Water-stable Aggregate Size Fractions Research Articles

Managing soil organic matter through biochar application and varying levels of N fertilisation increases the rate of water-stable aggregates formation

Abstract The formation of soil aggregates, including water-stable aggregates, is linked to soil organic matter (SOM). Biochar (B) is carbon-rich, which, in addition to storing carbon in a stable form for many years, has important benefits for soils and plants, but the mechanisms of soil structure formation after B and mineral fertiliser application are not sufficiently studied. For this reason, the study aimed to answer the following questions: How (1) the rate of B and (2) varying levels of nitrogen fertiliser (N) being applied to the soil affect the dynamics of soil aggregation due to the increase in the content of soil organic carbon, labile carbon in the bulk soil and in the content of water-stable aggregates (WSA) size-fractions. In 2014–2021, in Dolná Malanta (experimental site of Slovak University of Agriculture on silty loam Haplic Luvisol) during the growing seasons, soil samples were collected from all the B (0, 10 and 20 t ha–1) and N (0, 1st and 2nd level of N fertilisation) treatments. The results have shown that the highest values of many variables were associated with B20 treatment for all the N fertilisation levels. B compared to N more significantly affected the content of almost all the size-fractions of WSA. In all the treatments, the content of WSAma >5 mm, 5–3 mm, 3–2 mm and 1–0.5 mm in size was increasing over time – a yearly increase from 0.31 to 2.14% for 8-years. Based on the changes in the SOM content, WSA were divided into 3 groups: 1) Water-stable microaggregates (WSAmi < 0.25 mm), 2) Smaller size-fractions of water-stable macroaggregates (WSAma 1–0.25 mm), and 3) Medium and large fractions of WSAma (WSAma ≥1 mm).

Soil Slope Exposure Affects Physico-Chemical and Microbiological Properties in Soil Aggregate Size Fractions

Slope exposure is known to affect soil biogeochemical processes in mountainous forest ecosystems, but little attention has yet been paid to its influence at a soil aggregate scale. Therefore, we evaluated the effects of slope exposure (north- vs south-facing slope) on the physico-chemical and microbiological properties of bulk soil and dry-sieved and water-stable aggregate size fractions in both organic (OF) and mineral (AE) horizons in an Italian alpine forest. The changes in organic carbon (OC) and nitrogen (ON) fractions were assessed together with a battery of thirteen enzyme activities involved in the main nutrient cycles. In addition, soil biological properties including microbial biomass (estimated as double-stranded DNA content), and microbial activity (assessed as the ratio between the extra-(exDNA) and intracellular (iDNA) fractions of the total soil DNA pool) were determined. The OF horizon at the north-facing slope was enriched in recalcitrant and insoluble OC and ON fractions and characterized by a lower microbial activity, as indicated by the higher exDNA/iDNA ratio with respect to the south-facing slope. On the contrary, exDNA and iDNA contents, microbial biomass, as well as most of the enzyme activities, reached higher levels at the southern exposure in the AE horizon. These exposure-effects were bulk soil- and aggregate size fraction-specific. Overall, lower values of the chemical and microbiological parameters were found in the water-stable fraction. Our findings indicate that slope exposure (and thus topography), soil horizon, and aggregate size distinctly influence soil OC dynamics in mountain ecosystems.

Effects of Biochar on Properties of Tropical Sandy Soils Under Organic Agriculture

This study evaluated the influences of biochar made from local agricultural wastes on sandy soils in farmer fields where biochar has been used as a soil amendment for more than three years. The major objective of this study was to gain insight into the effects of long-term biochar application on properties of sandy soil.
 
 Unamended soil properties were compared to biochar-amended soils properties using the paired samples t-test (p < 0.05). The statistical results of the study indicated that cation exchange capacity, exchangeable potassium, available phosphorus, field capacity, plant available water, water-stable aggregate size fractions (> 1 and < 0.25 mm), median aggregate size and aggregate stability were significantly different at p < 0.05. Clearly, biochar present for 3 or more years can improve soil physicochemical properties.
 
 We conclude that sandy soil properties, especially soil physical properties, are very strongly affected by biochar application combined with conservative soil management. Biochars affect both physical and biological mechanisms of soil aggregate formation because the biochar particle sizes influence the arrangement of clay on biochar and biochar grains provide a favorable microbial habitat and food source for fungi creating microorganism-biochar-soil associations which enhance water-stable aggregates and water holding capacity.

The stability mechanism for organic carbon of aggregate fractions in the irrigated desert soil based on the long-term fertilizer experiment of China

Soil samples from the Hexi Corridor located in the arid regions of Northwestern China were collected from a 24–year–old field experiment. The distribution of organic carbon (OC) in the different size water stable aggregate (WSA) fractions and the stability mechanism of OC in WSA size fractions were studied. The results showed that long-term application of solo organic manures or in combination with inorganic N fertilizer significantly increased the OC concentrations of different size WSA fractions compared to the CK, while long-term application of solo N fertilizer had no significant effect. After fertilization for 24 years, the OC concentrations of 0.25–2 mm WSA fraction was significantly positively correlated with the percentages of this WSA fraction, while the OC concentrations of >2 mm, 0.053–0.25 mm and <0.053 mm were not significantly correlated with their corresponding percentages of WSA fractions (P > 0.05). In addition, the OC concentrations of loosely, stably and tightly bound humus in 0.25–2 mm WSA fraction were significantly higher than those in other size WSA fractions, which were also significantly correlated with the corresponding chemical components, e.g. different forms of calcium, in 0.25–2 mm WSA fraction (P < 0.05). It could be explained that affinities of OC to chemical components in 0.25–2 mm WSA fraction were stronger than those in other size WSA fractions. The results of NMR analysis also showed that the aromaticity and alkyl/O-alkyl ratio for the 0.25–2 mm WSA fractions were significantly lower than those for other size WSA fractions (P < 0.05). These results demonstrated that the 0.25–2 mm WSA fraction played an important role in stabilizing soil OC and protecting OC from degradation in the irrigated desert soil.

Effects of Organic Amendments on Soil Physical Attributes and Aggregate-Associated Phosphorus Under Long-Term Rice-Wheat Cropping

Abstract The quantification of phosphorus (P) in bulk soil and P distribution in different size fractions of water-stable aggregates (WSAs) are important for assessing potential P loss through runoff. We evaluated available and total P distribution within WSAs of a sitty clay to clay soil in a long-term fertility experiment of a rice-wheat cropping system in India. Surface soil samples were collected from seven plots amended with NPK fertilizers in combination with or without organic amendments, farmyard manure (FYM), green manure (GM), and paddy straw (PS). The plot with no NPK fertilizers or organic amendments was set as a control. The soil samples were separated by wet sieving into four soil aggregate size fractions: large macroaggregates (> 2.0 mm), small macroaggregates (0.25–2.0 mm), fine microaggregates (0.05–0.25 mm), and a silt + clay-sized fraction ( small macroaggregates > fine microaggregates > large macroaggregates. Within a size class, aggregate-associated available and total P contents in the organically amended soil were in the following order: FYM > PS ≥ GM. The available P content of the microaggregates ( 0.25 mm), and the total P content of the microaggregates was 4- to 5-times higher than that of the macroaggregates. Cultivation without organic amendments resulted in more microaggregates that could be checked by the application of organic amendments such as FYM and GM, which increased the proportion of water-stable macroaggregates by consolidating microaggregates into macroaggregates.

Potential harmful effects of landfill stabilized waste: semi-volatile organic compounds and salinity, a case study in Beijing, China

Landfill stabilized waste (LSW) is the aged municipal solid waste mined from old landfills. It is a soil-like material containing high value of organic matter and nutrients and thus may be used as topsoil substitute. To evaluate the potential harmful effects, LSW was sampled from five landfills around Beijing, China. The concentration of 150 semi-volatile organic compounds (SVOCs), total soluble salt, cation exchange capacity (CEC) and the distribution of water-stable aggregate size fractions were determined. According to the results, 14 SVOCs were detected in LSW, including phenol, cresol, 9 PAHs, 2 PAEs and 1 OCP. The concentrations of all detected SVOCs were below the critical limits for soil quality published by Chinese authority, but further studies are needed when it is applied in areas for residential use. The total soluble salt content of LSW ranged from 2.9 to 7.6 g kg−1, and CEC ranged from 3.13 to 6.06 cmol kg−1, indicating that LSW had a high level of salinity but a poor ability of holding and releasing positive-charged elements by clay minerals and humus colloids. Compared to agricultural soil (sandy clay), LSW had more and larger water-stable aggregates, and generally, plants grew better in LSW. LSW of 7–10 mm in size was best for plant growth. The results confirm LSW as a promising topsoil substitute which can be used in afforestation.

Soil aggregate stability and aggregate-associated carbon and nitrogen in natural restoration grassland and Chinese red pine plantation on the Loess Plateau

Artificial afforestation and natural recovery from abandoned cropland are two typical recovery types on the Loess Plateau, China. However, few studies have investigated the difference of natural secondary vegetation restoration and man-made plantation in soil aggregate physicochemical properties and soil aggregate stability. Therefore, we have selected natural restoration grassland and Chinese red pine plantation to study the differences of soil aggregate size distributions, aggregate carbon (C) and nitrogen (N) distributions, soil aggregate stability index (fractal dimension, D; mean weight diameter, MWD; geometric mean diameter, GMD; percentage of aggregation destruction, PAD) as well as their relationships. The results showed that after ~15years restoration from abandoned cropland, natural restoration grassland had higher soil organic carbon (SOC), total nitrogen (TN), ammonium nitrogen (AN), microbial biomass nitrogen (MBN) and MWD compared to Chinese red pine forest, but Chinese red pine forest had higher aggregate C and N, D, GMD and PAD. In addition, SOC positively correlated with MWD in natural restoration grassland but opposite in Chinese red pine forest. In detail, the differences of soil general properties and aggregate size fraction percentages between two land use types were found mainly in 2–5mm, 1–2mm, 0.25mm and clay water-stable aggregate size fractions. The results suggested that higher C content would further contribute the soil aggregate stability in natural restoration grassland, and higher N content would be more important in Chinese red pine plantation.

Stability of soil aggregates and their ability of carbon sequestration

One of the most important binding agents for forming stable aggregates is a soil organic matter (SOM), which can be retained in various size fractions of aggregates. If aggregates are water-resistant, they retain more carbon. Therefore, the aim of this study was to evaluate the stability of aggregates and their ability of carbon sequestration in different soil types and soil management systems in Slovakian vineyards. The highest content of water-stable macro-aggregates (WSA&lt;sub&gt;ma&lt;/sub&gt;) was determined in Cambisols, and the lowest in Fluvisols. The highest content of WSA&lt;sub&gt;ma&lt;/sub&gt; (size fraction 0.5&amp;ndash;3 mm) was determined in Chernozems, decreasing within the following sequence: Fluvisols &amp;gt; Leptosols &amp;gt; Cambisols &amp;gt; Luvisols. The soil type had a statistically significant influence on the re-distribution of soil organic matter in size fractions of water-stable aggregates. The highest content of SOM in water-stable aggregates of the vineyards was determined in grassy strips in-between the vineyard rows in comparison to intensively cultivated rows of vineyard. The highest values of carbon sequestration capacity (CSC) in WSA&lt;sub&gt;ma&lt;/sub&gt; were found in Cambisols &amp;gt; Leptosols and the lowest values of CSC were in Fluvisols. The micro-aggregates represented a significant carbon reservoir for the intensively cultivated soils (rows of vineyard). On the other hand, increasing of macro-aggregates (size fraction 0.5&amp;ndash;3 mm) was characteristic for grassland soils (between the rows of vineyard).

Soil structure stability and distribution of carbon in water-stable aggregates in different tilled and fertilized Haplic Luvisol

The influence of tillage and fertilization on soil structure stability and the distribution of carbon in water-stable aggregates of loamy Haplic Luvisol were studied. Soil samples from the locality of Dolná Malanta (experimental station of SUA Nitra) were collected (in 2007–2009) from a depth of 0–0.2 m in two tillage variants: (1. conventional tillage, 2. minimal tillage) and three treatments of fertilization: (1. without fertilization, 2. crop residues and NPK fertilizers, 3. NPK fertilizers). The minimal tillage system has a positive effect on both the aggregation processes and sequestration of carbon in size fractions of water-stable aggregates, as well as ploughing of crop residues together with NPK fertilizers. On the other hand, application of only NPK fertilizers had a negative effect on SOM content. Under the minimal tillage system and in treatment with crop residues together with NPK fertilizers, what has been observed is a statistically significant increase in the total organic carbon contents by increasing size fractions of water-stable aggregates. Organic carbon did not influence the aggregation processes with dependence on tillage systems. Under conventional tillage as well as in treatment with ploughing crop residues with NPK fertilizers, a very important effect on aggregation had bivalent cations Ca2+ and Mg2+.

Extraction of Heavy Metals from Water-Stable Soil Aggregates Using EDTA

Surface soil samples (0-15cm) were collected from a lead/zinc tailing site in Nanjing, China, and separated into four size fractions of water-stable aggregates. Metal contents in the bulk soil were 7479±370 mg·kg-1 for Zn, 4189±82 mg·kg-1 for Pb, 58.0±0.4 mg·kg-1 for Cd and 544±22mg·kg-1 for Cu, respectively. Contents of Zn, Pb, Cd and Cu in 0.02-0.002mm fractions were lower than them in other size fractions. Extraction of heavy metals from each size fractions of aggregates as function of extraction time and pH value of EDTA was performed. Also, the effects of the extraction process on metal fractions in the samples were investigated. The molar ratio of EDTA/total heavy metals (Zn+Pb+Cu+Cd) was adopted as 1 for all extraction experiments. The extraction kinetics of the four metals in each fraction could be well fitted with Elovich formula. Pb had the highest extraction efficiency at time 24h, 82.7% (2- 0.2mm), 73.6% (0.2-0.02mm), 75.0% (0.02-0.002), and 72.2% (<0.002mm), among the four tested metals. The highest desorption rate for four metals in different size fractions was Zn in <0.002mm fraction and Pb, Cd, and Cu in 2-0.2mm fraction. Extraction of the tested metals negatively depended on the original pH value of extraction solution and the relationship could be described linearly. Exchangeable/acid soluble Zn, exchangeable/acid soluble and reducible fraction of Pb, Cd and Cu were influenced greatly by the extraction using EDTA.

Black Spruce Soils Accumulate More Uncomplexed Organic Matter than Aspen Soils

Improving knowledge on the dynamics and maintenance of the boreal soil's C pool is of particular importance in response to climate change concerns. We hypothesized that different forest types (black spruce, trembling aspen, and mixedwood) found on a similar site type differentially affect soil organic carbon (SOC) distribution among physical fractions. The surface mineral soil (0-15 cm) of 24 plots differing in forest composition was sampled in forested Hapludalfs of the Abitibi-James Bay region, Canada. The soil was first separated into three water-stable aggregate size fractions (> 1000, 1000-250, and mixedwood (41.9) > trembling aspen (34.7). While similar amounts of SOC (∼30 Mg C ha ―1 ) were found in the S&C, more SOC was found in the less protected fractions (i.e., uncomplexed organic matter, UOM: LF and iPOM) under black spruce than under trembling aspen, the mixedwood being intermediate. This higher accumulation of UOM under black spruce suggests a slower C turnover that is probably induced by the low-quality C inputs and environmental constraints to decomposition found in these forests. These differences in the amounts of SOC stored within soil physical fractions might have strong repercussions on the SOC budget of the boreal forest of eastern Canada under climate change.

Effect of land use on the composition of soil organic matter in density and aggregate fractions as revealed by solid-state 13C NMR spectroscopy

Changes in soil organic carbon (SOC) contents with soil cultivation have been investigated extensively, but information on the influence of land use changes on the chemistry of soil organic matter (SOM) and SOM fractions is scarce. To make a contribution in this context, we sampled silty soils under different land use (spruce forest (A h horizon, 0–7 cm), continuous maize cropping (A p horizon, 0–30 cm), and grassland (A h horizon, 0–10 cm)) as well as the organic layer (L, O f, and O h) of the forest floor and collected plant materials from the three sites. Separation of SOM density fractions (free particulate organic matter, occluded particulate organic matter, and mineral-associated organic matter) and water-stable aggregate size fractions of the surface soils was performed in order to follow changes in the chemical composition of SOM in physical soil fractions using CPMAS 13C NMR spectroscopy and to determine land use effects on the chemistry of SOM fractions. The CPMAS 13C NMR spectra showed that maize litter had the highest content of O-alkyl-C and the lowest content of alkyl-C, aryl-C and carbonyl-C compared with the plant material from the spruce stand and the grassland. The forest litter had the largest content of aromatic and alkyl-C. Decomposition of spruce litter in the humus layer resulted in a decreasing O-alkyl-C content and an increasing alkyl-C content. The SOM of the acid forest soil consisted mainly of particulate organic matter (POM) with a high content of spruce litter-derived alkyl-C. The SOC stocks in the grassland and maize soil were dominated by mineral-associated SOM which contained relatively larger proportions of aryl and carbonyl-C. The decrease of the SOC concentration induced by cultivation resulted in a relative accumulation of aromatic C structures in the mineral-bound SOM. In all soils, the free POM had a smaller proportion of alkyl-C and a larger proportion of O-alkyl-C than the POM occluded in aggregates. The mean age of the SOM in the density fractions of the maize soil increased with increasing aromaticity in the order free POM < occluded POM < mineral-associated organic matter.

Preferential Accumulation of Microbial Carbon in Aggregate Structures of No‐Tillage Soils

We examined the effect of reduced tillage on the accumulation of fungal‐ versus bacterial‐derived organic matter within the soil matrix by quantifying the amino sugars glucosamine (Glc), galactosamine (Gal), and muramic acid (MurA) in aggregate‐size fractions isolated from no‐tillage (NT) and conventional‐tillage (CT) soil. Intact soil cores (0‐ to 5‐ and 5‐ to 20‐cm depth) were collected from the long‐term tillage experiment at Horseshoe Bend in Athens, GA. Four water‐stable aggregate‐size fractions were isolated: large macroaggregates (&gt;2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm), and the silt + clay fraction (&lt;53 μm). Small macroaggregates were further separated into coarse particulate organic matter (POM) (&gt;250 μm), microaggregates contained within macroaggregates, and the silt + clay fraction. Amino sugars were extracted from all fractions, purified, and analyzed by gas chromatography. Fungal‐derived amino sugar C (FAS‐C) comprised 63%, while bacterial‐derived amino sugar C (BAS‐C) accounted for 37% of the total amino sugar C pool under both tillage treatments. No‐tillage soil contained 21% more amino sugar C than the CT soil across the entire plow layer. Both, the percentage of total organic C as FAS‐C and BAS‐C were significantly higher in the silt + clay fraction of NT versus CT soil. The percentage of total organic C as FAS‐C was significantly higher in small macroaggregates of NT versus CT soil due to a preferential accumulation of FAS‐C in the microaggregates contained within these macroaggregates. These results indicate that microbial‐derived C is stabilized in NT soils, due primarily to a greater fungal‐mediated improvement of soil structural stability and concurrent deposition of fungal‐derived C in microaggregates contained within macroaggregates.

A modeling approach to quantifying soil macroaggregate dynamics

While several researchers have suggested that soil aggregate turnover is a significant control on organic matter dynamics, the quantification of soil aggregate dynamics has yet to be achieved. Quantification of soil aggregate turnover is essential to testing any hypothesis concerning the relationship between aggregate turnover and organic matter dynamics. The goal of the current work was to propose a modeling approach to the quantification of soil macroaggregate dynamics. The approach taken was to define model compartments representing water-stable soil aggregate size fractions and describing the flows between compartments using first-order kinetics. Soil aggregate data from a 2-yr field study on two contrasting soils were used to calibrate the model and yielded soil aggregate mean residence times ranging from 4 to 95 d, where aggregate dynamics were generally two to three times more rapid in a Gray Luvisol compared to a Black Chernozem. The model was subsequently used to predict the distribution of applied tracer spheres in water-stable aggregate size fractions from an initially free state. The models closely predicted the Dy "mean weight diameters" (Dy-MWD) after two growing seasons. While the models have several limitations, they offer the first attempt to quantitatively describe soil macroaggregate dynamics, which is essential to predicting the response in organic matter dynamics to changes in aggregate dynamics. Key words: Soil aggregation, macroaggregate turnover, tracer, model

Effects of slash-and-burn management on soil aggregate organic C and N in a tropical deciduous forest

Our study examined the effect of slash-and-burn management on the distribution of soil organic carbon (SOC) across water-stable aggregate size fractions. Macroaggregates (>250 μm) are an important source of SOC and soil organic nitrogen in forest soil: they account for approximately 80% of the total C and N content. Slashing and burning did not destroy macroaggregates, but the SOC associated with macroaggregates decreased by 32% due to combustion during burning. Fire also disrupted soil aggregate stabilization by changing the chemical nature of SOC. The largest changes were noted after the first growing season following fire: macroaggregates and associated C decreased 50% while microaggregates increased by the same proportion. The changes in organic C observed after the first growing season can be attributed to macroaggregate instability.

Water-stable aggregates and associated organic matter in forest, savanna, and cropland soils of a seasonally dry tropical region, India

The study was undertaken to quantify the distribution of soil in different size fractions of water-stable aggregates, and organic C, total N, and total P associated with these aggregates, along a gradient of forest-savanna-cropland in the Indian dry tropics. The effect of residue (wheat straw) amendment under dryland cultivation was also investigated. Proportions of macroaggregates (>0.3 mm) were highest in the forest and lowest in the cropland soil and ranged from 58–66% in forest, to 55% in savanna and 25–36% in cropland. In contrast, microaggregates ( 0.05) in the short term, although the proportion of macroaggregates increased, indicating an improvement in soil structure. Thus soil degradation after conversion of natural systems to cropland can be arrested up to some extent by residue input to the soil.

INFLUENCE OF CROPPING TREATMENTS ON THE MONOSACCHARIDE CONTENT OF THE HYDROLYSATES OF A SOIL AND ITS AGGREGATE FRACTIONS

The influence of three cropping treatments on the quantitative and qualitative composition of the carbohydrate fraction of a silt loam soil and its water-stable aggregates was assessed. Three cropping treatments were considered: bromegrass (Bromus inermis Leyss.) grown continuously for 15 yr (B15), grain-corn (Zea mays L.) grown continuously using conventional tillage for 15 yr (C15), and grain-corn grown continuously for 13 yr followed by 2 yr of bromegrass (B13C2). Six water-stable aggregate size fractions ranging from 8.00–4.70 to 0.25–0.10 mm in diameter were separated from the soil by wet sieving. The carbohydrate fraction was extracted by acid hydrolysis, reduced, acetylated to alditol acetates and analyzed by temperature-programmed capillary gas chromatography. Total carbohydrate content was not significantly influenced by cropping treatment but was observed to increase as aggregate size decreased irrespective of cropping treatment. The carbohydrate content in the different aggregate size fractions was positively correlated with both organic carbon and clay contents. With the exception of xylose and mannose, the monomeric sugars behaved similarly to total carbohydrates. Based on the magnitude of the ratio of the content of mannose to that of arabinose plus xylose, the contribution of plant carbohydrates to the total carbohydrate fraction increased as aggregate size decreased in the B15 treatment, but decreased in the C13B2 and C15 treatments. No significant correlations existed between either the total carbohydrate content or the content of individual sugars, and the stability of aggregates in the soils from the different cropping treatments. Key words: Wet aggregate stability, soil organic matter, soil structure, crop rotation, corn, bromegrass