The effect of initial concentration of carbofuran on the development and stability of its enhanced biodegradation in top-soil and sub-soil.

Carbofuran was incubated in top-soil and sub-soil samples from a pesticide-free site at a range of initial concentrations from 0.1 to 10 mg kg-1. Amounts of the incubated soils were removed at intervals over the subsequent 12 months, and the rate of degradation of a second carbofuran dose at 10 mg kg-1 was assessed. An applied concentration as low as 0.1 mg kg-1 to top-soil resulted in more rapid degradation of the fresh addition of carbofuran for at least 12 months. The degree of enhancement was generally more pronounced with the higher initial concentrations. When the same study was conducted in sub-soil samples from the same site, an initial dose of carbofuran at 0.1 mg kg-1 resulted in only small increases in rates of degradation of a second carbofuran dose. However, degradation rates in the sub-soil samples were, in many instances, considerably greater than in the corresponding top-soil samples, irrespective of pre-treatment concentration or pre-incubated period. Initial doses of 0.5 mg kg-1 and higher applied to sub-soil successfully activated the sub-soil microflora. Application of the VARLEACH model to simulate carbofuran movement through the soil profile indicated that approximately 0.01 mg kg-1 of carbofuran may reach a depth of 70 cm 400 days after a standard field application. The results therefore imply that adaptation of the sub-soil microflora (c 1 m depth) by normal field rate applications of carbofuran is unlikely to occur. In experiments to investigate this in soils exposed to carbofuran in the field, there was no apparent relationship between top-soil exposure and degradation rates in the corresponding sub-soils. The results further confirmed that same sub-soil samples have an inherent capacity for rapid biodegradation of carbofuran. The high levels of variability observed between replicates in some of the sub-soil samples were attributed to the uneven distribution of a low population of carbofuran-degrading micro-organisms in sub-surface soil. There was no apparent relationship between soil microbial biomass and degradation rates within or between top-soil and sub-soil samples.

Microsatellite analysis of the inbreeding grass weed Barren Brome (Anisantha sterilis) reveals genetic diversity at the within- and between-farm scales.

Nine microsatellites were used to screen 131 samples of Barren Brome (Anisantha sterilis: synonym Bromus sterilis) collected from within the fields of three English farms [from Oxfordshire (Oxon), Leicestershire (Leics) and Wiltshire (Wilts)] and eight seeds taken from samples of each of 10 farms across England, UK. Most individuals (approximately 97%) were homozygous. Polymorphism occurred at all nine loci in all three farms sampled at the field scale, and at most loci for nine of the other 10 farm samples. Between three and 11 alleles were found per locus. Gene diversity (D = 1 - summation operator p(i)2) ranged from 0.088 to 0.760. Polymorphism occurred among individuals within and among fields, and farms. Some alleles were found in only one farm. On the basis of the alleles at all nine loci in the 211 sampled plants, a total of 92 (44%) different genotypes was identified. Clustering analysis using the unweighted pair group method with arithmetic averages (UPGMA) for the combined Oxon, Wilts and Leics samples did not cluster them into their respective farms. Similarly, a phenogram of samples from all 10 farms showed considerable mixing of individuals with respect to farm origins. Identification of genotypes on field plans showed evidence of both spatial localization and mixing. Previous reports have suggested that A. sterilis is strictly inbreeding with little intrapopulation variation at the genetic level. Our data reveal that A. sterilis exists as numerous separate and genetically different lines, which are maintained by inbreeding but which very occasionally outcross. Possible explanations for this pattern of high genetic diversity are discussed.

EVIDENCE FOR AN ENDOGENOUS CYCLE OF DORMANCY IN DRY STORED SEEDS OF POA TRIVIALIS L.

Seeds of Poa trivialis L. (rough meadow-grass) were collected from two contrasting habitats during the summer of 1982. Collections from an arable population were obtained on three occasions and seeds from a grassland population were collected once. Although both populations responded similarly to various combinations of light, temperature and nitrate, the arable collections were markedly more dormant. Loss of dormancy of the grassland and early arable populations one month after collection was rapid in seeds stored at ambient temperatures and at 4 °C but more delayed in those stored at 23 °C. Despite a gradual loss of dormancy during storage, cyclic changes in dormancy were also evident, especially at 23 °C. The percentage germination of the various collections 7 d after incubation following storage at 23 °C increased over the first three months, but declined following six and 12 months storage and increased again after 15 months storage. At 4 °C the late arable collection lost dormancy more slowly than the other two collections and germination was greatest after six months storage; a reduction in percentage germination occurred after twelve months. Seeds buried at 5 and 15 cm showed cyclic changes of dormancy, being least dormant in autumn and spring, but most dormant in summer, corresponding to the natural periodicity of germination. It is proposed that cyclic changes in dormancy are regulated by endogenous rhythms and seasonal changes of temperature. The ecological implications of the germination behaviour of this species in various habitats are discussed.