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Cui et al. Carbon Footprints 2026, 5, 19 Page 9 of 16

DISCUSSION

Table 4. Effects of different farming practices on the SOC conversion efficiency
Organic material carbon input (Mg C ha ) Increase in SOC stock (Mg C ha ) Conversion efficiency (%)
-1
-1
0-20 cm CK 59.6 -0.24 ± 1.00c -0.40 ± 1.68c
FRM 58.1 5.54 ± 1.12b 9.53 ± 1.93a
OPT 130 10.1 ± 1.80a 7.75 ± 1.38ab
NoT 115 7.28 ± 0.95b 6.35 ± 0.83b
0-40 cm CK 59.6 2.75 ± 2.31c 4.60 ± 3.87c
FRM 58.1 9.80 ± 1.25b 16.9 ± 2.15a
OPT 130 17.2 ± 5.77a 13.2 ± 4.43ab
NoT 115 12.6 ± 2.24ab 11.0 ± 1.96b
0-70 cm CK 59.6 4.99 ± 1.50c 8.37 ± 2.51c
FRM 58.1 16.8 ± 1.75b 28.8 ± 3.01a
OPT 130 25.7 ± 2.94a 19.7 ± 2.26b
NoT 115 19.3 ± 0.97b 16.9 ± 0.85b
0-100 cm CK 59.6 20.31 ± 3.44c 34.1 ± 5.77b
FRM 58.1 27.8 ± 3.41b 47.9 ± 5.86a
OPT 130 39.6 ± 6.82a 30.4 ± 5.23b
NoT 115 30.0 ± 1.47b 26.2 ± 1.28b
The data shown are the mean ± SD (n = 4). Different letters for the same crop indicate significant differences at P < 0.05 among the four
treatments. CK represents the control treatment, FRM represents the local farmer operation treatment, OPT represents the optimized farming
operation treatment, and NoT represents the no-tillage treatment. SOC: Soil organic carbon.

Effects of farming practices on the vertical distribution of soil carbon
Overall, the SOC content decreased with soil depth, whereas the SIC content showed the opposite trend
[Figure 1], which is consistent with the findings of most other studies conducted in calcareous soils [41-43] .
When the SOC and SIC are integrated, the TC is more strongly controlled by the SOC in the upper soil
layers (0-70 cm) and by the SIC in the subsoil (70-100 cm). The SOC content in the OPT treatment was
greater than that in the other three treatments across all soil layers, indicating the importance of the subsoil
organic C stock . In addition, under appropriate conditions, such as the high and frequent water input in
[44]
the current study (2~4 times the amount of irrigation plus 600 mm of precipitation), high water leaching can
occur , and soluble OC and dissolved organic matter can be leached downwards into deeper soil layers [45,46] .
[45]
Both the annual SOC sequestration rates in Table 1 and the CEs in Table 4 doubled with every 20 cm of
depth, highlighting that for farmland organic C sequestration, the amount of soil (20/30 cm depth) is far less
than enough and that subsoil should also be considered [47,48] .

The downward transport of dissolved inorganic carbon may occur under long-term irrigation. In this study,
the SIC decreased in the upper layers but was relatively high at 70-100 cm, especially under the N-fertilized
treatments, which is consistent with the enhanced carbonate dissolution and leaching reported in intensively
managed calcareous soils [27,49] . N fertilization induces soil acidification through nitrification, the release of H⁺
and increased carbonate solubility [27,50] , whereas tillage may increase soil CO partial pressure and further
2
promote carbonate dissolution and migration . Dissolved Ca /Mg and HCO /CO may then move
2+
2+
[51]
2-
-
3
3
downwards with percolating water and contribute to secondary carbonate formation in deeper soil
layers [10,11,41] . Although these pathways cannot be directly verified here, the observed SIC redistribution along
the profile is consistent with fertilization-irrigation-driven carbonate dynamics and should be considered in
farmland soil C assessment [10,52] .