Soil organic carbon (SOC) pool not only acts as a buffer against the rising atmospheric CO2 but also plays an important role in sustaining the ecosystem productivity. Physiochemical protection of decomposing plant materials plays a critical role in the retention of soil organic matter, particularly in warm, humid tropical and subtropical regions. Red earth soils, characterized by high content of iron oxides and low content of SOC, cover over 204 million hectares in tropical and subtropical China. Understanding the mechanisms underlying physical and chemical stabilization of plant residues would help design management practices that facilitate soil C sequestration. We designed two incubation experiments to examine the impacts of litter quality as well as litter quantity and size on organic C retention in a typical red-earth soil in southeast China. While dual labeled (13C / 15N) corn leaf and root residues (0.2 mm length) were mixed into the soil in the 1st experiment (170 days), different quantities (0, 1 %, 2 % and 3 % residues to soil, w/w) or sizes (5 mm and 0.2 mm) of corn residues were incorporated into soil in the 2nd experiment (105 days). An array of soil and microbial parameters were quantified, including microbial biomass and activities, CO2 losses, and C distribution in different density fractions.
Results obtained showed that leaf and root materials were differentially retained in different size fractions. While more leaf materials were retained in the clay fraction, more root materials were found in the silt and sand fractions, suggesting that faster turnovers of leaves than roots by microbes. Surprisingly, the size of residue materials did not significantly affect the microbial respiration rate and the cumulative C losses. However, significantly higher dissolved organic C (DOC), formation of organic complexed iron and C accumulation in the heavy fraction were observed in the soil amended with fine (0.2 mm) than coarse (5 mm) residues. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) analysis further showed that smaller size of residues promoted stabilization of aliphatic and carboxylic compounds associated with iron oxide. In addition, correlation analysis revealed that the content of organic C in the clay fraction was positively correlated with DOC. Together, these results suggest that limited contacts between organic materials and soil minerals may hinder C sequestration in red-earth soils and incorporation of organic residues into the soil profile may facilitate the formation of organic-mineral complexes and the stabilization of organic C.