The reserve of weatherable primary silicates impacts the accumulation of biogenic silicon in volcanic ash soils

Banana plantlets (Musa acuminata cv Grande Naine) cultivated in hydroponics take up silicon proportionally to the concentration of Si in the nutrient solution (0–1.66 mM Si). Here we study the Si status of banana plantlets grown under controlled greenhouse conditions on five soils developed from andesitic volcanic ash, but differing in weathering stage. The mineralogical composition of soils was inferred from X-ray diffraction, elemental analysis and selective chemical/mineralogical extractions. With increasing weathering, the content of weatherable primary minerals decreased. Conversely, clay content increased and stable secondary minerals were increasingly dominant: gibbsite, Fe oxides, allophane, halloysite and kaolinite. The contents of biogenic Si in plant and soil were governed by the reserve of weatherable primary minerals. The largest concentrations of biogenic Si in plant (6.9–7 g kg⁻¹) and soil (50–58 g kg⁻¹) occurred in the least weathered soils, where total Si content was above 225 g kg⁻¹. The lowest contents of biogenic Si in plant (2.8–4.3 g kg⁻¹) and soil (8–31 g kg⁻¹) occurred in the most weathered desilicated soils enriched with secondary oxides and clay minerals. Our data imply that soil weathering stage directly impacted the soil-to-plant transfer of silicon, and thereby the stock of biogenic Si in a soil–plant system involving a Si-accumulating plant. They further imply that soil type can influence the silicon soil–plant cycle and its hydrological output.     

Effects of forest conversion into grassland on soil aggregate structure and carbon storage in Panama: evidence from soil carbon fractionation and stable isotopes

Land-use and land-cover strongly influence soil properties such as the amount of soil organic carbon (SOC), aggregate structure and SOC turnover processes. We studied the effects of a vegetation shift from forest to grassland 90 years ago in soils derived from andesite material on Barro Colorado Island (BCI), Panama. We quantified the amount of carbon (C) and nitrogen (N) and determined the turnover of C in bulk soil, water stable aggregates (WSA) of different size classes (<53 μm, 53–250 μm, 250–2000 μm and 2000–8000 μm) and density fractions (free light fraction, intra-aggregate particulate organic matter and mineral associated soil organic C). Total SOC stocks (0–50 cm) under forest (84 Mg C ha⁻¹) and grassland (64 Mg C ha⁻¹) did not differ significantly. Our results revealed that vegetation type did not have an effect on aggregate structure and stability. The investigated soils at BCI did not show higher C and N concentrations in larger aggregates, indicating that organic material is not the major binding agent in these soils to form aggregates. Based on δ¹³C values and treating bulk soil as a single, homogenous C pool we estimated a mean residence time (MRT) of 69 years for the surface layer (0–5 cm). The MRT varied among the different SOC fractions and among depth. In 0–5 cm, MRT of intra-aggregate particulate organic matter (iPOM) was 29 years; whereas mineral associated soil organic C (mSOC) had a MRT of 124 years. These soils have substantial resilience to C and N losses because the >90% of C and N is associated with mSOC, which has a comparatively long MRT.     

Degradation of p-nitrophenol and pentachlorophenol mixtures by Sphingomonas sp. UG30 in soil perfusion bioreactors

The degradation of mixtures of pentachlorophenol (PCP) and p-nitrophenol (PNP) were evaluated in pure cultures of Sphingomonas sp. UG30, statically incubated soils (60% water-holding capacity) and soil perfusion bioreactors where encapsulated cells of UG30 were used as a soil inoculant. In pure-culture studies, conditions were optimized for mineralization of PCP and PNP mixtures at concentrations of 30 mg l⁻¹ each. Optimum in vitro mineralization of PCP and PNP mixtures by UG30 was facilitated using ammonium phosphate as a nitrogen source, while inhibition was observed with ammonium nitrate. The bioreactor system used columns containing soil treated with mixtures of 100, 225 or 500 mg kg⁻¹ of PCP and PNP. Rapid dissipation of both substrates was observed at the 100 mg kg⁻¹ level. Inoculation with UG30 enhanced PCP degradation at the 100 mg kg⁻¹ level in bioreactors but not in static soil microcosms. At higher PCP and PNP concentrations (225 mg kg⁻¹), occasional complete degradation of PNP was observed, and PCP degradation was about 80% compared to about 25% in statically incubated soils after 20 days at 22°C. There was no additional degradation of the PCP and PNP mixtures attributable to inoculation with encapsulated cells of UG30 in either soil system at concentrations of 225 or 500 mg kg⁻¹. Journal of Industrial Microbiology & Biotechnology (2000) 25, 93–99. Received 25 February 2000/ Accepted in revised form 07 June 2000     

Effects of atmospheric CO2 enrichment on δ 13C, δ 15N values and turnover times of soil organic matter pools isolated by thermal techniques

CO₂ applied for Free-Air CO₂ Enrichment (FACE) experiments is strongly depleted in ¹³C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ ¹³C value. Simultaneous use of ¹⁵N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO₂ (540 ppm; for 3 years) treatments were analyzed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ ¹³C and δ ¹⁵N by IRMS. Energy consumption and mass losses in the temperature range 20–200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200–310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO₂, respectively. δ ¹³C values of SOM pools under elevated CO₂ treatment showed an increase from −34.3‰ of the pool decomposed between 20–200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20–200 and 200–310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310–400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO₂ treatments was not significantly different in SOM pools decomposed up to 480°C having MRT of about 60 years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400 years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. Responsible Editor: Bernard Nicolardot.     

Soil particle accumulation in termite (Macrotermes bellicosus) mounds and the implications for soil particle dynamics in a tropical savanna Ultisol

This study investigated the influence of mound-building termites on soil particle dynamics on the land surface and in soil-forming processes by examining the amount of soil particles in mound structures of Macrotermes bellicosus in a highly weathered Ultisol of tropical savanna. Soil particle turnover via the mounds was estimated using particle stock data and soil turnover data from previous studies. A 4-ha study plot with six mounds of relatively uniform shape and size was investigated. Soil mass constituting the mounds was 6,166 ± 1,581 kg mound⁻¹ within which the mound wall and nest body accounted for 5,002 ± 1,289 and 1,164 ± 293 kg, respectively. The mound wall contained a significantly larger amount of clay (252 ± 9.97 g kg⁻¹) balanced with a lower sand content (676 ± 26.5 g kg⁻¹) than in the adjacent surface (Ap1) horizon, (46.4 ± 12.8 g clay kg⁻¹; 866 ± 83.2 g sand kg⁻¹); the nest body had much higher clay content (559 ± 51.0 g kg⁻¹) but less sand (285 ± 79.2 g kg⁻¹) than the mound wall. As a result, the mounds of M. bellicosus accumulated clay of 2,874 ± 781 kg ha⁻¹ (corresponding to 2.52% of clay stock in the Ap1 horizon) along with an estimated clay turnover rate of 169 kg ha⁻¹ year⁻¹. These findings suggest a positive feedback effect from termite mound-building activity on soil particle dynamics in tropical savanna ecosystems: M. bellicosus preferentially use subsoil material for mound construction, resulting in relocation of illuvial clay in the subsoil to the land surface where clay eluviation from the surface soil and its illuviation in the subsoil are major soil-forming processes.     

Methane uptake rates in Japanese forest soils depend on the oxidation ability of topsoil, with a new estimate for global methane uptake in temperate forest

To clarify the reason for the higher CH₄ uptake rate in Japanese forest soils, twenty-seven sites were established for CH₄ flux measurement. The first order rate constant for CH₄ uptake was also determined using soil core incubation at 14 sites. The CH₄ uptake rate had a seasonal fluctuation, high in summer and low in winter, and the rate correlated with soil temperature at 17 sites. The annual CH₄ uptake rates ranged from 2.7 to 24.8 kg CH₄ ha⁻¹ y⁻¹ (the average of these rates was 9.7 or 10.9 kg CH₄ ha⁻¹ y⁻¹, depending on method of calculation), which is somewhat higher than the uptake rates reported in previous literature. The averaged CH₄ uptake rate correlated closely with the CH₄ oxidation rate of the topsoil (0–5 cm) in the study sites. The CH₄ oxidation constant of the topsoil was explained by a multiple regression model using total pore volume of the soil, nitrate content, and C/N ratio (p < 0.05, R ² = 0.684). This result and comparison with literature data suggest that the high CH₄ uptake rate in Japanese forest soils depends on the high porosity probably due to volcanic ash parent materials. According to our review of the literature, the CH₄ uptake rate in temperate forests in Europe is significantly different from that in Asia and North America. A new global CH₄ uptake rate in temperate forests was estimated to be 5.4 Tg y⁻¹ (1 SE is 1.1 Tg y⁻¹) on a continental basis.     

A Quantitative Model of Soil Organic Matter Accumulation During Floodplain Primary Succession

Texture is an important influence on organic matter (SOM) dynamics in upland soils but little is known about its role in riverine soils. We hypothesized that texture might be especially important to SOM accumulation in young alluvial soils. We combined the soil component of the CENTURY ecosystem model, which uses sand, silt, and clay concentration as primary variables, with a simple simulation model of fluvial deposition, and forest production to predict changes in soil carbon (C) and nitrogen (N) during primary succession on floodplains and terraces of the Queets River, Washington. Simulated soil C accumulated to a plateau of about 4000 g m⁻² at 110 years, closely matching observed patterns in an empirical chronosequence. Although direct fluvial OM deposition had only a small and short-lived influence on soil C, fluvial silt and clay deposition were an important influence on equilibrium C. The model underestimated soil N by about 35%, which appears to be due to failure of the model to account for N enrichment of an OM pool after its initial formation. These results suggest that basic influences on SOM retention in these young soils are not functionally different than those that apply to upland soils, but occur within highly dynamic physical contexts. Overbank deposition of silt and clay establishes a basic capacity for SOM retention. SOM, in turn, facilitates N retention. In this way, silt and clay are instrumental in propagating N forward from N-fixing red alder (Alnus rubra) stands to mature conifer forests that are frequently N-limited.     

Gaps and Soil C Dynamics in Old Growth Northern Hardwood–Hemlock Forests

Old growth forest soils are large C reservoirs, but the impacts of tree-fall gaps on soil C in these forests are not well understood. The effects of forest gaps on soil C dynamics in old growth northern hardwood–hemlock forests in the upper Great Lakes region, USA, were assessed from measurements of litter and soil C stocks, surface C efflux, and soil microbial indices over two consecutive growing seasons. Forest floor C was significantly less in gaps (19.0 Mg C ha⁻¹) compared to gap-edges (39.5 Mg C ha⁻¹) and the closed forest (38.0 Mg C ha⁻¹). Labile soil C (coarse particulate organic matter, cPOM) was significantly less in gaps and edges (11.1 and 11.2 Mg C ha⁻¹) compared to forest plots (15.3 Mg C ha⁻¹). In situ surface C efflux was significantly greater in gaps (12.0 Mg C ha⁻¹ y⁻¹) compared to edges and the closed forest (9.2 and 8.9 Mg C ha⁻¹ y⁻¹). Microbial biomass N (MBN) was significantly greater in edges (0.14 Mg N ha⁻¹) than in the contiguous forest (0.09 Mg N ha⁻¹). The metabolic quotient (qCO₂) was significantly greater in the forest (0.0031 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹) relative to gaps or edges (0.0014 mg CO₂ h⁻¹ g⁻¹/mg MBC g⁻¹). A case is made for gaps as alleviators of old growth forest soil C saturation. Relative to the undisturbed closed forest, gaps have significantly less labile C, significantly greater in situ surface C efflux, and significantly lower decreased qCO₂ values.     

Influence of maize root mucilage on soil aggregate stability

This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg⁻¹ dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting cycle after 21 d. Evolved CO₂ was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil. In mucilage-amended soils CO₂ evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase in evolved CO₂ up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of these established organo-mineral bounds.     

Reversibility of phosphorus sorption by ferruginous nodules

Ferruginous nodules sorb significant amounts of available soil and fertiliser phosphate. The effect of this sorption on phosphorus availability of an agricultural soil was tested by sequential extraction and by exhaustive cropping with millet (Pennisetum typhoides) in a greenhouse trial following fertilisation of the original soil containing 70% nodules and of prepared samples containing various mixes of separated soil fines and nodules. Phosphorus sorption maxima by the soil fines and nodules were 190 mg kg⁻¹ and 380 mg kg⁻¹ respectively. Samples of fines and nodules which had sorbed 110 and 194 mg kg⁻¹ were submitted to 8 successive extractions with 0.01 M KCl, after which P desorption amounted to 117 mg kg⁻¹ and 103 mg kg⁻¹ respectively. Hysteresis between sorption and desorption was negligible for the soil fines and increased with increasing nodule content of the samples. In the greenhouse experiment, P uptake at the first cropping was highest in the soil fines at all levels of phosphorus applied. Subsequent croppings, however, showed higher P uptake in the concretionary soils. These results indicate a higher initial P release from the soil fines with cropping followed by an earlier exhaustion of phosphorus. At the end of the greenhouse experiment, yields were low in spite of the large quantities of P still remaining in the soils. Phosphorus fractionation showed that, of the P left in the soil after cropping 20% was in labile, 29% in Fe or Al-associated, and 51% in low-availability forms.     

Insecticide-tolerant and plant growth promoting <Emphasis Type="Italic">Bradyrhizobium</Emphasis> sp. (<Emphasis Type="Italic">vigna</Emphasis>) improves the growth and yield of greengram [<Emphasis Type="Italic">Vigna radiata</Emphasis> (L.) Wilczek] in insecticide-stressed soils

This study was designed to identify rhizobial strains specific to greengram expressing higher tolerance against insecticides, fipronil and pyriproxyfen, and synthesizing plant growth regulators even amid insecticide-stress. Of the 50 bradyrhizobial isolates, the Bradyrhizobium sp. strain MRM6 showed tolerance up to 1,600 μg mL⁻¹ against each of fipronil and pyriproxyfen. The tolerant Bradyrhizobium sp. (vigna) produced plant growth promoting substances in substantial amounts, both in the presence and absence of insecticides. The strain MRM6 was further used to investigate its impact on greengram grown in soils treated with 200 (the recommended dose), 400 and 600 μg kg⁻¹ soil of fipronil and 1,300 (the recommended dose), 2,600 and 3,900 μg kg⁻¹ soil of pyriproxyfen. Fipronil at 600 μg kg⁻¹ soils and pyriproxyfen at 3,900 μg kg⁻¹ soils had greatest toxic effects and decreased plant biomass, symbiotic efficiency, nutrient uptake and seed yield of greengram plants. The Bradyrhizobium sp. (vigna) inoculant when used with fipronil and pyriproxyfen significantly increased the measured parameters compared to the plants grown in soils treated solely with the same concentration of each insecticide. This study inferred that the Bradyrhizobium sp. (vigna) strain MRM6 may be exploited as bio-inoculant to increase the productivity of greengram exposed to insecticide-stressed soils.     

Effects of nutrient additions on ecosystem carbon cycle in a Puerto Rican tropical wet forest

Wet tropical forests play a critical role in global ecosystem carbon (C) cycle, but C allocation and the response of different C pools to nutrient addition in these forests remain poorly understood. We measured soil organic carbon (SOC), litterfall, root biomass, microbial biomass and soil physical and chemical properties in a wet tropical forest from May 1996 to July 1997 following a 7‐year continuous fertilization. We found that although there was no significant difference in total SOC in the top 0–10 cm of the soils between the fertilization plots (5.42±0.18 kg m^?2) and the control plots (5.27±0.22 kg m^?2), the proportion of the heavy‐fraction organic C in the total SOC was significantly higher in the fertilized plots (59%) than in the control plots (46%) (P<0.05). The annual decomposition rate of fertilized leaf litter was 13% higher than that of the control leaf litter. We also found that fertilization significantly increased microbial biomass (fungi+bacteria) with 952±48 mg kg^?1soil in the fertilized plots and 755±37 mg kg^?1soil in the control plots. Our results suggest that fertilization in tropical forests may enhance long‐term C sequestration in the soils of tropical wet forests.     

亚热带不同林分土壤表层有机碳组成及其稳定性

在浙江临安玲珑山选取了常绿阔叶林、马尾松林、板栗林和雷竹林4种林分,采用传统的化学方法与固态13C核磁共振(NMR)技术研究其土壤有机碳在不同粒径土壤颗粒中的分布规律和结构特征,探讨林分类别和管理措施对土壤有机碳含量及其结构的影响,为亚热带地区森林固碳和土壤碳库管理提供科学依据。结果显示:(1)土壤表层(0—20 cm)有机碳含量按以下次序递减:雷竹林>常绿阔叶林>马尾松林>板栗林,且板栗林以粉黏粒结合态碳为主,其他林分土壤则以粗砂结合态碳为主;(2)13C NMR结果表明,阔叶林和马尾松林土壤有机碳中烷基碳所占比例最大,而雷竹林和板栗林则是烷氧碳比例最大,表明人工经营措施改变了土壤有机碳的成分组成;(3)随着土壤颗粒变细,有机碳中烷基碳比例增加,烷氧碳比例减少,A/O-A值和疏水碳/亲水碳值逐渐增大,表明颗粒越细,其结合的有机碳结构稳定性越高。     

Nitrogen Release from Decomposing Residues of Leguminous Cover Crops and their Effect on Maize Yield on Depleted Soils of Bukoba District, Tanzania

Nitrogen release patterns from decomposing shoot residues of Tephrosia candida, Crotalaria grahamiana, Mucuna pruriens, Macrotyloma axillare, Macroptillium atropurpureum and Desmodium intortum were studied in the laboratory for a period of 22 weeks in a sandy clay soil and 10 weeks in a clay soil using a leaching tube technique. The residual effect of soil incorporated shoot residues of T. candida, T. vogelii, C. grahamiana, M. pruriens and C. juncea on maize yield was evaluated at four sites each in the high rainfall zone (mean precipitation 2100 mm year⁻¹) and low rainfall zone (mean precipitation 800 mm year⁻¹) of Bukoba District, Tanzania. N mineralised from the legume residues ranged from 24 to 61% of the initial N after 22 weeks in a sandy clay soil and −1 to 34% after 10 weeks in a clay soil. The N mineralisation rates of the residues decreased in both soils in the order M. atropurpureum>M. axillare>C. grahamiana>D. intortum>T.␣candida>M. pruriens and were mostly strongly related to (polyphenols+lignin)-to-N ratio, lignin-to-N ratio and lignin. Relative to the control, legume residues resulted in two and threefold increase in maize grain yield i.e. from 1.1 to 3.2 Mg ha⁻¹ and from 1.4 to 3.8 Mg ha⁻¹ in a high and low rainfall zone respectively. However, maize yield response to legume residues was limited when compared with application of 50 kg N ha⁻¹ of mineral fertiliser. The % fertiliser equivalency (%FE) of legumes ranged between 25 and 59% with higher values recorded with C. grahamiana. At harvest, apparent N recoveries in maize ranged between 23 and 73% of the N applied in the legume residues. Highest recovery was found with application of C. grahamiana and least recovery from T. candida residues. These results suggested that application of legume residues alone might not be sufficient to meet N requirements and to achieve the yield potential of maize crop in Bukoba soils unless supplemented with small doses of mineral fertilisers.     

Biometric Based Carbon Flux Measurements and Net Ecosystem Production (NEP) in a Temperate Deciduous Broad-Leaved Forest Beneath a Flux Tower

Biometric based carbon flux measurements were conducted over 5 years (1999–2003) in a temperate deciduous broad-leaved forest of the AsiaFlux network to estimate net ecosystem production (NEP). Biometric based NEP, as measured by the balance between net primary production (including NPP of canopy trees and of forest floor dwarf bamboo) and heterotrophic respiration (RH), clarified the contribution of various biological processes to the ecosystem carbon budget, and also showed where and how the forest is storing C. The mean NPP of the trees was 5.4 ± 1.07 t C ha⁻¹ y⁻¹, including biomass increment (0.3 ± 0.82 t C ha⁻¹ y⁻¹), tree mortality (1.0 ± 0.61 t C ha⁻¹ y⁻¹), aboveground detritus production (2.3 ± 0.39 t C ha⁻¹ y⁻¹) and belowground fine root production (1.8 ± 0.31 t C ha⁻¹ y⁻¹). Annual biomass increment was rather small because of high tree mortality during the 5 years. Total NPP at the site was 6.5 ± 1.07 t C ha⁻¹ y⁻¹, including the NPP of the forest floor community (1.1 ± 0.06 t C ha⁻¹ y⁻¹). The soil surface CO₂ efflux (RS) was averaged across the 5 years of record using open-flow chambers. The mean estimated annual RS amounted to 7.1 ± 0.44 t C ha⁻¹, and the decomposition of soil organic matter (SOM) was estimated at 3.9 ± 0.24 t C ha⁻¹. RH was estimated at 4.4 ± 0.32 t C ha⁻¹ y⁻¹, which included decomposition of coarse woody debris. Biometric NEP in the forest was estimated at 2.1 ± 1.15 t C ha⁻¹ y⁻¹, which agreed well with the eddy-covariance based net ecosystem exchange (NEE). The contribution of woody increment (Δbiomass + mortality) of the canopy trees to NEP was rather small, and thus the SOM pool played an important role in carbon storage in the temperate forest. These results suggested that the dense forest floor of dwarf bamboo might have a critical role in soil carbon sequestration in temperate East Asian deciduous forests.     

Forest- and pasture-derived carbon contributions to carbon stocks and microbial respiration of tropical pasture soils

The clearing of tropical forest for pasture leads to important changes in soil organic carbon (C) stocks and cycling patterns. We used the naturally occurring distribution of¹³C in soil organic matter (SOM) to examine the roles of forest- and pasture-derived organic matter in the carbon balance in the soils of 3- to 81-year-old pastures created following deforestation in the western Brazilian Amazon Basin state of Rondônia. Different ¹³C values of C3 forest-derived C (-28) and C4 pasture-derived C (-13) allowed determination of the origin of total soil C and soil respiration. The ¹³C of total soil increased steadily across ecosystems from -27.8 in the forest to -15.8 in the 81-year-old pasture and indicated a replacement of forest-derived C with pasture-derived C. The ¹³C of respired CO₂ increased more rapidly from -26.5 in the forest to -17 in the 3- to 13-year-old pastures and indicated a faster shift in the origin of more labile SOM. In 3-year-old pasture, soil C derived from pasture grasses made up 69% of respired C but only 17% of total soil C in the top 10 cm. Soils of pastures 5 years old and older had higher total C stocks to 30 cm than the original forest. This occurred because pasture-derived C in soil organic matter increased more rapidly than forest-derived C was lost. The increase of pasture-derived C in soils of young pastures suggests that C inputs derived from pasture grasses play a critical role in development of soil C stocks in addition to fueling microbial respiration. Management practices that promote high grass production will likely result in greater inputs of grass-derived C to pasture soils and will be important for maintaining tropical pasture soil C stocks.     

Carbohydrates in water-stable aggregates and particle size fractions of forested and cultivated soils in two contrasting tropical ecosystems

Information on changes in storage and loss of soil organic carbon (SOC) when tropical forests are converted to cropland is needed for evaluating soil structural degradation and for selecting appropriate sustainable soil management practices. We evaluated changes in SOC storage of organic carbon and acid-hydrolyzable carbohydrates content of aggregated classes and particle size fractions of adjacent forested and cultivated soils in eight agroecosystems from Ethiopian highlands and Nigerian lowlands. In all agroecosystems, SOC content was two to four times higher in the forested than the cultivated soils. Higher SOC content was found in Ethiopian (20.2–47.3 g.kg^–1) than Nigerian (12.0–24.0 g.kg^–1) forested soils. The magnitude of reduction in SOC and total carbohydrates with cultivation was soil-specific, being generally higher in the sandy than the clayey soils. The smaller aggregate classes (< 1.00 mm) and the sand-sized particles (2000–63 µm) of the forested soils were preferentially enriched in carbohydrates relative to larger aggregates (4.75–1.00 mm). Carbohydrates were more concentrated in the clay-size fraction of the forested than in that of the cultivated soils. Cultivation reduced aggregate stability, increased the proportions of the smaller size aggregates and their associated carbohydrates relative to the forested soils. The susceptibility of the cultivated soils to loss in structural stability reflected this initial aggregation which was greater in the more stable clayey than the fragile sandy soils. The aggregate stability of either the forested or the cultivated soil could not be accounted for by the levels of OC or total carbohydrates in the soil.     

Isotopic signatures and nutrient relations of plants inhabiting brackish wetlands in the northeastern coastal plain of Venezuela

The semi-diurnal tidal regime (≥2 m) in the Paria Gulf on the Atlantic coast of Venezuela, and the flat landscape of the region, allow the penetration for tens of km of marine waters into the rivers draining the northeastern coastal plain of the country. The levels of salinity, tidal flooding, and sedimentation decrease perpendicularly from the river channel toward the back swamps. The vegetation varies sequentially from fringe mangroves along the river margins, to back swamps containing forests dominated by Pterocarpus officinalis, herbaceous communities of Lagenocarpus guianensis, and palm swamps with Mauritia flexuosa, Chrysobalanus icaco, and Tabebuia spp. This environmental structure was used to test the hypotheses that: (a) mangrove distribution is strongly associated with salinity of interstitial water, and (b) they occupy areas where tidal influence and sediment dynamics determine a relatively open N cycle. Analyses of soil, water, and plants along a 1.5 km transect located near the confluence of the Guanoco and San Juan Rivers (Sucre and Monagas States, Venezuela) revealed that: (a) conductivity decreased from 11 to 0.2 mmhos cm⁻¹ from the river fringe to the internal swamp, whereas Na in the same stretch decreased from 100 to 2 μM; (b) average leaf tissue concentrations of Na, P, and N decreased significantly along the transect; (c) P. officinalis showed a large Na-exclusion capacity indicated by positive K/Na ratios from 8 to 200, and Crinum erubescens counteracted Na by accumulating K above 1,000 mmol kg⁻¹; (d) leaves varied widely in δ ¹³C (−25.5 to −32‰) and δ ¹⁵N (4 to −10.5‰) values. Samples were aggregated according to soil carbon content corresponding to those of the mangrove forest belt (5–28 mol C kg⁻¹; 0–650 from river fringe) and those of the back swamps (40–44 mol C kg⁻¹; 700–1,500 m from river fringe). The concentrations of Na, P, and N (in mmol kg⁻¹) and δ ¹⁵N values (in ‰) were significantly higher in the mangrove forest compared to the back swamp (Na 213 vs. 88; P 41 vs. 16; N 1,535 vs. 727; δ ¹⁵N 1.5 vs. −3.7), indicating that the fringe forest was not nutrient limited. These results support the hypotheses that mangroves are restricted to the more-saline sections of the transect, and that the fringe forest has a more open N cycle, favoring ¹⁵N accumulation within the system.     

Bioremediation of atrazine-contaminated soil by repeated applications of atrazine-degrading bacteria

Bioaugmentation has previously been unreliable for the in situ clean-up of contaminated soils because of problems with poor survival and the rapid decline in activity of the bacterial inoculum. In an attempt to solve these problems, a 500-l batch fermenter was investigated for its ability to deliver inoculum repeatedly to contaminated soils via irrigation lines. In a field experiment, mesocosms were filled with 350 kg soil containing 100 mg kg⁻¹ atrazine, and inoculated one, four or eight times with an atrazine-degrading bacterial consortium that was produced in the fermenter. After 12 weeks, no significant degradation of atrazine had occurred in soil that was inoculated only once; whereas, mesocosms inoculated four and eight times mineralized 38% and 72% of the atrazine respectively. Similar results were obtained in a laboratory experiment using soil contaminated with 100 mg kg⁻¹ [¹⁴C]atrazine. After 35 days, soil that was inoculated once with 10⁸ cfu ml⁻¹ of the consortium or with the atrazine-degrading bacterium, Pseudomonas sp. strain ADP, mineralized 17% and 35% of the atrazine respectively. In comparison, microcosms inoculated every 3 days with the consortium or with Pseudomonas sp. (ADP) mineralized 64% or 90% of the atrazine over this same period. Results of these experiments suggest that repeated inoculation from an automated fermenter may provide a strategy for bioaugmentation of contaminated soil with xenobiotic-degrading bacteria. Received: 20 November 1998 / Received revision: 8 February 1999 / Accepted: 12 February 1999     

Decline of soil nitrogen mineralization and nitrification during forest conversion of evergreen broad-leaved forest to plantations in the subtropical area of Eastern China

We examined soil nitrogen (N) mineralization and nitrification rates, and soil and forest floor properties in one native forest: evergreen broad-leaved forest (EBLF), one secondary shrubs (SS), and three adjacent plantation forests: Chinese fir plantation (CFP), bamboo plantation (BP) and waxberry groves (WG) in Tiantong National Forest Park, Eastern China. All forests showed seasonal dynamics of N mineralization and nitrification rates. Soil N mineralization rate was highest in EBLF (1.6 ± 0.3 mg-N kg⁻¹ yr⁻¹) and lowest in CFP (0.4 ± 0.1 mg-N kg⁻¹ yr⁻¹). Soil nitrification rate was also highest in EBLF (0.6 ± 0.1 mg-N kg⁻¹ yr⁻¹), but lowest in SS (0.02 ± 0.01 mg-N kg⁻¹ yr⁻¹). During forest conversion of EBLF to SS, CFP, BP and WG, soil N mineralization rate (10.7%, 73%, 40.3% and 69.8%, respectively), soil nitrification rate (94.9%, 32.2%, 33.9% and 39%, respectively), and soil N concentration (50%, 65.4%, 78.9% and 51.9%, respectively) declined significantly. Annual soil N mineralization was positively correlated with total C and N concentrations of surface soil and total N concentration of forest floor, and negatively correlated with soil bulk density, soil pH and C:N ratio of forest floor across the five forests. Annual soil nitrification was positively correlated with total C concentration of surface soil and N concentration of forest floor, and negatively correlated with soil bulk density and forest floor mass. In contrast, annual soil nitrification was not correlated to pH value, total N concentration, C:N ratio of surface soil and total C concentration and C:N ratio of forest floor.