Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 84

fraction 5 is the residual organomineral component. This can be divided
further on the basis of particle size.

Chemical composition of solid fractions

Chemical characterization of the solid fractions (fractions 3, 4 and 5 above)
was done by 13 C cross-polarization magic angle spinning (CPMAS) NMR.
Material in the organomineral component was divided further on the
basis of size into sand, silt and clay fractions. Full experimental details are
published elsewhere (Gaunt et al ., 1999). Figure 2.6.1 shows the chemical
composition of the solid organic matter fractions separated by our
fractionation procedure and that of whole soil, using the 13 C CPMAS
NMR technique (Sohi et al ., 1998). Carbon in free organic matter was
located predominantly in O -alkyl structures. Intra-aggregate organic matter
contained a lesser proportion of C in O -alkyl groups, and more in aromatic
and alkyl groups. Lowering of O -alkyl to alkyl ratios is characteristic of
the early stages of the degradative process (Preston, 1996). This is
attributable to the transformation of readily metabolizable carbohydrates,
and production and persistence of biomass- and plant-derived alkyl C. Our
analysis suggests, therefore, that the free fraction represents less altered
organic matter (i.e. closer to the composition of incoming plant material) as
compared with the intra-aggregate.
The distribution of NMR-visible carbon for the clay organomineral
fractions appears more similar to that of intra-aggregate rather than free
organic matter (with slightly greater proportions of carbonyl C, less
aromatic and phenolic C). The observed differences between the free and
the intra-aggregate and organomineral fractions are potentially important,
since the chemical properties of a particular fraction will affect its reactivity.
We have found that these differences are consistent across soil type and
climate in soils taken from seven long-term experiments, where cereals are
grown under mineral fertilization (Gaunt et al ., 1999).

Isotope Tracing Through Fractions

If the proposed fractionation protocol and associated model is to be used
to predict C and N fluxes over a single crop-growing season, we need to
measure, and predict, fluxes of C and N over a period < 1 year. To establish
the sensitivity of our fractionation protocol, we set up incubation studies
using 13 C and 15 N tracers. To trace C, we used a 13 C natural abundance
technique. The stable isotope composition of plants differs depending on
the type of photosynthetic pathway (C 3 and C 4 ) of the plant. This results in
a difference in the

13 C content of 12-14‰, with C 4 plants being more