ausblenden:
Schlagwörter:
Soil moisture
Soil N2O emissions
Fertilized soils
Automated field measurements
Low bulk density soils
Time-domain reflectometry
Oxide emissions
Nitric-oxide
Costa-rica
Methane fluxes
Water-content
Forest
Probes
Fertilization
Conductivity
Zusammenfassung:
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 2-3 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 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 +/- 0.72 ng N2O-N cm(-2) h(-1) (mean +/- standard deviation) from unfertilized loam under annual crops compared to 3.54 +/- 4.31 ng N2O-N cm(-1) h(-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 Bur'). Emissions continued at a lower rate ('background flux') after fertilization effects faded. Mean post-fertilization fluxes were 6.3 +/- 6.5 ng N2O-N cm(-1) h(-1) while the background flux rate was 2.2 +/- 1.8 ng N2O-N cm h(-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 Aux. 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 fluxes from soils. (C) 2001 Elsevier Science Ltd. All rights reserved. [References: 49]
