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 seed-cake (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°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 was 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.