Partitioning of water resources among plants of a lowland tropical forest

Source water used by plants of several species in a semi-evergreen lowland tropical forest on Barro Colorado Island, Panama, was assessed by comparing the relative abundance of deuterium, D, versus hydrogen, H (stable hydrogen isotope composition, D) in xylem sap and in soil water at different depths, during the dry season of 1992. Ecological correlates of source water were examined by comparing xylem water D values with leaf phenology, leaf water status determined with a pressure chamber, and rates of water use determined as mass flow of sap using the stem heat balance method. Soil water D values decreased sharply to 30 cm, then remained relatively constant with increasing depth. Average D values were-13, for 0–30 cm depth and-36.7 for 30–100 cm depth. Soil water D values were negatively associated with soil water content and soil water potential. Concurrent analyses of xylem water revealed a high degree of partitioning of water resources among species of this tropical forest. Xylem water D of deciduous trees (average=-25.3±1.4) was higher than that of evergreen trees (average=-36.3±3.5), indicating that evergreen species had access to the more abundant soil water at greater depth than deciduous species. In evergreen shade-tolerant and high-light requiring shrubs and small trees, D of xylem water was negatively correlated with transpiration rate and leaf water potential indicating that species using deeper, more abundant water resources had both higher rates of water use and more favorable leaf water status.     

Ammonia volatilization from tropical legume mulches and green manures on unlimed and limed soils

In the humid tropics, legumes are harvested and surface applied as mulch or incorporated as green manure. Studies on N dynamics and budgets from these systems report unaccounted losses of N. Ammonia volatilization may account for a significant percentage of these unexplained N deficits. The main objectives of this study were to: 1) determine the rate and amount of ammonia volatilization from organic amendments, both incorporated (green manure) and unincorporated (mulch), 2) compare ammonia volatilization of organic amendments on both acid (unlimed) and limed soils, and 3) correlate quality, i.e. polyphenolic and lignin concentration and carbon-to-nitrogen ratio, of the organic amendments with ammonia volatilization and net N mineralization. In an incubation experiment, ammonia volatilization losses and net N mineralization were measured from fresh leaflets of 10 legumes over a three-week period. Ammonia volatilization losses for the 10 species ranged from 3.4 to 11.8% of the total N applied in the organic amendment. Lignin content was negatively correlated to ammonia volatilization. Ammonia volatilized from mulches but not green manures, on both unlimed and limed soils, suggesting that ammonia volatilization is a surface phenomenon and not affected by soil pH. Net N mineralization was affected by species and soil pH, but was unaffected by placement (green manure or mulch). For the farmer in low-input agriculture where N tends to be limiting, volatilization losses of N from legume mulch systems could be on the same order of magnitude as crop removal.     

Chronic photoinhibition in seedlings of tropical trees

Seedlings of five canopy species of tropical trees from Costa Rica and Puerto Rico were grown in full shade (midday range of photosynthetic photon flux density [PPFD], 100–140 μmol m^?2 s^?1), partial shade (midday PPFD, 400–600 μmol m^?2 s^?1) and full sun (midday PPFD, 1 500–1 800 μmol m^?2 s^?1) for 3 months. The species were Ochroma lagopus (Bombacaceae), a pioneer species; Inga edulis (Fabaceae), found in secondary forest; and Dipteryx panamensis (Fabaceae), Hampea appendiculata (Malvaceae), and Manilkara bidentata (Sapotaceae), three species characteristic of primary forest. After the plants were placed in the dark overnight, chlorophyll fluorescence characteristics were measured for recently expanded and mature leaves. The ratio of variable fluorescence to maximum fluorescence (F_v/F_m) was used to estimate the degree of chronic photoinhibition. Only individuals of one species, Dipteryx panamensis, showed significant depression of F_v/F_m after long-term exposure to full sun. The depression was highly correlated with quantum yield of O₂ evolution which also declined after exposure to full sun. The decline may have been related to foliar N concentration. Although all plants were supplied with ample nutrients, foliar N did not increase significantly for Dipteryx seedlings in full sun, whereas it did for Ochroma and Inga. Leaf age affected F_v/F_m only in the cases of Manilkara, where it was slightly lower in recently expanded leaves, and of Dipteryx where it interacted with the effects of light regime. We conclude that chronic photoinhibition is not common in seedlings of canopy trees of tropical rain forests except when availability of mineral nutrients may be limiting.     

The effects of grazing and burning on soil and plant nutrient concentrations in Colombian páramo grasslands

The impact of extensive livestock farming on the physical and chemical characteristics of the volcanic soils and on the nutrient status of green plant tissues of neotropical alpine grasslands (páramo) is studied. Soil and plant samples were taken over a one-year period at five sites with different agricultural (grazing and burning) management. In the undisturbed páramo ecosystem, soil moisture (50–250%) and organic matter content are high (7–27%) and decomposition (11–35% yr^-1) and element concentrations are low. Low temperatures (max < 10°C) and phosphorus fixation by the soil (5 mg P g^-1 soil) determine the low mineralization and turn-over rates.Multivariate analysis of laboratory results indicates that the season of sampling and the agricultural practice are the most important explanatory factors for variation of soil characteristics. After long-term heavy grazing, soils have a higher bulk density and a lower moisture content. The outcome of a litterbag experiment confirms the hypothesis of higher decomposition rates at grazed sites. In the intermediate (wet-dry) season, conditions were somewhat better for plant growth but the system remained nutrient limited.Surprisingly, no relation between soil density, moisture or carbon content and concentrations of available nutrients in the soil is found. This is supported by the rather uniform nutrient concentrations in green plant tissue among the sites. It is concluded therefore that the effect of burning and grazing on páramo soils is principally restricted to physical characteristics, and that differences in chemical characteristics of the soil do not cause differences in vegetation structure between grazed, burned and undisturbed sites.The Netherlands Centre for Geo-ecological Research, ICG.     

A comparative account of the microbiological characteristics of soils under natural forest,grassland and cropfield from Eastern India

Microbiological and physico-chemical characteristics of tropical forest, grassland and cropfield soils from India were investigated. The study revealed that the conversion of natural forest led to a reduction of soil organic C (26–36%), total N (26–35%), total P (33–44%), microfungal biomass (44–66%) and total microbial biomass C, N and P (25–60%) over a period of 30–50 years. Comparative analysis of microbial activity in terms of basal soil respiration revealed maximum activity in the forest and minimum in the cropfield soil. Analysis of microbial metabolic respiratory activity (qCO₂) indicated relatively greater respiratory loss of CO₂-C per unit microbial biomass in cropfield and grassland than in forest soil. Considering the importance of the microbial component in soil, we conclude that the conversion of the tropical forest to different land uses leads to the loss of biological stability of the soil.     

Changes in the origin and quality of soil organic matter after pasture introduction in Rondônia (Brazil)

We examined the effects of the conversion of tropical forest to pasture on soil organic matter (SOM) origin and quality along a chronosequence of sites, including a primary forest and six pastures. Bulk soil samples received a physical size-fractionation treatment to assess the contribution of each compartment to total SOM pool. Besides a general increase in total C and N stocks along the chronosequence, we observed a reduction of the relative contribution of the coarser fractions to total soil C content, and an increased concentration in the finer fractions. The origin of the C in each size fraction was established from measurements of¹³C abundance. After 80 years about 93% of the C in the least humified fraction of the top 10 cm of soil was of pasture origin, while in the most humified it was 82%. Chemical analyses indicated that the fine silt and coarse clay fractions contained the most refractory carbon.     

Somatic embryogenesis and plant regeneration from callus culture of Acacia catechu-a multipurpose leguminous tree

Plant regeneration via somatic embryogenesis was achieved from callus derived from immature cotyledons of Acacia catechu Willd. on Woody Plant Medium (WPM) supplemented with 13.9 M kinetin and 2.7 M 1-naphthaleneacetic acid. The addition of 0.9–3.5 mM L-proline to the medium influenced development of somatic embryos and also promoted secondary somatic embryogenesis. The light-green somatic embryos germinated on half-strength MS medium supplemented with 2% (w/v) sucrose. Somatic embryos germinated into plantlets that were acclimatized in the greenhouse and subsequently transferred to the field.     

Factors affecting in vitro shoot proliferation and ex vitro establishment of mangosteen

Shoot proliferation has been achieved in Garcinia mangostana L. using seed explants. Maximum mean number of shoots per explant (16.8) was obtained from cultures on Murashige and Skoog medium supplemented with 40 mM 6- benzyladenine, and 2.5 mM -naphthaleneacetic acid and kept at 30 °C under an 8 hour photoperiod. Cultures on the same medium but supplemented with 2 g l^-1 activated charcoal produced fewer shoots. However, growth of these shoots was more organized and 75% rooting was obtained. Woody Plant Medium was not a suitable medium for shoot proliferation. Ex vitro establishment was best obtained on planting medium consisting of sand, soil and organic material (3:2:1).Abbreviations BA 6-benzyladenine - IBA indole-3-butyric acid - NAA -naphthaleneacetic acid - MS Murashige & Skoog (1962) basal medium - WPM Woody Plant basal medium (Lloyd & Mc Cown 1980)     

Phosphorus limitation of forest leaf area and net primary production on a highly weathered soil

We tested the hypothesis that P was the nutrient limiting net primary production of a nativeMetrosideros polymorpha forest on a highly weathered montane tropical soil in Hawaii. A factorial experiment used all combinations of three fertilizer treatments: nitrogen (N), phosphorus (P) and a mix of other essential nutrients (OE), consisting primarily of mineral derived cations and excluding N and P. P addition, but not N or OE, increased leaf area index within 12 months, foliar P concentration measured at 18 months, and stem diameter increment within 18 months. Stem growth at 18 months was even greater when trees fertilized with P also received the OE treatment. N and P additions increased leaf litterfall and N and P in combination further increased litterfall. The sequence of responses suggests that increased available P promoted an increase in photosynthetic area which led to increased wood production. P was the essential element most limiting to primary production on old volcanic soil in contrast to the N limitation found on young volcanic soils.     

Effects of pasture introduction on soil CO2 emissions during the dry season in the state of Rondônia,Brazil

Soil CO₂ evolution rates, soil temperatures and moisture were measured during the dry season in two forest-to-pasture chronosequences in Rondônia, Brazil. The study included pastures ranging from 3 to 80 years-old. Mean dry-season CO₂ evolution from the forest in chronosequence 1, 88.8 mg CO₂-C m^–2h^–1 was lower than from the pastures which ranged from 111 to 158 mg CO₂-C m^–2h^–1. We found that temperature was not a good predictor of CO₂ emissions from pasture but that there was a significant relationship (r = 0.72,p < 0.05) between soil moisture and pasture emissions. The ¹³C of the soil CO₂ emissions also was measured on chronosequence I; ¹³C of the CO₂ emitted from the C3 forest was –29.43%. Pasture¹³CO₂ values increased from –17.91%. in the 3 year-old pasture to –12.86% in the 80 year-old, reflecting the increasing C₄ inputs with pasture age. Even in the youngest (3 year-old) pasture, 70 percent of the CO₂ evolved originated from C₄ pasture-derived carbon.