Utilization of Soil C and N by Microbial Groups in the Presence of Living Roots
Abstract
The effects of living plant roots and N on belowground C dynamics were examined in a CA annual grassland soil (Haploxeralf) during a 2-y greenhouse study. The fate of 13C-labeled plant roots ( Avena barbata L.) and soil were followed under planted and unplanted conditions; and with and without N addition (20 kg N ha-1 season-1). The treatments were applied during 2 growing seasons and each growing season was followed by a dry, fallow period (~ 150-d long). Living roots increased the turnover rate and loss of belowground 13 C during and after 2 seasons compared with unplanted soils. After 2 seasons, planted soils had 21% less belowground 13C present than in unplanted soils. However, total soil C increased in planted soils by 4.6% compared to unplanted after 2 seasons. N additions decreased belowground 13C turnover during the first treatment season in both planted and unplanted soils, however no effect of N on soil C was observed thereafter. Planted soils had larger microbial biomass and the community structure differed compared with unplanted soils. Planted soils had higher proportions of gram (-) bacteria, while unplanted soils had higher proportions of gram (+) bacteria, actinomycetes, and fungi. New root and exudate C supplied from living roots increased the turnover of microbial assimilated 13C compared with unplanted for all microbial groups. This greater turnover of belowground 13C was especially significant for gram (-) bacteria, which were stimulated in the planted soil. In contrast, the activity among microbial groups in unplanted soils was similar to that prior to the initiation of the treatments and soil wet-up. Our findings suggest that A. barbata roots increased soil C levels over time because root and exudate C inputs are significant, however that C increase will be moderated by an overall faster C mineralization rate of belowground C. Increased N deposition may slow soil C losses, however, they appear minor and temporary at the rates applied and for the plant-soil system studied.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.B13B1183B
- Keywords:
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- 0428 Carbon cycling (4806);
- 0454 Isotopic composition and chemistry (1041;
- 4870);
- 0465 Microbiology: ecology;
- physiology and genomics (4840);
- 0469 Nitrogen cycling