INTRODUCTION

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2.1 INTRODUCTION

Brahmaputra River is ranked one among the highest specific discharge system in the world. The gradient of the river is confirmed by the steepness recorded as 4.3-16.8 m km^-1 in the gorge section upstream of Pasighat and around 0.1 m km^-1 in Guwahati (Kamrup) (Goswami, 1998). The flow of nutrients is directed from upstream to downstream low-lying plains in the river. Gradient of Brahmaputra flowing from Pasighat through Majuli to downstream Kamrup, accompanies a huge sediment discharge of 1128 tonnes km^-2 year^-1. The sediment driven fertile alluvia are rich sources of nutrients desirable for crop management and developmental activities.

Keeping the geomorphological features in consideration, two study areas having unique topographies in the Brahmaputra floodplain were selected for the research project.

Majuli River Island lies at the upper middle stretch of Brahmaputra River at altitude of 84.50 m above the mean sea level (Phukan, 2005; Dutta et al., 2012). Ecological assessment in Majuli was compared to a similar downstream floodplain ecosystem in Brahmaputra Basin, Kamrup (Amingaon and Umananda River Island). Kamrup is located in the lower stretch of Brahmaputra River at altitude of 31 – 55 m above the mean sea level depending on variation in the hilly terrain. Majuli and Kamrup were considered as two different soil typologies in Brahmaputra River floodplain. Majuli represents the plains in upper Assam whereas Kamrup represents the hilly terrains in lower Assam.

In Majuli, annual tourism rush, agricultural activities, fishery, inland transportation and natural geophysical processes affect the aquatic as well terrestrial

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ecosystems (Block Development Office, Majuli 2011). In Kamrup, large scale industrial activities, occupational and trespassing events in Amingaon and high tourism rush in Umananda indicate an impending hazard to environmental quality. From the ecosystem health perspective, water and soil monitoring was performed in Majuli and Kamrup according standard methods of ASTM and APHA.

Soil serves as a medium for flow and regulation of water and determines the availability of nutrients. Most of the elements leach deeper into soil, ground water aquifer and water bodies. The processes that influence soil quality are more or less dependent on intrinsic and extrinsic factors (Svoray et al., 2004). Among the intrinsic factors, soil water holding capacity, nutrient status and soil porosity determine the occurrence of vegetation, while extrinsic factors are associated with the environmental determinants like topography of a particular area (Svoray et al., 2004). These factors are intertwined with the biotic components and they collectively contribute to soil fertility and productivity. This vital property of soil is influenced by process of humification and OM mineralization that occur progressively (Sinsabaugh et al., 1991).

Among the active pool of nutrients present in the soil, most important ones are Carbon, Nitrogen, Phosphorus and Sulphur. Biogeochemical cycles make these nutrients available to the living organisms through producer plants. The labile form of elements present in soil are fixed and made available to the living organisms through biochemical processes by microorganisms existing in soil most importantly bacteria, fungi and actinomycetes (Waldrop et al., 2000). Nutrients in soil are thus mineralized by microorganisms where a minor but significant quantity is retained by

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microorganisms as microbial biomass accounting to nearly 2-10% of the total nutrient content (Kujur and Patel, 2012). Soil microbial biomass content is directly or indirectly influenced by the enzymes present in soil (Bruce, 2005). The most important ones are classified as extracellular enzymes from diverse sources of plants or animal or microbial origin, and intracellular enzymes confined to cellular components of the living organisms in soil (Pancholy and Rice, 1973). However factors as vegetation, fertilizers, pesticides and agricultural practices influence enzyme activities in soil (Dinesh et al., 2004; Gundi et al., 2007; Hargreaves et al., 2003).

In an effort to understand this attribute, soil samples were analysed for geochemical parameters and microbial population from health as well as productivity viewpoint. Soil microbial population count, soil microbial biomass (MB) characterization and soil enzyme activities were checked in order to evaluate soil fertility and scope of productivity with respect to nutrient mineralization dynamics.

Statistical analyses helped to correlate the experimental variables and derive a functional relationship between the biotic and abiotic parameters in soil samples (all parameters contributing to nutrient mineralization).

Type of vegetation was studied in wetlands in Majuli and occurrence of similar species was checked in Kamrup. Brahmaputra River floodplain is rich in species composition. The fertile alluvial soil is homeland to a wide spectrum of endangered species of plants and animals with enormous variation in both vertical and horizontal distributions (Goswami, 1997). Vegetation study was carried out in Majuli by Quadrat Sampling Method. Occurrence of plant species recorded in Majuli was checked for

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existence in Kamrup. Vegetation profile gave an overview of landuse activities and soil quality. Majuli was dominated by grassland vegetation which has now been converted gradually to agricultural lands and residential areas (confirmed by social survey).

Vegetation in Majuli included some common plants as Daucas carota, Abelmoschus esculentus, Oxalis repens, Mimosa pudica, Chrysopogon aciculatus, Centella asiatica, Solanum nigrum, Andrographis paniculata, Imperata cylindrica, Spilanthes paniculata, Hemarthria sp., Commelina benghalensis, Hydrocotyle rotundifolia, Ageratum conyzoides, Ficus reliogiosa, Dichanthium annulatum, Polygonum hydropiper and Eichornia crassipes, etc. Plant species found in Majuli were also observed in Kamrup.