Search Results (613)

This study investigates the production and economic feasibility of biochar for smallholder and family farms in Central Amazonia, with potential implications for other tropical regions. The costs of construction of a prototype mobile kiln and biochar production were evaluated, using small-sized biomass from acai (Euterpe oleracea Mart.) agro-industrial residues as feedstock. The biochar produced was characterised in terms of its liming capacity (calcium carbonate equivalence, CaCO_3eq), nutrient content via organic fertilisation methods, and ash analysis by ICP-OES. Field trials with cowpea assessed economic outcomes, as well scenarios of fractional biochar application and cost comparison between biochar production in the prototype kiln and a traditional earth-brick kiln. The prototype kiln showed production costs of USD 0.87–2.06 kg⁻¹, whereas traditional kiln significantly reduced costs (USD 0.03–0.08 kg⁻¹). Biochar application alone increased cowpea revenue by 34%, while combining biochar and lime raised cowpea revenues by up to 84.6%. Owing to high input costs and the low value of the crop, the control treatment generated greater net revenue compared to treatments using lime alone. Moreover, biochar produced in traditional kilns provided a 94% increase in net revenue compared to liming. The estimated externalities indicated that carbon credits represented the most significant potential source of income (USD 2217 ha⁻¹). Finally, fractional biochar application in ten years can retain over 97% of soil carbon content, demonstrating potential for sustainable agriculture and carbon sequestration and a potential further motivation for farmers if integrated into carbon markets. Public policies and technological adaptations are essential for facilitating biochar adoption by small-scale tropical farmers. Full article

Soil organic carbon (SOC) pool plays an extremely important role in regulating the global carbon (C) cycle and climate change. Atmospheric nitrogen (N) and phosphorus (P) deposition caused by human activities has significant impacts on soil C sequestration potential of terrestrial ecosystem. To investigate the effects of N and P deposition on soil C sequestration and C-N coupling relationship in broad-leaved evergreen forests, a 6-year field nutrient regulation experiment was implemented in subtropical Castanopsis sclerophylla forests with four different N and P additions: N addition (100 kg N·hm⁻²·year⁻¹), N + P (100 kg N·hm⁻²·year⁻¹ + 50 kg P·hm⁻²·year⁻¹), P addition (50 kg P·hm⁻²·year⁻¹), and CK (0 kg N·hm⁻²·year⁻¹). The changes in the C and N contents and stable isotope distributions (δ¹³C and δ¹⁵N) of different soil organic fractions were examined. The results showed that the SOC and total nitrogen (STN) (p > 0.05) increased with N addition, while SOC significantly decreased with P addition (p < 0.05), and N + P treatment has low effect on SOC, STN (p > 0.05). By density grouping, it was found that N addition significantly increased light fraction C and N (LFOC, LFN), significantly decreased the light fraction C to N ratio (LFOC/N) (p < 0.05), and increased heavy fraction C and N (HFOC, HFN) accumulation and light fraction to total organic C ratio (LFOC/SOC, p > 0.05). Contrary to N addition, P addition was detrimental to the accumulation of LFOC, LFN and reduced LFOC/SOC. It was found that different reactive oxidized carbon (ROC) increased under N addition but ROC/SOC did not change, while N + P and P treatments increased ROC/SOC, resulting in a decrease in SOC chemical stability. Stable isotope analysis showed that N addition promoted the accumulation of new soil organic matter, whereas P addition enhanced the transformation and utilization of C and N from pre-existing organic matter. Additionally, N addition indirectly increased LFOC by significantly decreasing pH; significantly contributed to LFOC and ROC by increasing STN accumulation promoted by NO₃⁻-N and NH₄⁺-N; and decreased light fraction δ¹³C by significantly increasing dissolved organic C (p < 0.05). P addition had directly significant negative effect on LFOC and SOC (p < 0.05). In conclusion, six-year N deposition enhances soil C and N sequestration while the P enrichment reduces the content of soil C, N fractions and stability in Castanopsis sclerophylla forests. The results provide a scientific basis for predicting the soil C sink function of evergreen broad-leaved forest ecosystem under the background of future climate change. Full article

Urban expansion fragments once-contiguous forest patches, generating pronounced edge gradients that modulate soil physicochemical properties and biodiversity. We quantified how fragmentation reshaped the soil microbiome continuum and its implications for soil carbon storage in a temperate urban mixed deciduous forest. A total of 18 plots were considered in this study, with six plots for each fragment type. Intact interior forest (F), internal forest path fragment (IF), and external forest path fragment (EF) soils were sampled at 0–15, 15–30, and 30–45 cm depths and profiled through phospholipid-derived fatty acid (PLFA) chemotyping and amino sugar proxies for living microbiome and microbial-derived necromass assessment, respectively. Carbon fractionation was performed through the chemical oxidation method. Diversity indices (Shannon–Wiener, Pielou evenness, Margalef richness, and Simpson dominance) were calculated based on the determined fatty acids derived from the phospholipid fraction. The microbial biomass ranged from 85.1 to 214.6 nmol g⁻¹ dry soil, with the surface layers of F exhibiting the highest values (p < 0.01). Shannon diversity declined systematically from F > IF > EF. The microbial necromass varied from 11.3 to 23.2 g⋅kg⁻¹. Fragmentation intensified the stratification of carbon pools, with organic carbon decreasing by approximately 14% from F to EF. Our results show that EFs possess a declining microbiome continuum that weakens their carbon sequestration capacity in urban forests. Full article

Soil organic carbon (SOC) is an important part of the global C pool and is sensitive to climate change. The SOC content and fractions of rice paddies along four elevations (250, 1150, 1600 and 1800 m) on the same slope in four seasons (spring, summer, autumn and winter) at Yuanyang Terrace in southwest China were investigated, and their relationship with environmental factors was analyzed. The contents of SOC, unprotected SOC (uPOM), physically protected SOC (pPOM) and biochemically protected SOC (bcPOM) in rice paddies at a low elevation (250 m), were significantly lower by 49–51% than those at relatively high elevations (1600 m and 1800 m). Among the SOC fractions, the highest proportion (33–50%) was uPOM, followed by pPOM and bcPOM (accounting for 17–40%), and the lowest proportion was chemically protected SOC (cPOM). In addition, there were interseasonal differences among the contents of SOC fractions, with a significantly higher content of SOC, uPOM and pPOM at an elevation of 1600 m in summer than in the other three seasons, whereas the cPOM content at an elevation of 250 m in spring was significantly higher than in the other three higher elevations. According to the redundancy analysis (RDA), total nitrogen was the key environmental factor, with an explanatory degree of 56% affecting the contents of SOC and its fractions. Thus, the SOC content increased with increasing elevation, and physical and biochemical protection were potential stabilization mechanisms responsible for their stability in the rice paddy of Yuanyang Terrace. These results provides empirical evidence for the elevational distribution patterns and seasonal dynamics of SOC fractions in rice paddies across Yuanyang Terrace. These findings highlight the importance of physical and biochemical protection mechanisms in stabilizing SOC in rice paddies, which could enhance long-term C sequestration and contribute to climate change mitigation in terraced agroecosystems. Full article

An indicator-based approach was implemented to assess the contributions of soils in supplying ecosystem services, providing a scalable tool for modeling the spatial heterogeneity of soil functions at regional and local scales. The method consisted of (i) the definition of soil-based ecosystem services (SESs), using available point data and thematic maps; (ii) the definition of appropriate SES indicators; (iii) the assessment and mapping of potential SESs provision for the Emilia-Romagna region (22.510 km²) in NE Italy. Depending on data availability and on the role played by terrain features and soil geography and its complexity, maps of basic soil characteristics (textural fractions, organic C content, and pH) covering the entire regional territory were produced at a 1 ha resolution using digital soil mapping techniques and geostatistical simulations to explicitly consider spatial variability. Soil physical properties such as bulk density, porosity, and hydraulic conductivity at saturation were derived using pedotransfer functions calibrated using local data and integrated with supplementary information such as land capability and remote sensing indices to derive the inputs for SES assessment. Eight SESs were mapped at 1:50,000 reference scale: buffering capacity, carbon sequestration, erosion control, food provision, biomass provision, water regulation, water storage, and habitat for soil biodiversity. The results are discussed and compared for the different pedolandscapes, identifying clear spatial patterns of soil functions and potential SES supply. Full article

Grazing affects soil organic carbon (SOC) through plant removal, livestock trampling, and manure deposition. However, the impact of grazing on SOC is also influenced by multiple factors such as climate, soil properties, and management approaches. Despite extensive research, the mechanisms by which grazing intensity influences SOC density in grasslands remain incompletely understood. This study examines the effects of varying grazing intensities on SOC density (0–30 cm) dynamics in temperate grasslands of northern China using field surveys and experimental analyses in a typical steppe ecosystem of Inner Mongolia. Results show that moderate grazing (3.8 sheep units/ha/yr) led to substantial consumption of aboveground plant biomass. Relative to the ungrazed control (0 sheep units/ha/yr), aboveground plant biomass was reduced by 40.5%, 36.2%, and 50.6% in the years 2016, 2019, and 2020, respectively. Compensatory growth failed to fully offset biomass loss, and there were significant reductions in vegetation carbon storage and cover (p < 0.05). Reduced vegetation cover increased bare soil exposure and accelerated topsoil drying and erosion. This degradation promoted the downward migration of SOC from surface layers. Quantitative analysis revealed that moderate grazing significantly reduced surface soil (0–10 cm) organic carbon density by 13.4% compared to the ungrazed control while significantly increasing SOC density in the subsurface layer (10–30 cm). Increased precipitation could mitigate the SOC transfer and enhance overall SOC accumulation. However, it might negatively affect certain labile SOC fractions. Elucidating the mechanisms of SOC variation under different grazing intensities and precipitation regimes in semi-arid grasslands could improve our understanding of carbon dynamics in response to environmental stressors. These insights will aid in predicting how grazing systems influence grassland carbon cycling under global climate change. Full article

Analyzing the soil organic carbon (SOC) stock in dryland areas of southern Shanxi, particularly under the influence of fertilization and mulching conditions, is crucial for enhancing soil fertility and crop productivity and understanding the SOC pool’s resilience to future climate change scenarios in the region. In a long-term experimental site located in Hongtong County, Shanxi Province, soil samples were collected from the 0–100 cm depth over a nine-year period. These samples were analyzed to evaluate the impact of five treatments: no fertilization and no mulching (CK), conventional farming practices (FP), nitrogen reduction and controlled fertilization (MF), nitrogen reduction and controlled fertilization with ridge-film furrow-sowing (RF), and nitrogen reduction and controlled fertilization with flat-film hole-sowing (FH). The average annual yield of wheat grain, SOC stock, water-soluble organic carbon (WSOC), particulate organic carbon (POC), light fraction organic carbon (LFOC), mineral-associated organic carbon (MOC), and heavy fraction organic carbon (HFOC) stocks were measured. The results revealed that the FH treatment not only significantly increased wheat grain yield but also significantly elevated the SOC stock by 23.71% at the 0–100 cm depth compared to CK. Furthermore, this treatment significantly enhanced the POC, LFOC, and MOC stocks by 106.43–292.98%, 36.93–158.73%, and 17.83–81.55%, respectively, within 0–80 cm. However, it also significantly decreased the WSOC stock by 34.32–42.81% within the same soil layer and the HFOC stock by 72.05–101.51% between the 20 and 100 cm depth. Notably, the SOC stock at the 0–100 cm depth was primarily influenced by the HFOC. Utilizing the DNDC (denitrification–decomposition) model, we found that future temperature increases are detrimental to SOC sequestration in dryland areas, whereas reduced rainfall is beneficial. The simulation results indicated that in a warmer climate, a 2 °C temperature increase would result in a SOC stock decrease of 0.77 to 1.01 t·ha⁻¹ compared to a 1 °C increase scenario. Conversely, under conditions of reduced precipitation, a 20% rainfall reduction would lead to a SOC stock increase of 1.53% to 3.42% compared to a 10% decrease scenario. In conclusion, the nitrogen reduction and controlled fertilization with flat-film hole-sowing (FH) treatment emerged as the most effective practice for increasing SOC sequestration in dryland areas by enhancing the HFOC stock. This treatment also fortified the SOC pool’s capacity to withstand future climate change, thereby serving as the optimal approach for concurrently enhancing production and fertility in this region. Full article

The substrate-induced respiration-inhibition (SIRIN) method has been used to estimate fungi-derived N₂O emissions, but its contribution to soil N₂O emissions remains unclear. There is a need to quantify the fungal fraction of N₂O production more precisely. Here, using isotopocule analysis, we assessed the relative contribution of fungi to soil N₂O production potential under denitrifying conditions, where key limiting factors of denitrification (soil moisture, soil NO₃⁻, and electron donor) were removed. The result showed that the ratio of fungi-derived N₂O emissions (R_F) was 0.83~4.28% in paddy soils, 13.80~23.21% in vineyard soils, and 15.34~65.94% in vegetable field soils, respectively. This indicated that the bacteria were the dominator of soil N₂O production potential in most cases, but fungal pathways could be significant in vegetable field soils. The experiment with bactericide addition showed that inhibitors could affect non-target microorganisms in the SIRIN method. Our further analyses suggest that it is worth to explore the effect of soil organic carbon and microbial synergies on fungi-derived N₂O emissions. Full article

This study examined the regulatory mechanism of calcium (Ca) amendment on the dynamics of soil organic carbon (SOC) fractions and extracellular enzyme activities, elucidating the role of Ca in soil carbon cycling processes. A field experiment with maize was conducted, comparing treatments of low calcium (T1), high calcium (T2), and a calcium-free control (CK). Measurements included inter-root SOC fractions—soluble organic carbon (DOC), microbial biomass carbon (MBC), and readily oxidizable organic carbon (ROC)—and the activities of the following extracellular enzymes: β-xylanase, β-glucosidase (β-glu), phenol oxidase (Phox), peroxidase (Pero), phosphatase (Phos), acetylaminoglucosidase (NAG), and urease. The main findings indicated the following: (1) Calcium addition significantly increased SOC content (115.04% and 99.22% higher in T1 and T2, respectively, than CK during the entire reproductive period) and enhanced microbial activity (elevated DOC and MBC). However, SOC decreased by 8.44% (T1) and 16.38% (T2) relative to CK in the late reproductive stage (irrigation–ripening), potentially reflecting microbial utilization (supported by the inverse correlation between SOC and MBC/DOC), and maize carbon reallocation during grain filling. (2) Calcium activated β-glu, Phox, Phos, NAG, and urease (p < 0.05), with pronounced increases in Phox (241.13 IU·L⁻¹) and Phos (1126.65 U·L⁻¹), indicating enhanced organic matter mineralization and phosphorus availability. (3) Calcium-driven MBC and ROC accumulation was associated with the positive regulation of Phox (path coefficient > 0.8) and the negative regulation of Phos. SOC was co-regulated by β-glu and Phos (R² = 0.753). (4) Calcium dynamically optimized the short-term carbon distribution through enzyme activity while promoting long-term sequestration. Our study provides new evidence supporting multi-pathway interactions through which calcium mediates enzyme networks to influence the soil carbon cycle. The findings provide a theoretical foundation for calcium fertilizer management and soil carbon sequestration strategies in agriculture, advancing academic and practical goals for sustainable development and carbon neutrality. Full article

Effective management of straw in rice (Oryza sativa L.)–rape (Brassica napus L.) rotation systems is essential for optimising resource efficiency and improving soil quality. This two-year study investigated the impact of seven straw treatment methods on soil organic carbon (SOC) dynamics. The treatments examined were as follows: (1) control (CK); (2) rice straw (SF); (3) rapeseed straw (YF); (4) rice-straw-derived biochar (SB); (5) rapeseed-straw-derived biochar (YB); (6) mixed straw (YSF); (7) mixed biochar (YSB). Soil properties, enzyme activities and carbon fractions were subsequently analysed. During the canola growing season, the application of rice straw biochar increased oxidisable carbon (ROC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) by 25.7%, 61.7% and 67.2%, respectively, compared to the control. Notably, SB was more effective than unprocessed rice straw (SF) at increasing SOC and ROC. Furthermore, SB demonstrated superior performance in enhancing ROC (56.4%), MBC (36.0%) and DOC (12.2%) compared to hybrid biochar (YSB). SB consistently exhibited a higher carbon accumulation trend than the rapeseed-derived treatments (YF, YB and YSB). The results of the study indicated that applying rice straw biochar during the oilseed rape growing season was effective in increasing variable carbon pools and soil organic carbon accumulation. Full article

Soil organic carbon (OC) storage in crop rotation systems benefits soil productivity and global climate change. However, the regulatory mechanisms and pathways by which soil OC storage is affected under medium-term crop rotation at the aggregate level are not fully understood. Herein, fifteen soil samples from five cropping systems (abandoned farmland, continuous cropping of tobacco, tobacco–pea rotation, continuous cropping of dasheen, and dasheen–ryegrass rotation for over 10 years) were collected from soil at 0 to 20 cm depths in Miyi County, Sichuan Province, China. The soil aggregates and aggregate-associated OC, enzyme activities, and microbial biomass were evaluated. The effects of medium-term crop rotation on soil aggregate-associated OC content and biochemical properties varied between crop types. Specifically, tobacco–pea rotation significantly decreased the proportion of macro-aggregates (0.25–2 mm); the contents of OC, Ca-OC, aliphatic C, alcohols, and phenols; enzyme activities; and fungal biomass in the aggregate fractions, compared with those associated with the continuous cropping of tobacco. In contrast, dasheen–ryegrass rotation significantly increased the recalcitrant OC content, β-glucosidase and polyphenol oxidase activities, microbial biomass in mega-aggregates (>2 mm) and macro-aggregates, and the recalcitrant OC content and enzyme activity in microaggregates (0.053–0.25 mm) and slit clay (<0.053 mm), relative to those in the continuous cropping of dasheen. Moreover, for the continuous-cropping soils, the OC contents were positively correlated with POD activity but negatively correlated with other enzymes. For the rotational soils, the OC content was positively related to the Fe/Al-OC, aromatic-C, aliphatic-C, and microbial biomass contents but negatively related to the carbohydrate content. The increased OC content was driven by the microbial biomass in the aggregate fractions, and medium-term crop rotation changed the negative effect of microorganisms on the OC content into a positive effect at the aggregate level. Overall, medium-term crop rotation enhances OC storage by improving soil structural stability and microbial community dynamics. Full article

Replacing partial chemical fertilizers with organic fertilizer can increase organic carbon input, change soil nutrient stoichiometry and microbial metabolism, and then affect soil organic carbon (SOC) storage. A 6-year field experiment was used to explore the mechanism of SOC storage under different ratios of manure substitution in northeast China, with treatments including chemical fertilizer application alone (nitrogen, phosphorus, and potassium, NPK) and replacing 1/4 (1/4M), 2/4 (2/4M), 3/4 (3/4M), and 4/4 (4/4M) of chemical fertilizer N with manure N. Soil nutrients, enzymatic activity, and SOC fractions were analyzed to evaluate the effect of different manure substitution ratios on SOC storage. A high ratio of manure substitution (>1/4) significantly increased soil total N, total P, total K, and available nutrients (NO₃⁻-N, available P, and available K), and the 4/4M greatly decreased the C/N ratio compared to the NPK. Manure incorporation increased microbial biomass carbon (MBC) by 18.3–53.0%. Treatments with 50%, 75%, and 100% manure substitution (2/4M, 3/4M, and 4/4M) enhanced bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and total microbial necromass carbon (MNC) by 31.9–63.5%, 25.5–107.1%, and 27.4–94.2%, respectively, compared to the NPK treatment. Notably, the increase in FNC was greater than that of BNC as the manure substitution ratio increased. The increasing manure substitution significantly enhanced particulate organic C (POC) and total SOC but did not affect mineral-associated organic C (MAOC). High soil N and P supplies decreased leucine aminopeptidases (LAPs) and alkaline phosphatase activities but increased the activity ratio of β-glucosidase (BG)/(N-acetyl-glucosaminidase (NAG) + LAP). Treatments with 25% manure substitution (1/4M) maintained maize and soybean yield, but with increasing manure rate, the maize yield decreased gradually. Overall, the high ratio of manure substitution enhanced SOC storage via increasing POC and MNC, and decreasing the decomposition potential of manure C and soil C resulting from low N- and P-requiring enzyme activities under high nutrient supplies. This study provides empirical evidence that the rational substitution of chemical fertilizers with manure is an effective measure to improve the availability of nutrients, and its effect on increasing crop yields still needs to be continuously observed, which is still a beneficial choice for enhancing black soil fertility. Full article

Soil enzymes secreted by microorganisms play a key role in carbon (C), nitrogen (N), and phosphorus (P) metabolism in soil organic matter. As major drivers of climate change, warming and nitrogen addition affect soil physicochemical properties and enzyme activity, but their combined effects on these parameters across different soil aggregate size scales in desert steppes remain unclear. This study used a 2 × 2 factorial split-plot design (control; warming; nitrogen addition: warming + nitrogen addition) conducted from 2006 in Inner Mongolia’s desert steppe. Soil samples were collected in 2018–2019, and aggregates were fractionated into >2000 μm, 250–2000 μm, and <250 μm sizes using a modified dry-sieving method. Physicochemical properties and enzyme activities were measured. Our results show that warming significantly reduced the total nitrogen (TN) and organic carbon (SOC) content in aggregates, while nitrogen addition significantly decreased the pH value in aggregates but had no significant impact on other soil nutrient content indicators. For soil enzyme activity, warming significantly reduced the activity of Urease and Alkaline Phosphatase (ALP) in soil aggregates, and nitrogen addition significantly reduced the activity of Urease, ALP, and β-glucosidase (BG) in aggregates. However, the size of the aggregates had a significant impact on the activity of Urease and BG. The influence of soil physicochemical properties on different enzyme activities varied across different years. These findings indicate that under the global change scenario, the physicochemical properties and enzyme activity of desert steppe soils are affected by warming and nitrogen addition to varying degrees, and the impact of these two factors shows significant differences across different years. Moreover, the interactive effects of warming and nitrogen addition did not simply result in an additive effect influenced by single factors. Full article

In the context of China’s ecosystem facing a high intensity of nitrogen loads, carbon–nitrogen interactions are receiving increasing attention. Physical protection by soil aggregates is critical for soil carbon and nitrogen sequestration in terrestrial ecosystems; however, there is currently limited information on how nitrogen addition influences carbon and nitrogen dynamics across different stages of forest ageing. Herein, a field nitrogen manipulation experiment over 6 years was established in subtropical forests (46, 78, and about 200 years old) in China. Aggregate fractions and stable isotope analyses were used to assess the effects of nitrogen addition. The results show that forest soil was dominated by macroaggregates, and these increased with forest ageing (p > 0.05). The macroaggregates’ carbon content decreased with forest ageing (p > 0.05), while the macroaggregates’ nitrogen content was highest in the 200-year-old forest. Nitrogen addition increased the aggregates’ carbon and nitrogen concentrations in the 46- and 200-year-old forests. The macroaggregates, under nitrogen addition in the 78- and 200-year-old forests, were relatively weak, while forest age and nitrogen addition mainly affected macroaggregate carbon and nitrogen concentrations to promote their carbon and nitrogen storage, and the macroaggregates were the main storage unit for fixing and protecting new carbon in soils. Nitrogen addition increased the macroaggregates’ δ¹³C abundance in the 78- and 200-year-old forests and decreased it in the 46-year-old forest (p > 0.05); significantly increased the macroaggregates’ δ¹⁵N in the 46-year-old forest (p < 0.05), and decreased the macroaggregates’ δ¹⁵N in the 200-year-old forest (p > 0.05). Considering the distribution of δ¹³C and δ¹⁵N in the aggregates, the effect of nitrogen addition on the dynamic mechanism of soil aggregate carbon and nitrogen fractions varied based on forest age and aggregate size. Correlation analysis further revealed that soil total phosphorus, NH₄⁺-N, NO₃⁻-N, dissolved organic nitrogen, pH, texture, etc., were the primary predictors explaining most of the variation in aggregate fractions and their δ¹³C distribution. In summary, the effect of nitrogen deposition on the carbon and nitrogen stability of soil aggregates in broad-leaved forests depends on forest age. Full article

The interactions between iron oxides and organic carbon within the particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions in paddy soils remain insufficiently understood, yet they are likely crucial for unlocking the carbon sequestration potential of these systems. In this study, we investigated the distribution of soil iron oxides and organic carbon within POM and MAOM fractions following 10 years of continuous irrigation and organic amendment management. We also examined the relationship between iron oxide transformation and these two SOC (soil organic carbon) fractions. Our results demonstrated that, under both flooded irrigation and controlled irrigation regimes, straw return or manure application effectively enhanced soil carbon sequestration, as evidenced by increases in both POM-C (POM-associated organic carbon) and MAOM-C (MAOM-associated organic carbon) contents. Meanwhile, exogenous carbon inputs promoted the transformation of crystalline iron oxides into short-range ordered iron oxides and iron oxide colloids, thereby enhancing the activation and complexation degree of soil iron oxides and facilitating the formation of Fe-bound organic carbon. Further regression analysis revealed that the activation degree of iron oxides had a stronger influence on POM-C, whereas the complexation degree had a greater effect on MAOM-C. This implies that exogenous carbon inputs are effective in promoting soil carbon sequestration in both flooded and water-saving irrigated rice paddies and that iron oxide transformation plays a key role in mediating this effect. Full article