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Literature notes of Fontaine et al.(2011)

已有 2858 次阅读 2012-2-17 17:04 |系统分类:科研笔记

Literature notes of Fontaine et al.(2011) – Fungi mediate long term sequestration of carbon and nitrogen in soil through their priming effect

 

Shan Xu

 

A. Background of this study:

What is the importance of microbes in mediating SOM dynamics? Many biogeochemical models generally ignored the significance of microorganisms on the carbon and nutrients dynamics due to their high adaptability to environmental changes (Powlson et al., 1996) . However, some scientists suggested less than 2-3% of SOM compounds are colonized by microbes (Jenkinson et al., 1976). And the supply of labile carbon (plant litter and root exudates) stimulates microbial populations and degradation of SOM (Cheng et al., 2003; Kuzyakov et al., 2009). Therefore, the effects of microbes on SOM dynamics are most likely to enhanced due to the elevated microbial populations and increased proportion of microbial colonization on SOM compounds, which are derived from fresh energy-rich C supply.

   Then what are the mechanisms of this microbial limitation for ecosystem C and nutrient dynamics? A bank mechanism was suggested by recent theoretical work (Fontaine & Barot, 2005). In brief, microbial decomposition of recalcitrant SOM is mediated by the initial concentration of available soil nutrients. When soil is nutrient-limited, microbial mining of SOM would increase; in contrast, when soil nutrients are rich, microbial mining of SOM would decrease and it leads to a greater sequestration of nutrients in SOM. We can consider SOM as a pool of nutrients, microbe as a machine for releasing the nutrients from SOM pool, and fresh energy-rich C as the ignition of the machine.

 

B. Hypothesis of this study

   This study hypothesized that, i. the microbial degradation of SOM is stimulated by the fresh C-supply and, ii. the intensity of microbial degradation of SOM is controlled by the availability of soil nutrients and, iii. the dynamics of SOM pool are determined by the intensity of microbial degradation.

C. Specific actions of this study:

Fontaine et al. (2011) conducted a soil incubation of 161 days to investigate the existence of such a nutrient bank mechanism. The experimental design is a two-way factorial design with low and high nutrient treatments, with and without fresh C addition. The measured variables are cellulose-originated microbial biomass and soil-originated microbial biomass,  cellulose-derived CO2 efflux and soil-derived CO2 efflux (for PE measurement), soil mineral nitrogen and immobilized nitrogen, soil organic reserve, and soil microbial community structure. They used a technique based on dual labeling of the added fresh C. 13C-labeled cellulose is used for calculations of priming effect and 14C-labeled cellulose is used for calculations of changes in soil organic reserve. FAMEs and B- and F-ARISA were used for characterization of the soil microbial community.

 

D. Whether the results have verified the hypothesis:

   This study found the supply of fresh labeled C led to over-production of unlabeled CO2, and this cellulose-induced CO2 was derived from recalcitrant SOM, not from an acceleration of microbial turnover, which confirmed the first hypothesis.

   Cellulose-induced PE was strongly decreased when availability of mineral nutrients for soil fungi increased, which verified the second hypothesis.

   This study also found the content of mineral N was lower in soils with cellulose than that in soils without cellulose due to the immobilization of N by cellulolytic microbes. The incorporation of C decreased the sequestration of C in stable soil organic reserve in the low nutrient treatment; in contrast, the incorporation of C increased the sequestration of C in stable soil reserve in the high nutrient treatment. These results verified the third hypothesis.

   This study further suggested fungi are the predominant actors of cellulose decomposition and induced PE. Because both of the methods for characterization of the soil microbial community indicated fungal community was strongly affected by the supply of cellulose while bacterial community was not significantly affected by the supply of cellulose.

   All of these results supported the existence of a bank mechanism.

 

E. Gains from this study:

1. This study confirmed the critical role of microbe in the prediction of SOM dynamics.

2. This study also supported the existence of a bank mechanism that regulated nutrient and carbon dynamics in soil (see details above). 

3. Based on the dominant role of fungi in decomposition of polymerized fresh C, which confirmed in this study, agricultural practices with reduced fungi populations as a result of physical disturbances may increased the “leaky” nutrient cycling in cultivated soil.

4. 13C stable isotope method is frequently used to partition the origin of soil CO2 efflux. It can be realized by using 13C labeled litter or 13C labeled soil, which can separate litter-derived CO2 from SOM-derived CO2 based on mass balance equations. This study used cellulose extracted from a uniformly 13C labeled wheat straw. Differently, the 13C labeled cellulose was also labeled by 14C, which allowed them to calculate the MRT (mean residence time) of soil organic pool decomposed via the PE.

Although this dual labeling method is effective to explore the process and mechanism of PE in a laboratory incubation, it is difficult to apply 14C labeling, whether continuous labeling or pulse labeling, in field experiments. Because continuous 14C labeling requires special equipments and pulse 14C labeling does not uniformly label all plant C (Kuzyakov, 2006). 14C labeling rarely used in field experiments is also due to its strong radioactivity, which is related to safety issue. In field experiments, natural 13C tracer method are often used, i.e. either C3 plant or litter with C4 soil, or C4 plant or litter with C3 soil (Cheng et al., 2003).

5. This study ascribed the N-induced reduction of PE to microbial competition (r-strategist microbes & K-strategist microbes), i.e. r-strategist microbes, which immobilize soluble nutrients and only decompose cellulose, dominate under high N conditions, and K-strategist microbes, which could mine SOM, dominate under N-limited conditions. Whether this theory can explain the reduction of soil respiration under N enrichment or could be one of the mechanism accounting for that?

 

                                                     Jan 2nd, 2012

 



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