Assessing the effect of puddling on preferential flow processes through under bund area of lowland rice field

High water losses from rice fields and continued detection of nitrates in groundwater in rice growing areas are two important challenges of sustainable rice production. Recent studies have shown that preferential flow paths present in under-bund soils is a major reason behind such water and nitrogen losses in lowland rice soils. The present study was conducted to comprehensively characterize water flow and nitrogen transport processes in typical rice soils of eastern India. Detailed soil characterization and several flow and transport experiments conducted in undisturbed soil cores and directly under field conditions were used to estimate fate and transport parameters for preferential transport models. Experimental assessment also included data from plugged bunds where the under-bund soils were puddled to create a low-permeable soil layer. Results showed that the under-bund soils are generally more permeable than the within-field soils. The layer between the plow sole and the clay pan may have naturally evolved to serve as lateral flow path while the presence of isolated yet vertically connected high permeable zones through the clay pans may be serving as localized recharge pits in rice soils. Results of transport experiments suggested that although the presence of plow sole in within-field soil can significantly increase solute residence time, urea-N may bypass much of the soil matrix and get transported through the preferential flow paths of under-bund soil before it gets hydrolyzed. Such bypassing may also occur in rice vadose zone through localized patches of relatively high permeable soil or through leaky plow soles. Results further showed that bund plugging may effectively increase the retention time for solutes in within-field soils. Results of model parameterization suggested that preferential transport parameters estimated using the dual-permeability model could provide an acceptable prediction of urea-N transport behaviour in rice soils.