Soil Aggregate Dynamics Research Articles

A conceptual framework to study soil aggregate dynamics

The objective of this study was to develop a conceptual framework to study soil aggregate dynamics through the integration of aggregation, disruption, stabilization and destabilization processes of soil constituent units. We used aggregate‐size distribution and water stability of macroaggregates to evaluate changes in soil aggregates in two ecosystems where one is the ‘reference’ state and the other is the ‘new’ state. The framework was validated with data from (a) a non‐buffered annual row crop (ARC) system, (b) a no‐till (NT) system and (c) a bare fallow (BF) system. The dynamics of soil aggregates in the ARC were dominated by the disruption of aggregates (the aggregation–disruption index (ADI) was −0.08), in NT they were dominated by the aggregation process (ADI = 0.03) and in BF dominated by the disruption of aggregates (ADI = −0.20). Three pathways were observed: (a) expression of the aggregation process, which is essential in reestablishing soil structure and functionality, (b) expression of the disruption process, which is critical in the destruction of soil structure and (c) prevalence of mixed behaviour, where the dominant process depended on the relative rate of aggregation and disruption. The proposed framework and study of the carbon associated with aggregates could pave the way for the development of a methodology for the quantitative study of soil organic carbon (SOC) dynamics, carbon inputs (outputs), soil carbon fractions, and edaphic and biotic conditions that regulate SOC dynamics.Highlights Soil aggregate dynamics were assessed within multiple management systems. A new methodology is proposed for the quantitative study of soil aggregate dynamics. Provided an understanding of aggregation, disruption, stabilization and destabilization in aggregate dynamics. Aggregate dynamic pathways of value in land management can minimize deterioration of the environment.

Effects of Soil Management on Aggregation and Organic Matter Dynamics in sub-Saharan Africa

Maintenance of soil organic matter (SOM) is important for soil quality and agricultural productivity. However, little is known about the effects of management practices of different intensities on soil aggregation and SOM dynamics in tropical arable cropping systems of sub-Saharan Africa. We investigated the influence of land use practices and management intensity on soil aggregation and SOM dynamics across 12 long-term field experiments in eastern and western Africa. Aggregate size distribution and SOM were measured in arable systems under contrasting management intensities of high carbon, low carbon and a fallow. Aggregate stability indices and SOM were generally higher in the fallow compared to the arable systems. Fallowing and high carbon inputs in arable soils, significantly improved aggregate stability and carbon (C) and nitrogen (N) stabilization in whole soil, and in aggregate fractions. In contrast, no significant improvements in soil aggregation and C and N stabilization were found when organic inputs were either applied in low quantities or not applied at all, thus resulting in low carbon in soils. Our study showed that fallowing and long-term application of organic amendments alone or in combination with mineral fertilizers were the best among the practices tested in this study, for enhanced C and N stabilization in soils with the subsequent benefits of improving soil physical and chemical properties. These results emphasize the importance of management for sustaining soil quality. It is recommended that fallowing be an integral part of sustainable soil management strategies in these regions. Keywords: Soil aggregation, soil organic matter, carbon, nitrogen, management intensity

Comparative carbon stability in surface soils and subsoils under submerged rice and upland non-rice crop ecologies: A physical fractionation study

The role of subsoil to store stable carbon (C) has become an important research topic in recent decades. In doing so, pools of subsoil C are compared to surface soil. The present study identified differences in mineral associated C (as separated by density fractionation) and aggregate occluded C in surface and subsurface soils. It also considered the impact of submerged rice as well as upland non-rice ecology on soil C stability. Results indicated that subsoil had a significantly greater amount of soil microaggregates as well as microaggregates within macroaggregates compared to surface soils. Subsequently, subsoil also had higher microaggregate occluded C, inaccessible to microorganisms. The higher ‘ratio of C present in the heavy density fraction with C present in bulk soils’ suggested that more C was strongly associated with the subsoil mineral fraction. Besides, spectroscopic analysis of functional groups indicated higher stability of subsoil aggregate occluded C than the aggregate C of surface soils. Summarily, results indicated that subsoil is a potential niche for recalcitrant C storage. Contrariwise, submerged rice ecology resulted in marginally higher soil aggregation compared to upland non-rice soils. Crop ecology did not influence aggregate occluded C dynamics and mineral-C association. As soil physical and aggregation dynamics differ substantively in puddled rice soils and upland non-rice soils, more research is warranted before making a strong conclusion.

Can soil properties of Fluvisols be influenced by river flow gradient?

Can soil properties of Fluvisols be influenced by river flow gradient?

Soil Aggregation and Organic Carbon Dynamics in Poplar Plantations

Soil resident water-stable macroaggregates (diameter (Ø) > 0.25 mm) play a critical role in organic carbon conservation and fertility. However, limited studies have investigated the direct effects of stand development on soil aggregation and its associated mechanisms. Here, we examined the dynamics of soil organic carbon, water-stable macroaggregates, litterfall production, fine-root (Ø < 1 mm) biomass, and soil microbial biomass carbon with stand development in poplar plantations (Populus deltoides L. ‘35’) in Eastern Coastal China, using an age sequence (i.e., five, nine, and 16 years since plantation establishment). We found that the quantity of water-stable macroaggregates and organic carbon content in topsoil (0–10 cm depth) increased significantly with stand age. With increasing stand age, annual aboveground litterfall production did not differ, while fine-root biomass sampled in June, August, and October increased. Further, microbial biomass carbon in the soil increased in June but decreased when sampled in October. Ridge regression analysis revealed that the weighted percentage of small (0.25 mm ≤ Ø < 2 mm) increased with soil microbial biomass carbon, while that of large aggregates (Ø ≥ 2 mm) increased with fine-root biomass as well as microbial biomass carbon. Our results reveal that soil microbial biomass carbon plays a critical role in the formation of both small and large aggregates, while fine roots enhance the formation of large aggregates.

Linking organic matter chemistry with soil aggregate stability: Insight from 13C NMR spectroscopy

Soil aggregation is considered as a crucial process in agro-system sustainability due to the role in soil physical, chemical and biological dynamics. Here we tested the hypothesis that the initial chemical traits of organic matter (OM) may help to explain the variability of soil aggregation dynamics after organic amendment. We characterized ten OM types (alfalfa litter, biochar, cellulose, glucose, green compost, maize litter, manure compost, meat powder, sawdust, and solid digestate) by 13C-CPMAS NMR and elemental chemical features to investigate the effects of amendment quality on soil aggregation. In a manipulative factorial experiment, dry samples (200 g) of three soil types (S1, S2 and S3) with different texture, high pH (7–9), and similar OM content, were incorporated with 4 g (2% w/w) of dry, 2 mm-grounded OM, incubated in mesocosms for 300 days under controlled temperature (18 ± 2 °C night and 24 ± 2 °C day), and sampled at 4 dates for measuring aggregation index (AI), based on water stability of soil aggregates (WSA). We found that meat powder and alfalfa litter induced a rapid initial increase of AI, exceeding that of the controls by one to two orders of magnitude, likely acting as a C source for microbes. Biochar incorporation in soil barely affected AI, with intermediate effects with other OM types. Considering C bond types corresponding to OM 13C-CPMAS NMR spectral regions, carbonyl C was only correlated to early AI, possibly due to overlapping signals of amide structures; O-alkyl C and di-O-alkyl C (carbohydrate fraction) were positively associated to AI, indicating a promoting effect on soil structure, while aromatic C fractions showed an opposite pattern, possibly related to aggregate protection by coatings associated to water repellency, or to direct aggregate internal binding. This study demonstrates that OM chemical quality plays an important role in soil aggregation process, with the molecular composition defined by 13C-CPMAS NMR spectroscopy being more predictive of aggregation dynamics compared to classical elemental features. As such, this study provides a significant novel contribution to clarify the relationships between OM chemistry and soil aggregation.

Soil Aggregate Dynamics Influenced by Biochar Addition using the 13C Natural Abundance Method

Understanding the mechanisms of biochar C sequestration within soil aggregates requires us to investigate the role of biochar in the soil aggregation process. We aimed to compare aggregate dynamics in soils amended with organic matter or with biochar and to quantify the amount of biochar and organic matter included in different sizes of aggregate. In free‐draining incubation jars, soils collected from grassland and rice (Oryza sativa L.) paddy were amended with fresh corn stalks (Zea mays L) and leaves (CORN_RAW) and with biochar made from corn stalks and leaves (CORN_CHAR). Temporal change in the aggregates were investigated. Soils were fractionated into macroaggregates (250–1000 μm), microaggregates (53–250 μm), and organo‐mineral complexes (<53 μm). The added materials were measured using the difference in the relative depletion of 13C (δ13C) values between the soil and the amendments. Although the macroaggregates in the CORN_RAW treatment increased throughout the incubation period, those in the CORN_CHAR treatment only increased during the initial 60 d. This indicates that macroaggregates in biochar‐amended soil might not be stable enough. However, microaggregates in the CORN_CHAR treatment showed an increase of >200% throughout the incubation period, probably as a result of the breakdown of macroaggregates and formation through chemical adsorptive interactions between biochar surface and soil particles. The proportion of biochar‐derived C within aggregates was greatly increased by 204% on average from the initial to later periods, indicating that biochar C is preferentially enriched because of its chemical recalcitrance and physical occlusion within the aggregates. The overall results imply that biochar amendment could initiate macroaggregation; however, the temporal change pattern was different. Biochar C could be both chemically and physically protected from decomposition during long‐term sequestration.

Impact of land use change on soil aggregate dynamics in the dry tropics

AbstractA 2‐year study was conducted to elucidate land use change (LUC) impact on the distribution of aggregate size fractions and associated carbon (C) concentration involving natural forest (NF), degraded forest (DF), cropland (CL), and biofuel plantation (JP, Jatropha plantation) in the dry tropical region of India across the soil profile (0–10, 10–20, and 20–30 cm). Across the seasons and the land uses, the proportion of macro‐ and microaggregates was maximum at upper and minimum at lower layer whereas mesoaggregates increase with depth. The trend of macro‐ and microaggregate fractions through the soil profile was NF > JP > DF > CL whereas that of mesoaggregates was CL > DF < JP > NF. Dry mean weight diameter was highest at upper layer and decreased down the depth in all the land uses and followed the trend NF > JP > DF > CL. Aggregate associated organic carbon (OC) concentration in all the fractions decreased from NF to DF, CL, or JP indicates that macroaggregate associated OC concentration was more susceptible to loss than that of meso‐ and microaggregate associated OC concentration. LUC induced decline in macroaggregate associated OC stock and increase in meso‐ and microaggregate associated OC stock; indicated redistribution of OC stock among aggregate fractions. It may be concluded that JP in dry tropics can be an efficient strategy for rehabilitation of degraded land as it improves aggregate structure and stability in the whole profile and aggregate associated OC stock in upper layer of soil.

Elevated CO2 and Tree Species Affect Microbial Activity and Associated Aggregate Stability in Soil Amended with Litter

(1) Elevated atmospheric CO2 (eCO2) may affect organic inputs to woodland soils with potential consequences for C dynamics and associated aggregation; (2) The Bangor Free Air Concentration Enrichment experiment compared ambient (330 ppmv) and elevated (550 ppmv) CO2 regimes over four growing seasons (2005–2008) under Alnus glutinosa, Betula pendula and Fagus sylvatica. Litter from the experiment (autumn 2008) and Lumbricus terrestris were added to mesocosm soils. Microbial properties and aggregate stability were investigated in soil and earthworm casts. Soils taken from the field experiment in spring 2009 were also investigated; (3) eCO2 litter had lower N and higher C:N ratios. F. sylvatica and B. pendula litter had lower N and P than A. glutinosa; F. sylvatica had higher cellulose. In mesocosms, eCO2 litter decreased respiration, mineralization constant (respired C:total organic C) and soluble carbon in soil but not earthworm casts; microbial‐C and fungal hyphal length differed by species (A. glutinosa = B. pendula > F. sylvatica) not CO2 regime. eCO2 increased respiration in field aggregates but increased stability only under F. sylvatica; (4) Lower litter quality under eCO2 may restrict its initial decomposition, affecting C stabilization in aggregates. Later resistant materials may support microbial activity and increase aggregate stability. In woodland, C and soil aggregation dynamics may alter under eCO2, but outcomes may be influenced by tree species and earthworm activity.

Lasting effects of biosolids in agroecosystems

Lasting effects of biosolids in agroecosystems

Soil Aggregate Dynamics and Plant Community Response after Biosolids Application in a Semiarid Grassland.

Biosolids may improve the ecological function of degraded semiarid grasslands, but an understanding of the plant community is essential. An experiment was established in 2001 to determine the effects of a single surface application of biosolids on soil aggregate stability and the composition of the plant community in a semiarid grassland in British Columbia, Canada. Four treatments were evaluated: (i) surface biosolids application at 20 (Bio-20) and (ii) 60 Mg ha (Bio-60), (iii) mineral fertilizer, and (iv) a control. All treatments were replicated in four blocks. Soil was sampled during the spring (May), summer (June-July), and fall (October) in 2005, 2006, and 2009; the plant community was assessed in 2009. The greatest increases in size of stable aggregates relative to the control were in the spring and summer, which coincided with a 1.6- to 2.1-fold increase in the spring concentration of N within stable aggregates when biosolids were applied at 20 and 60 Mg ha , respectively. Nitrogen concentrations from the Bio-60 treatment were not different from the control, but the Bio-20 treatment had 42% greater N than all other treatments during summer. Biosolids application in this ecosystem did not increase perennial forage grass species relative to the control, and when biosolids were applied at a rate of 60 Mg ha there was a 75% reduction in the perennial forage plant species. The application of biosolids to native grasslands in semiarid environments should be done cautiously, especially when winter annual plant species (e.g., cheatgrass [ L.]) are present before application.

Dynamics of soil aggregate size in turbulent flow: Respective effect of soil type and suspended concentration

The fate of eroded soil particles impacts soil loss, river engineering and aquatic ecosystems. However, little is known about soil aggregate dynamics within the flow just after their detachment from the soil matrix. The relationship between particle size and turbulence has already been studied but few studies analysed the associated effect of particle concentration. The disaggregation/flocculation of three soils, two badland materials and a well developed calcareous brown soil, was studied by using a grid-stirred tank. An isotropic and homogeneous turbulence was generated to focus on the effects of suspended concentration on particle sizes. Increasing the suspended concentration in the range 1−10gL−1 leads to a decrease of the proportion of the medium size particles and of an increase of the proportion of the smallest particles, as a consequence to enhanced abrasion. The soil aggregates with the largest organic content had the highest strength but were still subject to disaggregation within the turbulent flow, the resulting particle size depending on the suspended concentration. This study demonstrates that soil aggregate characteristics are influenced by concentration, this behaviour being dependent upon the soil type.

Effect of biochar and organic fertilizers on C mineralization and macro-aggregate dynamics under different incubation temperatures

Understanding the effects of soil amendments on organic matter dynamics is essential for the sustainable management of soil. Less is known about the effects of biochar in comparison to commonly used organic fertilizers on soil aggregation and organic matter dynamics. Here, we aimed to analyze the effect of biochar, slurry, and manure on carbon (C) mineralization and macro-aggregate formation. Soil material whose macro-aggregates (>250μm) have been carefully crushed was mixed with biochar and/or slurry or manure and incubated at 5°C, 15°C, and 25°C over a period of eight weeks. We determined and modelled the CO2 emissions over the course of the incubation experiment and analyzed the contents of newly formed macro-aggregates, associated C and microbial biomass C after four and eight weeks. Temperature effects on the decomposition intensity were found to be highest for the manure and lowest for the biochar application. CO2 fluxes across treatments and temperatures were mostly satisfactorily described using 1- or 2-pool-models. Macro-aggregate yields were smaller at higher temperatures in the control and biochar (after 4 weeks) as well as manure (after 4 and 8 weeks) treatments, presumably because of a metabolization of binding agents. At 15°C, slurry addition resulted in significantly higher aggregate yields compared to all other treatments, which may be attributed to its more liquid state allowing an easier and more efficient contact with mineral particle surfaces. In combination with slurry, biochar resulted in lower aggregate yields than slurry alone at 15°C. Manure application did not significantly change aggregate yields compared to the control. Correlations between aggregate yields and the microbial biomass or CO2 production point towards a direct interaction of the respective substrates with soil particles to form aggregates followed by decomposition of the amendments and production of microbially derived aggregate binding agents.

Isotopic characterization of sequestration and transformation of plant residue carbon in relation to soil aggregation dynamics

Soil aggregates play a key role in preserving soil organic carbon (SOC). However, the mechanisms controlling aggregate formation and SOC distribution in different sizes of aggregates remain unclear. Here, we studied the dynamics of aggregate formation and associated SOC transferring among different aggregate fractions using an isotopic tracer technique (14C-labeled wheat residues). The year-long incubation results demonstrate that the wheat residue carbon applied to soil (Typic Hapludoll) was initially mainly stored in the pores between microaggregates that were agglomerated to form macroaggregates (>250μm). The wheat carbon started to transfer to the inside of microaggregates (250–53μm) after six months of the incubation. The newly formed humic acid carbon (HA14C) and fulvic acid carbon (FA14C) in the macroaggregates were 57–90% and 60–84% more than in microaggregates (250–53μm), respectively. Later, the sequestrated SOC was decomposed or chemically transformed, resulting in a decrease in the ratio of HA14C/FA14C in macroaggregates and an increase in the silt/clay fraction (<53μm). This result indicated that the SOC in the macroaggregates was vulnerable to degradation due to less protection by soil macrostructure, while it experienced a slow degradation due to strong surface adsorption or pore protection in soil microstructure. This study suggests that macroaggregates considerably control SOC turnover and thereby their stability considerably influences how much newly introduced organic carbon could be sequestrated into stable carbon pools like microaggregates. The turnover dynamics of macroaggregates provided insights into the potential of humic carbon formation that facilitates long-term carbon preservation in soil.

Aggregation and organic matter in subarctic Andosols under different grassland management

Quantity and quality of soil organic matter (SOM) affect physical, chemical, and biological soil properties, and are pivotal to productive and healthy grasslands. Thus, we analyzed the distribution of soil aggregates and assessed quality, quantity, and distribution of SOM in two unimproved and improved (two organic and two conventional) grasslands in subarctic Iceland, in Haplic and Histic Andosols. We also evaluated principal physicochemical and biological soil properties, which influence soil aggregation and SOM dynamics. Macroaggregates (>250 µm) in topsoils were most prominent in unimproved (62–77%) and organically (58–69%) managed sites, whereas 20–250 µm aggregates were the most prominent in conventionally managed sites (51–53%). Macroaggregate stability in topsoils, measured as mean weight diameter, was approximately twice as high in organically managed (12–20 mm) compared with the conventionally managed (5–8 mm) sites, possibly due to higher organic inputs (e.g., manure, compost, and cattle urine). In unimproved grasslands and one organic site, macroaggregates contributed between 40% and 70% of soil organic carbon (SOC) and nitrogen to bulk soil, whereas in high SOM concentration sites free particulate organic matter contributed up to 70% of the SOC and nitrogen to bulk soil. Aggregate hierarchy in Haplic Andosols was confirmed by different stabilizing mechanisms of micro- and macroaggregates, however, somewhat diminished by oxides (pyrophosphate-, oxalate-, and dithionite-extractable Fe, Al, and Mn) acting as binding agents for macroaggregates. In Histic Andosols, no aggregate hierarchy was observed. The higher macroaggregate stability in organic farming practice compared with conventional farming is of interest due to the importance of macroaggregates in protecting SOM and soils from erosion, which is a prerequisite for soil functions in grasslands that are envisaged for food production in the future.

Pasture degradation impacts soil phosphorus storage via changes to aggregate-associated soil organic matter in highly weathered tropical soils

Maintaining the productivity of tropical pastures is a major challenge for the sustainable management of tropical landscapes around the globe. To address this issue, we examined linkages between soil organic matter (SOM), aggregation, and phosphorus (P) dynamics by comparing productive vs. degraded pastures in the deforested Amazon Basin of Colombia. Paired plots of productive (dominated by planted Brachiaria spp.) vs. degraded pasture were identified on nine farms in the Department of Caquetá and sampled during the rainy season of 2011. Aboveground pasture biomass production and nutrient content were measured. Surface soils (0–10 cm) were also fractionated by wet sieving, and C, 13C, N and P contents were analyzed for the bulk soil and various aggregate size classes. Productive pastures yielded more than double the aboveground biomass compared to degraded pastures (during a 35 day regrowth period following cutting), with over 60% higher N and P contents in this material. Similar trends were observed for the standing litter biomass and nutrient contents. Soil aggregate stability was found to differ between pasture types, with a mean weight diameter of 3590 vs. 3230 μm in productive vs. degraded pastures, respectively. Productive pastures were found to have 20% higher total soil C and N contents than degraded pastures. While there was no difference in total P content between pasture types, organic P was found to be nearly 40% higher in soils of productive vs. degraded pastures. Differences in total SOM between pasture types were largely explained by a higher C content in the large macroaggregate fraction (>2000 μm), and more specifically in the microaggregates (53–250 μm) occluded within this fraction. These findings confirm the role of microaggregates within macroaggregates as a preferential site for the physical stabilization of SOM, and furthermore, suggest that it may serve as a useful diagnostic fraction for evaluating management impacts on SOM in tropical pasture systems. Similar to trends observed for C and N, total P content was 25% higher in the microaggregates within large macroaggregates of productive vs. degraded pasture soils. This correspondence between C and total P contents in large macroaggregate fractions, along with elevated levels of organic P in productive pastures, suggests that this P is likely in an organic form and that there is a close link between soil structure, SOM dynamics and the maintenance of organic P in these soils. Given the potential relevance of organic P for efficient P cycling in these soils, our findings offer critical new insight for the management of SOM and aggregate-associated P pools in tropical pasture systems.

Soil aggregation dynamics and carbon sequestration

The quantity and quality of residues determine the formation and stabilization of aggregate structure for soil organic carbon (SOC) sequestration. Plant roots and residues are the primary organic skeleton to enmesh the inorganic particles together and build macro- and microaggregates while sequestering SOC. There are three major organic binding agents of aggregation: temporary (plant roots, fungal hyphae, and bacterial cells), transient (polysaccharides), and persistent (humic compounds and polymers). Conversion of natural ecosystems into agricultural lands for intensive cultivation severely depletes SOC pools. Magnitude of SOC sequestration in the soil system depends on the residence time of SOC in aggregates. Microaggregates are bound to old organic C, whereas macroaggregates contain younger organic material. Many techniques have been used to assess the SOC distribution in aggregates. Classical methods include SOC determination in aggregate fractions by wet and dry sieving of bulk soil. Isotopic methods including the determination of 13C and 14C with mass spectrometry are techniques to quantify the turnover and storage of organic materials in soil aggregates. Other techniques involve the use of computed tomography, X-ray scattering, and X-ray microscopy to examine the internal porosity and interaggregate attributes of macro- and microaggregates. Current state-of-knowledge has not unravelled completely the underlying complex processes involved in the sequestration, stability, dynamics, and residence times of SOC in macro- and microaggregates. There is a need to develop a unique conceptual model of aggregate hierarchy.

AggModel: A soil organic matter model with measurable pools for use in incubation studies

Current soil organic matter (SOM) models are empirical in nature by employing few conceptual SOM pools that have a specific turnover time, but that are not measurable and have no direct relationship with soil structural properties. Most soil particles are held together in aggregates and the number, size and stability of these aggregates significantly affect the size and amount of organic matter contained in these aggregates, and its susceptibility to decomposition. While it has been shown that soil aggregates and their dynamics can be measured directly in the laboratory and in the field, the impact of soil aggregate dynamics on SOM decomposition has not been explicitly incorporated in ecosystem models. Here, we present AggModel, a conceptual and simulation model that integrates soil aggregate and SOM dynamics. In AggModel, we consider unaggregated and microaggregated soil that can exist within or external to macroaggregated soil. Each of the four aggregate size classes contains particulate organic matter and mineral-associated organic matter fractions. We used published data from laboratory incubations to calibrate and validate the biological and environmental effects on the rate of formation and breakdown of macroaggregates and microaggregates, and the organic matter dynamics within these different aggregate fractions. After calibration, AggModel explained more than 60% of the variation in aggregate masses and over 70% of the variation in aggregate-associated carbon. The model estimated the turnover time of macroaggregates as 31 and 181 days for microaggregates. Sensitivity analysis of AggModel parameterization supported the notion that macroaggregate turnover rate has a strong control over microaggregate masses and, hence, carbon sequestration. In conclusion, AggModel successfully incorporates the explicit representation for the turnover of soil aggregates and their influence on SOM dynamics and can form the basis for new SOM modules within existing ecosystem models.

Long‐Term Tillage Impacts on Soil Aggregation and Carbon Dynamics under Wheat‐Fallow in the Central Great Plains

Soil Sci. Soc. Am. J. 77:594–605 doi:10.2136/sssaj2012.0125 Received 12 Apr. 2012. *Corresponding author (maysoon.mikha@ars.usda.gov). Mention of commercial products and organization in this paper is solely to provide specific information. It does not constitute endorsement by USDA-ARS over other products and organization not mentioned. The US Department of Agriculture, Agricultural Research Service, is an equal opportunity–affirmative action employer and all agency services are available without discrimination.

Long-Term Impacts of Organic and Inorganic Fertilizers on Carbon Sequestration in Aggregates of an Entisol in Mediterranean Turkey

Soil management practices significantly impact soil aggregation and carbon (C) dynamics. Organic fertilizer and mycorrhizal inoculation also affect aggregate stability and soil C pool. Thus, this study was designed to assess the effects of inorganic and organic fertilizer treatments (control, chemical fertilizer, animal manure, compost, and compost + mycorrhizal inoculation) on bulk density (ρ b ), C and N concentrations, soil organic C (SOC) pool, C sequestration, total N, aggregate fractions, water-stable aggregates (WSA), and mean weight diameter (MWD) in the long-term field experiment initiated in 1996 on the Mediterranean coast of Turkey. Disturbed and undisturbed soil samples were collected from the 0- to 0.15-m and 0.15- to 0.30-m depths after wheat (Triticum aestivum) harvest in June 2010 to determine the soil physical and chemical characteristics. Soils that received animal manure, compost, and compost + mycorrhizae contained more C (in percent), N (in percent), SOC, and total N concentrations than control. In comparison with control, chemical fertilizers also affected soil C and N concentrations and pools by increasing plant biomass production. The C/N ratio of soil was less than 10 in control and chemical fertilizer treatments but more than 10 in animal manure, compost, and compost + mycorrhizae treatments. Organic fertilizers significantly reduced ρ b and increased porosity (in percent). Total amounts and rate of C and N sequestration increased with application of organic fertilizers. The rate (kg C ha ―1 year ―1 ) of C sequestration in 0- to 0.15-m depth was 7.19 for control, 329.6 for animal manure, and 273.4 for the compost + mycorrhizae treatment. Furthermore, organic fertilizers significantly increased the SOC concentrations in different aggregate size fractions. The C and N concentrations in the 0.25- to 1.0-mm aggregate size were higher than those in the less than 0.25-mm fraction. Concentrations of C and N decreased significantly with the decrease in aggregate size less than 0.25 mm (P < 0.0009). Application of organic fertilizers impacted soil ρ b , WSA, and MWD. Compost + mycorrhizae―treated soil had the highest MWD (3.09 mm), and the control treatment had the least (1.49 mm). Decline in aggregate size decreased WSA in compost and mycorrhizae treatments. The high values of MWD were associated with mycorrhizal inoculation rather than with application of organic fertilizer. Application of organic fertilizers including mycorrhizal inoculation contributed considerably to soil aggregation and soil C sequestration. There was more C concentration in 1- to 2-mm and 0.5- to 1-mm aggregate sizes than in the less than 0.25-mm size range. Additional research is needed to assess the role of mycorrhizae on macroaggregate development and C sequestration under long-term soil and crop management systems.