The effect of plant species,weather variables and chemical composition of plant material on decomposition in a tropical grassland

Summary The decomposition of litter and roots ofChenopodium album, Desmostachya bipinnata and mixed grass samples for a period of 402 days and ofDichanthium annulatum andSesbania bispinosa for a period of 278 days was studied in a tropical grassland. Litter bags positioned at midcanopy height, soil surface and at five cm depth below the soil surface and root bags placed at 5, 15, 25 and 35 cm depths belowground were used. For the total study period, the cumulative weight loss in litter bags was: Chenopodium=76–100%; Desmostachya=33–98%; Dichanthium=26–96%; mixed grass=43–99% and Sesbania=25–99%. The weight loss in root bags was: Chenopodium=93–100%; Desmostachya=47–56%; Dichanthium=71–87%; mixed grass=61–82%; Sesbania=87–100%. The nature of plant species affected decomposition rates. The position of litter/root bags also affected the decomposition rates. The mean relative decomposition rates of litter as well as of root material were found to be highest in rainy season and lowest in winter months. Rainfall, particularly the frequency of rainfall, was an important factor affecting decomposition rates. The litter species characterized by highest concentration of nitrogen, ash, acid detergent cell wall component and lowest concentration of carbon, cellulose and lignin, decomposed rapidly. In the case of roots, the material having high nitrogen, carbon, cellulose and ash content and low C/N ratio and lignin content decomposed rapidly.     

Nile perch,Lates niloticus,predation on the freshwater prawn,Caridina nilotica,in the Nyanza Gulf,Lake Victoria,East Africa

Synopsis The percentage of Nile perch, Lates niloticus, containing the prawn, Caridina nilotica, declined from 60–85%, for fish between 5 and 40 cm total length, to less than 10% for fish larger than 80 cm in length. The maximum number of prawns eaten by a Nile perch increased steadily, from 10 prawns for a fish in the 5–9.9 cm length group, to 913 prawns in a fish in the 60–69.9 cm length group, and then fell sharply for larger fish. The mean number of Caridina eaten by fish that were foraging on this animal followed a similar trend. Gill raker spacing increased in direct proportion to fish length and this may prevent fish larger than 70 cm in length from being effective prawn predators.     

The effect of the presence of a forage legume on nitrogen and carbon levels in soils under Brachiaria pastures in the Atlantic forest region of the South of Bahia,Brazil

The impact of forest clearance, and its replacement by Brachiaria pastures, on soil carbon reserves has been studied at many sites in the Brazilian Amazonia, but to date there appear to be no reports of similar studies undertaken in the Atlantic forest region of Brazil. In this study performed in the extreme south of Bahia, the changes in C and N content of the soil were evaluated from the time of establishment of grass-only B. humidicola and mixed B. humidicola/Desmodium ovalifolium pastures through 9 years of grazing in comparison with the C and N contents of the adjacent secondary forest. The decline in the content of soil C derived from the forest (C3) vegetation and the accumulation of that derived from the Brachiaria (C4) were followed by determining the ¹³C natural abundance of the soil organic matter (SOM). The pastures were established in 1987, 10 years after deforestation, and it was estimated that until 1994 there was a loss in forest-derived C in the top 30 cm of soil of approximately 20% (9.1 Mg C ha^–1). After the establishment of the pastures, C derived from Brachiaria accumulated steadily such that at the final sampling (1997) it was estimated 13.9 Mg ha^–1 was derived from this source under the grass-only pasture (0–30 cm). Samples taken from all pastures and the forest in 1997 to a depth of 100 cm showed that below 40 cm depth there was no significant contribution of the Brachiaria-derived C and that total C reserves under the grass/legume and the grass-only pastures were slightly higher than under the forest (not significant at P=0.05). The more detailed sampling under the pastures showed that to a depth of 30 cm there was significantly (P<0.05) more C under the mixed pasture than the grass-only pasture. It was estimated that from the time of establishment the apparent rate of C accumulation (0–100 cm depth) under the grass/legume pastures (1.17 Mg ha^–1 yr^–1) was almost double that under the grass-only pastures (0.66 Mg ha^–1 yr^–1). The data indicated that newly incorporated SOM derived from the Brachiaria had a considerably higher C:N ratio than that present under the forest.     

Trophic roles of particle feeders and detritus in a mangrove island prop root ecosystem

Swarms of Dioithona oculata, shoals of Mysidium columbiae, and schools of planktivorous fish were observed among mangrove prop roots on a lagoonal island. Detritus covering epiphytic and benthic macroalgae and invertebrates was a probable food source for pelagic mysids, and copepods, as well as benthic oysters and sponges. Open water blooms of dinoflagellate Amphidinium klebsii were probably only accessible to the swarming dioithonans which spend the night away from the mangrove prop roots. Dominant species were analyzed for stable carbon isotopic composition (expressed as ¹³C%.) to elucidate the origin and fate of detritus. Detrital components (–23.6 to –19.1%.), which included floating detritus, marine snow, prop root detritus, and detritus below prop roots, probably originated from the dominant subtidal macroalgal species (–24.2 to –14.6%.) and A. klebsii (–25.8 to –21.2%.), although other sources which are end-members could theoretically contribute since detrital isotopic ratios are half way between end-members. These other sources include the intertidal red algal Bostrychia spp. (–30.9 to –29.6%.), red mangrove Rhizophora mangle leaves (–28.2 to –27.0%.), and turtle grass Thalassia testudinum (–12.3 to –11.1%.). Particle feeders such as mangrove oysters, sponges, mysids, and dioithonans were usually enriched with ¹³C (–21.0 to –16.2%.) compared to their probable food sources, detritus and A. klebsii. At Anchovy Bay, adult mysids and planktivorous fish were more ¹³C enriched (1–3%.) than their probable prey, the dioithonans. Turtle grass may have a minor role in food webs, since dioithonans and mysids from Outer Twin and Anchovy Bays where turtle grass beds were abundant were consistently more ¹³C enriched (2–7%.) than in the Lair Channel where turtle grass beds were sparse.     

Levels of diurnal predation on a school of flat-iron herring,Harengula thrissina

Synopsis The incidence of diurnal predation (0800–1600h) on schools of several hundred thousand adult flat-iron herring,Harengula thrissina, was quantified. Nine species of piscine predators perpetrated 653 attacks and 158 captures during 42.5 h of observation. Four predators: cabrilla,Mycteroperca rosacea, cornetfish,Fistularia commersonii, green jack,Caranx caballus, and black skipjack,Euthynnus lineatus, were responsible for 92% of the attacks and 96% of the successes. Among these predators attack rates (attacks × time^–1) and success rates (successes × attacks^–1) varied with respect to time of day, although all predators were active throughout the day. As a consequence, the herring school experienced fairly homogeneous attack and success rates. These rates were estimated as roughly half the crepuscular attack rates but they were sustained over a much longer period. Piscivorous predators, particularly those species frequently found in association with schooling prey, may be focusing their attacks on opportunities, not specific times of day and/or light levels. More direct information needs to be gathered on diurnal as well as crepuscular attack and success rates in order to assess the impact that piscine predators have on schooling prey.     

Morphology of caryopses,seedlings and seedling emergence of the grass Calamovilfa longifolia from various depths in sand

Summary The spikelets of the grass Calamovilfa longifolia (Hook) Scribn. are one flowered and the dispersal unit is composed of a caryopsis (3.5 mm long and 0.9 mm wide) enclosed in lemma and palea. The highest germination of caryopses and emergence of seedlings occurred from 1–2 cm depths and seedling emergence decreased with increasing depth of burial. The maximum depth of sand from which a seedling can emerge is about 8 cm. Seedlings emerging from deep locations had first internodes which were more elongated than those of seedlings from more shallow plantings. Coleoptile lengths of seedlings from shallow or deeply buried caryopses were similar.     

Maintenance and termination of reproductive dormancy in an Australian stink bug, Biprorulus bibax

Reproductive dormancy in female Biprorulus bibax Breddin sampled during March-October in Southern New South Wales was terminated by exposure to 25°C under long (15h) and short (10h) photoperiods. 27% of individuals collected during March-May failed to oviposit under short days. Pre-oviposition period under short photoperiod was greates during March-June (22.4–61.7 d) and shortest during July-October (4.2–14.4 d). Under long photoperiod it was greatest in April and May (20.1–22.3 d) and shortest during June-October (4.9–16.6 d). Longevity ranged from 39.8–89.0 d, with longest lived females being collected in June and held under short days. Fecundity ranged from 51.6–164.4 and was greatest in individuals collected in June-July and held under short days. In females held under long days fecundity was greatest in March and lowest in May. These data indicate that female B. bibax pass the winter in a photoperiodically maintained reproductive diapause.     

Rapid community change at a tropical upwelling site in the Galápagos Marine Reserve

The high biodiversity of tropical marine communities has attractedconsiderable interest, yet we still lack a clear understanding of the tempo ofdiversity change in these systems []. Knowledge of the conditions associated with fast or slow community assembly inthe tropics would enhance our ability to predict recovery from natural andanthropogenic disturbance and to conserve biodiversity. Here we report anunusually rapid doubling of species richness within a year in a tropical,subtidal sessile invertebrate community in a protected (non-extractive) zone ofthe Galápagos Marine Reserve (GMR). Diversity changes in the rock wallcommunity were accompanied by large increases in the percent cover, densityand/or biomass of sponges, barnacles, ascidians, and an ahermatypic coral,Tubastrea coccinea, over the 1-year studyperiod (1999–2000). Barnacle (Megabalanuspeninsularis) and ascidian (Didemnum cineraceum)biomasses increased by an order of magnitude from 1999 to 2000. The greaterabundance of sessile invertebrate prey was accompanied by significant increasesin the abundance of barnacle and Tubastrea predators(Hexaplex princeps, Asperiscalabilleeana). An estimated 37% of barnacle tissue biomass productionwas consumed in 1 year. Temperature monitoring during the studyperiod showed that this site is characterized by strong upwelling, where rapid,3.0–9.0 °C decreases in temperature occurred at harmonicsof the semi-diurnal tidal periodicity during warm (January–February), butnot during cool months (June–July). Short-term acoustic current metermeasurements revealed strong, highly variable upwelling at the study site, withevents ranging from 2–111 min in duration and maximumupwelling velocities of 32.3 cm s^–1. Thesefindings suggest that the turnover of diversity and biomass may be unusuallyrapid at tropical upwelling sites, especially where invertebrate predators areprotected from harvesting. Consequently, upwelling sites may warrant specialconsideration in the planning of marine reserves to ensure the conservation ofbiodiversity.     

Gene-centromere mapping of medaka sex chromosomes using triploid hybrids between Oryzias latipes and O. luzonensis

Four sex-linked genetic markers (SL1, SL2, B2.38 and stsOPQ05-1) on the sex chromosomes of the medaka, O. latipes, were mapped in relation to the centromere by means of triploid hybrids between O. latipes and O. luzonensis. Female F₁ hybrid O. latipes of two inbred strains, Hd-rR and HNI, were crossed with male O. luzonensis. Triploidization was induced by heat-shock treatment. Hatching rate of heat-shock treated eggs was 59%, and that of untreated hybrid eggs was 2%, indicating that most of the hatched fry were triploid. Using these triploid hatched fry, the map distances between the four loci and the centromere were examined. The order was SL2 – centromere – SL1 – B2.38 – stsOPQ05-1 and the map distances were: SL2 – centromere, 1%; centromere – SL1, 18%; SL1 – B2.38, 19%; B2.38 – stsOPQ05-1, 9%. Previous studies using FISH showed that SL2 is located on the short arm of large submetacentric chromosomes, and SL1 was closely linked to SDF (sex-determining factor). The results of gene-centromere mapping of this study show that SL1, B2.38 and stsOPQ05-1 are located on the long arm, and that, SDF is thus also on the long arm of the sex chromosomes.     

Patterns of nocturnal movement of the long-spined sea urchin<Emphasis Type="Italic"> Diadema antillarum</Emphasis> (Philippi) in Gran Canaria (the Canary Islands,central East Atlantic Ocean)

We tagged individuals of the sea urchin Diadema antillarum (Philippi) around the island of Gran Canaria (The Canary Islands) during winter 2001–2002 using a new technique, consisting of the insertion of a hook fastened to a fishing line into the aboral pole (periproctal membrane). This allowed individual identification of tagged sea urchins. The goals were: (1) to quantify nocturnal movements and the homing behaviour of this echinoid on shallow rocky bottoms, and (2) to assess short term spatial and temporal variability of these movements. Tagged sea urchins displayed clear homing behaviour. The mean distance travelled at night was 3.7±1.2 m (range 1.0–5.1 m). Mean speed of nocturnal movement was 33±26 cm h^–1 (range 5–110 cm h^–1). We observed greater movement at midnight than at the beginning and the end of the night.Communicated by H.-D. Franke     

The role of <Emphasis Type="Italic">Calamagrostis</Emphasis> communities in preventing soil acidification and base cation losses in a deforested mountain area affected by acid deposition

The effects of grass growth and N deposition on the leaching of nutrients from forest soil were studied in a lysimeter experiment performed in the Moravian-Silesian Beskydy Mts. (the Czech Republic). It was assumed that the grass sward formed on sites deforested due to forest decline would improve the soil environment. Lysimeters with growing acidophilous grasses (Calamagrostis arundinacea and C. villosa), common on clear-cut areas, and with unplanted bare forest soil were installed in the deforested area affected by air pollution. Wet bulk deposition of sulphur in SO₄^2– corresponded to 21.6–40.1 kg ha^–1 and nitrogen in NH₄⁺ and NO₃^– to 8.9–17.4 kg N ha^–1, with a rain water pH of 4.39–4.59 and conductivity of 18.6–36.4 S cm^–1 during the growing seasons 1997–1999. In addition, the lysimeters were treated with 50 kg N ha^–1 yr^–1 as ammonium nitrate during the 3 years of the experiment. Rapid growth of planted grasses resulted in a very fast formation of both above- and below-ground biomass and a large accumulation of nitrogen in the tissue of growing grasses. The greatest differences in N accumulation in aboveground biomass were observed at the end of the third growing season; in C. villosa and C. arundinacea, respectively, 2.66 and 3.44 g N m^–2 after addition of nitrogen and 1.34 and 2.39 g N m^–2 in control. Greater amounts of nitrogen were assessed in below-ground plant parts (9.93–12.97 g N m^–2 in C. villosa and 4.29–4.39 g N m^–2 in C. arundinacea). During the second and third year of experiment, the following effects were the most pronounced: the presence of growing grasses resulted in a decrease of both the acidity and conductivity of lysimetric water and in a lower amount of leached nitrogen, especially of nitrates. Leaching of base cations (Ca²⁺ and Mg²⁺) was two to three times lower than from bare soil without grasses. An excess of labile Al³⁺ was substantially eliminated in treatments with grasses. Enhanced N input increased significantly the acidity and losses of nutrients only in unplanted lysimeters. The leaching of N from treatments with grasses (3.9–5.6 kg N ha^–1) was 31–46% of the amount of N in wet deposition. However, the amount of leached N (4.2–6.0 kg N ha^–1) after N application was only 7.1–8.9% of total N input. After a short three year period, the features of soil with planted grasses indicated a slight improvement: higher pH values and Ca²⁺ and Mg²⁺ contents. The ability of these grass stands to reduce the excess nitrogen in soil is the principal mechanism modifying the negative impact on sites deforested by acid depositions. Thus it is suggested that grass sward formation partly eliminates negative processes associated with soil acidification and has a positive effect on the reduction of nutrient losses from the soil.     

The use of carbon isotope ratios to evaluate legume contribution to soil enhancement in tropical pastures

Soil carbon distribution with depth, stable carbon isotope ratios in soil organic matter and their changes as a consequence of the presence of legume were studied in three 12-year-old tropical pastures (grass alone —Brachiaria decumbens (C₄), legume alone —Pueraria phaseoloides (C₃) and grass + legume) on an Oxisol in Colombia. The objective of this study was to determine the changes that occurred in the¹³C isotope composition of soil from a grass + legume pasture that was established by cultivation of a native savanna dominated by C₄ vegetation. The¹³C natural abundance technique was used to estimate the amount of soil organic carbon originating from the legume. Up to 29% of the organic carbon in soil of the grass + legume pasture was estimated to be derived from legume residues in the top 0–2-cm soil depth, which decreased to 7% at 8–10 cm depth. Improvements in soil fertility resulting from the soil organic carbon originated from legume residues were measured as increased potential rates of nitrogen mineralization and increased yields of rice in a subsequent crop after the grass + legume pasture compared with the grass-only pasture. We conclude that the¹³C natural abundance technique may help to predict the improvements in soil quality in terms of fertility resulting from the presence of a forage legume (C₃) in a predominantly C₄ grass pasture.     

Biological nitrogen fixation in grass–clover affected by animal excreta

Aiming at estimating the average N2-fixation in a pasture, ap preciating the great variability due to patchy urine and dung deposition, the in fluence of dairy cow excreta on biological N2-fixation in a perennial ryegrass–white clover mixture was studied using natural urine and dung. Application of urine as well as dung affected the N2-fixation by promoting the growth of grass and thereby the proportion of clover was significantly reduced. Also the proportion of clover-N derived from the atmosphere (pNdfa) was significantly reduced. In control plots clover dry matter constituted between 40 and 50% of the total dry matter production and the pNdfa ranged between 0.8 and 0.9. Addition of urine caused a significant increase in the grass growth rates, which was the primary reason for a decrease in proportion of clover. At the same time pNdfa decreased to 0.2–0.4 followed by an increase resulting in a total reduction of 45% in the N2-fixation in urine affected areas over a period of four months. The dung only affected the N2-fixation for a distance of up to 10 cm from the edge of the dung pats. In this border area the pNdfa decreased from 0.85 to 0.75 during one month after application followed by an increase, so that after three months there was no difference between pNdfa at 0–10 and 10–20 cm distance from the dung hill. The proportion of clover was lower in the 0–10 cm than in the 10–20 cm distance, which totally resulted in a total reduction of 20% in the N2-fixation over a period of four months in the 0–10 cm area around the dung pats. Considering the proportion of a pasture which may by affected by excreta at a stocking density of 4–6 cows ha-1, the length of the grazing period, the frequency of excretion and the area covered by individual patches, it was estimated that the N2-fixation in a grass-clover pasture would be reduced by 10–15% compared to the N2-fixation in a grass-clover sward not exposed to animal excreta.     

Digesta passage and functional anatomy of the digestive tract in the desert tortoise (Xerobates agassizii)

The herbivorous tortoise Xerobates agassizii contents with large fluctuations in the quality and abundance of desert pastures. Responses to grass (Schismus barbatus), herbage (Sphaeralcea ambigua) and pelleted diets were studied in captive animals. Digestive anatomy was investigated in wild tortoises. Cornified esophageal epithelia and numerous mucus glands along the digestive tract indicated a resistance to abrasive diets. Gastric contents were acidic whereas hindgut digesta were near neutral pH. The colon was the primary site of fermentation with short-chain fatty acids mainly comprised of acetate (69–84%), propionate (10–15%) and n-butyrate (1–12%). Fibre digestion was extensive and equivalent to 22–64% of digestible energy intakes. Large particles of grass (25 mm Crmordants) were excreted as a pulse but retained longer than either fluids (Co-EDTA) or fine particles (2 mm; Yb). Patterns of marker excretion suggested irregular mixing of only the fluid and fine particulate digesta in the stomach and the colon. Mean retention times of Crmordants were 14.2–14.8 days on the grass and highfibre pellets. Intakes of grass were low and accompanied by smaller estimates of digesta fill than for the high-fibre pellets. Digestive capacity was large and estimated at 11–21% of body mass on these diets. The capacious but simple digestive anatomy of the tortoise may provide the greatest flexibility in utilizing a variety of forages in its unreliable habitat.Abbreviations bm body mass - DM drymatter - EDTA ethylene-diamine tetra-acetic acid - MRT mean retention time - NDF neutral detergent fibre - SCFA short-chain fatty acid(s) - T _max time to maximum marker concentration     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Effects of different forms of Acacia saligna leaves inclusion on feed intake, digestibility and body weight gain in lambs fed grass hay basal diet

The study was undertaken to determine the effects of different forms of Acacia saligna leaves inclusion on feed intake, digestibility and body weight gain in lambs fed grass hay basal diet. Twenty local ‘Farta’ yearling male lambs weighing 17.4 ± 0.10 kg (mean ± SD) were randomly allocated to four dietary treatments consisting of daily feeding of lambs in T1 with 700 g grass hay (dry matter basis), and daily free choice feeding of lambs in T2, T3, and T4 with 700 g grass hay + 700 g fresh, wilted, and dried A. saligna leaves, respectively. Inclusion of different forms of A. saligna leaves improved (P<0.01) the daily intake of total DM by 8–9% and that of OM and CP by 7 and 38%, respectively compared to the control. The daily intake of NDFom was lower (P<0.01) by 1–2.3%, whereas that of the ADFom intake declined (P<0.01) by 1.3–3.4% for lambs on wilted and dried forms of A. saligna leaves. Inclusion of A. saligna leaves replaced the intake of the grass hay by 26–27% of the total diet and reduced (P<0.05) apparent digestibility coefficient of the total DM by 9, 7, and 4%, OM by 7, 6 and 4%, and CP by 12, 12, and 9%, but improved (P<0.05) apparent digestibility coefficient of NDFom by 3–4% compared to the control diet. Dried A. saligna leaves improved (P<0.05) apparent digestibility of DM, OM and CP compared to fresh A. saligna leaves. Lambs fed on hay alone lost (P<0.05) body weight, while the lambs supplemented with A. saligna leaves gained (P<0.05) weight. The results of this study revealed that A. saligna leaves could be best included in the grass hay based feeding in dried form at the rate of 265 g/kg DM for improved nutrient utilization and growth performance of yearling lambs.     

Does liquid fertilization affect fine root dynamics and lifespan of mycorrhizal short roots?

We studied effects of nitrogen, other nutrients and water (liquid fertilization; LF) on fine root dynamics (production, mortality) and life span of mycorrhizal short roots in a Norway spruce stand, using minirhizotrons. Data were collected and analyzed during a two-year period at depths of 0–20 cm, 21–40 cm and 41–85 cm, six years after the start of treatment. Relative to control (C), root production was lower in LF plots at depth 0–20 cm. Root production increased significantly at depth 41–85 cm. Fine root mortality in LF plots was higher at all depths. Life span of mycorrhizal short roots in LF plots was significantly lower than C plots and at the end of the study no mycorrhizal short roots were alive. It is suggested that the water and nitrogen input lower longevity of mycorrhizal short roots and promote fine root production at deeper soil layers.     

Soil respiration within riparian buffers and adjacent crop fields

We quantified rates of soil respiration among sites within an agricultural landscape in central Iowa, USA. The study was conducted in riparian cool-season grass buffers, in re-established multispecies (switchgrass + poplar) riparian buffers and in adjacent crop (maize and soybean) fields. The objectives were to determine the variability in soil respiration among buffer types and crop fields within a riparian landscape, and to identify those factors correlating with the observed differences. Soil respiration was measured approximately monthly over a two-year period using the soda-lime technique. Mean daily soil respiration across all treatments ranged from 0.14 to 8.3 g C m^–2 d^–1. There were no significant differences between cool-season grass buffers and re-established forest buffers, but respiration rates beneath switchgrass were significantly lower than those beneath cool-season grass. Soil respiration was significantly greater in both buffer systems than in the cropped fields. Seasonal changes in soil respiration were strongly related to temperature changes. Over all sites, soil temperature and soil moisture together accounted for 69% of the seasonal variability in soil respiration. Annual soil respiration rates correlated strongly with soil organic carbon (R = 0.75, P < 0.001) and fine root (<2 mm) biomass (R = 0.85, P < 0.001). Annual soil respiration rates averaged 1140 g C m^–2 for poplar, 1185 g C m^–2 for cool-season grass, 1020 g C m^–2 for switchgrass, 750 g C m^–2 for soybean and 740 g C m^–2 for corn. Overall, vegetated buffers had significantly higher soil respiration rates than did adjacent crop fields, indicating greater soil biological activity within the buffers.     

Phosphorus purification in buffer zones in cold climates

The retention of agricultural P by 10-m wide grass buffers (GBZ) and buffers under natural vegetation (VBZ) was studied for 10 years in southwestern Finland. The results were compared with those from 70-m long plots without buffers (NBZ). The GBZs were mowed but the VBZs were not managed. Surface waters were directed into a collector trench on each plot. Soil samples were taken from the buffers to determine easily soluble P by a Finnish method (0.5 M NH₄-acetate–0.5 M acetic acid, pH 4.6) and by the Olsen method.The highest losses of all P fractions were measured in spring, when the buffer vegetation had not yet started to grow. The mean annual total phosphorus (TP) loss from the GBZ and VBZ plots (0.7 kg ha⁻¹) was 40% lower than the TP loss from the NBZs (1.2 kg ha⁻¹). However, the loss of molybdate-reactive P (RP) was 70% higher from the VBZs than from the other plots. The concentration of Olsen-P was high (55.9 mg l⁻¹) on the soil surface, 0–2 cm, in the VBZs. The high loss of RP from the VBZs was most likely due to P leaching from the soil surface and decaying grass residue on the VBZs in spring.     

Arrangement and size distribution of repeat and single copy DNA sequences in four species ofCucurbitaceae

DNA sequence organization patterns have been studied in fourCucurbitaceae plant species, namely,Luffa cylindrica (sponge gourd),L. acutangula (ridge gourd),Benincasa hispida (ash gourd) andCoccinia indica (ivy gourd). Extensive interspersion of repeat and single copy sequences has been observed in sponge gourd and ridge gourd. In ash gourd and ivy gourd, however, there is a limited interspersion of these sequences and a large portion of the single copy DNA remains uninterspersed. The interspersed repetitive sequences are composed of a major class (75–80%) of short repeats (300 base pairs long) and a minor class (15–20%) of long repeats (2 000–4 000 base pairs) in all the four species. The average length of single copy sequences dispersed among repeats is 1 800–2 900 base pairs. In spite of these gross similarities in the genome organization in the four species, the fraction of repeats and single copy sequences involved in short and long period interspersion patterns, and fraction of single copy sequences remaining uninterrupted by repeats are vastly different. The probable implications of these differences with respect to speciation events and rates of genome evolution are discussed.Molecular Analysis ofCucurbitaceae Genomes, III. — NCL Communication No.: 3595.