On-Farm Assessment of Biosolids Effects on Nitrogen and Phosphorus Accumulation in Soils

A field plot experiment in a calcareous soil with wheat and maize rotation was carried out for 2 yr. The study aimed to investigate the effects of biosolids （sewage sludge or chicken manure） application on nitrogen （N） and phosphorus （P） accumulation in soils and to develop a model for the effects of biosolids application on available P （Olsen-P） accumulation in soils, by which the quantities of biosolids that can be safely applied to agricultural soils were estimated. The results showed that heavy application of biosolids to agricultural soils based on the N requirement of a wheat-maize rotation cropping system will oversupply P. Soil total N was increased by 0.010 g kg-1 at application rate of 1 ton sewage sludge per hectare. The high ratio of N to P in grains of wheat and maize （from 4.0 to 7.6） and low ratio of N to P in biosolids （〈2） led to more surplus P accumulated in soils. Although plant yields and P uptake by plants increased with increasing quantities of applied biosolids in soils, there was still an average 2.87 mg kg-1 increase in Olsen-P in the plough layer treated with biosolids for every 100 kg P ha-1 surplus. A predictive model was developed based upon the initial Olsen-P in soils, P input rates, crop yield, soil pH, and cultivation time. From the model, it is suggested that sewage sludge could be applied to calcareous soils for 12 yr using the recommended application rate （9 tons ha-1 yr-1）. The field results will be helpful in achieving best management of biosolids application for agricultural production and environmental protection.

Phytoavailability of Copper, Zinc and Cadmium in Sewage Sludge-Amended Calcareous Soils

The toxicity of trace elements （TEs）, such as copper （Cu）, zinc （Zn）, and cadmium （Cd）, often restrict land application of sewage sludge （SS） and there was little information about soil-plant transfer of TEs in SS from field experiments in China. In this study pot and field experiments were carried out for 2 years to investigate the phytoavailability of TEs in calcareous soils amended with SS. The results of the pot experiment showed that the phytoavailability of Zn and Cu in the SS was equal to 53.4%-80.9% and 54.8%-91.1% of corresponding water-soluble metal salts, respectively. The results from the field experiment showed that the contents of total Zn, Cu, and Cd in the soils increased linearly with SS application rates. With increasing SS application rates, the contents of Zn and Cu in the wheat grains initially increased and then reached a plateau, while there was no significant change of Cd content in the maize grains. The bioconcentration factors of the metals in the grains of wheat and maize were found to he in the order of Zn 〉 Cu 〉 Cd, but for the straw the order was Cd 〉 Cu 〉 Zn. It was also found that wheat grains could accumulate more metals compared with maize grains. The results will be helpful in developing the critical loads of sewage sludge applied to calcareous soils.