Carbon dioxide and nitrous oxide content in soils under corn and soybean

Agricultural practices affect the production and emission of C[O.sub.2] and [N.sub.2]O from soil. The purpose of this 2-yr field study was to determine the effects of tillage (conventionally tilled [CT] and no-till [NT]) and fertilizer source (composted cattle manure and inorganic N-P-K fertilizer) on the C[O.sub.2] and [N.sub.2]O content in soil profiles under corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. The mean C[O.sub.2] and [N.sub.2]O gas contents (i.e., mass of gas per unit soil volume) in the soil profile were determined periodically during two field seasons by sampling the soil atmosphere using plastic tubes installed at three depths (10, 20, and 30 cm) within the crop row. The soil C[O.sub.2] content was greater in CT than NT soil and in manure-amended than inorganically fertilized plots during 1 yr of the study. The soil [N.sub.2]O content was not affected by tillage practices or fertilizer sources. A significant autocorrelation between sampling dates in both years suggested that the C[O.sub.2] and [N.sub.2]0 contents in the soil profile were not erratic or random, but temporally dependent on site-specific factors. The peak C[O.sub.2] and [N.sub.2]0 levels were measured within 50 d after seeding, probably because soil moisture conditions slowed diffusive gas flux but were favorable for microbial activity. Fluctuations in soil C[O.sub.2] and [N.sub.2]O contents were not related to the seasonal variation in soil temperature. At most sampling dates, there was a significant (P < 0.05) positive correlation between the C[O.sub.2] and [N.sub.2]O content in the soil profile, suggesting similarity in the rate of gas accumulation and diffusive flux for C[O.sub.2] and [N.sub.2]O in soils. The C[O.sub.2] and [N.sub.2]O content in the soil profile appeared to be controlled more by soil moisture than soil temperature or agricultural practices. Abbreviations: CT, conventionally tilled; DAS, days after seeding; NT, no-till; WFPS, water-filled pore space.