Page 77 - Read online

Page 10 of 16 Cui et al. Carbon Footprints 2026, 5, 19

Table 5. Regression relationships between N inputs and C inputs and between the increase in the SIC stock over the 16-year
experimental period

Soil layer (cm) Regression equation R 2
0-20 Y = -0.02318 x 1 - 1.08131 x 2 + 0.00307 x 1 x 2 - 0.24864 0.659 (P< 0.01)
20-40 Y = -0.00748 x 1 + 0.41567 x 2 - 0.00165 x 1 x 2 - 10.96561 0.667 (P< 0.01)
40-70 Y = 0.03531 x 1 + 3.707172 x 2 - 0.01262 x 1 x 2 - 30.96895 0.593 (P < 0.01)
70-100 Y = -0.03011 x 1 - 5.08716 x 2 + 0.00614 x 1 x 2 - 6.95607 0.599 (P< 0.01)
Y, changes in the SIC stock (Mg C ha ); x 1 , fertilizer N input (kg N ha ); x 2, organic material carbon input (Mg C ha ). SIC: Soil inorganic carbon.
-1
-1
-1
Effects of intensive farming measures on soil organic and inorganic carbon
Enhancement of SOC is generally achieved through increased organic inputs, such as plant residues, root
exudates and animal manure, as well as through the suppression of SOC mineralization [53-55] . In agricultural
systems, greater straw return and organic fertilization not only increase C inputs but also improve soil
aggregate stability and stimulate root growth and rhizodeposition, thereby creating favourable conditions for
soil organic matter (SOM) formation [18,56,57] . However, the increase in SOC stocks under enhanced straw
return cannot be explained solely by greater C addition; it also reflects changes in the efficiency and pathways
of C transformation and stabilization in soil.

Upon incorporation, straw supplies abundant organic substrates that stimulate microbial metabolism, and
the proportion of residue-derived carbon retained in soil depends largely on microbial assimilation
efficiency. When residue availability is sufficient, a larger fraction of straw-derived carbon is incorporated
into microbial biomass rather than being rapidly mineralized as CO . As microbial turnover progresses, this
2
assimilated C is converted into microbial residues, which exhibit a strong affinity for mineral surfaces and
are preferentially stabilized as mineral-associated OC. This microbially mediated stabilization pathway [58]
provides a mechanistic explanation for sustained SOC accumulation. Moreover, improved aggregation
induced by straw return further enhances the physical protection of newly formed organic matter within soil
aggregates [56,57] , reducing its exposure to decomposition. Through the combined effects of enhanced
microbial transformation, mineral association and aggregate-mediated protection, increased straw return
ultimately improves C retention efficiency and promotes SOC sequestration.

In our study, OPT and NoT received approximately twice as much organic material as FRM and CK did,
which is the primary reason why the first two treatments resulted in a greater increase in the SOC stock. The
CK treatment received no N fertilizer during the 16-year experimental period; thus, the yields of straw, which
were all returned to the farmland, were much lower than those of the three N-fertilized treatments. In
addition to the straw from both crops incorporated into OPT, decomposed animal manure was also
incorporated, supplying N and C simultaneously. The integration of straw with animal manure fertilizer has
long been shown to increase SOC and agricultural productivity effectively, and the findings from the OPT
treatment provide further evidence.

Unlike the SOC stocks, the SIC stocks significantly decreased for all the treatments during the 16 years, and
the decrease in the SIC stocks in the CK and FRM treatments was considerably lower than those in the OPT
and NoT treatments [Figure 1B]. Regression analysis revealed no consistent relationship between changes in
the SIC stock and organic material C input across soil layers [Table 5], suggesting that OC input alone was
not a primary factor regulating SIC dynamics in this system. The potential neoformation of pedo-atmogenic
C was mainly defined by Ca /Mg from irrigation water and minor contributions from fertilizers on the
2+
2+
North China Plain . FRM received 25% more irrigation water during the experimental period and, thereby,
[10]
experienced lower SIC stock loss. As discussed in Section 4.1, N fertilization in croplands in China drives soil
acidification; no N input in the CK reduces acidification, carbonate dissolution, and loss.