Impact Of Subsurface Drainage On Nitrogen Transformation And Microbial Activity In A CoastalEcosystem

摘要

In South-east Asia, coastal tracts are often considered as homogeneous entities in terms of constraints to agricultural production. Most of the lands in these regions, and in India in particular, are saline to saline sodic and are faced with problemsof waterlogging and low carbon content in the root zone. In this study, an attempt is made to understand the nitrogen dynamics in salt-affected coastal rice wetlands that were reclaimed by subsurface drainage. The experimental area consist of fields at various stages of reclamation, equipped with subsurface drains with varying spacing. Date were collected from - four drain spacing/stages of reclamation combinations, namely 15 m (reclaimed), 25 m (reclaimed until1998 and left fallow in 1999 onwards), 35and 55 m (new area under reclamation since 1999), during 1999,2000 and 2001. The area with 25 m spacing was left fallow during 1999 and beyond, and the subsurface drainage system was blocked, to study the accumulation of toxic substance in the root zonein the absence of crops, irrigation, leaching and drainage. The study revealed that subsurface drainage helped to improve the fertility status of the soil by promoting ammonium storage on the clay complex. The process of nitrification was greatly reducedin saline coastal lands due to marginal presence of nitrifiers. Sustained operation of subsurface drainage systems served to maintain a favorable salt content in the upper soil layers, leading to enhanced microbial activity and nitrification. Consequently, nitrite concentration was limited to 0.3 mg l~(-1) in the root zone. Over the three years of the study, there was an exponential increase in the nitrite concentration in the root zone of the 25 m drain spacing area where the drainage system was blocked. Such an increasing trend of nitrite accumulation may give rise to severe nitrite toxicity and other environmental problems. The study concluded with the suggestion that researchers pay attention to nitrite accumulation in coastal land and its subsequent consequence on coastal ecosystems.