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International audience; The marks of prehistoric human societies on tropical forests can still be detected today. Levis et al. performed a basin-wide comparison of plant distributions, archaeological sites, and environmental data. Plants domesticated by pre-Columbian peoples are much more likely to be dominant in Amazonian forests than other species. Furthermore, forests close to archaeological sites often have a higher abundance and richness of domesticated species. Thus, modern-day Amazonian tree communities across the basin remain largely structured by historical human use.Science, this issue p. 925The extent to which pre-Columbian societies altered Amazonian landscapes is hotly debated. We performed a basin-wide analysis of pre-Columbian impacts on Amazonian forests by overlaying known archaeological sites in Amazonia with the distributions and abundances of 85 woody species domesticated by pre-Columbian peoples. Domesticated species are five times more likely than nondomesticated species to be hyperdominant. Across the basin, the relative abundance and richness of domesticated species increase in forests on and around archaeological sites. In southwestern and eastern Amazonia, distance to archaeological sites strongly influences the relative abundance and richness of domesticated species. Our analyses indicate that modern tree communities in Amazonia are structured to an important extent by a long history of plant domestication by Amazonian peoples.

Two experiments were undertaken to determine the effect of mycorrhizal inoculation and soil solution phosphorus (P) concentration on the growth of pejibaye (Bactris gasipaes, Palmae) seedling progenies from two Amazonian populations in a Hawaiian Oxisol. Mycorrhizal colonization and effectiveness were insignificant, perhaps because of residual fumigant toxicity and apparent water stress. Soil solution P concentration had highly significant effects on seedling leaf number, leaf area, plant biomass, anthracnose damage to leaves, and growth and physiological parameters in both experiments. Genotype effects were significant in the experiment involving progenies from two different populations, but not in the experiment involving two progenies from the same population. Native soil solution P concentration in this Oxisol was insufficient for acceptable pejibaye growth, while 0.2 mg/L soil P gave very good growth.

Barley, ryegrass, and fodder radish were grown in flowing nutrient solutions at four potassium concentrations, [K+], from 0-05 to 4 mg l-1. During the first 2 weeks after germination the response to [K+] (fodder radish > barley > ryegrass) depended on the potential relative growth rate, the ratio of root surface area to plant weight, and on the K+ flux into the roots. Subsequently, there was no effect of [K+] on growth rate within the range tested. The K+ flux decreased from 4-23 x 10"12 mol cm-2 s"1 in the first 2 weeks after germination, when it was concentration-dependent, to 2-5 x 10~12 mol cm-2 s"1 after 4-5 weeks, when it became independent of [K+] down to 0-05 mg 1_I. The results explain the importance of high [K+] and rapid root growth during the first 2 weeks after seed germination.

Perennial ryegrass was grown in flowing solution culture in a glasshouse, and during February lead was added to the nutrient solution and held at a constant concentration; uptake and transport of lead were followed in conditions of low intensity daylight or higher intensity artificial light. Uptake of lead by the roots was most rapid during the first 4 days after addition to the nutrient solution. After this time there was a steady increase in uptake per g dry weight of root with plants grown in artificial light having a much higher rate of uptake than plants grown in daylight. Roots always contained more lead than the corresponding shoots and concentration was always greater in the roots than in the shoots. The concentration in both roots and shoots increased with time but that in plants grown in artificial light was higher than that in plants grown in daylight. Two phases of uptake were identified, an initial rapid phase which is probably an exchange phenomenon, and a slow sustained phase which may be under metabolic control. A lower proportion of the total lead taken up remained in the roots of plants grown in artificial light than in those grown in daylight. This difference may have resulted from differences in (i) the production of organic carriers and/or (ii) transpiration. re]19750930

Summary. It is practicable to use herbicides as an alternative to ploughing grass swards for the preparation of a seedbed for the sowing of cereals and kale. The yields of spring barley and kale following a glaass/clover sward killed with 10 lb/ac amitrole-T were similar to those obtained following ploughing. Yields of tip to 40 cwt/ac of winter wheat were obtained following grass swards killed with amitrole-T compared with up to 45 cwt/ac after ploughing. The ideal herbicide must kill all plants in a sward and have no residual toxicity to the subsequent crop. At 10 lb/ac, amitrole-T killed most plants in the sword, but it was considered necessary to delay sowing for at least 3 weeks after spraying to minimize the danger of residual toxicity. Ploughing gave the lowest incidence of grasses persisting from the sward. Paraquat did not control clover and Agropyron repens, while broad-leaved weeds were more common on the ploughed than on the sprayed land. There were differences in the amount of nitrogen mineralized in ploughed and In undisturbed soil, but these were insufficient to affect crop yields materially. When a lower yield followed herbicidal destruction of the sward, this was not accompanied by a reduced percentage nitrogen content. L'emploi des deherbants pour la renovation des prairies Resume. On peut employer les desherbants, au lieu de retourner les prairies, pour peparer un lit de semis pour les cereales et le chou moellier. Les productions d'orge de printemps et de chou moellier obtenues apres avoir detruit une prairie (melange d'une graminee et de trefle) par l'application de 11,2 kg d'amitrole-T/ha furent a peu pres les memes qu'apres avoir retourne la prairie. Les productions de ble d'hiver obtenues apres la destruction de la prairie (melange de graminees) avec l'amitrole-T atteignirent 5020 kg/ha, en comparaison de 5650 kg/ha apres le labour. Un desherbant parfait doit detruire toutes les plantes d'une prairie et ne doit pas avoir dc toxicite residuelle pour la culture suivante. A la dose de 11,2 kg/ha, l'amitrole-T detruisit la plupart des plantes dec la prairie, mais on a juge qu'il etait necessaire d'attendre au moins trois semaines apres la pulverisation avant de semer pour reduirc le danger d'une toxicite residuclle. Le labour a donne le nombre le plus bas de graminees survivantes dans la prairie. Le paraquat n'a pas detruit le trefle ni le chiendent. Les mauvaises herbes a feuilles larges furent plus frequentes sur le terrain laboure que sur le terrain traite chimiquement. Les differences dans les quantites d'azote mineralise dans le sol retourne et dans le sol non retourne ne furent pas suffisantes pour influencer les recoltes d'une maniere importante. Quand une recolte plus faible fut obtenue apres la destruction chimique de la prairies, sa teneur en azote n'etait pas reduite. Die Verwendung von Herbiziden als Alternative zum Ackerweideumbruch in Wechselwirtschaften Zusammenfassung. Als Alternative zum mechanischen Umbruch einer Grasnarbe wird die Brauchbarkeit von Herbiziden fur die Vorbereitung eines Saatbetts zur Ansaat von Getreide und Markstammkohl bestatigt. Die Ertrage von Sommergerste und Markstammkohl entsprachen auf einer mit 11,2 kg/ha Amitrol-T abgetouteten Grasnarbe den nach Umbrueh erzielten. Im Vergleich zu 56,5 dz/ha nach Umbruch wurden auf einer mit Amitrol-T behandelten Flache Winterweizen-Ertrage bis zu 50,2 dz/ha erzielt. Das ideale Herbizid sollte ohne Nachwirkung auf die folgende Kultur alle Pflanzen einer Grasnarbe abtoten. Mit 11,2 kg/ba Amitrol-T wurden die meisten Pflanzen der Grasnarbe vernichtet. Es war jedoch notwendig, nach der Spritzung mindestens drei Wochen mit der Ansaat zu warten, um die Schadigungsgefahr durch Ruckstande zu vermindern. Von den untersuchten Verfahren wurde durch Umbruch die beste Vernichtung der Grasnarbe erreicht. Paraquat wirkte ungenugond gegen Klee und Quecke, BreitblaUttrige Unkrauter waren dagegen nach Umbruch haufiger als nach Herbizidbebandlung. In Boden mit und ohne Umbruch bestanden Unterscbiede in der Menge minerali-sierten Stickstoffs. Diese Unterschiede waren jedoch zu gering, um die Ertrae zu beeinRussen. Bei Minderertragen nach Abtotung der Narbe mit Herbiziden konnte keine entsprecbendc Reduzienjng des relativen Stickstoffgehaltes der Pftanzen beobachtet werden.

SUMMARYMacro-organic matter (roots and partially decomposed plant debris retained on a 0·25 mm mesh sieve) was measured in soils under various swards. Under a grazed perennial ryegrass/white clover sward, sown on arable land, macro-organic matter in the top 15 cm of soil rose steadily in the first 8 years to 15·8 t/ha, but subsequently declined. Under arable cropping there was great variation with crop and season. Under grass, most of the macro-organic matter accumulated in the top 2 cm of soil, particularly during the first 3 or 4 years. More macro-organic matter was found under perennial ryegrass/white clover than under cocksfoot/white clover swards.After 3 years under grass macro-organic matter accounted for 10% of the total soil carbon, and represented about half the increase in soil carbon.Half, or less, of the nitrogen which accumulated in soil under grass was in the macroorganic matter fraction. The differences between swards which received no N fertilizer and those which received 940 kg/ha over 3 years was small, 16–40 kg N/ha respectively for cut and frequently grazed swards. The ratio of C:N in macro-organic matter under different swards averaged 22:1.

Summary Wet sieving as a measure of the effect of leys on crumb stability is discussed, briefly nothing the significant details of sampling and analysis. The increase in water-stable aggregation following several ryegrass/white clover leys of four years duration was of the same magnitude for both clay and sandy loam soils. The lighter soil, having the lower initial stability, showed the greater proportionate increase. Perennial rycgrass/white clover swards were consistent in giving higher values for water-stable aggregation than other commonly sown grasses. It would seem that a large proportion of clover in a sward would not impair the stabilizing effect of a ley. There was no evidence of any significant variation resulting from different methods of grassland utilization. Data from Hurley soil suggest that intensive grazing did not reduce the effect of the grass ley on soil stability as long as a plant cover was maintained. Cocksfoot sown in drills for seed production had less effect on the soil than did swards of this grass sown with white clover. The application of nitro-chalk to grass leys over a three year period at annual rates up to 16 cwt. per acre had no significant effect on either of the soils examined. Profiles to 1 ft. depth showed that, whereas the high stability found under permanent pasture extended throughout this profile, the increase in water-stable aggregation under a three-year ley was largely confined to the top few centimetres.

A system is described for growing plants in flowing solution in which the concentration (activity) of some nutrient ions is continuously monitored and held almost constant by means of ion-selective electrodes which control the operation of nutrient pumps. By recording the concentration of a given ion in the flowing nutrient solution and the amount of that ion which is delivered by the nutrient pumps, it is possible to follow nutrient uptake over long and short periods. A brief account is given of the operation of the system using a nitrate ion-selective electrode to examine aspects of NO?~ uptake by perennial ryegrass (Lolium perenne L.).

Summary A model (based on the short- and long-term Q/I relations of K in a soil, on graphs of K-uptake by crop against post-uptake K-activity ratios, and the exhaustion value of K-activity ratios below which K can no longer be taken up) is presented by means of which to estimate the amount of labile K in a soil, and to compare the contributions of labile and fixed K to plant uptake. The application of the model was partially explored in a field experiment. Relative values for the magnitudes of the pools of labile and fixed K developed by 4 years’build-up treatments were calculated on the basis of the model. Their relative contributions to the K taken up by a ryegrass test crop in the following year were calculated from the soil Q/I relations, and the measured uptakes of K. While the data were incomplete and so could not provide conclusive evidence for the model, the values produced were sensible and apparently consistent.

1. The increase in soil carbon has been assessed in arable land sown to different leys variously managed.2. Under swards cut for herbage conservation, carbon in the top 15 cm. of soil increased by about 0-05 % each year. Although an increase of 0-3 % was found immediately under drills of cocksfoot grown for seed production, there was no increase midway between the drills and, for a given area of land, the carbon increment under such row-crops is probably similar to that under cut swards.

Mineralisable N, exchangeable K and the concentrations of K, Na, Mg and Ca in equilibrium soil extracts were determined after 4 years under cut grass, grazed grass and arable cropping with several levels of N and K applied as fertilisers. The yield of potatoes which followed these systems of cropping and manuring ranged from 60 t/ha. More than 50% of the yield variance was related to differences in the levels of mineralisable N and exchangeable K in the soil, compared with 17% which was related to levels of fertiliser (0 and 112 kg N/ha, 0-250 kg K/ha) applied direct to the potatoes. Levels of all nutrients measured in the soil and yields of potatoes were higher after grazed grass than after cut grass or arable cropping. Concentrations of Mg and Ca in the foliage of the potatoes were not, however, appreciably affected by previous systems of cropping and the yield of tubers seemed to be determined primarily by differences in the supply of N and K. Application of these two nutrients as fertilisers to the potatoes largely rectified any deficiency in their supply from the soil which resulted from the previous cropping.

1. The nitrogen content of an arable soil to 15 cm depth, sown to a ryegrass/white clover sward increased on average by 0·005 % N each year.2. Under a grass sward without clover and without fertilizer nitrogen, there was an increase of 0·017 % N in 3 yeais in the top 7·5 cm of soil; an annual addition of about 70 kg N/ha.3. Including white clover in grass swards not receiving nitrogen fertilizer resulted in an increase over 3 years of 0·026% N to 7·5 cm depth in one experiment, and 0·014% N to 15 cm depth in another, an annual increment of 100–110 kg N/ha.4. The increase was greater under ryegrass/white clover swards than under cocksfoot/white clover swards. A similar difference was found both in accumulation of organic carbon and in clover content. In an experiment comparing many different grasses it was not possible to distinguish differences in the level of soil nitrogen, after 4 years under grass, which could be ascribed to species or strains.There was no significant effect of nitrogen fertilizer applied at rates of up to 314 kg N/ha per year for 3 consecutive years.

Summary A technique is discussed whereby the level of mineral nitrogen present in the soil after incubation has been used to estimate differences in the nitrogen status of an arable soil after leys. There was an appreciable increase in the mineralization of nitrogen in arable soil after 3 years under ley, despite any immobilization which may have resulted from the decomposition of grass roots. The uptake of nitrogen by the winter wheat which followed the leys was closely related to the nitrogen status of soil so determined.

In a series of experiments with winter cereals growing in pots, mechanical compaction of moist soil over wheat seed severely inhibited germination — no plants emerged from a compacted sandy loam soil. Conversely, the destruction of the surface tilth by falling water drops had no effect on the rate of emergence of either wheat or oats.

Although it has been widely established that increased crop yields follow leys sown on arable land, the relative importance of the many causative factors which may be involved is obseure. An attempt has been made to evaluate the influence of 3- to 4-year leys on the organic matter in an arable soil and to distinguish also the relative importance of the resultant change in moisture-holding capacity, crumb stability and the nitrogen status of the soil as factors in the yield of winter wheat. An increase of about 4 tons per acre in total soil organic matter was recorded after 3 years under grazed swards. Moisture-holding capacity increased by an amount equivalent to the evapo-trans-piration loss from a full plant cover on one summer day. An increase in crumb stability was largely confined to the top inch of soil, which is effectively buried when leys are ploughed. The influence of variation in soil nitrogen status and in crumb stability on the yield of winter wheat after leys was examined by multiple regression analysis of data obtained over a number of yean from field experiments. After allowing for the effect of variation in the nitrogen status of the soil, the influence of variation in crumb stability was insignificant Conversely variation in the nitrogen status of soils, sampled under leys before ploughing, was closely correlated (r =+0.93***) with wheat yield.

Summary A tension table may be constructed from a Perspex tray ⅝ in. thick, approximately 12 in. wide and 19 in. long with 21 drainage channels. With a filter paper membrane, which gives rapid transmission of water and can be readily changed when contaminated it may be used to equilibrate 3–4 kg of moist soil overnight at suctions up to 100 cm of water. Several small samples may be handled at once by using small filter-paper trays.

Plant species and cultivars differ widely in their response to the concentration of nutrients in the soil (Vose 1963; Epstein & Jefferies 1964). These differences, which have important implications in agronomy and ecology (Rorison 1969) may be the result of differences in both nutrient uptake and nutrient utilization. For plants grown in soil the rate of nutrient uptake is the result of interactions between the rate of absorption by roots, and soil processes that determine the rate of transfer of nutrients to the root surface. Transfer processes in the soil include (a) diffusion induced by a change in ion activity at the root surface; and (b) mass flow induced by transpiration. The rate of transfer of nutrients by diffusion and mass flow is dependent on the ionic species involved and on a number of soil characteristics, including water content (Olsen & Kemper 1968; Barley 1970). Differences between species in rates of nutrient uptake can only be observed independently of transfer processes when nutrients are maintained at near constant concentration close to the absorbing root surface. This requirement is approached in systems of plant culture where nutrient solution of constant concentration flows continuously through the entire root system (Asher, Ozanne & Longeragan 1965). Our objective was to compare, under similar environmental conditions, the response of four herbage species to a range of potassium concentrations in flowing nutrient solution, where transfer processes are relatively unimportant, and in sand culture, where transfer processes might be more important. There is evidence that two of the species used (Dactylis glomerata and Trifolium pratense) require a high supply of potassium, and the other two species (Anthoxanthum odoratun and Medicago lupulina) require only a low supply (Brenchley & Warington 1958; Kruijne & de Vries 1963). Three (Dactylis glomerata, Trifolium pratense and Medicago lupulina) are cultivated species, whilst Anthoxanthum odoratum occurs as a common component of permanent grasslands in western Europe.

NORTH AFRICAN ecotypes of Festuca arundinacea Schreb (tall fescue) have been found capable of considerable growth during winter in Britain1. In some circumstances, during this period they have shown yellowing of foliage not seen in indigenous material. The yellowing was confined to regrowth when cold weather followed autumn or winter defoliation and was sometimes followed by death of tillers. Grobbelaar2 reported a reduced uptake of nitrate and other nutrients by maize when the root temperature was 15° C or less; Ayres and Doi3 considered that, below a soil temperature of 62° F, absorption of nutrients by sugar cane was depressed.