Effect of Azadirachta indica (neem), sodium thiosulphate and calcium chloride on changes in nitrogen transformations and inhibition of nitrification in soil incubated under laboratory conditions

A laboratory experiment was conducted to examine the effects of nitrification inhibitors (NIs) neem seedcake (Azadirachta indica) (NSC), sodium thiosulphate (Na2S2O3) and calcium chloride (CaCl2) on changes in NH4+-N, inhibition of nitrification and recovery of applied nitrogen (N) in soil. Surface soil samples of 0-15 cm were collected from an arable field, amended with urea N (UN) at the rate 200 mg N kg(-1), UN + NSC, UN + Na2S2O3 and UN + CaCl2 and incubated at 22 degrees C periodically over 50 d. Soil without any amendment was used as check (control). Results indicated that more than 58% of N applied as NH4- disappeared over a period of 50 d from the soil mineral-N pool. Some of this N (21%) was accumulated as NO3--N while the remaining N was unaccounted for. Addition of nitrification inhibitors NSC, Na2S2O3, and CaCl2 resulted in a decrease in the extent of NH4+ disappearance by 35%, 44% and 30%, respectively. In the treatment receiving UN alone, 56 mg NO3--N kg(-1) accumulated over 50 d (maximum 93 mg kg(-1)) indicated an active nitrification. Application of nitrification inhibitors NSC, Na2S2O3, and CaCl2 with UN inhibited nitrification by 54%, 64%, and 59%, respectively. Apparent N recovery (ANR) in the treatment receiving UN alone was 63% that substantially increased to 83%, 89% and 76% in the treatments receiving UN + NSC, UN + Na2S2O3, and UN + CaCl2, respectively indicating 32%, 41% and 20% increase in N recovery. Among three NIs tested, Na2S2O3 proved superior in inhibiting nitrification and increasing ANR. The study demonstrated that application of NSC, Na2S2O3, and CaCl2 which are cheap and easily available NIs inhibited nitrification and improved N recovery efficiency of applied N in an arable soil very effectively. It is suggested that these inhibitors should be tested under field conditions for increasing NUE and improving crop productivity. (C) 2010 Elsevier Ltd. All rights reserved.