The Mechanism of Hydrophobic Protection of SOM to Sequester Carbon in Soil

The recognized importance of hydrophobicity in stable SOM has a relevant implication in soil carbon sequestration. In fact, the hydrophobic character of OM represents a biochemical hindrance to microbial decomposition (Piccolo et al. 1999;

Spaccini et al. 2000), the basis for a persistent soil aggregate stability (Piccolo and Mbagwu 1999), and an overall SOM stabilization (Rumpel et al. 2004; Winkler et al. 2005; Zhou et al. 2010). The recalcitrant hydrophobic molecules are the constituents of the stable and humified SOM fraction (Piccolo 1996; Grasset et al. 2002; Deport et al. 2006), that enters in intimate association with fine soil particles, such as clay minerals and Fe and Al hydroxides, thus contributing to highly stabilize soil organo-mineral complexes (Mikutta et al. 2006; Schoning and Kogel-Knabner 2006; von Lutzow et al. 2006).

Furthermore, the porous architecture of hydrophobic domains of soil humus exerts a dynamic mechanism of hydrophobic protection toward the biolabile organic compounds released in soil solution by plant roots exudates and microbial degradation of crop biomolecules. It was experimentally verified by measuring the reduced degradation of 13C-labeled compounds in soils amended with humified matter at different degree of hydrophobicity (Spaccini et al. 2002). These authors synthesized a 13C-labeled 2-decanol as a model of an easily degradable molecule in soil. They partitioned the labeled molecule into solutions of two humic acids, one from compost (HA-C) and one from lignite (HA-L), of different degrees of hydrophobicity. The two labeled humic solutions and one solution containing only the labeled 2-decanol (soil + 13C) were added to a soil and incubated at field capacity for 3 months. The treated samples and a control soil were sampled periodically and the 13C content was measured by high-resolution mass spectrome-try. It was found that the biolabile 13C-labeled 2-decanol was protected from mineralization when incorporated into the hydrophobic domains of the HS. The highly hydrophobic and more aromatic humic acid from lignite was more effective than the one from compost in sequestering the carbon from 2-decanol. After incubation, the residual 13C-labeled OC recovered in bulk soil was equal to 28, 45, and 58% of the original content for samples containing the labeled alcohol alone or with HA from compost and lignite, respectively.

The same experiment by Spaccini et al. (2002) also followed the 13C-OC distribution in the particle-size fractions of the treated samples. The residual 13C-OC among soil particle sizes indicated that the hydrophobic protection was most effective in the silt- and clay-sized fractions (Fig. 1.4). This result confirms the importance of associations between fine textural fractions and microbially recalcitrant OM and suggests that SOM accumulation due to hydrophobic protection preferentially occurs within organo-mineral association of finer soil particles. Nevertheless, hydrophobic sequestration of carbon in soil may also take place within larger size fractions, provided that humified matter of large hydrophobic character is applied. In fact, the highly hydrophobic HA from lignite was able to reduce OC decomposition, with respect to treatments with HA from compost and 13C-2-decanol alone, even in the coarser fractions which are commonly associated with rapid cycling of SOM pools.

Exogenous organic matter (EOM), such as mature compost added to soils, may also be capable of reducing the biological mineralization of labile polysaccharides due to progressive incapsulation into hydrophobic domains of compost. In a long-term (1 year) experiment, Piccolo et al. (2004) treated both a sandy and a silty-loamy

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co160 1?

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Fig. 1.4 Variation in comparison to time 0 of 13C-SOM content in soil particle-size fractions according to treatments (13C-2dec., treatment with only 13C-labeled 2-decanol; 13C-HAC, treatment of HA from compost previously added with 13C-labeled 2-decanol; 13C-HAL, treatment of HA from lignite previously added with 13C-labeled 2-decanol). Bars in graph indicate standard deviation (n = 3)

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Fig. 1.4 Variation in comparison to time 0 of 13C-SOM content in soil particle-size fractions according to treatments (13C-2dec., treatment with only 13C-labeled 2-decanol; 13C-HAC, treatment of HA from compost previously added with 13C-labeled 2-decanol; 13C-HAL, treatment of HA from lignite previously added with 13C-labeled 2-decanol). Bars in graph indicate standard deviation (n = 3)

0 2 12 Incubation time (weeks)

0 2 12 Incubation time (weeks)

soil with a mature compost before and after addition of a labile polysaccharide to verify whether compost was able to reduce mineralization of the biolabile material. Mature compost induced a significant reduction of OC losses in both soils, thus confirming that labile organic matter in soils can be protected from biodegradation by repartition into the hydrophobic domains of the stable, humified organic matter. This study suggests that mature compost and humic acids may usefully integrate management practices aimed to sequester organic carbon in soils.

Thus, amendments of mature compost to soil is expected not only to improve the quantitative and qualitative status of SOM (Adani et al. 2006; Shindo et al. 2006; Spaccini et al. 2009), but to also increase the content of humified and hydrophobic organic components (Spaccini and Piccolo 2007; Caricasole et al. 2011), which can contribute to reduced OC mineralization and turn the soil to be a sink of organic carbon (Spaccini et al. 2002; Fortuna et al. 2003; Piccolo et al. 2004; Lal 2009).

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Organic Gardeners Composting

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