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Appendix C. Testing soil texture effects on soil respiration rates with validation dataset
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Prior to our validation, we examined the relationship between soil texture and
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respiration rates in our validation dataset. The soil respiration rates (Cmin) defined in
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Equation (6) in the main text can be simplified with assumptions of linear substrate
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consumption kinetics (Equation (1) in the main text), no C:N stoichiometry effects, and
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constant microbial growth efficiency 0.45:
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Cmin  (1  0.45)  DECS1  (1  0.45)  DECS2
 0.55   kS1,LIN  CS1  kS2,LIN  CS2 
Using the ki,LIN values approximated in Appendix S1, Cmin is rewritten as:

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Cmin  0.55  Td  Md  0.02  (1.4  Tex 2  3.6  Tex  2.3)  CS1  2.557 104  (1  0.00867  Clay )  CS2
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Assuming that CS1 is 1% of CTOT, CS2 equals to CTOT, no reduction effects by soil temperature
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and moisture (i.e. Md=Td=1), and clay effects on decomposition of S2 to be ignorable (i.e. 1-
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0.00867·Clay ≈ 1), Cmin is further simplified as:
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Cmin  0.55  CTOT  0.01 0.02  (1.4  Tex 2  3.6  Tex  2.3)  2.557 104
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Thus, the relationship between soil texture and measurable soil data (Cmin, CTOT) is expected
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to be:
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Cmin
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This shows that CENTURY model assumes an almost linear decrease in Cmin/CTOT as Tex
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increases from 0 to 1.

CTOT

 0.55 104   2.8  Tex 2  7.2  Tex  7.157 
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Contrary to our expectation, soil clay plus silt content was positively related to
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Cmin/CTOT (Spearman’s rank correlation ρ=0.299, P<0.001, Figure S4). This indicates that
1

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the assumption of texture effect in CENTURY (i.e. decomposition rate decreases with
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increasing clay and silt) is not appropriate, at least not for our dataset, possibly due to
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additional differences in soil organic matter quality co-varying with texture in our soils.
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Although stabilization effects of clay prevail on a global scale [Colman and Schimel, 2013;
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Schimel, 1994; Six et al., 2006], the functions of texture effects in CENTURY are based on a
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few empirical observations only. Also, a model validation study in Great plain revealed
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systematic overestimations of SOM pool in fine-structured sands [Parton et al., 1987],
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implying that the texture effect in CENTURY may be too strong. More detailed information
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on clay mineralogy, which is not considered in our study nor in CENTURY, might be
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necessary to better reflect stabilization mechanisms of SOM in mineral soils [Colman and
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Schimel, 2013; Cotrufo et al., 2013]. Therefore, to avoid that texture effects will adversely
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impact modeled relations, we employed an arbitrary-chosen fixed value of sand and clay
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content (50% and 5%, respectively) for all sites.
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Figure S4. The relationship of soil respiration rates per gram soil total C (Cmin/CTOT) vs. soil texture (fraction of
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silt plus clay). Symbols are identical to Figure S5.
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Reference
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continental scale, Soil Biology and Biochemistry, 60(0), 65-76.
Cotrufo, M. F., M. D. Wallenstein, C. M. Boot, K. Denef, and E. Paul (2013), The Microbial EfficiencyMatrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic
matter stabilization: do labile plant inputs form stable soil organic matter?, Global Change Biology,
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Parton, W. J., D. S. Schimel, C. V. Cole, and D. S. Ojima (1987), Analysis of Factors Controlling Soil
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Schimel, D. S. (1994), Climatic, edaphic, and biotic controls over storage and turnover of carbon in
soils, Global Biogeochemical Cycles, 8(3), 279-293.
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sequestration in agroecosystems, Soil Science Society of America Journal, 70(2), 555-569.
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