Spatial and temporal variations in nutrient dynamics in Pokkali soils of Kerala

Abstract

The Pokkali soils (Typic Sulfaquents) are low land soils situated below the mean sea level, located along the coastal tracts of Alappuzha, Ernamkulam and Thrissur districts. These soils are acid saline in nature where shrimp/prawn farming is practiced during high saline phase (December to April) coinciding with sea water entry followed by cultivating the salt tolerant Pokkali rice during low saline phase (June to October) when the dilution of salts occurs after South West monsoon. Only very few studies have been undertaken so far on nutrient status in Pokkali soils. Wide variations in nutrient content ranging from deficient to toxic level were reported in these studies. A comprehensive study is very much essential to unravel the seasonal and temporal variations of nutrient dynamics in Pokkali soils. The soil samples (lowland soils and neighbouring upland soils) from different land use systems and the water samples (both standing water and nearby brackish water inundating the Pokkali tract) were collected at bimonthly intervals starting from June 2017 to April 2018 to understand the nutrient dynamics in these soils with respect to spatial and temporal variation and to find out the influence of source water on them. Physico-chemical properties and biological properties of the collected soils were analysed. Salt water intrusion during high saline phase into Pokkali fields caused drastic increase in electrical conductivity, decrease in soil pH and associated changes in available plant nutrients. A gradual decrease in redox potential from June to October and increase from December to April were also observed as an influence of sea water intrusion. The south west monsoon received during the low saline phase played a significant role in diluting soil salinity and loss of H + ions from top soil, thus affecting the soil nutrient dynamics in total. High content of available plant nutrients and the change in nutrient content with respect to spatial and temporal variations were observed in low land soils compared to the nearby upland soils. Spatial variation of all available nutrients except phosphorus was highly influenced by the nearness of brackish water rather than the type of land use system. Highest available phosphorus recorded in paddy alone land use might be due to the phosphorus mineralisation from left out crop stubbles in paddy field. Some land uses in uplands showed deficiency of available Ca, Mg and B also. In terms of temporal variation, all nutrients except available phosphorus remained very high during high saline phase as influenced by marine water whereas availability of phosphorus was highly influenced by soil pH. High acidity and salinity during high saline phase adversely affected the soil biological properties. Fractionation of phosphorus and copper was carried out to study their major fractions and to ascertain their contribution to the available pool. All the P fractions were high in Pokkali soil. Temporal variation of all the P fractions was also prominent in Pokkali soil. Ws-P, Ca-P and residual-P were very high in October (low saline phase) whereas other fractions were high in April (high saline phase). Effect of land uses on P fractions was absent except for Org-P and Ca-P. Temporal variation of all the Cu fractions was clearly evident in Pokkali soil. Ws-Cu, Ex-Cu and residual Cu were high in April (high saline phase). All other fractions were high in October (low saline phase). Land uses did not show any direct effect on distribution of copper fractions. All the land uses showed same trend in copper fractions across the seasons. Phosphorus adsorption was high in April (high saline phase) whereas Cu adsorption was high in October (low saline phase). L and S-shaped curves were obtained for P and Cu adsorption respectively. Adsorption of P and Cu increased with increase in soil temperature. Adsorption of P and Cu among various soil textural classes followed the order clay>clay loam>sandy clay loam>loam>silt loam. Adsorption of P was mainly in inorganic forms ie. oxide and oxy hydroxides of Fe and Al surface whereas that of Cu was mainly in organic form. Freundlich adsorption isotherm was found as the best to explain the adsorption of P and Cu in Pokkali soils. Adsorption of both P and Cu was spontaneous and endothermic in nature. Low and high saline phases attributed to variations in physico-chemical and biological properties of Pokkali soils. South West monsoon caused leaching losses of nutrients particularly potassium, sulphur, magnesium and boron during low saline phase. The presence of brackish water source nearer to field had more effect on nutrient dynamics in Pokkali soils, rather the type of land use system. The influence of temperature on nutrient was visible in the quantity –intensity relations of P and Cu. The present study has clearly shown that the nutrient dynamics in Pokkali soils is influenced more by temporal variations than the spatial variations.