Soil nitrogen cycling and nitrogen oxide emissions along a pasture chronosequence in the humid tropics of Costa Rica

Our objectives were: (1) to measure how N2O and NO emissions from a chronos equence of forested land converted to pastures in the humid tropics of Cost a Rica had changed in 4 yr, and (?) to relate these emissions to indices of N availability. We observed lower mean N2O (11.9 ng N cm(-2) h(-1)) and NO (3.5 ng N cm(-2) h(-1)) emissions from pastures in 1996 compared to 1992 ( N2O, 39.9 ng N cm(-2) h(-1); NO: 5.8 ng N cm(-2) h(-1)). Even so, N2O emiss ions in recently formed pastures (13.8 ng N cm(-2) h(-1)) were still higher than previously measured emissions from forests (7.0 ng N cm(-2) h(-1)). I ndices of N cycling, such as net N mineralization, nitrification potential, and extractable soil nitrate, decreased with pasture age, which we attribu ted to a decrease in substrate availability. Denitrification enzyme activit y did not change significantly with pasture age, indicating that denitrific ation occurs at least sporadically at all sites and the presence of denitri fying enzymes is not as strongly linked to N availability as is the presenc e of nitrifying enzymes. There were no significant correlations between N2O and NO emissions and indices of N cycling. While this may indicate that th e processes are not closely related, we believe that sampling of nitrogen o xide emissions in 1996 was inadvertently biased towards exceptionally dry s oil conditions. This sampling bias limited the probability of observing lar ge nitrogen oxide emissions associated with episodic denitrification. Resul ts from chronosequence studies should be interpreted with caution especiall y for variables which depend on local weather conditions at time of measure ment. (C) 1999 Elsevier Science Ltd. All rights reserved.