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 agric ultural 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) e ach unfertilized and fertilized. Both soils are of andic origin. Their prop erties include relatively low bulk density and high organic matter content, water retention capacity, and hydraulic conductivity. The top 2-3 cm of th e 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 cl ay soil. Using automated sampling and analysis systems we measured N2O emis sions at 4.6 h intervals, meteorological variables, soil moisture, and temp erature 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 nea r saturation on loam during a wet period in February 1996. On both soils th e aggregated surface layer always remained unsaturated. Soils emitted N2O t hroughout 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 c ompared to 3.54 +/- 4.31 ng N2O-N cm(-1) h(-1) from the fertilized plot (35 1 days measurement). Fertilization dominated the temporal variation of N2O emissions. Generally fluxes peaked shortly after fertilization and were inc reased for up to 6 weeks ('post fertilization Bur'). Emissions continued at a lower rate ('background flux') after fertilization effects faded. Mean p ost-fertilization fluxes were 6.3 +/- 6.5 ng N2O-N cm(-1) h(-1) while the b ackground flux rate was 2.2 +/- 1.8 ng N2O-N cm h(-1). Soil moisture dynami cs affected N2O emissions. Post fertilization fluxes were highest from wet soils; fluxes from relatively dry soils increased only after rain events. N 2O emissions were weakly affected by soil moisture during phases of low N a vailability. Statistical modeling confirmed N availability and soil moistur e as the major controls on N2O Aux. Our data suggest that small-scale diffe rences in soil structure and moisture content cause very different biogeoch emical environments within the top 7 cm of soils, which is important for ne t N2O fluxes from soils. (C) 2001 Elsevier Science Ltd. All rights reserved .