Aggregate dynamics and associated soil organic matter in topsoils of two 2,000-year paddy soil chronosequences

Paddy chronosequence provides a unique way to investigate soil development. This study investigated dynamic profiles of aggregate and soil organic matter in topsoils of two 2,000-year paddy chronosequences. Based on an investigation of paddy soil surveys on the south bank of Hangzhou Bay, the paddy soil chronosequence derived from marine sediments in Shangyu City (5- to 2,000-year (yr) rice cultivation history) was newly identified, while the paddy soil chronosequence with similar parent materials in Cixi City (1- to 2,000-year rice cultivation history) was previously identified but with several paddy chronosequence sites modified in this work. Soil aggregates in topsoils of the two paddy soil chronosequences were subsequently fractioned using wet sieving method. The total organic carbon (TOC) and total nitrogen concentrations of bulk soil samples and the water-stable aggregate fractions were directly measured by dry combustion method or analyzed by Elementar cube after treatment with HCl in young paddy field. The different soil textures in two paddy chronosequences were supposed to be attributed to the different origins of deposits. During the first 300 years of rice cultivation history, the distribution within the aggregate classes changed dramatically, with the proportion of macroaggregates increasing significantly from 34.3 to 75.7 % (Cixi) and from 19.8 to 60.7 % (Shangyu) as the paddy soil age increased. TOC in all soil water-stable aggregate fractions increased as the rice cultivation time increased. The macroaggregate fraction (especially in the 500-2,000-mu m fraction) contained more organic carbon than the microaggregate fraction. This study determined that paddy fields with no less than 1,000-year histories had higher soil organic carbon (SOC) concentration than the young paddy fields with no more than 100-year histories. Soil decalcification and increased application of nitrogen fertilizer led to a significant chronological decrease of soil pH value in the topsoils of the two paddy soil chronosequences in which the rice cultivation time increased. Physical entrapment of SOC in macroaggregates may account for SOC sequestration even in paddy soils with 2,000-year history. The young paddy fields have a great potential for SOC sequestration.