Soil inorganic carbon loss offsets organic carbon sequestration in cropland
0 Abstract
Agricultural soils are vital for reducing atmospheric CO2; however, the effectiveness of farmland carbon sequestration, including soil organic carbon (SOC) and inorganic carbon (SIC), typically requires a lengthy period and varies with different farming practices. In a long-term study on the North China Plain, SOC and SIC changes due to farming practices involving N fertilization, organic materials, irrigation, and no-tillage were tracked. Four experimental treatments, including no N fertilizer input (CK), local farmer operation (FRM), optimized farming (OPT), and no-tillage (NoT), were selected for the study. From 2008 to 2024, the fertilized treatments sequestered SOC at rates of 0.35-0.63 Mg C ha-1 yr-1 in the 0-20 cm layer, which quadrupled in the 0-100 cm layer. Long-term high irrigation with N fertilization accelerated the leaching of SIC into the subsoil, and SIC losses ranged from 0.46 to 0.71 Mg C ha-1 yr-1 at 0-20 cm and from 1.88 to 2.42 Mg C ha-1 yr-1 at 0-100 cm. Organic materials and N fertilization interactively help sequester SOC, but excessive organic material input results lower conversion efficiency. Crucially, the substantial depletion of SIC across the whole profile largely counteracted the observed SOC gains, leading to a diminished or even negative net carbon balance. Ultimately, this study reveals that failing to account for whole-profile SOC-SIC co-dynamics leads to an overestimation of carbon sequestration in intensive agricultural systems, highlighting the necessity of integrated accounting for accurate climate mitigation assessments.
Keywords
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[1-3]. At 21st Conference of the Parties 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[3,4]. It has been estimated that up to 89% of global agricultural mitigation potential lies in enhancing C sequestration in croplands[5]. Despite ongoing debates about the magnitude and stability of C sequestration, abundant 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[7-9]. Soil inorganic carbon (SIC), mainly carbonates, 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 HCO3-/CO32- and Ca2+/Mg2+[10,20]. Moderate external N inputs 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[27], and frequent irrigation further promotes the leaching of HCO3- and CO32- into groundwater[13]. Secondary or pedogenic carbonate might be formed and sequestered if Ca2+/Mg2+ is available, as HCO3-/CO32- is generally readily present in the soil[28-30]. Conservation/reduced tillage enhances 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.
MATERIALS AND METHODS
Study region
A long-term field trial was conducted in Huantai County (117°50’00”-118°10’40”E, 36°51’50”-37°06’00”N), Shandong Province, on the North China Plain. The region is characterized by a temperate continental monsoon climate, with a mean annual temperature of 11.8-12.9 °C, a mean annual precipitation of 603.9 mm (primarily from June to August), and a frost-free period of 198 days per year. Before the trial was established, the farmland had been intensively operated since the early 1980s, when China initiated its reform and opening-up policy. Two crops, i.e., winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.), were planted annually in the region. Since the 1980s, crops in the area have been excessively fertilized (~300 kg N per crop season), frequently irrigated, and tilled, with high grain and straw yields (both > 15 t ha-1 yr-1). The soil is classified as an aquic inceptisol [according to United States Department of Agriculture (USDA) Soil Taxonomy, the soil is classified as an Inceptisol, suborder Aquepts, and great group Endoaquepts (aquic, calcareous, clay loam)]. The main properties of the ploughing soil (0-20 cm) in 2008 were a pH of 7.8 (CaCl2 method), a soil organic carbon content of 10.2 g kg-1, 30% sand (0.02-2 mm), 32% silt (0.002-0.02 mm), and 38% clay (< 0.002 mm), and the bulk density was 1.5 g cm-3.
Experimental design
From a long-term field trial initiated in 2008 and established using a randomized complete block design, 4 farming treatments (each with four replicates) were selected for this study [Supplementary Figure 1]. Two crops - winter wheat and summer maize - were planted for the four farming treatments. Each plot covered 450 m2 (25 m × 18 m), with 5 m buffer strips between treatments and 2 m intervals between subplots.
(1) Control (CK) treatment: No nitrogen (N) fertilizer was applied during either the wheat or the maize season. From 2008 to 2024, the average rates of P2O5 and K2O in the wheat season were 134 and 59.5 kg ha-1, respectively, and 102 and 76.9 kg ha-1, respectively, in the maize season, both of which were applied once as basal fertilizer. Straw from both crops was incorporated into the farmland, and rotary tillage was performed to a depth of 15 cm. Flood irrigation was adopted (75 mm per event) 2-4 times during the wheat season and 0-1 times during the maize season, depending on precipitation and soil moisture. The CK treatment received 59.6 Mg C of crop straw input per hectare.
(2) Local farmer operation treatment (FRM): The N input was 300 kg N ha-1 season-1 for both the wheat and maize seasons, which were equally split into basal and topdressing fertilization. Only wheat straw was incorporated. P and K fertilization and tillage were similar to those in the CK treatment. A similar flood irrigation treatment to that in the CK treatment was also applied, except with a water amount of 100 mm per event.
(3) Optimized farming operation treatment (OPT): A soil testing and formula fertilization program was employed for crop fertilization. A yield of 7.5 t ha-1 for both crops was targeted to determine the crop N requirements. The nitrate-N content in the soil was monitored as the soil N supply at the beginning of each crop season, and the fertilizer N input was calculated as “crop N requirement-soil N supply”[34]. From 2008 to 2024, the average N input in the OPT treatment was 156 kg N ha-1 during the wheat season and 186 kg N ha-1 during the maize season, with approximately 1/3 supplied by decomposed animal manure and 2/3 supplied by mineral fertilizer. The average P2O5 and K2O rates during the wheat season were 119 and 60.0 kg ha-1, respectively, whereas those during the maize season were 85 and 78.7 kg ha-1, respectively; all the treatments were applied as basal fertilizer. Straw of both wheat and maize was incorporated into the farmland, and deep tillage (~25 cm) was conducted prior to wheat sowing every autumn. Water-saving irrigation (75 mm per event) was implemented.
(4) No-tillage treatment (NoT): From 2008 to 2024, an Soil Testing and Fertilizer Formulation (STFF) fertilization program similar to OPT was adopted, resulting in average N inputs from mineral fertilizer of 155 kg N ha-1 in the wheat season and 175 kg N ha-1 in the maize season. Tillage was not performed. P and K fertilization was also similar to OPT. Straw return and irrigation were similar to those in the CK treatment.
Measurement and calculation
Sampling and analysis
Before the wheat sowing in 2008, soil samples were collected, and the bulk density was measured in the 0-20, 20-40, 40-60, 60-80, and 80-100 cm soil layers[35]. After the 2024 wheat harvest, soil samples were collected from the 0-20, 20-40, 40-70, and 70-100 cm layers. Soil bulk density was also determined during soil sampling. Total soil carbon (TC) was measured using an elemental analyser (Flash EA1112). The SOC content was determined using the wet oxidation method[36], and the SIC content was calculated as the difference between TC and SOC.
Calculation of carbon stock, annual change rate and carbon conversion efficiency
The farmland SOC, SIC, and TC stocks (D, kg C hm-2) are quantified as follows[37]:
where Ccontent is the SOC, SIC, or TC content at a given depth, g kg-1; H is the soil depth, m; and BD is the soil bulk density, g cm-3. Given that the depths of soil sampling at 40 to 100 cm for the years 2008 (40-60, 60-80, and 80-100 cm) and 2024 (40-70 and 70-100 cm) differed, we assumed that the soil C content and BD were the same across the 60-80 cm layer. In the calculation, the 60-80 cm layer was divided into two layers, i.e., 60-70 and 70-80 cm. The soil C in the 60-70 and 70-80 cm soil layers was calculated using the 40-60 and 80-100 cm soil layers, respectively, to form the 40-70 and 70-100 cm layers. Thus, the soil C stock in 4 layers (0-20, 20-40, 40-70 cm, and 70-100 cm) was obtained for 2008.
Annual changes in the SOC, SIC, and TC stocks (ΔC, kg C hm-2 yr-1) were calculated by dividing the difference in carbon stocks during the experimental period (2008-2024) by the experimental duration in years[38]:
where DT and DI are the SOC, SIC, and TC stocks at the end (2024) and beginning (2008) of the experiment, respectively (kg C hm-2); and n represents the number of experimental years, i.e., 16 in this study.
To evaluate the effects of different farming systems on the development of SOC, the carbon conversion efficiency (CE, i.e., the newly formed SOC from organic material input) was calculated as follows[39]:
where CE is the conversion efficiency (%); F is the amount of organic material C input (crop straw and composted animal manure) during the 16-year experimental period (kg hm-2); and C is the C content (%) of incorporated crop straw and composted animal manure, which in the current study, was 43.0% (wheat straw), 43.4% (maize straw), and 18.3% (organic fertilizer); and ΔSOC is the change in the SOC stock during the 16-year experimental period (kg C hm-2)[40].
Statistical analysis
Statistical analysis was performed using SPSS 26.0 (IBM Corporation, Armonk, NY, USA). One-way analysis of variance (ANOVA) followed by the least significant difference (LSD) test was used to test differences between treatments, and statistical significance was determined at P < 0.05. The figures were generated using Origin 2021 (Origin Lab Corporation, Northampton, MA, USA), and the data for each treatment are presented as the mean ± standard deviation of 4 replicates (n = 4).
RESULTS
Organic and inorganic carbon distribution across the soil profile
In the four experimental treatments, the SOC content decreased with increasing soil depth and tended to stabilize in the 40-100 cm layer [Figure 1A]. Compared with the CK treatment, the three N-fertilized treatments significantly increased the SOC content (P < 0.05) at 0-20 cm, by 20.5% (FRM), 36.7% (OPT) and 26.7% (NoT), respectively. This trend was also observed for the 40-70 cm layer; however, there were no significant differences in the SOC content among the four experimental treatments in the 20-40 and 10-100 cm layers (P > 0.05).
Figure 1. Effects of farming practices on the vertical distributions of soil organic carbon (A), soil inorganic carbon (B), and soil total carbon (C). The data shown are the mean ± SD (n = 4). The different lowercase letters indicate significant differences at P < 0.05. SOC: Soil organic carbon; SIC: soil inorganic carbon; TC: total soil carbon.
In contrast to those of the SOC, the SIC contents of the experimental soils were similar for the upper layers of 0-20, 20-40, and 40-70 cm, i.e., 1.4-3.9 g kg-1 [Figure 1B], and significantly increased to 6.5-8.7 g kg-1 in the 70-100 cm layer. The SIC contents of the CK treatment were the highest among those of the four treatments at 0-20 cm, but the differences from those of the other three N-fertilized treatments were not significant (P > 0.05), except for the lowest SIC contents of OPT and NoT at 20-40 and 70-100 cm.
The TC contents (i.e., the sum of SOC and SIC) of the four experimental treatments were higher in the 0-20 and 70-100 cm layers than in the 20-70 cm layer [Figure 1C]. The OPT treatment resulted in the highest TC content among the four treatments for the 0-100 soil profile. Compared with OPT and NoT, both CK and FRM had lower TC contents in the 0-20 cm layer, but in the 70-100 cm layer, the TC content of NoT was the lowest. SOC strongly controlled the TC content in the upper soil layers (0-70 cm), whereas SIC dominated the TC content in the subsoil (70-100 cm). Linear regression analysis revealed that the SOC content was significantly negatively correlated with the SIC content at 0-20 cm and 20-40 cm (R2 = 0.249 and 0.200, slope = -0.696 and -1.17, respectively; P < 0.05; Figure 2A and B), but the correlations were not significant at 40-70 cm or 70-100 cm (P > 0.05) [Figure 2C and D].
Figure 2. Correlations between soil organic and inorganic carbon contents at different soil depths: (A) 0-20 cm, (B) 20-40 cm, (C) 40-70 cm, and (D) 70-100 cm. SOC: Soil organic carbon; SIC: soil inorganic carbon.
Effects of experiment duration and farming practices on soil organic and inorganic carbon stocks
From 2008 to 2024, the SOC stocks in the 0-20 cm layer did not change in the CK treatment but increased by 19.5% for FRM, 35.6% for OPT and 25.6% for NoT [Table 1]; correspondingly, the annual change rates of the SOC stock were -0.01 ± 0.06 (CK), 0.35 ± 0.07 (FRM), 0.63 ± 0.11, and 0.46 ± 0.06 Mg C hm-2 yr-1, respectively. With respect to the 0-40 cm layer, the SOC stocks increased during the 16-year experimental period by 6.42% (CK), 22.9% (FRM), 40.2% (OPT), and 29.5% (NoT), and the annual increase rate was approximately 70% to 75% greater than that in the 0-20 cm layer (excluding the CK). With respect to the 0-100 cm soil layer, the SOC stocks in the four experimental treatments increased by 31.6% (CK), 43.3% (FRM), 61.6% (OPT), and 46.7% (NoT), with annual SOC sequestration rates of 1.27 ± 0.22, 1.74 ± 0.21, 2.48 ± 0.43, and 1.88 ± 0.09 Mg C hm-2 yr-1, respectively. OPT always had the highest SOC sequestration rate, followed by NoT, FRM, and CK. The annual SOC sequestration rates for the 1 m soil profile were approximately 4~5 times those of the 0-20 cm soil layer.
Changes in the SOC stocks across the 0-100 cm soil profile under different farming treatments over the 16-year experimental period
| SOC stock in 2008 (Mg C ha-1) | SOC stock in 2024 (Mg C ha-1) | Annual change (Mg C ha-1 yr-1) | ||
| 0-20 cm | CK | 28.4 ± 1.60 | 28.2 ± 2.00c | -0.01 ± 0.06c |
| FRM | 34.0 ± 0.58b | 0.35 ± 0.07b | ||
| OPT | 38.5 ± 3.04a | 0.63 ± 0.11a | ||
| NoT | 35.7 ± 1.17ab | 0.46 ± 0.06b | ||
| 0-40 cm | CK | 42.7 ± 3.56 | 45.5 ± 4.75b | 0.17 ± 0.14c |
| FRM | 52.5 ± 2.46ab | 0.61 ± 0.08b | ||
| OPT | 59.9 ± 8.25a | 1.07 ± 0.36a | ||
| NoT | 55.4 ± 4.40ab | 0.79 ± 0.14ab | ||
| 0-70 cm | CK | 57.1 ± 4.08 | 62.1 ± 5.32b | 0.31 ± 0.09c |
| FRM | 73.9 ± 4.95a | 1.05 ± 0.11b | ||
| OPT | 82.8 ± 6.89a | 1.61 ± 0.18a | ||
| NoT | 76.5 ± 3.25a | 1.21 ± 0.06b | ||
| 0-100 cm | CK | 64.3 ± 4.96 | 84.6 ± 6.75b | 1.27 ± 0.22c |
| FRM | 92.1 ± 6.67ab | 1.74 ± 0.21b | ||
| OPT | 104 ± 9.82a | 2.48 ± 0.43a | ||
| NoT | 94.3 ± 4.96ab | 1.88 ± 0.09b |
In contrast to the SOC stocks, the SIC stocks in the four experimental treatments decreased as the duration of the long-term experiment increased [Table 2]. For the 0-20 cm layer, the SIC stocks decreased annually by 0.27 ± 0.12 (CK), 0.71 ± 0.11 (FRM), 0.46 ± 0.11, and 0.53 ± 0.11 (NoT) Mg C hm-2 yr-1. As the soil depth increased, the SIC stock losses also increased. For the 0-100 cm layer, the increases were 1.62 ± 0.17 (CK), 1.88 ± 0.13 (FRM), 2.31 ± 0.11 (OPT), and 2.42 ± 0.46 (NoT) Mg C hm-2 yr-1, respectively. Both OPT and NoT resulted in higher SIC loss rates, followed by FRM and CK.
Changes in the SIC stocks across the 0-100 cm soil profile under different farming treatments over the 16-year experimental period
| SIC stock in 2008 (Mg C ha-1) | SIC stock in 2024 (Mg C ha-1) | Annual change (Mg ha-1 yr-1) | ||
| 0-20 cm | CK | 15.3 ± 0.68 | 11.0 ± 2.48a | -0.27 ± 0.12c |
| FRM | 3.93 ± 2.28b | -0.71 ± 0.11a | ||
| OPT | 7.86 ± 2.34ab | -0.46 ± 0.11b | ||
| NoT | 6.84 ± 2.27ab | -0.53 ± 0.11b | ||
| 0-40 cm | CK | 29.6 ± 2.13 | 20.2 ± 1.54a | -0.59 ± 0.07b |
| FRM | 12.1 ± 3.34b | -1.10 ± 0.09a | ||
| OPT | 14.6 ± 5.27ab | -0.94 ± 0.22a | ||
| NoT | 15.1 ± 3.27ab | -0.90 ± 0.08a | ||
| 0-70 cm | CK | 47.8 ± 2.64 | 30.8 ± 3.10a | -1.07 ± 0.03b |
| FRM | 24.1 ± 4.28ab | -1.48 ± 0.16a | ||
| OPT | 20.6 ± 1.81b | -1.71 ± 0.07a | ||
| NoT | 25.0 ± 7.63ab | -1.43 ± 0.32a | ||
| 0-100 cm | CK | 91.4 ± 5.10 | 65.5 ± 7.43a | -1.62 ± 0.17b |
| FRM | 61.3 ± 4.19a | -1.88 ± 0.13b | ||
| OPT | 54.5 ± 5.16a | -2.31 ± 0.11a | ||
| NoT | 52.7 ± 10.5a | -2.42 ± 0.46a |
Given that the SOC and SIC pools were considered together, we found that after the 16-year experimental period, the net C sequestration rates in the different soil layers were 0.17 ± 0.14 Mg C hm-2 yr-1 at 0-20 cm, 0.14 ± 0.47 Mg C hm-2 yr-1 at 0-40 cm, and 0.17 ± 0.64 Mg C hm-2 yr-1 at the 0-100 cm layer (Table 3). With respect to the other experimental treatments, i.e., FRM, NoT, and CK, the net C losses were similar across the 16-year experimental period, and the loss rates were similar across the three treatments, except for higher loss rates in the 0-20 cm layer in the CK and CON treatments.
Changes in the TC stocks across the 0-100 cm soil profile under different farming treatments over the 16-year experimental periods
| TC stock in 2008 (Mg C ha-1) | TC stock in 2024 (Mg C ha-1) | Annual change rate (Mg C ha-1 yr-1) | ||
| 0-20 cm | CK | 43.7 ± 1.02 | 39.2 ± 0.85bc | -0.28 ± 0.02c |
| FRM | 37.9 ± 2.81c | -0.36 ± 0.13c | ||
| OPT | 46.4 ± 1.30a | 0.17 ± 0.14a | ||
| NoT | 42.5 ± 2.95ab | -0.07 ± 0.13b | ||
| 0-40 cm | CK | 72.3 ± 1.74 | 65.7 ± 3.28a | -0.42 ± 0.11b |
| FRM | 64.6 ± 5.21a | -0.48 ± 0.22b | ||
| OPT | 74.5 ± 6.12a | 0.14 ± 0.47a | ||
| NoT | 70.5 ± 3.94a | -0.12 ± 0.14ab | ||
| 0-70 cm | CK | 105 ± 3.43 | 92.9 ± 4.93a | -0.75 ± 0.19b |
| FRM | 98.0 ± 8.25a | -0.44 ± 0.48ab | ||
| OPT | 103 ± 7.71a | -0.10 ± 0.43a | ||
| NoT | 101 ± 7.32a | -0.22 ± 0.24ab | ||
| 0-100 cm | CK | 156 ± 5.36 | 150 ± 8.51a | -0.35 ± 0.35b |
| FRM | 153 ± 10.5a | -0.14 ± 0.48b | ||
| OPT | 158 ± 6.91a | 0.17 ± 0.64b | ||
| NoT | 147 ± 10.6a | -0.54 ± 0.46b |
Effects of farming practices on soil organic carbon conversion efficiency
During the 16-year experimental period, the CK, FRM, OPT, and NoT treatments received OC inputs of 59.6, 58.1, 130, and 115 Mg C ha-1, respectively [Table 4]. The conversion efficiencies of the organic material inputs for the four experimental treatments were calculated. In the 0-20 cm layer, some of the SOC stock in the CK treatment decreased as the experiment progressed, and its CE was negative. Nevertheless, the CE values of the other three treatments were 9.53% (FRM), 7.75% (OPT), and 6.35% (NoT). For the 0-40 cm soil layer, the CE nearly doubled; for the 0-100 cm layer, the CE quadrupled and was highest in FRM (47.9%), followed by CK (34.1%), OPT (30.4%), and NoT (26.2%).
Effects of different farming practices on the SOC conversion efficiency
| Organic material carbon input (Mg C ha-1) | Increase in SOC stock (Mg C ha-1) | Conversion efficiency (%) | ||
| 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 |
DISCUSSION
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[44]. In addition, under appropriate conditions, such as the high and frequent water input in the current study (2~4 times the amount of irrigation plus 600 mm of precipitation), high water leaching can occur[45], and soluble OC and dissolved organic matter can be leached downwards into deeper soil layers[45,46]. 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 CO2 partial pressure and further promote carbonate dissolution and migration[51]. Dissolved Ca2+/Mg2+ and HCO3-/CO32- may then move 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].
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 CO2. As microbial turnover progresses, this 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 Ca2+/Mg2+ from irrigation water and minor contributions from fertilizers on the North China Plain[10]. FRM received 25% more irrigation water during the experimental period and, thereby, 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.
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 | R2 |
| 0-20 | Y = -0.02318 x1 - 1.08131 x2 + 0.00307 x1 x2 - 0.24864 | 0.659 (P < 0.01) |
| 20-40 | Y = -0.00748 x1 + 0.41567 x2 - 0.00165 x1 x2 - 10.96561 | 0.667 (P < 0.01) |
| 40-70 | Y = 0.03531 x1 + 3.707172 x2 - 0.01262 x1 x2 - 30.96895 | 0.593 (P < 0.01) |
| 70-100 | Y = -0.03011 x1 - 5.08716 x2 + 0.00614 x1 x2 - 6.95607 | 0.599 (P < 0.01) |
In addition to incorporating organic materials, N fertilizer is another essential agricultural practice that regulates SOC. We proposed a coupled impact of N fertilization and organic material input on SOC on the basis of a 224-day indoor incubation experiment[58]. In terms of this concept, mineral N fertilization inhibited crop straw decomposition and helped develop new SOC; N fertilization with straw incorporation did not affect SOC decomposition but did increase it when no crop straw was added. Our meta-analysis revealed that combining N fertilization with straw incorporation resulted in the best degree of SOM accrual[15]. Our results provide additional field-based evidence supporting this coupled effect. Regression analysis revealed a significant positive correlation (P < 0.01) between SOC and the interaction between N input and C input [Table 6]. Similarly, in the current study, although the SOC sequestration rate and CEs of the CK treatment were significantly lower than those of the FRM treatment [P < 0.05], again highlighting that N fertilizer is also essential for SOM accumulation. We believe that in the intensively managed region of the current study, the simultaneous supply of N fertilizer and organic materials effectively facilitated the build-up of SOM and reduced N losses, as the applied mineral N was not immobilized in a timely manner.
Relationships between N fertilizer inputs and C inputs and between N inputs and the increase in the SOC stock over the 16-year experimental period
| Soil layer (cm) | Regression equation | R2 |
| 0-20 | Y = -0.01581 x1 - 1.06857 x2 + 0.00696 x1 x2 + 3.75279 | 0.897 (P< 0.01) |
| 20-40 | Y = -0.02983 x1 - 1.99755 x2 + 0.01136 x1 x2 + 10.11617 | 0.725 (P< 0.01) |
| 40-70 | Y = -0.04077 x1 - 3.23455 x2 + 0.01648 x1 x2 + 17.04337 | 0.946 (P < 0.01) |
| 70-100 | Y = -0.08129 x1 - 6.91475 x2 + 0.02552 x1 x2 + 46.08679 | 0.725 (P < 0.01) |
The results of the present study revealed that increasing the amount of returned straw effectively increased the SOC stocks and carbon sequestration rates. However, Liu et al.[59] reported that the efficiency of SOC accumulation decreased with increasing straw addition rates and suggested that low-to-moderate straw return was more favourable for SOC enhancement. Moreover, straw decomposition rates are regulated by soil temperature, moisture, and N availability[60,61], and excessive straw input may stimulate SOC mineralization[62,63]. In this study, the SOC sequestration rates in the OPT treatment (combined organic and chemical fertilization) were significantly greater than those in the NoT treatment, which received chemical fertilizer alone, indicating that organic fertilizer substantially increased SOC accumulation.
The observed offset of SOC sequestration by SIC loss can be mechanistically explained by enhanced carbonate dissolution under intensive agricultural management, where coupled organic and inorganic carbon dynamics jointly regulate the net soil C balance[11,13]. High N fertilization promotes nitrification and associated proton production, which decreases soil pH and accelerates carbonate weathering[27]. Moreover, irrigation increases water flux and the leaching of dissolved inorganic carbon into deeper soil layers, and enhanced root and microbial respiration increases soil CO2 partial pressure, further driving carbonate dissolution[3,46]. These coupled biogeochemical processes contribute to SIC depletion and associated CO2 release, thereby partially offsetting SOC gains in calcareous soils[11,13].
Effects of farming measures on the soil carbon conversion efficiency
In this study, SOC sequestration increased with increasing organic material input, highlighting the dominant role of organic materials in SOM formation. However, these two parameters were negatively correlated (r = -0.185, P < 0.01) [Figure 3]. These findings indicate that excessively high organic material inputs may not consistently achieve the highest degree of SOC sequestration and that the efficiency of carbon conversion is not maximized[64]. This phenomenon often occurs in soils with high initial C content[48], suggesting a diminishing response as the SOM content approaches saturation. In the current study, the SOC content of OPT was 13.6 g kg-1 (SOM concentration of 23.4 g kg-1), suggesting that the farmland soil had not yet reached the SOM saturation level. No/reduced tillage helps increase the SOC stock, especially in surface soils[64], but for the 0-100 cm soil profile, NoT did not exhibit a CE comparable to that of the other treatments. These findings call for continuous monitoring of SOM dynamics in field experiments to validate the efficacy of no/reduced tillage on SOC sequestration and to assess potential saturation effects in these intensively farmed regions.
Figure 3. Regression analysis of organic material carbon inputs with carbon conversion efficiency.
Notably, the SIC in this study was not directly quantified using carbonate-specific analytical methods but was estimated as the difference between total carbon and wet-oxidation SOC. Although this approach has been widely adopted in studies of calcareous agricultural soils, it may introduce uncertainty associated with SOC analytical recovery and carbonate reactivity. Therefore, the absolute magnitude of SIC stocks should be interpreted with caution. However, because identical analytical procedures were consistently applied across all the treatments and sampling years, the comparative differences among the treatments and the observed temporal trends remain reliable.
CONCLUSIONS
This decadal field study demonstrated that intensive farming practices substantially altered both the SOC and SIC pools across the 0-100 cm soil profile. N fertilization and organic material incorporation interactively increased SOC sequestration, and SOC gains extended into subsoil layers, indicating the importance of whole-profile assessment. However, increased organic material input did not always result in proportionally higher SOC storage, highlighting the need to consider input efficiency and appropriate amendment levels. Long-term irrigation combined with N fertilization accelerated SIC leaching and depletion, and SIC losses offset a significant portion of newly formed SOC. The net soil C balance therefore differed among management systems. The optimized treatment achieved net carbon sequestration but required relatively high amounts of organic inputs, indicating a management trade-off. These results show that SOC-SIC co-dynamics must be considered when evaluating carbon sequestration under intensive agriculture.
DECLARATIONS
Authors’ contributions
Data curation, formal analysis, investigation, methodology, writing (original draft) and writing (review & editing) were performed: Cui, B.
Data curation, formal analysis and visualization were performed: Tang, X.; Ma, J.
Data conceptualization and methodology were performed: Kuang, L.
The investigation was performed: Yuan, N.
Conceptualization, funding acquisition, project administration, resources, supervision, writing, review and editing were performed: Meng, F.
All the authors contributed to the study conception and design. All the authors commented on previous versions of the manuscript. All the authors read and approved the final manuscript.
Availability of data and materials
The datasets used or analysed during the current study are available from the corresponding author upon reasonable request.
AI and AI-assisted tools statement
Not applicable.
Financial support and sponsorship
This research was supported by the National Key Research and Development Program of China (Grant No. 2023YFD1701701).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2026.
Supplementary Materials
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