Nitrogen supplying capacity of lowland rice soils in southern India
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Effective indigenous nitrogen (N) supply (EINS) was estimated in -N plots (no fertilizer N addition) of a multi-location field experiment with irrigated rice conducted in India. Dynamic soil tests, namely anaerobic incubation without (AI) or with K+-saturated cation exchange resin (AIR) and N release to a mixed-bed ion-exchange resin capsule (PST), were used and compared with total soil N, organic carbon (C), initial NH4-N and alkaline KMnO4-N as predictors for EINS. The pattern of net N mineralization was similar in all soils of 10 sites and fitted the two-pool first and zero order model: a rapid early phase from 0-14 d, a transition phase from 14-35 d and a slow, nearly linear phase from 35-56 d. In the rapid phase, average net NH4-N release with K-resin was 125% greater than the net NH4-N without K-resin. Static soil N tests were significantly correlated with net NH4-N of AIR only up to rapid and transition phases and also with the cumulative NH4-N adsorption measured by PST. Grain yield in the -N plots ranged from 1094 to 5707 kg . ha(-1) while EINS estimated by crop N uptake at first flowering (FF) varied from 24 to 107 kg . ha(-1) of N. All soil N tests were significantly correlated with N uptake at active tillering, panicle initiation (PI) and FF of rice, but none of the soil tests was correlated with average N uptake rates between PI and FF. Static soil N tests and short-term anaerobic incubation procedures did not provide enough information about soil N release rates during the reproductive growth period of rice. Only the late phase N between (35-56 d) measured by AIR was correlated with grain yield (r=0.67, P<0.01). Implications for use of soil tests in practical N management are discussed.
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