Soil carbon changes in cultivated and excavated land converted to grasses in east-central Saskatchewan

The conversion of annually cultivated or disturbed marginal land to forage grasses has the potential to accrete soil organic carbon (SOC) in the surface 0–15 cm depth. Soil organic carbon mass (Mg ha^–1) was measured in ten side-by-side cultivated versus forage grass seed-down restoration treatments on catenae at various sites in east-central Saskatchewan, Canada. Treatments were imposed for time periods ranging from five to twelve years. It was found that SOC mass was usually significantly higher in the grassland restorations versus the paired cultivated equivalents. Estimated SOC gain rates (0–15 cm) from grass seed-down in the region was estimated to be 0.6 to 0.8 Mg C ha^–1 yr^–1. Light fraction organic carbon (LFOC), the labile component of SOC, was more variable in the comparisons than SOC. Measured ¹³C natural abundance values in selected equivalent comparisons revealed a possible contribution from seeded warm season C₄ grasses and soil carbonate ¹³C to the C pools in upslope positions of the landscape. Overall, grassland restoration in this region appears to result in increased carbon storage in the surface soil.     

Dynamics of phosphorus fractions during fallow with natural vegetation and planted Pueraria phaseoloides in south-western Nigeria

A study was conducted in 1998 and 1999 on a long-term fallow management trial, established in 1989 at the International Institute of Tropical Agriculture (IITA), Ibadan, in the derived savanna of southwestern Nigeria, to quantify P fractions under natural fallow (NF) and a Pueraria cover crop fallow. Plots with previous 1:1, 1:2, and 1:3 crop/fallow ratios before reverting to fallow in either 1998 or 1999 were considered. Biomass accumulation under the two fallow types was comparable and increased linearly with fallow age, reaching slightly above 7 t dry matter ha^–1 after 8 months of fallow. Phosphorus accumulation in the fallow vegetation ranged from 2.1 – 9.1 kg ha^–1 for natural fallow (NF) and from 1.5 – 6.6 kg ha^–1 for Pueraria. Magnesium was also higher under NF (9.1 – 21 kg ha^–1) than under Pueraria (4.4 – 13 kg ha^–1), whereas N, Ca, and K contents were higher in Pueraria biomass than under NF at 1 year after fallow. Pueraria fallow tended to lower soil pH compared with NF. However, plots with less frequent cropping (1:3 crop/fallow ratio) did not have significantly different pH irrespective of the fallow vegetation type. Olsen extractable soil P increased as fallow length increased irrespective of the fallow system and previous crop/fallow ratio. For example, under NF (0–5 cm depth, 1:1 crop/fallow ratio in1998) Olsen P increased from 12 mg kg^–1 to 17 mg kg^–1 after 1 year of fallow and under Pueraria, it increased from 8 mg kg^–1 to 15 mg kg^–1. Fallow type and previous crop/fallow ratio had no significant and consistent effects on soil P fractions. However, NaOH- and concentrated HCl- extractable organic P fractions increased with fallow length. In 1998, under NF, NaOH- extractable organic P increased from 12 to 21 mg kg^–1 (1:1 crop/fallow ratio) and from 10 to 19 mg kg^–1 for both 1:2 and 1:3 crop/fallow ratio. HCl- extractable organic P increased from 11 to 30 mg kg^–1 (1:1 crop/fallow ratio), from 13 to 27 mg kg^–1 (1:2 crop/fallow ratio) and from 18 to 35 mg kg^–1 (1:3 crop/fallow ratio). Similar trend was observed under Pueraria fallow. These results suggest that P was reallocated to non-readily available organic P fractions irrespective of fallow type and previous land use. These organic P fractions, which are usually more stable, reflect the overall change in soil organic P levels when the soil was stressed by cultivation and then reverted to fallow. These pools may thus represent an active reservoir (source and sink) of P in shifting cultivation under tropical conditions without inorganic fertilizer application.     

Sedimentary Organic Carbon and Nitrogen Sequestration Across a Vertical Gradient on a Temperate Wetland Seascape Including Salt Marshes,Seagrass Meadows and Rhizophytic Macroalgae Beds

Coastal wetlands are key in regulating coastal carbon and nitrogen dynamics and contribute significantly to climate change mitigation and anthropogenic nutrient reduction. We investigated organic carbon (OC) and total nitrogen (TN) stocks and burial rates at four adjacent vegetated coastal habitats across the seascape elevation gradient of Cádiz Bay (South Spain), including one species of salt marsh, two of seagrasses, and a macroalgae. OC and TN stocks in the upper 1 m sediment layer were higher at the subtidal seagrass Cymodocea nodosa (72.3 Mg OC ha⁻¹, 8.6 Mg TN ha⁻¹) followed by the upper intertidal salt marsh Sporobolus maritimus (66.5 Mg OC ha⁻¹, 5.9 Mg TN ha⁻¹), the subtidal rhizophytic macroalgae Caulerpa prolifera (62.2 Mg OC ha⁻¹, 7.2 Mg TN ha⁻¹), and the lower intertidal seagrass Zostera noltei (52.8 Mg OC ha⁻¹, 5.2 Mg TN ha⁻¹). The sedimentation rates increased from lower to higher elevation, from the intertidal salt marsh (0.24 g cm⁻² y⁻¹) to the subtidal macroalgae (0.12 g cm⁻² y⁻¹). The organic carbon burial rate was highest at the intertidal salt marsh (91 ± 31 g OC m⁻² y⁻¹), followed by the intertidal seagrass, (44 ± 15 g OC m⁻² y⁻¹), the subtidal seagrass (39 ± 6 g OC m⁻² y⁻¹), and the subtidal macroalgae (28 ± 4 g OC m⁻² y⁻¹). Total nitrogen burial rates were similar among the three lower vegetation types, ranging from 5 ± 2 to 3 ± 1 g TN m⁻² y⁻¹, and peaked at S. maritimus salt marsh with 7 ± 1 g TN m⁻² y⁻¹. The contribution of allochthonous sources to the sedimentary organic matter decreased with elevation, from 72% in C. prolifera to 33% at S. maritimus. Our results highlight the need of using habitat-specific OC and TN stocks and burial rates to improve our ability to predict OC and TN sequestration capacity of vegetated coastal habitats at the seascape level. We also demonstrated that the stocks and burial rates in C. prolifera habitats were within the range of well-accepted blue carbon ecosystems such as seagrass meadows and salt marshes.

     

CENTURY ecosystem model application for quantifying vegetation dynamics in shifting cultivation areas: A case study from Rampa Forests, Eastern Ghats (India)

In India, slash and burn agriculture is one of the major factors contributing to deforestation, especially in the hilly north-eastern region and Eastern Ghats. Studies on vegetation dynamics associated with slash and burn agricultural practices have been intensively studied in the north-eastern part of India. These have covered semi-evergreen/evergreen vegetation, but similar studies on tropical mixed dry deciduous ecosystems are not as common. In the present study, we used the century ecosystem model to study vegetation dynamics in shifting cultivation areas on the mixed dry deciduous forests covering the Eastern Ghats of India. The site-specific parameters, temperature, precipitation, biomass and nutrient pools were used, and, by collecting information from local management practices, a 12-year shifting cultivation cycle during a 70-year period from 1960 to 2030 was simulated. century estimated a total loss of 239 tonnes carbon (tC) in soil organic matter over the simulation period, and the total nitrogen content of the soil organic matter showed an initial increase followed by a decline (344.3 g m² during 1960 to less than 318.3 g m² during 2030). century estimated that 66 tC ha^–1 would be lost from the forest system, reducing the initial forest system carbon level from 118.5 tC ha^–1. An increase in productivity from 0.49 tC ha^–1 during 1960 to 1.2 tC ha^–1 during the initial forest slash and burn in 1962 was observed, but thereafter productivity declined to 0.7 tC ha^–1 during the year 2030. Results obtained in other studies of similar types of agricultural practices are also reviewed.     

Chaff piles of harvester ant (Messor spp.) nests in a desert ecosystem

Summary The plant species composition of the chaff piles of three species of harvester ant (Messor spp.) and the contribution of the chaff to the organic pool were studied from August 1985 to July 1987. There were distinct differences in the plant species composition of the chaff of the three species. We attribute this to the different diets of the three species, which reflect the relative sizes of their individuals and their foraging strategies. The amount of chaff accumulated varies greatly between the three species (Messor rugossus: 127–196 g · ha^–1 · y^–1;Messor ebeninus: 2823–4437 g · ha^–1 · y^–1;Messor arenarius: 2165–2535 g · ha^–1 · y^–1), although the number of nests per hectare is virtually the same. We found that the amount of chaff is related to the rate of activity and the size of the individuals of each of the three ant species. The total chaff accumulated during the study period was 19.2 kg · ha^–1, which is an important contribution to the organic matter in the soil in the Negev desert ecosystem.     

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

Biomass,Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes

Tropical dry forest is the most widely distributed land-cover type in the tropics. As the rate of land-use/land-cover change from forest to pasture or agriculture accelerates worldwide, it is becoming increasingly important to quantify the ecosystem biomass and carbon (C) and nitrogen (N) pools of both intact forests and converted sites. In the central coastal region of México, we sampled total aboveground biomass (TAGB), and the N and C pools of two floodplain forests, three upland dry forests, and four pastures converted from dry forest. We also sampled belowground biomass and soil C and N pools in two sites of each land-cover type. The TAGB of floodplain forests was as high as 416 Mg ha^–1, whereas the TAGB of the dry forest ranged from 94 to 126 Mg ha^–1. The TAGB of pastures derived from dry forest ranged from 20 to 34 Mg ha^–1. Dead wood (standing and downed combined) comprised 27%–29% of the TABG of dry forest but only about 10% in floodplain forest. Root biomass averaged 32.0 Mg ha^–1 in floodplain forest, 17.1 Mg ha^–1 in dry forest, and 5.8 Mg ha^–1 in pasture. Although total root biomass was similar between sites within land-cover types, root distribution varied by depth and by size class. The highest proportion of root biomass occurred in the top 20 cm of soil in all sites. Total aboveground and root C pools, respectively, were 12 and 2.2 Mg ha^–1 in pasture and reached 180 and 12.9 Mg ha^–1 in floodplain forest. Total aboveground and root pools, respectively, were 149 and 47 kg ha^–1 in pasture and reached 2623 and 264 kg ha^–1 in floodplain forest. Soil organic C pools were greater in pastures than in dry forest, but soil N pools were similar when calculated for the same soil depths. Total ecosystem C pools were 306. The Mg ha^–1 in floodplain forest, 141 Mg ha^–1 in dry forest, and 124 Mg ha^–1 in pasture. Soil C comprised 37%–90% of the total ecosystem C, whereas soil N comprised 85%–98% of the total. The N pools lack of a consistent decrease in soil pools caused by land-use change suggests that C and N losses result from the burning of aboveground biomass. We estimate that in México, dry forest landscapes store approximately 2.3 Pg C, which is about equal to the C stored by the evergreen forests of that country (approximately 2.4 Pg C). Potential C emissions to the atmosphere from the burning of biomass in the dry tropical landscapes of México may amount to 708 Tg C, as compared with 569 Tg C from evergreen forests.     

Digital Mapping of Soil Organic Carbon Contents and Stocks in Denmark

Estimation of carbon contents and stocks are important for carbon sequestration, greenhouse gas emissions and national carbon balance inventories. For Denmark, we modeled the vertical distribution of soil organic carbon (SOC) and bulk density, and mapped its spatial distribution at five standard soil depth intervals (0−5, 5−15, 15−30, 30−60 and 60−100 cm) using 18 environmental variables as predictors. SOC distribution was influenced by precipitation, land use, soil type, wetland, elevation, wetness index, and multi-resolution index of valley bottom flatness. The highest average SOC content of 20 g kg⁻¹ was reported for 0−5 cm soil, whereas there was on average 2.2 g SOC kg⁻¹ at 60−100 cm depth. For SOC and bulk density prediction precision decreased with soil depth, and a standard error of 2.8 g kg⁻¹ was found at 60−100 cm soil depth. Average SOC stock for 0−30 cm was 72 t ha⁻¹ and in the top 1 m there was 120 t SOC ha⁻¹. In total, the soils stored approximately 570 Tg C within the top 1 m. The soils under agriculture had the highest amount of carbon (444 Tg) followed by forest and semi-natural vegetation that contributed 11% of the total SOC stock. More than 60% of the total SOC stock was present in Podzols and Luvisols. Compared to previous estimates, our approach is more reliable as we adopted a robust quantification technique and mapped the spatial distribution of SOC stock and prediction uncertainty. The estimation was validated using common statistical indices and the data and high-resolution maps could be used for future soil carbon assessment and inventories.     

The Carbon Balance of an Old-growth Forest: Building Across Approaches

The carbon budget of the Wind River old-growth forest is being addressed from a variety of perspectives and with a range of approaches. The goal of this comprehensive analysis is developing a thorough, general, and validated understanding of the carbon balance, as well as the processes controlling it. The initial results from studies addressing annual carbon (C) balance with ground-based methods, eddy flux, leaf-based models, and ecosystem models are consistent in some, but not all, respects. Net primary production is 500–600 g C m^–2 y^–1 (5–6 Mg C ha^–1 y^–1), consistent with estimates based on climate alone. The site appears to be close to carbon equilibrium, as a multiyear average, using ground-based methods but a sink of approximately 150–190 g C m^–2 y^–1 from eddy flux for a single year. An overview of the mechanisms that can drive forest carbon sinks illustrates why methods emphasizing different temporal and spatial scales, as well as different processes, can come to different conclusions, and it highlights opportunities in moving toward a truly integrated approach.     

Forest biomass estimation at regional and global levels, with special reference to China’s forest biomass

Accurate estimation of forest biomass size and regional distribution is a prerequisite in answering a long-standing debate on the role of forest vegetation in the regional and global carbon cycle. Appropriate biomass estimation methods and available forest data sources are two key factors for this purpose. Among the estimation methods, the continuous Biomass Expansion Factor (BEF; defined as the ratio of all stand biomass to stem volume or biomass) method is considered to be the best. We applied the continuous BEF to forest inventory data of China and estimated a biomass carbon of 4.6 PgC and a biomass carbon density of 38.4 Mg ha^–1. A review of recent literature shows that forest carbon density in major temperate and boreal forest regions in the Northern Hemisphere has a narrow variance ranging from 29 Mg ha^–1 to 50 Mg ha^–1, with a global mean of 36.9 Mg ha^–1. This suggests that the forest biomass density in China is closely coincident with the global mean.     

Seasonal Dynamics of Soil Labile Organic Carbon and Enzyme Activities in Relation to Vegetation Types in Hangzhou Bay Tidal Flat Wetland

Soil labile organic carbon and soil enzymes play important roles in the carbon cycle of coastal wetlands that have high organic carbon accumulation rates. Soils under three vegetations (Phragmites australis, Spartina alterniflora, and Scirpusm mariqueter) as well as bare mudflat in Hangzhou Bay wetland of China were collected seasonally. Seasonal dynamics and correlations of soil labile organic carbon fractions and soil enzyme activities were analyzed. The results showed that there were significant differences among vegetation types in the contents of soil organic carbon (SOC) and dissolved organic carbon (DOC), excepting for that of microbial biomass carbon (MBC). The P. australis soil was with the highest content of both SOC (7.86 g kg^-1) and DOC (306 mg kg^-1), while the S. mariqueter soil was with the lowest content of SOC (6.83 g kg^-1), and the bare mudflat was with the lowest content of DOC (270 mg kg^-1). Soil enzyme activities were significantly different among vegetation types except for urease. The P. australis had the highest annual average activity of alkaline phosphomonoesterase (21.4 mg kg^-1 h^-1), and the S. alterniflora had the highest annual average activities of β-glycosidase (4.10 mg kg^-1 h^-1) and invertase (9.81mg g^-1 24h^-1); however, the bare mudflat had the lowest activities of alkaline phosphomonoesterase (16.2 mg kg^-1 h^-1), β-glycosidase (2.87 mg kg^-1 h^-1), and invertase (8.02 mg g^-1 24h^-1). Analysis also showed that the soil labile organic carbon fractions and soil enzyme activities had distinct seasonal dynamics. In addition, the soil MBC content was significantly correlated with the activities of urease and β-glucosidase. The DOC content was significantly correlated with the activities of urease, alkaline phosphomonoesterase, and invertase. The results indicated that vegetation type is an important factor influencing the spatial-temporal variation of soil enzyme activities and labile organic carbon in coastal wetlands.     

Carbon balance of a tropical savanna of northern Australia

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO₂ efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204±53 ton C ha^–1, with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0–1 m) was 151±33 ton C ha^–1, accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53±20 ton C ha^–1, 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha^–1, of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha^–1 year^–1, of which 8.0 ton C ha^–1 (73%) was contributed by below-ground net primary production and 3.0 ton C ha^–1 (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha^–1 year^–1. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5–6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha^–1 year^–1.     

Effect of Vegetation Rehabilitation on Soil Carbon and Its Fractions in Mu Us Desert,Northwest China

Although vegetation rehabilitation on semi-arid and arid regions may enhance soil carbon sequestration, its effects on soil carbon fractions remain uncertain. We carried out a study after planting Artemisia ordosica (AO, 17 years), Astragalus mongolicum (AM, 5 years), and Salix psammophila (SP, 16 years) on shifting sand land (SL) in the Mu Us Desert, northwest China. We measured total soil carbon (TSC) and its components, soil inorganic carbon (SIC) and soil organic carbon (SOC), as well as the light and heavy fractions within soil organic carbon (LF-SOC and HF-SOC), under the SL and shrublands at depths of 100 cm. TSC stock under SL was 27.6 Mg ha^?1, and vegetation rehabilitation remarkably elevated it by 40.6 Mgha^?1, 4.5 Mgha^?1, and 14.1 Mgha^?1 under AO, AM and SP land, respectively. Among the newly formed TSC under the three shrublands, SIC, LF-SOC and HF-SOC accounted for 75.0%, 10.7% and 13.1% for AO, respectively; they made up 37.0%, 50.7% and 10.6% for AM, respectively; they occupied 68.6%, 18.8% and 10.0% for SP, respectively. The accumulation rates of TSC within 0–100 cm reached 238.6 g m^?2y^?1, 89.9 g m^?2y^?1 and 87.9 g m^?2y^?1 under AO, AM and SP land, respectively. The present study proved that the accumulation of SIC considerably contributed to soil carbon sequestration, and vegetation rehabilitation on shifting sand land has a great potential for soil carbon sequestration.     

Biogeochemical consequences of the transformation of native Cerrado into Pinus caribaea plantations in Brazil

Under the same climatic and edaphic conditions, native savanna vegetation in Brazil, the Cerrado, shows a lower stature and canopy cover than planted Pinus caribaea Morelet forests. To assess the differences in biogeochemical element cycling we compared the nutrient economy of Cerrado and Pinus on three replicate plots of each forest type. The mean nutrient storage in the soil organic layer under Pinus (N: 2630; P: 141; K: 103; Ca: 131; Mg: 20 kg ha^–1) was substantially higher than under Cerrado (N: 23; P: 1.2; K: 0.83; Ca: 5.8; Mg: 1.0 kg ha^–1) probably because the Pinus roots explored a larger soil volume. The Pinus trees had a higher nutrient-use efficiency as indicated by higher mean litter mass per unit nutrient in litter (N: 108; P: 2290; K: 729; Ca: 1360; Mg: 5420; S: 1190; Fe: 2960; Mn: 9990, Zn: 145000) than the Cerrado trees (N: 94; P: 1810; K: 619; Ca: 302; Mg: 938, S: 746; Fe: 1800; Mn: 7880; Zn: 63700). Mean annual small litterfall collected in 0.25-m² samplers between May 1997 and April 1999 was 2.1 Mg ha^–1 in Cerrado and 7.8 in Pinus. The litterfall rates of the 1–3 week collection intervals correlated negatively with the soil matric potential indicating that litterfall was partly related to water stress. The fluxes of N (73 kg ha^–1 year^–1), P (3.7), K (11), S (7.0), and Mn (0.83) to the soil with litterfall under Pinus were greater than the litterfall+turnover of the grass/herbs layer under Cerrado (N: 39, P: 2.8, K: 8.6, S: 5.4, Mn: 0.79 kg ha^–1 year^–1), those of Zn (0.06–0.07) were similar, and those of Ca (Pinus: 5.9/Cerrado: 10), Mg (1.5/4.4), and Fe (2.9/4.0) were smaller. Mean residence times of the organic matter and of all elements were longer in the soil organic layer under Pinus (3.7–26 years in the Oi horizon, 8.1–907 years in the whole organic layer) than under Cerrado (0.22–3.6 years in the Oi horizon, the only organic horizon under Cerrado). Our results demonstrate that the main differences in biogeochemical element cycling between the Pinus forest and the Cerrado consisted of a larger nutrient storage in the organic layer, a higher nutrient-use efficiency, and slower nutrient release rates from the organic layer in the Pinus forest than in the Cerrado. Nutrient cycling as assessed by the nutrient fluxes with litterfall was only partly faster in the Pinus forest than in the Cerrado.     

Tolerance of wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) to high soil and solution selenium levels

The fertilisation of wheat crops with Se is a cost-effective method of enhancing the concentration of organic Se in grain, in order to increase the Se intake of animals and humans. It is important to avoid phytotoxicity due to over-application of Se. Studies of phytotoxicity of Se in wheat grown in Australia, where rainfall and grain yield are usually relatively low, have not been reported previously, and overseas studies have had varied results. This study used trials conducted in the field, glasshouse and laboratory to assess Se phytotoxicity in wheat. In field trials that used rates of up to 120 g ha^–1Se as selenate, and in pilot trials that used up to 500 g ha^–1 Se soil-applied or up to 330 g ha^–1 Se foliar-applied, with soils of low S concentrations (2–5 mg kg^–1), no Se toxicity symptoms were observed. In pot trials of four weeks duration, the critical tissue level for Se toxicity was around 325 mg kg^–1 DW, a level attained by addition to the growth medium of 2.6 mg kg^–1 Se as selenate. Solution concentrations above 10 mg L^–1 Se inhibited early root growth of wheat in laboratory studies, with greater inhibition by selenite than selenate. For selenite, Se concentrations around 70 mg L^–1 were required to inhibit germination, while for selenate germination % was unaffected by a solution concentration of 150 mg L^–1 Se. Leaf S concentration and content of wheat increased three-fold with the addition of 1 mg kg^–1 Se as selenate to the growth medium. This effect is probably due to the induction of the S deficiency response of the main sulphate transporter. This study found wheat to be more Se-tolerant than did earlier studies of tobacco, soybeans and rice. We conclude that Se phytotoxicity in wheat will not be observed at the range of Se application rates that would be used to increase grain Se for human consumption (4–200 g ha^–1 Se as selenate, which would result in soil and tissue levels well below those seen in the above studies), even when – as is common in Australia – soil S concentration and grain yield are low.     

The effect of the presence of a forage legume on nitrogen and carbon levels in soils under Brachiaria pastures in the Atlantic forest region of the South of Bahia,Brazil

The impact of forest clearance, and its replacement by Brachiaria pastures, on soil carbon reserves has been studied at many sites in the Brazilian Amazonia, but to date there appear to be no reports of similar studies undertaken in the Atlantic forest region of Brazil. In this study performed in the extreme south of Bahia, the changes in C and N content of the soil were evaluated from the time of establishment of grass-only B. humidicola and mixed B. humidicola/Desmodium ovalifolium pastures through 9 years of grazing in comparison with the C and N contents of the adjacent secondary forest. The decline in the content of soil C derived from the forest (C3) vegetation and the accumulation of that derived from the Brachiaria (C4) were followed by determining the ¹³C natural abundance of the soil organic matter (SOM). The pastures were established in 1987, 10 years after deforestation, and it was estimated that until 1994 there was a loss in forest-derived C in the top 30 cm of soil of approximately 20% (9.1 Mg C ha^–1). After the establishment of the pastures, C derived from Brachiaria accumulated steadily such that at the final sampling (1997) it was estimated 13.9 Mg ha^–1 was derived from this source under the grass-only pasture (0–30 cm). Samples taken from all pastures and the forest in 1997 to a depth of 100 cm showed that below 40 cm depth there was no significant contribution of the Brachiaria-derived C and that total C reserves under the grass/legume and the grass-only pastures were slightly higher than under the forest (not significant at P=0.05). The more detailed sampling under the pastures showed that to a depth of 30 cm there was significantly (P<0.05) more C under the mixed pasture than the grass-only pasture. It was estimated that from the time of establishment the apparent rate of C accumulation (0–100 cm depth) under the grass/legume pastures (1.17 Mg ha^–1 yr^–1) was almost double that under the grass-only pastures (0.66 Mg ha^–1 yr^–1). The data indicated that newly incorporated SOM derived from the Brachiaria had a considerably higher C:N ratio than that present under the forest.     

Carbon cycling in cultivated land and its global significance

Long-term data from Sanborn Field, one of the oldest experimental fields in the USA, were used to determine the direction of soil organic carbon (SOC) dynamics in cultivated land. Changes in agriculture in the last 50 years including introduction of more productive varieties, wide scale use of mineral fertilizers and reduced tillage caused increases in total net annual production (TNAP), yields and SOC content. TNAP of winter wheat more than doubled during the last century, rising from 2.0–2.5 to 5–6 Mg ha^–1 of carbon, TNAP of corn rose from 3–4 to 9.5–11.0 Mg ha^–1 of carbon. Amounts of carbon returned annually with crop residues increased even more drastically, from less than 1 Mg ha^–1 in the beginning of the century to 3–3.5 Mg ha^–1 for wheat and 5–6 Mg ha^–1 for corn in the 90s. These amounts increased in a higher proportion because in the early 50s removal of postharvest residues from the field was discontinued. SOC during the first half of the century, when carbon input was low, was mineralized at a high rate: 89 and 114 g m^–2 y^–1 under untreated wheat and corn, respectively. Application of manure decreased losses by half, but still the SOC balance remained negative. Since 1950, the direction of the carbon dynamics has reversed: soil under wheat monocrop (with mineral fertilizer) accumulated carbon at a rate about 50 g m^–2 y^–1, three year rotation (corn/wheat/clover) with manure and nitrogen applications sequestered 150 g m² y^–1 of carbon. Applying conservative estimates of carbon sequestration documented on Sanborn Field to the wheat and corn production area in the USA, suggests that carbon losses to the atmosphere from these soils were decreased by at least 32 Tg annually during the last 40–50 years. Our computations prove that cultivated soils under proper management exercise a positive influence in the current imbalance in the global carbon budget.     

Cation distribution,cycling, and removal from mineral soil in Douglas-fir and red alder forests

Overstory species influence the distribution and dynamics of nutrients in forest ecosystems. Ecosystem-level estimates of Ca, Mg, and K pools and cycles in 50-year old Douglas-fir and red alder stands were used to determine the effect of overstory composition on net cation removal from the mineral soil, i.e. cation export from the soil in excess of additions. Net cation removal from Douglas-fir soil was 8 kg Ca ha^–1 yr^–1, 1 kg Mg ha^–1 yr^–1, and 0.3 kg K ha^–1 yr^–1. Annual cation export from soil by uptake and accumulation in live woody tissue and O horizon was of similar magnitude to leaching in soil solution. Atmospheric deposition partially off-set export by adding cations equivalent to 28–88% of cation export. Net cation removal from red alder soil was 58 kg Ca ha^–1 yr^–1, 9 kg Mg ha^–1 yr^–1, and 11 kg K ha^–1 yr^–1. Annual cation accumulation in live woody tissue and O horizon was three times greater than in Douglas-fir, while cation leaching in soil solution was five to eight times greater. The lack of excessive depletion of exchangeable cations in the red alder soil suggests that mineral weathering, rather than exchangeable cations, was the source of most of the removed cations. Nitric acid generated during nitrification in red alder soil led to high rates of weathering and NO₃-driven cation leaching.     

Comparison of soil nitrogen mineralization and nitrification in a mixed grassland and forested ecosystem in central Taiwan

We used the buried bag incubation method to study temporal patterns of net N mineralization and net nitrification in soils at Ta-Ta-Chia forest in central Taiwan. The site included a grassland zone, (dominant vegetation consists of Yushania niitakayamensis and Miscanthus transmorrisonensis Hayata) and a forest zone (Tsuga chinensis var. formosana and Yushania niitakamensis). In the grassland, soil concentration NH₄ ⁺ in the organic horizon (0.1–0.2 m) ranged from 1.0 to 12.4 mg N kg^–1 soil and that of NO₃ ^– varied from 0.2 to 2.1 mg N kg^–1 soil. In the forest zone, NH₄ ⁺ concentration was between 2.8 and 25.0 mg N kg^–1 soil and NO₃ ^–varied from 0.2 to 1.3 mg N kg^–1 soil. There were lower soil NH₄ ⁺ concentrations during the summer than other seasons. Net N mineralization was higher during the summer while net nitrification rates did not show a distinct seasonal pattern. In the grassland, net N mineralization and net nitrification rates were between –0.1 and 0.24 and from –0.04 to 0.04 mg N kg^–1 soil day^–1, respectively. In the forest zone, net N mineralization rates were between –0.03 and 0.45 mg N kg^–1 soil day^–1 and net nitrification rates were between –0.01 and 0.03 mg N kg^–1 soil day^–1. These differences likely result from differing vegetation communities (C₃ versus C₄ plant type) and soil characteristics.     

Preliminary compositional nutrient diagnosis norms for cowpea (Vigna unguiculata (L.) Walp.) grown on desert calcareous soil

This study calculated the compositional nutrient diagnosis (CND) norms of cowpea (Vigna unguiculata (L.) Walp.), as well as identified significant nutrient interactions of this crop growing in an irrigated calcareous desert soil. Three genotypes were distributed in rows in a 2-ha field. The soil showed high heterogeneity in its chemical properties. For statistical analysis, 86 foliar composite samples from healthy plants were used. Preliminary CND norms were developed using a cumulative variance ratio function and the ² distribution function. Means and standard deviations of row-centered log ratios V_X of five nutrients (N, P, K, Ca, and Mg) and a filling value R, which included all nutrients not chemically analyzed. Preliminary CND norms are: V_N^*=0.174±0.095, V_P^*=–2.172±0.234, V_K^*=–0.007±0.267, V_Ca^*=–0.022±0.146, V_Mg^*=–1.710±0.132, and V_R5^*=3.728±0.084. These CND norms are associated with dry bean yields higher than 1.88 t ha^–1, and are associated with the following foliar concentrations: 26.2 g N kg^–1, 2.5 g P kg^–1, 22.9 g K kg^–1, 21.6 g Ca kg^–1, and 4 g Mg kg^–1. Cowpea plants growing in desert calcareous soils took up lower amounts of N, P, and K than those considered as optimum in a previous report. Six interactions were strongly indicated for cowpea through principal component analyses: positive for Ca–Mg, and negative for N–Ca, N–Mg, Ca–P, Mg–P, and K–P. Furthermore, two interactions were identified using simple correlations, negative N–P and positive K–Ca.