Mineral-protection strength
We assumed that decomposing litter residues within mineral matrices
could be partitioned into free (not protected) and mineral- protected
litter pools (through mineral-organic association and aggregation).
During litter decomposition, the free litter pool would be transformed
into the mineral-protected litter pool, which could continue to
decompose, but at a slower rate than the free litter pool. Such mineral
protection would alter total soil respiration and relative contributions
from these pools. The stronger the mineral protection, the lower the
total soil respiration and the smaller the contribution from the
mineral-protected litter pool. Based on this feedback effect of mineral
protection on soil respiration, we developed a novel mineral-mediated
decomposition model (Eq. 1) to quantify mineral-protection strength,δ , for a specific mineral/soil:
\(C_{t}=\frac{1}{\delta}(1-e^{-k_{1}t})+(100-\frac{1}{\delta})(1-e^{-k_{2}t})\)(1)
where, Ct is the cumulative soil respiration (%
of litter C) at time t ; and k 1 andk 2 are the decomposition rate constants of the
mineral-protected and free litter pools, respectively. The two pools sum
to 100% of total soil respiration at any time.
Our model well (R2 > 0.96)
described the dynamics of cumulative soil respiration for all the soils
(Fig. 6a). The simulated mineral-protection strength ranged from 0.18 to
0.49 (Supplementary Table 2) and was affected by litter and clay mineral
types and their interaction. We observed that the vermiculitic soils had
larger mineral-protection strength than did the illite and kaolinite
soils for both litter types, and that the mineral-protection strength
was larger for maize litter than for soya litter in all model soils
except for the vermiculitic soil. The the vermiculitic soil had lower
protection strength for maize litter than for soya litter.
We found that the mineral-protection strength explained 67% to 96% of
the variation of the formation efficiency of SOM (Fig. 6b). In addition,
the mineral-protection strength of the model soil consisting of multiple
natural soil clay minerals (δ soil) was predicted
well from the mineral-protection strengths
(δclayi ) and the relative
abundances (%clayi) of the
compositional clay minerals in the soil using a multilinear regression
equation (Eq. 2). The absolute root-mean square error (ɛ) of the
simulation was < 0.05,
δ soil=∑%clayi *δclayi + ɛ (2)
The relative abundances of the clay minerals in the natural soil
material quantified by using the decomposition method of X-ray
diffraction spectra31 were 6% for kaolinite, 47% for
illite, 21% for vermiculite and 26% for a mixed layer mineral of
illite and vermiculite. The average of the mineral-protection strengths
of illite and vermiculite was used as the mineral-protection strength
for the mixed layer mineral of illite and vermiculite for either litter
type. As such, the mineral-protection strength of the mixed layer
mineral was the largest source of simulation error for either litter
type.