Silt-clay Fraction Research Articles

Divergent responses of soil aggregation and aggregate-carbon to fertilization regimes jointly explain soil organic carbon accrual in agroecosystems: A meta-analysis

Soil aggregation can be substantially affected by fertilizers and contributes to soil organic carbon (SOC) sequestration in agroecosystems. However, the mass and C distributions in different aggregates in responses to fertilization regimes are not often synchronous, which may largely affect soil C storage and stability. We conducted a meta-analysis of 2440 paired observations from 63 publications to assess the fertilization effects (i.e. inorganic, organic, and their combinations) on soil aggregation and aggregate-associated OC, as well as their linkages to the stimulated bulk soil C. Overall, fertilizer application significantly increased the mean weight diameter of soil aggregates by 27.8%. The proportion of large (> 2mm) and small (0.25-2mm) macroaggregates were significantly increased by 19.8% and 17.2%, and that of microaggregate (0.053-0.25mm) and silt-clay fraction (< 0.053mm) were significantly decreased by 6.0% and 18.4%, respectively. In contrast, fertilization significantly increased C concentration in all aggregates. Organic fertilizer applications had remarkably greater effects than inorganic fertilizer applications on soil C concentration but the effects declined with decreasing aggregate size (from 36.5% to 13.2%), while that of inorganic application changed very little among aggregates (from 14.1% to 10.0%). The fertilizer effects on soil aggregation and aggregate-associated OC divergently responded to the gradients of major agronomic conditions (i.e. climate, soil properties, and duration). Organic fertilizer applications tended to have distinctly greater promotion effect than solely inorganic fertilizer applications with temperate climate, neutral-to-alkaline pH and more sand-like texture of soil. The importance of mineral- rather than larger size aggregate-associated OC in contributing to the bulk SOC pool tended to increase in the long term. The inorganic-organic combinations exhibited the most lasting effect on SOC accrual. In conclusion, the responses of bulk soil C to fertilizer applications were not always in accordance with those of soil aggregation, but can be well explained when jointly considering soil aggregate C. Our findings highlight the varying contributions of aggregates to the soil C pool in diverse and complicated agronomic situations, which are important to the agricultural C sink stability.

Time-dependent regulation of soil aggregates on fertilizer N retention and the influence of straw mulching

Fertilizer nitrogen (N) turnover is highly controlled by soil aggregation. However, the functions of the various aggregates that regulate long-term fertilizer N retention under conservation management remain unexplored. In this study, 15N-labeled fertilizer was initially applied in situ to investigate the effects of maize straw mulching on fertilizer N allocation in soil aggregates at a decadal scale. The topsoil was fractionated into macroaggregate, microaggregate, and silt-clay (SC) fractions. Macroaggregate was further divided into particulate organic matter (POM) and mineral-associated organic matter (MAOM). A higher enrichment factor of fertilizer N than of soil total N in macroaggregate indicated that the fertilizer N was more apt to incorporation into macroaggregate. The fertilizer N in the bulk soil declined gradually to 84.0% by the 13th year. Temporally, the reduction proportion of fertilizer N in the SC fraction was the largest before 5th years, whereas macroaggregate was the main reactive spot for fertilizer N transformation from 9 to 13 years. Therefore, the function of aggregates was time-dependent in controlling fertilizer N retention and turnover via the release of previously entrapped fertilizer N, but encapsulated the subsequently applied N (i.e., unlabeled fertilizer), whereas mineral adsorption contributed to the long-term stabilization of fertilizer N. Compared with fertilization alone, straw mulching improved aggregates stability, favored the initial fertilizer N retention in macroaggregate by enriching fertilizer N in POM, and reduced the proportion of N loss in MAOM after 9 years. These finding indicate that the improvement in fertilizer N stability related to straw decomposition was sequentially attributed to the enhancement of aggregate encapsulation and persistent interaction with soil minerals. Therefore, this study provides new insights into the functional heterogeneity of soil aggregates at different time stages and the intricate interplay between carbon availability-controlled fertilizer N retention and the improvement in soil aggregation.

Improved Artificial Aggregates for Use in Green Roof Design

The development of surfaces in cities, as a result of progressive urbanization, not only reduces the natural retention capacity of the environment but also causes changes in the water balance. In urbanized areas, the amount and intensity of rainwater discharged to receivers increase, and the time of water outflow from the catchment area shortens. Low retention does not provide effective responses to the local water deficit and does not limit the effects of excess water during flood periods. Furthermore, aging drainage systems do not always have the required hydraulic efficiency in absorbing runoff after intense and heavy rainfall or snowmelt. The aim of the work was to determine the possibility of obtaining flat aggregates with a grain size of 2–16 mm from clay-silt fractions from sedimentation tanks using selected mechanical processing methods (crushing and screening in a crusher-screener system with recycling). An important issue was the examination of the physical and mechanical properties of the produced aggregates after firing, where the work required a detailed material analysis using various research techniques, such as XRD, XRF, SEM and digital microscopy. The obtained results will allow for further research on developing the concept of technology for the production of lightweight aggregates used, for example, on building roofs. Particular attention was paid to the flat shape of the aggregate, which affects a number of its properties. To obtain a flat-shaped aggregate, the authors used a patented sieving method. The obtained materials had high cavernosity of 69% on average, water absorption of 40.7% and low bulk density of 0.82 g/cm3.

Effects of slope aspect on soil aggregates humus on cut slopes in alpine areas of Southwest China

Large-scale road construction has resulted in bare cut slopes in alpine areas, leading to soil erosion and reduced fertility. Soil humus is the basis of soil fertility and an important carbon pool; it is affected by many factors. However, the effects of slope aspect on soil humus on cut slopes are still largely unclear. We studied the differences in the humus content in bulk soil and soil aggregates under four slope aspects (south-facing slope (SFS), west-facing slope (WFS), east-facing slope (EFS), and north-facing slope (NFS)) in alpine areas and analyzed the driving effects of soil physicochemical factors on humus distribution. The contents of fulvic acid carbon (FA-C), humic acid carbon (HA-C), humin carbon (HU-C), humus extractable carbon (HE-C) and total humus carbon (TH-C) differed among four slope aspects. Humus carbon did not differ among large macro-aggregates (LMA), small macro-aggregates (SMA) and micro-aggregates (MIA). Only the contents of humus carbon in silt + clay fraction (SCA) were significantly lower than those in the other soil aggregates for the NFS and EFS. Soil water content (SWC), soil organic matter (SOM), total nitrogen (TN), and available phosphorus (AP) were important soil physicochemical factors affecting the distribution of humus. Our results suggest a significant effect of slope aspect on the contents of humus carbon, and the silt + clay fraction had a weak ability to retain humus carbon. The study provides a theorical basis for future humus research and soil fertility management of cut slopes in alpine areas.

Estimation of fine sediment stocks in gravel bed rivers including the sand fraction

ABSTRACTFine sediment stored in the gravel bed is an important component of river systems. Current field protocols usually allow evaluation of the silt–clay fraction of fine sediment stocks only and neglect the sand fraction. This study proposes a new protocol to quantify fine sediment stocks, including the sand fraction inside the gravel bed matrix. Fine sediment stocks were sampled within patches of 0.30 m × 0.30 m on the dry gravel bed surface, separating the surface layer and the subsurface layer. The grain‐size distribution of the samples was obtained by field sieving (10 mm, 2 mm, 500 μm and 100 μm) over a bucket, using a known volume of water. The mass of the fraction below 100 μm was measured based on the concentration within the bucket. The local stocks were then integrated over the whole river reach by assigning local stocks to facies, in which fine sediment stocks were assumed to be homogeneously distributed. The methodology was applied to a 1 km long reach of the River Galabre (Southern French Alps), characterized by significant fine sediment stocks and upstream sediment input. Results from local measurements show a large amount of sand in both surface and subsurface layers. The quantity of sand can reach up to three times the quantity of silt–clay. An estimation of porosity showed that fine material may play an important role in structuring the bed, since porosity increases with increasing fine sediment content. The potential fine sediment stock that can be resuspended due to channel migration is found to be of the same order of magnitude as the sediment budget estimated from the measured flux in the upstream hydrometric station of the studied reach.

Soil Aggregation and Associated Organic Carbon and Total Nitrogen in a Sandy Loam Soil under Long-Term Tillage Effects

In Tunisia, climate change impacts that lead to the degradation of soil resources are considered to be a major limiting factor on socio-economic development. These impacts are exacerbated by the intensive plowing and cultivation practices used by Tunisian farmers, which expedite the depletion of soil organic matter (SOM), leading to changes in the physio-chemical properties of soil and consequently promoting soil erosion. In fact, the decrease in soil organic carbon (SOC) stocks affects soil’s fertility and the ability to regulate climate change. The objective of this study, which was conducted in Le Krib in the Siliana region of northwestern Tunisia, was to evaluate the effects of two cropping systems, consisting of durum wheat (Triticum aestivum) and oats (Avena sativa), and two types of tillage, no-till (NT) and mouldboard plowing (MP), on different soil aggregate classes (&gt;2000 µm, 2000–250 µm, 250–180 µm, 180–53 µm and &lt;53 µm) and soil physio-chemical properties, as well as the resulting effects on the carbon and nitrogen concentrations in these aggregates. The results showed that the carbon content of all soil aggregate classes was influenced by interactions between the previous crop and tillage regime. The clay-silt fraction had higher carbon concentrations under no-till and mouldboard plowing management. Furthermore, the previous crop and tillage type and their interactions had significant effects on nitrogen concentrations in micro-aggregates. The highest nitrogen concentrations (2846.6 ppm) were found in micro-aggregates in soils where the previous crop was durum wheat and mouldboard plowing was used, while the lowest concentrations (1297 ppm) were obtained in soils where the previous crop was oats and mouldboard plowing was used.

Long-term fertilization promotes soil organic nitrogen accumulation by increasing the abundance of keystone microbial cluster across aggregates

Nitrogen (N) in soil mainly occurs in organic forms, with soil organic nitrogen (SON) playing an essential role in supplying N to crops. However, it is unclear how the content and distribution of SON vary among different aggregate sizes, and how SON content is related to soil microorganisms. In this study, we investigated the associations between SON content and various microbial community characteristics across different aggregate sizes using data from a long-term field fertilization experiment in paddy soils. We also performed functional prediction to explore potential microbial mechanisms underlying SON accumulation. The results showed that SON content decreased as aggregate size decreased, with macroaggregates having the highest SON content and the silt-clay fraction having the lowest. Furthermore, our linear regression and structural equation modeling analysis revealed that keystone microbial clusters (highly interconnected taxa within Firmicutes, Proteobacteria, Chloroflexi, Actinobacteriota, and Ascomycota) were significantly correlated with SON content. Finally, functional prediction analysis suggested that this relationship may be due, at least in part, to the large number of metabolic pathways involved in biosynthesis of N-containing compounds present in keystone microbial clusters. Overall, our study highlights the importance of keystone microbial clusters in influencing the accumulation of SON and contributes to our understanding of soil N biogeochemical cycling.

Determination of Silt and Clay soil particle distribution using new Silt-Clay Separation Quick method

Hydrometer method is the standard method that normally used to determine fine particle size distributions for soil classification purposes. This study aims to develop new method that enable quick determination of silt-clay particle distribution compared to the hydrometer method that requires procedure of pretreatment and additional 24 hours required for the complete procedure of the test. The various soil samples were collected from seven different locations including Sunway Carnival (Marine Clay), Suling Hills (Residual Clay), Jawi Quarry (Residual Clay), Kelantan River (River Clay), Batu Kawan (Marine Clay), Kaolin Clay and Sungai Petani (Residual Clay). Three soil samples from each site were prepared for the hydrometer method and additional three samples for the new method silt-clay separation. Sample preparation and testing is repeated for all samples from seven site locations. The results of silt clay particle distribution obtained from both experimental works were compared to analyze the compatibility of data. The data analysis shows that there are no significant differences (P > 0.05) between the two methods in all examined soil samples. This indicates strong evidence to support this research that the silt-clay separation new method is comparable to the standard hydrometer method. Regression analysis between the two methods produced a coefficient of determination (R-square) values of 0.9912 respectively. The straight-line equation generated for all data produced the gradient equal to 0.8465, which is close to 1 indicates stronger evidence to supports the accuracy of the new method. In conclusion, the preliminary results of new method silt-clay separation confirmed its reliability to analyze the particle soil distribution in silt-clay fraction with 99.12% data accuracy compared to the standard hydrometer method.

Five-year warming does not change soil organic carbon stock but alters its chemical composition in an alpine peatland

Climate warming may promote soil organic carbon (SOC) decomposition and alter SOC stocks in terrestrial ecosystems, which would in turn affect climate warming. We manipulated a warming experiment using open-top chambers to investigate the effect of warming on SOC stock and chemical composition in an alpine peatland in Zoigê on the eastern Tibetan Plateau, China. Results showed that 5 years of warming soil temperatures enhanced ecosystem respiration during the growing season, promoted above- and belowground plant biomass, but did not alter the SOC stock. However, labile O-alkyl C and relatively recalcitrant aromatic C contents decreased, and alkyl C content increased. Warming also increased the amount of SOC stored in the silt-clay fraction (< 0.053 mm), but this was offset by warming-induced decreases in the SOC stored within micro- and macroaggregates (0.053–0.25 and > 0.25 mm, respectively). These changes in labile and recalcitrant C were largely associated with warming-induced increases in soil microbial biomass C, fungal diversity, enzyme activity, and functional gene abundance related to the decomposition of labile and recalcitrant C compounds. The warming-induced accumulation of SOC stored in the silt-clay fraction could increase SOC persistence in alpine peatland ecosystems. Our findings suggest that mechanisms mediated by soil microbes account for the changes in SOC chemical composition and SOC in different aggregate size fractions, which is of great significance when evaluating SOC stability under climate warming conditions.

Quantifying the relative contribution of driving factors to the transformation of bauxite residue into a plant growth substrate

The vast stock and increasing amount of bauxite residue with high alkalinity, low nutrient and poor structure poses a significant challenge to plant growth. Although additives have been recognized to have obvious effects on the transformation of bauxite residue to a soil-like media for plant growth, a clear understanding of the quantitative contribution of physicochemical properties to plant growth is missing. This study aimed to discuss the portrayed role and contributions of additives mediated alkalinity, nutrient, and aggregates component in the significant factors of aggregate structure formation and corresponding plants seed germination in bauxite residue. The regulation of pH and total alkalinity in bauxite residue depends on inorganic additives, while for ESP and nutrients, nitrohumic acid and vinegar residue are better than inorganic and citric acid. The silt-clay fraction and microaggregate increased from 35.1% and 25.6% to 40.8% and 31.9% within citric acid addition, respectively, while macroaggregate increased from 39.2% to 49.9% within nitrohumic acid application, indicating citric acid may be more involved in the formation of the fundamental building units of organo-mineral associations in bauxite residue, while nitrohumic acid and vinegar residue may play an important role in continuous aggregation of small particles. For aggregates component, calcium in silt-clay fraction had the largest explanatory power in aggregate stability and germination rate (R2 = 65.6%), followed by Al in 0.25–2 mm size fraction (R2 = 11.1%), while organic complex Fe oxides and Mg in microaggregate explained little variance (R2 = 3.89% and R2 = 1.25%), demonstrating Ca, Fe, and Mg contributed greatly to the formation of microaggregate, while Si and Al contributed greatly to the formation of large aggregate. Besides the dominant role of aggregates component, aggregate stability related to mean weight diameter, large macroaggregates, fractal dimension, and silt-clay fraction plays a more critical role in seed germination.

Quantifying simulated fine sand fraction in muddy sediment using laser diffraction

The objective of this study is to verify whether low amounts of fine sand added to a muddy sediment matrix can be detected and quantified with accuracy using a Mastersizer 3000 (Malvern Panalytical) laser diffraction particle-size analyzer equipped with a Hydro LV large volume liquid dispersion module. To achieve this goal, a postglacial sediment sample was sieved to recover naturally co-occurring sand and clay–silt fractions. Sand in the range of 1%–7% by weight was added to the clay–silt at three concentrations (88, 132, and 276 mg dry weight) and each sample was duplicated. A very strong linear relationship was found between the measured % volume of sand added and the actual weight of sand added to the mud. Sand representing as low as 1% by weight could be detected. On average, there was only a 0.7% absolute difference between the measured and actual % sand values (range 0.02%–1.65%). Sample concentration had a negligible impact on the measured % sand. A range of plausible values for the refractive and absorption indexes, essential parameters for the Mie theory calculation of the size distribution from the measured light scattering, also had very small impact on the measured % sand. The demonstrated possibility of detecting a small input of fine sand to muddy sediment provides a basis for studies using grain-size data to reconstruct past and modern detrital inputs and sediment transport variations.

Geotechnical characterization of the lateritic soil profile. Moa, Cuba

This paper presents the results of the study of the physical and geotechnical properties of the vertical profile of lateritic soils. The study analyses the different zones of the profile. The study of the physical-mechanical properties is carried out according to the unified soil classification system. The mineralogical composition is determined by DRx and the chemical composition by ICP-AES. Among the physical properties analysed are particle size distribution, specific weight of solid particles, dry density and porosity, pore index, moisture and degree of saturation, plasticity, angle of internal friction and cohesion. The upper ferricretic layer is characterized by a sandy-gravelly grain size, whilst in the limonitic, transition and saprolitic zones the silt-clay fraction predominates.

Do long-term N additions affect the soil organic carbon pool in temperate grasslands?

While the organic carbon stored in soil is a sizeable proportion of the total carbon stored in terrestrial ecosystems, it is also a considerable source of greenhouse gas emissions. In this study, we examined how the carbon pool had changed in an area of temperate grassland in Inner Mongolia, China, over a period of 14 years. A field experiment was set up in 2003 and was treated with 6 nitrogen (N) treatments:0, 2, 4, 8, 16, and 32 g·N·m−2·yr−1 as dry urea (CO(NH2)2). We collected soil samples in 2017, and divided them into three size aggregates: silt-clay fraction, microaggregate, and small macroaggregate. We determined various soil extracellular enzyme activities of these three categories, namely β-glucosidase (BG), N-acetyl-β-D-glucosaminidase (NAG), acid phosphatase (AP), peroxidase (PER), and phenol oxidase (POX). We found that the soil enzyme activities increased under N additions. In addition, the BG activity was higher, but the PER activity was lower, in the small macroaggregates than the silt-clay fraction. Furthermore, we found that the soil extracellular enzyme activities and soil physico-chemical properties in the small macroaggregates were not correlated. Rather, we found that some of the soil extracellular enzyme activities were negatively correlated with the pH value, microbial biomass carbon (MBC), total organic carbon (TOC), and positively correlated with the inorganic nitrogen content (IN), in the microaggregates and the silt-clay fraction. A key discovery was that the N additions had no effect on the 0–10 cm soil layer (fractions <2000 um) organic carbon pool or the distribution of different-sized aggregates, probably because they were regulated through the changes of plant, soil, and microbial interactions. The results will contribute to improve our understanding of how N additions affect TOC and different aggregate size classes in soil, and will support better predictions of how N deposition might contribute to future climate change.

Effect of land-use and land-cover change on mangrove soil carbon fraction and metal pollution risk in Zhangjiang Estuary, China

Land-use and land-cover change (LULCC) is the main cause of mangrove deforestation and degradation. However, the effect of LULCC on mangrove soil organic carbon (SOC) fractions and metal pollution risks, and the difference between the effects of those two soil evolutions are largely unknown. Here, we collected soil samples from natural systems (mangroves and mudflat), land-cover changes (Spartina alterniflora invasion), and anthropogenic land-use changes (cropland and culture pond) in Zhangjiang Estuary. We determined the soil aggregate fractions (macro-aggregate, micro-aggregate, and silt-clay fraction) and the associated carbon, and heavy metal dynamics. Our findings suggested that LULCC did not remarkably affect SOC contents, but changed the soil aggregate structures. LULCC significantly increased aggregate-associated carbon fractions, especially macro-aggregate carbon fraction. The large proportion of silt-clay fraction in natural systems was corresponding to a high percentage of mineral organic carbon, indicating that LULCC decreases the mangrove SOC stability. Land-cover change promoted the accumulation of SOC, nitrogen, and heavy metals compared with uninvaded mudflat. The heavy metal contents in mangrove soil were highest among all studied soils, expect for Cd, which suggested that mangrove soil had high metal accumulation. However, land-use changes could stimulate the mobility and dynamics of metals enriched in mangrove soils; these changes, especially in cropland, will also cause a large amount of exogenous Cd being exported into the adjacent aquatic environment. Thus, mangrove shifts metal pollutant from sink to source when affected by land-use changes. The contamination index demonstrated that heavy metals have posed ecological risks, especially for Cd in cropland. Compared with mangrove, land-use change was dominated by single-element pollution, but land-cover change showed low multiple-element complex pollution. These findings elucidate the effects of LULCC on mangrove SOC fraction and metal pollution risk, and are of great significance for designing the long-term management and conservation policies for mangrove managers.

Novel efficient capture of Cr(VI) from soil driven by capillarity and evaporation coupling

Soil contaminated by hexavalent chromium (Cr(VI)) poses a severe environmental threat owing to the carcinogenic and genotoxic characteristics of Cr(VI). Currently, field application of remediation technologies for Cr(VI) removal or detoxification fails to achieve optimum results owing to various limitations, such as high energy consumption, high chemical cost, secondary pollution, and long treatment duration. Herein, a novel strategy, namely, the capillary-evaporation membrane (CEM) method, which is based on the ubiquitous phenomena of capillarity and evaporation in natural soil environment without external forces, was applied to remove Cr(VI) from contaminated soil. The CEM method enables Cr(VI) dissolved in the soil solution to move upwards through soil pores and inter-particle spaces and get attached to the surface of adsorption membrane under the coupling action of capillarity and evaporation to achieve Cr(VI) removal. The CEM method showed high Cr(VI) removal capacity during 22 days of treatment of bulk soil (47.26%), sandy fraction (34.60%), and silt-clay fraction (52.50%), respectively. Further research on optimization of the CEM process conditions could remarkably improve Cr(VI) remediation performance. For example, the Cr(VI) removal rate increased to 89.04% in bulk soil through prolongation of the remediation period to 61 days. This study demonstrated a new environment-friendly remediation method driven by natural phenomena for Cr(VI)-contaminated soils.

Linking recent changes in sediment yields and aggregate-associated organic matter sources from a typical catchment of the Loess Plateau, China

The historically environmental information of soil erosion and human activity is thoroughly “recorded” in deposited organic matter (OM) in aggregates within the catchment. Although the changes in sediment-associated organic carbon (OC) concentration and source have great impact on the stability and fate of eroded OM on Earth, the response of OM sources of sediment aggregate fluxes in eroding landscapes to land uses still remain unclear. In this study, we selected three sediment deposit profiles in a typically dam-controlled catchment of the Chinese Loess Plateau to trace the 60-year changes in sediment yields and then explored the historical distributions of aggregate-associated OM sources through using the fallout radioisotope dating (137Cs and 210Pbex) and the isotope mixing model based on aggregate size-specific 13C and 15N isotope signatures. The results showed that the deposition rates of sediment trapped the check dam displayed a decreasing trend with year except for an abrupt increase in 2013 over the past 60 year, ranging from 4.47 × 103 to 89.16 × 103 t km−2 yr−1. The average SOC and TN concentrations in deposit profile showed the following order of macroaggregates (> 0.25 mm, 9.55 g kg−1 for SOC and 0.81 g kg−1 for TN) > microaggregates (0.053–0.25 mm, 4.16 g kg−1 for SOC and 0.42 g kg−1 for TN) > silt-clay (< 0.053 mm, 2.41 g kg−1 for SOC and 0.33 g kg−1 for TN), despite there was a opposite trend of mass fraction in aggregate sizes. The data of 13C and C/N among aggregate sizes indicated that the OM in silt-clay fraction had higher decomposition rate than that of other aggregates. The modeling outputs further showed that cropland was the primary contributing sources to transported OM of aggregate flux, accounting for > 20% of the total. Our results demonstrated the influence of erosion processes and changes in land use on the sediment yield and aggregate-associated OM dynamics in this Loess region.

Nitrogen addition stimulates soil aggregation and enhances carbon storage in terrestrial ecosystems of China: A meta‐analysis

China is experiencing a high level of atmospheric nitrogen (N) deposition, which greatly affects the soil carbon (C) dynamics in terrestrial ecosystems. Soil aggregation contributes to the stability of soil structure and to soil C sequestration. Although many studies have reported the effects of N enrichment on bulk soil C dynamics, the underlying mechanisms explaining how soil aggregates respond to N enrichment remain unclear. Here, we used a meta-analysis of data from 76N manipulation experiments in terrestrial ecosystems in China to assess the effects of N enrichment on soil aggregation and its sequestration of C. On average, N enrichment significantly increased the mean weight diameter of soil aggregates by 10%. The proportion of macroaggregates and silt-clay fraction were significantly increased (6%) and decreased (9%) by N enrichment, respectively. A greater response of macroaggregate C (+15%) than of bulk soil C (+5%) to N enrichment was detected across all ecosystems. However, N enrichment had minor effects on microaggregate C and silt-clay C. The magnitude of N enrichment effect on soil aggregation varied with ecosystem type and fertilization regime. Additionally, soil pH declined consistently and was correlated with soil aggregate C. Overall, our meta-analysis suggests that N enrichment promotes particulate organic C accumulation via increasing macroaggregate C and acidifying soils. In contrast, increases in soil aggregation could inhibit microbially mediated breakdown of soil organic matter, causing minimal change in mineral-associated organic C. Our findings highlight that atmospheric N deposition may enhance the formation of soil aggregates and their sequestration of C in terrestrial ecosystems in China.

Effects of intensive fish farming on sediments of a temperate bay characterised by polyculture and strong currents

Sansha Bay, facing the East China Sea, is characterised by high fish production (Pseudosciaena crocea, 91,000 t yr–1) and the polyculture of seaweeds (Saccharina japonica and Gracilaria lemaneiformis) and abalone (Haliotis discus hannai). To investigate the environmental effects of fish farming on sediments, surface (0–1 cm) sediments were collected from 41 stations grouped into four areas: fish, seaweed, and abalone farms and reference area. The physical and chemical parameters of these sediments were analysed. Nitrogen and phosphorus loadings from trash fish as feed were estimated to be 12,000 and 2,400 t yr–1, respectively, accounting for 90% and 86% of the total anthropogenic loading in Sansha Bay. However, no significant differences were observed in stable C and N isotope ratios, carbon-to-nitrogen ratio, total organic carbon, total nitrogen, total phosphorus, acid volatile sulfide, water content, and silt–clay fraction among the four areas (Kruskal–Wallis test, P > 0.05). Principal component and cluster analyses supported this conclusion, indicating that fish farming has no significant effect on the sediment. The amount of N removed by the cultured seaweed was estimated to be 288 t yr–1, accounting for only 2% of the N loaded from fish farming. The results suggested that the dispersion of fish farm-derived wastes were primarily facilitated by the topographic and hydrodynamic characteristics of Sansha Bay, such as its large depth (∼90 m), strong currents (1.4–1.9 m s–1), large tidal amplitude (5.4 m), and rapid exchange (<18 days) of a large volume of bay water (7.4 × 109 m3).

Aggregate Distribution and the Associated Carbon in Norfolk Soils under Long-term Conservation Tillage and Short-term Cover Cropping

ABSTRACT Conservation agriculture practices have been widely implemented to improve soils and their sustainability. Here, we investigated the impacts of long-term conservation tillage (CS) on soil aggregates and their associated carbon (C) in a typical sandy Ultisols. The short-term effect of cover cropping was also investigated. Soils (0–5 and 5–15 cm) were collected from fields under 40-years CS and conventional tillage (CV), in which cover crop and fallow treatments were embedded as a split-plot design for four years. Soils were tested for bulk density, pH, mean weight diameter, dry aggregate distribution, and C contents. The CS only resulted in higher total C (TC) concentrations (13.56 ± 1.14 g kg−1) than CV (10.06 ± 0.53 g kg−1) at 0–5 cm. Similar depth impacts were observed in various aggregate size fractions (2000–250 µm, macroaggregates; 250–53 µm, microaggregates; 0–53 µm, clay-silt fraction). Soils were dominated by macroaggregates (>50%) at both depths with associated C concentrations following the order of clay-silt fraction > microaggregates > macroaggregates. Nonetheless, after accounting for soil bulk density long-term CS did not result in higher TC stocks than CV at both depths. Cover crops demonstrated no effect on C stocks at both depths. Poor soil structure and low clay contents of the tested soils may partially explain these neutral impacts, while relatively lower residue returns of cover crops to cash crops may also confound the positive management outcomes. Novel management practices to increase soil organic C in the bulk soils by increasing organic inputs and the soils’ capacity to preserve the inputs are needed.

Landscape Effects on Decomposition

The average annual rainfall was close to the average for the Jornada Experimental Range basin (225 mm∙y−1). Decomposition of leaf litter bags on the soil surface was a function of the rainfall at the site and of soil texture. Sites with the highest splash erosion and infiltration (highest sand content) had the highest decomposition rates. There was no evidence that run-off, run-on processes had an effect on the decomposition of surface litter. Root decomposition was only different at one of the tarbush sites (p > 0.001) and that difference was primarily due to soil texture and spatial distribution of rainfall. High concentration of the clay-silt fraction resulted in differences in mass loss of surface litter at grassland, dry-lakes, and tarbush sites. One site at each of these was different from the other two sites because they are between 8 and 20 km from the other two sites.