A case study was undertaken to assess the risk of sewage-irrigated soils in relation to the transfer of trace elements to rice and wheat grain. For this purpose, peri-urban agricultural lands under the Keshopur Effluent Irrigation Scheme (KEIS) of Delhi were selected. These agricultural lands have been receiving irrigation through sewage effluents since 1979. Sewage effluent, groundwater, soil, and plant (rice and wheat grain) samples were collected with GPS coordinates from this peri-urban area. Under wheat crop, sewage irrigation for four decades resulted into a significant buildup of zinc (141 %), copper (219 %), iron (514 %), nickel (75.0 %), and lead (28.1 %) in sewage-irrigated soils over adjacent tube well water-irrigated ones. Under rice crop, there was also a significant buildup of phosphorus (339 %), sulfur (130 %), zinc (287 %), copper (352 %), iron (457 %), nickel (258 %), lead (136 %), and cadmium (147 %) in sewage-irrigated soils as compared to that of tube well water-irrigated soils. The values of hazard quotient (HQ) for intake of trace toxic elements by humans through consumption of rice and wheat grain grown on these sewage-irrigated soils were well within the safe permissible limit. The variation in Zn, Ni, and Cd content in wheat grain could be explained by solubility-free ion activity model (FIAM) to the extent of 50.1, 56.8, and 37.2 %, respectively. Corresponding values for rice grain were 49.9, 41.2, and 42.7 %, respectively. As high as 36.4 % variation in As content in rice grain could be explained by solubility-FIAM model. Toxic limit of extractable Cd and As in soil for rice in relation to soil properties and human health hazard associated with consumption of rice grain by humans was established. A similar exercise was also done in respect of Cd for wheat. The conceptual framework of fixing the toxic limit of extractable metals and metalloid in soils with respect to soil properties and human health hazard under the modeling framework was established.

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SU MMARYField experiments were conducted at the Crop Research Centre of Govind Ballabh Pant University of Agriculture and Technology, Pantnagar during 1996/97 and 1997/98. Each experiment comprised 10 crop sequences : (a) wheat-rice, (b) chickpea-rice, (c) lentil-rice, (d) pea-rice, (e) wheat-mungbean green manure-rice, (f ) wheat-Sesbania green manure-rice, (g) wheat-fodder-rice, (h) chickpeafodder-rice, (i) lentil-fodder-rice and ( j) pea-fodder-rice, in a randomized block design with four replications. The crop sequences were compared in terms of economic rice equivalent yield (REY), protein production, apparent nutrient balances and effect on soil fertility status. Amongst crop sequences involving two crops each year (200 % cropping intensity), chickpea-rice gave highest REY and protein production. Of the sequences involving three crops each year (300 % cropping intensity), chickpea-fodder-rice and wheat-fodder-rice were superior to others. The P balances were positive for all sequences, whereas K balances were generally negative except for sequences involving green manure legumes. Green manuring with Sesbania or mungbean helped restore soil fertility, indicating the advantage of green manure for higher productivity and sustainability of rice-wheat system. Chickpearice and chickpea-fodder-rice appeared promising alternatives to rice-wheat crop sequence.

A b s t r a c t. The study aims to elucidate the impact of organic inputs on strength and structural stability of aggregates in a sandy loam soil. Tensile strength, friability and water stability of aggregates, and the carbon contents in bulk soil and in large macro (>2 mm), small macro (0.25-2 mm), micro (0.053-0.25 mm) and silt+clay size (<0.053) aggregates were evaluated in soils from a long-term experiment with rice-wheat rotation at Modipuram, India, with different sources and amounts of organic C inputs as partial substitution of N fertilizer. Addition of organic substrates significantly improved soil organic C contents, but the type and source of inputs had different impacts. Tensile strength of aggregates decreased and friability increased through organic inputs, with a maximum effect under green gram residue (rice)-farmyard manure (wheat) substitution. Higher macroaggregates in the crop residue-and farmyard manure-treated soils resulted in a higher aggregate mean weight diameter, which also had higher soil organic C contents. The bulk soil organic C had a strong relation with the mean weight diameter of aggregates, but the soil organic C content in all aggregate fractions was not necessarily effective for aggregate stability. The soil organic C content in large macroaggregates (2-8 mm) had a significant positive effect on aggregate stability, although a reverse effect was observed for aggregates <0.25 mm. Partial substitution of nitrogen by organic substrates improved aggregate properties and the soil organic C content in bulk soil and aggregate fractions, although the relative effect varied with the source and amount of the organic inputs.K e y w o r d s: tensile strength, friability, aggregates, soil organic C, rice-wheat rotation mean weight diameter INTRODUCTIONSustainability of the rice-wheat system in the indo-gangetic plains (IGP) is crucial for meeting country food demand, and is also at the centre of global food security (Ladha et al., 2003a). This sustainability is arguably at stake, partly due to the staggering productivity, and partly due to degradation of soil quality (Ladha et al., 2003b). Continuous use of imbalanced fertilizers under intensive rice-wheat cultivation over the years had negative impacts on soil structure, organic carbon and nutrient supplying capacity of the soils (Bhandari et al., 2002;Regmi et al., 2002). Addition of organic inputs as a partial substitution of N has been one of viable options in restoring the soil quality and subsequently sustaining the system productivity.As the soil is collectively made of aggregates, the study at the single structural unit or aggregate has been useful to evaluate the soil response to management (Blanco-Canqui et al., 2005). Since the rice-wheat system involves two contrasting edaphic environments, the role of soil aggregation is of special importance. The mechanical properties of aggregates are clearly indicative of soil structural condition determining soil functions for plant growth. Tensile strength (TS), a fundamental property of ...