Macroaggregate Size Research Articles

Aggregate stability and carbon and N dymamics in macroaggregate size fractions with different soil texture

Soil nutrient cycling, the distribution of soil aggregates, and their stability are directly influenced by soil texture. Different sizes of soil aggregates provide microhabitats for microorganisms and therefore influence soil carbon (C) and nitrogen (N) mineralization. The purpose of the present study was to assess the aggregate stability and dynamics of carbon and nitrogen in macroaggregate size fractions (1-8 mm) with different clay content from meadow soils. Surface soil samples (0-15 cm) were collected from 4- to 5-year-old forage crops. Four macroaggregate size classes were isolated by dry sieving and analyzed for their mass proportions: fine macroaggregates (FM) (less than 1 mm), medium-fine macroaggregates (MFM) (1-2 mm), medium-coarse macroaggregates (MCM) (2-4 mm), and large-coarse macroaggregates (LCM) (4-8 mm). The dry mean weight diameter (MWD), organic carbon (OC), total nitrogen (TN), carbon and nitrogen of microbial biomass (C-MB, N-MB) were determined. CO2 emission and net nitrogen mineralized (NM) were measured after 14 weeks of incubation. The amounts of FM were significantly lower than those of intermediate macroaggregates (MCM and MFM) and decreased markedly with increasing clay content within soil macroaggregates. In general, the amounts of macroaggregate size fractions were lowest in soils with high clay content. MWD exhibited a significant correlation with particle size distribution, OC, and MB-C. OC, TN, MB-C, and MB-N contents within macroaggregates increased with decreasing macroaggregate size and increasing clay content of macroaggregate fractions. The CO2 emission and NM content increased with increasing macroaggregate size, indicating higher organic C and N mineralization activity in larger macroaggregates. Mineralization of OC was lowest in macroaggregate fractions with the highest clay content. We conclude that clay content can increase the protection of microbial biomass in meadow soils. Small macroaggregates tend to contain more recalcitrant organic matter compared to larger macroaggregates.

Impact of bare fallow management on soil carbon storage and aggregates across a rock fragment gradient

AbstractBackgroundOur understanding of C storage in soils lacks insights investigating organic matter (OM) depletion, often studied in bare fallow systems. The content of coarse rock fragments is often excluded, whereas it may affect C storage.AimsWe aim to contribute to a better understanding of the impact of bare fallow on C storage mechanisms in the soil as influenced by its coarse rock fragment contents. We investigated whether bare fallow induced a depletion of C in OM fractions and analyzed to which extent this affected soil aggregate size distribution and the C loading of the clay‐sized fraction.MethodsA comparison of 14 years bare fallow management with adjacent cropped soils located in Selhausen (Germany) provided a gradient of coarse rock fragments of 34%–71%, from which sites with three different fine earth (FE) contents were compared. Across the FE gradient, we isolated particulate OM and mineral‐associated OM fractions, obtained microaggregate and macroaggregate size fractions, and quantified the C loading.ResultsBare fallow management induced an OM depletion at lower contents of FE. There, the management influence was more concentrated onto less FE volume. The contribution of both particulate and mineral‐associated OM fractions to the C in the low‐FE soils decreased. The C loading increased under bare fallow, compared to cropped soil. In the low‐FE soil, we also found less macroaggregates, whereas the C content decreased in some microaggregate size fractions.ConclusionsA high content of coarse rock fragments can enhance OM depletion decreasing mineral‐associated and particulate C under bare fallow.

Production and abundance of macro-aggregate bed clasts from moderately consolidated cohesive beds and their implications for sediment management

ABSTRACT Flume-based research has demonstrated that large (> 1mm), high-water-content mud aggregates can be eroded from cohesive beds and transported in bedload. Studies have also documented these types of mud clasts preserved in the lithologic record. However, questions pertaining to the abundance of muddy bed aggregates and the physical properties that result in their production remain largely unaddressed. New flume experiments were conducted on materials from numerous locations of sediment management projects across the United States. Image analysis of eroded sediment particles demonstrated that macro-aggregated (> 250 μm) mud clasts were commonly produced in moderately consolidated sediment beds (1.2–1.5 g/cm3) with water content above the plastic limit. These macro-aggregates commonly account for more than 20% of the total eroded sediment mass. Physical properties commonly associated with cohesive behavior were evaluated for correlation to bed aggregate production and size. Clay content of the sediment bed was shown to have the greatest correlation to both macro-aggregate size and abundance.

Macroaggregates persist during wetting-drying cycles after glucose addition to a mineral soil

Soil with persistent macroaggregates should maintain its integrity following wetting disturbance. This study determined how wetting-drying cycles affect two pre-sieved macroaggregate size fractions (2–4 mm and 0.6–2 mm) of clay and organic soils pretreated with organic carbon substrates. Soil aggregates were unamended or treated with organic carbon (4 g C from soluble glucose or insoluble wheat straw per kg soil) before being exposed, during three months of incubation, to nine sequential wettings at three wetting intensities (no wetting, slow wetting at the rate of 1 g of water per second or fast wetting with 2 g water per second). The glucose-treated macroaggregates from the clay soil were minimally disturbed by wetting-drying cycles, maintained their shape (shown by high aggregate circularity and low major/minor axis ratio) and were more cohesive, possibly because of a higher microbial activity provided by the glucose treatment. In contrast, the clay soil macroaggregates mixed with wheat straw, which had the same carbon content as glucose, did not persist when exposed to wetting-drying cycles. Macroaggregates from organic soil resisted nine wetting events with or without glucose addition, implying a distinct aggregate binding mechanism in organic soil compared to the clay soil. The persistence of clay soil macroaggregates treated with glucose may be related to microbial processes that promote macroaggregate stability, with glucose physical adhesion expected to substantially improve aggregate stabilization in this mineral soil as well. Glucose-mediated aggregate stabilization can apparently persist for many weeks in an environment with recurring wetting-drying cycles.

Evaluation of soil macro-aggregate characteristics in response to soil macropore characteristics investigated by X-ray computed tomography under freeze-thaw effects

As two complementary aspects of soil structure, the importance of macropore and macro-aggregate characteristics in maintaining soil functions and productivity is paramount. However, the changes in macropores and macro-aggregates and their interactions induced by the effects of freeze–thaw action have rarely been evaluated. This study aimed to evaluate the freeze-thaw effects on soil macropore and macro-aggregate characteristics and their relationships using X-ray computer tomography and dry sieving. Thirty-nine undisturbed soil columns were derived from a depth of 0–15 cm in the Mollisol area of Northeast China. Seven freeze-thaw treatments, including 0 (CK), 1 (FT.1), 3 (FT.3), 5 (FT.5), 7 (FT.7), 10 (FT.10), and 15 (FT.15) freeze-thaw cycles, and two initial soil mass water contents (30% and 40% the initial soil water content (ISWC)) were selected under controlled laboratory conditions. Our results showed that freeze-thaw effects significantly increased the total image porosity, mean pore diameter, porosity with branches, and connectivity density (p < 0.05). However, the total number of pores showed a “first increase – then decrease” trend with an increasing number of freeze-thaw cycles. During the freeze-thaw cycles, the macropore and macro-aggregate size distributions exhibited dynamic processes. Compared to the CK treatment, soils for 30% and 40% ISWC under the FT.15 treatment exhibited a lower > 7 mm macro-aggregates by 27.9% and 43.1%, but a higher < 3 mm macro-aggregates by 78.0% and 88.4%, respectively. The total image porosity, porosity with branches, and connectivity density were significantly positively and negatively correlated (p < 0.05) with the mean weight diameter and fractal dimension of macro-aggregates, respectively. The increase in macroporosity and pore branches easily induced the breakdown of the macro-aggregates. This study can help improve the understanding of the effects of changes in macropore characteristics and the processes of macro-aggregate breakdown and further enable the utilisation of soil structure as an indicator for assessing the sustainability and erodibility of soils.

Soil structural stability following decades of straw incorporation and use of ryegrass cover crops

Maintaining good soil structural stability is an important element in agricultural sustainability. Incorporation of cereal straw and use of cover crops improve soil structural stability but the long-term individual and interactive effects of these management practices is poorly understood. We examined the impact of four rates of straw incorporation in spring barley (0, 4, 8 and 12 Mg ha−1, annually) combined with a ryegrass cover crop undersown in the barley in spring. Soil was sampled after four decades of treatments in the Askov (Denmark) straw incorporation experiment situated on a sandy loam with 12% clay. We assessed clay dispersibility measured on two macro-aggregate size fractions (ClayDis 1–2 mm and ClayDis 8–16 mm), wet-stability of aggregates, and clay dispersibility of < 8 mm field-moist soil and aggregate strength. Soil structural stability and strength were related to soil organic carbon (SOC), SOC/Clay and root biomass. The SOC content increased with straw rate and when ryegrass cover crops were grown. Root biomass was marginally greater (P = 0.068), when cover crops were included. The soils receiving 8 and 12 Mg straw ha−1 had a significantly lower ClayDis 1–2 mm and ClayDis 8–16 mm than soil with straw removal. Inclusion of a ryegrass cover crop did not affect ClayDis 1–2 mm, but decreased ClayDis 8–16 mm marginally (P = 0.054) and decreased clay dispersibility of field-moist soil. The results suggest, that binding agents from cover crops such as roots increase stabilization of large macro-aggregates, while the increased stability due to straw incorporation was related to SOC irrespective of aggregate size.

New insights into organic carbon stabilization in soil macroaggregates: An in situ study by optical microscopy and SEM-EDS technique

The purpose of this study was to investigate the in situ characterization of organic matter (OM) within soil macroaggregates, and to assess the relationships between OM characteristics and macroaggregate size indicating different OM stabilization mechanisms. Optical micro-morphological investigations, coupled to SEM-EDS (scanning electron microscopy and energy X-ray spectroscopy) technique, were carried out on thin sections of 1–4 and 0.25–1 mm soil aggregates (coarse and fine macroaggregates, respectively) from 0 to 20 cm soil layer corresponding to A horizon of four different sites in which soil structure were not disturbed by tillage.The intraggregate porosity, measured by image analysis of four different size classes (<50, 50–100, 100–200, >200 µm), showed that fine macroaggregates were significantly less porous (3.70–6.71% of total porosity) and had higher presence of the finest pore class (<50 μm) compared to coarse macroaggregates (5.93–9.08% of total porosity), independently from sites. The percentage of organic matter forms (OMFs) identified by optical investigation was significant higher in fine (13.5–17.7%) than in coarse (4.19–8.27%) macroaggregates. In particular, fine macroaggregates were richer in red and black amorphous organic forms, which were characterized by the highest values of Al:C, Fe:C and Ca:C molar ratios. These findings suggested thus an accumulation of OM in fine macroaggregates than in coarse macroaggregates occurred. It was probably due to a more efficient OM stabilization in fine than in coarse macroggregates related to both physical occlusion (lower porosity and smaller pore size) and organo-minerals interaction (higher presence of OMFs characterized by the highest Al:C, Fe:C and Ca:C ratios),The OM exposure index (EI), a measurement of the OM surface exposed to pores and thus potentially available for microbial activity, was unexpectedly higher in fine than in coarse macroaggregates (EI: 0.48–0.79 and 0.25–0.58 mm−1 in fine and coarse macroaggregates, respectively). However, the accessibility of OM defined by the EI seemed to facilitate neither the oxidative transformation nor the damage of enzyme activities, being the EI positively related to C:N ratio (r = 0.66), negatively to δ13C values (r = -0.74) and positive to the geometric mean of the five assayed enzyme activities related to C-cycle (r = 0.79). Therefore, even more potentially exposed, in fine macroaggregates the OM was not accessible to microorganisms due to the effective physical occlusion, and thus both accumulation of few transformed OM and maintenance of functionality related to C-cycle occurred.The OM stabilization in macroaggregates thus involved both physical occlusion and organo-metals/mineral phase interactions processes. Both these processes are often related to microaggregates rather than macroaggregates. Our findings thus seem to provide a new insight for studying the potentiality of OM stabilization and C sequestration in soil macroaggregates.

Macroaggregate persistence: Definition and applications to describe soil surface dynamics

Macroaggregates (diameter > 0.25 mm) help the soil surface to resist erosive forces, but their contribution to soil surface stability changes with time because macroaggregate formation and disintegration is a dynamic process. Surface macroaggregates can be visualized by advanced image analysis, a non-invasive method to track aggregates. The objective of this study was to develop a mathematical method to describe the spatial and temporal dynamics of surface macroaggregates observed in digital images. We define aggregate persistence as the ability of aggregates to remain in a pre-determined spatial unit throughout a given time span. The first index explains how many aggregates with the same size distribution remain on a soil surface area through time, which we call the Grouped Aggregate Persistence Index (GAPI). The proportion of individual aggregates with the same size, shape and location at the beginning and end of a measurement period is the Individual Aggregate Persistence Index (IAPI). We calculate the GAPI and IAPI for macroaggregates on the surface of a clay agricultural soil, as an example. Photographs of the soil surface (55 cm2) are analyzed with a customized MATLAB program that uses the watershed method to calculate the macroaggregate size distribution for the GAPI and identify the size, shape and location of macroaggregates for the IAPI. These persistence indices are a non-destructive way to describe dynamic changes in macroaggregates at the soil surface, which is complementary to other methods that visually evaluate the soil structure.

Fertilization and Tree Species Influence on Stable Aggregates in Forest Soil

Background and objectives: aggregation and structure play key roles in the water-holding capacity and stability of soils and are important for the physical protection and storage of soil carbon (C). Forest soils are an important sink of ecosystem C, though the capacity to store C may be disrupted by the elevated atmospheric deposition of nitrogen (N) and sulfur (S) compounds by dispersion of soil aggregates via acidification or altered microbial activity. Furthermore, dominant tree species and the lability of litter they produce can influence aggregation processes. Materials and methods: we measured water-stable aggregate size distribution and aggregate-associated organic matter (OM) content in soils from two watersheds and beneath four hardwood species at the USDA Forest Service Fernow Experimental Forest in West Virginia, USA, where one watershed has received (NH4)2SO4 fertilizer since 1989 and one is a reference/control of similar stand age. Bulk soil OM, pH, and permanganate oxidizable carbon (POXC) were also measured. Research highlights: fertilized soil exhibited decreased macro-aggregate formation and a greater proportion of smaller micro-aggregates or unassociated clay minerals, particularly in the B-horizon. This shift in aggregation to soil more dominated by the smallest (&lt;53 µm) fraction is associated with both acidification (soil pH) and increased microbially processed C (POXC) in fertilized soil. Intra-aggregate OM was also depleted in the fertilized soil (52% less OM in the 53–2000 µm fractions), most strongly in subsurface B-horizon soil. We also document that tree species can influence soil aggregation, as soil beneath species with more labile litter contained more OM in the micro-aggregate size class (&lt;250 µm), especially in the fertilized watershed, while species with more recalcitrant litter promoted more OM in the macro-aggregate size classes (500–2000 µm) in the reference watershed. Conclusions: long-term fertilization, and likely historic atmospheric deposition, of forest soils has weakened macro-aggregation formation, with implications for soil stability, hydrology, and storage of belowground C.

Dynamics of soil aggregate-associated organic carbon based on diversity and high biomass-C input under conservation agriculture in a savanna ecosystem in Cambodia

No-till (NT) cropping systems have the potential to enhance soil aggregation, providing physical protection and soil C sequestration. The existence of discrepancies in the impact of tillage on soil aggregation and soil C sequestration warrants further studies, particularly for different crop rotations. We hypothesized the following: a) NT biannual crop rotations tend to be more effective in restoring large macroaggregation and the concentrations of soil organic C (SOC), total N and permanganate oxidizable C (POXC) associated with macroaggregates than NT systems with a one-year frequency pattern and conventional tillage (CT); b) the continuous biomass-C inputs via crop residues in large macroaggregates under NT tend to increase the proportion of aliphatic C than those under CT. Therefore, the objectives of this study were: (i) to assess changes in the aggregate size distribution and levels of aggregate-associated total SOC, total N and POXC and (ii) to characterize humic acid (HA) using 13C CP-MAS nuclear magnetic resonance (NMR) spectra of 8- to 19-mm soil aggregate size class in a reference vegetation (RV) and in rice-, soybean- and cassava-based cropping systems (RcCS, SbCS and CsCS, respectively) in a clayed Oxisol after tillage and crop rotation management. We evaluated four treatments in each cropping system: 1) CT, and 2) three NT systems in a randomized complete block design with three replicates. Soil aggregate samples were collected at depths of 0–5, 5–10 and 10–20 cm. The conversion of RV to agricultural land influenced the distribution of aggregate size classes, soil aggregation indices and aggregate-associated SOC, total N and POXC in the two surface layers. The formation of large macroaggregates (8–19 mm) dominated the aggregate size distribution with a relatively higher proportion under RV and NT than under CT. Across all soil depths, the proportions of the 8- to 19-mm aggregate size fraction were 59% (NV), 43% and 47% (RcCS), 45% and 53% (SbCS) and 34% and 37% (CsCS) for the CT and NT systems, respectively. Among the three NT systems, the biannual crop rotations in the three cropping systems (NT2-Rice, NT2-Soybean, NT2-Cassava; NT3-Rice, NT3-Soybean and NT3-Cassava) indicated better performance than the one-year frequency pattern in restoring large macroaggregation and the concentrations of SOC, total N and POXC associated with large macroaggregates. Additionally, in the surface (0–5 cm) and subsurface (10–20 cm) soil layers, the SbCS with a high rate (7.32 Mg C ha−1 year−1) and diversity [Pennisetum typhoides) (Pearl millet)/maize + Brachiaria ruziziensis (Brz), Stylosanthes guianensis (St)] of biomass-C inputs reached the highest levels of lability of SOC and POXC in the macroaggregate size classes of 0.25–0.5 and 8–19 mm, respectively. The CP-MAS 13C NMR measurement suggests that the continuous and high biomass-C inputs with diverse crop residues under NT, such as millet, maize, Brz, St and Crotalaria juncea, tended to increase the proportion of aliphatic C than under CT; an opposite trend was observed for aromatic C. 13C NMR revealed an advance caused by the association between the quantity and quality of C addition via cultural residues in the discrimination of the composition of C in the macroaggregation in the tropical region.

The distribution of soil microbial parameters based on aggregate fractions in successional grassland restoration ecosystems on the Loess Plateau

Soil microbial biomass and microbial metabolic quotients have been identified as the most effective indicators of a response to ecosystem development and disturbance. In order to determine the effects of grassland restoration programs, we analyzed the distribution of soil microbial parameters in different aggregate fractions at 1 yr. (1 year), 10 yr. (10 years) and 30 yr (30 years). Results show that Cmic (soil microbial biomass carbon) and Pmic (soil microbial biomass phosphorus) increased with grassland succession, the greatest values were concentrated at a micro-aggregate size. However, qCO2 (soil microbial metabolic quotient) decreased as the years under restoration increased. qCO2 was significantly correlated with SOC (soil organic carbon), TN (soil total nitrogen), Pmic (negatively) and Cmic (positively). Natural succession of grassland alters soil microbial properties positively, which plays a major role in aggregate formation. The time under vegetation rehabilitation impacts soil microbial parameters at different aggregate sizes, macro-aggregate size is the key factor in soil stability improvement in the process of natural restoration, especially for the aggregate size of 2–3 mm.

Long-term effects of nitrogen fertilization on aggregation and localization of carbon, nitrogen and microbial activities in soil

Long-term nitrogen (N) fertilization affects soil aggregation and localizations of soil organic carbon (SOC), N and microbial parameters within aggregates. The mechanisms of these N effects are poorly understood. We studied these processes in a loamy soil from a 23-year repeated N addition field experiment under a rice–barley rotation. Nitrogen fertilization increased plant productivity and the portion of large macroaggregates (>2mm). However, SOC contents in macro- and micro-aggregates remained constant despite an N-induced increase of 27% in root C input into soil. Therefore, N fertilization accelerated SOC turnover. Nitrogen addition increased total N (TN) content in bulk soil and two macroaggregates (>2, and 1–2mm), but not in microaggregates (<0.25mm). Also, N fertilization increased the phospholipid fatty acids (PLFAs) contents of fungi in the large macroaggregates, but not in the microaggregates. In contrast, the effect of N addition on contents of bacterial and total microbial PLFAs was not apparent. Nitrogen fertilization increased N-acetyl-β-D-glucosaminidase (NAG) activities in the two larger macroaggregate size classes (>2, and 1–2mm), but not in the aggregates (<1mm). In both control and N fertilization, the large macroaggregates localized more TN, microbial PLFAs, and NAG activities than the microaggregates. In conclusion, long-term N fertilization not only directly promotes soil N resource but also indirectly improves soil structure by forming large macroaggregates, accelerates SOC turnover, and shiftes localization of microorganisms to the macroaggregates.

Macroaggregation and soil organic carbon restoration in a highly weathered Brazilian Oxisol after two decades under no-till

Conclusions based on studies of the impacts of soil organic carbon (SOC) fractions and soil texture on macroaggregation and SOC stabilization in long-term (>20years) no-till (NT) fields remain debatable. This study was based on the hypothesis that the amount and frequency of biomass-C input associated with NT can be a pathway to formation of macroaggregates and to SOC buildup. The objectives were to: 1) assess the macroaggregate distribution (proportional mass, class mass) and the SOC and particulate organic carbon (POC) stocks of extra-large (8–19mm), large (2–8mm) and small (0.25–2mm) macroaggregate size classes managed for two decades by NT, and 2) assess the recovery of SOC stocks in extra-large macroaggregates compared to adjacent native vegetation (Andropogon sp., Aristida sp., Paspalum sp., and Panicum sp.). The crop rotation systems were: soybean (Glycine max L.), maize (Zea mays L.) and beans (Phaseolus vulgaris L.) in summer; and black oat (Avena strigosa Schreb), white oat (Avena sativa), vetch (Vicia sativa L.), black oat.+vetch (Avena strigosa Schreb+vetch) and wheat (Triticum aestivum L.) in winter. The experimental was laid out as 2×2 randomized block factorial with 12 replicates of a NT experiment established in 1997 on two highly weathered Oxisols. The factors comprised of: (a) two soil textural types: clay loam and sandy clay, and (b) two sampling depths: 0–5 and 5–20cm. The three classes of macroaggregates were obtained by wet sieving, and the SOC content was determined by the dry combustion method. The extra-large macroaggregate classes in 0–20cm depth for sandy clay (SdC) and clay loam (CL) Oxisol represented 75.2 and 72.4% of proportional mass, respectively. The SOC and POC stocks among macroaggregate classes in 0–5 and 5–20cm depths decreased in the order: 8–19mm>2–8mm ≈ 0.25–2mm. The SdC plots under soybean/maize at 3:1 ratio recovered 58.3%, while those at 1:1 ratio (high maize frequency) in CL recovered 73.1% of SOC stock in the extra-large macroaggregates compared with the same under native vegetation for 0–20cm depth. Thus, partial restoration of the SOC stock in original extra-large macroaggregate confirms the hypothesis that NT through higher maize cultivation frequency can be a pathway to fomation of macroaggregates and SOC buildup.

Effects of reclamation years on composition and diversity of soil bacterial communities in Northwest China.

The objective of this study was to evaluate bacterial community structure and diversity in soil aggregate fractions when salinized farmland was reclaimed after >27 years of abandonment and then farmed again for 1, 5, 10, and 15 years. Illumina MiSeq high-throughput sequencing was performed to characterize the soil bacterial communities in 5 aggregate size classes in each treatment. The results indicated that reclamation significantly increased macro-aggregation (>0.25 mm), as well as soil organic C, available N, and available P. The 10-year field had the largest proportion (93.9%) of soil in the macro-aggregate size classes (i.e., >0.25 mm) and the highest soil electrical conductivity. The 5 most dominant phyla in the soil samples were Proteobacteria, Actinobacteria, Gemmatimonadetes, Acidobacteria, and Bacteroidetes. The phylogenetic diversity, Chao1, and Shannon indices increased after the abandoned land was reclaimed for farming, reaching maximums in the 15-year field. Among aggregate size classes, the 1-0.25 mm aggregates generally had the highest phylogenetic diversity, Chao1, and Shannon indices. Soil organic C and soil electrical conductivity were the main environmental factors affecting the soil bacterial communities. The composition and structure of the bacterial communities also varied significantly depending on soil aggregate size and time since reclamation.

Regional-scale variability of soil properties in Western Cuba

The soils of Artemisa and Mayabeque provinces, in western Cuba, have a historical importance in Cuban agricultural productivity and have been subject to production- oriented practices and fragile protection from degradation. Very few contemporary studies about these soils are available. As a support to near-future management decisions, the objectives of this work are: i) to characterize the variability of some physical and chemical properties in the main agricultural lands of the Cuban provinces of Mayabeque and Artemisa, and ii) to identify relevant features associated to soil property changes and their possible impacts on near future decision making. Soil samples were taken in a 116 data point stratified-random-unaligned grid at two depths: 10–15cm and 35–40cm. The measured properties were: sand (Sa), silt (Si) and clay (Cl) content, organic matter content (SOM), bulk density (ρb), particle density (ρs), exchangeable cation content (Na+, K+, Ca2+, and Mg2+), pH, and the macroaggregate size distribution. An exploratory analysis illustrates the most outstanding issues of the variable distribution, their dependence on the involved soil depths and types, and the correlation between measurements. Some relevant spatial features associated to the behavior of the properties are identified with kriging spatial interpolation. Calcium content is shown to be a key property in soil functioning, mainly for its effect on macroaggregation. In contrast, Cl and SOM did not affect the soil structure patterns. Analyses suggest that Cuba’s most productive soils are subject to severe degradation mainly from increasing pH and compaction, and therefore the adoption of conservation practices is crucial for their sustainability on the medium and long term.

Soil CO2–C flux and carbon storage in the dry tropics: Impact of land-use change involving bioenergy crop plantation

The study was conducted to evaluate the impact of land-use change in the dry tropics on soil CO2–C flux and soil organic carbon (SOC) storage, along with the major factors controlling them i.e. soil microbial biomass (SMB), belowground net productivity (BNP), and soil aggregate size fractions. Land-use change, in this study, involved conversion of natural forest, to degraded forest and then to an agroecosystem or a bioenergy crop plantation of Jatropha curcas. The soil CO2–C flux was highest in the agroecosystem followed in decreasing order by degraded forest, bioenergy crop plantation and smallest in the natural forest. The inverse trend was found in case of SOC storage, SMB and BNP. The proportion of macroaggregate in the soil follows the trend of SOC storage, whereas mesoaggregate follows the trend of CO2–C flux across all land-use types. The CO2–C flux showed significant negative correlation with BNP, SMB, macroaggregate size fraction, and SOC. Our study suggests that the flux of soil CO2–C was regulated directly by the soil aggregate fraction and not by SMB or BNP. However, soil aggregate formation was, in turn, related to the SMB and/or below BNP. Macroaggregates appear to protect the SOC, which results in lower CO2–C flux and higher SOC storage. It can be concluded that the bioenergy crops plantation on degraded forest lands in the dry tropics, may increase C storage in soil and reduce soil CO2–C flux, thereby helping in the mitigation of global climate change in addition to providing feed stocks for fossil fuel substitution.

Organic matter accumulation post‐mineral sands mining

AbstractSoil development and organic matter (OM) accumulation are vital for sustainability in reclaimed prime farmlands following mineral sands mining. Additionally, the effectiveness of soil reconstruction techniques on soil development greatly influences crop productivity. Soil development and management effects following mineral sands mining were evaluated in years 1 (2005), 4 (2009) and 6 (2011) at the Carraway‐Winn Reclamation Research Farm, VA, USA. Treatments for this full scale agricultural experiment are as follows: biosolids applied at a rate of 78 Mg/ha managed with conventional tillage (BIO‐CT) and no‐till (BIO‐NT), a 15‐cm topsoil cap (TS), and a lime + fertilized control. Crop yields were determined annually, and soils were collected and analysed for aggregate size distributions and OM pools (available, aggregate‐protected and mineral‐bound). Crop yields (Corn‐Zea mays and wheat/soybean‐Triticum aestivum/Glycine max) were generally larger in the biosolids treatments from 2005 to 2008, with no difference among treatments from 2009 to 2011. Whole soil carbon (C) and nitrogen (N) rapidly increased between 2005 and 2009, mainly in the large macroaggregate (2000–8000 μm) size fraction. Carbon accumulation rates in the whole soils ranged from 2.85 to 3.58 Mg C/ha in the first 4 yr of soil development (similar trends were observed for N). There were no differences for soil aggregate parameters among treatments until year 6, where biosolids treatments contained more microaggregate (53–250 μm) and mineral‐bound C and N relative to other treatments. Short‐term increases in crop yields and long‐term increases in stable soil C and N make biosolids applications a viable alternative to traditional TS replacement strategies for this mining land use scenario.

Development of aggregates after application of maize residues in the presence of mycorrhizal and non-mycorrhizal pea plants

It has been demonstrated before that growing pea (Pisum sativum L.) plants reduce the decomposition of 15N labeled maize residues. This reduction was especially strong in pots planted with the mycorrhizal wild-type Frisson (myc+) in comparison with the non-mycorrhizal mutant P2 (myc−). The aim of the present experiment was to assess the contribution of AMF and saprotrophic microorganisms to the formation of aggregates and accumulation of C and N as occluded light fraction organic matter in the soil of this pot experiment. In the myc+ treatment of the size class<53μm, the dry weight yield and the δ13C value of soil organic C were significantly higher than in the myc− treatment. After application of maize leaf residues, the percentage of dry weight yield generally increased in the two macroaggregate size classes at the expense of the small and large microaggregates. This demonstrates the important role of saprotrophic microorganisms in macroaggregate formation. Consistently, the content of maize-derived C declined significantly in the occluded light fraction of the myc+ treatment, showing that less plant residues were accumulated in aggregates. Overall, the presence of saprotrophic microorganisms induced aggregate formation and accumulation of C within aggregates. The presence of mycorrhiza did not affect aggregation or soil C partitioning, despite a higher root biomass. It should be further investigated whether the positive effects of the maize-decomposing microorganisms on aggregation persist for longer periods.

Aggregate C depletion by plowing and its restoration by diverse biomass-C inputs under no-till in sub-tropical and tropical regions of Brazil

Encapsulation of soil organic carbon (SOC) within aggregates is one of the principal mechanisms for long-term C sequestration, macroaggregate formation and stabilization. Our objectives were to quantify the changes in aggregate size distribution, aggregate-C concentrations and stocks upon conversion of native vegetation (NV) to conventional plow-based tillage (CT), and to assess the rate of aggregation and SOC recovery with no-till (NT) under diverse biomass-C inputs. The study was conducted at both sub-tropical (Ponta Grossa – PG, State of Paraná) and tropical (Lucas do Rio Verde – LRV, State of Mato Grosso) sites in Brazil. The SOC content under NV was used as a baseline to evaluate the depletion rate under CT and the restoration rate under NT. A specific emphasis was given to the largest macroaggregate size class (8–19mm) because of its importance to protecting the recently deposited labile SOC. A discriminant analysis of principal components (DAPC) indicated that NV soil is modified by conversion to an arable land use and that, mechanical tillage, biomass input, and their interactions drastically influence the distribution of aggregate-size classes, aggregation indices, and SOC distribution within aggregates. At both sites, soil aggregation indices were positively impacted by NT and associated with SOC concentration in the labile fractions (e.g., total polysaccharides (TPS), hot water extractable organic C (HWEOC), particulate organic C (POC)). At the PG site, the 8–19mm aggregate size fraction was significantly affected by land use and tillage treatments and represented 54%, 43%, and 72%, under NV, CT, and NT in 0–20cm depth, respectively. Furthermore, the 8–19mm size fraction stored 55%, 45%, and 71% of the total SOC stock under NV (53.8MgCha−1), CT (28.5MgCha−1) and NT (51.2MgCha−1), respectively. At the LRV site, the 8–19mm aggregate size fraction decreased from 50% under Cerrado NV to 35% under CT, and ranged from 33% to 51% under diverse biomass-C input under NT in 0–20cm depth. The 8–19mm size fraction stored 52%, 37%, and 41% of the total SOC stock across all aggregate sizes under NV (25.4MgCha−1), CT (11.7MgCha−1), and NT (9.9–18.1MgCha−1), respectively. The difference in SOC stock among land uses is largely attributed to storage in the 8–19mm aggregate size class, indicating that NT cropping systems rebuilt the largest macroaggregates, which are crucial for stabilization of SOC.

Soil aggregation and distribution of carbon and nitrogen in different fractions under long-term application of compost in rice–wheat system

Soil organic matter improves the physical, chemical and biological properties of soil, and crop residue recycling is an important factor influencing soil organic matter levels. We studied the impact of continuous application of rice straw compost either alone or in conjunction with inorganic fertilizers on aggregate stability and distribution of carbon (C) and nitrogen (N) in different aggregate fractions after 10 cycles of rice–wheat cropping on a sandy loam soil at Punjab Agricultural University research farm, Ludhiana, India. Changes in water stable aggregates (WSA), mean weight diameter (MWD), aggregate-associated C and N, total soil C and N, relative to control and inorganically fertilized soil were measured. Total WSA were significantly ( p = 0.05) higher for soils when rice straw compost either alone or in combination with inorganic fertilizers was applied as compared to control. The application of rice straw compost either alone or in combination with inorganic fertilizers increased the macroaggregate size fractions except for 0.25–0.50 mm fraction. The MWD was significantly ( p = 0.05) higher in plots receiving rice straw compost either alone at 8 tonnes ha −1 (0.51 mm at wheat harvest and 0.41 mm at rice harvest) or at 2 tonnes ha −1 in combination with inorganic fertilizers (0.43 and 0.38 mm) as compared to control (0.34 and 0.33 mm) or inorganically fertilized plots (0.33 and 0.31 mm). The macroaggregates had higher C and N density compared to microaggregates. Application of rice straw compost at 2 tonnes ha −1 along with inorganic fertilizers (IN + 2RSC) increased C and N concentration significantly over control. The C and N concentration increased further when rice straw compost at 8 tonnes ha −1 (8RSC) was added. It is concluded that soils can be rehabilitated and can sustain the soil C and N levels with the continuous application of rice straw compost either alone or in combination with inorganic fertilizers. This will also help in controlling the rising levels of atmospheric carbon dioxide.