Nitrogen transformation and growth-yield-quality responses of rice in the neem leaf extract applied and water-lowered soils

Neem leaf extract applied to lower soil moisture could inhibit nitrification, increase nitrogen (N) availability, and improve rice yield. The study aimed to evaluate the effects of neem leaf extract on soil N transformation, rice growth, yield, and quality in different soil moisture contents. Two factors were studied under a greenhouse pot experiment: soil moisture contents (field capacity, saturated, and flooded) and nitrification inhibitors (none, nitrapyrin, and neem leaf extract at 0.2, 0.4, and 0.8 g dry neem leaf basis 100 g⁻¹ soil), with a total of fifteen treatment combination. Lowering soil moisture increased urea hydrolysis and nitrification rates and NH₄⁺-N and NO₃⁻-N concentrations, promoting rice vegetative growth and delaying harvest dates (138 and 151 days for saturated and field capacity soils, compared to 130 days for flooded soil). Yet grain yield (in g pot⁻¹) decreased (from 34.3-40.3 in flooded soil to 33.3-37.2 in saturated soil and 15.0-24.9 in field capacity soil). Neem leaf extract generally depressed grain yield, specifically at higher rates in lower soil moisture. In flooded soil, neem extract maintained grain yield (35.9-40.3 vs. 35.9 without it), while in saturated soil, it decreased (33.3-37.2 vs. 36.8 without extract). In field capacity soil, neem extract decreased yield (18.9-21.4 vs. 24.9 without it). The grain yield depression was linked to high soil NH₄⁺-N due to enhanced urea hydrolysis and inhibited nitrification by neem. Rice quality remained unaffected by soil moisture and neem extract, including grain shape, elongation, amylose, and amylopectin contents.