Isotopic characterization of sequestration and transformation of plant residue carbon in relation to soil aggregation dynamics

Abstract

Soil aggregates play a key role in preserving soil organic carbon (SOC). However, the mechanisms controlling aggregate formation and SOC distribution in different sizes of aggregates remain unclear. Here, we studied the dynamics of aggregate formation and associated SOC transferring among different aggregate fractions using an isotopic tracer technique (14C-labeled wheat residues). The year-long incubation results demonstrate that the wheat residue carbon applied to soil (Typic Hapludoll) was initially mainly stored in the pores between microaggregates that were agglomerated to form macroaggregates (>250μm). The wheat carbon started to transfer to the inside of microaggregates (250–53μm) after six months of the incubation. The newly formed humic acid carbon (HA14C) and fulvic acid carbon (FA14C) in the macroaggregates were 57–90% and 60–84% more than in microaggregates (250–53μm), respectively. Later, the sequestrated SOC was decomposed or chemically transformed, resulting in a decrease in the ratio of HA14C/FA14C in macroaggregates and an increase in the silt/clay fraction (<53μm). This result indicated that the SOC in the macroaggregates was vulnerable to degradation due to less protection by soil macrostructure, while it experienced a slow degradation due to strong surface adsorption or pore protection in soil microstructure. This study suggests that macroaggregates considerably control SOC turnover and thereby their stability considerably influences how much newly introduced organic carbon could be sequestrated into stable carbon pools like microaggregates. The turnover dynamics of macroaggregates provided insights into the potential of humic carbon formation that facilitates long-term carbon preservation in soil.