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Mineral Horizons (mineral + horizon)
Selected AbstractsGross rates of ammonification and nitrification at a nitrogen-saturated spruce (Picea abies (L.)Karst.) stand in southern GermanyEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 5 2010P. Rosenkranz We investigated the magnitudes of temporal and spatial variabilities of gross ammonification and nitrification, in an N-saturated temperate forest ecosystem. Forest soil gross ammonification, gross nitrification and heterotrophic soil respiration were measured in the forest floor and uppermost mineral layer over a period of 3 years. Total annual gross fluxes for the organic layer and uppermost mineral horizon (0,4 cm) were in the range of 800,980 kg N ha,1 year,1 for gross ammonification and 480,590 kg N ha,1 year,1 for gross nitrification. Annual heterotrophic soil respiration was 8000,8900 kg C ha,1 year,1. Highest soil C and N turnover rates occurred in summer, and a consistent pattern was observed throughout the observation period, with highest values for plots located at a clear-cut area and lowest values for plots located at an unmanaged, approximately 100-year-old, spruce control site. Soil moisture, soil temperature and substrate availability accounted for most of the observed variability of C and N turnover rates. Because gross rates of inorganic N production were more than an order of magnitude larger than ecosystem N losses along hydrological and gaseous pathways, our study underlines the importance of internal microbial N turnover processes for ecosystem N cycling and retention. [source] Mineral surfaces and soil organic matterEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 2 2003K. Kaiser Summary The organic carbon content of soil is positively related to the specific surface area (SSA), but large amounts of organic matter in soil result in reduced SSA as determined by applying the Brunauer,Emmett,Teller (BET) equation to the adsorption of N2. To elucidate some of the controlling mechanisms of this relation, we determined the SSA and the enthalpy of N2 adsorption of separates with a density > 1.6 g cm,3 from 196 mineral horizons of forest soils before and after removal of organic matter with NaOCl. Likewise, we investigated these characteristics before and after sorption of increasing amounts of organic matter to four mineral soil samples, oxides (amorphous Al(OH)3, gibbsite, ferrihydrite, goethite, haematite), and phyllosilicates (kaolinite, illite). Sorption of organic matter reduced the SSA, depending on the amount sorbed and the type of mineral. The reduction in SSA decreased at larger organic matter loadings. The SSA of the mineral soils was positively related to the content of Fe oxyhydroxides and negatively related to the content of organic C. The strong reduction in SSA at small loadings was due primarily to the decrease in the micropores to which N2 was accessible. This suggests preferential sorption of organic matter at reactive sites in or at the mouths of micropores during the initial sorption and attachment to less reactive sites at increasing loadings. The exponential decrease of the heat of gas adsorption with the surface loading points also to a filling or clogging of micropores at early stages of organic matter accumulation. Desorption induced a small recovery of the total SSA but not of the micropore surface area. Destruction of organic matter increased the SSA of all soil samples. The SSA of the uncovered mineral matrix related strongly to the amounts of Fe oxyhydroxides and the clay. Normalized to C removed, the increase in SSA was small in topsoils and illuvial horizons of Podzols rich in C and large for the subsoils containing little C. This suggests that micropores preferentially associate with organic matter, especially at small loadings. The coverage of the surface of the soil mineral matrix as calculated from the SSA before and after destruction of organic matter was correlated only with depth, and the relation appeared to be linear. We conclude that mineralogy is the primary control of the relation between surface area and sorption of organic matter within same soil compartments (i.e. horizons). But at the scale of complete profiles, the surface accumulation and stabilization of organic matter is additionally determined by its input. [source] Effects of soil frost on soil respiration and its radiocarbon signature in a Norway spruce forest soilGLOBAL CHANGE BIOLOGY, Issue 4 2009JAN MUHR Abstract Apart from a general increase of mean annual air temperature, climate models predict a regional increase of the frequency and intensity of soil frost with possibly strong effects on C cycling of soils. In this study, we induced mild soil frost (up to ,5 °C in a depth of 5 cm below surface) in a Norway spruce forest soil by removing the natural snow cover in the winter of 2005/2006. Soil frost lasted from January to April 2006 and was detected down to 15 cm depth. Soil frost effectively reduced soil respiration in the snow removal plots in comparison to undisturbed control plots. On an annual basis 6.2 t C ha,1 a,1 were emitted in the control plots compared with 5.1 t C ha,1 a,1 in the snow removal plots. Only 14% of this difference was attributed to reduced soil respiration during the soil frost period itself, whereas 63% of this difference originated from differences during the summer of 2006. Radiocarbon (,14C) signature of CO2 revealed a considerable reduction of heterotrophic respiration on the snow removal plots, only partly compensated for by a slight increase of rhizosphere respiration. Similar CO2 concentrations in the uppermost mineral horizons of both treatments indicate that differences between the treatments originated from the organic horizons. Extremely low water contents between June and October of 2006 may have inhibited the recovery of the heterotrophic organisms from the frost period, thereby enhancing the differences between the control and snow removal plots. We conclude that soil frost triggered a change in the composition of the microbial community, leading to an increased sensitivity of heterotrophic respiration to summer drought. A CO2 pulse during thawing, such as described for arable soils several times throughout the literature, with the potential to partly compensate for reduced soil respiration during soil frost, appears to be lacking for this soil. Our results from this experiment indicate that soil frost reduces C emission from forest soils, whereas mild winters may enhance C losses from forest soils. [source] Patterns of rhizosphere carbon flux in sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) saplingsGLOBAL CHANGE BIOLOGY, Issue 6 2005Richard P. Phillips Abstract Despite its importance in the terrestrial C cycle rhizosphere carbon flux (RCF) has rarely been measured for intact root,soil systems. We measured RCF for 8-year-old saplings of sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) collected from the Hubbard Brook Experimental Forest (HBEF), NH and transplanted into pots with native soil horizons intact. Five saplings of each species were pulse labeled with 13CO2 at ambient CO2 concentrations for 4,6 h, and the 13C label was chased through rhizosphere and bulk soil pools in organic and mineral horizons for 7 days. We hypothesized yellow birch roots would supply more labile C to the rhizosphere than sugar maple roots based on the presumed greater C requirements of ectomycorrhizal roots. We observed appearance of the label in rhizosphere soil of both species within the first 24 h, and a striking difference between species in the timing of 13C release to soil. In sugar maple, peak concentration of the label appeared 1 day after labeling and declined over time whereas in birch the label increased in concentration over the 7-day chase period. The sum of root and rhizomicrobial respiration in the pots was 19% and 26% of total soil respiration in sugar maple and yellow birch, respectively. Our estimate of the total amount of RCF released by roots was 6.9,7.1% of assimilated C in sugar maple and 11.2,13.0% of assimilated C in yellow birch. These fluxes extrapolate to 55,57 and 90,104 g C m,2 yr,1 from sugar maple and yellow birch roots, respectively. These results suggest RCF from both arbuscular mycorrhizal and ectomycorrhizal roots represents a substantial flux of C to soil in northern hardwood forests with important implications for soil microbial activity, nutrient availability and C storage. [source] Extraction of water-soluble organic matter from mineral horizons of forest soilsJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 4 2007Thilo Rennert Abstract Dissolved organic matter (DOM) is involved in many important biogeochemical processes in soil. As its collection is laborious, very often water-soluble organic matter (WSOM) obtained by extracting organic or mineral soil horizons with a dilute salt solution has been used as a substitute of DOM. We extracted WSOM (measured as water-soluble organic C, WSOC) from seven mineral horizons of three forest soils from North-Rhine Westphalia, Germany, with demineralized H2O, 0.01 M CaCl2, and 0.5 M K2SO4. We investigated the quantitative and qualitative effects of the extractants on WSOM and compared it with DOM collected with ceramic suction cups from the same horizons. The amounts of WSOC extracted differed significantly between both the extractants and the horizons. With two exceptions, K2SO4 extracted the largest amounts of WSOC (up to 126 mg C,kg,1) followed by H2O followed by CaCl2. The H2O extracts revealed by far the highest molar UV absorptivities at 254 nm (up to 5834 L mol,1,cm,1) compared to the salt solutions which is attributed to solubilization of highly aromatic compounds. The amounts of WSOC extracted did not depend on the amounts of Fe and Al oxides as well as on soil organic C and pH. Water-soluble organic matter extracted by K2SO4 bore the largest similarity to DOM due to relatively analogue molar absorptivities. Therefore, we recommend to use this extractant when trying to obtain a substitute for DOM, but as WSOM extraction is a rate-limited process, the suitability of extraction procedures to obtain a surrogate of DOM remains ambiguous. [source] | ||||||||||