N₂O fluxes in soils of contrasting textures fertilized with liquid and solid dairy cattle manures

Manure is known to increase soil N 2 O emissions by stimulating nitrification and denitrification processes. Our objective was to compare soil-surface N 2 O emissions following the application of liquid and solid dairy cattle manures to a loamy and a clay soil cropped to silage maize. Manures were applied in 2 consecutive years at rates equivalent to 150 kg total N ha -1 and compared with a control treatment receiving an equivalent rate of synthetic N. Soil-surface N 2 O fluxes, soil temperature, and soil water, nitrate and ammonium contents were monitored weekly in manured and control plots. From 60 to 90% of seasonal N 2 O emissions occurred during the first 40 d following manure and synthetic fertilizer applications, indicating that outside that period one or several factors limited N 2 O emissions. The period of higher emissions following manure and fertilizer application corresponded with the period when soil mineral N contents were highest (up to 17 g NO 3 − -N m -2 ) and water-filled pore space (WFPS) was greater than 0.5 m 3 m -3 . The absence of significant N 2 O fluxes later in the growing season despite high WFPS levels indicated that the stimulating effect of organic and synthetic N additions on soil N 2 O production was relatively short-lived. Fertilization of silage maize with dairy cattle manure resulted in greater or equal N 2 O emissions than with synthetic N. This was observed despite lower overall soil mineral N contents in the manured plots, indicating that other factors affected by manure, possibly additional C substrates and enhanced soil respiration, resulted in greater denitrification and N 2 O production. Silage maize yields in the manured soils were lower than those receiving synthetic N, indicating that the N 2 O emissions per kilogram of harvested biomass were greater for manures than for synthetic N. Our results also suggest that the main source of N 2 O was nitrification in the loam and denitrification in the clay soil. There was no clear difference in N 2 O emissions between liquid and solid manures. The variable effects of liquid and solid manure addition reported in the literature on soil N 2 O emissions likely result from the variable composition of the manures themselves as well as from interactions with other factors such as soil environment and farming practices. A better characterization of the availability of manure C and N is required to assess the impact of manure application on soil N 2 O emissions under field conditions.