The Effect of Experimentally Induced Root Mortality on Trace Gas Exchange

Soil-atmosphere exchange of carbon dioxide (CO₂), nitric oxide (NO), nitrous oxide (N₂O) and methane (CH₄) was measured following a root exclusion experiment in the Tapajos National Forest near Santarem, Para, Brazil. The sampling period (June 4 - August 14, 2000) coincided with the beginning of the dry season. The experiment was set up as a randomized complete block design with 5 pairs of 2.5 x 2.5 m plots in both sand and clay soils. Trenches were dug around one plot in each pair for screen installation. Trace gas fluxes were measured weekly for ten weeks following the trenching. Duplicate flux measurements were made for each of the trenched and non-trenched plots. Enclosures made of 0.25 m diameter PVC pipe were placed on a base imbedded in the soil. Dynamic measurements using a portable backpack system equipped with an NO₂ chemiluminescent detector for NO and an infrared gas analyzer for CO₂ were completed in the field. CH₄ and N₂O fluxes were measured through a static enclosure method. Syringe samples of the enclosure headspace were analyzed by GC-FID (CH₄) and ECD (N₂O) the following day. Daily average fluxes ranged between -0.01 and 60.3 ng-N cm^-2 hr^-1 for N₂O. NO fluxes ranged between 0.58 and 8.74 ng-N cm^-2 hr^-1. CH₄ fluxes varied between net consumption and production from -1.73 to 0.912 mg m^-2 d^-1. Soil respiration ranged from 1.34 to 5.12 umoles CO₂ m^-2 s^-1. Significant differences were seen between trenched and non-trenched plots in both clay and sand soils for N₂O emissions only. Hourly field standardization of the NO₂ chemiluminescent analyzer resulted in lower variability than the traditional method of standardization which is completed at the beginning and end of the measurement day. Frequent field standardization of the analyzer is necessary to reduce measurement error due to intra-day variability.