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    Author(s): A.M. Weitza; E. Linderb; S. Frolkingc; P.M. Crillc; M. Keller
    Date: 2001
    Source: Soil Biology & Biochemistry 33 :1077-1093
    Publication Series: Miscellaneous Publication
    PDF: View PDF  (1 B)

    Description

    We studied soil moisture dynamics and nitrous oxide (N2O) ¯uxes from agricultural soils in the humid tropics of Costa Rica. Using a splitplot 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±3 cm of the soils consists of distinct small aggregates (dia. ,0.5 cm). We measured a strong gradient of bulk density and moisture within the top 7 cm of the clay soil. Using automated sampling and analysis systems we measured N2O emissions at 4.6 h intervals, meteorological variables, soil moisture, and temperature at 0.5 h intervals. Mean daily soil moisture content at 5 cm depth ranged from 46% water ®lled 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 ¯uxes were 1.040.72 ng N2O-N cm22 h21 (meanstandard deviation) from unfertilized loam under annual crops compared to 3.544.31 ng N2O-N cm22 h21 from the fertilized plot (351 days measurement). Fertilization dominated the temporal variation of N2O emissions. Generally ¯uxes peaked shortly after fertilization and were increased for up to 6 weeks (`post fertilization ¯ux'). Emissions continued at a lower rate (`background ¯ux') after fertilization effects faded. Mean post-fertilization ¯uxes were 6.36.5 ng N2O-N cm22 h21 while the background ¯ux rate was 2.21.8 ng N2O-N cm22 h21. Soil moisture dynamics affected N2O emissions. Post fertilization ¯uxes were highest from wet soils; ¯uxes 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 con®rmed N availability and soil moisture as the major controls on N2O ¯ux. Our data suggest that small-scale differences in soil structure and moisture content cause very different biogeochemical environments within the top 7 cm of soils, which is important for net N2O ¯uxes from soils.

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    Citation

    Weitza, A.M.; Linderb,E.; Frolkingc, S.; Crillc, P.M.; Keller, M. 2001. N2O emissions from humid tropical agricultural soils: effects of soil moisture, texture and nitrogen availability. Soil Biology & Biochemistry 33 :1077-1093

    Keywords

    Soil moisture, Soil N2O emissions, Fertilized soils, Automated ®eld measurements, Low bulk density soils

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