National-scale estimation of changes in soil carbon stocks on agricultural lands

Average annual net change in soil carbon stocks under past and current management is needed as part of national reporting of greenhouse gas emissions and to evaluate the potential for soils as sinks to mitigate increasing atmospheric CO2. We estimated net soil C stock changes for US agricultural soils during the period from 1982 to 1997 using the IPCC (Intergovernmental Panel on Climate Change) method for greenhouse gas inventories. Land use data from the NRI (National Resources Inventory; USDA-NRCS) were used as input along with ancillary data sets on climate, soils, and agricultural management. Our results show that, overall, changes in land use and agricultural management have resulted in a net gain of 21.2 MMT C year(-1) in US agricultural soils during this period. Cropped lands account for 15.1 MMT C year(-1), while grazing land soil C increased 6.1 MMT C year(-1). The land use and management changes that have contributed the most to increasing soil C during this period are (1) adoption of conservation tillage practices on cropland, (2) enrollment of cropland in the Conservation Reserve Program, and (3) cropping intensification that has resulted in reduced use of bare fallow.     

Changes in the carbon and nitrogen status of forest soil organic horizons between 1949/50 and 1987

Forest soil organic horizons from 15 profiles in NE Scotland originally sampled in 1949/50, were resampled in 1987. Analyses of both sets of soils for organic C and N show that although concentrations of the two elements have decreased with time, there has been a large increase in storage due to an increase in O horizon thickness. In most cases surface organic horizons have become more acid between 1949/50 and 1987. Calculated mean accumulation rates for C and N are 353.4 kg ha(-1) year(-1) and 21.2 kg ha(-1) year(-1) respectively. Changes in the C/N ratio with time give no indication of progressive N saturation and suggest sudden breakthrough of N in drainage water is not imminent.     

Influence of livestock grazing on C sequestration in semi-arid mixed-grass and short-grass rangelands

We evaluated the effects of livestock grazing on C content of the plant-soil system (to 60 cm) of two semi-arid grasslands: a mixed-grass prairie (grazed 12 years), and a short-grass steppe (grazed 56 years). Grazing treatments included season-long grazing at heavy and light stocking rates, and non-grazed exclosures. Significantly higher soil C (0-30cm) was measured in grazed pastures compared to non-grazed exclosures, although for the short-grass steppe higher soil C was observed with the heavy grazing treatment only. Excluding grazing caused an immobilization of C in excessive aboveground plant litter, and an increase in annual forbs and grasses which lack dense fibrous rooting systems conducive to soil organic matter formation and accumulation. Our data indicate that higher soil C with grazing was in part the result of more rapid annual shoot turnover, and redistribution of C within the plant-soil system as a result of changes in plant species composition.     

Soil carbon dynamics in cropland and rangeland

Most soils in the Midwestern USA have lost 30 to 50% of their original pool, or 25 to 40 Mg C/ha, upon conversion from natural to agricultural ecosystems. About 60 to 70% of the C thus depleted can be resequestered through adoption of recommended soil and crop management practices. These practices include conversion from plow till to no till, frequent use of winter cover crops in the rotation cycle, elimination of summer fallow, integrated nutrient management along with liberal use of biosolids and biological nitrogen fixation, precision farming to minimize losses and enhance fertilizer use efficiency, and use of improved varieties with ability to produce large root biomass with high content of lignin and suberin. The gross rate of soil organic carbon (SOC) sequestration ranges from 500 to 800 kg/ha/year in cold and humid regions and 100 to 300 kg/ha/year in dry and warm regions. The rate of SOC sequestration can be measured with procedures that are cost effective and credible at soil pedon level, landscape level, regional or national scale. In addition to SOC, there is also a large potential to sequester soil inorganic carbon (SIC) in arid and semi-arid regions. Soil C sequestration has numerous ancillary benefits. It is truly a win-win situation: extremely cost-effective, and a bridge to the future until alternative energy options take effect.     

Long-term effect of sewage sludge application on soil humic acids

Sewage sludges are used in agriculture because they act as a fertilizer. Long-term studies are needed to evaluate the effect of sewage sludge on soil properties by paying particular attention to the soil organic matter. Soil plots were amended for 10 years with 1Mg dry matter ha(-1)year(-1) of sewage sludge. Chemical parameters such as total organic carbon (TOC), N, C/N ratio and CEC were determined when this period ended. Moreover, TOC was fractionated into humified and non-humified fractions. Humic acids (HA) were isolated and studied by elemental analysis, DRIFT, (1)H NMR and CPMAS 13C NMR spectroscopies. At the end of the tests, compared to the control soil, the sludge-amended soil did not exhibit change in total organic C and related humified fractions. However, the HA composition of the soil treated with sludge had developed an HA composition closer to that of the HA-sludge as a result of the enrichment of recalcitrant fractions contained in the sludge.     

Dissolved inorganic nitrogen budget for a forested catchment at Beddgelert, North Wales

An input-output budget for dissolved inorganic-N in a small forested catchment in North Wales is presented. From 1982 to 1990, bulk precipitation inputs averaged 10.3 kg ha(-1) year(-1), whereas throughfall inputs in 1983-1984 were 20.3 kg ha(-1) year(-1). Streamwater outputs were consistently larger than bulk precipitation inputs, averaging 14.6 kg ha(-1) year(-1). Inorganic-N in the forest stream was predominantly nitrate and concentrations were substantially higher than in a nearby moorland stream. Both streams showed seasonal trends in nitrate concentration, with highest concentrations occurring in summer in the forest stream but in winter in the moorland stream. Nitrate concentration in the forest stream increased with increasing soil temperature up to approximately 7 degrees C and decreased at higher temperatures. Nitrification is thought to be responsible for nitrate production at temperatures both below and above 7 degrees C, but root uptake becomes significant only at the higher temperatures. In the forest, dry deposition and cloudwater inputs of inorganic-N are responsible for increased nitrogen fluxes in throughfall compared with wet deposition. Mineralization and nitrification in excess of plant needs causes the organic soil horizons to act as a net source of dissolved inorganic-N. Nitrogen transformations in the soil lead to soil acidification at a rate of 1.0 keq ha(-1) year(-1).     

Assessment of chemical, biochemical and ecotoxicological aspects in a mine soil amended with sludge of either urban or industrial origin

A greenhouse pot experiment was conducted to evaluate the effect of sewage sludge (SS), of sugar beet sludge (SBS), or of a combination of both, in the remediation of a highly acidic (pH 3.6) metal-contaminated soil, affected by mining activities. The SS was applied at 100 and 200 Mg ha(-1) (dry weight basis), and the SBS at 7 Mg ha(-1). All pots were sown with Italian ryegrass (Lolium multiflorum Lam.). After 60 d of growth, shoot biomass was quantified and analysed for Cu, Pb and Zn. The pseudo-total and bioavailable contents of Cu, Pb and Zn and the enzymatic activities of beta-glucosidase, acid phosphatase, cellulase, protease and urease were determined in the soil mixtures. Two indirect acute bioassays with leachates from the soil (luminescent inhibition of Vibrio fischeri and Daphnia magna immobilization) were also used. The SS, in particular when in combination with SBS, corrected soil acidity, while increasing the total organic matter content and the cation exchange capacity. The application of SS led to a decrease in the level of effective bioavailable metals (extracted by 0.01 M CaCl(2), pH 5.7, without buffer), but caused an increase in their potential bioavailability (extracted by a solution of 0.5M NH(4)CH(3)COO, 0.5 M CH(3)COOH and 0.01 M EDTA, pH 4.7). Plant biomass increased more than 10 times in the presence of 100 Mg SS ha(-1), and more than five times with the combined use of 100 Mg SS ha(-1) and SBS, but a considerable phytotoxic effect was observed for the application rate of 200 Mg SS ha(-1). Copper, Pb and Zn concentrations in the shoots of L. multiflorum decreased significantly when using 100 Mg SS ha(-1) or SBS. The activities of beta-glucosidase, urease and protease increased with increasing SS applications rates, but cellulase had a reduced activity when using 200 Mg ha(-1)SS. Both amendments were able to suppress soil toxicity to levels that did not affect D. magna, but increased the soil leachate toxicity towards V. fischeri, especially with the application of 200 Mg SS ha(-1). This study showed that for this type of mine soils, and when using SS of similar composition, the maximum SS application rate should be 100 Mg ha(-1), and that liming the SS amended soil with SBS did not contribute to a further improvement in soil quality.     

Carbon storage in northeast China as estimated from vegetation and soil inventories

We have estimated the stocks of carbon in vegetation and soil in northeast China based on data for 122 plots from the fourth national forest inventory, and for 388 soil profiles from the second national soil survey. The techniques of Geographic Information System (GIS) have been used to extrapolate site-specific estimates of vegetation and soil organic carbon to the entire area of northeast China. Our estimate indicates that the amount of carbon in vegetation and soil for the region are 2.81 PgC (10(15) g C) and 26.43 PgC, respectively, and that the area weighted average density of vegetation and soil organic carbon are 22.7 MgC/ha and 212.7 MgC/ha, respectively. The eastern and northern parts of the region show much higher carbon storage than the rest of the region. Substantial spatial variations in vegetation and soil organic carbon across northeast China suggest that regional estimates on carbon stocks and fluxes should take into account these spatial variations. We suggest that the methodology developed can be used for the entire nation of China as well as other regions of the world.     

Soil-extractable phosphorus and phosphorus saturation threshold in beef cattle pastures as affected by grazing management and forage type

Grazing can accelerate and alter the timing of nutrient transfer, and could increase the amount of extractable phosphorus (P) cycle from soils to plants. The effects of grazing management and/or forage type that control P cycling and distribution in pasture's resources have not been sufficiently evaluated. Our ability to estimate the levels and changes of soil-extractable P and other crop nutrients in subtropical beef cattle pastures has the potential to improve our understanding of P dynamics and nutrient cycling at the landscape level. To date, very little attention has been paid to evaluating transfers of extractable P in pasture with varying grazing management and different forage type. Whether or not P losses from grazed pastures are significantly greater than background losses and how these losses are affected by soil, forage management, or stocking density are not well understood. The objective of this study was to evaluate the effect of grazing management (rotational versus “zero” grazing) and forage types (FT; bahiagrass, Paspalum notatum, Flugge versus rhizoma peanuts, Arachis glabrata, Benth) on the levels of extractable soil P and degree of P saturation in beef cattle pastures. This study (2004–2007) was conducted at the Subtropical Agricultural Research Station, US Department of Agriculture–Agricultural Research Service located 7 miles north of Brooksville, FL. Soil (Candler fine sand) at this location was described as well-drained hyperthermic uncoated Typic Quartzipsamments. A split plot arrangement in a completely randomized block design was used and each treatment was replicated four times. The main plot was represented by grazing management (grazing vs. no grazing) while forage types (bahiagrass vs. perennial peanut) as the sub-plot treatment. Eight steel exclosures (10?×?10 m) were used in the study. Four exclosures were placed and established in four pastures with bahiagrass and four exclosures were established in four pastures with rhizoma peanuts to represent the “zero” grazing treatment. The levels of soil-extractable P and degree of P saturation (averaged across FT and soil depth) of 22.1 mg kg^?1 and 11.6 % in pastures with zero grazing were not significantly (p?≤?0.05) different from the levels of soil-extractable P and degree of P saturation of 22.8 mg kg^?1 and 12.9 % in pastures with rotational grazing, respectively. On the effect of FT, levels of soil-extractable P and degree of P saturation were significantly higher in pastures with rhizoma peanuts than in pastures with bahiagrass. There was no net gain of soil-extractable P due to the presence of animals in pastures with rotational grazing. Averaged across years, soil-extractable P in pastures with rotational grazing and with “zero” grazing was less than 150 mg kg^?1, the water quality protection. There had been no movement of soil-extractable P into the soil pedon since average degree of P saturation in the upper 15 cm was 14.3 % while the average degree of P saturation in soils at 15–30 cm was about 9.9 %. Overall, average extractable P did not exceed the crop requirement threshold of 50 mg P kg^?1 and the soil P saturation threshold of 25 %, suggesting that reactive P is not a problem. Our study revealed that rhizoma peanuts and bahiagrass differ both in their capacity to acquire nutrients from the soil and in the amount of nutrients they need per unit growth. Rhizoma peanuts, which are leguminous forage, would require higher amounts of P compared with bahiagrass. The difference in the amount of P needed by these forages could have a profound effect on their P uptake that can be translated to the remaining amount of P in the soils. Periodic applications of additional P may be necessary especially for pastures with rhizoma peanuts to sustain their agronomic needs and to potentially offset the export of P due to animal production. Addition of organic amendments could represent an important strategy to protect pasture lands from excessive soil resources exploitation.     

Soil carbon pools and fluxes in urban ecosystems

The transformation of landscapes from non-urban to urban land use has the potential to greatly modify soil carbon (C) pools and fluxes. For urban ecosystems, very little data exists to assess whether urbanization leads to an increase or decrease in soil C pools. We analyzed three data sets to assess the potential for urbanization to affect soil organic C. These included surface (0-10 cm) soil C data from unmanaged forests along an urban-rural gradient, data from "made" soils (1 m depth) from five different cities, and surface (0-15 cm) soil data of several land-use types in the city of Baltimore. Along the urban-rural land-use gradient, we found that soil organic matter concentration in the surface 10 cm varied significantly (P=0.001). In an analysis of variance, the urban forest stands had significantly (P=0.02) higher organic C densities (kg m(-2) to 1 m depth) than the suburban and rural stands. Our analysis of pedon data from five cities showed that the highest soil organic C densities occurred in loamy fill (28.5 kg m(-2)) with the lowest occurring in clean fill and old dredge materials (1.4 and 6.9 kg m(-2), respectively). Soil organic C densities for residential areas (15.5 +/- 1.2 kg m(-2)) were consistent across cities. A comparison of land-use types showed that low density residential and institutional land-uses had 44 and 38% higher organic C densities than the commercial land-use type, respectively. Our analysis shows that as adjacent land-use becomes more urbanized, forest soil C pools can be affected even in stands not directly disturbed by urban land development. Data from several "made" soils suggests that physical disturbances and inputs of various materials by humans can greatly alter the amount C stored in these soils.     

Cumulative impact of 40 years of industrial sulfur emissions on a forest soil in west-central Alberta (Canada)

The impact of 40 years of sulfur (S) emissions from a sour gas processing plant in Alberta (Canada) on soil development, soil S pools, soil acidification, and stand nutrition at a pine (Pinus contorta x Pinus banksiana) ecosystem was assessed by comparing ecologically analogous areas subjected to different S deposition levels. Sulfur isotope ratios showed that most deposited S was derived from the sour gas processing plant. The soil subjected to the highest S deposition contained 25.9 kmol S ha(-1) (uppermost 60 cm) compared to 12.5 kmol S ha(-1) or less at the analogues receiving low S deposition. The increase in soil S pools was caused by accumulation of organic S in the forest floor and accumulation of inorganic sulfate in the mineral soil. High S inputs resulted in topsoil acidification, depletion of exchangeable soil Ca2+ and Mg2+ pools by 50%, podzolization, and deterioration of N nutrition of the pine trees.     

Impact of soil movement on carbon sequestration in agricultural ecosystems

Recent modeling studies indicate that soil erosion and terrestrial sedimentation may establish ecosystem disequilibria that promote carbon (C) sequestration within the biosphere. Movement of upland eroded soil into wetland systems with high net primary productivity may represent the greatest increase in storage capacity potential for C sequestration. The capacity of wetland systems to capture sediments and build up areas of deposition has been documented as well as the ability of these ecosystems to store substantial amounts of C. The purpose of our work was to assess rates of sediment deposition and C storage in a wetland site adjacent to a small first-order stream that drains an agricultural area. The soils of the wetland site consist of a histosol buried by sediments from the agricultural area. Samples of deposited sediments in the riparian zone were collected in 5 cm increments and the concentration of 137Cs was used to determine the 1964 and 1954 deposition layers. Agricultural activity in the watershed has caused increased sediment deposition to the wetland. The recent upland sediment is highly enriched in organic matter indicating that large amounts of organic C have been sequestered within this zone of sediment deposition. Rates of sequestration are much higher than rates that have occurred over the pre-modern history of the wetland. These data indicate the increased sedimentation rates in the wetland ecosystem are associated with increased C sequestration rates.     

Ammonia emission, deposition and impact assessment at the field scale: a case study of sub-grid spatial variability

A local ammonia (NH3) inventory for a 5x5 km area in central England was developed, to investigate the variability of emissions, deposition and impacts of NH3 at a field scale, as well as to assess the validity of the UK 5-km grid inventory. Input data were available for the study area for 1993 and 1996 on a field by field basis, allowing NH3 emissions to be calculated for each individual field, separately for livestock grazing, livestock housing and manure storage, landspreading of manures and fertiliser N application to crops and grassland. An existing atmospheric transport model was modified and applied to model air concentrations and deposition of NH3 at a fine spatial resolution (50 m grid). From the mapped deposition estimates and land cover information, critical loads and exceedances were derived. to study the implications of local variability for regional NH3 impacts assessments. The results show that the most extreme local variability in NH3 emissions, deposition and impacts is linked to housing and storage losses. However, landspreading of manures and intensive cattle grazing are other important area sources, which vary substantially in the landscape. Overall, the range of predicted emissions from agricultural land within the study area is 0-2000 kg N ha(-1) year(-1) in 1993 and 0-8000 kg N ha(-1) year(-1) in 1996, respectively, with the peak at a poultry farm located in the study area. On average, the estimated field level NH3 emissions over the study area closely match the emission for the equivalent 5-km grid square in the national inventory for 1996. Deposition and expected impacts are highly spatially variable, with the edges of woodland and small "islands" of semi-natural vegetation in intensive agricultural areas being most at risk from enhanced deposition. Conversely the centres of larger nature reserves receive less deposition than average. As a consequence of this local variability it is concluded that national assessments at the 5 km grid level underestimate the occurrence of critical loads exceedances due to NH3 in agricultural landscapes.     

Effect of arsenic-contaminated irrigation water on agricultural land soil and plants in West Bengal, India

Total arsenic withdrawn by the four shallow tubewells, used for agricultural irrigation in the arsenic-affected areas of Murshidabad district per year is 6.79 kg (mean: 1.79 kg, range: 0.56-3.53 kg) and the mean arsenic deposition on land per year is 5.02 kg ha(-1) (range: 2-9.81 kg ha(-1)). Mean soil arsenic concentrations in surface, root of plants, below ground level (0-30 cm) and all the soils, collected from four agricultural lands are 14.2 mg/kg (range: 9.5-19.4 mg/kg, n = 99), 13.7 mg/kg (range: 7.56-20.7 mg/kg, n = 99), 14.8 mg/kg (range: 8.69-21 mg/kg, n = 102) and 14.2 mg/kg (range: 7.56-21 mg/kg, n = 300) respectively. Higher the arsenic in groundwater, higher the arsenic in agricultural land soil and plants has been observed. Mean arsenic concentrations in root, stem, leaf and all parts of plants are 996 ng/g (range: <0.04-4850 ng/g, n = 99), 297 ng/g (range: <0.04-2900 ng/g, n = 99), 246 ng/g (range: <0.04-1600 ng/g, n = 99) and 513 ng/g (range: <0.04-4850 ng/g, n = 297) respectively. Approximately 3.1-13.1, 0.54-4.08 and 0.36-3.45% of arsenic is taken up by the root, stem and leaf respectively, from the soil.     

Carbon dioxide fluxes over a grazed prairie and seeded pasture in the Northern Great Plains

Temperate grasslands are vast terrestrial ecosystems that may be an important component of the global carbon (C) cycle; however, annual C flux data for these grasslands are limited. The Bowen ratio/energy balance (BREB) technique was used to measure CO2 fluxes over a grazed mixed-grass prairie and a seeded western wheatgrass [Pascopyrum smithii (Rybd) L?ve] site at Mandan, ND from 24 April to 26 October in 1996, 1997, and 1998. Above-ground biomass and leaf area index (LAI) were measured about every 21 days throughout the season. Root biomass and soil organic C and N content were determined to 110 cm depth in selected increments about mid-July each year. Peak above-ground biomass and LAI coincided with peak fluxes and occurred between mid-July to early August. Biomass averaged 1227 and 1726 kg ha(-1) and LAI 0.44 and 0.59, for prairie and western wheatgrass, respectively. Average CO2 flux for the growing season was 279 g CO2 m(-2) for prairie and 218 g CO2 m(-2) for western wheatgrass (positive flux is CO2 uptake and negative flux is CO2 loss to the atmosphere). Using prior measured dormant season CO2 fluxes from the prairie sites gave annual flux estimates that ranged from -131 to 128 g CO2 m(-2) for western wheatgrass and from -70 to 189 g CO2 m(-2) for the prairie. This wide range in calculated annual fluxes suggests that additional research is required concerning dormant season flux measurements to obtain accurate estimates of annual CO2 fluxes. These results suggest Northern Great Plains mixed-grass prairie grasslands can either be a sink or a source for atmospheric CO2 or near equilibrium, depending on the magnitude of the dormant season flux.     

An inventory of nitric oxide emissions from soils in China

The emission of NO was parameterized using empirical relationships with landuse type, fertilization rate and soil temperature. Eight landuse types (including four arable lands) were considered. Fertilization rates were distinguished for different regions and crops. A typical summer period of July in 1999 was chosen for detailed calculations. The total NO emission in the July is 141.1 Gg N, with 73.7% from arable lands and 22.0% from grasslands. The highest emission intensity can be more than 40 ng N m(-2) s(-1) in the heavily fertilized North China Plain, and the average of the whole lands is 6.5 ng N m(-2) s(-1). The annual emission was roughly estimated to be 657 Gg N, about 11.7% of the global total (5600 Gg N, reported by IPCC in 2000), and about 12.5% of the anthropogenic origin in China. Our results were compared with some earlier findings, and uncertainties were discussed.     

Soil quality and vulnerability in a Mediterranean natural ecosystem of Central Italy

Wet and dry atmospheric depositions and soil chemical and microbiological properties were determined in a Mediterranean natural ecosystem of Central Italy near Rome (Castelporziano Estate). The monitoring of depositions permitted us to quantify the exceedances of S and N compounds (expressed as eqH(+)ha(-1)year(-1)) over the critical loads of acidity. Critical loads, i.e. the quantity of a substance which a part of the environment can tolerate without adverse effects occurring, were determined adopting the level 0 methodology following the UN/ECE Convention on Long-range Transboundary Air Pollution. Deposition data were available for the period 1992-1997, and acidity exceedances were referred to the main vegetation types present in the area. Results showed that most part of the Estate has a medium degree of vulnerability to acidification, and the corresponding risk of acidification deriving from the exceedances of atmospheric deposition was rather low. The study of soil chemical and microbiological properties included mainly total soil organic carbon (SOC), microbial biomass-C, biomass-C/SOC, soil respiration, and metabolic quotient (qCO2). Soil organic C metabolism has been discussed on the basis of the results from eight sampling sites.     

Soil carbon protection in podocarp/hardwood forest, and effects of conversion to pasture and exotic pine forest

A combination of paired site, time series, and survey approaches were used to estimate the effect of land use change on mineral soil carbon (C), and to identify factors associated with variation. Land-uses compared included podocarp/hardwood forest, improved pasture, and pine plantation. Soil C was significantly related to soil pH that ranged between 3.9-5.9 (0-0.05 m), 3.6-6.0 (0.05-0.10 m), and 4.5-6.1 (0.10-0.50 m) in indigenous forest. Time series data obtained by periodically re-sampling soil (0-0.10 m) in permanent plots in a pine forest previously under pasture showed that mineral soil C decrease by approximately 4 Mg ha(-1) by the end of the first rotation. The time series data compared closely with mean results obtained at paired-site throughout New Zealand. Soil C concentration was highly variable in all land-uses, and the evidence suggests that chemical stabilisation of C occurred under acid conditions in native forest, through complexation with Al, and that effects persisted long after conversion of the native forest to other land-uses. The implications of these findings for the design of sampling protocols for soil C are discussed.     

Net carbon flux from agricultural ecosystems: methodology for full carbon cycle analyses

Agricultural ecosystems have the potential to sequester carbon in soils by altering agricultural management practices (i.e. tillage practice, cover crops, and crop rotation) and using agricultural inputs (i.e. fertilizers and irrigation) more efficiently. Changes in agricultural practices can also cause changes in CO2 emissions associated with these practices. In order to account for changes in net CO2 emissions, and thereby estimate the overall impact of carbon sequestration initiatives on the atmospheric CO2 pool, we use a methodology for full carbon cycle analysis of agricultural ecosystems. The analysis accounts for changes in carbon sequestration and emission rates with time, and results in values representing a change in net carbon flux. Comparison among values of net carbon flux for two or more systems, using the initial system as a baseline value, results in a value for relative net carbon flux. Some results from using the full carbon cycle methodology, along with US national average values for agricultural inputs, indicate that the net carbon flux averaged over all crops following conversion from conventional tillage to no-till is -189 kg C ha(-1) year(-1) (a negative value indicates net transfer of carbon from the atmosphere). The relative net carbon flux, using conventional tillage as the baseline, is -371 kg C ha(-1) year(-1), which represents the total atmospheric CO2 reduction caused by changing tillage practices. The methodology used here illustrates the importance of (1) delineating system boundaries, (2) including CO2 emissions associated with sequestration initiatives in the accounting process, and (3) comparing the new management practices associated with sequestration initiatives with the original management practices to obtain the true impact of sequestration projects on the atmospheric CO2 pool.     

Effect of biosolid incorporation on arsenic distribution in Mollisol soils in central Chile

The effect of biosolid incorporation on arsenic distribution in Mollisol soils in central Chile was studied. Two soils were sequentially extracted for arsenic with a five-step method that accounts for the following arsenic forms: non-specifically adsorbed (F1), specifically adsorbed (F2), amorphous and poorly crystallized Fe and Al oxides (F3), well-crystallized Fe and Al oxides (F4) and residual (F5). The arsenic residual fraction was predominant in Pintué soil, whereas in Graneros soil, arsenic was mostly associated to amorphous Fe and Al oxides. Graneros soil exhibited a higher As(V) adsorbing capacity than Pintué soil, which relates to the higher clay and iron and aluminum oxides contents, confirming that these components participation is essential for the adsorption of this metalloid. Biosolid application at a rate of 100Mg ha(-1) caused an increase in arsenic bound to amorphous Fe and Al oxides and in the residual fraction, in Pintué soil. When Pintué soil was spiked with arsenic, aged for two months, and treated with biosolid (100Mg ha(-1)), the content of arsenic in the most labile fractions decreased, thus showing a favorable effect in its application to soils with few specific sites for arsenic adsorption. Arsenic speciation was carried out in the first two fractions of the sequential extraction procedure. As(V) was the main form in both fractions. Biosolid incorporation at a rate of 100Mg ha(-1) caused a significant increase in organic arsenic forms.