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The decline of soil organic matter (SOM) and micro and macronutrients is among the most serious threats that many agricultural soils in the world are facing. In many countries, soil amendments from a wide range of organic wastes are increasingly used as a win-win strategy to mitigate global warming while enhancing soil fertility and, in turn, food security. Digestate, a by-product of the anaerobic digestion, is often used as a fertilizer and an amendment. However, an in-depth understanding of its impacts on native SOM and organic carbon (OC) stabilization mechanisms is still needed. The aim of this work was to investigate the influence of solid vs. liquid digestates on both SOM accrual and distribution in particulate organic matter (POM) and mineral-associated organic matter (MAOM) pools, after 0, 3 and 6 months from their application. None of the amendments had significant effects on soil OC and total nitrogen concentrations and stocks. However, after 6 months, a clear difference in OC distribution between SOM pools was observed as a function of the digestate phase applied to soil. In particular, the MAOM to POM ratio decreased in the soil amended with solid digestate with respect to the unamended control soil, while this ratio did not differ following liquid digestate amendment. Thermal analysis suggested that the interaction between SOM and mineral surfaces, rather than the biochemical recalcitrance, is the major SOM stabilization mechanism in both unamended and amended soils. This study highlighted that a more in-depth evaluation of OC accrual through digestate application, especially in the short term, can be achieved by investigating functionally defined fractions rather than bulk SOM. [Display omitted] • Solid and liquid digestates had no significant effects on SOC concentration and stock. • Digestate phase affects SOC distribution between SOM pools. • The MAOM to POM ratio decreased in soil amended with solid digestate. • The MAOM to POM ratio in soil amended with liquid digestate and in the control was similar. • Further studies based on longer-term observations and multiple applications are needed. [ABSTRACT FROM AUTHOR]

The strategies of substituting inorganic fertilizers with organic fertilizers have been shown to significantly impact the dynamics of soil organic carbon (SOC) combating soil degradation and climate changes in intensive vegetable production. However, the underlying mechanisms and comprehensive assessment of environmental consequences are not fully understood. We conducted a five-year field experiment in the Yangtze River Delta Alluvial Plain, China, four fertilization treatments were established at equal nitrogen (N) levels: sole mineral fertilizers (SN); organic substituting 20 % (M1N4), 50 % (M1N1), and 100 % (SM) of mineral N fertilizer with organic fertilizer. Soil carbon (C) pools, physical and chemical composition of SOC, and C footprint were measured based on soil aggregates, 13C nuclear magnetic resonance spectroscopy, and life cycle assessment to investigate the effects of organic substitution on SOC quantity, quality, and environmental implications. Our results showed a significant increase in SOC content, particularly in the M1N1 treatment. This increase was attributed to changes in soil properties, an elevation in dissolved organic C content, and improved SOC stability. Specifically, organic substitution increased the proportion of macroaggregates, content of lignin phenols, and enrichment of alkyl C groups. Based on life cycle assessment incorporating field measurements, the average C footprint ranged from 1.82 to 2.58 CO 2 kg eq kg−1 vegetable in the five-year experiment, primarily influenced by transportation and fertilizer manufacturing. Partial organic substitution reduced the C footprint by 6.2–10.1 %, while the SM treatment increased it by 0.54 CO 2 eq kg−1 vegetable, and the M1N1 had the lowest C footprint at 1.82 CO 2 eq kg−1 vegetable. As a whole, this approach of substituting 50 % of mineral N with organic fertilizers serves as a viable strategy to bolster soil C stabilization, promote C accumulation, and mitigate the C footprint linked to intensive vegetable production in subtropical regions. [Display omitted] • SOC content increased in proportion to 0–20-50–100 % organic substitution strategies. • SOC changes were driven by soil physicochemical properties and compositions. • Organic substitutions promoted SOC stabilization and reduced the C footprint. • 50 % substitution had a higher SOC increase and generated the minimum C footprint. [ABSTRACT FROM AUTHOR]

Long-term fertilization regimes can substantially change soil organic carbon (SOC) dynamics within different soil fractions. However, the underlying mechanisms of these effects, particularly under manure substitution regimes, remain elusive. Soils from different fertilization regimes [i.e., control, mineral NPK fertilization, and NPK with three manure substitution rates (30%, 50%, and 70%)] in a 33-year red paddy field experiment in southern China were sampled. The mean weight diameter (MWD) of soil aggregates and soil microbial properties (PLFAs), SOC content, and chemical composition of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) under different treatments were determined to investigate the effects of manure substitution regimes on SOC quantity and quality. The results showed mineral NPK fertilization alone significantly (p < 0.05) decreased the MWD of soil aggregates by 14.4%, whereas 30%, 50%, and 70% manure replacement significantly (p < 0.05) increased the MWD by 12.0%, 19.9% and 27.2%, respectively. Compared to the mineral NPK fertilization regime, manure substitution practices continuously and significantly (p < 0.05) increased SOC along with its content in microbial biomass C and POC when the substitution rate of manure increased from 30% to 70%. Regardless of the substitution rates, manure substitution regimes decreased the O-alkyl C and aryl C proportions of POC, but increased the recalcitrant alkyl C resulting in higher alkyl C to O-alkyl C ratios compared with mineral NPK fertilization. Pearson analysis further showed that the O-alkyl C of POC were more associated with the soil microbial biomass C and N and total PLFAs. Partial least squares path modeling analysis indicated that microbial biomass C, POC, and MAOC were positive factors impacting SOC, whereas the promotion of manure substitution on SOC stabilization progressed mainly through increasing POC content. Our results suggested that substitution of 70% mineral N with manure N is the most effective fertilizer practice for improving POC-related SOC sequestration in rice paddy soil of subtropical regions. • Manure substitution significantly increased SOC relative to NPK fertilization. • The functional groups from POC were more associated with the soil microbial traits. • Manure promoted SOC stabilization mainly by increasing POC formation. • Manure substitution was more beneficial to form POC with abundant recalcitrant C levels. [ABSTRACT FROM AUTHOR]

Abstract: The study was conducted to investigate the effects of slope gradient and land use change on soil structural stability, and soil organic carbon (SOC) and total nitrogen (TN) pools in aggregate-size fractions in western Iran. Three land uses in the selected site were natural forest (NF), disturbed forest (DF) and cultivated land (CL); and three classes of slope gradient (0–10%, S1; 10–30%, S2; and 30–50%, S3) were used as a basis for soil sampling. The results showed that DF and CL treatments significantly decreased soil structural stability indices in the three slope classes. The highest percentages of macro-aggregates (i.e. 2.00–4.75mm) and meso-aggregates (0.25–2.00mm) were found in the lowest slope class (S1) which was related to high SOC stock in this position. The highest percentage of macro-aggregates was observed in the NF soil; but the highest percentages of micro-aggregates (0.053–0.25mm) were observed in the CL treatment. Micromorphological observations confirmed that topsoil under natural forest mainly consisted of highly-porous crumb microstructure, excremental pedo-features or passage features, which are indicators of enhanced SOC and biological activity. The lowest values of SOC and TN were observed at the steep slope class (S3) presumably coincided with accelerated soil erosion. Overall, enhanced aggregation and aggregate-associated organic carbon pools were observed in the forest soils on the steep slopes indicating the importance of land management on C sequestration in natural environments. [Copyright &y& Elsevier]

• Reduced mineral fertilizers (F) with green manure were investigated for 10 years. • All treatments significantly improved soil macroaggregation (0.25−2 mm). • 37.5 t + F80 and 30 t + F60 plots were prominent to sequester SOC within aggregates. • The SOC within aggregates was mainly distributed in mSOC <0.053 mm density fraction. • c-iPOC 0.25−2mm and f-iPOC 0.053−0.25mm were most sensitive indicators to SOC sequestration. A 10-year field experiment with different inorganic fertilizer rates, NPK (F; 100 %, 80 % and 60 %) supplemented with Chinese milk vetch, Astragalus sinicus L. (MV; 15 t ha−1, 22.5 t ha−1, 30 t ha−1 and 37.5 t ha−1) as green manure was conducted in a fluvo-aquic paddy soil in Southern China. The study aimed to assess soil water stable aggregates (WSA) and their stability indices, aggregate associated SOC and physical protection mechanisms of SOC within aggregate associated density fractions (fLOC, c-iPOC, f-iPOC and mSOC) at 0−20 cm soil depth. The results showed that soil macroaggregation was significantly improved (P < 0.05) with combined application of NPK and green manuring, by 76.7 % and 10.1 % with 37.5t + F80, and by 80.6 % and 12.5 % with 30t + F60, compared to the control (CK) and F (100 % NPK) treatments, respectively. Aggregate stability indices such as mean weight diameter (MWD) and geometric mean diameter (GMD) were increased under 37.5t + F80 (51.4 % and 22.7 %) and 30t + F60 (54.3 % and 22.7 %) treatments compared to CK. Bulk SOC concentration was positively correlated with MWD (R2 = 0.62) and GMD (R2 = 0.59). Compared to CK, bulk soil OC was significantly increased by 31.0 % with 22.5 t + F80, 38.8 % with 37.5 t + F80 and by 18.0 % with 30 t + F60. With reduced mineral fertilization combined with green manure (MV), SOC concentrations of the macroaggregates (0.25−2 mm), microaggregates (0.053−0.25 mm) and silt + clay fraction (<0.053 mm) were 18.0 %, 11.2 % and 6.8 % higher than CK. The highest proportion of OC added with MV residues was distributed in mSOC fraction isolated from macroaggregates (0.25−2 mm) and microaggregates (0.053−0.25 mm). However, c-iPOC 0.25−2 mm and f-iPOC 0.053−0.25 mm fractions were the most sensitive indicators of the increased SOC, suggesting a shift of SOC towards microaggregates and thereby long-term SOC sequestration within the microaggregates through physical protection mechanisms. Moreover, f-iPOC was positively correlated (slope = 0.051; R2 = 0.17) with bulk soil OC indicating that f-iPOC was main SOC accumulating fraction within the microaggregates. It can be concluded that reduced mineral fertilization combined with green manuring (especially, 22.5 t + F80, 37.5 t + F80 and 30 t + F60) could be an efficient strategy to improve soil structure and intra-aggregate SOC sequestration in fluvo-aquic paddy soil. [ABSTRACT FROM AUTHOR]

• The fine silt and clay (FSi + Cl) particle size fractions were reacted with Fe(III). • Samples were investigated by EXAFS spectroscopy. • Fe(III)-SOM complexes and Fe(III) polymerization can vary in different environments. • Pairing LCF with WT allowed to differentiate lighter backscattering elements. • Fe speciation is not clearly related to SOM amount or quality. Iron (Fe) speciation in soils is highly dependent on environmental conditions, mineralogy, and chemical interactions with soil organic matter (SOM). The fine silt and clay (FSi + Cl) particle size fraction of soils constitutes a primary organo-mineral fraction and contains SOM with long turnover time. In this study, the FSi + Cl particle size fractions isolated from a coniferous forest, a grassland, a technosol, and an agricultural soil were reacted with Fe(III) at pH 7. Unreacted and reacted samples were then investigated by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. Statistical methods were used to determine goodness-of-fit parameters for linear combination fitting (LCF) and wavelet transformation (WT) of the Fe K-edge EXAFS data. WT separated spectral contributions from different backscattering atoms in higher coordination shells located at similar interatomic distances from the central absorbing Fe atom. LCF results paired with WT showed that the FSi + Cl particle size fractions consisted of a mixture of Fe phyllosilicates, Fe (hydr)oxides, and organically complexed Fe in different proportions. Our research revealed that after sorption experiments, in which Fe(III) was added to the system, increasing amounts of Fe(III)-SOM complexes were found in the solid phase of grassland and agricultural soils, whereas the precipitation of Fe(III) led to the preferential formation of ferrihydrite in the coniferous forest soil and in the technosol. Although the quantitative Fe-mediated organic carbon stabilization effect after Fe(III) addition is shown in this work, Fe speciation is not clearly related to SOM amount or quality (i.e. , carbon-to-nitrogen ratio). The variation of Fe chemical speciation among the soil fractions likely translates into differences in their environmental fate. [ABSTRACT FROM AUTHOR]

• SOC stock was higher in conservation tillage than conventional tillage. • Light fractions can be responsive indicators of the changes in SOC. • OLF in conventional tillage was richer in aromatic C than in conservation tillage. The adoption of conservation tillage is promising for soil management system in the Loess Plateau of China; however, how conservation tillage influences the molecular structure contributes and long-term stabilization of soil organic matter (SOM) is still not clear in this area. In this study, experimental plots were continually cultivated with winter wheat (Triticum aestivum L.) and soil samples were collected in September 2009 and September 2015, 10 and 16 years after the experiment was initiated, respectively. Four treatments were applied: reduced tillage (RT), no tillage (NT), sub-soiling (SS) and conventional tillage (CT). Soil samples were physically fractionated into five fractions: free light fraction (FLF), occluded light fraction (OLF), coarse sand particulate organic matter (c-POM), fine sand particulate organic matter (f-POM) and mineral associated organic matter + silt + clay (m-SOM). The soil organic carbon (SOC) stocks were analysed in SOM fractions and bulk soil (BS). Solid state 13C cross polarization/total sideband suppression nuclear magnetic resonance (13C CP/TOSS NMR) spectroscopy was applied to determine the abundance of different forms of carbon (C) (carboxyl C, O/N-alkyl C, aromatic C and alkyl C) in the soil physical fractions. The SOC stocks in the 0–30 cm layer were influenced by the tillage systems. Although there were no significant differences in SOC stocks among these four treatments in the 0–10 and 10–20 cm layers in first 10 years, after 16 years, the SOC stocks were highest in the SS and NT treatments in the 0–10 cm layer. As the duration of the experiment increased from 2009 to 2015, the SOC content of m-SOM-C fraction under RT and CT also increased. As determined by 13C NMR, in the light fractions (FLF to OLF), the abundance of aromatic C increased when the abundance of O/N-alkyl C declined and there was a higher aromatic C/O/N-alkyl C ratio. A gradual increase in the abundance of alkyl C of heavy fractions was accompanied by a decrease in aromatic C, which might be due to the microbial by-products deposited on mineral surface in the soils. In addition, a higher SOM preservation were shown in conservation tillage (NT and SS) by our NMR data. Particular interesting were the results of the light and c-POM fractions, in which conservation tillage showed a higher abundance of O/N-alkyl C and a lower aromatic C/O/N-alkyl C ratio in these two fractions compared with conventional tillage (RT and CT).However, these changes were not evident in the heavy fractions. The changes in the chemical structures of the light and c-POM fractions were possibly due to the retention of crop residues and low levels of soil disturbance. In conclusion, long-term conservation tillage affected the fractions and the chemical functional groups of SOM and could be ideal to preserve SOC in the Loess Plateau. [ABSTRACT FROM AUTHOR]