Mixed Plots Research Articles

The incidence of barley scald in cultivar mixtures

A field trial incorporating resistant and susceptible plants in mixed plots, where the proportion of resistant plants was 0%, 50%, or 100%, indicated that cultivar mixtures were not particularly effective for the control of barley scald disease. Combining 2 scald resistance genes into a single line was more effective in controlling disease than deploying the same genes singly in different lines in a mixture. The lack of benefit from mixtures in this experiment differed from the findings of 2 previous studies investigating scald resistance in mixtures. Although the study included only one growing season and one location, the findings suggest that the use of mixtures for controlling this disease may carry some risk and that breeding for a resistance pyramid is likely to be more widely effective.

A study on the behaviour of pesticides and their transformation products in the Scheldt estuary using liquid chromatography-electrospray tandem mass spectrometry.

Off-line solid-phase extraction (SPE) combined with liquid chromatography-electrospray tandem mass spectrometry (LC-ESI-MS-MS) was used to study the estuarine behaviour of the polar pesticides, atrazine, chloridazon, diuron and metolachlor, and their transformation products (TPs), hydroxyatrazine (HA), desisopropylatrazine (DIA), desethylatrazine (DEA), 3,4-dichlorophenylmethylurea (DPMU) and monuron. The compounds were identified by comparing their LC retention times and product-ion spectra with those of standard solutions. In all but one case the detection limits of the method were sufficient to determine the compounds of interest over the entire salinity range in the estuary. The concentrations of the dissolved pesticides ranged from 70 ng l-1 for chloridazon to 1350 ng l-1 for diuron. The levels of TPs were 3-8% of the levels of their parent pesticide. The mixing plots of polar pesticides and their TPs indicated that TPs, which are present in fresh river water, are conservatively transported to the sea and that no additional amounts of TPs are formed during their transport through the estuary. The one exception was HA, of which approximately 10% of the amount transported to the North Sea is formed in the lower part of the estuary by photochemical oxidation of atrazine. The latter was concluded from the ratios of each analyte over the sum total of the parent pesticide and all TPs along the salinity gradient, which proved to be a useful tool for identifying such estuarine transformations.

Growth response of loblolly pine, sweetgum, and water oak in a pine-hardwood density study

A study was established with loblolly pine (Pinus taeda L.) planted at 1683 trees/ha competing with sweetgum (Liquidambar styraciflua L.), water oak (Quercus nigra L.), or loblolly pine at densities of 0, 1794, 2692, and 5382 trees/ha. The study is a split-split plot with three blocks. Main plots are the three competitor species; subplots are with or without 2 years of herbaceous vegetation control (HVC); and sub-subplots are competitor densities. The study was measured at 6 years after establishment. Pines in pure plots responded to HVC with a mean basal area gain of 1.8 m2/ha (13%). With HVC, basal area increased 2.7 m2/ha (70%) for sweetgum and 2.3 m2/ha (58%) for water oak. Pines in mixed plots did not respond, or responded negatively, to HVC. Pine height, basal area, volume index, and canopy projection were only slightly affected by sweetgum density above zero. Pine characteristics decreased as water oak density increased. Adding sweetgum or water oak to pine reduced total (pine + hardwood) volume index and basal area.

Root distributions in a Grevillea robusta-maize agroforestry system in semi-arid Kenya

Limited knowledge of root distributions in agroforestry systems has resulted in assumptions that various tree species are more suited to agroforestry than others, because they are presumed to have few superficial lateral roots. This assumption was tested for Grevillea robusta when grown with maize (Zea mays) in an agroforestry system in a semi-arid region of Kenya. At a site with a shallow soil, root lengths of both species between the soil surface and bedrock were quantified by soil coring, at intervals over four cropping seasons, in plots containing sole stands and mixtures of the trees and crop; the trees were 4–6 years old and they were severely pruned before the third season. Profiles of soil water content were measured using a neutron probe. Prior to pruning of the trees, recharge of soil water below the deepest maize roots did not occur, resulting in significant (P<0.05) suppression of maize root lengths and downward root growth. Maximum root length densities for both species occurred at the top of the soil profile, reaching 1.1–1.7 cm cm-3 for G. robusta, but only 0.5 cm cm-3 for maize grown with trees. Root populations in mixed plots were dominated by G. robusta at all times, all depths and all distances from trees and maize and, thus, there was no spatial separation of the rooting zones of the trees and crop. Competition between G. robusta and maize for soil water stored near the surface was unavoidable, although pruning reduced its impact; complementary use of water by the trees and crop would only have been possible if alternative sources of water were available.

LITTERFALL, LITTER DECOMPOSITION AND MAIZE BIOASSAY OF MULCHES FROM FOUR INDIGENOUS TREE SPECIES IN MIXED AND MONOSPECIFIC PLANTATIONS IN COSTA RICA

SUMMARY Three plantations with native timber species were established in the Atlantic humid lowlands of Costa Rica to compare growth and nutrient cycling in pure and mixed stands. As part of the project, leaf litter decomposition, litterfall and accumulation of leaf litter on the plantation floor were studied in a young plantation of Pithecellobium elegans D. C. Benth, Genipa americana L., Vochysia ferruginea Mart., and Hyeronima alchorneoides Fr. Allemao. Of the four species studied, V. ferruginea had the highest annual litterfall (867.6 g m-2), G. americana had the lowest (386.7 g m-2), and the mixed plots had an intermediate level (660 g m-2). P. elegans ‘leaves decomposed the most rapidly, V. ferruginea's leaves decomposed the slowest, and the mixed litter decomposed at an intermediate rate. Mulch bioassays testing maize seedling growth showed higher survival and growth rates than the unmulched control in every treatment except for the G. americana mulch. Overall, P. elegans appears to be a good candi...

Plant Effects on Spatial and Temporal Patterns of Nitrogen Cycling in Shortgrass Steppe

Because of the water-limited nature and discontinuous plant cover of shortgrass steppe, spatial patterns in ecosystem properties are influenced more by the presence or absence of plants than by plant type. However, plant type may influence temporal patterns of nutrient cycling between plant and soil. Plants having the carbon-3 (C3) or carbon-4 (C4) photosynthetic pathway differ in phenology as well as other attributes that affect nitrogen (N) cycling. We estimated net N mineralization rates and traced nitrogen-15 (15N) additions among plant and soil components during May, July, and September of 1995 in native plots of C3 plants, C4 plants, or mixtures of C3 and C4. Net N mineralization was significantly greater in C3 plots than in C4 plots during both July and September. C3 plots retained significantly more 15N in May than did mixed and C4 plots; these differences in 15N retention were due to greater 15N uptake by C3 plants than by C4 plants during May. There were no significant differences in total 15N retention among plant communities for July and September. Soil 15N was influenced more by presence or absence of plants than by type of plant; greater quantities of 15N remained in soil interspaces between plants than in soil directly under plants for July and September. Our results indicate that plant functional type (C3 versus C4) can affect both the spatial and the temporal patterns of N cycling in shortgrass steppe. Further research is necessary to determine how these intraseasonal differences translate to longer-term and coarser-scale effects of plants on N cycling, retention, and storage.

Evaluating the Role of Plantations as Carbon Sinks: An Example of an Integrative Approach from the Humid Tropics

/ Despite their fast growth, tropical plantations are a small sink of atmospheric carbon because they occupy only a small area in relation to other land uses worldwide. Proper design and management of plantations can increase biomass accumulation rates, making them more effective C sinks. However, fast-growing plantations can extract large amounts of nutrients from the soil, and site fertility declines may limit sustained plantation forestry after a few rotations. We measured aboveground biomass accumulation, carbon sequestration, and soil chemistry in three young plantations of 12 indigenous tree species in pure and mixed designs in the humid lowlands of Costa Rica. Annual biomass increments for the three mixed plantations ranged from 10-13 Mg/ha. The mixtures of four species gave higher biomass per hectare than that obtained by the sum of one fourth hectare of each species in pure plots. At this early age of the plantations, estimated annual C sequestration values were comparable to other reports from young plantations of exotic species commonly grown in the tropics. Four years after planting, decreases in soil nutrients were apparent in pure plots of some of the fastest growing species, while beneficial effects on soils were noted under other species. The mixed plots showed intermediate values for the nutrients examined and, sometimes, improved soil conditions. A mixture of fast and slower growing species yields products at different times, with the slower growing species constituting a longer term sink for fixed carbon. Examination of the role of tropical plantations as C sinks necessitates integrative approaches that consider rates of C sequestration, potential deleterious effects on ecosystem nutrients, and economic, social, and environmental constraints.KEY WORDS: Native trees; Aboveground biomass; Stem increments; Rotation length; Soil nutrients; Economics

Leaf Litter Decomposition, Litterfall, and Effects of Leaf Mulches from Mixed and Monospecific Plantations in Costa Rica

Rates of litter decomposition and nutrient release from litter provide valuable information on the capacity of different tree species to replenish soil nutrients in degraded tropical areas. Leaf litter decomposition, leaf litterfall, plantation floor leaf litter, and mulch performance were studied for four indigenous timber species, Virola koschnyiWarb, Dipteryxpanamensis(Pittier) Record and Mell, Terminalia amazonia(J.F. Gmel.) Exell., and Albizia gua-chapele(H.B.K.) Little, grown in mixed and monospecific plantations in the Atlantic humid lowlands of Costa Rica. Terminalia amazonialitter decomposed the fastest: no litter remained after 6 months. After 12 months, D. panamensis, A. guachapele, and the mixed litter decomposed completely, while 15% of the original weight of V. koschnyilitter remained. Differences in decomposition rates were closely related to leaf nutrient content. Total annual leaf litterfall was highest in T. amazonia(872.9 g/m2), followed by D. panamensis, V. koschnyi, and the mixed plots. A. guachapelehad the lowest leaf litterfall (236.0 g/m2). The highest plantation-floor leaf litter was found in V. koschnyiand D. panamensis.Both litterfall and plantation-floor litter accumulation fluctuated least in the mixed plots. A. guachapeleand D. panamensismulch most positively affected maize seedling growth, followed by the mixed mulch. Recommendations are drawn from the results to suggest species choice for sustainable land management in the region.

Plant functional type effects on trace gas fluxes in the shortgrass steppe

Plant community structure is expected to regulate the microbial processes of nitrification and denitrification by controlling the availability of inorganic N substrates. Thus it could also be a factor in the concomitant release of NO and N2O from soils as a result of these processes. C3 and C4 plants differ in several attributes related to the cycling of nitrogen and were hypothesized to yield differences in trace gas exchange between soil and atmosphere. In this study we estimated fluxes of NO, N2O and CH4 from soils of shortgrass steppe communities dominated by either C3 plants, C4 plants or mixtures of the two types. We collected gas samples weekly from two sites, a sandy clay loam and a clay, throughout the growing seasons of 1995 and 1996. Plant functional type effects on gas fluxes at the clay site were not apparent, however we found several differences among plant communities on the sandy clay loam. CH4 uptake from atmosphere to soil was significantly greater on C4 plots than C3 plots in both years. NO fluxes were significantly greater from C4 plots than from C3 plots in 1995. NO fluxes from C3 and mixed plots were not significantly different between 1995 and 1996, however fluxes from C4 plots were significantly greater in 1995 compared to 1996. Results indicate that under certain environmental conditions, particularly when factors such as moisture and temperature are not limiting, plant community composition can play an important role in regulating trace gas exchange.

Ectomycorrhizal diversity on Betula papyrifera and Pseudotsuga menziesii seedlings grown in the greenhouse or outplanted in single-species and mixed plots in southern British Columbia

Habituellement, le potentiel d'inoculum et la diversite des champignons ectomycorhiziens sur un site diminuent suite a la recolte. L'objectif de cette etude consistait a determiner si la plantation d'un melange d'especes d'arbres suite a la coupe permettrait de maintenir une plus grande diversite de champignons ectomycorhiziens que la plantation d'une seule espece. Le bouleau a papier (Betula papyrifera Marsh.) et le douglas taxifolie (Pseudotsuga menziesii (Mirb.) Franco) furent plantes seuls ou en melange, dans differentes proportions et densites, dans trois sites situes dans la zone continentale au sud de la Colombie-Britannique. Dans le cas de l'essai au champ, les types d'ectomycorhizes furent caracterises apres un examen morphologique detaille de 12 semis par parcelles, 4, 16 et 28 mois apres la plantation. Les champignons mycorhiziens Thelephoia, la race E, Rhizopogon (sur le douglas taxifolie uniquement), Mycelium radicis atrovirens et Cenococcum etaient les plus abondants. La dominance de Thelephora a diminue au cours de la periode d'echantillonnage de 82 a 41% des ectomycorhizes chez le bouleau et de 26 a 15% chez le douglas taxifolie. Rhizopogon est demeure aussi abondant chez le douglas taxifolie (de 36% des ectomycorhizes a 4 mois jusqu'a 37% a 28 mois). Vers le 28 e mois, 91% des mycorhizes chez le bouleau et 56% chez le douglas taxifolie etaient communes aux deux especes. Cela a d'importantes implications dans le transfert potentiel des nutriments et du carbone entre les deux especes. A 16 (P = 0,068) et 28 (P = 0,088) mois apres la plantation, l'homogeneite de la population d'ectomycorhizes associee au systeme racinaire du douglas taxifolie etait plus grande dans les parcelles ou il y avait un melange d'especes que dans celles ou il n'y avait qu'une seule espece. La richesse (nombre de types d'ectomycorhizes) et l'indicc de diversite de Simpson par semis n'etaient pas affectes par le melange des especes d'arbre. La densite de la plantation n'a pas affecte la richesse, l'homogeneite ou la diversite. Les ectomycorhizes qui se sont developpes sur les semis de douglas taxifolie ou de bouleau cultives en serres dans des sols provenant de ces sites etaient tres semblables a ceux qui se sont developpes sur le terrain. Cela demontre qu'on peut faire des essais en serres pour predire quel type d'ectomycorhizes se formeront sur les semis cultives sur des sites perturbes, au moins pour les quelques premieres annees apres la plantation.

Soil respiration following site preparation treatments in boreal mixedwood forest

The effects of experimental site preparation treatments on soil respiration were studied in a boreal mixedwood forest. The treatments were: (1) intact forest (uncut); (2) clearcut without site preparation (cut); (3) clearcut followed by mixing of organic matter with mineral horizons (mixed); and (4) plots from which all organic matter was removed (screefed). Soil respiration was measured as carbon dioxide (CO 2) evolution from surface soil once a month from June to October, 1994 in the field using infra-red gas analyzer (IRGA). In addition, soil temperature and moisture content were determined once a month during the 1994 growing season and soil organic matter content was determined once in July 1994. Mixed plots had the highest soil respiration rates (0.86 to 0.98 g m −2 h −1), followed by the clearcut (0.68 to 0.84 g m −2 h −1) and uncut plots (0.56 to 0.82 g m −2 h −1), with screefed plots having the lowest respiration rates (0.24 to 0.52 g m −2 h −1) from June to September. Soil respiration of the cut plots was not significantly different from that of the uncut control. The site preparation treatments reduced soil moisture and soil organic matter contents significantly. Changes in soil temperature within treatment at 0, 5 and 10 cm depths and between the treatments were not significant. Observed soil respiration patterns were attributed to changes in soil moisture and organic matter content associated with the various treatments. A laboratory incubation experiment elucidated the effects of organic matter, soil moisture, and temperature on soil respiration rates. Site preparation treatments in boreal mixedwood forests affect soil respiration by modifying the moisture and organic matter content of the soil.

Impact of Intraplot Mixtures of Toxic and Nontoxic Plants on Population Dynamics of Diamondback Moth (Lepidoptera: Plutellidae) and Its Natural Enemies

Studies were conducted in 1993 and 1994 to investigate the effects of intraplot mixtures of toxic (with Bacillus thuringiensis Berliner) and nontoxic plants on population dynamics of diamondback moth, Plutella xylostella (L.), and its natural enemies. In 1993, diamondback moth densities in all plots (mixed and nonmixed) declined after generation 2, apparently because of poor plant quality. In 1994, mixed plots with 83.3% toxic and 16.7% nontoxic plants caused diamondback moth densities on nontoxic plants to decline after 2 generations, whereas population densities in plots of 100% nontoxic plants continued to rise. Diamondback moth, Plutella xylostella (L.), dynamics in plots with 50% toxic plants were intermediate compared with dynamics in the other treatment plots. Percentage of parasitism by the parasitoid Diadegmo insulare Cresson was not significantly different between mixed and nonmixed plots. Incidence of the 4 most abundant diamondback moth predators was not different in mixed and nonmixed plots, nor was there a difference in numbers of predators on toxic versus nontoxic plants. These data suggest that intrafield mixtures could serve to decrease density of a target pest such as the diamondback moth, while not adversely affecting natural enemies.

Ectomycorrhizal diversity on &lt;I&gt;Betula papyrifera&lt;/I&gt; and &lt;I&gt;Pseudotsuga menziesii&lt;/I&gt; seedlings grown in the greenhouse or outplanted in single-species and mixed plots in southern British Columbia

Ectomycorrhizal diversity on &lt;I&gt;Betula papyrifera&lt;/I&gt; and &lt;I&gt;Pseudotsuga menziesii&lt;/I&gt; seedlings grown in the greenhouse or outplanted in single-species and mixed plots in southern British Columbia

Total, organic and extractable-P in humus and soil beneath Sitka spruce planted in pure stands and in mixture with Scots pine

Total, organic and extractable P were measured in the humus and underlying soil to 10 cm depth beneath Sitka spruce (SS) and mixed Sitka spruce and Scots pine (SS+SP) stands planted on upland heath. The humus beneath SS+SP contained significantly (p<0.01) greater amounts of total and organic-P than that in SS and the mixed stands had more effectively retained approximately 87 per cent of previously applied fertilizer-P, totalling 100 kg P ha−1, compared with 70 per cent in SS. Despite the larger amounts of total-P in the mixed plots 0.01 M CaCl2 extractable molybdate reactive phosphorus (MRP) was significantly (p<0.05) greater in SS+SP humus only during March and April. Greater concentrations of MRP were released from the humus and soil during July and August at a mean rate of 58 g P ha−1 day−1. This coincided with drying of the soil during the summer and the rate of release, attributed to death of fine roots and microorganisms, was 4 to 30 times greater than reported values for rates of net mineralization of P from forest soils.

Bias in the estimation of horizontal resistance to airborne fungi in plant breeding

Summary The question is posed: do mixed plots of plant breeding materials selected for horizontal resistance to airborne fungal pathogens provide unbiased estimates? Alternatively, is the common belief that resistance is often under-estimated in mixed plantings justified? A review of the literature and some simple calculations suggest that systematic bias is indeed to be expected, provided that the components of resistance are positively correlated. If they are not, resistance is likely to be unpredictably mis-estimated. There is room for much more critical experimentation and better theory on what is, after all, an important matter.

Corn and Sorghum as a Winter Food Source for Ring-Necked Pheasants

In many localities, winter food plots planted to corn and/or sorghum grains are used to enhance overwinter survival of ring-necked pheasants (Phasianus colchicus). We used organ size and carcass composition to evaluate this strategy with 110 pen-reared, female ring-necked pheasants fed ad libitum or restricted (65, 85, and 100% of a 173 kcal.bird -1 day -1 maintenance ratio [MR]) corn, sorghum, or corn/sorghum (mixed) diets in South Dakota during winters 1990-92. We also evaluated 97 wild females collected from areas with corn, sorghum, corn/sorghum (mixed), and areas without (control) food plots in east-central South Dakota. Except for ceca length (P = 0.059), intestinal masses and lengths did not differ (P > 0.10) among pen-reared females fed restricted or ad libitum diets. Females fed 100% MR corn diets were heavier (P ≤ 0.10) and fatter (P ≤ 0.10) than females fed the 100% MR sorghum or mixed diet. Females fed 85% MR sorghum diets were fatter than females fed corn (P = 0.020) or mixed (P = 0.056) diets. Pen-reared females fed 65% MR sorghum diets were heavier than females fed corn (P = 0.017) but not mixed diets (P = 0.133). Wild females collected from control plots had longer (P ≤ 0.10) and heavier (P ≤ 0.10) small intestine and ceca than did females collected from food plots. Females collected from sorghum and control plots contained less (P ≤ 0.10) fat than did females collected from corn or mixed plots. Diets of females from corn and mixed plots were mostly food plot grains, whereas wild foods, soybeans, and corn composed most of the diet of pheasants on sorghum and control plots. Comparisons of the 3 grain diets (corn/sorghum/mixed), with pen-reared females, did not suggest any 1 diet was nutritionally better than another. We postulate wild females feeding within food plots had larger lipid reserves than did females not feeding in food plots because of higher quality diets.

Mixing Model Approaches to Estimate Storm Flow Sources in an Overland Flow‐Dominated Tropical Rain Forest Catchment

Previous hydrometric studies demonstrated the prevalence of overland flow as a hydrological pathway in the tropical rain forest catchment of South Creek, northeast Queensland. The purpose of this study was to consider this information in a mixing analysis with the aim of identifying sources of, and of estimating their contribution to, storm flow during two events in February 1993. K and acid‐neutralizing capacity (ANC) were used as tracers because they provided the best separation of the potential sources, saturation overland flow, soil water from depths of 0.3, 0.6, and 1.2 m, and hillslope groundwater in a two‐dimensional mixing plot. It was necessary to distinguish between saturation overland flow, generated at the soil surface and following unchanneled pathways, and overland flow in incised pathways. This latter type of overland flow was a mixture of saturation overland flow (event water) with high concentrations of K and a low ANC, soil water (preevent water) with low concentrations of K and a low ANC, and groundwater (preevent water) with low concentrations of K and a high ANC. The same sources explained the streamwater chemistry during the two events with strongly differing rainfall and antecedent moisture conditions. The contribution of saturation overland flow dominated the storm flow during the first, high‐intensity, 178‐mm event, while the contribution of soil water reached 50% during peak flow of the second, low‐intensity, 44‐mm event 5 days later. This latter result is remarkably similar to soil water contributions to storm flow in mountainous forested catchments of the southeastern United States. In terms of event and preevent water the storm flow hydrograph of the high‐intensity event is dominated by event water and that of the low‐intensity event by preevent water. This study highlights the problems of applying mixing analyses to overland flow‐dominated catchments and soil environments with a poorly developed vertical chemical zonation and emphasizes the need for independent hydrometric information for a complete characterization of watershed hydrology and chemistry.

The effects of soil mixing on soil nutrient status, recovery of competing vegetation and conifer growth on cedar-hemlock cutovers in coastal British Columbia

In 1988 an experiment was established to stimulate the effect of windthrow on low-and high-productivity forest types in coastal British Columbia. It was hypothesized that site productivity may be improved by mixing the upper 1 m of the organic matter and mineral soil. Results of this study indicated that soil mixing (1) slightly increased soil pH and temperature on both forest types after 2 and 5 years, (2) decreased all soil nutrient availability indices on the high productivity type after 2 years, but resulted in no difference from the control after 5 years, and (3) decreased microbial activity and cellulose loss rate and most soil nutrient availability indices on the low-productivity type after 2 and 5 years. Soil mixing greatly reduced Gaultheria shallon above-ground biomass on both low- and high productivity forest types, whereas biomass of Epilobium angustifolium and other plant species increased slightly on the high-productivity type after 2 and 5 vears. Tsuga heterophylla was taller on high-productivity type and on mixed plots in both types after 2 and 5 years. Thuja plicata was taller on the high-productivity type after 5 years only. The increased conifer growth measured on the mixed low-productivity type was attributed to higher levels of available nutrients due to reduced G. shallon competition.

Effects of clearcutting and soil mixing on soil properties and understorey biomass in western red cedar and western hemlock forests on northern Vancouver Island, Canada

On northern Vancouver Island in British Columbia it has been hypothesised that the cultivation effect of repeated windthrow is the cause of the higher nutrient availability in windstorm-derived, natural second-growth stands of western hemlock and amabilis fir (the HA type), compared with adjacent old-growth stands of western red cedar with a smaller component of hemlock, and a dense understorey of the ericaceous shrub, salal (the CH type). In 1988 an experiment was established in a clearcut area containing examples of these two forest types. The experiment was designed to simulate the effects of a broadscale windthrow by mixing mineral and organic horizons using a large rake attached to an excavator. In this study, a range of soil physical and chemical properties, and the biomass of understorey plants, were measured 4.5 years after treatment in uncut, clearcut, and clearcut and mixed plots of the two types. In the HA type, clearcutting and soil mixing decreased surface organic matter and moisture content, and increased the rate of decomposition of cellulose. It had little effect on the rate of microbial activity measured using CO 2 evolved in laboratory incubation, or N and P measured after KCl extraction, anaerobic incubation, or using ion-exchange resin bags in situ. In the CH type, the treatment decreased mineralisable N and phosphate-P held on resin bags, and the rates of cellulose decomposition and CO 2 evolution. Mixing markedly decreased the cover of salal compared with both the clearcut only and the uncut treatment. The anticipated benefits of mixing and soil disturbance (increased soil nutrient availability brought about by the mixing of mineral and organic horizons) have not occurred. This was probably because the mixing treatment brought humus material from deeper in the soil profile to the surface. Decomposition of this material is limited more by its poorer quality for decomposers (lower N concentration, and possibly higher tannins) than by soil microclimate.

Interactions of N7-guanyl methyl- and thioether-substituted d(CATGCCT) derivatives with d(AGGNATG).

Enzymatic reaction of the carcinogen ethylene dibromide with GSH yields the half-mustard S-(2-bromoethyl)GSH, which reacts with DNA to form S-[2-(N7-guanyl)ethyl]GSH as the major adduct. Varying ratios of mutations to N7-guanyl DNA adduct levels had been observed when bacteria were treated with various half-mustards [Humphreys, W. G., Kim, D.-H., Cmarik, J. L., Shimada, T., and Guengerich, F.P. (1990) Biochemistry 29, 10342-10350]. Derivatives of the model oligonucleotide d(CATGCCT) were prepared with the single Gua substituted with N7-methylGua,N-acetyl-S-[2-(N6-guanyl)ethyl]Cys methyl ester, S-[2-(N7-guanyl)ethyl]GSH, or S-[2-(N7-guanyl)ethyl]GSH dimethyl ester. The hybridization of these oligonucleotides with the complement d(AGGCATG) was examined using UV melting profiles. The melting temperature (Tm) was reduced in the order N7-methylGua (least change) << N-acetyl-S-[2-(N7-guanyl)ethyl]Cys methyl ester < S-[2-(N7-guanyl)ethyl]GSH methyl ester < S-[2-(N7-guanyl)ethyl]GSH. These results indicate interaction of the adduct side chain with the rest of the oligonucleotide that was not observed in previous NMR studies [Oida, T., Humphreys, W. G., and Guengerich, F. P. (1991) Biochemistry 30, 10513-10522]. UV mixing plots indicated that d(CATGCCT) oligonucleotides containing the Gua adducts still pair with d(AGGNATG) containing Cyt but not the other three bases (at "N"). CD spectroscopic studies showed considerably reduced molar ellipticity for all oligonucleotide pairs containing the N7-guanyl adducts, even under conditions where duplex formation was extensive as judged by the UV melting and mixing studies.(ABSTRACT TRUNCATED AT 250 WORDS)