Comparative analysis of soil organic carbon and soil properties in landscapes of Kerala: insights from the Western Ghats of India.

Soil organic carbon (SOC) is known to vary among different ecosystems and soilscapes, yet the degree of variation remains uncertain. Comparing SOC levels in undisturbed ecosystems like forests with those in gradually altered ecosystems can provide valuable insights into the impact of land use on carbon dynamics. This study aimed to evaluate the effects of different land uses on soil fertility parameters in the tropical region of Kerala, focusing on forests as well as cultivated agricultural landscape such as coconut, pepper, tapioca, acacia plantations, and mixed home garden cropping systems. Significant variations were observed among different crops and land use systems in terms of soil fertility. Forests exhibited the highest SOC content at 3.78 g kg-1, while acacia plantations showed the lowest at 0.76 g kg-1. Additionally, various soil properties such as different carbon fractions (e.g., humic acid, fulvic acid), total nitrogen, carbon, available nutrients, physical properties, aggregate size fractions, microbial biomass carbon, and spectral signatures differed significantly across the different land uses. These findings suggest a decline in soil fertility in altered ecosystems compared to adjacent forest soils, highlighting the vital role of forests in conserving natural resources and maintaining soil health. In addition, among the different landscapes studied, mixed cropping systems of home gardens sustained soil fertility better than monocropping systems. The observed variations in soil physicochemical properties among different land use types indicate a threat to sustainable crop production. Effective management practices aimed at improving soil fertility and sustaining crop production in these altered ecosystems are essential. This study highlights the importance of adopting appropriate management strategies to conserve soil health and ensure sustainable crop production in tropical landscapes like Kerala. The holistic approach adopted in this study, encompassing a wide range of soil fertility parameters across various land uses, along with its implications for sustainable land management, adds significant novelty and relevance to the existing literature on soil dynamics in tropical regions like Kerala.

The impact of agricultural management on soil aggregation and carbon storage is regulated by climatic thresholds across a 3000 km European gradient.

Organic carbon and aggregate stability are key features of soil quality and are important to consider when evaluating the potential of agricultural soils as carbon sinks. However, we lack a comprehensive understanding of how soil organic carbon (SOC) and aggregate stability respond to agricultural management across wide environmental gradients. Here, we assessed the impact of climatic factors, soil properties and agricultural management (including land use, crop cover, crop diversity, organic fertilization, and management intensity) on SOC and the mean weight diameter of soil aggregates, commonly used as an indicator for soil aggregate stability, across a 3000 km European gradient. Soil aggregate stability (-56%) and SOC stocks (-35%) in the topsoil (20 cm) were lower in croplands compared with neighboring grassland sites (uncropped sites with perennial vegetation and little or no external inputs). Land use and aridity were strong drivers of soil aggregation explaining 33% and 20% of the variation, respectively. SOC stocks were best explained by calcium content (20% of explained variation) followed by aridity (15%) and mean annual temperature (10%). We also found a threshold-like pattern for SOC stocks and aggregate stability in response to aridity, with lower values at sites with higher aridity. The impact of crop management on aggregate stability and SOC stocks appeared to be regulated by these thresholds, with more pronounced positive effects of crop diversity and more severe negative effects of crop management intensity in nondryland compared with dryland regions. We link the higher sensitivity of SOC stocks and aggregate stability in nondryland regions to a higher climatic potential for aggregate-mediated SOC stabilization. The presented findings are relevant for improving predictions of management effects on soil structure and C storage and highlight the need for site-specific agri-environmental policies to improve soil quality and C sequestration.

Minimal Impacts of Microplastics on Soil Physical Properties under Environmentally Relevant Concentrations.

Agricultural soils are a major reservoir of microplastics, and concerns have arisen about the impacts of microplastics on soil properties and functioning. Here, we measured the physical properties of a silt loam in response to the incorporation of polyester fibers and polypropylene granules over a wide range of concentrations. We further elucidated the underlying mechanisms by determining the role of microplastic shape and the baseline effects from the amendment of soil particles. The incorporation of microplastics into soil tended to increase contact angle and saturated hydraulic conductivity and decrease bulk density and water holding capacity, but not affect aggregate stability. Polyester fibers affected soil physical properties more profoundly than polypropylene granules, due to the vastly different shape of fibers from that of soil particles. However, changes in soil properties were gradual, and significant changes did not occur until a high concentration of microplastics was reached (i.e., 0.5% w/w for polyester fibers and 2% w/w for polypropylene granules). Currently, microplastic concentrations in soils not heavily polluted with plastics are far below these concentrations, and results from this study suggest that microplastics at environmentally relevant concentrations have no significant effects on soil physical properties.

Soil aggregate disintegration effects on soil erodibility in the water level fluctuation zone of the Three Gorges Reservoir, China.

Spatial hydrological alterations can affect soil structural stability. Over time, forces induced by water weaken soil aggregates and this has a negative implication to soil health. The Three Gorges Reservoir (TGR) in particular, experienced a long-term hydrological condition and repetitive seasonal water level fluctuations that could affect soil health. The present study was conducted to investigate the effects of different water levels on soil aggregate disintegration rate over time and its relation to soil erosion susceptibility in water reservoirs. Samples from different elevations (155 m, 160 m, 163 m, 166 m, 172 m, and 180 m) in the water level fluctuation zone (WLFZ) were exposed to continuous wet-shaking for 3, 9, 27, 54, and 81 min resulted to different WLF intensity accordingly. The results showed a comparative difference between aggregates size before and after the experiment where micro-aggregates (<0.25 mm) increased with respect to elevations increase. The exponential prediction proved that aggregate stability decreased with the increase of WLF intensity, insisting the effects of continuous hydrological stress to aggregate break-down. A couple of factors definitely confirmed that soil erodibility (k) is primarily determined by disintegration of soil aggregates for the surface soil of the TGR. Despite the fact that Disintegration rate (Dr) and k showed a positive relationship, R2 = 0.73 (p < 0.05), the results showed that the soil properties decreasing Dr also decreases soil erodibility in the study area. Non-effective role of soil organic matter (SOM) for stabilizing soil aggregates was primarily related to water level fluctuations inhibiting decomposition. Relying on the present findings, environmental problems mostly soil erosion in the TGR could be therefore linked to excessive destabilization of soil aggregates. Therefore, the results of this study should play a major role in determining the factors primarily inducing soil erosion in river reservoirs.

Formation of stable aggregates by fluid-assembled solid bridges.

When a colloidal suspension is dried, capillary pressure may overwhelm repulsive electrostatic forces, assembling aggregates that are out of thermal equilibrium. This poorly understood process confers cohesive strength to many geological and industrial materials. Here we observe evaporation-driven aggregation of natural and synthesized particulates, probe their stability under rewetting, and measure bonding strength using an atomic force microscope. Cohesion arises at a common length scale (∼5 µm), where interparticle attractive forces exceed particle weight. In polydisperse mixtures, smaller particles condense within shrinking capillary bridges to build stabilizing "solid bridges" among larger grains. This dynamic repeats across scales, forming remarkably strong, hierarchical clusters, whose cohesion derives from grain size rather than mineralogy. These results may help toward understanding the strength and erodibility of natural soils, and other polydisperse particulates that experience transient hydrodynamic forces.

Evaluating the effects of formation and stabilization of opal/sand aggregates with organic matter amendments.

Opal (SiO2·nH2O, amorphous silica), the by-product of alumina extraction from coal fly ash (CFA), has a strong adsorption capacity and is also an important component of clay minerals in soils. The combining of opal with sand to form artificial soils is an effective disposal strategy for large-scale CFA stockpiles and reduction of environmental risk. Nevertheless, its poor physical condition limits plant growth. Organic matter (OM) amendments have broad potential applications for water-holding and improving soil aggregation. Effects of OMs (vermicompost (VC), bagasse (BA), biochar (BC) and humic acid (HA)) on the formation, stability and pore characteristics of opal/sand aggregates were evaluated through 60-day laboratory incubation experiments. Results demonstrated that four OMs could reduce pH, with BC having the most significant effect, VC significantly increasing the electrical conductivity (EC) and TOC content of the aggregates. Except for HA, other OMs could improve the aggregates' water-holding capacity. The mean weight diameter (MWD) and percentage of >0.25 mm aggregates (R0.25) of BA-treated aggregates were the largest, and BA had the most noticeable contribution to macro-aggregate's formation. The best aggregate stability was obtained with HA treatment, meanwhile the percentage of aggregate destruction (PAD0.25) decreased with the addition of HA. After amendments, the proportion of organic functional groups increased, which favored aggregate's formation and stability; the surface pore characteristics were improved, with the porosity ranging from 70% to 75%, reaching the level of well-structured soil. Overall, the addition of VC and HA can effectively promote aggregates' formation and stabilization. This research may play a key role in converting CFA or opal into artificial soil. The combining of opal with sand to form artificial soil will not only solve the environmental problems caused by large-scale CFA stockpiles but will also enable the comprehensive utilization of siliceous materials in agriculture.

Synergistic effects of biochar and carboxymethyl cellulose sodium (CMC) applications on improving water retention and aggregate stability in desert soils.

Making improvements to the water-holding characteristics and water-erosion resistance of desert soils, particularly in inland extremely arid areas, is vital for achieving both sustainable water resource utilisation and food security. The aim of this study is to evaluate the effects of the co-application of biochar and carboxymethyl cellulose sodium (CMC) on the physical properties of sandy desert soil, including infiltration rate, saturated water conductivity, field water-holding capacity and aggregate stability. Sandy desert soil samples were collected from jujube plantations on the southern edge of the Taklimakan Desert in the Hotan Prefecture, Xinjiang, China. Five CMC application ratios (C0:0, C1:0.01 g/kg, C2:0.02 g/kg, C3:0.04 g/kg and C4:0.08 g/kg) and five biochar application ratios (B0:0, B1:1.0 g/kg, B2:2.0 g/kg, B3:4.0 g/kg and B4:8.0 g/kg) were designed and a total of 11 experimental treatments were performed, which were labelled as CK (control group), B2C0, B2C1, B2C2, B2C3, B2C4, B4C4, B0C2, B1C2, B3C2 and B4C2. Compared with CK, the combined application of biochar and CMC reduced the soil bulk density (BD) by 1.29-9.41% and the saturated hydraulic conductivity (Ks) by 29.64-94.98%, and increased the soil saturated water content (SSWC) by 8.81-30.74% and the water holding capacity (WHC) by 13.91-36.87%. Similarly, the water-stable aggregates that were co-applied with biochar and CMC increased by 29.10-256.86%. This resulted in significant improvement in the stability of sandy desert soil against water erosion. The principal component analysis (PCA) results found B4C4 to have the best comprehensive improvement effect. Therefore, 0.08 g/kg of CMC and 8.0 g/kg of biochar were used as recommended for improving the hydraulic properties of desert soils. Generally, CMC and biochar have a mutually complementary effect on improving sandy desert soil, providing new ideas and approaches for the improvement of soil and the sustainable development of agriculture in desert areas.

Optimizing residue and tillage management practices to improve soil carbon sequestration in a wheat-peanut rotation system.

The sustainable development of agriculture has been challenged by the decline of soil quality and the change of climate. It is well known that soil carbon (C) sequestration plays crucial roles in improving soil structural stability, mitigating greenhouse emissions, and promoting plant nutrient supply. Therefore, a 3-year field experiment was conducted to evaluate the effects of different residue and tillage management practices on soil C sequestration in a wheat-peanut rotation system. Four treatments were studied: moldboard plow tillage with wheat residue returning (PTS), rotary tillage with wheat residue returning (RTS), no tillage with wheat residue mulching (NTS), and no tillage with wheat residue removal (NT). Our results indicated that residue return favored the improvement of soil C sequestration capacity relative to residue removal. In addition, NTS improved soil C sequestration in the surface soil layer (0-5 cm), but markedly reduced soil C sequestration in the deeper soil layers (5-30 cm). NTS thus caused a more obvious soil stratification phenomenon, which was not conducive to improving soil quality. At the 5-30 cm soil depths, the soil labile organic C fractions concentrations, carbon pool management index (CPMI), macroaggregates-associated C storage, intra-aggregate C fractions concentrations, and soil total organic carbon (TOC) storage under PTS were all higher than those under other treatments. Overall, a peanut strategic cultivation management mode that combines moldboard plow tillage and wheat residue return may be used as a reference for optimizing agricultural soil management to achieve the improvement of soil C sequestration capacity in a wheat-peanut rotation system.

Accumulation and relationship of metals in different soil aggregate fractions along soil profiles.

Different aggregates vary in their ability to retain or adsorb metals in soil. Five soil profiles were sampled from different soil horizons and grouped, and the concentrations of Al, Mg, Ca, Fe, Mn, Cd, Cu and Pb were determined in six sizes of aggregates (> 2, 2-1, 1-0.6, 0.6-0.25, 0.25-0.053, < 0.053 mm). Significantly high (p < 0.05) structural stability indexes (SSI) and aggregate stability indexes (ASI) were recorded in the topsoil horizon, which may be attributed to the high soil organic matter (SOM) content in aggregates from topsoil. In addition, ASI and SSI were positively correlated (r = 0.569, p < 0.05) with each other, which indicated that the stability of soil aggregates could contribute to the structural stability of bulk soil. Moreover, accumulation factors (AF), principal component analysis (PCA) and Pearson's correlation coefficients were used for metal element assessment. The results indicated that SOM was not a key factor affecting the accumulation of Ca, Mg, Al, Fe, Mn, Pb, Cd and Cu in soil aggregates. In general, AF values for metal elements in microaggregates (< 0.25 mm) were high, which showed that metals preferred to accumulate in fine soil aggregates. The PCA and Pearson's correlation coefficients indicated that soil parent materials primarily controlled the distribution of Al, Ca, Fe, Mg and Mn, while materials derived from technogenic sources have important impacts on the distribution of Cd, Cu and Pb in soil aggregates along the soil profile.

Interactive global change factors mitigate soil aggregation and carbon change in a semi-arid grassland.

The ongoing global change is multi-faceted, but the interactive effects of multiple drivers on the persistence of soil carbon (C) are poorly understood. We examined the effects of warming, reactive nitrogen (N) inputs (12 g N m-2  year-1 ) and altered precipitation (+ or - 30% ambient) on soil aggregates and mineral-associated C in a 4 year manipulation experiment with a semi-arid grassland on China's Loess Plateau. Our results showed that in the absence of N inputs, precipitation additions significantly enhanced soil aggregation and promoted the coupling between aggregation and both soil fungal biomass and exchangeable Mg2+ . However, N inputs negated the promotional effects of increased precipitation, mainly through suppressing fungal growth and altering soil pH and clay-Mg2+ -OC bridging. Warming increased C content in the mineral-associated fraction, likely by increasing inputs of root-derived C, and reducing turnover of existing mineral-associated C due to suppression of fungal growth and soil respiration. Together, our results provide new insights into the potential mechanisms through which multiple global change factors control soil C persistence in arid and semi-arid grasslands. These findings suggest that the interactive effects among global change factors should be incorporated to predict the soil C dynamics under future global change scenarios.

Validation of RUSLE K factor using aggregate stability in contrasted mediterranean eco-geomorphological landscapes (southern Spain).

Mediterranean mountains are facing great environmental and socioeconomic challenges in the current framework of Global Change. One of these is soil degradation, which is one of the major threats in those territories. Soil degradation is more dramatic where eco-geomorphology and land uses with less vegetation cover promote soil erosion. Soil erosion is influenced by soil erodibility, which can be assessed by different methodologies, e.g. RUSLE K factor and aggregate stability of soils. This study deals with the validation of RUSLE K factor by means of soil aggregate stability analysed in two-contrasted watersheds from one Mediterranean mountainous region in South of Spain, under sub-humid and semiarid climatic conditions. In both of them, landscape dynamic from 1956 to 2016 was analysed in order to characterize the modifications in land uses. A total of 361-soil samples was also taken covering all land uses for analysing aggregate stability of soils as well as those soil properties needed to calculate the RUSLE K factor. The results indicated that: i) landscape dynamic was influenced by changes in land uses contributing mainly to an increment in vegetation cover in the rainiest watershed; ii) the analysed soil properties showed very few significant differences between watersheds and between land uses, especially regarding organic matter content; and iii) the validation of K Factor using aggregate stability was better in the rainiest watershed and, within this one, in the natural land uses and irrigated cultivations, meaning where the biotic factors were more influential. These results implicated more researches are necessary, principally, focussed on the validation of the RUSLE K parameter using different fractions of aggregates as well as considering other eco-geomorphological parameters.

Effect of phosphogypsum and poultry manure on aggregate-associated alkaline characteristics in bauxite residue.

Bauxite residue is a highly alkaline solid waste with poor physical structure which ultimately limits plant growth. Ecological reconstruction is an effective strategy to improve its environmental management, although soil formation process still requires further investigation. Here, an incubation experiment was used to investigate the effects of phosphogypsum and poultry manure, on aggregate size distribution and aggregate-associated exchangeable bases of bauxite residue. Phosphogypsum and poultry manure additions significantly increased the proportion of 2-1 mm residue aggregates and enhanced mean weight diameter (MWD) of residues in the 0-20 cm and 20-40 cm layers, although little effect was evident in the 40-60 cm layer. Phosphogypsum addition reduced pH and EC values to approximately 8.5 and 200 mS/cm in different size aggregates at 0-20 cm. Exchangeable Ca2+ concentration was improved, especially in 0.25-0.05 mm and <0.05 mm aggregates, following amendment additions. The relative contents of katoite and cancrinite in >0.25 mm aggregate fractions were relatively higher, which was consistent with changes in pH. Phosphogypsum and poultry manure changed the microstructure and surrounding pores of residue aggregates, whilst the concentration of Ca on microaggregate surfaces was higher than that on macroaggregates. These findings reveal that application of phosphogypsum and poultry manure directly alter the distribution of exchangeable bases and alkaline indicators within residue aggregates, resulting in aggregate size distribution and microstructure variations.

Use of a stabilized sewage sludge in combination with gypsum to improve saline-sodic soil properties leached by recycled wastewater under freeze-thaw conditions.

Soil salinization is a major environmental hazard that limits agricultural production. Using sewage sludge and recycled wastewaters in amelioration of saline-sodic soils is one of the most effective ways to dispose waste. However, a very low initial permeability of soil in the freeze-thaw conditions can make improvement difficult. Therefore, column experiments at a soil depth of 15 cm have been conducted to determine the effects of the combination of four stabilized sewage sludge doses (0, 50, 100, 150 t ha-1), three freeze-thaw cycles (0, 5, 10 times) and two water types (FW: freshwater, RWW: recycled wastewater) on gypsum-treated saline-sodic soil properties. The effects of non-saline-sodic RWW on the soil properties were similar to the FW in total 22.5 cm leaching amount. Compared to gypsum alone and initial values, sewage sludge increased wet aggregate stability, organic matter, total N and exchangeable Ca + Mg while it decreased pH, exchangeable Na and CaCO3. Saturated hydraulic conductivity was not induced by sewage sludge although exchangeable sodium percentage and electrical conductivity were reduced by 44% and 63.6%, respectively. Negative effects of freeze-thaws on hydraulic conductivity and salinity and sodicity elimination were not observed, while pH and aggregate stability were negatively affected from ten freeze-thaws. Overall, it can be concluded that the improvement of hydraulic conductivity is attributed to the further improvement of soil structure from more strong wet aggregate stability via additional sewage sludge and leaching amounts.

Dairy cattle slurry fertilization management in an intensive Mediterranean agricultural system to sustain soil quality while enhancing rapeseed nutritional value.

Animal excreta are commonly recycled as fertilizers, although attention should be given to environmental impacts. Legislation must also be adapted to new research findings. The framework of this study is an intensive fodder Mediterranean agricultural system affected by EU legislation on the protection of waters against nitrate pollution. This paper studies the effect of two N based dairy cattle slurry (DCS) rates (170 vs. 250 kg N ha-1 yr-1) plus additional mineral N (up to 450 kg N ha-1 divided between two crops), on different soil quality parameters. A control (no N applied) was included. The experiment, which lasted for 8 years, included forage maize followed by ryegrass, grain maize and rapeseed. In the whole period, the organic carbon inputs from the DCS treatments comprised C slurry inputs (14.8 or 21.9 Mg ha-1) plus the C input difference in crop residues (8.3 Mg ha-1) between DCS and the control treatment. In the 0-0.3 m soil depth, slurries significantly increased soil organic carbon (SOC) from by 2.3 or 2.7% yearly (c. 2.8 Mg C with 10 Mg C ha-1 input) mainly in its light fraction. The size of the microbial biomass increased by 5.1% yearly (c. 0.12 Mg C with 10 Mg C ha-1 input). A higher aggregate stability against slaking disruption was observed. Soil pH slightly decreased, P (Olsen) fertility increased (up to 10 mg P kg-1) as did K availability (up to 140 mg K kg-1) and Mn and Ni bioavailability. In rapeseed plants, seed Ca, S, Cu and Mn content increased as did K, S, Fe, Mn and Zn in the rest of the plant biomass. These changes were within acceptable concentration ranges. The higher N rate from DCS has proved useful for the circular nutrient economy, while improving soil physical and chemical quality and the sustainability of the agricultural system as a whole.

Impact of land cover types on soil aggregate stability and erodibility.

Gökçeada is the biggest island, and it is also known as the organic island of Turkey. Approximately 65% of the Gökçeada lands have slope > 12%. Climate, topography, land cover, and soil characteristics are considered to be the main natural factors affecting soil erosion severity in the Gökçeada. Prevention of soil degradation, hence the preservation or improvement of the overall quality of the soil, is directly related to the presence of stable soil aggregates. In addition, the resistance to weathering and replacement of soil particles are also relevant aspects in terms of sustainability. Aggregate stability (AS) and erodibility of land (Kfac) are related to soil properties. However, this relationship can vary under different circumstances. In this study, 248 surface soil samples have been taken from forest and semi-natural areas (FSNA) and agricultural areas (AGRA) according to CORINE 2006. Eleven selected soil properties were measured, and their impacts on AS and Kfac (RUSLE-K) were determined by using the CRT (classification and regression tree) in Gökçeada. Results showed that the relations among soil characteristics changed according to the land cover classes. Total organic carbon is much more associated with AS in AGRA, while total carbon is associated with AS in FSNA. The effect of calcium carbonate on Kfac was higher than other soil properties when the land cover type was ignored. On the other hand, in AGRA, the effect of between clay content on Kfac was greater than those of FSNA.

Soil organic matter as sole indicator of soil degradation.

Soil organic matter (SOM) is known to play vital roles in the maintenance and improvement of many soil properties and processes. These roles, which largely influence soil functions, are a pool of specific contributions of different components of SOM. The soil functions, in turn, normally define the level of soil degradation, viewed as quantifiable temporal changes in a soil that impairs its quality. This paper aims at providing a generalized assessment of the current state of knowledge on the usefulness of SOM in monitoring soil degradation, based on its influence on the physical, chemical and biological properties and processes of soils. Emphasis is placed particularly on the effect of SOM on soil structure and availability of plant nutrients. Although these properties are discussed separately, the soil system is of dynamic and interactive nature, and changes in one property will likely affect other soil properties as well. Thus, functions of SOM almost always affect various soil properties and processes and engage in multiple reactions. In view of its role in soil aggregation and erosion control, in availability of plant nutrients and in ameliorating other forms of soil degradation than erosion, SOM has proven to be an important indicator of soil degradation. It has been suggested, however, that rather than the absolute amount, temporal change and potential amount of SOM be considered in its use as indicator of soil degradation, and that SOM may not be an all-purpose indicator. Whilst SOM remains a candidate without substitute as long as a one-parameter indicator of soil degradation is needed, narrowing down to the use of its labile and microbial components could be more appropriate, since early detection is important in the control and management of soil degradation.

Plant diversity and root traits benefit physical properties key to soil function in grasslands.

Plant diversity loss impairs ecosystem functioning, including important effects on soil. Most studies that have explored plant diversity effects belowground, however, have largely focused on biological processes. As such, our understanding of how plant diversity impacts the soil physical environment remains limited, despite the fundamental role soil physical structure plays in ensuring soil function and ecosystem service provision. Here, in both a glasshouse and a long-term field study, we show that high plant diversity in grassland systems increases soil aggregate stability, a vital structural property of soil, and that root traits play a major role in determining diversity effects. We also reveal that the presence of particular plant species within mixed communities affects an even wider range of soil physical processes, including hydrology and soil strength regimes. Our results indicate that alongside well-documented effects on ecosystem functioning, plant diversity and root traits also benefit essential soil physical properties.

Effects of three different biochars on aggregate stability, organic carbon mobility and micronutrient bioavailability.

Previous studies have demonstrated both beneficial and detrimental effects on soil properties from biochar incorporation. Several biochars, with different feedstock origins, were evaluated for their effectiveness at improving soil quality of a sandy agricultural soil. A pot trial was used to investigate aggregate stability and microbial activity, pore water trace element mobility and micronutrient concentrations in grain of spring wheat after incorporation of three biochars. The feedstocks for biochar production were selected because they were established UK waste products, namely oversize woody material from green waste composting facilities, and rhododendron and soft wood material from forest clearance operations. Biochars were incorporated into the soil at a rate of 5% v/v. Aggregate stability was improved following addition of oversize biochar whilst microbial activity increased in all treatments. Dissolved organic carbon (DOC) concentrations in soil pore water from biochar-treated soils were raised, whilst micronutrient concentrations in wheat grain grown in the treated soils were significantly reduced. It was concluded that incorporation of biochar to temperate agricultural soils requires caution as it may result in reductions of essential grain micronutrients required for human health, whilst the effect on aggregate stability may be linked to organic carbon functional groups on biochar surfaces and labile carbon released from the char into the soil system.

Study of soil aggregate breakdown dynamics under low dispersive ultrasonic energies with sedimentation and X-ray attenuation.

It has been increasingly recognized that soil organic matter stabilization is strongly controlled by physical binding within soil aggregates. It is therefore essential to measure soil aggregate stability reliably over a wide range of disruptive energies and different aggregate sizes. To this end, we tested high-accuracy ultrasonic dispersion in combination with subsequent sedimentation and X-ray attenuation. Three arable topsoils (notillage) from Central Europe were subjected to ultrasound at four different specific energy levels: 0.5, 6.7, 100 and 500 J cm-3, and the resulting suspensions were analyzed for aggregate size distribution by wet sieving (2 000-63 µm) and sedimentation/X-ray attenuation (63-2 µm). The combination of wet sieving and sedimentation technique allowed for a continuous analysis, at high resolution, of soil aggregate breakdown dynamics after defined energy inputs. Our results show that aggregate size distribution strongly varied with sonication energy input and soil type. The strongest effects were observed in the range of low specific energies (< 10 J cm-3), which previous studies have largely neglected. This shows that low ultrasonic energies are required to capture the full range of aggregate stability and release of soil organic matter upon aggregate breakdown.

Vegetation restoration improved aggregation stability and aggregated-associated carbon preservation in the karst areas of Guizhou Province, southwest China.

Background: The change in the soil carbon bank is closely related to the carbon dioxide in the atmosphere, and the vegetation litter input can change the soil organic carbon content. However, due to various factors, such as soil type, climate, and plant species, the effects of vegetation restoration on the soil vary. Currently, research on aggregate-associated carbon has focused on single vegetation and soil surface layers, and the changes in soil aggregate stability and carbon sequestration under different vegetation restoration modes and in deeper soil layers remain unclear. Therefore, this study aimed to explore the differences and relationships between stability and the carbon preservation capacity (CPC) under different vegetation restoration modes and to clarify the main influencing factors of aggregate carbon preservation. Methods: Grassland (GL), shrubland (SL), woodland (WL), and garden plots (GP) were sampled, and they were compared with farmland (FL) as the control. Soil samples of 0-40 cm were collected. The soil aggregate distribution, aggregate-associated organic carbon concentration, CPC, and stability indicators, including the mean weight diameter (MWD), fractal dimension (D), soil erodibility (K), and geometric mean diameter (GMD), were measured. Results: The results showed that at 0-40 cm, vegetation restoration significantly increased the >2 mm aggregate proportions, aggregate stability, soil organic carbon (SOC) content, CPC, and soil erosion resistance. The >2 mm fractions of the GL and SL were at a significantly greater proportion at 0-40 cm than that of the other vegetation types but the CPC was only significantly different between 0 and 10 cm when compared with the other vegetation types (P < 0.05). The >2 mm aggregates showed a significant positive correlation with the CPC, MWD, and GMD (P < 0.01), and there was a significant negative correlation with the D and K (P < 0.05). The SOC and CPC of all the vegetation types were mainly distributed in the 0.25-2 mm and <0.25 mm aggregate fractions. The MWD, GMD, SOC, and CPC all gradually decreased with increasing soil depth. Overall, the effects of vegetation recovery on soil carbon sequestration and soil stability were related to vegetation type, aggregate particle size, and soil depth, and the GL and SL restoration patterns may be more suitable in this study area. Therefore, to improve the soil quality and the sequestration of organic carbon and reduce soil erosion, the protection of vegetation should be strengthened and the policy of returning farmland to forest should be prioritized.