MEASUREMENT OF NITRATE LEACHING POTENTIAL OF A VERTISOL USING BROMIDEAS A TRACER UNDER RAIN-FED CONDITIONS OF THE INDIAN SEMIARID TROPICS

High temperatures and evaporation in summer (April-June) and erratic r ainfall in the wet season (July-September) are common features of the harsh environment of Indian semiarid tropics (SAT). The rainfall of th is region often occurs in large storms punctuated by discrete dry spel ls. During excessive rainy periods there is a strong possibility that a large part of the resident soil-N, especially NO3-, NO2- etc., may m ove beyond the root zone with the excess water percolating down the so il profile. To understand the N losses through leaching in a deep Vert isol (depth > 150 cm) of the Indian SAT, a field experiment was conduc ted during the ''wet'' season (June-September) of 1992 under rainfed c onditions, using bromide (Br-) as a tracer for NO3-. Bromide (as NaBr) at 200 kg ha(-1) was applied onto the surface of a bare fallow soil, in microplots (200 x 200 cm(2)), on June 15, and its vertical movement to 100 cm was monitored weekly. Results showed that Br- distribution was influenced strongly by rainfall. After 1 week (June 15-22), with a rainfall of 64 mm, Br- was found to a depth of 60 cm, and whereas mos t (40%) was in the top layer (0-10 cm), a total of 90% of applied Br- was recovered to a depth of 60 cm. With large storms occurring in earl y July (e.g., 68 mm on July 2), Br- was moved further down the profile and accumulated largely in the 30 to 40-cm layer. In early August, be cause of continuous heavy rainfall, almost all Br- had migrated beyond 50 cm depth. Approximately 40% was not found within the profile (0-10 0 cm) on September 11. In dry spells during late August and early Sept ember, upward movement of 10 to 20% of Br- was also noticed at 60 to 1 00 cm depth. Thus, the results demonstrate that early August may be a critical period for maximum NO3- leaching in this soil and emphasize t he need of advance strategies to reduce these effects in view of incre asing need for N use efficiency and reduction of ground water contamin ation.