Aggregate separation method (1)

Shan Xu

In the review of Kuzyakov (2010) suggested, “Only a few studies have evaluated the contribution of individual stabilization mechanisms to the extra CO₂ released during PE”. In addition, Janssens et al. (2010) suggested “If nitrogen additions were to interact with these stabilization mechanisms, decomposition and heterotrophic respiration could well be affected.” Then, how can the stabilization mechanisms of soil organic matter be defined? More than three definitions were concluded in the review of von. Lutzow et al. (2006):

      1) Selective preservation: which process leads to the relative accumulation of recalcitrant molecules, including primary recalcitrance and secondary recalcitrance.

      2) Spatial inaccessibility: the spatial localization of OM that influences access by microbes and enzymes.

      3) Interactions with surfaces and metal ions: inter-molecular interactions between OM and either inorganic soil components or other     OM that alter the rate of decomposition.

Other definitions…

One of the mechanisms that reduce soil OM accessibility is occlusion of OM by aggregation.

Aggregate hierarchy theory was suggested by Tisdall & Oades. (1982):

Primary mineral particles------(c)persistent-------> microaggregates--------(b) temporary & (a) transient---------->macroaggregates

 1)  The organic binding agents have been classified into (a) transient, mainly polysaccharides, (b) temporary, roots and fungal hyphae, and (c) persistent, resistant aromatic components associated with polyvalent metal cations, and strongly sorbed polymers (Tisdall & Oades, 1982).

  2) Aggregate size class (um):

<53um(Microaggregate, M2)

53-250um (Microaggregate, M1)

250-2000um (Macroaggregate, A2)

>2000um (Macroaggregate, A1)

   3) Three consequences of this aggregate hierarchy are:

(i) a gradual breakdown of macroaggregates into microaggregates before they dissociate into primary particles, as increasing dispersive energy is applied to soil;

(ii) an increase in C concentration with increasing aggregate-size class because large aggregate-size classes are composed of small aggregate-size classes plus organic binding agents;

(iii) younger and more labile organic matter is contained in macroaggregates than in microaggregates.

The mechanisms that occluded OM is spatially protected against decomposition included:

      reduced access for the microorganisms and their enzymes, which control further food web interactions;

      reduced diffusion of enzymes into the intra-aggregate space;

      restricted aerobic decomposition due to reduced diffusion of oxygen (von. Lutzow et al., 2006)

Then how do nitrogen additions interact with soil OM aggregation?

How can we use aggregate separation method to test the hypothesis that nitrogen additions suppress the decomposition of soil organic carbon through occluding OM by aggregation?

The proportion of every aggregate size class

The total organic C content of every aggregate size class

……

                            Jan.15, 2012