1230 cm−1 CH bands. The soils from the different depths had different spectral properties, with the soils 0–5 cm deep having stronger absorbance at the 1055 cm−1 carbohydrate band, at 1270–1460 cm−1, and at the 1730 cm−1 ester band. The soils 5–15 cm deep are characterized by greater absorbance at the clay band. Soil C and nitrogen (N) correlated negatively with the 3700 cm−1 clay band and the 1830 cm−1 quartz band and correlated positively with the 2920 cm−1 because of aliphatic CH absorbance.

Impact of Long-Term Alfalfa Cropping on Soil Potassium Content and Clay Minerals in a Semi-Arid Loess Soil in China

Alfalfa cropping has been considered an efficient method of increasing soil fertility. Usually nitrogen increase in root nodules is considered to be the major beneficial effect. A 21-year time series (five sampling periods) of alfalfa cultivation plots on a loess soil, initially containing illite and chlorite, in Lanzhou of northwestern China was selected to investigate the relationships among alfalfa cropping, soil potassium (K) content and soil clay minerals. The results indicated that soil K significantly accumulated after cropping, with a peak value at about 15 years, and decreased afterwards. The accumulated K was associated with the K increase in the well-crystallized illite, which was not extracted by the traditional laboratory K extraction methods in assessing bioavailability. The steep decline in soil K content after 15-year cropping was in accord with the observed fertility loss in the alfalfa soil. Plant biomass productivity peaked at near 9 years of culture, whereas soil K and clay minerals continued to increase until cropping for 15 years. This suggested that K increased in the topsoil came from the deep root zone. Thus alfalfa continued to store K in clays even after peak production occurred. Nitrogen did not follow these trends, showing a general decline compared with the native prairie soils that had not been cropped. Therefore, the traditional alfalfa cropping can increase K content in the topsoil.

Resource Amendments Influence Density and Competitive Phenotypes of Streptomyces in Soil

Carbon from plant rhizospheres is a source of energy for soil microbial communities in native habitats. Soil amendments have been used as a means for deliberately altering soil community composition in agricultural soils to enhance plant health. However, little information is available in agricultural or natural soils on how specific carbon compounds or quantities influence soil microbial communities. Streptomyces are important soil saprophytes noted for their ability to produce antibiotics and influence plant health. To explore how specific types and amounts of carbon compounds influence Streptomyces in soil, glucose, cellulose, and lignin were added alone and in combination with six other carbon substrates of varying complexity to mesocosms of native prairie soil for 9 months at amounts equivalent to natural inputs from plants. Estimated culturable population densities, antibiotic inhibitory phenotypes, and resource utilization profiles were examined for Streptomyces communities from each treatment. The type and quantity of carbon compounds influenced densities, proportions, antibiotic phenotypes, and substrate utilization profiles of Streptomyces. Cellulose and lignin inputs produced the largest Streptomyces densities. Also, Streptomyces communities receiving high-resource inputs were more inhibitory whereas those receiving low-resource inputs used substrates more efficiently. Knowledge of how the availability and quantity of particular carbon compounds influences Streptomyces communities and their function, specifically resource use and inhibitory phenotypes, may be helpful in understanding the roles of resource availability in Streptomyces community dynamics and the potential of Streptomyces to suppress pathogens and enhance plant fitness in native and agricultural soils.

The invasive Sorghum halepense harbors endophytic N2-fixing bacteria and alters soil biogeochemistry

Exoticplantsinvadingnewhabitatsfrequently initiate broad changes in ecosystem functioning. Sorghum halepense is an invasive grass capable of growing in nitrogen (N)-poor tallgrass prairie soils that creates near monocultures in once phylogenetically diverse-communities. The biogeochemistry of soils invaded by S. halepense was compared to that of un- invaded native prairie soils. Invaded soils contained two to four times greater concentrations of alkaline metals, micronutrients, and essential plant nutrients than native prairie soils. The notable exception was Ca +2 ,w hich was always significantly lower in invaded soils. The N- content of S. halepense above-ground biomass was 6.4 mg g −1 (320 mg N plant −1 ) and suggested a supplemental N source supporting plant growth. Altered soil biogeochemistry in invaded areas coupled with high above-ground biomass in N-poor soils suggested N2- fixing activity associated with S. halepense. Nitrogenase activity of plant tissues indicated that N2-fixation was occurring in, and largely restricted to, S. halepense rhizomes and roots. A culture approach was used to isolate these N2-fixing bacteria from plant tissues, and 16S rRNA gene sequencing was used to identify these bacterial isolates. Nitrogenase activity of bacterial isolates indicated several were capable of N2-fixation. In addition to N2-fixation, other roles involved in promoting plant growth, namely mobilizing phosphorus and iron chelation, are known for closest matching relatives of the bacterial isolates identified in this work. Our results indicate that these plant growth-promoting bacteria may enhance the ability of S. halepense to invade and persist by altering fundamental ecosystem properties via significant changes in soil biogeochemistry.

The Magruder Plots: Untangling the Puzzle

Long‐term experiments play a vital role in revealing dynamic soil and weather processes that directly influence sustainable crop production and ecosystem health. The objective of this review paper is to present one of the oldest long‐term continuous winter wheat (Triticum aestivum L.) experiments, the Magruder Plots, and their contribution to the scientific community regarding questions of yield response to amendments, yield stability, percentage organic matter (OM) lost over time, soil nutrient status, microbial activity, weed population, and information on the economic return from winter wheat production. Alexander C. Magruder initiated the experiment in 1892 and is in progress to date. The original plot was started to evaluate wheat production on native prairie soils without fertilization. After 6 yr the principal investigator split the initial plot into two and fertilized one half with cattle manure. The experiment was then modified to include 10 treatments in 1930 by Dr. Horace J. Harper to answer several soil fertility related questions. Since 1947, six treatments have remained intact that evaluate simple combinations of manure, and inorganic N, P, K, and lime. Following 114 yr of continuous winter wheat production under conventional tillage, the check plot that has never received any fertilizer addition continues to produce wheat grain yields of >1 Mg ha−1. Despite the decline in soil organic matter from 4 to 1% during this time period, wheat grain yields continue to show slight increases with time, likely due to improved genetics. While continuous wheat without rotation is not recommended, this 114‐yr study documents the feasibility.

Nodulation and Growth of Alnus nitida and Alnus maritima Inoculated with Species-specific and Nonspecific Frankia

Abstract Actinorhizal plants form N2-fixing symbioses with soil-borne bacteria of the genus Frankia. Potential exists for development of sustainable, actinorhizal nursery crops that obtain most of their required N through N2 fixation, but information on host-symbiont specificity, presence of compatible Frankia in soils, and techniques to inoculate during plant production is lacking. Our objectives were to determine the effect of inoculum type and source and the effect of supplemental N on nodulation, growth, and N content of two actinorhizal species, Alnus nitida (Spach) Endl. and Alnus maritima (Marsh.) Muhl. ex Nutt. Plants of both species were subjected to one of four inoculum treatments (two crushed-nodule inocula: species-specific and cross inoculation, and two soil inocula: soil collected beneath native Alnus rubra Bong. in Washington state and native prairie soil from Iowa), were supplied fertilizer with or without N, and were grown in a greenhouse for 22 weeks. Inoculated plants nodulated, grew larger and faster, and accrued greater N content than uninoculated controls in both fertilizer treatments. Plants that received species-specific inoculum grew larger, acquired more dry weight from symbioses, and accumulated higher N content than cross-inoculated plants. Plants of A. nitida inoculated with soil from Washington state grew larger and accumulated more dry weight from symbioses than those inoculated with prairie soil, but A. maritima grew similarly with soil inoculum from both sources. Our results demonstrate that A. nitida and A. maritima can benefit from N2-fixing symbiosis during production and that potential exists for development of superior inocula and inoculation techniques.

The Magruder Plots: Untangling the Puzzle

Long‐term experiments play a vital role in revealing dynamic soil and weather processes that directly influence sustainable crop production and ecosystem health. The objective of this review paper is to present one of the oldest long‐term continuous winter wheat (Triticum aestivum L.) experiments, the Magruder Plots, and their contribution to the scientific community regarding questions of yield response to amendments, yield stability, percentage organic matter (OM) lost over time, soil nutrient status, microbial activity, weed population, and information on the economic return from winter wheat production. Alexander C. Magruder initiated the experiment in 1892 and is in progress to date. The original plot was started to evaluate wheat production on native prairie soils without fertilization. After 6 yr the principal investigator split the initial plot into two and fertilized one half with cattle manure. The experiment was then modified to include 10 treatments in 1930 by Dr. Horace J. Harper to answer several soil fertility related questions. Since 1947, six treatments have remained intact that evaluate simple combinations of manure, and inorganic N, P, K, and lime. Following 114 yr of continuous winter wheat production under conventional tillage, the check plot that has never received any fertilizer addition continues to produce wheat grain yields of >1 Mg ha−1. Despite the decline in soil organic matter from 4 to 1% during this time period, wheat grain yields continue to show slight increases with time, likely due to improved genetics. While continuous wheat without rotation is not recommended, this 114‐yr study documents the feasibility.

Relationships Among Soil Carbon and Physiochemical Properties of a Typic Albaqualf as Affected by Years Under Cultivation

Soil organic carbon (SOC) is a dynamic soil property and a key component of the quality and sustainability of agricultural soils. Predictions of SOC change often rely on correlations between SOC and other soil physical and chemical properties that are assumed to be more stable over time. The objective of this study was to examine the long-term stability of relationships between SOC, physiochemical properties, and years under cultivation in a common rice (Oryza sativa L.) soil (i.e., a fine, montmorillonitic, thermic, Typic Albaqualf) of the Mississippi Delta region of the south-central United States. The field site consisted of an undisturbed native prairie and three agricultural fields that have been cultivated for 15, 26, and 44 years, respectively, in 2001. Neither SOC concentration nor mass was significantly correlated with sand, silt, or clay content, but both were significantly correlated with pH in 1987. Soil OC concentration and mass were significantly correlated with both silt and clay, but not sand content, and pH in 2001. However, the relationship between SOC concentration and sand content changed significantly, while the relationships between SOC and pH and silt and clay content remained stable and did not change significantly from 1987 to 2001. Soil OC decreased exponentially as years under cultivation increased, but the modeled relationship appeared to remain stable from 1987 to 2001. Results indicate that continuous cultivation of the Typic Albaqualf of this study will result in SOC stabilizing around 8–10 g kg−1, which is less than 50% of the SOC in the adjacent undisturbed, native prairie soil. The results of this study can be used to strengthen predictions of how ecosystems, particularly flood-irrigated rice agroecosystems, will respond to future changes in management practices towards more sustainable use of the soil resource.

Organic carbon storage and stable isotope composition of soils along a grassland to forest environmental gradient in Saskatchewan

Limited information is available about soil organic carbon accumulation rates and stable isotope composition in the boreal region of the Canadian prairies. The objectives of the study were to document soil development, measure carbon storage and accumulation rates, and determine the 13C/12C ratio of organic matter in native prairie soils in the major soil-climatic zones of Saskatchewan. The mean thickness of the Ah horizon increases from 5 cm in the Brown Chernozems to 14 cm in Black Chernozems, and this horizon is absent in Gray Luvisols. The thickness and degree of development of B horizons increase from Brown to Gray soils. Total organic C storage to 1.2 m depth in Brown, Dark Brown, Black Chernozems, and Gray Luvisols is 9.08, 11.72, 14.88, 9.63 kg C m-2, respectively. The long-term mean annual accumulation rates of organic C for Brown, Dark Brown, Black, and Gray soils are 0.57, 0.90, 1.18, and 0.84 g m-2 yr-1, respectively. For a Rego Black Chernozem the rate is 1.83 g m-2 yr-1. All these values are higher than those reported for temperate grasslands in the United States of America. The ỏ13C values of organic C (an average of all profiles in each soil zone to 1.2-m depth) range from -22.9 ‰ for Dry Brown soils, -24.3‰ for Brown soils, -24.8‰ for Dark Brown soils, -25.3‰ for Black soils, and -26.8‰ for Gray soils. The relative contribution of C4 plants to soil organic C decreases from the warm semiarid grassland to the moist Boreal region, where C4 plants have not influenced organic C at all. Considering the net primary production (NPP) estimated for the soil zones, average aboveground carbon sequestration is estimated to be about 0.46% of NPP. These data provides a realistic assessment of C balances in native prairie soils of Saskatchewan. Key words: Rate of carbon accumulation, stable isotope of soil carbon, soil zones of Saskatchewan, grassland soils, forest soils, Rego Black Chernozem soils

Nitrogen Balance in the Magruder Plots Following 109 Years in Continuous Winter Wheat#

The Magruder plots are the oldest continuous soil fertility wheat research plots in the Great Plains region, and are one of the oldest continuous soil fertility wheat plots in the world. They were initiated in 1892 by Alexander C. Magruder who was interested in the productivity of native prairie soils when sown continuously to winter wheat. This study reports on a simple estimate of nitrogen (N) balance in the Magruder plots, accounting for N applied, N removed in the grain, plant N loss, denitrification, non‐symbiotic N fixation, nitrate (NO3 −) leaching, N applied in the rainfall, estimated total soil N (0–30 cm) at the beginning of the experiment and that measured in 2001. In the Manure plots, total soil N decreased from 6890 kg N ha−1 in the surface 0–30 cm in 1892, to 3198 kg N ha−1 in 2002. In the Check plots (no nutrients applied for 109 years) only 2411 kg N ha−1 or 35% of the original total soil organic N remains. Nitrogen removed in the grain averaged 38.4 kg N ha−1 yr−1 and N additions (manure, N in rainfall, N via symbiotic N fixation) averaged 44.5 kg N ha−1 yr−1 in the Manure plots. Following 109 years, unaccounted N ranged from 229 to 1395 kg N ha−1. On a by year basis, this would translate into 2–13 kg N ha−1 yr−1 that were unaccounted for, increasing with increased N application. For the Manure plots, the estimate of nitrogen use efficiency (NUE) (N removed in the grain, minus N removed in the grain of the Check plots, divided by the rate of N applied) was 32.8%, similar to the 33% NUE for world cereal production reported in 1999. #Contribution from the Oklahoma Agricultural Experiment Station.

Carbon and Nitrogen Storage in a Typic Albaqualf as Affected by Assessment Method

Accurate quantification of soil organic carbon (OC) and nitrogen (N) concentrations are necessary to ascertain the effects of land use, crop rotation systems, and management practices on soil C and N sequestration potential. Soil OC and total N were determined by various methods in a Typic Albaqualf under native tallgrass prairie and agricultural soil cropped to a rice (Oryza sativa L.)–soybean (Glycine max L.)–wheat (Triticum aestivum L.)/soybean rotation that has been annually cultivated for 15, 26, and 44 years. Two wet-oxidation methods, the Walkley-Black (WB) and modified Walkley-Black (mod WB), and high-temperature combustion using a Carlo-Erba and LECO analyzer were used to determine the effects of assessment method on soil OC concentration, while the high-temperature combustion method using the Carlo-Erba and LECO analyzer were used to determine the effects of assessment method on total soil N concentration. Soil OC and total N concentrations determined by high-temperature dry combustion using the Carlo-Erba and LECO analyzers did not differ significantly. Soil OC concentrations determined by the modWB method were generally significantly higher than those from the WB or high-temperature combustion methods. Despite significant linear correlation (r>0.74;p<0.001) between soil OC concentrations by wet-oxidation and dry-combustion methods, assessment methodology significantly affected interpretations regarding changes in soil OC storage over time. The results of this study indicate that the choice of assessment methodology is a critical decision for the accurate quantification of soil OC concentration, content, and change over time.

Effects of Grazing on Restoration of Southern Mixed Prairie Soils

AbstractA comparative analysis of soils and vegetation from cultivated areas reseeded to native grasses and native prairies that have not been cultivated was conducted to evaluate restoration of southern mixed prairie of the Great Plains over the past 30 to 50 years. Restored sites were within large tracts of native prairie and part of long‐term grazing intensity treatments (heavy, moderate, and ungrazed), allowing evaluation of the effects of grazing intensity on prairie restoration. Our objective was to evaluate restored and native sites subjected to heavy and moderate grazing regimes to determine if soil nutrients from reseeded cultivated land recovered after 30 years of management similar to the surrounding prairie and to identify the interactive influence of different levels of grazing and history of cultivation on plant functional group composition and soils in mixed prairies. For this mixed prairie, soil nitrogen and soil carbon on previously cultivated sites was 30 to 40% lower than in uncultivated native prairies, indicating that soils from restored sites have not recovered over the past 30 to 50 years. In addition, it appears that grazing alters the extent of recovery of these grassland soils as indicated by the significant interaction between grazing intensity and cultivation history for soil nitrogen and soil carbon. Management of livestock grazing is likely a critical factor in determining the potential restoration of mixed prairies. Heavy grazing on restored prairies reduces the rate of soil nutrient and organic matter accumulation. These effects are largely due to changes in composition (reduced tallgrasses), reduced litter accumulation, and high cover of bare ground in heavily grazed restored prairies. However, it is evident from this study that regardless of grazing intensity, restoration of native prairie soils requires many decades and possibly external inputs to adequately restore organic matter, soil carbon, and soil nitrogen.

History studies of the hermaphrodite nematode Acrobeloides sp. with Pseudomonas cepacia as a food source on agar

AbstractThe life cycle and reproduction of a species of Acrobeloides isolated from native North American prairie soil was studied in monoxenic cultures with Pseudomonas cepacia as a food source at 23°C. The demographic parameters were calculated from cultures for the period from 0 to 15 days. The intrinsic rate of natural increase (rm) was 0.38 days, mean total productivity of Acrobeloides sp. without predation (R0) in 15 days was 87.7, and capacity of increase (rc) was 0.36. The minimum generation time (Tmin) was 10 days, cohort generation time (Tc) 6.8 days and mean generation time (T) 18.5 days. The age of an adult female at median egg laying (T) was 7.9 days, and mortality (lx) 0.4. The estimation of consumption of Acrobeloides sp. by nematophagous mites, expressed in organic carbon content, was 0.22 μg day-1. Based on laboratory experiments, this Acrobeloides sp. from native prairie soil does not have a high ability for population growth with Pseudomonas cepacia as food source at 23°C.

Effects of mycorrhizae on plant growth and Dynamics in Experimental Tallgrass Prairie Microcosms

Experimental microcosms (40 X 52 X 32 cm) containing an assemblage of eight tallgrass prairie grass and forb species in native prairie soil were maintained under mycorrhizal (untreated control) or mycorrhizal-suppressed (fungicide-treated) conditions to examine plant growth, demographic, and community responses to mycorrhizal symbiosis. The fungicide benomyl successfully reduced mycorrhizal root colonization in the fungicide-treated microcosms to only 6.4% (an 83% reduction relative to mycorrhizal controls). Suppression of mycorrhizas resulted in a 31% reduction in total net aboveground plant production and changes in the relative production of C4 and C3 plants. The C4 tallgrasses Andropogon gerardi and Sorghastrum nutans produced less plant biomass in the fungicide-treated microcosms, and had a greater ratio of reproductive to vegetative biomass. Cool-season C3 grasses, Koeleria pyramidata and Poa pratensis accumulated more biomass and were a significantly greater proportion of total community biomass in mycorrhizal-suppressed microcosms. Forbs showed variable responses to mycorrhizal suppression. The two legumes Amorpha canescens and Dalea purpurea had significantly lower survivorship in the fungicide-treated microcosms, relative to the controls. The results confirm the high mycorrhizal dependency and growth responsiveness of dominant prairie grasses, and indicate that differential growth and demographic responses to mycorrhizal colonization among species may significantly affect plant productivity and species relative abundances in tallgrass prairie.

Vegetation and Cropping Effects on Pedogenic Processes in a Sandy Prairie Soil

The presence of native prairies in the Lower Wisconsin River valley that have not been cropped affords a mean of comparison across undisturbed and managed ecosystems. A native prairie soil and a prairie converted to corn (Zea mays L.) and red pine (Pinus resinosa Aiton) plantations were studied to determine the influence of vegetation on soil-forming processes in the sandy landscape of the Lower Wisconsin River valley. Effects of afforestation and cultivation were determined by separating organic matter among particle-size and chemical fractions, differentiating among forms of extractable Fe and Al, and measuring dissolved organic carbon (DOC), Fe and Al in bulk precipitation, throughfall, and soil solutions at three depths. The soil under native prairie vegetation showed distinctive horizonation, including an umbric epipedon formed by melanization. Thickness of the A horizon was significantly less in the forested and cultivated soils than in the prairie soil. Greater amounts of the floatable fraction of organic C, fulvic acids, and Fe-organic complexes were present in the forested soils than in the prairie soil. Carbon concentration in the A horizon of the cultivated soil was significantly lower than in the prairie and forested soils, due to losses by erosion, decomposition, and DOC leaching. In the cultivated soil, the floatable and humin fractions showed the greatest decrease in C concentration compared with the prairie soil. Analysis of the solution data indicated leaching of soluble Fe-organic complexes under coniferous and corn vegetation, suggesting that afforestation and cultivation initiated incipient podzolization.

Biological Condition of an Agricultural Soil Six Years after Conservation Reserve

Microbial properties of Kenoma silt loam soil in Coffey County, Kansas, were investigated for a native tallgrass prairie, a field currently under cultivation, and a previously cultivated field that was reseeded to native grasses in 1986 as part of the Conservation Reserve Program (CRP). Soil samples were taken from the surface 10 cm and analyzed for soil respiration, microbial biomass carbon (C) and nitrogen (N), N availability, and dehydrogenase activity. In comparison to soils in native prairie, measures of microbial quality in soils under cultivation decreased as follows: microbial biomass C and N by 67%, nitrogen availability by 37%, dehydrogenase activity by 68%, and respiratory rate by 69%. When CRP land was compared with soils in cultivation, no significant differences could be detected in terms of microbial biomass N, N availability, and dehydrogenase activity. The CRP land showed lower values of microbial biomass C, but a significantly higher respiratory rate. These results led to three important conclusions: First, the results strongly confirm that cultivation has a profound impact on native prairie soils. Second, the levels of soil microbial quality parameters in the reseeded prairie were not similar to those in the native prairie after a period of 6 years. Third, a higher respiratory rate in the reseeded prairie indicates a response of the soil microbial community to the reestablishment of permanent prairie vegetation. The latter point also was supported by a greater proportion of active pools of C and N in the reseeded prairie.

Characterization and growth response of bacteria in soil following application of carbofuran.

Enhanced biodegradation of carbofuran (2, 3-dihydro-2, 2 dimethyl-7-benzofuranyl methyl carbamate) is an economically significant, but poorly understood, microbial phenomenon in soil. A series of experiments was conducted to examine short term changes in soil bacterial populations stimulated by carbofuran application at field rates. In the field experiment, commercially formulated carbofuran and butylate (S-ethyl diisobutyl carbamothioate) were applied at 5.6 kg ai ha-1 and 8.4 kg ai ha-1, respectively, on a soil (Putnam silt loam) exhibiting enhanced degradation of carbofuran. In laboratory studies, technical grade carbofuran (20 mg kg-1 soil) was applied to samples of the field soil. Bacterial populations were estimated using non-selective (tryptic soy agar) and selective media containing carbofuran or butylate. Largest population increases in pesticide-treated soil were observed between 7 and 15 days after treatment (DAT) compared to populations in non-treated soil. Significant increases (P less than 0.05) in total bacterial populations and presumed carbofuran-degraders due to carbofuran application were associated with increased populations of Pseudomonas spp. and Flavobacterium spp. Application of carbofuran appeared to provide a competitive advantage to these species over actinomycetes persisting beyond 20 DAT. Growth responses of bacteria to carbofuran in the Putnam soil were compared to those in a native prairie soil (Mexico silt loam), which exhibited a much slower rate of carbofuran degradation. Bacterial population response to carbofuran was measurable, but small and short-lived. Perpetuation of the enhanced degradation phenomenon may lie in a persistent pesticide-induced competitive advantage given to a very small segment of the microbial population. This advantage may not be detectable after 20 days using conventional plating techniques.

Methane and nitrous oxide fluxes in native, fertilized and cultivated grasslands

METHANE and nitrous oxide are long-lived, radiatively active trace gases that account for ∼20% of the total anticipated atmospheric warming1. The atmospheric concentrations of both gases have increased dramatically over the past few decades, and continue to increase at a rate of ∼1.1 and 0.25% yr−1 for CH4 (ref. 2) and N2O (ref. 3) respectively. Increased biospheric production is gen-erally suggested as the reason for the increases, but decreases in global sinks may also be important. It has been suggested, for example, that nitrogen fertilization may decrease the rate at which tropical4,5 and temperate forest soils6 take up methane from the atmosphere. Furthermore, the recent extensive changes in land management and cultivation could be contributing to the observed increases in both atmospheric CH4 and N2O, as has been suggested for tropical soils7. Little information exists on CH4 uptake in temperate grasslands (which currently occupy ∼8% of the Earth's surface), its relation to N2O production, or the effect of land management or cultivation8,9. Here we report measurements of CH4 uptake and N2O emissions in native, nitrogen-fertilized and wheat-growing prairie soils from spring to late autumn, 1990. We found that nitrogen fertilization and cultivation can both decrease CH4 uptake and increase N2O production, thereby contributing to the increasing atmospheric concentrations of these gases.

Effects on plant growth of inoculation of stored stripmining topsoil in North Dakota with mycorrhizal fungi contained in native soils

Stored topsoil from stripmining operations in western North Dakota was inoculated with mycorrhizal fungi contained in native prairie soil. The effects on plant mycorrhizal infection percentage, growth as shoot dry weight, and phosphorus uptake were determined. The studied topsoil piles were found to contain little or no vesicular arbuscular mycorrhizal (VAM) fungal inoculum at a depth of 120 cm. The inoculum soil was mixed into the stored soil at rates of 10% and 1%, or surface-applied at 1%. In control pots, sterilized inoculum soil was used. Corn plant (Zea mays) bioassays were used. After 30 days growth the percent VAM fungal infection of the test plants increased with both the 10% and 1% soil inocula. Phosphorus concentrations were generally increased by inoculation with 10% soil mixtures but not 1%. Shoot dry weights of the plants were not measurably different between 10% and 1% inoculation. However, when the plant growth period was increased to 60 days, all three parameters were increased over the check plants. When the inoculum was not mixed into the soil, but layered on the surface, there were no differences in any of the parameters.