Stabilisation of precipitates of pedogenic dissolved organic matter by multivalent cations

Purpose Precipitation of dissolved organic matter (DOM) by multivalent cations is important for biogeochemical cycling of organic carbon. We investigated to which extent cation bridges are involved in DOM precipitation and how cross-links by cations and water molecule bridges (WaMB) stabilise the matrix of precipitated DOM. Materials and methods DOM was precipitated from the aqueous extract of a forest floor layer adding solutions of Ca(NO3)2, Al(NO3)3 and Pb(NO3)2 with different initial metal cation/C (Me/C) ratios. Precipitates were investigated by differential scanning calorimetry before and after ageing to detect cation bridges, WaMB and restructuring of supramolecular structure. Results and discussion Twenty-five to sixty-seven per cent of the dissolved organic carbon was precipitated. The precipitation efficiency of cations increased in the order Ca���<���Al���<���Pb, while the cation content of precipitates increased in the order Pb���<���Ca���<���Al. The different order and the decrease in the WaMB transition temperature (T*) for Al/C���>���3 is explained by additional formation of small AlOOH particles. Thermal analysis indicated WaMB and their disruption at T* of 53–65 ��C. Like cation content, T* increased with increasing Me/C ratio and in the order Ca���<���Pb���<���Al for low Me/C. This supports the general assumption that cross-linking ability increases in the order Ca���<���Pb���<���Al. The low T* for high initial Me/C suggests less stable and less cross-linked precipitates than for low Me/C ratios. Conclusions Our results suggest a very similar thermal behaviour of OM bound in precipitates compared with soil organic matter and confirms the relevance of WaMB in stabilisation of the supramolecular structure of cation-DOM precipitates. Thus, stabilisation of the supramolecular structure of the DOM precipitates is subjected to dynamics in soils.

Stand scale variability of topsoil organic matter composition in a high-elevation Norway spruce forest ecosystem

Our knowledge about the effect of single-tree influence areas on the physicochemical properties of the underlying mineral soil in forest ecosystems is still limited. This restricts our ability to adequately estimate future changes in soil functioning due to forest management practices. We studied the stand scale spatial variation of different soil organic matter species investigated by 13C NMR spectroscopy, lignin phenol and neutral sugar analysis under an unmanaged mountainous high-elevation Norway spruce (Picea abies L.) forest in central Europe. Multivariate geostatistical approaches were applied to relate the spatial patterns of the different soil organic matter species to topographic parameters, bulk density, oxalate- and dithionite-extractable iron, pH, and the impact of tree distribution. Soil samples were taken from the mineral top soil. Generally, the stand scale distribution patterns of different soil organic matter compounds could be divided into two groups: Those compounds, which were significantly spatially correlated with topography/altitude and those with small scale spatial pattern (range ��� 10 m) that was closely related to tree distribution. The concentration of plant-derived soil organic matter components, such as lignin, at a given sampling point was significantly spatially related to the distance of the nearest tree (p ��� 0.05). In contrast, the spatial distribution of mainly microbial-derived compounds (e.g. galactose and mannose) could be attributed to the dominating impact of small-scale topography and the contribution of poorly crystalline iron oxides that were significantly larger in the central depression of the study site compared to crest and slope positions. Our results demonstrate that topographic parameters dominate the distribution of overall topsoil organic carbon (OC) stocks at temperate high-elevation forest ecosystems, particularly in sloped terrain. However, trees superimpose topography-controlled OC biogeochemistry beneath their crown by releasing litter and changing soil conditions in comparison to open areas. This may lead to distinct zones with different mechanisms of soil organic matter degradation and also stabilization in forest stands.

Pedogeomorphology, geochronology and paleobotany of soil profiles obtained in the Nagqu area, central Tibet

Vast areas on the Tibetan Plateau are covered by alpine sedge mats consisting of different species of the genus Kobresia. These mats have topsoil horizons rich in rhizogenic organic matter which creates turfs. As the turfs have recently been affected by a complex destruction process, knowledge concerning their soil properties, age and pedogenesis are needed. In the core area of Kobresia pygmaea mats around Nagqu (central Tibetan Plateau, ca. 4500 m a.s.l.), four profiles were subjected to pedological, paleobotanical and geochronological analyses concentrating on soil properties, phytogenic composition and dating of the turf. The turf of both dry K. pygmaea sites and wet Kobresia schoenoides sites is characterised by an enrichment of living (dominant portion) and dead root biomass. In terms of humus forms, K. pygmaea turfs can be classified as Rhizomulls mainly developed from Cambisols. Wet-site K. schoenoides turfs, however, can be classified as Rhizo-Hydromors developed from Histic Gleysols. At the dry sites studied, the turnover of soil organic matter is controlled by a non-permafrost cold thermal regime. Below-ground remains from sedges are the most frequent macroremains in the turf. Only a few pollen types of vascular plants occur, predominantly originating from sedges and grasses. Large amounts of microscopic charcoal (indeterminate) are present. Macroremains and pollen extracted from the turfs predominantly have negative AMS 14C ages, giving evidence of a modern turf genesis. Bulk-soil datings from the lowermost part of the turfs have a Late Holocene age comprising the last ca. 2000 years. The development of K. pygmaea turfs was most probably caused by an anthropo(zoo)-genetically initiated growth of sedge mats replacing former grass-dominated vegetation ('steppe'). Thus the turfs result from the transformation of pre-existing topsoils comprising a secondary penetration and accumulation of roots. K. schoenoides turfs, however, are characterised by a combined process of peat formation and penetration/accumulation of roots probably representing a (quasi) natural wetland vegetation.

Bioavailability and extraction of heavy metals from contaminated soil by Atriplex halimus

Pot experiments were performed to evaluate the phytoremediation capacity of plants of Atriplex halimus grown in contaminated mine soils and to investigate the effects of organic amendments on the metal bioavailability and uptake of these metals by plants. Soil samples collected from abandoned mine sites north of Madrid (Spain) were mixed with 0, 30 and 60 Mg ha?1 of two organic amendments, with different pH and nutrients content: pine-bark compost and horse- and sheep-manure compost. The increasing soil organic matter content and pH by the application of manure amendment reduced metal bioavailability in soil stabilising them. The proportion of Cu in the most bioavailable fractions (sum of the water-soluble, exchangeable, acid-soluble and Fe?Mn oxides fractions) decreased with the addition of 60 Mg ha?1 of manure from 62% to 52% in one of the soils studied and from 50% to 30% in the other. This amendment also reduced Zn proportion in water-soluble and exchangeable fractions from 17% to 13% in one of the soils. Manure decreased metal concentrations in shoots of A. halimus, from 97 to 35 mg kg?1 of Cu, from 211 to 98 mg kg?1 of Zn and from 1.4 to 0.6 mg kg?1 of Cd. In these treatments there was a higher plant growth due to the lower metal toxicity and the improvement of nutrients content in soil. This higher growth resulted in a higher total metal accumulation in plant biomass and therefore in a greater amount of metals removed from soil, so manure could be useful for phytoextraction purposes. This amendment increased metal accumulation in shoots from 37 to 138 mg pot?1 of Cu, from 299 to 445 mg pot?1 of Zn and from 1.8 to 3.7 mg pot?1 of Cd. Pine bark amendment did not significantly alter metal availability and its uptake by plants. Plants of A. halimus managed to reduce total Zn concentration in one of the soils from 146 to 130 mg kg?1, but its phytoextraction capacity was insufficient to remediate contaminated soils in the short-to-medium term. However, A. halimus could be, in combination with manure amendment, appropriate for the phytostabilization of metals in mine soils.

Initializing the DSSAT-Century model: inverse calibration of carbon pools from apparent soil N mineralization

The CENTURY soil organic matter model was adapted for the DSSAT (Decision Support System for Agrotechnology Transfer), modular format in order to better simulate the dynamics of soil organic nutrient processes (Gijsman et al., 2002). The CENTURY model divides the soil organic carbon (SOC) into three hypothetical pools: microbial or active material (SOC1), intermediate (SOC2) and the largely inert and stable material (SOC3) (Jones et al., 2003). At the beginning of the simulation, CENTURY model needs a value of SOC3 per soil layer which can be estimated by the model (based on soil texture and management history) or given as an input. Then, the model assigns about 5% and 95% of the remaining SOC to SOC1 and SOC2, respectively. The model performance when simulating SOC and nitrogen (N) dynamics strongly depends on the initialization process. The common methods (e.g. Basso et al., 2011) to initialize SOC pools deal mostly with carbon (C) mineralization processes and less with N. Dynamics of SOM, SOC, and soil organic N are linked in the CENTURY-DSSAT model through the C/N ratio of decomposing material that determines either mineralization or immobilization of N (Gijsman et al., 2002). The aim of this study was to evaluate an alternative method to initialize the SOC pools in the DSSAT-CENTURY model from apparent soil N mineralization (Napmin) field measurements by using automatic inverse calibration (simulated annealing). The results were compared with the ones obtained by the iterative initialization procedure developed by Basso et al., 2011.

Amelioration of a degraded acid soil from SW Spain by no-tillage and Ca-amendment

En los suelos, el exceso de acidez lleva asociado deficiencias en ciertos nutrientes y una alta disponibilidad de aluminio, t��xico para los cultivos propios del ambiente mediterr��neo. Su laboreo, provoca la p��rdida de materia org��nica (MO), deteriora su estructura y reduce la actividad biol��gica, provocando en ��ltima instancia una menor calidad del suelo. Es de esperar pues que cuando se labran suelos ��cidos, sus problem��ticas particulares tiendan a agravarse. En nuestra zona de estudio, la ���ra��a��� de Ca��amero (Extremadura, Espa��a), predominan los suelos muy ��cidos y degradados por un laboreo inadecuado. Las ra��as constituyen amplias plataformas casi horizontales, con unos suelos muy viejos (Palexerults), que se caracterizan por tener el complejo de cambio dominado por el aluminio, y un pH ��cido que decrece en profundidad. Poseen un potente horizonte Bt rico en arcillas caolin��ticas, que propicia que en periodos con exceso de lluvia, se generen capas colgadas de agua cercanas a la superficie. En torno a los a��os 1940���s estos suelos, que previamente sosten��an un alcornocal, o su matorral de sustituci��n, se pusieron en cultivo. El laboreo aceler�� la mineralizaci��n de la materia org��nica, agrav�� los problemas derivados del exceso de acidez y condujo al abandono de los campos cultivados por falta de productividad. Para recuperar la calidad de estos suelos degradados y obtener unos rendimientos compatibles con su uso agr��cola es necesario, por un lado, aplicar enmiendas que eleven el pH y reduzcan la toxicidad del aluminio y, por otro, favorecer el incremento en el contenido en MO. En 2005 se implant�� en esta ra��a un ensayo de campo para estudiar la influencia del no laboreo y de la utilizaci��n de una enmienda c��lcica en par��metros relacionados con la calidad del suelo en un cultivo forrajero. El dise��o experimental fue en parcelas divididas con cuatro repeticiones donde el factor principal fue el tipo de laboreo, no laboreo (NL) frente a laboreo convencional (LC), y el factor secundario el uso o no de una enmienda c��lcica. La enmienda consisti�� b��sicamente en una mezcla de espuma de azucarer��a y yeso rojo y se incorpor�� al comienzo del ensayo hasta los 7 cm de profundidad. Desde el comienzo del ensayo el NL influy�� positivamente en el contenido de carbono org��nico total (COT) y particulado (COP), mientras que la enmienda tuvo una ligera influencia al principio del ensayo en ambos pero su efecto positivo se desvaneci�� con el paso del tiempo. Los mayores contenidos en COT y POC se observaron cuando se combin�� el NL con la enmienda. La enmienda increment�� con rapidez el pH, y el Ca, y disminuy�� el contenido en aluminio hasta una profundidad de 50 cm, incluso en NL, y mejor�� ligeramente la agregaci��n del suelo. El NL por s�� solo, gracias al aumento en POC, TOC y las prote��nas del suelo relacionadas con la glomalina (PSRG), que son capaces de formar compuestos estables no t��xicos con el aluminio, tambi��n contribuy�� a la reducci��n de la toxicidad de aluminio en la capa m��s superficial. Cuando en las campa��as con exceso de precipitaciones se generaron capas colgadas de agua pr��ximas a la superficie, el NL gener�� unas condiciones m��s favorables para la germinaci��n y desarrollo del cultivo, resultando en una producci��n m��s alta que el LC. A ello contribuy�� la mayor capacidad de almacenamiento de agua y la mayor transmisividad de esta hacia abajo, en la capa m��s superficial (0-5 cm) que propici�� una menor saturaci��n por agua que el LC. Respecto a los par��metros relacionados con la agregaci��n, el NL aument�� los macroagregados hasta los 10 cm de profundidad y favoreci�� la acumulaci��n de CO y N en todas las fracciones de tama��o de agregados. Sin embargo, la recuperaci��n del grado de macroagregaci��n tras el cese del laboreo resulta lenta en comparaci��n con otros suelos, posiblemente debido al bajo contenido en arcilla en el horizonte Ap. En comparaci��n con el NL, la enmienda mostr�� tambi��n un efecto positivo, aunque muy ligero, en la agregaci��n del suelo. En contradicci��n con otros estudios en suelos ��cidos, nuestros resultados indican la existencia de una jerarqu��a de agregados, y destacan el papel importante de la MO en la mejora de la agregaci��n. Tanto el NL como la enmienda favorecieron por separado varias propiedades qu��micas, f��sicas y biol��gicas del suelo, pero, en general, encontramos los mayores beneficios con su uso combinado. Adem��s, a largo plazo el efecto positivo de NL en las propiedades del suelo fue en aumento, mientras que el efecto beneficioso de la enmienda se limit�� b��sicamente a las propiedades qu��micas y se desvaneci�� en pocos a��os. Destacamos que las condiciones meteorol��gicas a lo largo del ensayo beneficiaron la producci��n de biomasa en NL, y en consecuencia las propiedades relacionadas con la materia org��nica, por lo que son un factor a tener en cuenta a la hora de evaluar los efectos de la enmienda y el laboreo sobre las propiedades del suelo, especialmente en zonas donde esas condiciones son muy variables entre una campa��a y otra. Los resultados de este estudio han puesto de manifiesto que el NL no ha mermado la eficacia de la enmienda caliza, posiblemente gracias a la alta solubilidad de la enmienda aplicada, es m��s, el manejo con NL y enmienda es el que ha favorecido en mayor medida ciertos par��metros de calidad del suelo. Por el contrario el LC s�� parece anular los beneficios de la enmienda en relaci��n con las propiedades relacionadas con la MO. Por tanto, cabe concluir que la combinaci��n de NL y la enmienda es una pr��ctica adecuada para mejorar las propiedades qu��micas y f��sicas de suelos ��cidos degradados por el laboreo. ABSTRACT Excessive acidity in soils is associated with deficiencies in certain nutrients and high concentrations of available aluminum, which is toxic for most Mediterranean crops. Tilling these soils results in the loss of soil organic matter (SOM), damages soil structure and reduces biological activity, ultimately degrading soil quality. It is expected, therefore, that when acid soils are tilled, their particular problems will tend to get worse. In our study area, the "Ca��amero���s Ra��a��� (Extremadura, Spain), acid soils degraded by an inappropriate tillage prevail. Ra��as are large and flat platforms with very old soils (Palexerults), which are characterized by an exchange complex dominated by aluminum and an acid pH which decreases with depth. These soils have a strong Bt horizon rich in kaolinite clays, which encourages the formation of perched water-tables near the soil surface during periods of excessive rain. During the first third of the 20th century, these soils, that previously supported cork oak or its scrub replacement, were cultivated. Tillage accelerated the mineralization of the SOM, aggravating the problems of excessive acidity, which finally led to the abandonment of the land due to low productivity. To recover the quality of these degraded soils and to obtain consistent yields it is necessary, first, to apply amendments to raise the pH and reduce aluminum toxicity, and second to encourage the accumulation of SOM. In 2005 a field trial was established in the Ra��a to study the influence of no-tillage and the use of a Ca-amendment on soil quality related parameters in a forage crop agrosystem. The experimental design was a split-plot with four replicates where the main factor was tillage type, no-tillage (NT) versus traditional tillage (TT) and the secondary factor was the use or not of a Ca-amendment. The Ca-amendment was a mixture of sugar foam and red gypsum that was incorporated into the top 7 cm of the soil. Since the beginning of the experiment, NT had a positive influence on total and particulate organic carbon (TOC and POC, respectively), while the Ca-amendment had a small positive influence at the beginning of the study but its effect diminished with time. The highest TOC and POC contents were observed when NT and the Ca-amendment were combined. The Ca-amendment, even under NT, rapidly increased pH and Ca, and decreased the aluminum content to a depth of 50 cm, as well as improving soil aggregation slightly. NT, due to the increased POC, TOC and Glomalin-related soil proteins (GRSP), which can form stable non-toxic compounds with aluminum, also contributed to the reduction of aluminum toxicity in the upper layer. When perched water-tables near the soil surface were formed in campaigns with excessive rainfall, NT provided more favorable conditions for germination and crop development, resulting in higher yields compared with TT. This was directly related to the higher water storage capacity and the greater transmissivity of the water downwards from the upper layers, which led to lower water saturation under NT compared with TT. With regards to the aggregation-related parameters, NT increased macroaggregation to a depth of 10 cm and favored the accumulation of OC and N in all aggregate size fractions. However, the degree of recovery of macroaggregation after tillage ceased was slow compared with other soils, possibly due to the low clay content in the Ap horizon. Compared with NT, the Ca-amendment had a slight positive effect on soil aggregation. In contrast to other studies in acid soils, our results indicate the existence of an aggregate hierarchy, and highlight the important role of SOM in improving aggregation. Both NT and the Ca-amendment separately favored various chemical, physical and biological soil properties, but in general we found the greatest benefits when the two treatments were combined. In addition, the positive effect of NT on soil properties increased with time, while the beneficial effect of the Ca-amendment, which was limited to the chemical properties, vanished after a few years. It is important to note that the meteorological conditions throughout the experiment benefited biomass production under NT and, as a consequence, organic matter related properties. This suggests that meteorological conditions are a factor to consider when evaluating the effects of Ca-amendments and tillage on soil properties, especially in areas where such conditions vary significantly from one campaign to another. The results of this study show that NT did not diminish the effectiveness of the Ca-amendment, possibly due to the high solubility of the selected amendment. Moreover, the combination of NT and the Ca-amendment was actually the management that favored certain soil quality parameters the most. By contrast, TT seemed to nullify the benefits of the Ca-amendment with regards to the OM related properties. In conclusion, the combination of NT and the application of a Ca-amendment is an advisable practice for improving the chemical and physical properties of acid soils degraded by tillage.

Qu��mica da mat��ria org��nica e pedog��nese em Latossolos h��micos sob vegeta����o de cerrado

A mat��ria org��nica do solo (MOS) representa um importante reservat��rio de carbono (C) nos ecossistemas terrestres. O conte��do de C estocado no solo pode ser liberado para a atmosfera na forma de CO2, com a decomposi����o da MOS, ou pode ser aumentado com a entrada de res��duos e reten����o da MOS. Nesse sentido, �� importante entender os mecanismos de estabilidade e reten����o da MOS para predizer como os solos respondem a mudan��as, quer sejam elas induzidas por altera����es clim��ticas ou por pr��ticas de manejo. Dentro dos Latossolos, classe que ocupa cerca de 32 % do territ��rio brasileiro, h�� aqueles que possuem horizonte A h��mico hiper espesso e, portanto, com maior estoque de C. Aspectos sobre a origem, forma����o e preserva����o do horizonte A h��mico destes solos em suas ocorr��ncias em diferentes biomas ainda n��o foram completamente elucidados e est��o estritamente ligados �� fonte, din��mica e mecanismos de preserva����o e distribui����o da MOS no solo. O objetivo deste trabalho �� entender a g��nese da MO dos Latossolos h��micos que ocorrem no Bioma Cerrado, por meio da caracteriza����o molecular pela t��cnica da pir��lise acoplada �� cromatografia gasosa e espectroscopia de massas (pir��lise - CG/EM). Para isso, foram coletadas amostras dos horizontes A em dois perfis de Latossolos com horizonte A h��mico (LH1, LH2) e um perfil de Latossolo com horizonte A moderado (solo de refer��ncia; LNH) situados em superf��cie de aplanamento adjacente �� Serra do Espinha��o, no munic��pio de Gr��o Mogol - MG, sob clima tropical semi-��mido e vegeta����o de cerrado sensu strictu. Por meio da descri����o morfol��gica dos solos em diferentes n��veis de observa����o (campo, lupa e microsc��pio) procurou-se entender melhor os mecanismos de espessamento do horizonte A e a distribui����o de part��culas de carv��o ao longo do perfil. As amostras dos horizontes foram submetidas ao fracionamento f��sico e extra����o da MOS, gerando as seguintes fra����es: fra����o leve livre (FLL); fra����o leve oclusa (FLO), fra����o extra��vel com NaOH (EXT) e res��duo (RES). A morfologia dos perfis evidencia a intensa e longa atividade biol��gica (fauna e ra��zes) a que esses solos foram e est��o submetidos. Isso explica a abund��ncia de microagregados e a consequente macropososidade elevada, assim como a ampla distribui����o de fragmentos de carv��o em todo o horizonte A, e parte do B, com dimens��es milim��tricas a submilim��tricas, sugerindo a fragmenta����o destes ao longo do tempo. Foi evidenciado o maior conte��do de carv��es nos dois LHs em compara����o ao LNH. A distribui����o da MOS nas fra����es estudadas foi a mesma para os tr��s perfis estudados: RES>EXT>FLL>FLO, que mostra a import��ncia da fra����o RES para estes solos. Produtos da carboniza����o (Black carbon; BC: hidrocarbonetos poliarom��ticos) foram mais abundantes na fra����o RES e FLO, no entanto, a maior diferen��a qualitativa entre a MOS de LHs e LNH diz respeito �� abund��ncia de BC na fra����o RES, que �� maior em LHs do que LNH; confirmando a maior quantidade de carv��es em LHs verificada na morfologia. Um ��ndice de degrada����o do BC foi estabelecido com base em an��lise fatorial com os todas as fra����es estudadas e produtos poliarom��ticos. Este ��ndice, aplicado ��s fra����es EXT e RES, mostrou que a degrada����o do BC aumenta com a profundidade/idade, e n��o houve diferen��as significativas entre os perfis estudados. Portanto, LHs provavelmente tem maior entrada de carv��es, o que deve estar ligado a um hist��rico de maior incid��ncia de inc��ndios ou maior abund��ncia local de esp��cies arb��reas.

Climate and Terrain Characteristics Linked to Mud Ejection Occurrence in the Canadian High Arctic

Pressurised slurries of fine-grained sediment expelled from the base of the active layer have been observed in recent years in the High Arctic. Such mud ejections, however, are poorly understood in terms of how exactly climate and landscape factors determine when and where they occur. Mud ejections at the Cape Bounty Arctic Watershed Observatory, Melville Island, Nunavut, were systematically mapped in 2012 and 2013, and this was combined with observations of mud ejection activity and climatic measurements carried out since 2003. The mud ejections occur late in the melt season during warm years and closely following major rainfall events. High-resolution satellite imagery demonstrates that mud ejections are associated with polar semi-desert vegetative settings, flat or low-sloping terrain and south-facing slopes. The localised occurrence of mud ejections appears to be related to differential soil moisture retention.

Facilitating soil-and plant-water research through the use of TDR

The development of TDR for measurement of soil water content and electrical conductivity has resulted in a large shift in measurement methods for a breadth of soil and hydrological characterization efforts. TDR has also opened new possibilities for soil and plant research. Five examples show how TDR has enhanced our ability to conduct our soil- and plant-water research. (i) Oxygen is necessary for healthy root growth and plant development but quantitative evaluation of the factors controlling oxygen supply in soil depends on knowledge of the soil water content by TDR. With water content information we have modeled successfully some impact of tillage methods on oxygen supply to roots and their growth response. (ii) For field assessment of soil mechanical properties influencing crop growth, water content capability was added to two portable soil strength measuring devices; (a) A TDT (Time Domain Transmittivity)-equipped soil cone penetrometer was used to evaluate seasonal soil strengthwater content relationships. In conventional tillage systems the relationships are dynamic and achieve the more stable no-tillage relationships only relatively late in each growing season; (b) A small TDR transmission line was added to a modified sheargraph that allowed shear strength and water content to be measured simultaneously on the same sample. In addition, the conventional graphing procedure for data acquisition was converted to datalogging using strain gauges. Data acquisition rate was improved by more than a factor of three with improved data quality. (iii) How do drought tolerant plants maintain leaf water content? Non-destructive measurement of TDR water content using a flat serpentine triple wire transmission line replaces more lengthy procedures of measuring relative water content. Two challenges remain: drought-stressed leaves alter salt content, changing electrical conductivity, and drought induced changes in leaf morphology affect TDR measurements. (iv) Remote radar signals are reflected from within the first 2 cm of soil. Appropriate calibration of radar imaging for soil water content can be achieved by a parallel pair of blades separated by 8 cm, reaching 1.7 cm into soil and forming a 20 cm TDR transmission line. The correlation between apparent relative permittivity from TDR and synthetic aperture radar (SAR) backscatter coefficient was 0.57 from an airborne flyover. These five examples highlight the diversity in the application of TDR in soil and plant research.

Organic Matter Dynamics in the Wet Tropics Under Different Sugarcane Residue Management Regimes

Retention of sugarcane leaves and tops on the soil surface after harvesting has almost completely replaced pre- and post-harvest burning of crop residues in the Australian sugar industry. Since its introduction around 25 years ago, residue retention has increased soil organic matter to improve soil fertility as well as improve harvest flexibility and reduce erosion. However, in the wet tropics residue retention also poses potential problems of prolonged waterlogging, and late-season release of nitrogen which can reduce sugar content of the crop. The objective of this project is to examine the management of sugarcane residues in the wet tropics using a systems approach. Subsidiary objectives are (a) to improve understanding of nitrogen cycling in Australian sugarcane soils in the wet tropics, and (b) to identify ways to manage crop residues to retain their advantages and limit their disadvantages. Project objectives will be addressed using several approaches. Historic farm production data recorded by sugar mills in the wet tropics will be analysed to determine the effect of residue burning or retention on crop yield and sugar content. The impact of climate on soil processes will be highlighed by development of an index of nitrogen mineralisation using the Agricultural Production Systems Simulator (APSIM) model. Increased understanding of nitrogen cycling in Australian sugarcane soils and management of crop residues will be gained through a field experiment recently established in the Australian wet tropics. From this experiment the decomposition and nitrogen dynamics of residues placed on the soil surface and incorporated will be compared. The effect of differences in temperature, soil water content and pH will be further examined on these soils under glasshouse conditions. Preliminary results show a high ammonium to nitrate ratio in tropics soils, which may be due to low rates of nitrification that increase the retention of nitrogen in a form (ammonium) that is less subject to leaching. Further results will be presented at Congress.

Patterns of Soil Bacteria and Canopy Community Structure Related to Tropical Peatland Development

Natural environmental gradients provide important information about the ecological constraints on plant and microbial community structure. In a tropical peatland of Panama, we investigated community structure (forest canopy and soil bacteria) and microbial community function (soil enzyme activities and respiration) along an ecosystem development gradient that coincided with a natural P gradient. Highly structured plant and bacterial communities that correlated with gradients in phosphorus status and soil organic matter content characterized the peatland. A secondary gradient in soil porewater NH4 described significant variance in soil microbial respiration and ��-1-4-glucosidase activity. Covariation of canopy and soil bacteria taxa contributed to a better understanding of ecological classifications for biotic communities with applicability for tropical peatland ecosystems of Central America. Moreover, plants and soils, linked primarily through increasing P deficiency, influenced strong patterning of plant and bacterial community structure related to the development of this tropical peatland ecosystem.

Effect of Water Management on Interannual Variation in Bulk Soil Properties from the Eastern Coastal Everglades

We examined interannual variation in soil properties from wetlands occurring in adjacent drainage basins from the southeastern Everglades. Triplicate 10-cm soil cores were collected, homogenized, and analyzed during the wet season 2006–2010 from five freshwater sawgrass wetland marshes and three estuarine mangrove forests. Soil bulk density from the Taylor Slough basin ranged from 0.15 gm-cm���3 to 0.5 gm-cm���3, was higher than from the Panhandle basin every year, and generally increased throughout the study period. Organic matter as a percent loss on ignition ranged from 7 % to 12 % from freshwater marshes and from 13 % to 56 % from estuarine mangroves. Extractable iron in soils was similar among drainage basins and wetland types, typically ranging from 0.6 to 2.0 g Fe kg���1. In contrast, inorganic sulfur was on average over four times higher from estuarine soils relative to freshwater, and was positively correlated with soil organic matter. Finally total soil phosphorus (P) was lower in freshwater soils relative to estuarine soils (84��������5 versus 326��������32 mg P kg���1). Total P from the freshwater marshes in the Panhandle basin rose throughout the study period from 54.7��������8.4 to 107��������17 mg P kg���1, a possible outcome of differences in water management between drainage basins.

Geochemical data and radiocarbon ages of organic matter fractions and bacteriohopanepolyol data of sediment core GeoB6518-1 from the Congo River basin

The age of organic material discharged by rivers provides information about its sources and carbon cycling processes within watersheds. While elevated ages in fluvially-transported organic matter are usually explained by erosion of soils and sediments, it is commonly assumed that mainly young organic material is discharged from flat tropical watersheds due to their extensive plant cover and high carbon turnover. Here we present compound-specific radiocarbon data of terrigenous organic fractions from a sedimentary archive offshore the Congo River in conjunction with molecular markers for methane-producing land cover reflecting wetland extent in the watershed. We find that the Congo River has been discharging aged organic matter for several thousand years with increasing ages from the mid- to the Late Holocene. This suggests that aged organic matter in modern samples is concealed by radiocarbon from nuclear weapons testing. By comparison to indicators for past rainfall changes we detect a systematic control of organic matter sequestration and release by continental hydrology mediating temporary carbon storage in wetlands. As aridification also leads to exposure and rapid remineralization of large amounts of previously stored labile organic matter we infer that this process may cause a profound direct climate feedback currently underestimated in carbon cycle assessments.

ELUM: a spatial modelling tool to predict soil greenhouse gas changes from land conversion to bioenergy in the UK

Acknowledgements This work is based on the Ecosystem Land Use Modelling & Soil Carbon GHG Flux Trial (ELUM) project, which was commissioned and funded by the Energy Technologies Institute (ETI). The authors are grateful to Niall McNamara (Centre for Ecology & Hydrology, Lancaster) for coordinating the project and to Dagmar Henner (University of Aberdeen) for project assistance. We are also grateful to staff at the ETI, particularly to Geraldine Newton-Cross, Geraint Evans and Hannah Evans for constructive advice and feedback, and to Jonathan Oxley for project support. The ELUM Software Package contains Ordnance Survey data �� Crown copyright and database right 2012.

Radiocarbon dating, carbon storage and soil properties of samples from Tulemalu Lake, central Canadian Arctic

We investigated total storage and landscape partitioning of soil organic carbon (SOC) in continuous permafrost terrain, central Canadian Arctic. The study is based on soil chemical analyses of pedons sampled to 1 m depth at 35 individual sites along three transects. Radiocarbon dating of cryoturbated soil pockets, basal peat and fossil wood shows that cryoturbation processes have been occurring since the Middle Holocene and that peat deposits started to accumulate in a forest-tundra environment where spruce was present (~6000 cal yrs BP). Detailed partitioning of SOC into surface organic horizons, cryoturbated soil pockets and non-cryoturbated mineral soil horizons is calculated (with storage in active layer and permafrost calculated separately) and explored using principal component analysis. The detailed partitioning and mean storage of SOC in the landscape are estimated from transect vegetation inventories and a land cover classification based on a Landsat satellite image. Mean SOC storage in the 0-100 cm depth interval is 33.8 kg C/m**2, of which 11.8 kg C/m**2 is in permafrost. Fifty-six per cent of the total SOC mass is stored in peatlands (mainly bogs), but cryoturbated soil pockets in Turbic Cryosols also contribute significantly (17%). Elemental C/N ratios indicate that this cryoturbated soil organic matter (SOM) decomposes more slowly than SOM in surface O-horizons.