Denitrification characteristics of subtropical soils in China affected by soil parent material and land use

Forty‐five soil samples were collected from rice paddy land (R), tea garden land (T), forestland (F), brush land (B), and upland (U) in Jiangxi province, a subtropical region of China. These soils were derived from Quaternary red earth (Q), Tertiary red sandstone (S), and granite (G). Their denitrification capacities were determined after treatment with 200 mg NO₃⁻‐N kg⁻¹ soil by measuring changes in NO₃⁻‐N content during a 28‐day anaerobic incubation under N₂ gas in the headspace, at 30°C. The subtropical soils studied here were characterized by generally small denitrification capacities, ranging from no denitrification capacity to complete disappearance of added NO₃⁻‐N within 11 days of incubation. With few exceptions, NO₃⁻‐N reduction with incubation time followed a first‐order relationship with reaction constants of 0 – 0.271 day⁻¹, but the data could be simulated better by a logarithmic relationship. Thus, denitrification capacity was determined by the reaction constant of the first‐order reaction, the slope of the logarithmic relationship, and the averaged NO₃⁻‐N reduction rate in the first 7 days of anaerobic incubation (ranging from 0 to 28.5 mg kg⁻¹day⁻¹), and was significantly larger in the soils derived from G than from Q and S for all land uses except for rice paddy land. Soil organic carbon and nitrogen availability are the key factors that determine differences in denitrification capacity among the three soil parent materials. Rice cultivation significantly promoted denitrification capacity compared with the other four land uses and masked the effect of soil parent materials on denitrification capacity. This is most likely due to increases in organic carbon and total N content in the soil, which promoted the population and biological activities of microorganisms which are able to respire anaerobically when the rice soil is flooded. Neither the increased pH of upland soil caused by the addition of lime for upland crop production, nor the decreased pH of the tea garden soil by the acidification effect of tea plants altered soil denitrification capacity. Our results suggest that land use and management practices favour soil carbon and/or nitrogen accumulation and anaerobic microorganism activities enhance soil denitrification capacity.     

Aluminum solubility of strongly acidified allophanic Andosols from Kagoshima Prefecture,southern Japan

Abstract

The aluminum solubility of acidified soils both from furrows and under tree canopies of a tea garden was studied using equilibrium experiments in 0.01 mol L^?1 CaCl₂ solution systems. The soils were originally classified as allophanic Andosols. The furrow soils were more severely acidified because of the heavy application of nitrogen fertilizer, especially in the upper soil horizons (pH[H₂O] of 3.6–3.8 in the A1 and 2A2 horizons). These acidified soils were characterized by the dissolution of allophanic materials (allophane, imogolite and allophane-like materials) and by an increase in Al–humus complexes. Ion activity product (IAP) values of the strongly acidified soil horizons were largely undersaturated with respect to imogolite (allophanic clay) or gibbsite. Plots of p(Al³⁺) as a function of pH strongly indicated that Al solubility of the soils was largely controlled by Al–humus complexes, especially in the A1 horizon. In the canopy soils, which were more weakly acidified (pH[H₂O] 4.9–5.0), Al solubility was close to that of gibbsite and allophanic materials, indicating that the solubility is partly controlled by these minerals.     

Ground water preservation by soil protection: Determination of tolerable total Cd contents and Cd breakthrough times

Taking Cd as an example we introduce a procedure to estimate tolerable total content of heavy metals in soils with regard to a specific ground water quality criterion. Furthermore, we present a piston‐flow approach to estimate breakthrough times of a sorptive solute to the ground water. Both procedures are applied to the sandy soils in the 4300 ha wastewater irrigation area Braunschweig, Germany. Applicability of these procedures is tested by numerical simulations. The calculated breakthrough times of Cd for an input concentration of 3 μg L⁻¹ and a mean water flux density of 570 mm yr⁻¹ varies, as a function of depth of water table and sorption characteristics, between 10 and 805 years (mean = 141 years). The deviation between the piston‐flow approach and the numerical simulation is on the average 1.6%. We determined a mean tolerable total Cd content of 0.61 mg kg⁻¹ with regard to a ground water quality criterion of 3 μg L⁻¹. The limit of the German sewage sludge regulation (AbfKlärV, 1992) of 1 mg Cd kg⁻¹ exceeds the calculated tolerable total content in 90% of the investigated Ap horizons. Moreover, the results of the numerical simulations show that the limit of 1 mg Cd kg⁻¹ would lead to a concentration in seepage water significantly above 8 μg Cd L⁻¹. We conclude that in the sandy soils of the wastewater irrigation area the current limit of 1 mg Cd kg⁻¹ is not sufficient to keep the Cd concentration in seepage water below 3 μg L⁻¹ and, thus, to ensure ground water protection in the long run.     

Unusually high levels of bio-available phosphate in the soils of Ogasawara Islands, Japan: Putative influence of seabirds

Ogasawara Islands are important ecosystems sustaining many indigenous spices. To clarify the indigenous soil environments of Ogasawara Islands, we studied the chemistry of the soils. Many surface soils were low in bio-available P (0 to 0.55 g P₂O₅ kg⁻¹, average: 0.04 g P₂O₅ kg⁻¹ as Bray II P, n = 22), but several soils were found to contain extremely large amounts of bio-available P (1.36 to 6.98 g P₂O₅ kg⁻¹, average: 2.93 g P₂O₅ kg⁻¹, n = 5). From soil profile analyses, the authors concluded that the extremely large amount of bio-available P could not be explained by the effects of parent materials with high P contents nor the effect of fertilizations by human activity, but the effects of natural seabird activities in the past could be the cause. The soil profiles with large amounts of bio-available P indicate deep migration of soil materials from A horizons, which could be a result of intensive mixing of upper horizons by seabird activities. The intensive mixing was supported by the low mechanical impedance of the horizons for the P-accumulating soils (8.17 ± 2.54 kg cm⁻², n = 8) than those for the non-P-accumulating soils (17.46 ± 3.52 kg cm⁻², n = 36). It is likely that in the past seabirds, such as shearwaters, made burrows in the soils for nesting and propagating and inadvertently transported a large amount of P from the sea to the soils, resulting in the extremely large amounts of bio-available P in the present soils.     

Microbial CO2 production, CH4 dynamics and nitrogen in a wetland soil (New York State, USA) associated with three plant species (Typha, Lythrum, Phalaris)

Plants furnish soil with organic carbon (OC) compounds that fuel soil microorganisms, but whether individual plant species – or plants with unique traits – do so uniquely is uncertain. We evaluated soil microbial processes within a wetland in which areas dominated by a distinct plant species (cattail –Typha sp.; purple loosestrife –Lythrum salicaria L.; reed canarygrass –Phalaris arundinacea L.) co‐mingled. We also established an experimental plot with plant shoot removal. The Phalaris area had more acidic soil pH (7.08 vs. 7.27–7.57), greater amount of soil organic matter (19.0% vs. 9.0–11.5%), and the slowest production rates of CO₂ (0.10 vs. 0.21–0.46 μmol kg⁻¹ s⁻¹) and CH₄ (0.040 vs. 0.054–0.079 nmol kg⁻¹ s⁻¹). Nitrogen cycling was dominated by net nitrification, with similar rates (17.2–18.9 mg kg⁻¹ 14 days⁻¹) among the four sampling areas. In the second part of the study, we emplaced soil cores that either allowed root in‐growth or excluded roots to evaluate how roots directly affect soil CO₂ and CH₄. The three plant species had similar amounts of root growth (ca 290 g m⁻² year⁻¹). Fungal biomass was similar in soils with root in‐growth versus root exclusion, regardless of dominant plant species. Rates of soil CO₂ production did not differ with root in‐growth versus root exclusion, and added glucose increased CO₂ production rates by only 35%. Root in‐growth did lead to greater rates of CH₄ production; albeit, addition of glucose had much greater effect on CH₄ production (1.24 nmol kg⁻¹ s⁻¹) compared with controls without added glucose (0.058 nmol kg⁻¹ s⁻¹). Our data revealed relatively few subtle differences in soil characteristics and processes associated with different plant species; albeit, roots had little effect, even inhibiting some microbial processes. This research highlights the need for both field and experimental studies in long‐established monocultures of plant species to understand the role of plant biodiversity in soil function.     

Role of soil constituents in fixation of soluble Zn, Cu, Ni and Cd added to soils

Slow immobilization of trace metals in soil, termed ‘fixation’, affects their natural attenuation but it is still unclear which reactions occur. Twenty‐eight soils were selected to assess the role of Fe oxides and carbonates on fixation of Cu, Cd, Zn and Ni. Soils included samples from 2 toposequences (Vietnam, Spain) and 13 European topsoils with different soil characteristics (pH 3.4–7.7). Samples were amended with 250 mg Zn kg⁻¹, 100 mg Cu kg⁻¹, 80 mg Ni kg⁻¹ and 2.5 mg Cd kg⁻¹ as metal salts and incubated for 850 days. Fixation was measured as the increase of the fraction of added metals that were not isotopically exchangeable. Fixation increased with time and was, averaged over all the soils, 43% (Cu), 41% (Zn), 41% (Ni) and 28% (Cd) after 850 days. Metal fixation within samples from each toposequence was generally positively related to total Fe oxide concentration (Fe_d) for Zn, Ni and Cd. However, the fixation of Cd, Zn and Ni was mainly explained by pH and not by Fe_d when considering all soils. Fixation of Zn and Cd in soils with pH >7.0 increased with increasing concentrations of carbonates at initial ageing times. Fixed fractions of Zn, Ni and Cd were significantly released when experimentally removing 50% of carbonates by acidification. Fixation of Cu was most poorly related to soil properties. Our data suggest that fixation of Cd, Zn and Ni is related to a pH‐dependent diffusion into oxides and that of Cd and Zn also to diffusion and/or coprecipitation in carbonates. Fixation of Ni at neutral pH may also be related to stabilization of precipitates that form readily in soil.     

Organic C levels of Ap horizons in North German Pleistocene sands as influenced by climate,texture, and history of land‐use

We evaluated the contents of organic carbon (C_org) of Ap horizons from 11 North German study areas along a Southeast to Northwest precipitation gradient with respect to their general levels and as related to C : N ratio, soil texture (clay content), bulk soil density, climate, and historical land‐use since 1780. The focus was on sandy soils, with the largest group of samples originating from 308 km² of the Fuhrberg catchment north of Hannover/Lower Saxony. Data from loess areas were used for comparisons. Major aims were (1) to quantify current C_org stocks, (2) to provide data on site‐specific, steady‐state C_org levels in old arable soils, and (3) to identify the main controls of C_org levels in the studied sands. The mean C_org content in sandy, well‐drained, old Ap horizons (uplands, > 200 years under cultivation, near steady‐state) increased with precipitation from < 8 g kg^—1 in the dry eastern parts of the study area (530 mm year^—1, 8.3°C) to 25 g kg^—1 in the moist Northwest (825 mm year^—1, 8.4°C). The C_org levels in lowlands which have been drained for more than 40 years were approximately 3 g kg^—1 higher than those of uplands under a similar climate. The factor clay content had no predictive value because low contents were associated with high C_org levels. Large proportions of refractory organic matter in sands resulting from specific features of historical land‐use and soil development (calluna heathland, heath plaggen fertilization, podzolization) appeared to be the most probable reason for such high C_org levels. However, the high C_org levels of these old arable sites were still exceeded by those of younger arable areas formerly under continuos grassland. A chrono‐sequence suggested that a period of about 100 years is necessary until a new steady‐state C_org level is established after conversion of grassland into arable land. Elevated C_org levels in current Ap horizons were also found for former woodland and heathland soils. The main conclusion is that sands can contain a lot of stable organic matter, sometimes more than finer textured soils.     

Heavy Metal Concentrations,Partitioning, and Storage in Slovak Forest and Arable Soils Along a Deposition Gradient

Along a heavy metal deposition gradient, caused by a Cu smelter, heavy metal concentrations, partitioning, and storage in forest and arable soils were examined. We sampled organic and mineral soil horizons (0—50 cm) at ten pairs of forest and arable sites derived from the same parent material. A-horizons were extracted with a seven-step sequence; O- and subsoil horizons were digested with strong acids (HNO₃/HClO₄). We found high concentrations of Cd (up to 17.38 mg kg^—1 in the O horizons/up to 2.44 mg kg^—1 in the A horizons), Cu (8437/415), Pb (3343/126), and Zn (1482/637) which decreased exponentially with distance from the smelter and with soil depth. The metal concentrations in the organic layers indicate that the average transport distance decreases in the order Cd > Zn > Pb > Cu. With regard to metal partitioning, NH₄NO₃- + NH₄OAc-extractable forms in the A horizons were most affected by the deposition being more pronounced under forest. In the uppermost 50 cm of the four soils nearest to the smelter two to four times higher Cd, Cu, Pb, and Zn storages were found in forest than in arable soils. At greater distance, the higher deposition onto forest soils due to the scavenging effect of the canopy obviously was compensated by stronger leaching.     

SYNOPSIS Polycyclic Aromatic Hydrocarbons (PAHs) in Soil — a Review

PAHs are mainly produced by combustion processes and consist of a number of toxic compounds. While the concentrations of individual PAHs in soil produced by natural processes (e.g., vegetation fires, volcanic exhalations) are estimated to be around 1—10 μg kg⁻¹, recently measured lowest concentrations are frequently 10 times higher. Organic horizons of forest soils and urban soils may even reach individual PAH concentrations of several 100 μg kg⁻¹. The PAH mixture in temperate soils is often dominated by benzofluoranthenes, chrysene, and fluoranthene. The few existing studies on tropical soils indicate that the PAH concentrations are relatively lower than in temperate soils for most compounds except for naphthalene, phenanthrene, and perylene suggesting the presence of unidentified PAH sources. PAHs accumulate in C‐rich topsoils, in the stemfoot area, at aggregate surfaces, and in the fine‐textured particle fractions, particularly the silt fraction. PAHs are mainly associated with soil organic matter (SOM) and soot‐like C. Although the water‐solubility of PAHs is low, they are encountered in the subsoil suggesting that they are transported in association with dissolved organic matter (DOM). The uptake of PAHs by plants is small. Most PAHs detected in plant tissue are from atmospheric deposition. However, earthworms bioaccumulate considerable amounts of PAHs in short periods. The reviewed work illustrates that there is a paucity of data on the global distribution of PAHs, particularly with respect to tropical and southern hemispheric regions. Reliable methods to characterize bioavailable PAH pools in soil still need to be developed.     

SPATIAL DISTRIBUTION OF COPPER FRACTIONS IN A VINEYARD SOIL

The aim of this work is to assess the effect of planting patterns on the spatial distribution of total copper and other Cu fractions in vineyard soils. Both classical and geostatistical tools were used for the study. The soil of the plot had a loam texture and was strongly acid. The mean total Cu concentration (Cu_T) was 368 mg kg⁻¹. The mean value of potential available fractions was 188 mg kg⁻¹ for Cu_EDTA and 122 mg kg⁻¹ for Cu_DPTA, whereas the mean exchangeable Cu (Cu_EX) was 5·2 mg kg⁻¹. All Cu measurements exhibited a wide variation. These values are very high compared with those found in non‐polluted soils, and they can affect the soil, plants and microorganisms. The best correlation for Cu_EX was with soil pH, whereas for Cu_EDTA, Cu_DPTA, and Cu_T, the best correlation was with soil organic carbon. Directional semivariograms were fitted with a spherical model (parallel to plant rows) and a periodic model (perpendicular) showing a dependence on orientation and distance. All Cu measurements were higher along plant rows than among them, finding a periodic pattern in the variance for the normal direction from plant rows. However, in site‐specific management, it is crucial not only to describe the pattern of variation but also to estimate the Cu content in the soil. Copper concentration maps were estimated by kriging interpolation. These maps show a higher Cu accumulation along the cultivated rows than the uncultivated rows. Copyright © 2011 John Wiley & Sons, Ltd.     

广东大宝山矿区土壤重金属污染

Soil contamination in the vicinity of the Dabaoshan Mine, Guangdong Province, China, was studied through determi- nation of total concentrations and chemical speciation of the toxic metals, Cu, Zn, Cd, and Pb, using inductively coupled plasma mass spectrometry. The results showed that over the past decades, the environmental pollution was caused by a combination of Cu, Zn, Cd, and Pb, with tailings and acid mine drainage being the main pollution sources affecting soils. Significantly higher levels （P ≤ 0.05） of Cu, Zn, Cd, and Pb were found in the tailings as compared with paddy, garden, and control soils, with averages of 1486, 2516, 6.42, and 429 mg kg^-1, respectively. These metals were continuously dispersed downstream from the tallings and waste waters, and therefore their concentrations in the paddy soils were as high as 567, 1 140, 2.48, and 191 mg kg^-1, respectively, being significantly higher （P ≤ 0.05） as compared with those in the garden soils. The results of sequential extraction of the above metals from all the soil types showed that the residual fraction was the dominant form. However, the amounts of metals that were bound to Fe-Mn oxides and organic matter were relatively higher than those bound to carbonates or those that existed in exchangeable forms. As metals could be transformed from an inert state to an active state, the potential environmental risk due to these metals would increase with time.     

Forms and profile distribution of potassium in typical soils of the lower Benue Valley of Nigeria

Abstract

Six profiles, derived from Precambrian Basement Complex rocks (mainly gneiss), Cretaceous sediments (mainly shale and sandstone), and Quaternary alluvium, and which are typical of the major agricultural soils in the Lower Benue Valley (Nigeria) were studied with the objective to determine their overall potassium (K) reserves and any relationship between these and other soil properties including their parent materials. Total K in the soils varies from 0.13–27.1 g kg^‐1 with average 6.64 g kg‐¹. This correlates positively with the clay, and negatively with the sand contents of the soils and is also influenced by their parent materials. The order of abundance according to parent material is: alluvium‐ > Basement Complex (gneiss)‐ ≈ shale‐ > sandstone‐derived soils. The concentrations of readily available K (RAK) in the soils are quite low, accounting for only between 0.30 and 7.8% of the total K in the soils and less than 4.0% of their exchange capacities. Based on critical limits established for many Nigerian soils, the soils derived from sandstone are clearly deficient in RAK, while soils developed from gneiss, shale and alluvium parent materials have moderate to sufficient levels for a wide range of crops. Non‐exchangeable or moderately available K (MAK) in the soils is also relatively low (0.020–8.59 mmol_c kg^‐1); while the sandstone‐derived soils have the least MAK, the alluvial soils have the most levels. However, the potassium supplying power (KSP) of the soils may be considered to be generally high. Although this bears no particular relationship to soil parent materials, the sandstone‐derived soils have the lowest KSP. The bulk of the total K reserves in the soils (55–88%) exists as difficultly available or structural K (DAK). The alluvial soils first, then the gneiss‐ and shale‐derived soils next have the highest contents of DAK, while the highly weathered sandstone soils have the lowest. Simple correlation analysis shows that, irrespective of parent material and K form, clay content and CEC are the most important soil properties influencing the overall K supplying status of these soils. It is concluded that in major agricultural soils of the Lower Benue Valley of Nigeria K exists mostly in the lattice structures of K‐bearing minerals, with accumulations in the subsurface horizons. Its plant‐available or supplying status is low on sandstone‐derived soils and moderate to sufficient on soils derived from Basement Complex rocks, shales and alluvium.     

Biochemical characterization of urban soil profiles from Stuttgart, Germany

The knowledge of biochemical properties of urban soils can help to understand nutrient cycling in urban areas and provide a database for urban soil management. Soil samples were taken from 10 soil profiles in the city of Stuttgart, Germany, differing in land use—from an essentially undisturbed garden area to highly disturbed high-density and railway areas. A variety of soil biotic (microbial biomass, enzyme activities) and abiotic properties (total organic C, elemental C, total N) were measured up to 1.9 m depth. Soil organic matter was frequently enriched in the subsoil. Microbial biomass in the top horizons ranged from 0.17 to 1.64 g C kg⁻¹, and from 0.01 to 0.30 g N kg⁻¹, respectively. The deepest soil horizon at 170-190 cm, however, contained 0.12 g C kg⁻¹ and 0.05 kg N kg⁻¹ in the microbial biomass. In general, arylsulphatase and urease activity decreased with depth but in three profiles potentially mineralizable N in the deepest horizons was higher than in soil layers directly overlying. In deeply modified urban soils, subsoil beside topsoil properties have to be included in the evaluation of soil quality. This knowledge is essential because consumption of natural soils for housing and traffic has to be reduced by promoting inner city densification.     

Depth distribution of aluminum and heavy metals in soils of Costa Rican coffee cultivation areas

Depth distributions of metals in soil profiles are indicative of weathering and soil genesis and anthropogenic pollution. We studied the depth distribution of total Al, Cd, Cu, Fe, Mn, Pb, and Zn concentrations in 8 Oxisols, 5 Andisols, 2 Mollisols, and 2 Alfisols of coffee plantation areas in Costa Rica. The concentrations of the mainly geo‐/pedogenic Al (means of 76 g kg^—1 in the A horizons and of 106 g kg^—1 in the lowermost sampled B horizons) and Fe (A: 56 g kg^—1, B: 66 g kg^—1) generally increased with profile depth. In spite of the regular application of Cu‐containing fungicides, Cu (A: 135 mg kg^—1, B: 158 mg kg^—1) showed accumulations in the A horizons of only three profiles. Higher Cd (A: 0.14 mg kg^—1, B: 0.09 mg kg^—1) and Pb concentrations (A: 7.3 mg kg^—1, B: 5.5 mg kg^—1) in most topsoils compared to the subsoils indicated anthropogenic inputs. The mean Mn (A: 1190 mg kg^—1, B: 1150 mg kg^—1) and Zn (A: 59 mg kg^—1, B: 66 mg kg^—1) concentrations varied little with depth. In general, the metal depth distribution in the studied tropical soils was similar to that of temperate soils although the weathering regime is quite different.     

Lead and Cadmium in Soils of La Rioja Vineyards,Spain

Land degradation can be triggered by the abuse of chemicals that damage soil quality. Agriculture is changing the chemical and physical properties of soils, and in vineyards, those changes are due to the use of pesticides. In order to assess the Pb and Cd content, 212 soil samples from La Rioja D.O.Ca were analysed. Concentrations of Pb in soil ranged from 0·96 to 64·31 mg kg⁻¹ with a mean concentration of 21·26 mg kg⁻¹ in the surface layer, while they ranged from 7·97–43·93 mg kg⁻¹ with a mean of 20·83 mg kg⁻¹ in the subsurface layer. The mean content of bioavailable lead was 1·03 mg kg⁻¹ in the surface layer and 0·76 mg kg⁻¹ in the subsurface. Cd overall average concentration was 0·29 mg kg⁻¹ in the surface; in the subsurface, the mean was 0·31 mg kg⁻¹ and ranged from 0·10 to 1·22 mg kg⁻¹. The values in the surface layers were 0·15 mg kg⁻¹ and in the subsurface layer 0·01 of Cd bioavailability. On the basis of pedogeochemical Pb and Cd distribution, balanced fertilization will be of great importance for sustainable development of agricultural wine‐producers. Copyright © 2016 John Wiley & Sons, Ltd.     

Adsorption complex,pH relationships in some acid mediterranean forest soil profiles

Acid soils in some mediterranean forests were investigated for the composition of the adsorption complex and the gradients in soil pH. The effective CEC (235–838 mmol_c kg^?1) and base saturation (93–98 %) are highest in ectorganic horizons. In the mineral horizons the effective CEC (23–52 mmol_c kg^?1) and base saturation (11–40 %) are much lower. The exchange complex of mineral horizons consists for 90 (AEh) to 40 percent (Bw2) of organic matter. The effective CEC of the mineral clay fraction is low (60 mmol_c kg^?1 clay). The clear trends in soil pH within the ectorganic layer of deciduous and sclerophyllous oak forests are attributed to vertical spatial separation of nitrogen mineralization (ammonification and strongly impeded nitrification) and nutrient uptake by roots (mainly NH₄). This leads to a high effective CEC in the fermentation layer and acidification of the uppermost part of the mineral soil. In contrast to the situation in temperate forests this process is impeded in mediterranean coniferous forests, where litter decomposition is extremely slow and both proton production and consumption take place in the organic rich mineral horizon.     

Effect of land‐use on soil nutrients and microbial biomass of an alpine region on the northeastern Tibetan plateau,China

Land‐use patterns affect the quantity and quality of soil nutrients as well as microbial biomass and respiration in soil. However, few studies have been done to assess the influence of land‐use on soil and microbial characteristics of the alpine region on the northeastern Tibetan plateau. In order to understand the effect of land‐use management, we examined the chemical properties and microbial biomass of soils under three land‐use types including natural grassland, crop‐field (50 + y of biennial cropping and fallow) and abandoned old‐field (10 y) in the area. The results showed that the losses of soil organic carbon (SOC) and total nitrogen (TN) were about 45 and 43 per cent, respectively, due to cultivation for more than 50 y comparing with natural grassland. Because of the abandonment of cultivation for about a decade, SOC and TN were increased by 27 and 23 per cent, respectively, in comparison with the crop field. Microbial carbon (ranging from 357·5 to 761·6 mg kg⁻¹ soil) in the old‐field was intermediate between the crop field and grassland. Microbial nitrogen (ranging from 29·9 to 106·7 mg kg⁻¹ soil) and respiration (ranging from 60·4 to 96·4 mg CO₂‐C g⁻¹ C_mic d⁻¹) were not significantly lower in the old‐field than those in the grassland. Thus it could be concluded that cultivation decreased the organic matter and microbial biomass in soils, while the adoption of abandonment has achieved some targets of grassland restoration in the alpine region of Gansu Province on the northeastern Tibetan plateau. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of changes in pH, ionic strength, and sulphate concentration on the CEC of temperate acid forest soils

As the acidity of rain diminishes, changes in the pH, ionic strength, and ion activities of the soil solution will influence the charge characteristics of soil. We have investigated the response of cation exchange capacity (CEC) of three acid forest soils of variable charge to small changes in pH, ionic strength, and SO^2?₄ concentration. The variable charge for these temperate soils has the same significance as for tropical soils and those from volcanic ash. Maximum absolute increase in CEC on increasing pH by 0·2–0·5 units reached 5 cmol_c kg^-1 in O horizons. The increase in CEC on doubling ionic strength in EA and Bsh horizons of a Cambic Podzol was about half that amount, but relative gains compared to effective CEC were 65 and 46%, respectively. For other soil horizons, absolute changes were smaller, and relative changes were between 10 and 30%. Halving the SO^2?₄ concentration significantly influenced CEC only in some samples. Both pH and ionic strength must be adjusted with care when determining CEC_c of acid forest soils. Decreasing acid deposition will not inevitably increase CEC_c because in some soils pH effects may be balanced by simultaneous decrease in ionic strength.     

Phosphorus mineralization in subarctic agricultural and forest soils

Summary Potential P and C mineralization rates were determined in a 12-week laboratory incubation study on subarctic forest and agricultural soil samples with and without N fertilizer added. There was no significant difference in net inorganic P produced between N fertilized and unfertilized soils. The forest soil surface horizons had the highest net inorganic P mineralized, 32 mg P kg^-1 soil for the Oie and 17 mg P kg^-1 soil for the Oa. In the cropped soils net inorganic P immobilization started after 4 weeks and lasted through 12 weeks of incubation. Cumulative CO₂–C evolution rates differed significantly among soils, and between fertilizer treatments, with the N-fertilized soils evolving lower rates of CO₂–C than the unfertilized soils. Soils from the surface horizons in the forest evolved the highest rates of CO₂–C (127.6 and 89.4 mg g^-1 soil for the Oie and Oa horizons, respectively) followed by the cleared uncropped soil (42.8 mg g^-1 soil C), and the cropped soils (25.4 and 29.0 mg g^-1 soil C). In vitro soil respiration rates, or potential soil organic matter decomposition rates, decreased with increasing time after clearing and in accord with the degree of disturbance. Only soils with high potential C mineralization rates and high organic P to total P ratios, mineralized P by the end of the study. Mineralizable P appeared to be associated with readily mineralizable organic C.     

Errata

The content of fixed ammonium was analyzed for 12 samples of upland soils, including Saline soils, Sols lessives, Meadow soils, Black-colored soils, and Dark-brown forest soils, collected from Jilin and Liaoning provinces, Northeast China. The content of fixed NH₄ ⁺ -N ranged from 0.11 to 0.27 g kg^-1 and no appreciable differences among the soil types were observed. Fixed NH₄ ⁺-N accounted for 9 to 23% of total N in the Ap horizons.