Agro-Ecosystems最新文献

This study evaluated the persistence of atrazine in the soil and movement in surface runoff and shallow phreatic flow. Atrazine persisted in the surface 10 cm of soil for over 4 months. However, it could not be detected at greater depths after approximately 2 months. Persistence (time) in the soil did not vary significantly with application rate. Initial concentrations in the soil did, however, vary significantly. Initial atrazine concentrations in the soil for the 4.48 kg ha⁻¹ application rate were approximately twice those at the 2.24 kg ha⁻¹ rate. Atrazine was not detected in significant amounts in surface runoff after 26 days and was not observed at any time in the subsurface flow from the treated area. Total losses for the 2 rates in 1974 and 1975 ranged from 0.22–2.24%. Concentrations in the soil and surface runoff were directly related to application rates.

The dynamics in tiller population and dry-matter accumulation of a grass sward, cut at different time intervals, was compared with a descriptive simulation model. The model was validated against experimental data of tiller number, and dry-matter accumulation for irrigated cv. Katambora Rhodes grass (Chloris gayana Kunth), cut at intervals of 1, 2, 3 or 4 weeks. The model predicted that the initial rate of regrowth following long cutting intervals was much slower than the regrowth following short cutting intervals. The simulations also showed that relatively high dry-matter yields alternated with relatively low dry-matter yields when the interval between cuttings was long. These results were in reasonable agreement with the measured values. The fluctuation in the number of tillers and dry-matter yield was attributed mainly to the formation of culmed-vegetative tillers, common to Rhodes grass at an early stage of development, which renders them incapable of regrowth after defoliation.

This paper is concerned with reinterpretation of published data on efficiency of nutrient utilisation in agro-ecosystems. A close examination of nutrient cycles both under pastoral and arable farming in various countries shows conclusively that the utilisation of phosphorus and potassium in the majority of ecosystems is very high, more than 70% of the added and recycled and K being either utilised by plants or remaining in a chemical form available to plants. This conclusion is in conflict with the traditional belief that due to the process of large-scale P fixation the major part of the added P is fixed in the soil in compounds not available to plants.

The crop growth simulation model BACROS describing production under optimum conditions of water and nutrient supply, is tested with data collected in field experiments with Rhodes grass (Chloris gayana Kunth). The choice of specific parameters for the plant species is discussed and special attention is paid to stomatal behaviour. It was concluded that in all situations examined (climate room, greenhouse and field), stomatal aperture was determined by the CO₂ concentration inside the stomatal cavity and hence by the rate of assimilation. Measured and calculated rates of assimilation, respiration and transpiration of an artificial sward showed good agreement. In the field situation the maximum rate of dry matter accumulation was predicted with reasonable accuracy as was the rate of water use. Initial growth rates and the rate of growth towards the end of the growing period showed deviations. The former inaccuracies were associated with the death of tillers, the latter with the formation of flowering stems.

Bromus tectorum L., the most ubiquitous alien in steppe vegetation in the intermountain West of North America, entered British Columbia, Washington, and Utah ca. 1889–1894. By ca. 1928 the grass had reached its present distribution occupying much of the perennial grasslands in Washington, Idaho, Oregon, Nevada, Utah and British Columbia as native grasses dwindled with overgrazing and cultivation. In the process this cleistogamous winter annual may have competitively displaced both native colonizers (including cleistogamous us annual grasses) as well as the dominants of climax stands. The spread of B. tectorum demonstrates the degree of success an alien may achieve when preadaption, habitat alteration simultaneous with entry, unwitting conformation of agricultural practices to the plant's ecology and apparent susceptibility of the native flora to invasion, are all in phase.

As part of a study of triazine resistance in weed species, the authors have collected several resistant populations of Chenopodium album in different areas in France. A study of intra- and inter-population variability between different susceptible and resistant populations has been made using polyacrylamide gel electrophoresis of foliar isoenzymes. The separation of esterases and malate dehydrogenase isoenzymes was carried out in a modified vertical apparatus.

Within each resistant population, all individuals have the same pattern of esterase and populations collected in the same area have the same zymogram. However, populations from different areas have distinct zymograms. These data suggest that the resistant populations originated in the region where they now occur, and that there is a high rate of self-pollination in each resistant population.

After treating a susceptible population with a low dose of atrazine, about 1% of plants survived and these have a high frequency of the esterase zymogram typical of resistant individuals. The offspring of these plants are completely resistant. The resistant and susceptible populations grew near each other so that they may have a common origin. The pattern of occurrence of resistance in relation to its inheritance and the level of inbreeding in populations is discussed.