Aggregate Size Classes Research Articles

Cover crop diversity improves multiple soil properties via altering root architectural traits

Cover crops may influence soil health and functioning. However, little is known about the role of belowground root architectural traits in linking cover crop diversity with rhizosphere soil ecosystem properties. We hypothesize that cover crop diversity may improve root traits, which in return, could influence its effects on essential indicators of soil physicochemical heterogeneity, such as the composition of soil aggregate-size classes and nutrients, and the soil organic matter (SOM) and soil organic carbon (SOC) contents. We studied the impact of plant diversity on root traits and soil properties. The four soil aggregate-size classes, such as large macro- (>2000 μm), small macro- (<2000-500 μm), meso- (<500-250 μm), and micro-aggregates (<250 μm) were separated by the dry sieving. Root traits such as surface area (cm2) and length (cm) were quantified by image analysis using Winrhizo. The soil nutrient, SOM, and SOC contents were determined by standard methods. We used Fox's (2005) method for partitioning the selection and complementarity effects on plant productivity. Plant diversity improved productivity, root architectural traits, root coverage, the composition of soil aggregate-size classes and nutrients, SOM and SOC contents across the soil depth. It increased and decreased the relative abundance of meso- and micro-aggregates, respectively. Both root architectural traits and SOC contents positively correlated with several soil edaphic properties essential for physicochemical heterogeneity, health, and functioning. Our results suggest that competition among plant roots in species-rich than poor communities may improve rhizosphere soil carbon storage, the composition of soil aggregate-size classes, and nutrients.

In situ effects of biochar field-aged for six years on net N mineralization in paddy soil

An understanding of nitrogen (N) mineralization is essential for tracing the supply of inorganic N for plant uptake. However, how microorganisms regulate N mineralization for carbon (C) sequestration under field-aged biochar amendment remains unclear. To address this, we investigated the soil net N mineralization rate (net Nmin), contents and hydrolytic enzyme activities of C and N, microbial biomass N, and native 15N values in bulk soil and aggregate size classes for six years after biochar application (20 and 40 t ha-−1) in a typical rice-wheat rotation. The results showed that aged biochar decreased net Nmin (normalized by total N content) by 10.5 %–69.9 %, and C and N hydrolytic enzyme activities per unit of microbial biomass C by 4.8–71.1 % and 24.0–77.8 %, respectively, compared with N fertilization in all soil aggregates except for ClayF size class (<2 μm). Microbial biomass N (MBN) increased by 21.5–130.9 % in soil aggregates, while the δ15N values decreased following biochar addition compared with those under N fertilization. The labile C:N ratios were higher in the bulk soil and MacroA size class (250−2000 μm) following biochar addition than under N fertilization, which would increase microbial N demand as evidenced by the lower enzymatic C:N ratios and higher MBN. Microorganisms obviously restrained net Nmin but did not increase N hydrolytic enzyme activity to meet their stoichiometric N demands. Structural equation modeling revealed that enzymatic C:N stoichiometry is a dominant indicator of net Nmin in bulk soil and the >53 μm size class, while the MBN is more important to net Nmin in the <53 μm size class. We conclude that the addition of aged biochar could meet microbial stoichiometric requirements and regulate extracellular enzyme production, resulting in the decline of net Nmin in soil aggregates, especially in MacroA size class.

Rhizosphere effects promote soil aggregate stability and associated organic carbon sequestration in rocky areas of desertification

Soil aggregate stability is an important index for predicting soil water loss and soil erosion resistance. Plant roots effectively control soil erosion and stabilize soil structure, which has a crucial influence on the formation of aggregates and soil organic carbon (SOC) sequestration. We examined how rhizosphere effects influence soil aggregate stability and its associated SOC contents and δ13C values, following a proposed extended model of carbon (C) flows between the aggregate size classes in the root systems based on 13C fractionation in each step of SOC formation. The results show that the rhizosphere effects significantly improved the stability of aggregates. The mean weight diameter (MWD) and geometric mean diameter (GMD) of rhizosphere soil aggregates were significantly higher than those of non-rhizosphere soil aggregates associated with plants with fibrous roots. SOC levels of all size aggregates in the rhizosphere soil of both fibrous and tap root plants were higher than those of non-rhizosphere soil. Moreover, SOC contents increased in the order of silt-clay particles (SCP, <0.05 mm), microaggregates (MIA, 0.05-0.25 mm) and small macroaggregates (SMA, 0.25-1 mm), and then decreased from SMA to large macroaggregates (LMA,> 1 mm). The δ13C values in non-rhizosphere soil were generally higher than those in rhizosphere soil in aggregates of the same size class, especially in the tap root plants. Except for the rhizosphere soil of fibrous root plants, the other three soil types (rhizosphere and non-rhizosphere soil of tap root plants, and non-rhizosphere soil of fibrous root plants) were shown to have aggregate δ13C values that decreased with increasing soil aggregate size. Δ13C enrichment of the SOC fractions showed that the general flow direction of SOC was from rhizosphere to non-rhizosphere, and from large aggregates to small aggregates. The C flow in the aggregates of rhizosphere soil was clearly greater than that in the non-rhizosphere soil, especially with the fibrous root plants. These findings suggest that plant roots have the potential to regulate soil structural stability, and enhance soil erosion resistance and SOC sequestration.

Carbon mineralization and its temperature sensitivity under no-till and straw returning in a wheat-maize cropping system

Soil organic carbon (SOC) mineralization is regulated by temperature and moisture. No-till (NT) and straw returning (SR) have been widely adopted to sequester SOC, but information about the effects of temperature and moisture on SOC mineralization in NT and SR is limited, particularly SOC mineralization in different aggregate size classes. To identify the responses of SOC mineralization to temperature and moisture in NT and SR, undisturbed soils were sampled from a factorial experiment of tillage (NT and plow tillage [PT]) and straw (SR and straw removal [S0]) in a wheat-maize cropping system and incubated at moisture levels of 40%, 70%, and 100% at 15 °C and 25 °C. The results showed that maize season increased SOC mineralization but decreased its temperature sensitivity (Q10) compared with the wheat season under different moisture levels. Because NT and SR promoted macro-aggregation and low mineralization was observed in macro-aggregates, NT significantly decreased the absolute (per unit soil) and specific (per unit SOC) mineralization compared with PT, regardless of the soil temperature, moisture, depth, and sample date (P < 0.05). The SR significantly increased absolute SOC mineralization; Whereas, there was no significant difference in specific mineralization and was even 24.4% lower at 5–10 cm depth in the maize season than that of S0 (P < 0.05). However, macro-aggregates had 13.6%-37.3% and 11.9%-19.9% higher Q10 values than the micro-aggregates in the wheat and maize seasons, respectively. This led to a higher Q10 under NT and SR at 40% moisture level, indicating that NT and SR constrained SOC accumulation under global warming. Increases in moisture ranging from 40% to 100% could decrease Q10, thus no significant difference between NT and PT, and SR and S0 at 100% level. Conclusively, NT with SR decreases SOC mineralization but increases its Q10, indicating that the loss of SOC is currently low but could accelerate under future warming. Our study also provided an effective approach that increased moisture could constrain the improvement of Q10 induced by NT with SR.

Organic matter stabilization in aggregates and density fractions in paddy soil depending on long-term fertilization: Tracing of pathways by 13C natural abundance

Previous studies on upland soils showed that 13C natural abundance can successfully reveal C stabilization pathways between aggregates and soil organic matter (SOM) density fractions. The direction of C stabilization in paddies can, however, deviate from that in upland soils owing to i) periodic drying–rewetting cycles, with oxygen pulses under oxic conditions, and thus, shifts in microbial processing of organic residues, and ii) intensive organic and mineral fertilization. To trace C stabilization in paddies, soil was sampled from a long-term field experiment under an unfertilized Control and NPK, NPK + straw, and NPK + manure fertilizer regimes. Soil was analyzed for total C, microbial biomass (MB), and dissolved organic C, and separated into three classes based on aggregate size (>250 μm, 53–250 μm, and <53 μm) followed by density fractionation of each class to obtain free and occluded light fractions as well as dense and mineral heavy fractions. Pathways of C were determined based on C content and δ13C in all pools. The highest increase in total C (69%) was in NPK + manure, whereas the MBC increased with fertilization by at least 29% compared with the Control. All fertilizers increased macro-aggregation by at least 111% compared with the Control. The highest C content in the aggregates was in the mineral fractions of macroaggregates. Fertilization decreased the δ13C of total SOM compared with the Control, indicating suppressed decomposition of organic compounds. Aggregate size classes showed a typical δ13C enrichment trend from macro-to microaggregates, reflecting similarities between paddy and upland soils. A detailed scheme of C flows within aggregates and SOM fractions based on the δ13C natural abundance revealed the following general sequence: mineral → dense → free light → occluded light fractions. This trend, which is partly opposite to that observed in upland soils, reflects the anoxic and variable redox conditions of paddies. This facilitates the predominant stabilization of recent C input in the mineral fraction with Fe oxides due to Fe2+/Fe3+ dynamics, whereas light fractions are processed by microorganisms mainly in periods without overflooding. The C pathways in the two heavy fractions were separate from those in the two light fractions, which also indicates differences in the C stabilization processes between paddy and upland soils. Thus, the present study provides further detailed insights into the C stabilizing mechanisms in paddy soils which depend on management.

Carbon sequestration in aggregates from native and cultivated soils as affected by soil stoichiometry

Quantitative influence and underlying mechanisms of nutrient stoichiometry on mineralization of native Soil organic C (SOC) and straw C in different aggregate classes from cultivated and non-cultivated soils are still unclear. Soil samples (Mollisols) from a native woodlot and a farmland converted from woodlot were sieved into three aggregate classes (mega-aggregates (6.3-2 mm), macro-aggregates (2-0.25 mm), and micro-aggregates (< 0.25 mm)) and incubated (180 days) under different nutrient rates (nil, low, and high supplies of N and P) with or without 13C-enriched straw amendment. Significantly higher percentage of native SOC was mineralized from mega- and macro-aggregates (65.8-82.2%) compared with micro-aggregates (48.3-52.0%) in woodlot soil. Nutrient addition significantly increased aggregate-associated C in both soils with straw, and the increase was greater in farmland than in woodlot soil and in large-sized aggregates than in micro-aggregates. These results suggested that large aggregates serve as a C reservoir of labile C, while micro-aggregates that contained C was not easily mineralized even with abundant nutrients. Differences of C mineralization among aggregate size classes were significant in woodlot soil but not in farmland soil. Depletion of SOC was greater with increasing nutrient addition rates in farmland aggregates without straw, while the depletion in woodlot aggregates showed no difference among nutrient treatments, suggesting that microbial activity was nutrient-limited in farmland aggregates. The results improved our knowledge on SOC mineralization in response to residue-nutrient management in different aggregate classes from cultivated and non-cultivated soils, which have important implications for strategies to improve soil fertility or mitigate climate change via increased SOC.

Effects of freeze-thaw cycles and soil moisture content on soil available micronutrients on aggregate scale in natural grassland and Chinese pine forestland on the Loess Plateau, China

Micronutrient presence and concentration are strongly influenced by soil aggregate size. Seasonal freeze-thaw patterns are known to modify soil aggregate distributions, thereby contributing to the transformation and redistribution of available soil micronutrients. Few studies, however, have evaluated the response of aggregate-associated available micronutrients to freeze-thaw cycles (FTCs) under different types of vegetation restoration. We designed a laboratory experiment to simulate the three typical types of ecosystems on the Loess Plateau in China (cropland, Chinese pine forestland, and natural grassland). In this experiment, we measured the effects of number of FTCs (0, 1, 3, and 9) and soil moisture content (SMC) (40 and 80% field capacity) on available micronutrients (Cu, Fe, Mn, and Zn) in bulk soil and three soil aggregate size classes. FTCs significantly increased available micronutrient content in bulk soil by increasing > 2000 μm aggregate-associated available micronutrient contents and decreasing < 250 μm aggregate-associated available micronutrient contents. In most cases, the influences of FTCs on bulk soil and aggregate-associated available micronutrient content were enhanced by increased SMC. Compared with cropland soil, the bulk soil available Cu, Fe, Mn, and Zn in natural grassland soil and available Cu in Chinese pine forestland soil were significantly increased. The increasing effects of FTCs on the available micronutrient content in cropland and natural grassland soils were larger than that in Chinese pine forestland soil. The results indicated that increasing FTCs and SMC was beneficial for alleviating available micronutrient deficiencies in soil, and natural grassland was better than Chinese pine forestland for increasing available micronutrients in this loess-derived soil.

Effect of Tillage and Water Management on Aggregate Stability of a Gleyic Cambisols

The objective of this study was to determine the effect of tillage and water management on the physical stability of irrigated lowland rice field, in Kwalkwalawa, Sokoto State. The experiment was carried out in a farmer`s field, near the Usmanu Danfodiyo University Teaching and Research Farm, Kwalkwalawa, Sokoto State. The coordinates of the area were taken using global positioning system (GPS) model Garmin etrex 20.0, which shows that the area is located on (N13°05.963”E005°12.650” and at 252 m asl). The soils of the study area were classified as Aeric Endoaquepts at subgroup level in the USDA Soil Taxonomy System which correlated with Gleyic Cambisols in the World Reference Base. The treatments consisted of factorial combination of two tillage systems (conventional tillage (CT) and reduced tillage (RT), three irrigation water managements (Alternate one, two and three days irrigation intervals, which were carried out from one week after transplanting to hard dough stage) and three rice varieties (FARO 44, 60 and 61) all laid in a split-plot design and replicated three times. After harvest, disturbed soil samples were collected with the aid of soil auger, prepared and passed through 5 mm sieve for aggregate stability determination. Result reveals that a consistent trend in aggregate size fraction was observed between the two tillage systems in both years, were a significant decrease in values of aggregate fractions of both the CT and RT in 2019 compared to in 2018. Aggregate size fraction of 5-2 mm had a significantly high value of RT compared to CT, 2-0.25 mm fractions were at par (0.41) while a greater value of aggregate fractions for CT in both 0.25-0.005 mm and ˂0.005 mm were observed compared to RT. Alternate days to irrigation and sampling depths increase with a corresponding in all the aggregate size classes in the two years of this study. A significant difference in mean weight diameter (MWD) and geometric mean diameter (GMD) between the two tillage, water management and depth was noticed in both years of the trial where RT and alternate one day had high value while MWD and GMD increase with increasing depth.

Characteristics of magnetic susceptibility on cropland and pastureland slopes in an area influenced by both wind and water erosion and implications for soil redistribution patterns

Many areas are influenced by both wind erosion and water erosion, and the overlap of wind and water erosive forces results in serious land degradation. Magnetic susceptibility (MS) is an effective tool for quantifying soil redistribution. This method has been applied successfully in wind and water erosion research, but not in research on soil erosion induced by both wind and water yet. This study attempts to investigate the characteristics of MS in different land uses in a wind and water eroded area and explore their implications for wind and water erosion. A cropland and a pastureland slope in the Bashang area of the Agro-Pastoral Transition Zone of North China were selected, and samples from the two transects were measured for mass-specific low-frequency MS (χlf), frequency-dependent MS (χfd), particle size distribution and the MS of five aggregate size classes. The results show that there is a high value concentration of χlf in the middle slope of the cropland, indicating that the soil is transported by water and accumulated on the slope. The correlation between χlf and χfd in the cropland is obviously weaker than the pastureland, which is a result of SP loss due to wind erosion in the cropland. The mean χlf of the surface pastureland is half that of the cropland, which suggests there is deposition of exogeneous wind eroded particles with low MS in the pastureland, and they include coarse particles by saltation and creep from local areas and fine particles transported from remote areas. Due to the different sources of the soil particles, the contribution mechanisms of MS on the two slopes are distinct. Human activities associated with tillage also influence the soil redistribution in the cropland. These preliminary results indicate that MS could be applied to the study of soil erosion induced by both wind and water to provide a supplementary perspective achieving spatially explicit results.

Inconsistent response of soil bacterial and fungal communities in aggregates to litter decomposition during short-term incubation.

BackgroundSoil aggregate-size classes and microbial communities within the aggregates are important factors regulating the soil organic carbon (SOC) turnover. However, the response of soil bacterial and fungal communities in aggregates to litter decomposition in different aggregate-size classes is poorly understand.MethodsSoil samples from un-grazed natural grassland were separated into four dry aggregate classes of different sizes (2–4 mm, 1–2 mm, 0.25–1 mm and <0.25 mm). Two types of plant litter (leaf and stem) of Leymus chinensis were added to each of the four aggregate class samples. The CO2 release rate, SOC storage and soil microbial communities were measured at the end of the 56-day incubation.ResultsThe results showed that the 1–2 mm aggregate had the highest bacterial Shannon and CO2 release in CK and leaf addition treatments, and the SOC in the <0.25 mm aggregate was higher than that in the others across the treatments. The relative abundance of Ascomycota was higher in the 2–4 mm and <0.25 mm aggregates than in the 1–2 mm and 0.25–1 mm aggregates in the treatment without litter addition, and the relative abundance of Aphelidiomycota was lower in the 2–4 mm and <0.25 mm aggregates than in the 1–2 mm and 0.25–1 mm aggregates. Also, litter addition increased the relative abundance of Proteobacteria and Bacteroidetes, but decreased the relative abundance of Acidobacteria, Gemmatimonadetes, and Actinobacteria. The relative abundance of Ascomycota and Aphelidiomycota increased by more than 10% following leaf litter addition. The bacterial Shannon index had a significantly positive and direct effect on SOC concentration and CO2 release, while the fungal Shannon index was significantly correlated with SOC concentration. Our results indicate that the soil bacterial diversity contributes positively to both carbon emissions and carbon storage, whereas soil fungal diversity can promote carbon storage and decrease carbon emissions.

A Comparison of Physical Soil Organic Matter Fractionation Methods for Amended Soils

Selecting a suitable physical fractionation method, to investigate soil organic matter dynamics, from the plethora that are available is a difficult task. Using five different physical fractionation methods, on soils either nontreated or with a history of amendment with a range of exogenous organic matter inputs (Irish moss peat; composted horse manure; garden compost) and a resulting range of carbon contents (6.8 to 22.2%), we show that method selection had a significant impact on both the total C recovered and the distribution of the recovered C between unprotected, physically protected, or chemically protected conceptual pools. These between-method differences most likely resulted from the following: (i) variation in the methodological fractions obtained (i.e., distinguishing between aggregate size classes); (ii) their subsequent designation to conceptual pools (e.g., protected versus unprotected); and (iii) the procedures used in sample pretreatment and subsequent aggregate dispersion and fractionation steps. The performance of each method also varied depending on the amendment in question. The findings emphasise the need for an understanding of the nature of the soil samples under investigation, and the stabilisation mechanism of interest, both prior to method selection and when comparing and interpreting findings from literature studies using different fractionation methods.

Soil macroaggregates and organic-matter content regulate microbial communities and enzymatic activity in a Chinese Mollisol

The formation and turnover of macroaggregates are critical processes influencing the dynamics and stabilization of soil organic carbon (SOC). Soil aggregate size distribution is directly related to the makeup and activity of microbial communities. We incubated soils managed for >30 years as restored grassland (GL), farmland (FL) and bare fallow (BF) for 60 days using both intact and reduced aggregate size distributions (intact aggregate distribution (IAD)<6 mm; reduced aggregate distribution (RAD)<1 mm), in treatments with added glucose, alanine or inorganic N, to reveal activity and microbial community structure as a function of aggregate size and makeup. Over a 60-day incubation period, the highest phospholipid fatty acid (PLFA) abundance was on day 7 for bacteria and fungi, on day 15 for actinomycete. The majority of the variation in enzymatic activities was likely related to PLFA abundance. GL had higher microbial abundance and enzyme activity. Mechanically reducing macroaggregates (>0.25 mm) by 34.7% in GL soil with no substrate additions increased the abundance of PLFAs (average increase of 15.7%) and activities of β-glucosidase (increase of 17.4%) and N-acetyl-β-glucosaminidase (increase of 7.6%). The addition of C substrates increased PLFA abundance in FL and BF by averages of 18.8 and 33.4%, respectively, but not in GL soil. The results show that the effect of habitat destruction on microorganisms depends on the soil aggregates, due to a release of bioavailable C, and the addition of substrates for soils with limited nutrient availability. The protection of SOC is promoted by larger size soil aggregate structures that are important to different aggregate size classes in affecting soil C stabilization and microbial community structure and activity.

Soil aggregates indirectly influence litter carbon storage and release through soil pH in the highly alkaline soils of north China.

BackgroundSoil aggregate-size classes, structural units of soil, are the important factors regulating soil organic carbon (SOC) turnover. However, the processes of litter C mineralization and storage in different aggregates-size classes are poorly understood, especially in the highly alkaline soils of north China. Here, we ask how four different aggregate sizes influence rates of C release (Cr) and SOC storage (Cs) in response to three types of plant litter added to an un-grazed natural grassland.MethodsHighly alkaline soil samples were separated into four dry aggregate classes of different sizes (2–4, 1–2, 0.25–1, and <0.25 mm). Three types of dry dead plant litter (leaf, stem, and all standing dead aboveground litter) of Leymus chinensis were added to each of the four aggregate class samples. Litter mass loss rate, Cr, and Cs were measured periodically during the 56-day incubation.ResultsThe results showed that the mass loss in 1–2 mm aggregates was significantly greater than that in other size classes of soil aggregates on both day 28 and day 56. Macro-aggregates (1–2 mm) had the highest Cr of all treatments, whereas 0.25–1 mm aggregates had the lowest. In addition, a significant negative relationship was found between Cs/Cr and soil pH. After incubation for 28 and 56 days, the Cs was also highest in the 1–2 mm aggregates, which implied that the macro-aggregates had not only a higher CO2 release capacity, but also a greater litter C storage capacity than the micro-aggregates in the highly alkaline soils of north China.

Impact of soil use on aggregate stability and its relationship with soil organic carbon at two different altitudes in the Colombian Andes

The stability of soil aggregates depends on the organic matter, and the soil use and management can affect the soil organicmatter (SOM) content. Therefore, it is necessary to know therelationship between aggregate stability and the content of SOMin different types of soil use at two different altitudes of theColombian Andes. This study examined the conditions of soilaggregate stability expressed as a distribution of the size classes of stable aggregates (SA) and of the mean weighted diameter of the stable aggregates (MWD). To correlate these characteristics with the soil organic carbon (OC), we measured the particulate organic matter pool (POC), the OC associated with the mineral organic matter pool (HOC), the total organic carbon content (TOC), and the humification rate (HR). Soils were sampled at two altitudes: 1) Humic Dystrudepts in a cold tropical climate (CC) with three plots: tropical mountain rainforest, pastures, and crops; 2) Fluvaquentic Dystrudepts in a warm tropical climate (WC) with three plots: tropical rainforest, an association of oil palm and pastures, and irrigated rice. Soils were sampled at three depths: 0-5, 5-10 and 10-20 cm. The physical properties, mineral particle size distribution, and bulk density were measured. The content of SA with size&gt;2.36 mm was higher in the CC soil (51.48%) than in the WC soil (9.23%). The SA with size 1.18-2.36 mm was also higher in the CC soil (7.78%) than in the WC soil (0.62%). The SA with size 0.60-1.18 mm resulted indifferent. The SA with size between 0.30 and 0.60 mm were higher in the WC soil (13.95%) than in the CC soil (4.67%). The SA&lt;0.30 mm was higher in the WC soil (72.56%) than in the CC soil (32.15%). It was observed that MWD and the SA&gt;2.36 mm increased linearly with a higher POC, but decreased linearly with a higher HR. For the SA&lt;0.30 mm, a linear decrease was observed at a higher POC, while it increased at a higher HR.

The fractionation of soil aggregates associated with primary particles influencing wind erosion rates in arid to semiarid environments

Soil particle size distribution (PSD) plays an important role in wind erosion studies and sediment transport modeling. Few studies have been done focusing on the susceptibility of different soil fractions to wind erosion under natural conditions. This study was performed to measure in-situ wind erosion rates using a portable wind tunnel facility on 60 undisturbed soils of arid to semiarid regions. The effects of primary (i.e. clay, silt, and sand particles) and 16 secondary (aggregate) size classes were attributed to wind erosion rates. Moreover, the boundaries between major fractions of the soils (i.e. micro-aggregates, meso-aggregates and macro-aggregates) were determined. The result showed that clay and silt particles and also surface rock fragment were inversely proportional to wind erosion rates as power functions, whereas sand content showed a positive exponential relationship. The size ranges of <53 μm, 53–300 μm and >300 μm were distinguished for the micro-aggregates, meso-aggregates, and macro-aggregates, respectively. The meso-aggregate fraction was proportional and the micro-aggregate and macro-aggregate fractions were inversely proportional to wind erosion rates. Furthermore, a narrower size range of 75–150 μm was detected as the most erodible soil fraction. The distribution of the primary particles in the major aggregate fractions was different, so that in the meso-aggregates, sand particles and in the micro-aggregates and macro-aggregates, silt and especially clay particles predominated. It is concluded that the presence of macro-aggregates (>300 μm) associated with fine primary particles (clay) can significantly reduce wind erosion rate rather than the sandy soils containing weaker aggregates.

Physical and chemical processes determining soil organic matter dynamics in a managed vertisol in a tropical dryland area

Vertisols are soils with a high potential for carbon (C) sequestration. However, these soils undergo swelling and shrinkage cycles, which exert a strong influence on soil organic matter (SOM) dynamics. The long-term effects of agricultural conservation practices on SOM content and stability on a Vertisol located in a tropical dryland area was evaluated.Two tillage systems (conventional tillage [CT], disc ploughing and extraction of 70% harvest residues, and no tillage [NT], incorporating 100% of the harvest residues) in combination with three different crop rotations (sorghum-soyabean [SrSy], maize-maize [MM] or soyabean-maize [SyM]) were tested. All plots were irrigated during dry periods. The effects on soil organic C, N, microbial biomass and aggregate size class distribution were evaluated. Carbon sink capacity were assessed by studying the aggregate-associated SOM content and quality (thermal recalcitrance), as determined by thermo-calorimetry.Relative to the CT system, NT yielded a higher percentage of small macroaggregates (2-0.25 mm). It also promoted the accumulation of aggregate-associated SOM, especially in the small macroaggregates and microaggregates. Thermal analysis revealed that the aggregate-associated SOM was more recalcitrant (higher values of T50 and more energy released at 475 °C) in NT than in CT. Coinciding with these effects, the soil microbial biomass/SOC ratio was lower in NT than in CT. The gain of SOM and the higher recalcitrance of OM in the NT soils may be related to different characteristics of NT Vertisols, such as poor aeration or disruption of the swelling and shrinkage cycles. The findings show that soil conservation practices increase the capacity for long-term C sequestration in Vertisols via organomineral interactions and SOM recalcitrance, which are determined by the formation of large cracks as a result of the swelling and shrinkage cycles that are some of the specific properties and processes of these soils.

Short-Term Effect of Nitrogen Intensification on Aggregate Size Distribution, Microbial Biomass and Enzyme Activities in a Semi-Arid Soil Under Different Crop Types

There is a lack of quantitative assessments available on the effect of agricultural intensification on soil aggregate distribution and microbial properties. Here, we investigated how short-term nitrogen (N) intensification induced changes in aggregate size distribution and microbial properties in a soil of a hot moist semi-arid region (Bangalore, India). We hypothesised that N intensification would increase the accumulation of macroaggregates > 2 mm and soil microbial biomass and activity, and that the specific crop plant sowed would influence the level of this increase. In November 2016, surface (0–10 cm) and subsurface (10–20 cm) soil samples were taken from three N fertilisation treatments, low N (50 kg N ha−1), medium N (75 and 100 kg N ha−1 for finger millet and maize, respectively), and high N (100 and 150 kg N ha−1 for finger millet and maize, respectively). Distribution of water-stable aggregate concentrations, carbon (C) and N dynamics within aggregate size class, and soil microbial biomass and activity were evaluated. The high-N treatment significantly increased the concentration of large macroaggregates in the subsurface soil of the maize crop treatment, presumably due to an increased C input from root growth. Different N fertilisation levels did not significantly affect C and N concentrations in different aggregate size classes or the bulk soil. High-N applications significantly increased dehydrogenase activity in both the surface soil and the subsurface soil and urease activity in the surface soil, likely because of increased accumulation of enzymes stabilised by soil colloids in dry soils. Dehydrogenase activity was significantly affected by the type of crop, but urease activity not. Overall, our results showed that high N application rates alter large macroaggregates and enzyme activities in surface and subsurface soils through an increased aboveground and corresponding belowground biomass input in the maize crop.

Carbon in aggregate size classes in a Rhodic Eutrudox under different cropping systems

Soil productive capacity is related to levels of carbon (C) in aggregates of different sizes. The aim of this study was to assess total organic carbon levels in different size classes of water-stable aggregates in a Rhodic Eutrudox under different production systems. The cropping systems assessed were no-tillage (NT); no-tillage scarified every three years (NTS); disk plowing (DP) and heavy disking (HD). All systems were subjected to crop succession (S) (soybean - Glycine max / wheat - Triticum aestivum) and rotation (R) (soybean, maize (Zea mays), wheat) and cover and green manure (Lupinus albus, Raphanus sativus and Avena strigosa). Intact soil samples were collected in trenches at depths of 0–0.10; 0.10-0.20; 0.20–0.30 and 0.30-0.40 m. The highest levels of carbon were found under no-tillage, irrespective of the aggregate size class. In all treatments, the top layer (0.0–0,10 m) under crop succession showed the highest carbon content for all aggregate size classes. However, at depths below 0.10 m, crop rotation exhibited the highest carbon levels (between 12 and 20 g kg-1). After 29 years of trials, cropping systems with the lowest soil disturbance combined with crop rotation were found to contribute to raising the level of carbon in the soil and maintaining stable aggregates.

The transport of aggregates associated with soil organic carbon under the rain‐induced overland flow on the Chinese Loess Plateau

AbstractOrganic carbon (OC) is easily enriched in sediment particles of different sizes due to aggregate breakdown and selective transport for sheet erosion. However, the transport of aggregate‐associated OC has not been thoroughly investigated. To address this issue, 27 simulated rainfall experiments were conducted in a 1 m × 0.35 m box on slope gradients of 15°, 10°, and 15°and under three rainfall intensities of 45 mm h−1, 90 mm h−1 and 120 mm h−1. The results showed that OC was obviously enriched in sediment particles of different sizes under sheet erosion. The soil organic carbon (SOC) concentrations of each aggregate size class in sediments were different from those in the original soil, especially when the rainfall intensity or slope was sufficiently low, such as 45 mm h–1 or 5°, respectively. Under a slope of 5°, the SOC enrichment ratios (ERocs) of small macroaggregates and microaggregates were high but decreased over time. As rainfall intensity increased, OC became enriched in increasingly fine sediment particles. Under a rainfall intensity of 45 mm h–1, the ERocs of the different aggregate size classes were always high throughout the entire erosion process. Under a rainfall intensity of &gt; 45 mm h–1 and slope of &gt; 5°, the ERocs of the different aggregate size classes were close to 1.0, especially those of clay and silt. Therefore, the high ERocs in sediments resulted from the first transport of effective clay. Among total SOC loss, the proportion of OC loss caused by the transport of microaggregates and silt plus clay‐sized particles was greater than 50%. We also found that low stream power and low water depth were two requirements for the high ERocs in aggregates. Stream power was closely related to sediment particle distribution. Flow velocity was significantly and positively related to the percentage of OC‐enriched macroaggregates in the sediments (P &gt; 0.01). Our study will provide important information for understanding the fate of SOC and building physical‐based SOC transport models. © 2019 John Wiley &amp; Sons, Ltd.

Soil Carbon and Nitrogen Dynamics in Two Agricultural Soils Amended with Manure-Derived Biochar.

Biochar has been promoted as a means to sequester C and improve soil quality. Biochar produced from agricultural waste streams and recycled as a soil amendment also provides a strategy for improved nutrient management in agricultural systems. The effects of biochar amendment on soil C and N cycling remain poorly constrained. This study aimed to examine the effects of biochar on soil C and N storage, N mineralization, and soil physiochemical properties. Soils were collected from a field experiment in which biochar derived from poultry manure was applied for 2 yr in two croplands differing in soil texture (sandy and silt-loam). Samples from biochar-amended and control soils were physically fractionated to separate water-stable soil aggregates and analyzed for C and N. Biochar amendments increased total soil C by 16 (sandy soil) and 30% (silt-loam soil). These increases were observed in aggregate size classes associated with short-term C and N storage in silt-loam soils and intermediate-term C and N storage in sandy soils. Net N mineralization rates observed in a short-term incubation were small or negative (1.79 and -24.7 μg N g soil for sandy and silt-loam soils, respectively), indicating little or no new N mineralization from biochar over short timescales. Biochar amendment had a positive impact on cation exchange capacity at both sites, increasing it by 7 and 11% in the silt-loam soil and sandy soil, respectively. These results suggest that biochar amendments to cropping systems can improve the capacity of soil to retain nutrients and store C and N.