Linkage of aggregate formation, aggregate-associated C distribution, and microorganisms in two different-textured ultisols: A short-term incubation experiment

Abstract

The formation and stability of aggregate is considered to be controlled by both mineral particles and microorganisms. However, the effect of soil texture on aggregate development and especially the associated retention of carbon (C) derived from plant residues remains unclear for Ultisols in which abundant oxides functioning as inorganic adhesive agents are important for aggregation. Further, little is known about whether different microbial populations are responsible for the aggregation process in different-textured soils. In this study, two soils with distinctive textures (sandy loam and clay) were incubated with and without the addition of 13C-labeled Chinese fir leaves for 29 days to examine the dynamics of soil aggregates and the fate of leaf-derived C. This was done by determining the mass, total C and δ13C signatures of four aggregate size classes after 1, 7, and 29 day (d) of incubation. To analyze the role of microorganisms in aggregate formation, regular CO2 efflux, dissolved organic carbon (DOC), microbial biomass carbon (MBC), inorganic nitrogen, hot-water-extractable polysaccharides, and phospholipid fatty acid (PLFA) were measured to represent microbial activity, communities, and growing conditions. The results showed that sandy loam produced larger new macroaggregates (>250 μm) than the clay in the first 7 d. However, differences in mean weight diameter (MWD) and the proportion of macroaggregates were not maintained by the end of the incubation. Fresh leaf-derived C incorporated into different-sized aggregates was not significantly different between the sandy loam and clay at all sampling times. The majority of retained residue-C (56% and 45% in the sandy loam and clay, respectively) was eventually stored in macroaggregate fractions (>250 μm) that were extensively formed by the clustering of microaggregates and silt-and-clay between 1 and 7 d after incubation, which indicated that soil aggregation had a functional control over the occlusion of leaf residue-derived C. The MWD of the aggregates was strongly related to cumulative CO2-C, MBC, DOC, and NO3−−N in both soils (P < 0.001). The biomass of bacteria and gram-positive (G + ) bacteria significantly explained the variation in MWD in the clay (P < 0.001) whereas the fungi and fungal: bacterial (F:B) ratio were significantly correlated with MWD in the sandy loam (P < 0.05). We conclude that soil texture had a negligible effect on macroaggregate formation and leaf-derived C incorporation into aggregates in this short-term incubation experiment. The study underpins the importance of microbial biomass, growth, and activities in structure formation in soils irrespective of the clay content. The distinct abundance of specific microbial groups or the microbial structure between the two prepared soils might be related to asynchronous aggregation process in these Ultisols.