Chemical and microbial compositions of SOM
To characterize the contributions of litter residues to SOM, we measured
the chemical composition of original litters and post-incubation SOM
using solid-state 13C nuclear magnetic resonance
(13C NMR). The model soils were pretreated with 2%
hydrofluoric acid to concentrate the SOM and reduce the interference in
NMR spectra from paramagnetic materials such as Fe(III).
The 13C NMR spectra of the SOM samples displayed the
same types of functional C groups as those in the original litters, but
in varying proportions (Fig. 2a). The relative abundances of anomerics
and O-alkyls from carbohydrates decreased and those of other functional
groups increased compared to the original litters, regardless of litter
or clay mineral type (Supplementary
Table 3). The chemistry of litter-derived SOM differed between litter
types in each model soil and among the model soils for either litter
type (Fig. 2b). All the SOM samples from soya litter had higher
abundances of N-containing compounds (NCH/OCH3 and
COO/N–C=O) and less aromatics and aromatic C–O groups than those from
maize litter. For either litter type, the SOM had higher relative
abundances of aromatics and aromatic C–O groups in the illite soils
than in the vermiculitic soils. The litter type effect on the NMR
signals for newly forming SOM was less for illite soils than for
vermiculitic soils. These findings suggest that litter and clay mineral
types may interactively affect the chemical composition of SOM, rather
than litter type alone, and that different clay minerals may
discriminatively protect labile and recalcitrant litter residues to
different degrees.
To characterize microbial contributions to SOM, we measured
post-incubation phosphorous lipid fatty acids (PLFAs) and amino sugars
as indicators of microbial biomass and necromass of different functional
communities, respectively. The total amounts of PLFAs and amino sugars
accounted for 1.2-1.8 g-C kg-1 litter C and 3.0-8.5
g-C kg-1 litter C, respectively and were affected
inconsistently by litter or clay mineral types (Fig. 3). The total
amount of PLFAs was not different among all eight soils except for the
vermiculitic soil, which had the lowest total PLFAs for maize litter and
the highest total PLFAs for soya litter (Fig. 3a). The total amount of
PLFAs was dominated by bacterial PLFAs in all eight soils. In contrast,
the total amount of amino sugars was greatest in the kaolinite soils,
followed by the vermiculitic soils, and least in the illite and mineral
s for both litter types (Fig 3b). The total amount of amino sugars was
dominated by fungal amino sugars in all eight model soils except the
illite soil mixed with soya litter. The amount of fungal amino sugars
was greatest in the vermiculitic soils, followed by the kaolinite soils
and the natural mineral soils, and least in the illite soils for both
litter types. The vermiculitic soils contained solely fungal amino
sugars. This trend suggests that different clay minerals types may also
discriminatively protect fungal and bacterial residues to different
degrees.