Humid Tropics Of Costa Rica Research Articles

Enteric Methane Emissions in Dairy Cows with Different Genetic Groups in the Humid Tropics of Costa Rica

Enteric methane (CH4) is one of the main greenhouse gases emitted in livestock production systems with ruminants. Among the options to reduce such emissions, animal genetics is one of the factors that is taking relevance in recent years. The aim of the present study was to assess the emission of enteric CH4 in dairy cows with different genetic backgrounds. Sixteen cows belonging to the following three genetic groups were selected for this study: seven F1 (50% Jersey × 50% Gyr), five Triple cross (50% Jersey × 31% Holstein × 19% Sahiwal) and four Jersey. Enteric CH4 emissions were measured in all cows for 15 months, at the middle of each month, using the SF6 technique. Enteric CH4 emissions did not differ (p > 0.05) among genetic groups, although it varied with the stage of lactation, due to differences in milk yield and dry matter intake (DMI). Pasture DMI and the intensity of CH4 emissions (g kg-1 DMI) differed (p < 0.05) between dry and lactating cows, with higher DMI in the lactation period, while CH4 emission intensity was higher for dry cows. Cows with the highest proportion of Bos taurus genes presented a higher annual mean methane conversion factor (Ym), with 7.22, 7.05 and 5.90% for the Triple cross, purebred Jersey and F1, respectively. In conclusion, non-significant differences in enteric CH4 emissions and Ym were detected among dairy cows with different genetic backgrounds. However, F1 cows tended to show lower enteric CH4 emission and Ym, compared to those with more Bos taurus genes.

Implications of land use change on runoff generation at the plot scale in the humid tropics of Costa Rica

Recent land use changes in Central America have involved the abandonment of marginal farmland activities, the regeneration of secondary forest and the spread of high return crops such as oil palm plantations. The potential impacts of land use change on overland flow are evaluated using data from Tinoco Experimental Catchment (South Pacific Costa Rica). Our main hypothesis is that secondary forest overland flow is lower than the one generated under the other land cover types. For this purpose, runoff responses at plot scale are analyzed for different land uses: secondary forests, forest plantations, oil palm plantations and grasslands. Runoff plots were situated over former grasslands, abandoned 8–15years prior to plot settlement. Measurements were conducted at two complementary spatial scales i) the plot (150m2) under natural precipitation and ii) rainfall simulation on microplots (0.0625m2). The combination of natural and simulated rain runoff response measurements provides a more accurate picture of the overland flow generation in the study site. Secondary forest shows a significantly lower runoff response than grassland and oil palm plantations, although there are no significant differences among the plots in variables such as saturated hydraulic conductivity (Ks). The oil palm plantation plot presented the highest runoff coefficient (mean RC=32.6%), twice that measured under grasslands (mean RC=15.3%) and 20-fold greater than in secondary forest (mean RC=1.7%). The runoff plots part of the Tinoco Experimental Catchment provide valuable data and coefficients for evaluating the influence on overland flow of secondary forest recovery and oil palm plantation expansion over hillsides, contributing to a better understanding of the effects of land cover dynamics on water resources in the humid tropics.

Banana leaf and glucose mineralization and soil organic matter in microhabitats of banana plantations under long-term pesticide use.

Soil organic matter (SOM) and microbial activity are key components of soil quality and sustainability. In the humid tropics of Costa Rica 3 pesticide regimes were studied-fungicide (low input); fungicide and herbicide (medium input); and fungicide, herbicide, and nematicide (high input)-under continuous banana cultivation for 5 yr (young) or 20 yr (old) in 3 microhabitats-nematicide ring around plants, litter pile of harvested banana, and bare area between litter pile and nematicide ring. Soil samples were incubated sequentially in the laboratory: unamended, amended with glucose, and amended with ground banana leaves. Soil organic matter varied with microhabitat, being greatest in the litter pile, where microbes had the greatest basal respiration with ground banana leaf, whereas microbes in the nematicide ring had the greatest respiration with glucose. These results suggest that soil microbes adapt to specific microhabitats. Young banana plantations had similar SOM compared with old plantations, but the former had greater basal microbial respiration in unamended and in glucose-amended soil and greater first-order mineralization rates in glucose-amended soil, thus indicating soil biological quality decline over time. High pesticide input did not decrease microbial activity or mineralization rate in surface soil. In conclusion, microbial activity in tropical volcanic soil is highly adaptable to organic and inorganic inputs.

Energycane (Saccharum spp. × Saccharum spontaneum L.) Biomass Production, Reproduction, and Weed Risk Assessment Scoring in the Humid Tropics and Subtropics

There is growing interest in biofuel production, and energycane (Saccharum spp. × S. spontaneum L.) has been proposed as an important biofuel and biomass crop. However, little is known about the growth and ecology of this new crop, especially in the tropics. The present study evaluated the performance of 14 energycane clones, elephantgrass (Pennisetum purpureum Schumach.), and two sugarcane (S. officinarum L.) varieties in the humid tropics of Costa Rica, and eight energycane clones in the subtropics of Florida. In the tropics, energycane's growth and biomass production were highly variable when comparing clones. However, the best performing clones US85‐1006, US88‐1006, and US78‐1014 produced almost twice the dry biomass (&gt;64 Mg ha−1) compared with sugarcane varieties Pindar and Q‐132 (21–39 Mg ha−1). In the subtropics, energycane fresh (52–79 Mg ha−1) and dry (20–30 Mg ha−1) weights were less than half of those in the tropics. Energycane clones flowered in both environments, but pollen viability was three to four times higher (&gt;40%) in the tropics than in the subtropics, although viable seeds were found only in the tropics. Weed risk assessment (WRA) scores were higher in the tropics than in the subtropics and varied among clones. The results confirmed that energycane is a promising feedstock for biomass production and could play an important role as a bioenergy crop when grown in the tropics and subtropics, but due to genotype × environment interactions, the tradeoff between biomass production and weedy and invasive risk must be assessed for each individual clone and environment.

A hydrologic tracer study in a small, natural wetland in the humid tropics of Costa Rica

Growing populations and food demand in the tropics are leading to increased environmental pressures on wetland ecosystems, including a greater reliance on natural wetlands for water quality improvement. Effective assessment of wetland treatment potential requires an improved understanding of the hydraulic and biogeochemical factors that govern contaminant behavior, however detailed studies of flow through natural, tropical wetlands are scarce. We performed a tracer study using a conservative salt (potassium bromide) to examine the hydraulic behavior of a small, natural wetland in the Costa Rican humid tropics and modeled observed breakthrough curves using the 1-D advection–dispersion equation. Velocities in the wetland were extremely slow, from less than 4 m day−1 to a maximum of ~30 m day−1, and were distributed across several flowpaths, illustrating a spatial heterogeneity of flow and velocities. Modeled dispersion coefficients were also low (33 ± 33 m2 day−1). Estimated residence times suggested high potential pollutant removal capacity over a range of influent concentrations, reinforcing the environmental services provided by this and other small tropical wetlands. The study also highlighted how small variations in wetland topography and vegetation yield strong differences in transport patterns that affect transport and mixing in densely vegetated, heterogeneous wetland systems. Empirical data on the hydraulics, and resulting ecosystem functions, of small, distributed wetlands may provide support for improved conservation and management of these important ecosystems.

Distribution of coarse and fine roots of Theobroma cacao and shade tree Inga edulis in a cocoa plantation

Cocoa (Theobroma cacao L.) is often cultivated below legume shade trees, but root interactions between the species are not well known. The first step in understanding these interactions is the description of spatial root system distribution of cocoa and shade trees. We studied the root distributions of cocoa and Inga edulis Mart. (inga) in an agroforestry plantation in the humid tropics of Costa Rica. Fine roots were sampled by soil coring, and a fractal root architecture model parameterised with data from partial root excavations was used for determining coarse root distribution. Fine root length in the 0–50 cm soil layer was 26,762 and 22,026 km ha−1 for cocoa and inga, respectively, with 24 % and 23 % of fine root length of cocoa and inga, respectively, in the 0–2 cm layer. Horizontally, root distributions of the two species with cocoa at 3 × 3 m and inga at 9 × 9 m spacing overlapped strongly, while inga did not cover all points at 18 × 18 m spacing. Both species seem to efficiently capture nutrients released from the litter layer on the soil surface, and nutrient leaching in shaded cocoa plantations is unlikely. The proximity of root systems may facilitate N exchange between the N2-fixing inga and cocoa, but competition for other nutrients is likely.

Hydrological Importance and Water Quality Treatment Potential of a Small Freshwater Wetland in the Humid Tropics of Costa Rica

Rapid increases in population and growing food demand are causing widespread deterioration of tropical wetlands globally, and an increased focus on the role and function of these imperiled ecosystems is required. Objectives of this study were to investigate the hydrological dynamics and water quality treatment potential of a small freshwater wetland in the humid tropics of Costa Rica. High-resolution, spatially distributed surface water and meteorological data were combined with a detailed topographical survey to quantify the wetland water balance, hydroperiod, and seasonal variability of wetland area, volume, and residence time. The water balance was dominated by precipitation and outflow, with little contribution from runoff, except during the largest storms. Over 80% of the wetland was flooded continuously; hydroperiods in remaining areas were bi-modal. Small seasonal variations in wetland area, volume, and residence times yielded high and sustained water quality treatment potential. Potential pollutant removal efficiencies were 63.6–99.8% for biological oxygen demand; 60.0–99.8% for total suspended solids; 51.1–98.5% for total nitrogen; and 34.2–99.7% for total phosphorous. The study provides insights into the hydrological functions of this and similar small Central American wetlands and provides a template for extending in-depth hydrological monitoring to other tropical wetland sites.

Carbon accumulation in the biomass and soil of different aged secondary forests in the humid tropics of Costa Rica

Efforts are needed in order to increase confidence for carbon accounts in the land use sector, especially in tropical forest ecosystems that often need to turn to default values given the lack of precise and reliable site specific data to quantify their carbon sequestration and storage capacity. The aim of this study was then to estimate biomass and carbon accumulation in young secondary forests, from 4 and up to 20 years of age, as well as its distribution among the different pools (tree including roots, herbaceous understory, dead wood, litter and soil), in humid tropical forests of Costa Rica. Carbon fraction for the different pools and tree components (stem, branches, leaves and roots) was estimated and varies between 37.3% (±3.3) and 50.3% (±2.9). Average carbon content in the soil was 4.1% (±2.1). Average forest plant biomass was 82.2 (±47.9) Mg ha −1 and the mean annual increment for carbon in the biomass was 4.2 Mg ha −1 yr −1. Approximately 65.2% of total biomass was found in the aboveground tree components, while 14.2% was found in structural roots and the rest in the herbaceous vegetation and necromass. Carbon in the soil increased by 1.1 Mg ha −1 yr −1. Total stored carbon in the forest was 180.4 Mg ha −1 at the age of 20 years. In these forests, most of the carbon (51–83%) was stored in the soil. Models selected to estimate biomass and carbon in trees as predicted by basal area had R 2 adjustments above 95%. Results from this study were then compared with those obtained for a variety of secondary and primary forests in different Latin-American tropical ecosystems and in tree plantations in the same study area.

Effect of protein supplementation on reproductive and productive performance in Bos indicus × Bos taurus heifers raised in the humid tropics of Costa Rica

In order to evaluate the effect of protein supplementation on the productive and reproductive performance of heifers, 45 Bos indicus x Bos taurus heifers, 673 +/- 146 days of age and weighing about 340 kg, were divided into two groups. The control group (n = 23) continued without supplementation, but the supplemented group (SG; n = 22) received concentrate at a rate of 1% BW kg per day. Animals were adapted to the concentrate over a 15-day period and then supplemented for 30 days, after which estrus was synchronized using a progesterone implant. Back fat thickness (BFT) was assessed by ultrasound. The SG had better average daily weight gain than the nonsupplemented group (0.63 +/- 0.16 vs. 0.51 +/- 0.13 kg/day, P < 0.05) and tended to show a better percentage ovulation (P < 0.10, 77% vs. 57%, respectively). However, this tendency was not reflected in the final pregnancy rates. No differences were observed between groups in dry matter intake and body condition scores. SG with a high BFT (>0.70 cm) showed a better ovulation percentage than those with low BFT (<0.69 cm; P < 0.01). Additionally, the pregnancy rate of supplemented heifers with high BFT tended to be better than heifers with low BFT in this same group (67% vs. 30%, respectively, P < 0.10). It is concluded that protein supplementation improved reproductive performance and that ultrasonography to measure BFT may be used to estimate reproductive performance.

The Discontinuance of Environmental Technologies in the Humid Tropics of Costa Rica: Results from a Qualitative Survey

Previous studies in the Parismina watershed, Costa Rica, have revealed a high rate of discontinuance by one-time adopters of a suite of conservation farm technologies currently promoted by EARTH University. In the case of such technologies, the environmental benefits only accrue as long as the technology is in use. Most diffusion-adoption research is concerned with the process of initial adoption or rejection of particular innovations, with very few studies concentrating on the post-adoption stage which includes the continuance or discontinuance of the innovation. The objective of this study is to investigate why some farmers discontinue previously adopted environmental technologies while others continue to use them. Our results identify two general categories of factors influencing discontinuance: 1) factors related to specific characteristics of each technology and the additional labor required to maintain them; 2) factors springing from the wider socioeconomic context such as a change in farming practices or the devolution of responsibility for maintenance to a sole individual. We conclude by offering specific suggestions to extension agencies hoping to reduce levels of innovation discontinuance.

Evaluation of early embrionic development after natural mating using ultrasonography in bos indicus cows in the humid tropics of Costa Rica

Fifty-eight nursing Zebu cows averaging 5922 days postpartum were used to evaluate the early embryonic development by ultrasound under a natural mating system (bulls A to F). Three consecutive mating periods of 21 days were established. Sixty two percent of the animals were diagnosed as pregnant (36/58). During the first period (AB) 76% (27/36) of the animals become pregnant, 17% (6/36) during the second (CD) and 9% (3/36) in the last mating period (EF). Sixty two percent of the animals were true positives (animals found pregnant diagnosed by ultrasound (US), and conforming with the embryonic scale proposed by Rosiles et al. (2006); 12% (n = 7) of false negatives (pregnant cows by US but not conforming with the scale); 2% (n = 1) of false positives (pregnant cows, evaluated with US and conforming with the embryonic scale but not pregnant at the end of the study); and a 26% (n = 15) of true negatives (not pregnancy evaluated with US). The sensitivity of the test was 97%, with a specificity of 68% with a positive predictive value of 83% and a negative predictive value of 6%. Relation between the measuring scale of embryo development and the results obtained by ultrasonography was k = 0.69. No significant correlation were observed between body condition score (BCS) evaluation and body fat (BF) measurement at any of the three evaluations performed after mating, however, measurements of BCS and BF among time showed a positive correlation (0.48 P < 0.05, 0.29, P < 0.07, respectively) from the beginning of the evaluation period to the end of the trial. Ultrasonography to monitor embryonic development to determine the early establishment of pregnancy in natural mating reproductive programs can be a valuable technique to monitor reproductive efficiency.

Effect of ruminally protected Methionine on the productive and reproductive performance of grazing Bos indicus heifers raised in the humid tropics of Costa Rica

With the objective of evaluating the effect of methionine supplementation prior to a breeding program, thirty one heifers (Bos taurus x Bos indicus) were used averaging 386 +/- 29 days of age and a mean body weight of 402.6 +/- 28 kg. Fifteen of the animals received a supplement (SG) during 45 days with molasses-urea mixture (2 kg molasses + 407 g urea/head/day), plus 10 g of ruminally protected methionine. The other sixteen heifers did not receive supplement (CG). Fecal and pastures samples were collected to assess dry herbage intake and digestibility. Serial ultrasound measurements from the ovary were performed in both groups to evaluate follicular dynamics. The heifers were categorized according to their follicular size and presence of a CL. Forage intake and dry matter digestibility were reduced (P < 0.05) and body condition tended to improve (P = 0.07) in the supplemented heifers, however, total intake, final weight, daily gain and dorsal back fat were not affected. After the supplementation period, the percentage of females in the categories < 3 mm and 3 to < 6 mm, was greater (P < 0.05) in CG (25% and 43.7%) that in SG (0% and 26.6%) but in the follicle category of > or = 9 mm, the percentage of animals was 60% in SG and 18.8% in CG (P < 0.05). The percentage of ovulation for the SG and CG was 86.7% and 62.5%, respectively (P < 0.05). The combination of supplementation with methionine-urea and molasses at the end of the dry season and the anticipated onset of the rainy season favored the establishment of ovarian activity and follicular dynamics.

EARTH to farmers: Extension and the adoption of environmental technologies in the humid tropics of Costa Rica

The diffusion of innovations model has been used by social scientists for decades to understand the adoption of new agricultural technologies, but its applicability to environmental as opposed to commercial technologies has been the source of much debate. The “classic” model's ability to account for the diffusion of environmental innovations is hampered by its social–psychological roots and voluntarist assumptions. A model more appropriate to the adoption of environmental technologies in a developing country setting should take into account institutional factors, including the mode of interaction between farmers and extension agencies. This paper examines patterns of adoption of a set of environmental farm technologies in the humid tropics of Costa Rica, paying particular attention to the role of a local agricultural engineering university's outreach activities. Our findings indicate that overall patterns of adoption remain low; that farm size is the only structural variable significantly related to adoption; and that of the various extension activities analyzed, farmer attendance at a university meeting or workshop is by far the strongest predictor.

The Smectite-to-Disordered Kaolinite Transition in a Tropical Soil Chronosequence, Pacific Coast, Costa Rica

AbstractSoils developed on Quaternary fluvial fill terraces in the humid tropics of Costa Rica display progressive changes in mineral assemblage, chemical composition and particle size with age. Clay minerals from B horizons of active floodplains are predominantly smectite with lesser amounts of disordered kaolinite. B horizons in 5 to 10 ka soils consist of sub-equal amounts of smectite and disordered kaolinite, and soils on 37–125 ka terraces consist of disordered kaolinite with only traces of smectite. The composition of the smectite, as determined by EDX scans of smectite-rich pore space, is [(Mg0.2,Ca0.1)(Fe0.6Al1.4)(Si3.6Al0.4)O10(OH)2], consistent with ferruginous beidellite.Bulk mineral assemblage varies from a smectite-plagioclase-augite-quartz-magnetite assemblage in ⩽ 10 ka terrace soils to a disordered kaolinite-goethite-hematite-quartz-magnetite assemblage in ⩾37 ka terrace soils. Leaching results in rapid loss of soluble base cations and residual concentration of Ti and Zr indicates mass losses of ∼50% by chemical denudation by 125 ka. Plots of terrace age vs. various measures of clay mineralogy, chemical composition, and particle size produce parabolic curves consistent with rapid chemical weathering pre-37 ka and slower to imperceptible rates of change from 37 to 125 ka. For some pedogenic properties, particularly particle size and concentrations of base cations and Zr, soils appear to reach steady-state conditions within 37 ka.These results were applied to interpretation of landscape evolution in this tectonically active region by: (1) facilitating identification of two Holocene (5 ka and 10 ka) terraces on the Esterillos Block 5–30 m above sea level (masl), and two Pleistocene terraces ⩾ 125 ka on the Parrita Block 30 masl, and, in turn, (2) documenting uplift rates as high as 4.4 m/ka between 37 and 10 ka on the Esterillos Block, and as low as 0.1 m/ka over the past 125 ka on the adjacent Parrita Block. These findings are consistent with previous work indicating that the subduction of anomalous bathymetric features at the Middle America Trench is having a significant impact on fore-arc dynamics and topography over relatively short geological time periods and spatial scales.

Reproductive response in supplemented heifers in the humid tropics of Costa Rica

To evaluate the effect of two supplementary diets to determine the consequence on productive and reproductive performance in heifers ( Bos indicus × Bos taurus) averaging between 24 and 36 months of age and grazing tropical pastures, two trials were conducted. Thirty animals (initial BW 325.1 ± 33.6 kg) were divided in two groups in the initial study: supplemented (SG) and control (CG); SG received a concentrate (5.5% CP and 2.85 Mcal/kg of DE dry matter basis) at 1% of body weight (BW). In the second study, 45 heifers (initial BW 332.6 ± 29.3 kg) were assigned in two treatments, with the same amount of supplement (1% BW) but with a greater nutrient content (13% CP and 3.15 Mcal/kg of DE). The proportion of animals with a corpus luteum at the end of each study was greater in the supplemented groups ( P < 0.05). Ovarian follicular dynamics was similar between groups in the first study, but in the second study there were more heifers in the SG group with follicles larger than 9 mm in diameter ( P < 0.05). Pregnancy rate was similar for SG and CG ( P > 0.05). The response to a regimen of estrous synchronization in both trials was numerically superior in the SG group. No differences were observed in the length of estrus. Daily gain and body condition score were similar for supplemented and control groups (0.27 compared with −0.06 in the first study and 0.90 kg compared with 0.60 in the second study, respectively). Dietary supplementation improved the number of animals initiating estrous cycles and the expression of estrus when compared with unsupplemented control heifers. The dietary regimens imposed in these studies appear to be an adequate for the management of growing heifers destined to a reproductive program.

Dry‐matter yields and crude protein and rumen‐degradable protein concentrations of three Arachis pintoi ecotypes at different stages of regrowth in the humid tropics

AbstractWell‐established stands of three ecotypes of Arachis pintoi (CIAT 17434, 18744 and 18748) were harvested from replicated plots (three blocks, each containing three plots for each ecotype) during the two dominant seasons (dry and wet) of the low altitude, humid tropics of Costa Rica. Each plot was further divided into six subplots so that, within each season, samples corresponding to 4, 6, 8, 10, 12 or 14 weeks of regrowth could be collected. For each harvest, dry matter (DM) yield of the leaf, stem and whole plant, and the leaf:stem ratio, were recorded. Samples of the whole plant were analysed for crude protein (CP), rumen‐degradable (RDP) and rumen‐undegradable protein (RUP) concentrations. DM yield of the leaf, stem and whole plant increased with advancing period of regrowth but the effects of period of regrowth varied somewhat among ecotypes and across seasons. Generally, DM yield was greater during the wet than during the dry season. The greatest difference between ecotypes for stem and total DM yields was evident during the dry season. In general, DM contents were low in the whole plant, leaf and stem samples (&lt;220 g kg−1) and increased with increasing period of regrowth. Increases in leaf:stem ratio were most dramatic during the dry season with greater periods of regrowth, although the ratio was fairly constant during the wet season. Whole‐plant CP concentration was relatively high after short periods of regrowth (up to 279 g kg−1 DM) but declined with longer periods of regrowth; the relative decline was much greater during the dry season. The RDP concentration was relatively constant during the wet season (mean 115 g kg−1 DM), but declined with longer periods of regrowth during the dry season (range 194–111 g kg−1 DM). In general, the concentrations of RDP, on a CP basis, were greater during the dry season and ranged from 590 to 700 g kg−1 CP. Season, ecotype and period of regrowth all exerted an effect on RUP concentrations.

Effect of alternating bulls as a management tool to improve the reproductive performance of suckled Zebu cows in the humid tropics of Costa Rica

A study was undertaken to evaluate the effect of alternating bulls between a single and a multiple sire mating (MSM) program on the reproductive performance of suckled Zebu cows raised under range conditions in the humid tropics of Costa Rica. Multiparous Zebu cows ( n=94) suckling calves were distributed between two experimental trials (A and B) consisting of 47 animals each. A single sire mating (SSM) system was alternated weekly with a MSM system with three bulls. This period lasted for 8 weeks. To facilitate estrous expression, four cows were strategically synchronized (estrus-stimulated) in alternate weeks. Courtship predominated over mounting under non-stimulated estrus, for each mounting performed an average of 6.0 and 6.3 courtship activities were recorded in the SSM and MSM, respectively. Under the influence of strategic synchronization corresponding values were 3.9 and 4.2 in the SSM and MSM, respectively ( P>0.05). Blood samples for progesterone evaluation were taken twice weekly. All cows in trial A were in anestrus at the start of the study. By second week, 5 out of the 47 cows had initiated estrous cycles and by the third week six were pregnant. In contrast in trial B, 9 out of 47 had initiated estrous cycles before interacting with the bulls and on week 3, only two females had become pregnant and three had initiated estrous cycles. Significant differences were found in the cumulative percentage of cows pregnant between trials A and B ( P<0.05). Even though these results occurred, the rotation of the bulls (one or three), or the type of cows (estrus-stimulated or not) did not influence the results in this study.

N2O emissions from humid tropical agricultural soils: effects of soil moisture, texture and nitrogen availability

We studied soil moisture dynamics and nitrous oxide (N2O) fluxes from agricultural soils in the humid tropics of Costa Rica. Using a split-plot design on two soils (clay, loam) we compared two crop types (annual, perennial) each unfertilized and fertilized. Both soils are of andic origin. Their properties include relatively low bulk density and high organic matter content, water retention capacity, and hydraulic conductivity. The top 2–3cm of the soils consists of distinct small aggregates (dia. <0.5cm). We measured a strong gradient of bulk density and moisture within the top 7cm of the clay soil. Using automated sampling and analysis systems we measured N2O emissions at 4.6h intervals, meteorological variables, soil moisture, and temperature at 0.5h intervals. Mean daily soil moisture content at 5cm depth ranged from 46% water filled pore space (WFPS) on clay in April 1995 to near saturation on loam during a wet period in February 1996. On both soils the aggregated surface layer always remained unsaturated. Soils emitted N2O throughout the year. Mean N2O fluxes were 1.04±0.72ng N2O-N cm−2h−1 (mean±standard deviation) from unfertilized loam under annual crops compared to 3.54±4.31ng N2O-N cm−2h−1 from the fertilized plot (351 days measurement). Fertilization dominated the temporal variation of N2O emissions. Generally fluxes peaked shortly after fertilization and were increased for up to 6 weeks (‘post fertilization flux’). Emissions continued at a lower rate (‘background flux’) after fertilization effects faded. Mean post-fertilization fluxes were 6.3±6.5ng N2O-N cm−2h−1 while the background flux rate was 2.2±1.8ng N2O-N cm−2h−1. Soil moisture dynamics affected N2O emissions. Post fertilization fluxes were highest from wet soils; fluxes from relatively dry soils increased only after rain events. N2O emissions were weakly affected by soil moisture during phases of low N availability. Statistical modeling confirmed N availability and soil moisture as the major controls on N2O flux. Our data suggest that small-scale differences in soil structure and moisture content cause very different biogeochemical environments within the top 7cm of soils, which is important for net N2O fluxes from soils.

Survival, growth, timber productivity and site index of Cordia alliodora in forestry and agroforestry systems

Matching tree species to appropriate site conditions and stand management is crucial for sound agroforestry production. In this study, survival, growth and site index for laurel ( Cordia alliodora (Ruiz and Pavon) Oken.) were measured between 1987‐1999 in two forestry (line plantings and pure plantations) and four agroforestry systems (taungya and three laurel ‐ cacao (Theobroma cacao L.) systems) in the lowland humid tropics of Costa Rica and Panama. Mortality ranged between 2 and 52% at age nine years. Poor drainage, flooding and high water tables resulted in low laurel survival on some sites. At age 5, laurel site index was 21 m in cacao ‐ plantain (Musa AAB) ‐ laurel associations (CLP), but only 15 m in line plantings. Diameter at breast height at that age was 28.5 cm in CLP and only 15.6 cm in pure plantations. Laurel growth and site index were high when planted in association with intensively managed crops (e.g., cacao, plantain or taungya sequences).

Sexual behaviour of Zebu bulls in the humid tropics of Costa Rica: single versus multiple-sire groups

This study compares the courtship and mounting behaviour of Bos indicus bulls in single- and multiple-sire groups in Costa Rica (latitude 10°25′N, longitude 84°32′W, annual precipitation of 3096 mm, temperature of 24°C and humidity of 85.3%). Four, 3–4 year-old Brahman bulls with previous sexual experience were used to sire a group of 120 multiparous cows (average of 128 days post-partum and a body condition score of 2.5) allocated to two groups of 60 each: (1) single-sire mating group (SSM) and (2) multiple-sire mating group of three bulls (MSM). Bulls were rotated among groups every 7 days for 28 days. The frequency, type and duration of sexual activities (mounting and mounting attempts) and courtship activities (smelling and licking genital area, butting, supporting the head over a female and the sign of Flehmen) were calculated for each mating group. Descriptive and non-parametric statistics (Wilcoxon, Mann-Withney tests) were used to calculate differences between mating programmes. Sexual activities tended to be more frequent in the SSM group than the MSM group (267 versus 124, P>0.05). Differences in the ratios of sexual to courtship activities between both groups were significant (1:3 in SSM and 1:6 in MSM, P>0.05). Pregnancy rate averages were 28 and 37%, respectively, ( P>0.05). It is concluded that under these conditions multiple-sire mating and single-sire mating achieved similar pregnancy rates.