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Page 2 of 16 Cui et al. Carbon Footprints 2026, 5, 19

INTRODUCTION
Agricultural ecosystems are key components of the global terrestrial carbon (C) cycle, and variations in soil
C stocks directly influence regional and global C sink-source dynamics . At 21st Conference of the Parties
[1-3]
to the United Nations Framework Convention on Climate Change (COP21), the French Ministry of
Agriculture launched the ambitious “4 per 1000” initiative under the Lima-Paris Action Agenda, aiming to
increase the global soil C stock by 0.4% annually to offset anthropogenic greenhouse gas emissions . It has
[3,4]
been estimated that up to 89% of global agricultural mitigation potential lies in enhancing C sequestration in
croplands . Despite ongoing debates about the magnitude and stability of C sequestration, abundant
[5]
evidence confirms the high sequestration potential of agricultural soils .
[6]
Soil comprises organic carbon (OC) and inorganic carbon stocks. Soil organic carbon (SOC) is characterized
by rapid turnover, high sensitivity to agricultural management practices, and critical importance for crop
productivity, soil health, and agrarian C sequestration . Soil inorganic carbon (SIC), mainly carbonates,
[7-9]
persists over much longer timescales and, although less responsive to farming practices, is increasingly
recognized as an essential contributor to C sequestration in the agricultural sector [10-13] . Understanding the
dynamics of the soil C pool, therefore, requires consideration of both SOC and SIC responses to natural
factors and farming practices. However, most prior studies focus on the SOC pool and seldom investigate, in
a synchronous manner, the changes in both the SOC and SIC stocks in farmland.

Agricultural practices, including straw incorporation, fertilization, irrigation, tillage and others, regulate
organic matter inputs, mineralization rates, aggregate stability, and carbonate dissolution-precipitation
equilibria, thereby reshaping SOC and SIC trajectories [14-17] . C input via organic materials controls the SOC
level [17-19] and affects SIC formation by providing HCO /CO and Ca /Mg 2+[10,20] . Moderate external N inputs
-
2+
2-
3
3
increase biomass production and microbial C use efficiency, promoting the transition from particulate
organic C to mineral-associated organic C [21-23] . Excessive N, however, accelerates SOC mineralization, alters
microbial communities, and may limit the formation of mineral-associated organic C because of cation
leaching [24-26] . Soil acidification from excessive N fertilization is a major driver of lithogenic carbonate
dissolution and loss , and frequent irrigation further promotes the leaching of HCO and CO into
[27]
2-
-
3
3
groundwater . Secondary or pedogenic carbonate might be formed and sequestered if Ca /Mg is available,
2+
[13]
2+
as HCO /CO is generally readily present in the soil [28-30] . Conservation/reduced tillage enhances
2-
-
3
3
macroaggregate formation and SOC protection, whereas intensive tillage accelerates SOC mineralization and
might reduce subsoil SOC accumulation [31-33] . SIC contributes to C sequestration through the formation of
pedogenic carbonate or the leaching of bicarbonate into groundwater, which is not readily lost [14,28] .
Despite numerous intensive studies on SOC evolution in agricultural soils, a critically important scientific
question remains to be answered: has the SIC pool also been substantially affected by the aforementioned
farming measures to the same extent as the SOC pool? Owing to the slow response of the soil C pool to
agricultural practices, it is not easy to obtain a robust answer to the question raised. Usually, a long-term field
trial is employed to observe changes in the SOC pool, excluding the SIC pool, under different farming
practices. Accordingly, we monitored SOC and SIC changes under different farming management practices
through a field trial initiated in 2008 in northern China, where agriculture has been intensively practised
since the 1990s. We assumed that the SOC pool responded faster than the SIC pool did, as a large amount of
organic material was returned to the farmland in this intensive farming region. Therefore, the objectives of
this study were to investigate how the soil C pool was affected by intensive farming measures and to examine
the roles of the SOC and SIC pools in farmland C sequestration.