No abstract

Agricultural soils often receive annual applications of manure for long periods. Our objective was to quantify the effects of 19 consecutive years of pig (Sus scrofa) slurry (PS) application to a loamy soil (loamy, mixed, frigid Aeric Haplaquept) on N2O emissions. Soil surface N2O fluxes (FN2O) were measured 36 times in 1 yr. Nitrous oxide concentration profiles, soil NH+4‐ and NO−3‐N contents, denitrifying enzyme activity (DEA), and denitrification rate (DR) in soil were also determined to explain the variation in FN2O Long‐term (19 yr) treatments on continuous silage maize (Zea mays L.) were 60 (PS60) and 120 Mg ha−1 yr−1 (PS120) of pig slurry and a control receiving mineral fertilizer at a dose of 150 kg ha−1 each of N, P2O5, and K2O. Denitrifying enzyme activity, soil N2O concentrations, and FN2O (<25 ng m−2 s−1) were low in the control plots receiving mineral fertilizer. Annual applications of PS to the soil for 18 yr had positive residual effects on the DEA compared with the long‐term fertilized control plots. Following PS application, there was a strong and rapid increase of FN2O (up to 350 ng m−2 s−1) on manured plots. The PS‐induced FN2O increased with increasing quantity of PS, probably as the result of a greater availability of NO−3‐N for denitrification. The effects of PS on FN2O were mostly limited to the 30 d following application, with low fluxes (<10 ng m−2 s−1) during the rest of the measurement period. Total N2O–N emissions represented 0.62, 1.23, and 1.65% of total N applied in control, PS60, and PS120 plots, respectively. These emission factors for the PS plots agreed with values previously suggested for N‐fertilized soils (1.25%).

Incorporation of urea decreases ammonia (NH) volatilization, but field measurements are needed to better quantify the impact of placement depth. In this study, we measured the volatilization losses after banding of urea at depths of 0, 2.5, 5, 7.5, and 10 cm in a slightly acidic (pH 6) silt loam soil using wind tunnels. Mineral nitrogen (N) concentration and pH were measured in the top 2 cm of soil to determine the extent of urea N migration and the influence of placement depth on the availability of ammoniacal N for volatilization near the soil surface. Ammonia volatilization losses were 50% of applied N when urea was banded at the surface, and incorporation of the band decreased emissions by an average of 7% cm (14% cm when expressed as a percentage of losses after surface banding). Incorporating urea at depths >7.5 cm therefore resulted in negligible NH emissions and maximum N retention. Cumulative losses increased exponentially with increasing maximum NH-N and pH values measured in the surface soil during the experiment. However, temporal variations in these soil properties were poorly related to the temporal variations in NH emission rates, likely as a result of interactions with other factors (e.g., water content and NH-N adsorption) on, and fixation by, soil particles. Laboratory and field volatilization data from the literature were summarized and used to determine a relationship between NH losses and depth of urea incorporation. When emissions were expressed as a percentage of losses for a surface application, the mean reduction after urea incorporation was approximately 12.5% cm. Although we agree that the efficiency of urea incorporation to reduce NH losses varies depending on several soil properties, management practices, and climatic conditions, we propose that this value represents an estimate of the mean impact of incorporation depth that could be used when site-specific information is unavailable.

Incorporation of broadcast pig slurry and urea into soil is incompatible with no-till production systems and alternative application methods that reduce NH 3 -N loss are required. The objective of this study was to assess the impact of incorporating urea and pig slurry in shallow furrows (banding) on NH 3 volatilization. A field study was conducted on a silty loam soil that had been under no-till for 2 years. Ammonia volatilization was measured for 29 days after urea and pig slurry (140 kg N ha -1 ) were broadcast or incorporated (5 cm) in bands. High urease activity and soil temperatures as well as an absence of rainfall combined to result in large losses of NH 3 -N from all treatments. Broadcast urea lost the greatest proportion of applied N (64%) followed by banded urea (31%), broadcast pig slurry (29%) and banded pig slurry (16%). High emissions from broadcast urea were consistent with previous reports of large volatilization losses on no-till soils. Presence of crop residues and associated high urease activity (288 lg NH 4 -N g -1 h -1 ) at the surface of no-till soils were likely important factors contributing to these high emissions. Incorporation of slurry and urea in bands was not as efficient in reducing volatilization as expected but not for the same reason. Relatively high emissions from banded slurry were the result of an incomplete incorporation of slurry in the shallow bands and indicate that the benefit of this practice is limited at high slurry application rates. In banded urea plots, hydrolysis of concentrated urea likely resulted in high localized NH 4 ? concentrations and pH, which increased NH 3 source strength and emissions. Our results therefore suggest that incorporating urea in bands may not be as efficient for reducing NH 3 emissions as incorporation of broadcasted urea which results in lower soil urea concentrations.

Volatilization of ammonia following application of urea contributes to smog formation and degradation of natural ecosystems. The objective of this study was to evaluate the impact of (i) incorporation and banding of urea and (ii) surface broadcast of slow-release urea types on NH(3) volatilization in a dry acidic soil. Volatilization was measured using wind tunnels for 25 d after standard urea (140 kg N ha(-1)) was broadcast, broadcast and incorporated (0-5 cm), or incorporated in shallow bands (3-5 cm) to a conventionally tilled silty loam soil. Urea supplemented with a urease inhibitor or coated with a polymer was also broadcast at the soil surface. Little N diffused out of the polymer-coated granules and ammonia losses were low (4% of applied N). Use of a urease inhibitor also resulted in a low NH(3) loss (5% of applied N) while maintaining soil mineral N at levels similar to plots where untreated urea was broadcast. The rate of hydrolysis of urea broadcast at the soil surface was slowed by the lack of moisture and NH(3) loss (9% applied N) was the lowest of all treatments with standard urea. Incorporation of broadcast urea increased emissions (16% applied N) by increasing urea hydrolysis relative to surface application. Furthermore, incorporation in band also increased emissions (27% applied N) due to a localized increase in soil pH from 6.0 to 8.7. We conclude that incorporating urea in bands in a dry acidic soil can increase NH(3) volatilization compared to broadcast application followed by incorporation.