Soil Mineral and Nitrate Nitrogen, Plant N Uptake, N Use Efficiencies and Yield as Influenced by Tillage and Nitrogen Management under Wheat Crop in Sub-Tropical Eastern India

Accumulation of mineral nitrogen (min-N) in soil profile (0-90 cm) was significantly (p<0.05) higher in conventional tillage (CT) than zero tillage (ZT) treatment plots both at 42 days after sowing (DAS) and 84 DAS of wheat, the increase over ZT being 58% and 44% respectively; but at the harvest stage an opposite trend was noted. Min-N accumulation in soil also varied significantly (p<0.05) amongst N application rates with its highest value at N150 followed by subsequent reductions with decreasing N levels at all stages of wheat growth, except at harvest where N0 level had the highest accumulation (341.06 kg ha-1) which is ascribed to poor growth and very low plant N uptake. Significant effect of tillage was found on grain yield, dry matter yield and higher plant N uptake recorded under ZT plots over CT. Of the N application levels, N150 produced the highest grain yield and it was significantly (p<0.05) higher than all other N treatments. ZT showed highest nitrogen use efficiencies [agronomic efficiency (AE), physiological efficiency (PE) and apparent recovery of nitrogen (ARN)] as compared to CT, whereas maximum values of AE (36.28) and PE (29.59) was observed at N120 and these were significantly higher than all other N treatments. As anticipated, highest ARN value was seen at N60 followed by its decrease with increasing levels of N application. Relative proportion of residual inorganic nitrogen (mineral vs. nitrate) in soil profile when compared at the harvest of wheat, a very high proportion of mineral (NH4+ + NO3-) as compared to nitrate nitrogen was evident under both the tillage treatments and more so in ZT than CT plots irrespective of N levels, indicating thereby the predominance NH4+ form of nitrogen in all the layers up to 90 cm soil profile which may be explained by lower nitrification rate due to high water storage in the soil profile during the entire wheat growing season. From the results, it is inferred that plots under ZT combined with N application @150 kg ha-1 proved superior to all other treatment combinations in respect of crop yield, nitrogen use efficiencies, plant N uptake and water storage in soil profile.