Effects of Organo-mineral Complexes on Flocculation, Settlement and Vertical Distribution of Bioelements in Soil Suspensions

Abstract

Soil organic matter that could be combined with soil particles into large aggregates, may reduce soil erosion. However, soil organic matter itself is a possible source of water pollution. The objective of the present study was to analyze organo-mineral complexes and their effects on flocculation and settling characteristics of soils. Particle-size fractionation and flocculation/settlement experiment were conducted by using 7 types of soil samples (smectic heavy clay soils with 5 types of agricultural practices, alluvial clay loam soil, and volcanic heavy clay soil). All the samples were taken from paddy fields in Japan. Particle size fractionation showed that short-term (4 years) organic matter application to the smectic soil had increased the number of coarse organic particles. Puddled ponding water in the smectic heavy clay field under 4 years of organic matter application led to the highest levels of total organic carbon and particulate organic carbon among the sampling fields. Therefore, the coarse organic particles were considered to be easily suspended by puddling. The high floatability of coarse organic particles was consistent with their low density and weak association with minerals. Flocculation/settlement experiments showed the significant effect of pH decrease by organic matter application on the settlement of the smectic soil particles. The alluvial soil, volcanic soil, and smectic soils under continuous (4 years and 19 years) organic matter application could form large floes even at 50 mM NaCl, while the smectic soils under conventional tillage, no-tillage, and no-puddling required more than 200 mM NaCl for rapid settlement. Sodium pyrophosphate treatment of the dispersive soils resulted in higher concentrations of carbon and nitrogen in the upper layer (0–4 cm) of their suspension after 8 h of settlement, although the treatment had reduced the organic matter content of the soils. The reduction in the hetero-flocculation of the soil particles with different charge characteristics was considered to be the possible process for the increased dispersibility of the soils by the removal of organo-mineral complexes, because the smectic soil did not show a rapid settlement at the average point of zero salt effect for the whole soil components. Based on the settlement profiles, we estimated that the reduction of nutrient losses (suspended load) from surface ponding water in puddled fields was 74–112 kgC ha−1 and 6–7 kgN ha−1 by 8 h of settlement, and at least 100–157 kgC ha−1 and 9–10 kgN ha−1 by 8 h of settlement with an improvement in the pH and electrolyte concentrations.