Microbial community composition and carbon cycling within soil microenvironments of conventional, low-input, and organic cropping systems

A. Y. Y. Kong^1*, K. M. Scow², A. L. Córdova-Kreylos³, W. E. Holmes¹ and J. Six¹

Abstract
This study coupled stable isotope probing with phospholipid fatty acid analysis (13C-PLFA) to describe the role of microbial community composition in the short-term processing (i.e., C incorporation into microbial biomass and/or deposition or respiration of C) of root- versus residue-C and, ultimately, in long-term C sequestration in conventional (annual synthetic fertilizer applications), low-input (synthetic fertilizer and cover crop applied in alternating years), and organic (annual composted manure and cover crop additions) maize-tomato (Zea mays – Lycopersicum esculentum) cropping systems. During the maize growing season, we traced 13C-labeled hairy vetch (Vicia dasycarpa) roots and residues into PLFAs extracted from soil microaggregates (53–250 μm) and silt-and-clay (<53 μm) particles.

Total PLFA biomass was greatest in the organic (41.4 nmol g⁻¹ soil) and similar between the conventional and low-input systems (31.0 and 30.1 nmol g⁻¹ soil, respectively), with Gram-positive bacterial PLFA dominating the microbial communities in all systems.

Although total PLFA-C derived from roots was over four times greater than from residues, relative distributions (mol%) of root- and residue-derived C into the microbial communities were not different among the three cropping systems. Additionally, neither the PLFA profiles nor the amount of root- and residue-C incorporation into the PLFAs of the microaggregates were consistently different when compared with the silt-and-clay particles. More fungal PLFA-C was measured, however, in microaggregates compared with silt-and-clay.

The lack of differences between the mol% within the microbial communities of the cropping systems and between the PLFA-C in the microaggregates and the silt-and-clay may have been due to (i) insufficient differences in quality between roots and residues and/or (ii) the high N availability in these N-fertilized cropping systems that augmented the abilities of the microbial communities to process a wide range of substrate qualities.

The main implications of this study are that (i) the greater short-term microbial processing of root- than residue-C can be a mechanistic explanation for the higher relative retention of root- over residue-C, but microbial community composition did not influence long-term C sequestration trends in the three cropping systems and (ii) in spite of the similarity between the microbial community profiles of the microaggregates and the silt-and-clay, more C was processed in the microaggregates by fungi, suggesting that the microaggregate is a relatively unique microenvironment for fungal activity.


Source
Soil Biology and Biochemistry (2010) 43: 20-30
DOI: 10.1016/j.soilbio.2010.09.005 


Author Locations and Affiliations
(1) Department of Plant Sciences, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
(2) Land, Air, and Water Resources Department, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
(3) Marrone Bio Innovations, Inc., 2121 Second Street, Suite 107B, Davis, CA 95618, USA
* Corresponding author, E-mail aykong@ucdavis.edu


Posted December 2010