THE EFFECTS OF SOIL PHYSICAL PROPERTIES AND IRRIGATION METHOD ON DENITRIFICATION

Concern for denitrification loss of plant available N after irrigation led to field and laboratory denitrification studies. The objectives of the studies were to evaluate the effect of sprinkle and flood irrigation on soil physical properties and denitrification losses. Acetylene-inhibition techniques were used to monitor the effects of soil physical properties, such as bulk density and crusting, on field denitrification losses under flood and sprinkle irrigation. In one flood irrigation treatment, polyacrylamide (PAM) was incorporated at the rate of 650 kg ha -1 into a clay loam soil to enhance soil aggregate stability. In additional flood and sprinkle irrigation treatments, PAM was not applied. All treatments were irrigated on a weekly basis with 5 cm of water. Bulk density, soil moisture, and soil crusting were 1.2, 1.5, and 19.2 times higher, respectively, under flood irrigation than under sprinkle irrigation, and infiltration rates were 4.7 times lower under flood irrigation. Soil physical properties of flood-irrigated soils treated with PAM were similar to sprinkle-irrigated soils. Penetrometer measurements of the surface of PAM-treated and sprinkle-treated soils were not significantly different, and bulk density differences were less than 6%. The poor physical properties and welter conditions of the flood-irrigated plots produced a fourfold increase in total cumulative nitrate losses by denitrification compared with sprinkle-irrigated plots. In field experiments, the highest denitrification rates were from flood-irrigated soils, without PAM, initiated within 24 h after irrigation. Rates decreased rapidly beyond that time, whereas PAM-treated soils evolved high amounts of N 2 O throughout both field and laboratory experiments. The continuously elevated denitrification activity in PAM-treated soils is believed to be caused more by increased anaerobic microsite activity within PAM-stabilized aggregates than by irrigation method. The regression coefficients of the relationship between percent water-filled pore space (WFPS) and denitrification rate were significant. Soil moisture content was not the only contributing factor to denitrification activity, however, as denitrification from sprinkle-irrigated soils remained significantly lower than from flood-irrigated soils when held at 80% WFPS. These results suggest that soil physical properties, such as structure and aggregate stability, have a large influence on denitrification activity.