Nutrient Availability Under Trickle Irrigation: II. Mineral Nitrogen

Abstract

AbstractThe effects of irrigation and fertilizer application on nitrate, nitrite, and ammonium concentration in a Chromic Luvisol were studied in an orchard that had been trickle irrigated for 5 years. Immediately before the first irrigation of the season, mean plot nitrate‐N was 5 ppm; 8 h of irrigation reduced this to 0.5 ppm. Nitrite concentration fell from 0.8 to 0.4 ppm in the same period. During irrigation ammonium concentration rose from 7.1 to 13.5 ppm (the average rate of increase being 0.8 ppm/h) in the surface 6‐cm depth of soil extending up to a distance varying from 30 to 65 cm from the outlet. There was no evidence that nitrate or nitrite accumulated at the A/B horizon boundary and it is likely that the loss was due to denitrification.Ammonium concentration decreased rapidly as the soil dried out and 8 h after the end of irrigation it had fallen from 13.5 to 8.5 ppm. As there was no change in the nitrate concentration in this region during the same 8‐h period the ammonium was being immobilized rather than nitrified. The only place where nitrification took place during the first 3‐d cycle was immediately below the fertilizer band at site D where nitrate concentration rose from 0 to 15 ppm in 3 d. Mean plot nitrate‐N at the end of the first cycle was 2.6 ppm. Some 32 cycles later at the beginning of the midseason 3‐d cycle nitrate‐N had fallen to 0.6 ppm. Neither nitrate nor nitrite varied with time during the midseason 3‐d cycle. Ammonium concentration rose significantly over the region extending from at least 12 cm below the surface under the outlet to the 3‐ to 6‐cm depth 65 cm away from the outlet. Over this volume of soil ammonium‐N increased from 3.9 to 19.4 ppm during irrigation. In the 4 h after the end of irrigation the ammonium concentration decreased and there was no significant difference between the pre‐ and post‐irrigation ammonium concentrations within the volume defined above. Apparently all the ammonium released during irrigation and not taken up by plants was immobilized by the soil microbial population. The amount of ammonium released was greater in the midseason (15.5 ppm) than in the first 3‐d cycle (6.4 ppm). Ammonium release rate during the midseason irrigation was 1.3 ppm N · h−1. There were no significant changes in nitrate, nitrite, or ammonium during the midseason 1‐d cycle.The results obtained for five 3‐d irrigation cycles in early summer again demonstrated that the decrease in nitrate during the irrigation cycle was accompanied by an increase in ammonium. The phenomena was part of a continuous cycle which increased the mineral nitrogen availability during the wet portion of every 3‐d cycle.