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This study evaluates the impact of conservation agriculture (CA) on soil carbon sequestration and crop productivity in the Indo-Gangetic Plains, focusing on short-term effects over 3 and 5 years. Conducted at two distinct sites, Karnal and Samastipur, the research compares zero tillage, permanent raised beds, and conventional tillage systems across diverse cropping patterns. Initial findings after 3 years showed no significant differences in carbon and nitrogen stocks at Karnal, while Samastipur's maize-mustard-mungbean rotation on permanent raised beds showed increased carbon stocks. Notably, after 5 years, significant differences in soil carbon stocks emerged at both sites, with improved organic matter input indicated by coarse particulate organic matter (cPOM) formation. The study confirms the potential of POXC and POC as early indicators for carbon sequestration in CA systems, highlighting the role of CA practices in enhancing soil health and crop productivity sustainably. [ABSTRACT FROM AUTHOR]

How different fertilization practices modify soil organic carbon (SOC) sequestration is still unclear. Our study aimed to evaluate the changes in SOC stocks and their physical fractions after 10 years of organic and inorganic fertilization. Five treatments were established under a wheat-maize system in Northern China: control (CK), chemical fertilizer (F), straw plus chemical fertilizer (SF), manure plus chemical fertilizer (MF), and straw and manure plus chemical fertilizer (SMF). The results showed that the SOC sequestration rate at 0–20 cm depth decreased in the following order: SMF (1.36 Mg C/ha/yr) > MF (1.13 Mg C/ha/yr) > SF (0.72 C/ha/yr) > F (0.15 Mg C/ha/yr) > CK (−0.25 Mg C/ha/yr). The values indicated that straw returning and manure application were important measures to achieve the "4 per 1000" target, and the application of manure was a more effective strategy. The high input of chemical fertilizer only maintained the initial SOC level and was not a powerful C-farming practice. A minimum input of 4.93 Mg C/ha/yr was required to keep the initial SOC storage. The SOC associated with small macroaggregate (0.25–2 mm) was the most sensitive indicator for the changes of bulk SOC. In addition, the accumulation of SOC under SMF, MF, and SF treatments mainly occurred in the occluded particulate organic C (oPOC) in small macroaggregates, indicating that the physical protection of macroaggregates played a predominant role in SOC sequestration. The SMF, MF, and SF treatments also displayed higher mineral organic C (mSOC) in soil aggregates than the CK and F treatments. A transformation of oPOC towards the mSOC fraction indicated that exogenous C further shifted into stable C pools under the physical protection of soil aggregates. In conclusion, these findings confirmed the important role of straw returning and manure application in SOC accumulation and stabilization, highlighting that a combination strategy of straw + manure + chemical fertilizer had the best effect. [ABSTRACT FROM AUTHOR]

This study evaluates the impact of conservation agriculture (CA) on soil carbon sequestration and crop productivity in the Indo-Gangetic Plains, focusing on short-term effects over 3 and 5 years. Conducted at two distinct sites, Karnal and Samastipur, the research compares zero tillage, permanent raised beds, and conventional tillage systems across diverse cropping patterns. Initial findings after 3 years showed no significant differences in carbon and nitrogen stocks at Karnal, while Samastipur’s maize-mustard-mungbean rotation on permanent raised beds showed increased carbon stocks. Notably, after 5 years, significant differences in soil carbon stocks emerged at both sites, with improved organic matter input indicated by coarse particulate organic matter (cPOM) formation. The study confirms the potential of POXC and POC as early indicators for carbon sequestration in CA systems, highlighting the role of CA practices in enhancing soil health and crop productivity sustainably.

How different fertilization practices modify soil organic carbon (SOC) sequestration is still unclear. Our study aimed to evaluate the changes in SOC stocks and their physical fractions after 10 years of organic and inorganic fertilization. Five treatments were established under a wheat-maize system in Northern China: control (CK), chemical fertilizer (F), straw plus chemical fertilizer (SF), manure plus chemical fertilizer (MF), and straw and manure plus chemical fertilizer (SMF). The results showed that the SOC sequestration rate at 0–20 cm depth decreased in the following order: SMF (1.36 Mg C/ha/yr) > MF (1.13 Mg C/ha/yr) > SF (0.72 C/ha/yr) > F (0.15 Mg C/ha/yr) > CK (−0.25 Mg C/ha/yr). The values indicated that straw returning and manure application were important measures to achieve the “4 per 1000” target, and the application of manure was a more effective strategy. The high input of chemical fertilizer only maintained the initial SOC level and was not a powerful C-farming practice. A minimum input of 4.93 Mg C/ha/yr was required to keep the initial SOC storage. The SOC associated with small macroaggregate (0.25–2 mm) was the most sensitive indicator for the changes of bulk SOC. In addition, the accumulation of SOC under SMF, MF, and SF treatments mainly occurred in the occluded particulate organic C (oPOC) in small macroaggregates, indicating that the physical protection of macroaggregates played a predominant role in SOC sequestration. The SMF, MF, and SF treatments also displayed higher mineral organic C (mSOC) in soil aggregates than the CK and F treatments. A transformation of oPOC towards the mSOC fraction indicated that exogenous C further shifted into stable C pools under the physical protection of soil aggregates. In conclusion, these findings confirmed the important role of straw returning and manure application in SOC accumulation and stabilization, highlighting that a combination strategy of straw + manure + chemical fertilizer had the best effect.

Purpose: The mechanisms responsible for soil organic matter (SOM) stabilization under long-term organic amendment (OA) in Andosols remain elusive. Thus, this study was aimed at unraveling the mechanisms underlying the stabilization of SOM in Andosols as a consequence of long-term OA application. Methods: The SOM in Andosols from three treatments, chemical fertilizer (CF), fish cake (FC), or rapeseed oil cake (ROC) applied for 32 years, was examined. The SOM was fractionated using heavy liquid separation into free SOM (fSOM), occluded light SOM (oSOM), weakly bound form SOM (wSOM), and strongly bound form SOM (sSOM) fractions. Each fraction was characterized using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and thermally assisted hydrolysis and methylation combined with gas chromatography/mass spectrometry (THM-GC/MS). The δ13C signatures were also determined. Results: The total C in ROC and FC soils was equivalent to 112% and 103% of that in CF soil, respectively. The δ13C suggested that OA-derived C had accumulated in fSOM, oSOM, and wSOM fractions (FC soil) or fSOM (ROC soil). 13C NMR spectra revealed that O-alkyl C was the major C group (24–47%), especially in fSOM and sSOM fractions. THM-GC/MS data indicated that the yield of N-containing compounds was the highest in sSOM fraction while that of aromatic compounds other than lignin-phenols was the highest in fSOM fraction. The yield of other aromatic compounds in SOM fractions was larger in ROC soil than in other soils. Conclusion: These findings suggest that the mechanisms responsible for stabilizing SOM in Andosols as influenced by the application of OA involve the selective preservation of OA-derived aromatic components in the fSOM and oSOM fractions and the inaccessibility of metabolic products due to the intimate association with mineral surfaces. [ABSTRACT FROM AUTHOR]

Physical soil organic matter (SOM) fractions provide increased insight into the biogeochemical functioning of soils. Several fractionation methodologies have been developed to separate particulate (POM) from mineral-associated organic matter (MAOM) fractions either by particle size, particle density, or some combination of these two properties. The proliferation of approaches to separation has led to ambiguity regarding what these methodologically defined fractions should be understood to conceptually represent, hindering robust data synthesis analyses and model development. Here we attempt to identify chemical similarities and differences amongst POM and MAOM fractions separated by different physical approaches, with the aim of guiding fractionation choices for future research. We obtained soils from 11 farms across the United States with variable parent materials, textures, and pHs, and fractionated them using 4 methods. We tested two single-step methods: a size fractionation (53 µm cutoff) and a density separation (1.85 g cm−3 cutoff), and two multi-step methods: a combined size and density separation after full dispersion, which isolates a third pool of organic matter associated with coarse, heavy particles (coarse heavy associated organic matter; CHAOM), and a combined method with delayed dispersion to separate the free POM from the occluded POM + CHAOM fraction. We analyzed all fractions for C and N concentrations, their isotopic composition, and for their chemical composition via mid-infrared spectroscopy. We found that MAOM fractions tended to be very homogenous in character regardless of fractionation schemes, while POM varied widely. In particular, to POM isolated by density floatation was distinct in both isotopic and spectroscopic signature compared to POM isolated by size fractionation alone. Indeed, our results indicated that POM isolated by size is a composite fraction of light POM and CHAOM, and does not map well onto a conceptual pool. We found the CHAOM fraction to be more similar to MAOM in terms of chemical and spectroscopic composition, indicating that one-step density separations may be the most effective means of isolating contrasting SOM pools in a time and cost-efficient matter. However, our analysis highlights that POM, CHAOM and MAOM exhibit distinct features, and the three fractions are worth separating via a combined size-density fractionation when possible.

The effect of drip irrigation on the origin and size fraction of soil organic carbon was studied in the soils of an apple orchard (Malus domestica Borkh.) on hilly (20%) terrain in northeastern Slovenia in three slope positions (upslope, midslope and downslope), comparing irrigated with non-irrigated soils. Physical fractionation of soil organic carbon was performed on three soil layers (0-0.05, 0.05-0.15 and 0.15-0.30 m) in three size fractions: fraction A (> 0.0002 m), fraction B (0.0002‒0.00005 m) and fraction C (< 0.00005 m). Fraction A was the richest in soil organic carbon (7.7%), but fraction C was the dominant fraction in the total soil volume (86‒92%), making it the largest source of soil organic carbon (73%). The δ13C signature was performed to determine the existence of two different types (origins) of soil organic carbon: fresh and sedimentary. Fresh organic carbon dominates in the A fraction, while sedimentary organic carbon dominates in the C fraction and may contribute to higher structural stability, besides higher carbonates in the finest fraction. Irrigation mainly contributes to the higher stock of soil organic carbon (predominantly fresh and less sedimentary) in the coarse A fraction (21.14 t/ha in irrigated and 14.17 t/ha in non-irrigated soils).

To investigate the stabilization mechanism of microbial necromass carbon (C) in rice paddy soil, the accumulation of microbial necromass and its contribution to SOC in bulk soil and in various fractions were addressed under a 40-year fertilization trial. The fertilization regimes included no fertilizer (Control), nitrogen, phosphorus and potassium inorganic fertilizer (NPK), and inorganic fertilizer plus manure (NPKM). Coarse particulate organic matter (cPOM), fine inter-microaggregate POM (fPOM), intra-microaggregate POM (iPOM), non-occluded silt plus clay fraction (s + c_f), and silt plus clay occluded within microaggregates (s + c_m) fractions were isolated. The results indicated that fungal necromass were the main component of microbial necromass across the fractions; however, the bacterial role was enhanced in mineral fractions (s + c_f and s + c_m) compared to POM fractions (cPOM, fPOM, and iPOM). The iPOM and mineral fractions stored more than 90 % of microbial necromass C due to the physical protection (organo-mineral associations and occlusion within microaggregates). Compared to Control, the significantly increased content of microbial necromass C and its contribution to SOC in bulk soil were attributed to the enhanced content of fungal necromass in iPOM fraction under NPK treatment. The highest content of microbial necromass C was found under NPKM treatment; moreover, the increased content of microbial necromass C in iPOM and s + c_m fractions made up to 82.80 % of that in bulk soil compared to Control, while a saturation behavior was found in s + c_f fraction. The contribution of microbial necromass to SOC was not altered under NPKM treatment compared to Control. However, a reduced fungal/bacterial necromass C ratio was found in bulk soil and iPOM fraction under NPKM treatment compared to Control, mainly due to the supply of higher quality substrates and the increased soil pH. As a whole, this study revealed the diverse responses of microbial necromass C accumulation and its contribution to SOC under different fertilization regimes in paddy soil.

The effect of drip irrigation on the origin and size fraction of soil organic carbon was studied in the soils of an apple orchard (Malus domestica Borkh.) on hilly (20%) terrain in northeastern Slovenia in three slope positions (upslope, midslope and downslope), comparing irrigated with non-irrigated soils. Physical fractionation of soil organic carbon was performed on three soil layers (0-0.05, 0.05-0.15 and 0.15-0.30 m) in three size fractions: fraction A (> 0.0002 m), fraction B (0.0002±0.00005 m) and fraction C (< 0.00005 m). Fraction A was the richest in soil organic carbon (7.7%), but fraction C was the dominant fraction in the total soil volume (86±92%), making it the largest source of soil organic carbon (73%). The δ13C signature was performed to determine the existence of two different types (origins) of soil organic carbon: fresh and sedimentary. Fresh organic carbon dominates in the A fraction, while sedimentary organic carbon dominates in the C fraction and may contribute to higher structural stability, besides higher carbonates in the finest fraction. Irrigation mainly contributes to the higher stock of soil organic carbon (predominantly fresh and less sedimentary) in the coarse A fraction (21.14 t/ha in irrigated and 14.17 t/ha in non-irrigated soils). [ABSTRACT FROM AUTHOR]

This study quantified the C content of the chemical and physical fractions of SOM in different management systems in an Argisoil of sandy texture. The study was carried out in a reference area of Native Forest (NF), and in three managed areas: Permanent Pasture (PP), No-Tillage System (NTS) and an area of Private Natural Heritage Reserve (PNHR) in the process of natural regeneration. Soil samples were collected in the layers 0.0-0.05, 0.05-0.10 and 0.10-0.20 m. We assessed the soil density (Sd), total organic carbon (TOC) content, chemical fractionation of SOM with determination of the C contents of the fulvic acids (FA), humic acids (HA) and humin (HUM), with subsequent calculations of the HA/FA ratios, AE/HUM, stock (StockC), physical granulometric fractionation and determination of C contents of particulate organic matter (C-POM) and carbon management index (CMI). Higher TOC contents were observed for the NF area. The C-HA and C-HUM contents were higher in the NF and NTS. NF showed higher C-POM levels in all layers evaluated. For the C-MOM, the NTS area was superior to the other managed areas. The managed areas had lower StockPOM values than the NF. The managed areas had lower CMI values in relation to NF. The NTS area showed that, even in crop succession, it contributes to the improvement of the soil organic fraction over the adoption time. On the other hand, the areas of PP and PNHR showed that inadequate management favors the reduction of edaphic quality.

[Display omitted] • Applies a novel approach based on the check dam deposit of an individual event. • Establishes the inter-gully areas as the main source of the sediment-associated TOC. • The main sources of the two organic C fractions (POC and MOC) were different. • Approximately 10% of the TOC mobilized by erosion was lost during delivery to the dam. • The lost carbon was dominated by POC, while MOC accounted for ca. 13% of the loss. Information on the sources and fate of organic carbon mobilized by soil erosion is crucial for understanding the impact of erosion on carbon (C) cycling. However, few studies have evaluated this in low-order stream systems. In this study, we adopted a novel approach, that used the sediment deposited behind a check dam constructed at the outlet of the Fengzigou catchment (0.91 km2) on the Chinese Loess Plateau by a single high magnitude storm event, to simplify the field sampling requirements. The total sediment-associated organic carbon (TOC) was fractionated into particulate organic carbon (POC) and mineral associated organic carbon (MOC). These results were combined with those from a sediment source tracing exercise. This made it possible to explore further the source and loss of the sediment-associated organic carbon. The results showed that the inter-gully areas contributed the most eroded TOC (52.3 ± 7.6 %) and, together with the gully walls, represented the main TOC source. The primary contributor of the POC fraction in the deposited sediment was the inter-gully areas (80.5 ± 12.6 %), whereas the gully walls were the main contributor of the MOC fraction (54.9 ± 10.2 %). The total output of sediment produced by the event was 1304 ± 48 t and the equivalent value for sediment-associated TOC was 2.94 ± 0.26 t, of which the POC fraction accounted for 27.6 ± 3.6 % and the MOC fraction 72.4 ± 9.8 %. The TOC lost (0.3 ± 0.02 t) through the mobilization and delivery processes was 10.2 ± 1.2 % of the TOC mobilized by erosion, and the lost carbon was dominated by the POC fraction (86.7 ± 8.8 %). The specific TOC output from the catchment associated with the event documented was 0.3 t km−2. This work demonstrates the potential for erosion-induced carbon redistribution to provide a source for atmospheric CO 2 in the study area. [ABSTRACT FROM AUTHOR]

As atmospheric carbon (C) concentrations increase, so too has interest in understanding the mechanisms that preserve C within the soil organic matter (SOM). Mineral-associated organic matter (MAOM) is one pool of SOM recently shown to protect soil organic C from mineralization. However, most MAOM research has been in agriculture and forest mineral soils. Given the magnitude of soil C stored in wetlands, this study sought to determine the abundance of MAOM in wetlands. A standard method for quantifying MAOM in terrestrial soils was tested, modified, and optimized with three unique wetland soils. Using a physical fractionation method followed by a density fractionation, it was concluded that field moist soils and a dispersant were necessary for fractionating wetland soils (0–50 cm). The Bayhead Swamp soil had the greatest total C (484.07 ± 1.25 mg C g soil -1), but the smallest MAOM pool (0.63 %); most of the C was in the light fraction as particulate organic matter (POM, >98 %). The sandy-peat Cypress Dome soil also lacked MAOM (1.09 %) and was dominated by POM (66.26 %). The silt-rich Brackish Marsh had the least total C (94.37 ± 1.67 mg C g soil -1), but the largest fraction of protected C (24.60 % MAOM). Nitrogen (N) followed similar trends to C, except in the Brackish Marsh where nearly half the total N was MAOM. This research represents the first step in evaluating the role of MAOM in C persistence in organic-rich wetland soils; future work should consider C physical fraction, in addition to C quantity.

Soil organic carbon (OC) is one of the most important soil components regulating soil quality, fertility and agronomic productivity as well as the global carbon (C) cycle. Soil acts as a sink for global C, which can be influenced by the water regime and organic matter (OM) management in field. The aim of this study is to evaluate the effect of the application of different organic amendments on C sequestration in paddy soils under contrasting irrigation regimes. A 4-month pot experiment was conducted under net house conditions and the treatments were composed of two organic amendments: rice straw (RS) and poultry manure (PM); four application rates of amendment: 0 g (control), 2.5 g, 5.0 g and 15.0 g kg−1 soil; and two irrigation regimes: (i) continuous waterlogging condition (CWL) and (ii) alternate wetting and drying (AWD). After the incubation period, soil samples were collected from the pot and isolated into labile (>53 µm) and mineral-associated (<53 µm) OM. Bulk (before and after incubation) and fractionated soil samples were analyzed for OC, total nitrogen (N), C:N ratio; and C sequestration percentage was calculated. Relatively higher amounts of soil OC were present in CWL condition (1.23%) than AWD (1.13%). The C sequestration potential also showed the similar trend (CWL: 47% > AWD: 35%). This was explained by the induced aerobic condition in between the anerobic condition in AWD and the continuous anaerobic condition in CWL which resulted in a difference in OM decomposition. The mineral-associated OM fraction (<53 µm) was higher in the CWL condition than AWD condition which also indicated the importance of the chemical stabilization of OC (OC bound to minerals) in the CWL condition. The application of PM led to a significant increase (45%) in C sequestration potential than RS (37%). This could be attributed to C:N ratio and probable biochemical composition of amendments which resulted in lower decomposability of PM than RS, and also in line with the higher distribution of OC in mineral-bound OM than labile fraction. The application of higher organic amendments did not increase OC content, and declined C sequestration potential in soils as the microbial activity presumably did not match with the amendment amount. Overall, C sequestration potential was higher with 5 g PM kg−1 soil application under CWL-irrigated paddy soil. The findings indicated the need to pay more attention to the selection of the proper type and rate of organic amendments for higher C sequestration in soil under a specific irrigation system for sustainable agriculture. [ABSTRACT FROM AUTHOR]

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Soil organic carbon (SOC) plays a key role in improving soil quality and optimizing crop yield. Yet little is known about the fate of macroaggregates (>0.25 mm) under long-term fertilization and their relative importance in SOC sequestration in reclaimed calcareous soil. Therefore, the effects of mineral fertilizers and organic manure on the mechanisms of organic carbon (OC) stabilization in macroaggregates were investigated in this study. Four treatments were used: unfertilized control (CK), mineral fertilizer (NPK), compost chicken manure alone (M), and mineral fertilizers plus manure (MNPK). Samples from the 0–20 cm layer of soil receiving 11-year-long fertilization were separated into four fractions based on the macroaggregates present (unprotected coarse and fine particulate organic matter, cPOM and fPOM; physically protected intra-microaggregate POM, iPOM; and biochemically protected mineral associated OM, MOM) by the physical fractionation method. Compared with the control, the long-term application of NPK had little effect on SOC content, total nitrogen (TN) content, and OC and TN contents of macroaggregate fractions. In contrast, incorporation of organic manure (MNPK) significantly increased SOC (45.7%) and TN (24.3%) contents. Application of MNPK increased OC contents within macroaggregate-extracted fractions of cPOM (292.2%), fPOM (136.0%) and iPOM (124.0%), and TN contents within cPOM (607.1%), fPOM (242.5%) and iPOM (127.6%), but not the mineral associated organic carbon (MOM-C) and nitrogen (MOM-N) contents. Unprotected C fractions were more strongly and positively correlated with SOC increase than protected C fractions, especially for cPOM-C, indicating that SOC sequestration mainly occurred via cPOM-C in the studied calcareous soil. In conclusion, MNPK increased the quantity and stability of SOC by increasing the contents of cPOM-C and cPOM-N, suggesting that this management practice (MNPK) is an effective strategy to develop sustainable agriculture.

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]

Soils represent important pools of soil organic carbon (SOC) that can be greatly influenced by labile C inputs, which are expected to increase in future due to CO2 enrichment of atmosphere and a concomitant rise in plant primary productivity. Studying effects of variable labile C inputs on SOC pool helps to understand how soils respond to global change. However, this knowledge is missing for coniferous forest soils despite being widespread throughout the northern temperate zone. We conducted a 7-month field manipulation experiment to study the effects of variable labile C inputs (simulated by additions of C4 sucrose) on the C content in soil fractions and on microbial abundance in the organic (O), surface mineral (A), and subsoil mineral (B) horizons of a temperate coniferous forest soil. SOC in less-protected soil fractions and total organic C were substantially decreased by labile C additions that simulated future increases in C inputs. The SOC losses were comparable between the A and B horizon (40% vs. 30%). However, because sucrose availability estimated from its incorporation into soil fractions and microbial biomass sharply decreased with soil depth, the loss of C was higher in the B than in the A horizon when related to the amount of sucrose added. Utilization of sucrose was highest by fungi in the O horizon and by bacteria in the mineral soil horizons. The results indicate that future increases in labile C inputs to coniferous forest soils will cause rapid and substantial losses of SOC in both the surface and subsoil mineral horizons. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Applied Ecology / Yingyong Shengtai Xuebao is the property of Chinese Journal of Applied Ecology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Density separations isolated a POM fraction distinct from size separations. • Size fractionated POM represented a composite fraction. • Both size- and density-based separations isolated similar MAOM fractions. • Soil properties did not affect fractionation results in a consistent manner. • Fractionations employing a density separation step are recommended for future research. Physical soil organic matter (SOM) fractions provide increased insight into the biogeochemical functioning of soils. Several fractionation methodologies have been developed to separate particulate (POM) from mineral-associated organic matter (MAOM) fractions either by particle size, particle density, or some combination of these two properties. The proliferation of approaches to separation has led to ambiguity regarding what these methodologically defined fractions should be understood to conceptually represent, hindering robust data synthesis analyses and model development. Here we attempt to identify chemical similarities and differences amongst POM and MAOM fractions separated by different physical approaches, with the aim of guiding fractionation choices for future research. We obtained soils from 11 farms across the United States with variable parent materials, textures, and pHs, and fractionated them using 4 methods. We tested two single-step methods: a size fractionation (53 µm cutoff) and a density separation (1.85 g cm−3 cutoff), and two multi-step methods: a combined size and density separation after full dispersion, which isolates a third pool of organic matter associated with coarse, heavy particles (coarse heavy associated organic matter; CHAOM), and a combined method with delayed dispersion to separate the free POM from the occluded POM + CHAOM fraction. We analyzed all fractions for C and N concentrations, their isotopic composition, and for their chemical composition via mid-infrared spectroscopy. We found that MAOM fractions tended to be very homogenous in character regardless of fractionation schemes, while POM varied widely. In particular, to POM isolated by density floatation was distinct in both isotopic and spectroscopic signature compared to POM isolated by size fractionation alone. Indeed, our results indicated that POM isolated by size is a composite fraction of light POM and CHAOM, and does not map well onto a conceptual pool. We found the CHAOM fraction to be more similar to MAOM in terms of chemical and spectroscopic composition, indicating that one-step density separations may be the most effective means of isolating contrasting SOM pools in a time and cost-efficient matter. However, our analysis highlights that POM, CHAOM and MAOM exhibit distinct features, and the three fractions are worth separating via a combined size-density fractionation when possible. [ABSTRACT FROM AUTHOR]

Abstract When aboveground materials are harvested for fuel production, such as with Sorghum bicolor, the sustainability of annual bioenergy feedstocks is influenced by the ability of root inputs to contribute to the formation and persistence of soil organic matter (SOM), and to soil fertility through nutrient recycling. Using 13C and 15N labeling, we traced sorghum root and leaf litter‐derived C and N for 19 months in the field as they were mineralized or formed SOM. Our in situ litter incubation experiment confirms that sorghum roots and leaves significantly differ in their inherent chemical recalcitrance. This resulted in different contributions to C and N storage and recycling. Overall root residues had higher biochemical recalcitrance which led to more C retention in soil (27%) than leaf residues (19%). However, sorghum root residues resulted in higher particulate organic matter (POM) and lower mineral associated organic matter (MAOM), deemed to be the most persistent fraction in soil, than leaf residues. Additionally, the overall higher root‐derived C retention in soil led to higher N retention, reducing the immediate recycling of fertility from root as compared to leaf decomposition. Our study, conducted in a highly aggregated clay‐loam soil, emphasized the important role of aggregates in new SOM formation, particularly the efficient formation of MAOM in microaggregate structures occluded within macroaggregates. Given the known role of roots in promoting aggregation, efficient formation of MAOM within aggregates can be a major mechanism to increase persistent SOM storage belowground when aboveground residues are removed. We conclude that promoting root inputs in S. bicolor bioenergy production systems through plant breeding efforts may be an effective means to counterbalance the aboveground residue removal. However, management strategies need to consider the quantity of inputs involved and may need to support SOM storage and fertility with additional organic matter additions.

No‐tillage farming combined with temporary cattle grazing is becoming a common practice in parts of South America. We quantified the effects of no‐tillage and winter grazing of annual ryegrass (Lolium multiflorum Lam.) on soil carbon and nitrogen stock, stratification ratio (concentration in 0–5 to 10–20‐cm layer) and lability (based on particulate organic matter, POM), relative to conventional tillage or ungrazed ryegrass. A 9‐year‐old experiment was conducted in a southern Brazilian Ferralsol. Soil under no‐tillage accumulated 1.11 Mg C ha−1 year−1 and 0.10 Mg N ha−1 year−1 to 100‐cm depth relative to conventional tillage when ryegrass was not grazed, and also increased the stratification ratio of carbon (1.48 vs. 1.11) and nitrogen (1.66 vs. 1.17). The carbon and nitrogen lability was also greater in no‐tillage soil, as carbon and nitrogen stocks increased proportionally more in sand‐POM than in silt or clay size fractions (carbon lability index = 1.57 vs. 1.00; nitrogen lability index = 1.57 vs. 1.00). Semiquinone concentrations in physical fractions were 4–28% smaller in no‐tillage, suggesting less organic matter aromaticity in this treatment. Grazing, however, caused no further improvement of stocks of carbon and nitrogen in the 0–100‐cm layer of no‐tillage soil (e.g., 209 vs. 212 Mg C ha−1, ns), or enhancement of stratification or lability of those elements, relative to ungrazed ryegrass. Overall, no‐tillage effectively promoted soil carbon and nitrogen accumulation and lability in this subtropical Ferralsol, whereas grazing did not compromise the gains of no‐tillage. Highlights: Capacity of no‐tillage and grazing to enhance stocks, stratification and lability of soil C and N.C sequestration with no‐tillage is being debated, and little is known when combined with grazing.No‐tillage accumulated 1.11 Mg C ha−1 year−1 but grazing promoted no further C accumulation.No‐tillage effectively accumulated C in soil and grazing did not compromise such gain of no‐tillage. [ABSTRACT FROM AUTHOR]

Manure application and no‐till management improve soil quality, crop yields and C inputs, but little specific knowledge is available regarding the combined effect of long‐term dairy liquid manure (DLM) application and no‐till management on soil organic matter status and soil structure in topsoils. Here, we assess the impacts of surface‐applied DLM for a 10‐year period on soil total organic carbon (TOC) and nitrogen (TN) concentrations and stocks, on organic matter fractions and on soil physical‐structural attributes in a no‐till crop rotation system on clayey Ferralsol under no‐till crop rotation management in a subtropical climate. Four rates of DLM (0, 60, 120 and 180 m3 ha−1 year−1) were applied in a randomized block design with four replications. Soil samples were collected to calculate TOC and TN stocks to 100‐cm depth and organic matter fractions in the 0–5‐cm layer, and physical indicators of soil quality: bulk density, macroporosity and microporosity and aggregate stability. The largest DLM rates (120 and 180 m3 ha−1 year−1) added 3.8–5.8 Mg C ha−1 year−1 as manure, increased crop residue input by 0.6 Mg C ha−1 year−1, and increased TOC stock by 17% and TN stock by 27% in the top 10 cm of soil, at annual accumulation rates that averaged 0.72 Mg C ha−1 year−1 and 86 kg N ha−1 year−1. Stocks of TOC and TN in the sand‐POM (particulate organic matter) fraction under the 180 m3 ha−1 year−1 rate increased by 49 and 63%, and in the silt‐MOM (mineral‐associated organic matter) fraction by 30 and 47%, respectively. In the clay‐MOM fraction, the change in TOC was not significant, but the TN increased by 16%. Soil physical‐structural attributes (bulk density, porosity and aggregates) were improved by DLM application in the 10‐cm layer and followed the soil carbon and nitrogen increments. Overall, the DLM amendment applied on the soil surface of no‐till proved to be a strategy to ameliorate soil organic matter status, an important factor for agronomic and environmental benefits, besides improving crop biomass production. Highlights: Long‐term application of liquid manure increased concentration and stock of SOC and TN in the 0–10‐cm layer.Increases of 0.72 Mg C ha−1 year−1 and 86 kg N ha−1 year−1 were observed in the 10‐cm topsoil with manure.SOC and TN increased in the sand‐POM and silt‐MOM size fractions but not the clay‐MOM fraction.Improvements to soil physical‐structural attributes were related to organic matter changes. [ABSTRACT FROM AUTHOR]

The extent by which the contribution of eucalypt harvest residues (HR) to soil organic carbon (SOC) is impacted by soil disturbance during and/or after harvesting is unclear. We addressed this question by following the decomposition of HR in microplots (15-cm diameter, 15-cm height) and determining their contribution to SOC in 10 sites across southeastern Brazil. The experiment was set up according to a complete randomized block design arranged into a 3 × 2 factorial scheme including: HR removal (HR0), only bark removal (HR − B) and HR including bark (HR + B) applied to: undisturbed soil (US) with HR left on the soil surface or disturbed soil with HR mixed into the first 5 cm of the topsoil. We had eight blocks as replicates. Following a 12-month field decomposition, soil samples were fractionated to isolate SOC within the particle-size fractions (PSF) greater and lower than 53 µm. Subsequently, we used 13C to quantify the effects of the treatments in C3-derived SOC content (C3-SOC) within each PSF. For both PSF, their C3-SOC content increased in response to HR − B or HR + B relative to HR0, depending on soil disturbance and HR half-life time (t0.5). For HR − B, net increments in C3-SOC within the PSF > 53 µm increased significantly with HR t0.5, regardless of soil disturbance. Otherwise, the C3-SOC content within the PSF < 53 µm increased with HR t0.5 in response to both HR types, but only for US treatments. Overall, in regions where climate drives fast decomposition rates, on-site disturbance during and/or after harvesting should be minimized to favor the contribution of HR to SOC, particularly within the PSF < 53 µm. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC), the largest terrestrial carbon pool, plays a significant role in soil‐related ecosystem services such as climate regulation, soil fertility and agricultural production. However, its fate under land use change is difficult to predict. A major issue is that SOC comprised of numerous organic compounds with potentially distinct and poorly understood turnover properties. Here we use spatiotemporal measurements of the particulate (POC), mineral‐associated (MOC) and charred SOC (COC) fractions from 176 trials involving changes in land use to assess their underlying controls. We find that the initial pool sizes of each of the three fractions consistently and dominantly control their temporal dynamics after changes in land use (i.e. the baseline effects). The effects of climate, soil physicochemical properties and plant residues, however, are fraction‐ and time‐dependent. Climate and soil properties show similar importance for controlling the dynamics of MOC and COC, while plant residue inputs (in term of their quantity and quality) are much less important. For POC, plant residues and management practices (e.g. the frequency of pasture in crop‐pasture rotation systems) are substantially more important, overriding the influence of climate. These results demonstrate the pivotal role of measuring SOC composition and considering fraction‐specific stabilization and destabilization processes for effective SOC management and reliable SOC predictions. [ABSTRACT FROM AUTHOR]

Reconstructing the provenance of siliciclastic marine sediment is important for understanding sediment pathways and constraining palaeoclimate and erosion records. However, physical fractionation of different size fractions can occur during sediment transport, potentially biasing records derived from bulk sediment. In this study, records of radiogenic Sr and Nd isotopic composition and K/Al ratio of the separated clay fraction, as well as bulk grain size, are presented, measured from deep-sea sediments recovered from International Ocean Discovery Program (IODP) Sites U1456 and U1457 in the Arabian Sea. These new records are compared with published bulk sediment records to investigate the influence of sediment transport on these proxies and to constrain provenance evolution and its relationship to climate variability since middle Miocene time. Correlations between grain size and the bulk sediment isotopic composition confirm that transport processes are influencing the bulk sediment record. This relationship, although present, is not as strong in the clay-fraction isotopic records. Heterogeneity of bulk sediment likely drives differences between bulk and clay records, thought to be largely controlled by sediment transport processes. The isotopic records reveal variations in provenance that correlate with climatic change at 8–7 Ma, as well as an increase in overall provenance variability beginning at c. 3.5 Ma, likely linked to monsoon strength and glacial–interglacial cycles. The clay-fraction records highlight the potential value of measuring proxy records from multiple size fractions to help constrain provenance records as well as investigate sediment transport and/or weathering and erosion processes recorded in deep-sea sediment archives. [ABSTRACT FROM AUTHOR]

The distribution of total organic matter (Corg), particulate organic matter (CPOM), and potentially mineralizable organic matter (C0) in mega- (10–5 and 5–2 mm), macro- (2–0.25 mm), and microaggregates (<0.25 mm) isolated by dry sieving from gray and agrogray soils (Luvic Retic Greyzemic Phaeozems) of different land uses was studied. In the soils under forest and meadow, the highest Corg content was detected in the megaaggregates of 5–2 mm in size (43 and 37% of Corg in whole soil sample, respectively), whereas in the arable soil, it was found in macroaggregates (45%). In the uncultivated soil, the CPOM was mainly accumulated in megaaggregates (65–72% of the CPOM in the whole sample); in the arable soil, it was equally distributed between mega- and macroaggregates (46–45%). The fine (0.25–0.05 mm) CPOM subfraction in the uncultivated and arable soils contained 1.3- and 2.3-fold more carbon, respectively, as compared with the coarse (2–0.25 mm) CPOM subfraction. The C0 content in the aggregates correlated with CPOM and Corg. Decomposition of plant residues with a wide C : N ratio in soil increased with a decrease in the aggregate size. However, aggregate size did not influence the decomposition of plant residues with a narrow C : N ratio. The effect of aggregate size on the decomposition rate mainly appeared at the early stages of transformation of plant residues. [ABSTRACT FROM AUTHOR]

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This research provides an assessment of the Ethiopian sugarcane bagasse (ESCB) and examines its suitability for the pulp and paper production. The study is focused toward carrying out the physical fractionation pretreatment on the ESCB aiming at the reduction of its lignin, extractive, silica and ash content. A measurement on the chemical composition was carried out to determine the cellulose, lignin, holocellulose, ash and silica content. Furthermore, cold and hot water solubility, 1% NaOH solubility and the presence of ethanol–toluene extractives were measured. Chemical composition analysis of sugarcane bagasse revealed a good level of cellulose (≈ 50%) and Klason lignin content (< 30%). The measurement of the ESCB fiber dimensions (fiber length of 1.86 mm, fiber diameter of 30.02 µm, cell-wall thickness of 2.53 µm) advocates its application in the paper and pulp industry. The physical fractionation pretreatment had a significant effect on reducing the lignin, extractive, ash and silica content in the ESCB, as could be evidenced from the FTIR analysis. These results demonstrate the efficacy of Ethiopian sugarcane bagasse and validate it as a suitable raw material for pulp and paper industry. [ABSTRACT FROM AUTHOR]

Agricultural soils have lost enormous amounts of soil organic matter (SOM) as the result of conventional agriculture. Nowadays, returning plant biomass in the form of crop residues is used as an efficient strategy to increase the amount of SOM. In addition, earthworms help increase the SOM content by transforming OM into stable forms. There is, however, limited information on how earthworms transform and stabilize various forms of crop residues and what the potential feedbacks on soil quality are. In a five-month laboratory manipulation experiment, we added maize biomass and/or biochar to a temperate agricultural soil both in the presence and absence of earthworms and carried out physical fractionation to analyse the stabilization of OM either as aggregate-occluded particulate OM (oPOM) or mineral-associated OM (MAOM). We show that the combination of maize biomass, maize biochar and earthworms proved to be highly efficient in increasing the OM content in agricultural soils and that earthworms can simultaneously enhance both OM decomposition and stabilization. In the absence of earthworms, the OM was stabilized more in the oPOM fraction, suggesting that both maize substrates acted as aggregation agents. In the presence of earthworms, however, maize substrates were stabilized more in the MAOM fraction, either through direct sorption as plant-derived substrates or as microbial necromass, suggesting that earthworms facilitate stabilization of both more-available maize biomass and more-recalcitrant maize biochar on mineral particles. We provide evidence that plant-derived rather than microbial-derived MAOM is present in the earthworm-affected agricultural soil and is thus important in increasing the SOM content and stability. • Earthworms can simultaneously enhance both OM decomposition and stabilization. • In the absence of earthworms, the OM was stabilized more in the oPOM fraction. • In the presence of earthworms, the OM was stabilized more in the MAOM fraction. • Plant-rather than microbial-derived MAOM is present in the earthworm-affected soil. [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]

The application of biochar to soils, either alone or combined with other amendments, represents a management practice aimed at storing carbon (C) while enhancing soil fertility. However, the long-term effects of biochar application on soil organic C protection against microbial decomposition are uncertain. This study investigated, in a 9-year-long-term field experiment, the protection of organic C by minerals and iron (Fe) in soils amended with biochar alone or combined with either municipal solid waste compost or sewage sludge. Particulate (not protected by minerals) and mineral-associated organic matter fractions were separated, quantified, and finally the Fe-mediated protection mechanisms were examined by Fe K-edge extended X-ray absorption fine structure spectroscopy. With respect to the unamended control soil, soils amended with biochar, especially when combined with municipal solid waste compost and sewage sludge, had 3 times greater content of particulate organic C. Biochar combined with municipal solid waste compost and sewage sludge also increased mineral-associated organic C content (1.5×), although the magnitude of the effect was smaller than for the particulate organic C fraction. The contents of Fe(III)-organic matter complexes in particulate and mineral-associated organic matter fractions of the amended soils were similar to those of the unamended soils. Hematite represented the main Fe oxide phase in the particulate organic matter fraction of all the soils as well as in the mineral-associated organic matter fraction of the unamended soils, whereas ferrihydrite was more abundant in the mineral-associated organic matter fraction of the amended soils. As a whole, the obtained results in general, and the positive effect on the mineral-associated organic matter possibly mediated by ferrihydrite occurrence in particular, highlight the potential of biochar, alone or in combination with other amendments, as a strategy to store and preserve C in soils. • The effect of biochar and other amendments on SOM pools after 9 years was evaluated. • Biochar increased the MAOM pool especially when co-applied with other amendments. • Ferrihydrite played a fundamental role in the organic C stabilization in the MAOM. • Long-term (co)application of biochar positively affected soil organic C accrual.. [ABSTRACT FROM AUTHOR]

Theories and points of view of soil science classics, such as V. V. Dokuchaev, I. V. Turin and M. M. Kononva on chernozem organic matter are reviewed in this work. The achievements of the leading researches of soil biology and biochemistry of Dokuchaev Soil Science Institute (D. V. Khan, M. Sh. Shaimuhammetov, N. A. Titova etc.) in the investigation of chemical and physical functioning of organic matter and the products of organo-mineral interaction of chernozemic soils. The author variant of level system for organic matter content in chernozem is presented in the work. The level system consists of minimal, median, maximal and critical levels. The convergent scheme of the investigation of the organo-mineral interactions of soils is presented. This scheme combines the Savvinov sieve method in modification of Khan and granulo-densimetric fractionation functioning by Shaimuhammetov-Travnikova, chemical and biokinetic fractionation. The particular examples of the application of this scheme on chernozems in conditions of long-term experiments is presented.

The long-term stabilization of organic matter in mineral soils is widely believed to be mainly driven by its interaction with the mineral matrix rather than by its recalcitrance. This interaction involves several processes, and the relative importance of each one may vary according to the ecosystem type. Here we investigate organo-mineral interactions in soils across different ecosystems (coniferous and broadleaved forests, grasslands, and technosols), widely differing in their soil organic matter (SOM) content, by an approach involving two consecutive steps: (i) a size fractionation by wet sieving after sonication, to isolate the particulate organic matter (POM, >20 μm) from the organo-mineral complex (OMC, <20 μm), and (ii) a characterization of the OMC by sequential extractions with different chemicals, each one disrupting a specific kind of bond between SOM and active mineral surfaces. The OMC accounted for >60% of total organic C and >75% of total N contents. Differences among ecosystems affected the total amount of SOM, but barely its distribution between POM and OMC. The sequential extractions showed that a substantial amount of the organic matter in the OMC (20–30%) was weakly bound to minerals. Other components of the OMC, in contrast, were much less dominant than expected, i.e., the organic matter stabilized by carbonate coating (5–6% of total organic C in the OMC) and by iron oxyhydroxydes (3–6%). By far, the main pool of the OMC was the final residue remaining after all extractions (34–48% of total organic C in the OMC). Because this residue remains unextractable with NaOH after removing all the active mineral components, such unextractability can be due only to its own chemical characteristics. Future research should aim at its in-depth characterization and at elucidating its ecological functions. • Size fractionation was coupled with an extensive set of chemical extractions. • The distribution of organic C and N among size fractions is little affected by land use. • Irrespective of land use, the organo-mineral complex (OMC) accounts for most of TOC and N. • A substantial part of the organic matter in the OMC is bound loosely. • For all land uses, the highest organic C contents are found in humin. [ABSTRACT FROM AUTHOR]

• The mango residues deposition influenced the C from the SOM fractions. • The increase of C from the SOM fractions was observed only in the mango planting row. • Even in fragile soil, management practices in mango results in sequestration of TOC. The Sub-middle São Francisco Valley is a center of intense activity of irrigated fruit farming, which may lead to alterations in soil attributes and processes compared with areas under native vegetation, in this case Caatinga. One of the attributes sensitive to such alterations is soil organic matter (SOM), considering the climatic semi-arid conditions, fragile soils (sandy soils) and the adopted management such as the organic residues deposition from pruning, goat manure fertilization, high availability of water, and nutrients. Thus, in order to evaluate the influence of irrigated mango cultivation on the C stocks of the SOM fractions in a Neossolo Quartzarênico in Petrolina – PE, Brazil, in two consecutive years (2014 and 2015), soil samples were collected in the 0–0.1 and 0.1–0.2 m layers in areas under irrigated mango (planting row and interrow) and Caatinga (taken as reference). The following variables were determined: contents of total organic carbon (TOC); carbon associated with humic substances – HS (fulvic acid fraction – FAF, humic acid fraction – HAF and humin fraction – HF); carbon associated with light fraction (LtF) and heavy fraction (HvF); and labile C (LC). Samples were collected to determine soil density, and the stocks of TOC and C associated with SOM fractions were calculated. Irrigated mango cultivation (soils collected in the row) led to higher stocks of TOC, C of HF and HS, compared with the Caatinga in both years evaluated. Irrigated mango cultivation (soils collected in the row) also resulted in increase in the stock of C in LtF in the 0–0.1 m layer in 2014. In the second year of evaluation, irrigated mango cultivation (soils collected in the row) caused increments of 26.09 and 35.29% in the TOC of the 0–0.1 and 0.1–0.2 m layers, respectively. There were no differences in the C of HS in the soil along the row of irrigated mango trees in the 0-0.1 and 0.1–0.2 m layers between 2014 and 2015. On the other hand, for soils of the interrow of irrigated mango, the C stocks of HS, HF and HvF were lower in 2015 than in 2014. In the semiarid climate and fragile soils, the increase in C stocks of SOM fractions is more limited to the row of irrigated mango, compared with the Caatinga. [ABSTRACT FROM AUTHOR]

Evidence is accumulating that belowground crop residues contribute more efficiently to the build‐up and maintenance of native soil organic carbon (SOC) than aboveground plant parts. We corroborated previous testing of the hypothesis that root‐C would preferentially accumulate in soil microaggregates, where it is physically protected against microbially mediated decomposition. In three European field trials with C3 to C4 crop transitions, we compared the content of maize‐C (Zea mays L.) in soil from rotations with grain maize (MG) or silage maize (MS) (i.e. with incorporation of roots and shoots or roots only). After decades of maize cultivation, SOC content did not differ within three out of four MG–MS pairs, although obviously larger amounts of shoot biomass were added to soil in the case of MG. We found that relative contribution of roots was on average 3.5 times more than shoots to the build‐up of SOC per equivalent mass of residue C added to soils. Preferential occlusion of root‐C as silt‐sized intra‐microaggregate particulate organic matter (iPOM) was not observed. There were much larger effects from shoot incorporation on maize‐C in the >53‐μm fraction and free silt and clay. Storage of root‐C as sand‐sized iPOM was not quantified here, but first estimates suggested that physical entrapment at this level could only partly explain the longevity of root‐C in soil. We reconfirm the relative stability of root‐C in soil, but do not conclude that this stems from preferential physical entrapment over shoot‐C. Future work should investigate the cause of preferential root‐C association with the clay‐sized fraction and if this occurs before or after microbial processing. Highlights: The hypothesized preferential physical stabilization of root‐C was assessed.Aboveground biomass had a minor effect on SOC storage in European long‐term trials.Contribution of maize root‐C to maintenance of SOC was three‐fold that of shoot‐C inputs.There was no preferential occlusion of root‐C in silt‐sized microaggregates. [ABSTRACT FROM AUTHOR]

Moso bamboo (Phyllostachys edulis) is widely distributed in southern China, and is one of the fastest growing plants worldwide; however, information remains limited on the impact of converting secondary broad-leaved evergreen forests to Moso bamboo plantations, and how the soil organic carbon (SOC) pool and its chemical composition should be managed subsequently. To elucidate these effects, three representative areas were chosen, all with very similar site conditions. In each area, four comparable stands were selected; namely, undisturbed (M0), extensively managed (M1), and intensively managed (M2) stands in each Moso bamboo plantation, and a secondary broad-leaved evergreen forest (CK). Soil samples were collected and examined from depths of 0–20 and 20–40 cm in all 12 stands. The results showed that, SOC and mineral-associated organic matter C (MOM-C) stocks in 0–40 cm soil depths were significantly higher in M0 and M1 than in CK; however, these two parameters were significantly lower in M2. M0 and M1 showed a significant decline in the ratio of microbial biomass C (MBC) to total organic C (TOC), hot-water-extractable organic C (DOC) to TOC, and the C mineralization rate. However, M2 showed a significant increase compared to CK for all of these parameters. Fourier-transform infrared spectroscopy (FTIR) showed that land-use conversion also changed SOC chemical composition. Compared with CK and M2, M0 and M1 showed lower relative content of polysaccharides and higher content of recalcitrant compounds and soil hydrophobicity. Aliphatic and aromatic compounds were positively correlated with accumulated C sequestration in all fractions but negatively correlated with microbial activity in both soil layers; thus, chemical protection mechanism was important for stabilizing the soil in M0 and M1. Overall, Moso bamboo plantations with management strategies M0 and M1 could stabilize more C through promoting the formation of stable organic-mineral complexes and the accumulation of resistant organic components, showing much higher potential in terms of soil C sequestration than M2. • Undisturbed (M0), extensive managed (M1) bamboo plantations could accumulate C significantly in soil. • Mineral associated organic C was the major mechanism for C sequestration in bamboo forest soils. • M0 and M1 significantly increased the abundance of stable chemical groups of SOC by forming high soil hydrophobicity. • Management strategies influenced chemical composition and stability of soil organic C. [ABSTRACT FROM AUTHOR]

When aboveground materials are harvested for fuel production, such as with Sorghum bicolor, the sustainability of annual bioenergy feedstocks is influenced by the ability of root inputs to contribute to the formation and persistence of soil organic matter (SOM), and to soil fertility through nutrient recycling. Using 13C and 15N labeling, we traced sorghum root and leaf litter‐derived C and N for 19 months in the field as they were mineralized or formed SOM. Our in situ litter incubation experiment confirms that sorghum roots and leaves significantly differ in their inherent chemical recalcitrance. This resulted in different contributions to C and N storage and recycling. Overall root residues had higher biochemical recalcitrance which led to more C retention in soil (27%) than leaf residues (19%). However, sorghum root residues resulted in higher particulate organic matter (POM) and lower mineral associated organic matter (MAOM), deemed to be the most persistent fraction in soil, than leaf residues. Additionally, the overall higher root‐derived C retention in soil led to higher N retention, reducing the immediate recycling of fertility from root as compared to leaf decomposition. Our study, conducted in a highly aggregated clay‐loam soil, emphasized the important role of aggregates in new SOM formation, particularly the efficient formation of MAOM in microaggregate structures occluded within macroaggregates. Given the known role of roots in promoting aggregation, efficient formation of MAOM within aggregates can be a major mechanism to increase persistent SOM storage belowground when aboveground residues are removed. We conclude that promoting root inputs in S. bicolor bioenergy production systems through plant breeding efforts may be an effective means to counterbalance the aboveground residue removal. However, management strategies need to consider the quantity of inputs involved and may need to support SOM storage and fertility with additional organic matter additions. [ABSTRACT FROM AUTHOR]

Abstract Soil organic matter (SOM) changes with land use and soil management, yet the controlling factors over the chemical composition of SOM are not fully understood. We applied quantitative 13C nuclear magnetic resonance and spectral editing techniques to measure chemical structures of SOM from different land use types. The land use types included a native grassland (nGL), a crop land with straw burning in the field (bCL), a restored grassland (rGL) and a cropland with straw removed out of the field (rCL) for 28 years. The abundances of O CH groups from carbohydrates were higher in the SOMs of the nGL and rGL than in those of the rCL and bCL, while the abundances of OCH 3 and aromatic C O groups from lignin were higher in the SOMs of the three-ever cultivated lands (rGL, rCL and bCL) than in that of the nGL. Although aromatic C C groups were most dominant in the Mollisols, they did not consistently decrease after the burnings of straw were ceased in the fields of the rCL and rGL compared to the bCL with continuous burning. In addition, the COO groups were bound with the aromatic C C groups in all the land use types, and the sizes of the aromatic clusters were affected by the land use types. The labile and recalcitrant components were correlated with SOC contents the mineral-associated and particular SOM in a contrasting way. Our results suggested that the chemical composition of SOM in the Mollisol depended on land use types, and that labile and recalcitrant components might be protected through mineral associations and aggregation, respectively. The most abundant aromatics in the Mollisols might not just be pyrogenic and could be oxidized to different extents, depending on field drainage conditions. Graphical abstract Unlabelled Image Highlights • Restored and native grasslands were compared with croplands with straw removed and burnt • Long-term cropping from native grassland resulted in decreases in carbohydrates and increases in lignin • No straw burning increased lignin oxidization and reduced the size of aromatic cluster • Labile and recalcitrant components may be protected in light and heavy fractions, respectively [ABSTRACT FROM AUTHOR]

Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40 year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ15N and ∆14C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ15N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆14C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change. We investigated effects of tree species planted in replicated plots of a common garden on five indices of soil organic matter (SOM) stability (heterotrophic soil respiration, bulk soil δ15N and ∆14C, and C in particulate and mineral associated organic matter). Our results suggest tree species regulate SOM stability via the chemical composition of their tissues, especially roots. Some of our stability indices (C in mineral associated SOM and bulk soil ∆14C), though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change. [ABSTRACT FROM AUTHOR]

Data on the contents of particulate organic matter (POM) in the humus horizons of different soil types were obtained. The influence of the land use system and fertilizers on the carbon of particulate organic matter (CPOM) in gray forest soil and typical chernozem was shown. The mineralization rate of POM under constant conditions of temperature and moisture was estimated, and the ratios between the pools of POM and biologically active organic matter in the soils were calculated. In the soils of natural cenoses, the percentage of POM together with the sand particle-size fraction varied within 12–36% of the soil mass, and the CPOM content varied from 1.66 to 8.03% of the fraction mass. The portion of CPOM in the total Corg ranged from 20 to 48%. Cultivated gray forest soil and typical chernozem were depleted of CPOM by 3.0–3.2 and 2.0–2.8 times, respectively, in comparison with the samples of noncultivated soils. The mineralization rate of POM in the gray forest soil and typical chernozem was about the same (7.9–12.1 and 7.2–8.2% of CPOM, respectively), which was about 1.2–2.4 times higher than the mineralization rate of the total soil organic matter. It was noted that POM could be a sensitive indicator of the Corg dynamics in the soil, a good predictor of changes in the content and quality of soil organic matter, and a significant source of potentially mineralizable organic matter in the soil. [ABSTRACT FROM AUTHOR]

The state of Rio Grande do Sul has about 20% of the total area as lowland soils, suitable for flooded rice (Oryza sativa). In order to mitigate damage caused by rice monoculture, new crops such as sorghum (Sorghum bicolor) and soybean (Glycine max) have been cultivated in these areas. With different qualities of crop residues, it is expected a change in soil organic matter (SOM) dynamics and consequently, nitrogen (N) availability. The objective of this study was to evaluate the influence of rice, soybean and sorghum crop residues on the N present in physical fractions of SOM of two lowland soils, using labeled 15N technique, under incubation for 180 days in aerobic condition and more 180 days in anaerobic condition. At 30, 180 and 360 days of incubation the remaining N of the plant residues and N destination from the residues in both soils were quantified in the physical fractions of SOM >250 µm, 250-53 µm and <53 µm. The soil with higher amount of clay+silt received a larger quantity of 15N from residues, while flooding of the soil after 180 days caused a loss of N added to the soil by the soybean and sorghum residues. In general, larger amounts of 15N were reported in the fraction <53 µm, associated with clay minerals, throughout the incubation period. These N losses should be considered in N fertilization for the following crops in rotation with flooded rice. [ABSTRACT FROM AUTHOR]

Abstract Elevated CO 2 (eCO 2) may enhance soil organic carbon (SOC) sequestration via greater input of photosynthetic carbon (C). However, greater rhizodeposits under eCO 2 may stimulate microbial decomposition of native SOC. This study aimed to examine the status and stability of SOC in three Australian cropping soils after long-term CO 2 enrichment. Samples (0–5 cm) of Chromosol, Vertosol and Calcarosol soils were collected from an 8-year Free-air CO 2 Enrichment (SoilFACE) experiment and were used to examine SOC dynamics by physical fractionation and incubation with 13C-glucose. Compared to the ambient CO 2 (aCO 2) (390–400 μmol mol−1), 8 years of elevated CO 2 (eCO 2) (550 μmol mol−1) did not increase SOC concentration of all soils, but changed SOC distribution with 12% more C in coarse soil fractions and 5% less C in fine fractions. Elevated CO 2 also enhanced the susceptibility of SOC to 13C-glucose-induced priming, but this effect was only significant in the coarse-textured Calcarosol topsoil. The eCO 2 history increased labile C (coarse C fraction, +13%) and soil pH (+0.25 units), and decreased available N (−30%) in the Calcarosol, which stimulated microbial biomass C by 28%, leading to an enhanced priming effect. Despite with greater total primed C, the Chromosol that had the highest amount of native C, had lower primed C per unit of SOC when compared to the low-C Calcarosol. In conclusion, the effect of long-term eCO 2 enrichment on soil C and N availability in cropping soils depended on soil type with the coarse-textured Calcarosol soil being more susceptible to substrate-induced decomposition of its SOC. Graphical abstract Unlabelled Image Highlights • Elevated CO 2 (eCO 2) did not increase soil organic C in topsoil after 8 years. • Long-term eCO 2 increased C distribution to the >53 μm fraction. • Elevated CO 2 increased the priming effect in the coarse-textured Calcarosol. • Soil organic C stability: Chromosol >Vertosol > Calcarosol [ABSTRACT FROM AUTHOR]

Abstract Soil organic matter (SOM) protection, stability and long-term accumulation are controlled by several factors, including sorption onto mineral surfaces. Iron (Fe) has been suggested as a key regulator of SOM stability, both in acidic conditions, where Fe(III) is soluble, and in near-neutral pH environments, where it precipitates as Fe(III) (hydr)oxides. The present study aimed to probe, by sorption/desorption experiments in which Fe was added to the system, the mechanisms controlling Fe(III)-mediated organic carbon (C) stabilization; fine silt and clay (FSi + Cl) and fine sand (FSa) SOM fractions of three soils under different land uses were tested. Fe(III) addition caused a decrease in the organic C remaining in solution after reaction, indicating an Fe-mediated organic C stabilization effect. This effect was two times larger for FSa than for FSi + Cl, the former fraction being characterized by both low specific surface area and high organic C content. The organic C retained in the solid phase after Fe-mediated stabilization has relatively low sensitivity to desorption. Moreover, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy indicated that Fe-mediated organic C stabilization can be mainly ascribed to the formation of complexes between carbohydrate OH functional groups and Fe oxides. These results demonstrate that the binding of labile SOM compounds to Fe(III) contributes to its preservation, and that the mechanisms involved (flocculation vs. coating) depend on the size fractions. Graphical abstract Unlabelled Image Highlights • Fe(III) controls organic C stabilization in fine silt and clay and fine sand SOM fractions. • Fe(III) addition caused a decrease in the organic C in the solution. • The stabilization effect was larger for the fine sand SOM fraction. • The reaction with Fe(III) may promote either flocculation or coating phenomena. • The organic C retained after Fe-mediated stabilization has low sensitivity to desorption. [ABSTRACT FROM AUTHOR]

Highlights • Organic manure applied for 15 years at different application rates in high-altitude region. • Soil available N and K changed little but crop yields increased with increasing application rates. • SOC storages increased only in >0.25 mm aggregates with increasing application rates. • Macroporosity and drainage capacity increased during crop growth period only at high application rates. Abstract Organic amendments may increase crop yields by improving soil nutrient availability and soil structures by enhancing soil organic matter (SOM). Yet how soil structures are improved by increasing organic application rates during crop growth periods and affect crop yields are not well understood. By using a 14-yr field experiment, the objectives of this study were 1) to examine long-term effects of amendment rates of organic manure on crop yields, soil organic carbon (SOC) contents in aggregates and bulk soil, and soil structural and hydraulic properties, and 2) to investigate the effects of SOC changes on the seasonal dynamics of soil aggregates, soil pore structures and hydraulic conductivity. The organic manure was incorporated into soil at the rates of 0 (OM0), 7.5 (OM7.5), 15 (OM15) and 22.5 (OM22.5) Mg ha−1 in combination with balanced mineral N, P and K fertilizers. The manure application rates had negligible effects on soil N and K availability, but increased crop yields. The SOC contents, aggregate stabilities, and hydraulic conductivities changed with time in all the treatments during the soybean growth period in 2014, but in different manners as affected by the manure application rates. With the increase of manure application rates, SOC storages increased in all physical fractions and in the > 0.25-mm aggregates. However, the saturated hydraulic conductivity and effective porosity of > 1-mm macropores increased profoundly only in the treatments of OM15 and OM22.5 after June. The correlation analyses suggested that all the SOC pools might contribute to the stabilization of macroaggregates and that the transformation of organic manure into SOC in macroaggregates might influence the formation of the stable, > 1-mm macropores. These findings indicated that sufficient organic materials (equivalent to total annual crop straw biomass) should be incorporated into soil to maintain the resilience of soil structures in addition to high water retention and drainage capacities during the crop growth period in the high-altitude Mollisol region of China. [ABSTRACT FROM AUTHOR]