Physical Protection Of Organic Matter Research Articles

Distribution of Organic Carbon Fractions in Soil Aggregates and Their Contribution to Soil Aggregate Formation of Paddy Soils

ABSTRACT Different soil organic matter fractions play different roles in governing soil aggregation. Therefore, this study determined the distribution of labile organic carbon (OC) and stable OC fractions in soil aggregates and their contribution to the soil aggregation of paddy soils in the Central Plain of Thailand. Quantitative analysis of the OC fractions in soil aggregates was determined using wet oxidation method and their changes in the chemical composition of organic compounds were investigated using FTIR spectroscopy. The results showed that the large macroaggregate (LMA: 2–8 mm) content of studied soils generally increased with increasing soil organic carbon, clay, polyvalent cations and aluminum (Al) oxides contents. Generally, both the labile OC and stable OC fractions increased with increasing aggregate size classes, suggesting a hierarchical order of soil aggregation and physical protection of organic matter against microbial decomposition. The labile OC fraction in all aggregate size classes was positively correlated with the content of LMA (r = 0.54*–0.77**), suggesting that the labile OC fraction represented a major organic cementing agent that contributed to LMA formation. FTIR analysis revealed that the labile OC fraction mainly consisted of plant-derived and microbial-derived polysaccharides. Therefore, paddy soils with higher organic cementing agent (labile OC fraction) and inorganic cementing/flocculating agents may provide a stronger intimate association of these transient binding agents, particularly the microbial-derived polysaccharides with clay mineral surfaces and metal cations/Al oxides. In turn, the greater amounts of stabilized, microbial-derived polysaccharides also efficiently promoted the formation of LMA from a combination of microaggregates.

Combination of Imaging Infrared Spectroscopy and X-ray Computed Microtomography for the Investigation of Bio- and Physicochemical Processes in Structured Soils

Soil is a heterogeneous mixture of various organic and inorganic parent materials. Major soil functions are driven by their quality, quantity and spatial arrangement, resulting in soil structure. Physical protection of organic matter (OM) in this soil structure is considered as a vital mechanism for stabilizing organic carbon turnover, an important soil function in times of climate change. Herein, we present a technique for the correlative analysis of 2D imaging visible light near-infrared spectroscopy and 3D X-ray computed microtomography (mCT) to investigate the interplay of biogeochemical properties and soil structure in undisturbed soil samples. Samples from the same substrate but different soil management and depth (no-tilled topsoil, tilled topsoil and subsoil) were compared in order to evaluate this method in a diversely structured soil. Imaging spectroscopy is generally used to qualitatively and quantitatively identify OM with high spatial resolution, whereas 3D X-ray mCT provides high resolution information on pore characteristics. The unique combination of these techniques revealed that, in undisturbed samples, OM can be found mainly at greater distances from macropores and close to biopores. However, alterations were observed because of disturbances by tillage. The correlative application of imaging infrared spectroscopic and X-ray mCT analysis provided new insights into the biochemical processes affected by soil structural changes.

Changing soil hydraulic properties and water repellency in a pomegranate orchard irrigated with saline water by applying polyacrylamide

Soil wettability and water repellency are important physical properties which greatly affect soil-water relations. This study was conducted to investigate the effects of polyacrylamide (PAM) on soil water repellency and wettability in a pomegranate orchard drip irrigated with saline water in Isfahan Province, central Iran. The experiment was a randomized complete block design conducted within each “durations of drip irrigation” (considered as the environment) including control (uncultivated), and 8 and 15 years old trees under drip irrigation system (DIS). PAM concentrations were 0 (control), 10 and 20mgl−1 arranged in each block at three replications. The PAM (solubilized in saline water with electrical conductivity of 6.2dSm−1) with the mentioned concentrations was applied once during the irrigation time. A week after applying the PAM, soil samples were taken from 0 to 0.3, 0.3–0.6 and 0.6–0.9m depths under the emitters. Water repellency index (WRI) and soil-water contact angle (β) were determined using intrinsic sorptivity method by measuring the water and ethanol sorptivities in all soil samples. Increasing the duration of irrigation with saline water increased electrical conductivity (ECe), concentrations of (Ca2++Mg2+) and Na +, and sodium adsorption ratio (SAR) of the saturated extract in the 0–0.3m soil layer under the emitters. Increasing the duration of irrigation also increased WRI and β, and decreased water infiltration, especially in the 0–0.3m soil layer, presumably through enhancing effects of long-term use of saline water on surface tension of water, soil aggregate stability, physical protection of organic matter in soil and stability of hydrophobic coatings. PAM application significantly reduced soil water repellency and increased soil water sorptivity. In the 0–0.3m, 8 and 15 years of drip irrigation increased the WRI by 56 and 134 percent, respectively, compared with the control. PAM application at rates of 10 and 20mgl−1 decreased WRI by 27 and 40 percent, respectively, when compared with the control. Application of 20mgl−1 of PAM could completely ameliorate the degradative effects of drip irrigation with saline water for 8 years, whereas amelioration in the 15-year treatment remained incomplete.

Effects of cover crops on the physical protection of organic matter and soil aggregation

Effects of cover crops on the physical protection of organic matter and soil aggregation

Physical protection of organic matter in minesoils assessed by low-temperature ashing (LTA)

Physical stabilization of soil organic matter (SOM) was assessed by a novel combination of methods in minesoils of Central Italy, cultivated or afforested with different species by 30years. We studied this stabilization in: 1) a managed (thinned and mowed) English oak (Quercus robur L.) plantation; 2) a similarly managed 1:1 mixed plantation of Italian alder (Alnus cordata Loisel.) and English oak; 3) an unmanaged portion of the mixed plantation; 4) a yearly tilled and manured cropland; 5) an adjacent forest growing on a natural soil, which served as a term of reference. We focused on water-stable soil aggregates, in which SOM is physically protected from decay. Aggregates of 0.5 to 1.0mm in diameter were subjected to Low-Temperature Ashing (LTA) by oxygen plasma, a technique able to progressively remove SOM with minimal or no damage to mineral constituents and soil fabric. All minesoils had behaved as C sinks, although to a different extent depending on land use, with cropland storing around half soil C than the afforested areas. Carbon enrichment improved soil structure promoting the formation of large and water-stable soil aggregates. The C contained in the core of these aggregates, measured here as the one able to resist 48h of LTA treatment and confidently assumed as the C fraction best protected from decay, ranged from 43 to 70% of total C, depending on land use and horizon. The strict relationships found between the amount of “protected” SOM and the median diameter (d50) of water-stable soil aggregates (R2=0.88), on the one hand, and the larger class of pores in water-stable soil macroaggregates (R2=0.99), on the other hand, demonstrated the crucial role of this SOM pool in soil structure formation. This study showed the potential of LTA used in combination with laser diffraction analysis, FTIR-PAS spectroscopy, and mercury intrusion porosimetry to distinguish C stocks benefiting from different physical protection in aggregates and to assess their role in soil structure formation. Such a methodological approach could be highly useful to understand the mechanisms by which soils at the first stages of pedogenesis, such as reclaimed minesoils, sequester C and to reveal how much tillage and other types of soil disturbances expose the protected C to decay.

Soil pores and their contributions to soil carbon processes

It has been generally recognized that micro-scale heterogeneity in soil environments can have a substantial effect on many soil physical, chemical, and biological processes driving physical protection of soil carbon (C). However, only recently the development of tools for micro-scale soil analyses, including X-ray computed micro-tomography (μCT), enabled quantitative analyses of these effects. X-ray μCT application to soil science is arguably one of the newest and fastest growing areas of soil science research; and its methodology is still being actively developed. The large amount of spatially explicit data that μ-CT scanning generates coupled with specially designed experiments can open new avenues for improved understanding of soil functioning and soil-plant interactions. Pores are both drivers and products of a variety of soil processes that ultimately determine physical protection of organic matter in soil by influencing its accessibility to microorganisms. The μCT tools are well suited for providing information on characteristics of soil physical micro-environments, a.k.a. soil pores. Here we review the experimental approaches currently employed by research groups around the world in exploring the role of pore characteristics in soil C processes, with specific focus on soil C decomposition and protection at 5–1000μm spatial scale. We discuss pore/C/microbe relationships with emphasis 1) on direct and indirect effects of pore characteristics on soil microorganisms and subsequent microbial effects on decomposition of organic inputs; 2) on presence of feed-back effects of microorganisms on soil pore architecture; and 3) on importance of pore characteristics for decomposition of freshly added organic inputs/plant residues. The pore-oriented perspective will contribute to better structuring of the research efforts in deciphering the mechanisms of soil C protection/sequestration by soils.

Factors influencing soil aggregation and particulate organic matter responses to bioenergy crops across a topographic gradient

Bioenergy crops have the potential to enhance soil carbon (C) pools from increased aggregation and the physical protection of organic matter; however, our understanding of the variation in these processes over heterogeneous landscapes is limited. In particular, little is known about the relative importance of soil properties and root characteristics for the physical protection of particulate organic matter (POM). We studied short-term (3-year) changes in aggregation and POM-C pools under three cropping systems (switchgrass, a triticale/sorghum double crop, continuous corn) replicated across five landscape positions along a topographic gradient in Iowa, USA. We isolated POM associated with three aggregate fractions (>2mm, 0.25–2mm, and 0.053–0.25mm) to determine the relative influence of ten soil and three root properties. Aggregation increased in all cropping systems and was greatest under switchgrass; however cropping system effects were not consistent among positions. Total soil organic C stocks did not change, but C within both physically protected (iPOM-C) and unprotected (frPOM) C pools increased. Shifts in iPOM-C were concurrently influenced by soil properties and root traits. Soil texture had the strongest influence (65% relative importance), with finer-textured soils showing greater gains in total iPOM-C, while greater root biomass influenced (35% relative importance) accrual of total iPOM-C. Aggregate fractions varied in their iPOM-C response to soil and root variables, however individual pools similarly showed the importance of soil texture and root biomass and annual root productivity (BNPP). Changes in frPOM-C were strongly correlated with BNPP. Our data suggest that macroaggregate formation drives short-term responses of POM, which are influenced by both soil and root system properties. Crops that maximize root biomass and BNPP will lead to the largest increases in protected soil C stocks. However, C storage rates will vary across landscapes according to soil conditions, with texture as the primary influence.

O uso de amostras peneiradas a dois milímetros subestima a proteção física da matéria orgânica no solo

O fracionamento físico densimétrico é uma importante ferramenta no estudo da proteção física da matéria orgânica (MO) em agregados de solo, porém seus resultados podem ser alterados pela malha da peneira utilizada no preparo da amostra. A hipótese do presente estudo é de que a desagregação excessiva do solo pelo uso de peneira de 2 mm libera MO originalmente protegida dentro de agregados e, portanto, subestima o teor de C na fração leve-oclusa (C-FLO) e a magnitude da proteção física da MO intra-agregados, em comparação à peneira de 9,5 mm. Amostras (0-5 cm) de cinco solos foram coletadas em experimentos de longa duração (9-18 anos) nas regiões Centro-Oeste (quatro solos) e Sul do Brasil (um solo), sob sistemas de manejo convencional e conservacionista. Tomando as amostras < 9,5 mm como referência, verificou-se subestimação de 2,9 a 15,7 % no C-FLO quando utilizadas amostras < 2,0 mm, a qual foi mais expressiva nos sistemas conservacionistas de manejo de solo e positivamente correlacionada com os teores de C no solo (r = 0,92, p < 0,04), massa de macroagregados > 2 mm (r = 0,90, p < 0,02) e índice de estabilidade de agregados (r = 0,94, p < 0,01). Em três dos cinco solos estudados, o uso de amostras < 2,0 mm levou à conclusão errônea de que os sistemas conservacionistas de manejo do solo não são efetivos na proteção física da MO do solo. Em estudos da proteção física da MO intra-agregados indica-se o uso da peneira de malha de 9,5 mm no preparo da amostra ou, alternativamente, da mesma malha da peneira empregada na preparação de amostras para avaliação da estabilidade de agregados, usualmente 8 mm ou maior.

Effects of residue location on soil organic matter turnover: results from an incubation experiment with 15N‐maize

AbstractDifferences in the mechanisms of storage and decomposition of organic matter (OM) between minimum tillage (MT) and conventional tillage (CT) are generally attributed to differences in the physical impact through tillage, but less is known about the effects of residue location. We conducted an incubation experiment at a water content of 60% of the maximum water‐holding capacity and 15°C with soils from CT (0–25 cm tillage depth) and MT fields (0–5 cm tillage depth) with 15N‐labeled maize straw incorporated to different depths (CT simulations: 0–15 cm; MT simulations: 0–5 cm) for 28 d in order to determine the effects of the tillage simulation on (1) mineralization of recently added residues, (2) the dynamics of macroaggregate formation and physical protection of OM, and (3) the partitioning of maize‐derived C and N within soil OM fractions. The MT simulations showed lower relative C losses, and the amount of maize‐C mineralized after 28 d of incubation was slightly but significantly lower in the MT simulations with maize added (MTmaize) than in the respective CT (CTmaize) simulations. The formation of new water‐stable macroaggregates occurred during the phase of the highest microbial activity, with a maximum peak 8 d after the start of incubation. The newly formed macroaggregates were an important location for the short‐term stabilization of C and N with a higher importance for MTmaize than for CTmaize simulations. In conclusion, our results suggest that a higher amount of OM in MT surface soils compared with CT surface soils may not only result from decreased macroaggregate destruction under reduced tillage but also from a higher efficiency of C retention due to a more concentrated residue input.

No-till soil management increases microbial biomass and alters community profiles in soil aggregates

Aggregation is important for soil functioning, providing physical protection of organic matter and microbial inhabitants. Tillage disrupts aggregates, increases wind and water erosion of soils and exposes formerly protected organic matter to decomposition and losses. Microbial biomass and community dynamics in dry-sieved aggregate-size classes from long-term no-till (NT) and conventionally tilled (CT) soils were examined using phospholipid fatty acid analysis (PLFA). Bacterial, fungal, and total biomass were up to 32% greater in NT compared to CT aggregates. Aggregate size also affected microbial biomass, which was highest in the 1–2 mm size class. Arbuscular mycorrhizal fungi (AMF) were particularly affected by tillage disturbance with increases of 40–60% among aggregate-size classes in NT vs. CT, but glomalin related soil protein concentration was not different between tillage treatments or among aggregate-size classes. Bacterial stress biomarkers were higher in CT than NT aggregates but were not significantly correlated with total C, total N or C:N ratio, indicating that the physiological status of bacteria within aggregates was not simply governed by the quantity of available resources. Ordination analysis of PLFA profiles demonstrated a shift in microbial community structure between NT and CT aggregates, correlated with AMF abundance in NT aggregates and increased bacterial stress biomarkers in CT aggregates. Our results demonstrated greater microbial biomass and altered microbial community structure in NT vs. CT aggregates. This work demonstrates that tillage management influences microbial community structure within aggregates and may provide a potential explanation for differences in process rates observed in NT vs. CT soils. Further research into the processes that govern community structure in aggregates from NT and tilled soils is needed to better understand how the interaction of microorganisms with their physical environment affects nutrient turnover and availability.

Sistemas de manejo e a matéria orgânica de solo de várzea com cultivo de arroz

Solos de várzea representam um importante depósito de C atmosférico, cujo efeito de práticas agrícolas na dinâmica da matéria orgânica (MO) necessita ser mais bem entendido. Este estudo foi realizado em experimento de longa duração (11 anos), localizado na Estação Experimental do Arroz de Cachoeirinha (RS), e teve como objetivo avaliar o efeito do sistema plantio direto (SPD) nos estoques de C orgânico, na labilidade da MO e na proteção física da MO em agregados de um Gleissolo Háplico (200 g kg-1 de argila) cultivado com arroz (Oriza sativa L.) irrigado por inundação, em comparação ao sistema de preparo convencional (SPC). O estoque de C orgânico da camada de 0-20 cm não foi afetado pelos sistemas de manejo de solo (38,39 Mg ha-1 no solo em SPC e 37,36 Mg ha-1 em SPD), o que indica que as taxas de decomposição da MO nesse ambiente anaeróbio não foram influenciadas pelo revolvimento do solo. A labilidade da MO nesse solo de várzea, avaliada pela razão C orgânico particulado (COP)/C orgânico associado aos minerais, foi bem superior à labilidade normalmente verificada em solos aerados e, na média da camada de 0-20 cm, não foi alterada pelos sistemas de manejo de solo (0,24 em SPC e 0,28 em SPD). O aumento da fração leve-oclusa (FLO) foi apenas uma pequena proporção (9 %) do aumento do teor de C orgânico total no SPD, em comparação ao SPC, na camada superficial (0-5 cm), indicando que a proteção física da MO no interior de agregados foi pouco expressiva na estabilização da MO neste ambiente de drenagem restrita. Isso se deve, provavelmente, à efêmera agregação e, ou, à facilidade de acesso dos microrganismos e de suas enzimas à decomposição da MO intra-agregados nesse solo devido à saturação por água.

Soil carbon sequestration in a pine forest after 9 years of atmospheric CO2 enrichment

AbstractThe impact of anthropogenic CO2 emissions on climate change may be mitigated in part by C sequestration in terrestrial ecosystems as rising atmospheric CO2 concentrations stimulate primary productivity and ecosystem C storage. Carbon will be sequestered in forest soils if organic matter inputs to soil profiles increase without a matching increase in decomposition or leaching losses from the soil profile, or if the rate of decomposition decreases because of increased production of resistant humic substances or greater physical protection of organic matter in soil aggregates. To examine the response of a forest ecosystem to elevated atmospheric CO2 concentrations, the Duke Forest Free‐Air CO2 Enrichment (FACE) experiment in North Carolina, USA, has maintained atmospheric CO2 concentrations 200 μL L−1 above ambient in an aggrading loblolly pine (Pinus taeda) plantation over a 9‐year period (1996–2005). During the first 6 years of the experiment, forest‐floor C and N pools increased linearly under both elevated and ambient CO2 conditions, with significantly greater accumulations under the elevated CO2 treatment. Between the sixth and ninth year, forest‐floor organic matter accumulation stabilized and C and N pools appeared to reach their respective steady states. An additional C sink of ∼30 g C m−2 yr−1 was sequestered in the forest floor of the elevated CO2 treatment plots relative to the control plots maintained at ambient CO2 owing to increased litterfall and root turnover during the first 9 years of the study. Because we did not detect any significant elevated CO2 effects on the rate of decomposition or on the chemical composition of forest‐floor organic matter, this additional C sink was likely related to enhanced litterfall C inputs. We also failed to detect any statistically significant treatment effects on the C and N pools of surface and deep mineral soil horizons. However, a significant widening of the C : N ratio of soil organic matter (SOM) in the upper mineral soil under both elevated and ambient CO2 suggests that N is being transferred from soil to plants in this aggrading forest. A significant treatment × time interaction indicates that N is being transferred at a higher rate under elevated CO2 (P=0.037), suggesting that enhanced rates of SOM decomposition are increasing mineralization and uptake to provide the extra N required to support the observed increase in primary productivity under elevated CO2.

Indications for Soil Carbon Saturation in a Temperate Agroecosystem

The soil C saturation concept postulates that there is an upper limit to the equilibrium soil C level of mineral soils even when soil C input is increased. To test this concept, we analyzed the relationship between steady‐state soil C input and soil organic C (SOC) concentration in a temperate corn (Zea mays L.) agroecosystem experiment located in Lexington, KY. In this experiment, a gradient in plant productivity and consequent C input to the soil was produced with four N application rates (0, 84, 168, and 336 kg N ha−1 yr−1) under two disturbance regimes, no‐till (NT) and moldboard plowing (MP). We examined whether physical protection of organic matter by soil aggregation and chemical protection by association with silt and clay particles led to SOC saturation. We used wet sieving to physically separate SOC pools that differ in C stabilization potential, and determined the C sequestration in each fraction. Total SOC was positively related to C input, and this was primarily due to C stabilization in small macroaggregates. In both tillage systems, however, microaggregate C and silt‐plus‐clay C did not increase with greater C input. Within the small macroaggregates, coarse particulate organic matter C and microaggregate C increased with C input, but there was no increase in silt‐plus‐clay C. Our results indicate that soil fractions with low C stabilization potential exhibit C saturation behavior. Apparent C saturation of some of the fractions indicates that SOC pools have a limited capacity to stabilize added C and that such a limit to C stabilization will constrain the ecosystem services provided by these SOC pools.

Carbon and nitrogen stocks in relation to organic matter fractions, aggregation and pore size distribution in no‐tillage and conventional tillage in northern France

SummaryThe magnitude of and mechanisms for long‐term differences in soil organic matter stocks under no‐tillage and conventional tillage are still relatively poorly known. We quantified differences in total C and N stocks after 32 years of no‐tillage (NT) and conventional tillage (CT) in plots with a long‐term cultivation history before differentiation and the same annual C and N returns to the soil. The role of physical protection of organic matter (OM) in these stock differences was further investigated by examining the changes at different levels of structural complexity, i.e. organic matter fractions, aggregation and pore‐size distribution. Four structural zones were sampled: loose and dense soil zones under CT and the 0–5 cm (rich in OM) and 5–20 cm (massive structure) soil layers under NT. The C and N stocks, calculated for an equivalent mass of dry soil, were only 10–15% larger under NT than under CT. Mineral‐associated N and particulate organic matter accounted for about 50% of the difference in N stocks. However, 66% of the total difference in C stocks was due to differences in the particulate organic matter (58%) and free residues (8%) fractions. The additional C and N under NT were almost exclusively situated in aggregates larger than 250 μm in diameter. Our results suggest that physical protection of OM under NT contributes significantly to the differences in C and N stocks between NT and CT by (i) enhanced macroaggregate formation in the 0–5 cm layer due to greater microbial activity and OM content and (ii) a better protection of soil organic matter in the 5–20 cm layer due to the presence of small pores and lack of soil disruption by tillage or climate.

Influence of organic-mineral aggregates on microbial degradation of the dinoflagellate Scrippsiella trochoidea

Aggregation of particulate organic matter (POM) and mineral grains may result in physical protection of organic matter (OM). To test this, phytoplankton cells of the dinoflagellate Scrippsiella trochoidea were inoculated with a natural bacterial assemblage and incubated with or without the clay montmorillonite. Within 5 h, aggregation of phytoplankton OM and clay resulted in transfer of the majority (∼80%) of OM into the >1.6 g cm −3 density fraction. Degradation of particulate organic carbon (POC), particulate nitrogen (PN), dissolved organic carbon (DOC), and dissolved and particulate total hydrolyzable amino acids (THAA), were modeled with a multi-G approach. Quantity of resistant OM was between two and four times larger during clay incubation relative to clay-free incubation. The two incubations did not exhibit significant differences in degradation state of particulate amino acids nor were there indications of preferential sorption of basic amino acids. The results suggest that a considerable fraction of phytoplankton OM can become resistant, at least on a timescale of weeks, mostly due to aggregation of POM and clay mineral grains.

Organo‐mineral associations in sandy acid forest soils: importance of specific surface area, iron oxides and micropores

SummaryOrgano‐mineral associations stabilize soil organic matter, though the mechanisms by which they do so are unclear. We used particle‐size fractions < 6.3 μm of two soils to examine the importance of Fe oxides, short‐range order Al silicates and the surface areas of minerals and micropores on the formation of organo‐mineral associations. In the subsoil Fe oxides were most strongly statistically correlated with the mineral‐bound organic carbon. We therefore assume that they are the most important substrates for the formation of organo‐mineral associations. There is no indication that this is caused by physical protection of organic matter in their micropores (< 2 nm). In the Haplic Podzol, dithionite–citrate–bicarbonate‐soluble short‐range order Al silicates may also play a role. Fe oxide particles were calculated to offer specific surface areas of ∼ 200 m2 g−1 (goethite) and ∼ 800 m2 g−1 (ferrihydrite), corresponding to crystal diameters of only a few nm. We assume that the resulting large amount of oxide‐specific reactive surface sites (conditionally charged hydroxyl groups) is responsible for their dominant role as sorbents. With maximum C loadings of 1.3 mg C per m2 Fe oxide for the Dystric Cambisol and 1.1 mg C per m2 Fe oxide + short‐range order Al silicates for the Haplic Podzol the subsoils of both soils seem to have reached saturation with respect to organic matter sorption. In contrast to subsoil horizons, organo‐mineral associations from topsoils contain much larger amounts of organic matter. Here a larger C loading on Fe oxides or a greater importance of other sorbents in addition to the oxides must be assumed.

A quantification of short-term macroaggregate dynamics: influences of wheat residue input and texture

Soil structure and soil aggregation play an important role in an array of processes such as soil erodibility, organic matter protection and soil fertility. Modeling attempts of these processes would benefit substantially from including soil structural parameters such as soil aggregation. However, quantitative data on soil structural dynamics is lacking. Therefore, we conducted short-term (3 weeks) incubations to acquire necessary soil structural parameters for modeling purposes. Prior to incubation, all structures >53 μm were destroyed from three soils with varying texture but under similar management. Five different amounts of wheat residue, ranging from 0 to 3 wt%, were added to each of these soils. After 3 weeks, samples were analyzed for large water-stable macroaggregates (>2000 μm) using a wet sieving method and for fungal growth using epifluorescence microscopy. Aggregate formation increased linearly with increasing amounts of residue at a rate of 12.0±1.24 g aggregates g −1 residue added. We found no differences in aggregate formation among the three soils, even though the equilibrium level of macroaggregates differed in the field. While amounts of water-stable macroaggregates in the sandy loam and the silt loam soil corresponded well with fungal lengths, this was not the case for the silty clay loam soil. This suggests that fungi are less important in aggregate formation in more clayey soils. Cumulative respiration correlated well ( r=0.89–0.91) with water-stable macroaggregates for all three soils. A model assuming an aggregate formation rate proportional to the respiration rate was very successful in fitting the measured aggregate amounts. This model predicted about 65% of the changes in aggregation when different amounts were added, and about 85% of the changes in aggregation over time. This model yielded a macroaggregate turnover time of 40–60 days. The quantitative results presented here can directly be incorporated into models describing and predicting soil aggregate dynamics, as a determining factor for physical protection of organic matter within a soil.

Physical protection of mineralizable C in aggregates from long-term pasture and arable soil

Depending on agricultural management, soil aggregation can provide physical protection of organic matter against rapid decomposition. Within a given soil series, farm management affects the quality and quantity of organic inputs, soil disturbance and biological activity, and thereby the processes of aggregate formation ( biogenic vs. physicogenic). We determined the physical protection of readily mineralizable organic matter against mineralization in undisturbed aggregates from a conventional arable field and a permanent pasture (>70 years). Soil samples from the two fields were incubated at constant temperature and moisture content, corresponding to field capacity. The increase in CO 2 evolution due to crushing (<250 μm) of the aggregates was used to estimate the macroaggregate-protected C fraction. The fraction of C protected at the microstructural level was estimated from the increase in CO 2 evolution after ball-mill grinding. In addition, aggregate size distribution and bulk density and porosity of undisturbed soil and macroaggregates were determined. Unprotected C fractions were not significantly different between the management systems and ranged from 1.9% to 2.4% of total organic C. In the arable soil, 1.4% of total soil organic C was physically protected in macroaggregates. Crushing of macroaggregates did not significantly increase C mineralization in the pasture soil. The results indicate that mineralization was considerably suppressed in the dominantly large and dense physicogenic macroaggregates from the arable field, but not in the dominantly porous, biogenic macroaggregates of the pasture soil. However, the protection in macroaggregates from the arable soil is not likely to be effective on the long-term because of the low water stability and the disrupting forces of cultivation under field conditions. A relatively high additional C mineralization from ground compared to crushed soil material, especially in the upper layer of the pasture soil, suggests a more important C protection at the microstructural level. Higher C protection in microaggregates from the pasture soil was supported by a previous micromorphological study of soil microstructures in thin sections of the considered management systems.

Soil structure and characteristics of organic matter in two orchards differing in earthworm activity

By consuming plant remains and soil, earthworms incorporate organic matter (OM) into the soil and form biogenic soil structures, which can affect OM dynamics. We carried out a (micro)morphological study of soil structure development and OM distribution in two orchards (45 year) in a Dutch calcareous marine loam: RI − without, and KR + with high earthworm activity, the result of different levels of heavy metal contamination from fungicides. In both soils, sedimentary–stratification was absent to 60 cm depth and equal amounts of biogenic calcite spheroids were counted, suggesting similar earthworm activity in the past. In RI − the current vol.% of worm-worked groundmass in thin sections was 6% in the Ah and 7% in the Bw horizon compared with 51% in the Ah and 16% in the Bw horizon of KR +. Disappearance of earthworms with time in RI − gave rise to a compacted physicogenic soil structure with angular and prismatic aggregates and the absence of earthworm biopores. Due to restricted fragmentation and incorporation of OM fragments in casts, a litter layer formed at the soil surface. OM coatings were present in fissures and root pores of the Ah horizon, indicating the absence of mixing of organic and mineral soil materials. OM fragments were relatively coarse (>50 μm) and heterogeneously distributed through the Ah groundmass. Stronger decalcification in RI − than in KR + could be ascribed to higher production of organic acids in the litter layer of RI − and the absence of soil homogenisation by earthworms. In KR + earthworm activity was high, which has resulted in a biogenic structure with granular and subangular blocky aggregates and many worm casts and biopores. Particulate OM was relatively fine (<50 μm) and encapsulated with clayey material in casts and micro-aggregates. The organic C content was not significantly ( P<0.05) higher in the Ah horizon of KR +, than in the Ah horizon of RI − (15.7 and 13.7 g kg −1, respectively). The lower C mineralization rates in KR + below 6 cm depth, however, might be an indication of higher microbial substrate-use efficiency or physical protection of OM against decomposition. The latter explanation would accord with the observed encapsulation of OM in micro-aggregates, and with studies on other management systems that favour biogenic aggregate formation. The quality of the soil macro- and microstructures, degree of soil compaction and decalcification and soil OM dynamics were strongly determined by the occurrence of earthworms in soils.

Preservation of organic matter in mound-forming coral skeletons

This study demonstrates that intracrystalline organic matter in coral skeletons is well preserved over century timescales. The extent of preservation of organic matter in coral skeletons was investigated by measuring total organic carbon (TOC), total hydrolyzable amino acid (THAA), chloropigment, and lipid concentrations in 0–300 year old annual growth bands from Montastraea annularis (Florida Keys) and Porites lutea (Red Sea). Organic matter intrinsic to the calcium carbonate mineral (intracrystalline) was analyzed separately from total skeletal organic matter. The Red Sea coral had less TOC (0.02–0.04 wt%) than the Florida Keys coral (0.04–0.11 wt%), but a higher percent of intracrystalline organic matter in all annual bands measured. Carbon in the form of THAA, most likely from mineral-precipitating proteins, contributed 30–45% of the TOC in both corals. Carbon in lipids represented about 3% of the TOC in the coral skeletons. Chlorophyll-a and b were present in annual bands where endolithic algae were present, but these compounds were minor contributors to TOC. The distribution of specific organic compounds showed that organic matter was well preserved throughout the time period sampled in both the total and intracrystalline pools. Variations in THAA were not correlated with TOC over time, suggesting that organic matter that is involved in biomineralization, like amino acids, may be deposited in response to different environmental factors than are other components of skeletal organic matter. Differences in the quantity and composition of organic matter between the two corals investigated here were assessed using principal components analysis and suggest that location, species and skeletal structure may all influence organic matter content and possibly the degree of physical protection of organic matter by the coral skeleton. Further, our study suggests that intracrystalline organic matter may be better protected from diagenesis than non-intracrystalline organic matter and may therefore be a more reliable source of organic matter for paleoceanographic studies than total skeletal organic matter.