Chronosequence of paddy soils and phosphorus sorption–desorption properties

Abstract

Accumulated soil phosphorus (P) in agricultural lands due to long-term organic manure and mineral phosphate fertilizer input is considered one of the main non-point pollution sources to surrounding surface water bodies. A chronosequence of soils is a potent instrument for pedological investigation and allows assessment of the effect of duration of agricultural cultivation on the environmental risk to water bodies of P loss from soil. The present study was conducted to evaluate the impact of pedogenesis on the retention of P in a chronosequence (25, 100, 300, 700, and 2,000 years) of rice paddy soils located on the south bank of Hangzhou Bay. Batch sorption–desorption experiments were performed on the samples of surface horizons from this chronosequence. P retention differed moderately among the studied soils ranging from 17.8% to 22.6% when 2,500 mg P kg−1 was added. Subsequent desorption experiment revealed that 11.2% to 21.9% of the recently sorbed P might move into solution. The maximum P sorption capacity as estimated by Langmuir isotherm ranged from 521.9 to 850.9 mg kg−1, with a mean value of 726.2 mg kg−1, while the degree of P saturation was less than 5%. The results indicate that P sorption is influenced by soil development with the maximum sorption capacity of studied soils significantly positively correlated to the organic carbon content of these soil materials. The pH of the soil decreased from alkaline to neutral with increasing duration of rice cultivation due to continuing decalcification. The organic carbon content in the paddy surface soil materials increased with increasing duration of rice cultivation. The maximum P sorption capacity also slightly increased with increasing duration of rice cultivation likely due to organic carbon accumulation in topsoils. There is a very low risk of P in these paddy soils moving to the surrounding surface water bodies. This study of some possible long-term impacts helps to indicate that rice paddy agriculture is an environment friendly and sustainable land use.