Temporal variation in N 2O and N 2 fluxes from a permanent pasture in Switzerland in relation to management, soil water content and soil temperature

Abstract

Nitrous oxide (N 2O) is an important greenhouse gas, and as such it contributes to the global greenhouse effect. Agro-ecosystems, including fertilized grasslands, represent a major source of N 2O, which is emitted together with N 2. In order to reduce the loss of fertilizer-N in the form of N 2O and N 2 by adjusting grassland management, it is important to understand the relationship between the emission of these gasses after fertilization and the conditions in the soil–plant system. This relationship was studied by determining the seasonal patterns of N 2O and N 2 fluxes, soil temperature, and soil water content based on weekly measurements between May and November in 1993, and between April and November in 1994. Measurements were carried out in fertilized and regularly cut pasture at a site located in the foothills of the Swiss Alps with a humus rich sandy loam Gleyic Cambisol. N fluxes were measured using soil cores incubated in the field either with or without acetylene inhibition. Highest N fluxes were measured from wet soil after plant cutting, and after N fertilizer application. For individual N applications, estimates of N loss relative to the amount of N applied ranged from 0.05% to 5.2% for N 2O and from 0% to 25.5% for N 2. The largest portion of the annual N loss as N 2O occurred during early spring (about 66%) and coincided with soil thawing, and as N 2 during autumn (44–62%) in combination with high soil water contents and advanced plant development. The magnitude of the fluxes of N 2O and N 2 were not directly correlated with soil temperature, but increased with increasing soil water content. N 2/N 2O ratio was largest at soil water contents above 60% water-filled pore space, and was further enhanced in the presence of a well developed plant canopy. The data suggest that in this managed grassland system soil moisture content and the stage of canopy development are the main factors regulating the seasonal pattern of N 2O and N 2 losses, and their ratio.