Release of dissolved organic carbon (DOC) and nitrogen (DON) from subtropical urban soils has had little investigation, even though the growth of metropolitan areas is unprecedented in southern states. The time of initial disturbance from native or agricultural to urban land use and irrigation with sodic water may compromise urban soils' ability to retain DOC and DON. We examined 21 soils from beneath remnant native land use and urban land use in south-central Texas. Soils were collected from five classes of urban land use and four classes of remnant native land use and analyzed using initial mass isotherms. We found that release of DOC was significantly higher from soils under urban parks (range, 433–782 mg kg−1) relative to soils from residential lawns (range, 89–228 mg kg−1) and remnant native soils (range, 43–264 mg kg−1). Release of DON and DON reactive soil pools were also significantly increased in soils under urban relative to remnant land use (P < 0.05). Older urban soils with minimal irrigation tended to have DOC and DON reactive soil pools similar to remnant soils, perhaps illustrating that equilibrium is reached over time.

Long-term changes in sulfate concentrations and soil acidification in two adjacent forest soils (Andosols and Umbrisols) were investigated using soil samples archived from 1967 to 2009. The amounts of phosphate-extracted sulfate in Andosols (≤51.0 mmol kg) were generally higher than that in Umbrisols (≤13.7 mmol kg). Both soils contained extremely high amounts of sulfate compared with the soils of polluted regions of Europe and North America in the 1980s and 1990s. A strong positive correlation was obtained between iron oxide minerals (acid oxalate–extractable Fe minus pyrophosphate-extractable Fe [Feo - Fep]) and phosphate-extracted sulfate, whereas soil organic matter was suggested to inhibit sulfate adsorption especially by allophane (acid oxalate–extractable Al minus pyrophosphate-extractable Al [Alo - Alp]). The amounts of phosphate-extracted sulfate in Umbrisols have gradually decreased by about one third between 1967 and 2009, resulting from the decline of sulfur emissions. However, those in Andosols have remained at high levels, indicating the very slow or irreversible equilibration of adsorbed sulfate in these soils. In addition, surface soils of Andosols from 1994 and 2009 showed soil acidification indicated by the decrease in soil pH, the dissolution of aluminum, and the formation of the precipitates, such as aluminum hydroxysulfate and basic iron sulfate. It is considered that irreversible adsorption of sulfate is an important factor contributing to the decrease in acid-neutralizing capacity. Although acidification in Andosols is slow to appear because of their high acid-neutralizing and sulfate adsorption capacities, acidification persists for a long time once the critical loads of acid deposition have occurred.

Gas transport parameters and X-ray computed tomography (CT) measurements in porous medium under controlled and identical conditions provide a useful methodology for studying the relationships among them, ultimately leading to a better understanding of subsurface gaseous transport and other soil physical processes. The objective of this study was to characterize the relationships between gas transport parameters and soil-pore geometry revealed by X-ray CT. Sands of different shapes with a mean particle diameter (d 50) ranging from 0.19 to 1.51 mm were used as porous media under both air-dried and partially saturated conditions. Gas transport parameters including gas dispersivity (α), diffusivity (D P/D 0), and permeability (k a) were measured using a unified measurement system (UMS). The 3DMA-Rock computational package was used for analysis of three-dimensional CT data. A strong linear relationship was found between α and tortuosity calculated from gas transport parameters ( ), indicating that gas dispersivity has a linear and inverse relationship with gas diffusivity. A linear relationship was also found between k a and d 50/T UMS 2, indicating a strong dependency of k a on mean particle size and direct correlation with gas diffusivity. Tortuosity (T MFX) and equivalent pore diameter (d eq.MFX) analyzed from microfocus X-ray CT increased linearly with increasing d50 for both Granusil and Accusand and further showing no effect of particle shape. The T UMS values showed reasonably good agreement with T MFX values. The k a showed a strong relationship when plotted against d eq.MFX/T MFX 2, indicating its strong dependency on pore size distribution and tortuosity of pore space.

Iron (Fe) oxides were found to inhibit natural acidification of highly weathered soils from tropical and subtropical regions. The mechanism for the inhibition of the acidification has not been well documented. Electrodialysis was used to simulate natural acidification process in soils under intensive leaching and to investigate mechanism of Fe oxides inhibiting acidification of kaolinite and an Alfisol subsoil. Results indicate that the electrodialysis treatment could release exchangeable base cations from kaolinite and phyllosilicate surfaces of the Alfisol subsoil gradually, produce exchangeable acidity, and ultimately result in strong acidification of the near-neutral kaolinite and Alfisol subsoil. Contrary to the traditional soil acidification principle, addition of Fe oxides to the kaolinite and Alfisol subsoil led to a decrease in effective cation exchange capacity (ECEC) and exchangeable acidity but inhibited the decrease in pH of the kaolinite and Alfisol subsoil during the electrodialysis process. Thus, the presence of Fe oxides could inhibit the acidification of the kaolinite and the Alfisol subsoil. The inhibition of acidification by Fe oxides was related to the amount of Fe oxides added and the surface charges per unit mass of negatively charged kaolinite or Alfisol subsoil and the positively charged Fe oxides carried. Higher content of Fe oxides and larger surface charges per unit mass resulted in a lower ECEC and exchangeable acidity but higher pH and thus weaker acidification of kaolinite or Alfisol subsoil. Changes in ECEC of kaolinite and Alfisol subsoil induced by Fe oxides were completely reversible with changes in ionic strength of the bathing solutions. These results confirm the occurrence of a diffuse layer overlapping on oppositely charged particles and support the hypothesis that Fe oxides inhibit the natural acidification process through an electric double layer overlapping in highly weathered soils.

Wind erosion has become a very serious environmental, agricultural, and social problem in arid and semiarid regions such as the agro-pastoral ecotone, North China. Wind erosion is caused by near-surface interaction of wind turbulence and soil particles. Numerous factors influence it, such as wind velocity, precipitation, and temperature; their interplay contributes to the complex height variation of wind turbulence and particle distribution. To determine the spatial distribution of the saltation particles, we used a field experiment using a passive sediment collector and a laboratory experiment using a push-type wind tunnel coupled with particle composition measurements. Based on the principle of maximum entropy, a new model is proposed to describe the distribution of saltation particles moved by wind turbulence. By comparing the model results with both the field experimental data and the data from the indoor wind tunnel, the model was validated. The results showed that the maximum entropy principle is suitable to model the processes governing sediment particle density and lays a solid foundation to describe the coupling dynamics of wind-sand movement.

Land use conversion induces quantitative change of soil water-soluble organic matter (WSOM), but knowledge of such change is still limited. In this study, field moist and air-dried soils sampled from subarctic Alaska under three land use managements (i.e., forest, agriculture, and grassland converted from agricultural use and under a Conservation Reserve Program [CRP]) were extracted with deionized water and separated by filtration into different size fractions (2.5 μm, 0.45 μm, and 1 kDa). Water-soluble organic C (WSOC), water-soluble organic N, and fluorescence spectroscopy of each fraction were determined. There were few differences in quantitative data between samples from different land uses with air-dried samples, implying that air-dried samples were not suitable for characterizing the impact of soil management practices on soil WSOM. For field moist soil samples, the WSOC contents decreased in the order forest > CRP > agricultural land. Furthermore, WSOC was dominated by large (>0.45 μm) and small (<1 kDa) size molecules in CRP and forest soils, whereas small molecules predominated in agricultural soils. The WSOM of different size fractions and land use displayed three similar fluorophore components (two humic-like and a tyrosine-like), indicating that the impact of land use was mainly on the quantity, rather than on the composition, of WSOM. In conclusion, our data suggested that the long-term agricultural land use could lower the WSOM levels in soils; however, the decreasing trend could be reversed by conversion of agricultural land to grassland under conservation.

Soil compaction negatively influences many important soil functions, including crop growth. Compaction occurs when the applied stress, R, overcomes the soil strength. Soil strength in relation to com- paction is typically expressed by the soil precompression stress, Rpc. Deformation is assumed to be elastic and reversible as long as R e Rpc. This work examined soil stress-strain behavior as measured in situ during wheeling experiments and related it to the stress-strain behavior and Rpc measured on soil cores in uniaxial compression tests in the laboratory. The data analyzed were from a large number of wheeling experiments carried out in Sweden and Denmark on soils with a wide range of texture. Contradicting the concept of precompression stress, we observed residual strain, ?res ,a tR e Rpc. These observations were supported by stress-strain data measured in uniaxial compression tests, which likewise showed ?res 9 0a tR e Rpc. Residual strain was observed in the field whenR exceeded approximately 40 kPa, and when the ratio R/Rpc exceeded roughly 0.1, although ?res was very small at R/Rpc G 0.5. These values were similar to those obtained on confined uniaxial compression curves. On the basis of our findings, we question the use of Rpc as a measure of soil strength and call for a reevaluation of the precompression stress concept.

In this study, we evaluated the boron (B) adsorption- desorption reaction of six soils and examined the extent to which organic matter content, as well as incubation time, affected B release. Six soils varying in initial pH, clay content, and organic matter content were se- lected for the study. Adsorption experiments were carried out to deter- mine B adsorption isotherms using 0.01 M NaCl solutions containing 0 to 100 mg B kg j1 . The soils were equilibrated for 23 h. Boron de- sorption was investigated after replacing aliquots with B-free 0.01 M NaCl solution three times. Three of the soils were also spiked with 0 to 100 mg B kg j1 and equilibrated for up to 23 months at field capacity water content in a temperature-controlled room at 25-C. At various in- tervals, sub-samples were extracted with DTPA-sorbitol, boiling water, and 0.1 M NaCl. The B adsorption-desorption behavior of all six soils was found to be nonhysteretic in the short-term experiment, regardless of organic matter content. Apparent hysteresis of B adsorption can result from difficulties in the experimental procedure itself. Some B adsorption hysteresis was found for two soils with all three extractants after long- term equilibrations of 1 to 23 months. Our results do not support the hypothesis that B desorption hysteresis increases in soils with increasing organic matter content.

Abstract Charcoal is the result of incomplete combustion of plant material. It is common as residue from wild and prescribed fires in many soil surface horizons, including those of the extensive Ponderosa pine forests in the western United States, but its effects on soil properties are incompletely u