摘要:

Dissolved organic matter (DOM) in soils plays an important role in the biogeochemistry of carbon, nitrogen, and phosphorus, in pedogenesis, and in the transport of pollutants in soils. The aim of this review is to summarize the recent literature about controls on DOM concentrations and fluxes in soils. We focus on comparing results between laboratory and field investigations and on the differences between the dynamics of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP).Both laboratory and field studies show that litter and humus are the most important DOM sources in soils. However, it is impossible to quantify the individual contributions of each of these sources to DOM release. In addition, it is not clear how changes in the pool sizes of litter or humus may affect DOM release. High microbial activity, high fungal abundance, and any conditions that enhance mineralization all promote high DOM concentrations. However, under field conditions, hydrologic variability in soil horizons with high carbon contents may be more important than biotic controls. In subsoil horizons with low carbon contents, DOM may be adsorbed strongly to mineral surfaces, resulting in low DOM concentrations in the soil solution. There are strong indications that microbial degradation of DOM also controls the fate of DOM in the soil.Laboratory experiments on controls of DOM dynamics have often contradicted field observations, primarily because hydrology has not been taken into account. For example, laboratory findings on the effects of plant species (conifer vs. deciduous) on DOM release from forest floors and on the effects of substrate quality (e.g.: C/N ratio) or pH on DOC concentrations were often not confirmed in field studies. The high adsorption capacity of soil clay minerals and oxides for DOM shown in laboratory studies may not control the transport of DOM in soils in the field if macropore fluxes dominate under field conditions. Laboratory findings about the biodegradability of DOM also await verification under field conditions.Studies that include DON and DOP dynamics in addition to DOC are few. The rate of release and the fate of DOC, DON, and DOP in soils may differ to a far greater extent than previously assumed. Controls established for DOC might thus be not valid for DON and DOP.Despite intensive research in the last decade, our knowledge of the formation and fate of DOM in soils and its response to changing environmental conditions is still fragmented and often inconsistent. Predictions at the field scale are still very uncertain, and most of the information available today is the result of studies on temperate soils and forest ecosystems. Thus, future research on controls of DOM dynamics should be extended to soils under different land uses and in other climate zones. Emphasis should also be given to: (i) the effects of soil organic matter properties on the release of DOM (ii) environmental factors controlling DOM quantity and quality (iii) the assessment of biological versus physico-chemical controls on the release and retention of DOM in soils, and (iv) the differences between DOC, DON, and DOP. Finally, if our goal is to predict DOM concentrations and fluxes in soils, future research on the controls of DOM dynamics should have a strong focus on field studies.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Recent studies have shown that soil organic carbon (OC) may either hinder or favor the sorption of dissolved organic matter (DOM) in soils. Our concept was that the nature of soil OC determines these contrasting findings. To test this hypothesis, we compared the DOM sorption in soils with OC derived from biomass decomposition with that in soils with OC more likely derived from charred materials (black carbon). All the mineral soil samples in the study were from Spodosols, and the DOM was from an aqueous extract of a mor forest floor layer. Sorption was determined in batch experiments. The sorption in soils that contain large amounts of black carbon was, in general, less than the sorption in soils with decomposition-derived OC. When the DOM sorption parameters of the soils were correlated to the OC content, the black carbon soils showed a positive effect of the OC content on the DOM sorption. In the soils lacking the features of black carbon residues, the DOM sorption was negatively influenced by OC. These results lead us to assume that the nature of soil OC is a soil property that needs to be considered in the DOM sorption of soils, especially when soils have large amounts of highly aromatic OC.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

The characterization of soil organic matter, and of humic substances in particular, has always been made on fine earth, i.e., the <2-mm fraction. The fraction larger than 2 mm, known as rock fragments or skeleton, is commonly discarded. We have extracted the humic acids (HAs) and fulvic acids (FAs) from fine earth, rock fragments, and rock fragments washing (i.e. the fine material adhering to the rock fragments) of the upper forest soil horizons, A1 and A2. The substances were characterized using wet-chemical analyses, FT-IR, and liquid state13C NMR spectroscopies. HAs of the rock fragments have higher N and H content, paraffinic chains, aliphatic-OH, and highly substituted aromatic groups than those of the fine earth. These features indicate that carbohydrates, lipids, and proteinaceous residues are incorporated in the humic acids of the rock fragments. The fresh biological material residues present in the structure of the skeleton HAs may be attributable to: (i) the selective preservation from microbial and chemical attack offered by the skeletal environment; (ii) the more rapid cycling of the organic matter inside the rock fragments compared with that of the fine earth.The FAs show greater homogeneity than the HAs, probably because of their mobility among the different soil compartments. However, the FAs of the rock fragments have more carboxyl and acidic-OH groups than found in the fine earth. Greater differences are also observed between the two horizons. The FAs extracted from the A2 horizon show less mineralization than those from the A1 horizon.The humic substances extracted from the rock fragments washing show characteristics more similar to those of the skeleton than to those of the fine earth. This suggests that most rock fragments washing originate from the weathering of the rock fragments and, therefore, may be regarded as an intermediate phase between the skeleton and the fine earth.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Aggregate-size fractions of two cultivated calcareous Mediterranean soils (Mevo Horon and Palmahim) were characterized with regard to organic matter and other soil components involved in soil structure stabilizing reactions. The parameters determined in each aggregate-size fraction included: particle size distribution and contents of CaCO3, organic C and N, fulvic acid fraction (FF), humic acid (HA), total humic substances (HS), and extractable Al and Fe. Except for the organic C and HS in the Mevo Horon (MH) soil, all of the calculated ratios between the contents of each of the components in any size fraction and that calculated for the same component in the <2-μm fraction, exhibited a parabolic type curve, reaching a maximum for the 20- to 50-μm aggregates. It is hypothesized that this ratio is a reflection of the degree of surface coating (or association) of the clay by these structure-stabilizing components. The values of coating observed in each of the aggregate-size fractions were normalized with that observed for the <2-μm fraction (which presumably exhibits the highest degree of association). In practically all combinations of aggregate-size fractions and all soil components (in both soils), the individual stabilizing component is present at higher relative concentration than in the <2-μm fraction. These components, therefore, act first as surface coating materials, with the excess material forming interparticle associations.13C-NMR spectra of both soils (OM not extracted) displayed similar characteristics, with generally high content of both aliphatic (50-60%) and aromatic carbons (22-30%) in both the largest (>250 μm) and the smallest (<2 μm) aggregate-size fractions. Higher polysaccharide content was observed in the larger particle size fraction, reflecting the influence of recent plant material.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Redistribution and depletion of soil water in a Tavares fine sand (Typic Quartzipsamments) profile were evaluated after irrigation or rain in a commercial citrus grove with under-tree microsprinkler irrigation. Tensiometers were installed at 15-, 30-, 90-, and 150-cm depths in five clusters along the dripline of 25-year-old Hamlin orange trees on Cleopatra mandarin rootstock. Irrigation was scheduled when the soil water potential at the 15- and 30-cm depths exceeded either −10 KPa (Jan. to June) or −15 KPa (July to Dec.) to replenish the water deficit (below field capacity) in the top 90 cm of the soil profile. The tensiometers placed at 15- and 30-cm depths responded to changes in soil water regardless of irrigation or rainfall. Tensiometer readings at various depths were used to estimate the water content at corresponding depths using the van Genuchten analytical relationship equations. Total soil water contents within the rootzone (0 to 90 cm) and below the rootzone (90 to 150 cm) within the monitoring depth (0 to 150 cm) were also calculated to estimate the water available for the trees and water that drained below the rootzone. Results showed some leaching does occur during months when there is heavy rainfall. This study demonstrated that tensiometer readings can be used to calculate the soil water content at various depths within the soil profile. Therefore, duration of irrigation can be adjusted to minimize leaching below the rootzone. Because optimal irrigation management is important to minimize nutrient leaching below the rootzone, tensiometer-based irrigation scheduling is an important component of nutrient and irrigation best management practices for citrus in sandy soils.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Seedling emergence is affected by soil crusting through two mechanisms-crust impedance (CI) and crust effect on evaporation rate that determine the moisture content in the seed bed. The relative importance of these two mechanisms was evaluated in this study. The effects of soil sodicity, phosphogypsum (PG), polyacrylamide (PAM), and raindrop impact energies on evaporation and CI of two soils, hamra (mixed, loamy, typic Rhodoxeralf) and loess (mixed, silty loam, calcic Haploxeralf), and their consequential effects on seedling emergence, were studied under high and low evaporation conditions (8.5 and 5.6 mm day−1, respectively). Increasing the exchangeable sodium percentage (ESP) from 3.0 to 7.0 decreased the rate of evaporation from the loess by 10% and doubled the soil CI. Under mist (low raindrop energy), crust formation did not occur, and evaporation from both soils was higher than that from crusted soils.Rain energy and PG had a more significant effect on the evaporation rate (ER) and CI of the hamra than that of the loess. The effect of PG on ER in the loess was negligible. Cotton seedling emergence was affected by evaporation rate and the moisture content in the seed bed, but it was not affected by crust impedence. Conversely, onion seeds were affected by both, moisture content in the seed bed and by crust strength. Increasing the crust ESP from 3 to 7 and maintaining adequate moisture reduced the onion seedling emergence rate by 50%. Application of PAM plus PG to the soil surface doubled the emergence rate of onions in loess with an ESP of 3.0 but was not effective in loess with an ESP of 7.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID

摘要:

Experiments were conducted to assess the effects of soil water content on denitrification during hairy vetch (Vicia villosa) decomposition. Hairy vetch plants were grown from seed to maturity in soil cores. Before and after kill, simulated rainfall was applied to cores weekly and leachate was analyzed for NO−3and NH+4. Denitrification incubations (ca. 48 h duration) were conducted 3, 17, 31, and 45 days after kill using the acetylene block method. Soil water content was varied systematically to give a range of percent water-filled pore space (WFPS) values from field capacity (60%) to saturation (100%). Little denitrification occurred on Day 3 (<2 mg N2O-N). Substantial denitrification occurred on Days 17, 31, and 45, with maxima of 44, 27, and 30 mg N2O-N produced in saturated cores, respectively, accounting for approximately 60 to 75% of the total inorganic N (NO−3+ NH+4+ N2O) present in cores. There was an apparent linear relationship between denitrification and soil water content (WFPS), with a threshold for denitrification at ca. 60% WFPS. Cumulative N lost from cores during four denitrification incubations ranged from 1 to 48 Kg N ha−1, depending on percent WFPS. Rates of N mineralization were relatively linear after denitrification incubations (55 days). Cumulative N mineralized from unsaturated cores was ca. 190 Kg N ha−1through 120 days after kill. These data indicate that substantial quantities of vetch-N may be lost during decomposition.

ISSN:0038-075X    

出版商:OVID    

年代:2000

数据来源: OVID