Abstract
Physical protection of soil carbon (C) is one of the important components of C storage. However, its exact mechanisms are still not sufficiently lucid. The goal of this study was to explore the influence of soil structure, that is, soil pore spatial arrangements, with and without presence of plant residue on (i) decomposition of added plant residue, (ii) CO2 emission from soil, and (iii) structure of soil bacterial communities. The study consisted of several soil incubation experiments with samples of contrasting pore characteristics with/without plant residue, accompanied by X-ray micro-tomographic analyses of soil pores and by microbial community analysis of amplified 16S–18S rRNA genes via pyrosequencing. We observed that in the samples with substantial presence of air-filled well-connected large (>30 µm) pores, 75–80% of the added plant residue was decomposed, cumulative CO2 emission constituted 1,200 µm C g-1 soil, and movement of C from decomposing plant residue into adjacent soil was insignificant. In the samples with greater abundance of water-filled small pores, 60% of the added plant residue was decomposed, cumulative CO2 emission constituted 2,000 µm C g-1 soil, and the movement of residue C into adjacent soil was substantial. In the absence of plant residue the influence of pore characteristics on CO2 emission, that is on decomposition of the native soil organic C, was negligible. The microbial communities on the plant residue in the samples with large pores had more microbial groups known to be cellulose decomposers, that is, Bacteroidetes, Proteobacteria, Actinobacteria, and Firmicutes, while a number of oligotrophic Acidobacteria groups were more abundant on the plant residue from the samples with small pores. This study provides the first experimental evidence that characteristics of soil pores and their air/water flow status determine the phylogenetic composition of the local microbial community and directions and magnitudes of soil C decomposition processes.
Highlights
Soil is a critical component of terrestrial ecosystems, harboring enormous microbial diversity, moderating the decomposition cycle and greatly influencing atmospheric CO2 concentrations [1,2]
Since ground samples were identical in their chemical composition and mineralogy to their intact aggregate fraction counterparts, we can safely conclude that the differences in pore characteristics among the intact aggregate fraction samples are the most likely cause for the differences in residue decomposition observed in this study
The results of this study demonstrated that soil pore structure greatly influences the magnitude of decomposition of the added plant residue, likely simultaneously affecting the decomposition of the native Soil organic matter (SOM)
Summary
Soil is a critical component of terrestrial ecosystems, harboring enormous microbial diversity, moderating the decomposition cycle and greatly influencing atmospheric CO2 concentrations [1,2]. Soil structure defines sizes and characteristics of soil pores, creating physical micro-environments for microbial activities and playing a major role as both an arena and a product of SOM stabilization and dynamics [5,6,7,8]. Microorganisms are vital actors of soil systems. Their ability to move and grow in a structurally and chemically heterogeneous soil environment governs belowground processes of biomass transformation into SOM and defines its further fate, e.g., stabilization within soil matrix or loss to the atmosphere as emitted CO2 [9]. Pores control micro-scale soil air, water, and nutrient fluxes; influence micro-environmental conditions for microbial functioning, e.g., [16,17,18]. It has been shown that micro-environmental conditions may exert even a stronger influence on SOM decomposition than microbial community composition [19]
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