Abstract
Understanding the factors controlling nitrogen (N) mineralization in paddy soils
and the development of useful indicators is essential to improve N use efficiency
(NUE) in paddy rice production and thereby reduce the application of relatively
costly N fertilizer for rice production. The present study aimed to assess NUE for
paddy rice production, determining the ability of some chemical and biological N
mineralization indices to predict N mineralization and subsequently validate
newly developed N mineralization predictors. To achieve these objectives, A
series of field, laboratory and pot experiments were carried out to achieve these
objectives. Varietal and seasonal variations in NUE were determined through a
field experiment at BAU farm, Mymensingh with 11 popularly grown rice
varieties. After that, soil variations in NUE were examined by field trials with T.
aman rice (cv BINA dhan-7) in eight locations representing eight soil series. For
determining N mineralization, a set of 18 paddy soils were incubated for 14 weeks
under saturated condition at 25°C. For predicting N mineralization, various
chemical indices such as Hot KCl extraction, Phosphate-borate buffer extraction
(PBB), acid and Alkaline KMnO4 extraction, and biological indices viz. Bglucosaminidase, Arylamidase, Protease, ẞ-glucosidase, Phosphomonoesterase, Lglutaminase and Urease activity of soil were tested. This experiment was
validated using another set of soils (nine soil series) through laboratory N
mineralization and bioassay test (N uptake study) in pot culture and field
conditions. Results revealed a wide variability in N use efficiency among rice
varieties, seasons and soils. There was no correlation between total N in soil and
N mineralization indicating that currently used total N is not a good predictor for
determining soil N availability in paddy soil. A significant variation was observed
among the soil series of both the soil sets for the chemical and biological N
mineralization indices. The combination of both chemical (Acid KMnO4
extractable N) and biological indicators (ẞ-glucosidase and Arylamidase)
significantly improved the N mineralization prediction ability in the larger soil
set. However, Acid KMnO4 extractable N appeared to be a better predictor for
determining N availability for both the paddy soil sets and formulating N
fertilizer recommendation in paddy soils.