Ammonia volatilization from soils with surface rice straw residue

Abstract

Reduced tillage rice production can result in rice straw and other residue on the soil surface which may enhance NH, volatilization of surface-applied N. Laboratory, greenhouse, and field experiments were conducted to determine the effect of surface rice residue and related factors on NH3 volatilization from an acid Beaumont clay (pH 5.4) and an alkaline Lake Charles clay (pH 7.4). The treatments in the greenhouse and lab consisted of all possible combinations of the following variables: surface rice straw vs. no straw, (NH4)2SO4 VS-urea, surface broadcast vs. deep placement of N, immediate flooding vs. delayed flooding. The treatments in the field consisted of all possible combinations of the following variables: surface rice straw vs. no straw, (NH4)2SO4 VS-urea, and single vs. two equal split applications of the N. The presence of surface straw increased NH3 loss in all studies on both soils. Urea tended to volatilize more than (NH4)2SO4 due to greater straw urease activity. Subsurface application of N virtually eliminated NH3 1oss. Generally,a delayed flood increased NH3 loss- Subsurface N application increased N uptake in the greenhouse. No variable affected N uptake in the field, possibly due to low NH3 losses in the field (O to 6% of applied N). The only variable to affect grain yield was the N application method. Split-applied N increased yield in the Beaumont field study, but decreased it in the Lake Charles study. Reduction in grain yield was not related to increase in NH3 loss. Factors specific to each field may have affected grain yield. A high urease activity (1 980 mg urea kg-' hr'), pH (7.2), and H-ion buffering capacity (73 mmol kg-' (pH unit)-') of rice straw created a conducive environment for NH3 volatilization of surface-applied N, particularly urea. Soil CEC's of over 50 cmol kg-' bound NH4+ from deep-applied N before volatilization could occur.