No abstract

A decline in pasture productivity is often associated with a reduction in vegetative cover. We hypothesize that nitrogen (N) in urine deposited by grazing cattle on degraded pastures, with low vegetative cover, is highly susceptible to losses. Here, we quantified the magnitude of urine-based nitrous oxide (N2O) lost from soil under paired degraded (low vegetative cover) and non-degraded (adequate vegetative cover) pastures across five countries of the Latin America and the Caribbean (LAC) region and estimated urine-N emission factors. Soil N2O emissions from simulated cattle urine patches were quantified with closed static chambers and gas chromatography. At the regional level, rainy season cumulative N2O emissions (3.31 versus 1.91 kg N2O-N ha−1) and emission factors (0.42 versus 0.18%) were higher for low vegetative cover compared to adequate vegetative cover pastures. Findings indicate that under rainy season conditions, adequate vegetative cover through proper pasture management could help reduce urine-induced N2O emissions from grazed pastures.

Savanna ecosystems cover ∼20% of the total land surface and account for ∼30% of the terrestrial global net primary production. They are also highly sensitive to climate change, since their carbon (C) sink capacity may decline under rising temperatures and irregular rainfall. These responses, which will define the future climate role of the savanna ecosystems, are currently not well understood. The Colombian Orinoco River basin (“Llanos”) natural savannas are being rapidly converted to agriculture. The impact of this transformation on C fluxes and accumulation is not clear. It is thus urgent to understand the Llanos natural savanna ecosystem services, including their C cycle and underlying mechanisms. Here we report and analyze 2 years of measurements of carbon dioxide fluxes from a naturally-restored (secondary) Llanos High Plains savanna ecosystem, using eddy covariance. Meteorological conditions, particularly rainfall, were quite variable during the measurement period. During the first year of measurements, the savanna was a weak carbon source (35 gC m−2), while during the second year, the system was a comparatively strong carbon sink (−273 gC m−2), despite receiving less rainfall than during the first year. As expected, the savanna net ecosystem exchange (NEE) was highly dependent on global solar radiation, soil water content, and ecosystem respiration. We found that after ∼10 days of nominal drought, i.e., with less than ∼5 mm/day of precipitation, the NEE became highly dependent on drought duration. The ecosystem reached a critical condition of low photosynthetic activity after ∼60 days of nominal drought. Based on these findings, we developed and applied a simple standard meteorology-based model that properly reproduced the observations. Our results indicate that a shift to a climate with similar total precipitation but split into extreme dry and wet seasons might eventually suppress the savanna C uptake capacity.

La turbulencia generada por las interacciones superficie-atmósfera, y su disipación, determinan en gran parte la dispersión de contaminantes y, por lo tanto, su impacto en la salud pública. La parametrización de la turbulencia en los modelos de transporte y dispersión de contaminantes ha sido observacionalmente obtenida y validada principalmente en latitudes medias, pero muy poco en el trópico. En este artículo se reporta la caracterización estadística de la turbulencia atmosférica en un sitio representativo del Área Metropolitana de Bogotá – Colombia (4°39’30.48”N, 74°5’2.38”O, 2577 m s.n.m.m.), una megaciudad andina tropical, a partir de mediciones de alta frecuencia con un anemómetro ultrasónico de tres ejes, validado en exposición y operado a 10 Hz, a 7.8 m de altura, entre agosto y noviembre del 2014. El análisis revela que, durante el periodo de medición, la turbulencia lateral en Bogotá, expresada como desviación estándar, fue elevada y comparable a la turbulencia longitudinal (σv ≅ σu ≈ 0.2-1.3 m s-1), lo que caracteriza su atmósfera como horizontalmente isotrópica. Las intensidades turbulentas observadas (Iu ≈ Iv ≈ 0.6, Iw ≈ 0.3) fueron ~2 a ~3 veces más altas que las reportadas para latitudes medias a las bajas velocidades del viento en Bogotá. El análisis espectral es consistente con el estadístico, con espectros longitudinales y verticales comparables a los reportados para condiciones neutrales a ligeramente inestables, y espectros laterales con frecuencias pico de un orden de magnitud menor que los reportados en la literatura, lo que indica una elevada producción de turbulencia lateral. Se estima que los remolinos al inicio de la cascada de energía tuvieron longitudes características de ~120 m y ~10 m en los planos horizontal y vertical, respectivamente. © 2019. Acad. Colomb. Cienc. Ex. Fis. Nat.