Sorption of Glyphosate on Soil Components: The Roles of Metal Oxides and Organic Materials

Predicting the behavior, fate, and transport potential of a herbicide in any soil involves understanding the sorption characteristics. The sorption characteristics of glyphosate (GPS) on soil and their main components were investigated, indicating that the mineral phase is more important than the organic carbon in adsorption of GPS. Sorption isotherms were determined from each component using the batch equilibrium method at various concentrations (5, 10, 15, 20, 25, and 30 mg L^?1) and sorption affinity of GPS was approximated by the Freundlich equation. The sorption strength K _f [mg kg^?1 (L mg^?1)^?n] across the various components ranged from 2.1–134.9 while the organic carbon-normalized Freundlich sorption capacity values, K _foc, ranged from 1.28–3.53 mg kg^?1-OC/(mg L^?1)ⁿ. Infrared Fourier transform spectroscopy (FTIR) of the components showed significant structural differences. The results suggest that the presence of the oxides and hydroxides iron, in particular in soil solutions, enhanced GPS adsorption. They also suggest that reduction in OC% due to various treatments may enhance the remobilization of GPS into the aqueous phase (i.e., groundwater), though at different rates. Comparatively, contribution of surface area to the adsorption of GPS on the various components proved more significant than contents of organic carbon.     

The critical soil P levels for crop yield, soil fertility and environmental safety in different soil types

Background and aims

Sufficient soil phosphorus (P) is important for achieving optimal crop production, but excessive soil P levels may create a risk of P losses and associated eutrophication of surface waters. The aim of this study was to determine critical soil P levels for achieving optimal crop yields and minimal P losses in common soil types and dominant cropping systems in China.

Methods

Four long-term experiment sites were selected in China. The critical level of soil Olsen-P for crop yield was determined using the linear-plateau model. The relationships between the soil total P, Olsen-P and CaCl₂-P were evaluated using two-segment linear model to determine the soil P fertility rate and leaching change-point.

Results

The critical levels of soil Olsen-P for optimal crop yield ranged from 10.9 mg kg^?1 to 21.4 mg kg^?1, above which crop yield response less to the increasing of soil Olsen-P. The P leaching change-points of Olsen-P ranged from 39.9 mg kg^?1 to 90.2 mg kg^?1, above which soil CaCl₂-P greatly increasing with increasing soil Olsen-P. Similar change-point was found between soil total P and Olsen-P. Overall, the change-point ranged from 4.6 mg kg^?1 to 71.8 mg kg^?1 among all the four sites. These change-points were highly affected by crop specie, soil type, pH and soil organic matter content.

Conclusions

The three response curves could be used to access the soil Olsen-P status for crop yield, soil P fertility rate and soil P leaching risk for a sustainable soil P management in field.     

Phosphorus in pasture plants: potential implications for phosphorus loss in surface runoff

Phosphorus (P) loss from land can impair surface water quality. Losses can occur from soil and plant components. While it is known that P losses increase with soil P concentration, it is not known how losses from pasture plants vary with soil P concentration or between different forages. We examined total P and filterable reactive P (FRP) in water extracts of plant shoots, used as a measure of potential P loss to surface runoff, in different forage species relative to soil P concentration in field trials and a glasshouse experiment. The mean total P concentration of 16 forage species in grazed field plots was greater (P?<?0.01; LSD₀₅?=?117 mg kg^?1) in legumes (3,480 mg kg^?1) than for grasses (3,210 mg kg^?1). Total plant P concentrations of grasses and legumes increased with soil Mehlich-3 P concentrations in both glasshouse and field trials with concentrations close to 6,000 mg kg^?1 in arrowleaf clover at 680 mg kg^?1 Mehlich-3 soil P. FRP in water extracts of plant shoots increased relative to plant total P as soil Mehlich-3 P increased, with the greatest concentrations shown by crimson clover and arrowleaf clover. Analysis of water extracts of ryegrass and clover herbage from a field trial showed that while FRP was increasing, phytase-available-P decreased significantly from about 70% of filterable unreactive P at the lowest Mehlich-3 P concentrations, to close to zero at 200 mg kg^?1 Mehlich-3 P. The wide variation, and enrichment of FRP in water extracts and total P with increasing Mehlich-3 P among species, indicates that cultivar and site selection and sward management provide a potential option to mitigate P loss to surface waters.     

Factors affecting soil acidification under legumes I. Effect of potassium supply

This study examined the effect of K (as K₂SO₄) supply on acid production under N₂-fixing plants of lupin (Lupinus angustifolius L. cv. Gungurru) and clover (Trifolium subterraneum L. cv. Dalkeith) grown in a K-deficient soil with a low pH buffer capacity for 55 days in the glasshouse at 20/12 °C (day/night). Increasing K supply up to 240 mg K kg^-1 soil markedly increased plant growth of both species but clover growth was more responsive than lupin. Growing plants for 55 days decreased soil pH by 0.65–0.85 units under lupin and 0.45–0.83 units under clover. The amounts of H⁺ produced per kg biomass (specific acid production) were the highest at the nil K supply, generally decreased with increasing K level up to 30 mg K kg^-1 under lupin and up to 120 mg K kg^-1 soil under clover and only slightly increased with further increasing K under lupin. Increasing K₂SO₄ supply proportionally increased plant uptake of K and SO ₄ ^2- but generally decreased concentrations of Ca, Mg, Na, P and Cl. Specific acid production correlated well with concentrations of excess cations and ash alkalinity, and total acid production was strongly correlated with total excess cations and total ash alkalinity in plants. These relationships were not affected by K treatment and species. Specific acid production also correlated with plant Ca concentration but not with K concentration. In addition, lupin and clover extruded similar amounts of H⁺ per kg biomass produced. It is suggested that application of K₂SO₄ does not have a significant impact on acid production by lupin and clover.     

Soil mineralizable nitrogen and soil nitrogen supply under two-year potato rotations

Two-year potato rotations were evaluated for their effects on soil mineralizable N and soil N supply. Pre-plant soil samples (0–15 cm) collected from the potato year after seven rotation cycles were used to estimate soil mineralizable N using a 24 week aerobic incubation. Potentially mineralizable N (N ₀ ) ranged from 102 to 149 kg N ha^?1, and was greater after pea/white clover and oats/Italian ryegrass than after oats by an average of 35 and 22%, respectively. Labile, intermediate and stable mineralizable N pools were increased after pea/white clover compared with oats, whereas only the stable mineralizable N pool was increased after oats/Italian ryegrass. Potato plant N uptake with no fertilizer applied was greater in potato-pea/white clover compared with the three other rotations (126 vs. average of 67 kg N ha^?1). Choice of rotation crop in potato production influences both the quantity and quality of soil mineralizable N.     

Carbon balances in US croplands during the last two decades of the twentieth century

Carbon (C) added to soil as organic matter in crop residues and carbon emitted to the atmosphere as CO₂ in soil respiration are key determinants of the C balance in cropland ecosystems. We used complete and comprehensive county-level yields and area data to estimate and analyze the spatial and temporal variability of regional and national scale residue C inputs, net primary productivity (NPP), and C stocks in US croplands from 1982 to 1997. Annual residue C inputs were highest in the North Central and Central and Northern Plains regions that comprise ~70% of US cropland. Average residue C inputs ranged from 1.8 (Delta States) to 3.0 (North Central region) Mg?C?ha^?1?year^?1, and average NPP ranged from 3.1 (Delta States) to 5.4 (Far West region) Mg?C?ha^?1?year^?1. Residue C inputs tended to be inversely proportional to the mean growing season temperature. A quadratic relationship incorporating the growing season mean temperature and total precipitation closely predicted the variation in residue C inputs in the North Central region and Central and Northern Plains. We analyzed the soil C balance using the crop residue database and the Introductory Carbon Balance regional Model (ICBMr). Soil C stocks (0–20?cm) on permanent cropland ranged between 3.07 and 3.1?Pg during the study period, with an average increase of ~4?Tg?C?year^?1, during the 1990s. Interannual variability in soil C stocks ranged from 0 to 20?Tg?C (across a mean C stock of 3.08?±?0.01?Pg) during the study period; interannual variability in residue C inputs varied between 1 and 43?Tg C (across a mean input of 220?±?19?Tg). Such interannual variation has implications for national estimates of CO₂ emissions from cropland soils needed for implementation of greenhouse gas (GHG) mitigation strategies involving agriculture.     

Degradation of Metolachlor in Tobacco Field Soil

The extensive use of metolachlor to control weeds in tobacco fields in China has aroused concern about its environmental fate. The aim of this study was to investigate the degradation and residue fate of metolachlor in tobacco field soil (silt loam) under laboratory and field conditions. In laboratory experiments, metolachlor in bulk soil exhibited fast degradation in a temperature range from 10 to 35°C and a soil moisture level of 20–80%, with half-lives (T_1/2) from 66.7 to 28.8 days. The degradation rate of metolachlor decreased as the application dose increased. Owing to higher microbial populations and enzymatic activities, metolachlor rapidly dissipated in rhizosphere soil as compared to bulk soil. Field persistence of metolachlor was evaluated in the same soil during the tobacco (Nicotiana tabacum K326) growing season in 2012 and 2013. The dissipation of metolachlor followed the first-order kinetics and its T_1/2 values were 11.7–13.5 days in soil and 9.0–9.6 days in green tobacco leaves, respectively. At harvest time, the residual levels of metolachlor in soil and green tobacco leaves were in the range of 0.626–1.623 and 0.083-0.481 mg kg^?1, respectively. These findings might have practical implications for the fate of metolachlor residue in tobacco fields. Environmental factors, especially temperature and moisture, should be considered in combination with the appropriate application dose of metolachlor for achieving satisfactory weed-control efficacy, reducing runoff, and minimizing effects on environmental quality.     

Determining critical values of soil Olsen-P for maize and winter wheat from long-term experiments in China

The critical value of soil Olsen-P is the point above which the probability of crop yield response to fertilizer P is small or nil. Determining this critical value is fundamental when making appropriate P fertilizer recommendations. In this study, the critical values were determined for continuous maize (Zea mays L.)-winter wheat (Triticum aestivum L.) cropping systems from a 15-year field experiment across three sites in China using linear-linear, linear-plateau and Mitscherlich models. The mean critical values for maize using the three models ranged from 12.1 to 17.3 mg P kg^?1 (average 15.3 mg P kg^?1) and for winter wheat from 12.5 to 19.0 mg P kg^?1 (average 16.3 mg P kg^?1) among study sites. The mean critical value for maize was approximately 7% lower than that for winter wheat across all sites based on the three models. Critical values identified by the Mitscherlich model were 1.4 to 2.1 times those from linear-linear and 1.3 to 1.9 times of those from linear-plateau and were crop and site dependent. There was a significant negative correlation (P?<?0.05) between the mean critical value from the three models and the observed P uptake by either maize or wheat. Our study shows that the critical values can vary with sites, crops and models used, and thus caution should be taken when selecting the most appropriate one when making P fertilizer recommendations for agronomic return and to minimize chances of negative environment impact from overfertilization.     

Instances of Soil and Crop Heavy Metal Contamination in China

Both general and specific investigations of soil and crop heavy metal contamination were carried out across China. The former was focused mainly on Cd, Hg, As, Pb, and Cr in soils and vegetables in suburbs of four large cities; the latter investigated Cd levels in both soils and rice or wheat in contaminated areas throughout 15 provinces of the country. The results indicated that levels of Cd, Hg, and Pb in soils and some in crops were greater than the Governmental Standards (Chinese government limits for soil and crop heavy metal contents). Soil Cd ranged from 0.46 to 1.04?mg kg^?1, on average, in the four cities and was as high as 145?mg kg^?1 in soil and 7?mg kg^?1 in rice in the wide area of the country. Among different species, tuberous vegetables seemed to accumulate a larger portion of heavy metals than leafy and fruit vegetables, except celery. For both rice and wheat, two staple food crops, the latter seemed to have much higher concentrations of Cd and Pb than the former grown in the same area. Furthermore, the endosperm of both wheat and rice crops had the highest portion of Cd and Cr. Rice endosperm and wheat chaff accumulated the highest Pb, although the concentrations of all three metals were variable in different parts of the grains. For example, 8.3, 6.9, 1.4, and 0.6?mg kg^?1 of Pb were found in chaff, cortex, embryo, and endosperm of wheat compared with 0.11, 0.65, 0.71, and 0.19?mg kg^?1 in the same parts of rice, respectively. Untreated sewage water irrigation was the major cause of increasing soil and crop metals. Short periods of the sewage water irrigation increased individual metals in soils by 2 to 80% and increased metals in crops by 14 to 209%. Atmospheric deposition, industrial or municipal wastes, sewage sludge improperly used as fertilizers, and metal-containing phosphate fertilizers played an important role as well in some specific areas.     

Influence of Lime and Organic Matter on the Mobility of Cadmium in Cadmium-Contaminated Soil in Relation to Nutrition of Spinach

Cadmium and cadmium compounds are water soluble, mobile in most soils, bio-available, and tend to bio-accumulate. A pot culture experiment was conducted on contaminated soil to study the influence of lime and organic matter on the mobility of cadmium in spinach and its rhizosphere soil. Application of lime (50% and 100% lime requirement) and organic matter (0.5 and 1% by weight of soil) to soil decreased the availability of Cd to the soil and plant throughout the crop growth. The highest diethylene triamine penta-acetic acid (DTPA) extractable Cd was 10.84 mg kg^?1 in the treatment OM₀ L₀ (No application of organic matter and lime) at 20 days after sowing of spinach. Likewise, the highest Cd concentration in spinach roots and shoots were 19.80 and 17.0 mg kg^?1 in the treatment OM₀ L₀ at 20 days after sowing. The Cd concentration in spinach roots and shoots were decreased by 63.23 and 71.88%, respectively, in the treatment OM₁ L₁₀₀ (application of FYM at 1.0% by weight of soil and lime at 100% lime requirement) after 60 days of growth. The lowest concentrations of Cd in the soil and plant after the harvest of the crop were 2.88 and 4.27 mg kg^?1, respectively, in the treatment OM₁ L₁₀₀ and resulted in 65.75 and 71.55% decrease over control (OM₀ L₀). The highest total chlorophyll content of leaves was 2.19 mg kg^?1 of fresh weight in the treatment OM₁ L₁₀₀ at 40 days of crop growth.     

Promotion of AM utilization through reduced P fertilization 2. Field studies

The hypothesis of this study was that cumulative P fertilization decreases the contribution of arbuscular mycorrhiza (AM) to crop growth and nutrient uptake in Northern European field conditions. The modes of action of P fertilization were evaluated through effects on mycorrhization, crop dependence on AM, and AM fungal (AMF) community. Field studies were carried out within long-term experiments on soils with low and intermediate initial content of extractable P, where no P fertilization and 45 kg ha^–1 a^–1 P were applied for 20 years. AM effectiveness in terms of growth and nutrient uptake of flax, red clover and barley, percentage root length colonized by AMF, P response of flax, and spore densities and species composition of the AMF communities, were assessed. In the soil with low initial P supply, cumulative P fertilization decreased AM contribution to crop growth and nutrient uptake. The higher AM effectiveness in soil with no added P compensated the cumulative P fertilization (soil P_H2O 2.5 v. 9.5 mg kg^–1) for flax, but not completely for clover. In contrast, barley obtained no benefit from AM at harvest and only a slight benefit from cumulated P. In the soil with intermediate initial P supply, AM reduced growth of flax and barley, especially with no added P, and no response to AM was obtained on clover due to retarded mycorrhization. Cumulative P fertilization reduced yield losses of flax by AM (P_H2O 18.8 v. 5.4 mg kg^–1), because fertilization inhibited mycorrhization. In both soils, root colonization and spore density were decreased by cumulative P fertilization, but no changes in AMF species composition were observed.     

Carbon Sequestration in Fine Roots and Foliage Biomass Offsets Soil CO2 Effluxes along a 19-year Chronosequence of Shrub Willow (Salix x dasyclados) Biomass Crops

Previous greenhouse gas (GHG) assessments for the shrub willow biomass crops (SWBC) production system lacked quantitative data on the soil CO₂ efflux (F_c). This study quantifies the mean annual cumulative F_c, the C sequestration in the above- and belowground biomass, and the carbon balance of the production system. We utilized four SWBC fields, which have been in production for 5, 12, 14, and 19 years. Two treatments were applied: continuous production (CP)—shrub willows were harvested, and stools were allowed to regrow, and tear-out (TO) (crop removal)—shrub willows were harvested, and stools were sprayed with herbicide following spring, crushed, and mixed into the soil. Mean annual cumulative F_c were measured using dynamic closed chambers (LI-8100A and LI-8150). Across different age classes, the mean cumulative F_c ranged from 27.2 to 35.5 Mg CO₂ ha^?1 year^?1 for CP and 26.5 to 29.3 Mg CO₂ ha^?1 year^?1 for TO. The combined carbon (C) sequestration of the standing above- and belowground biomass, excluding stems, ranged from 50.6 to 94.8 Mg CO₂ eqv. ha^?1. In the CP treatment, the annual C sequestration in the fine roots and foliage offsets the annual cumulative F_c. Across different age classes, C balances ranged from ?21.5 to ?59.3 Mg CO₂ ha^?1 for CP and 26.5 to 29.3 Mg CO₂ ha^?1 for TO. The GHG potential of SWBC is about ?36.3 Mg CO₂ eqv. ha^?1 at the end of 19 years, suggesting that the SWBC system sequesters C until termination of the crop.     

Carbon and nitrogen pools and mineralization in a grassland gley soil under elevated carbon dioxide at a natural CO2 spring

The growth and chemical composition of most plants are influenced by elevated CO₂, but accompanying effects on soil organic matter pools and mineralization are less clearly defined, partly because of the short‐term nature of most studies. Herein we describe soil properties from a naturally occurring cold CO₂ spring (Hakanoa) in Northland, New Zealand, at which the surrounding vegetation has been exposed to elevated CO₂ for at least several decades. The mean annual temperature at this site is ≈ 15.5 °C and rainfall ≈ 1550 mm. The site was unfertilized and ungrazed, with a vegetation of mainly C3 and C4 grasses, and had moderate levels of ‘available’ P. Two soils were present ? a gley soil and an organic soil – but only the gley soil is examined here. Average atmospheric CO₂ concentrations at 17 sampling locations in the gley soil area ranged from 372 to 670 ppmv. In samples at 0–5 cm depth, pH averaged 5.4; average values for organic C were 150 g, total N 11 g, microbial C 3.50 g, and microbial N 0.65 g kg^?1, respectively. Under standardized moisture conditions at 25 °C, average rates of CO₂‐C production (7–14 days) were 5.4 mg kg^?1 h^?1 and of net mineral‐N production (14 ?42 days) 0.40 mg kg^?1 h^?1. These properties were all correlated positively and significantly (P < 0.10) with atmospheric CO₂ concentrations, but not with soil moisture (except for CO₂‐C production) or with clay content; they were, however, correlated negatively and mainly significantly with soil pH. In spite of uncertainties associated with the uncontrolled environment of naturally occurring springs, we conclude that storage of C and N can increase under prolonged exposure to elevated CO₂, and may include an appreciable labile fraction in mineral soil with an adequate nutrient supply.     

Carbon footprint of rice production under biochar amendment – a case study in a Chinese rice cropping system

As a controversial strategy to mitigate global warming, biochar application into soil highlights the need for life cycle assessment before large‐scale practice. This study focused on the effect of biochar on carbon footprint of rice production. A field experiment was performed with three treatments: no residue amendment (Control), 6 t ha^?1 yr^?1 corn straw (CS) amendment, and 2.4 t ha^?1 yr^?1 corn straw‐derived biochar amendment (CBC). Carbon footprint was calculated by considering carbon source processes (pyrolysis energy cost, fertilizer and pesticide input, farmwork, and soil greenhouse gas emissions) and carbon sink processes (soil carbon increment and energy offset from pyrolytic gas). On average over three consecutive rice‐growing cycles from year 2011 to 2013, the CS treatment had a much higher carbon intensity of rice (0.68 kg CO₂‐C equivalent (CO₂‐C_e) kg^?1 grain) than that of Control (0.24 kg CO₂‐C_e kg^?1 grain), resulting from large soil CH₄ emissions. Biochar amendment significantly increased soil carbon pool and showed no significant effect on soil total N₂O and CH₄ emissions relative to Control; however, due to a variation in net electric energy input of biochar production based on different pyrolysis settings, carbon intensity of rice under CBC treatment ranged from 0.04 to 0.44 kg CO₂‐C_e kg^?1 grain. The results indicated that biochar strategy had the potential to significantly reduce the carbon footprint of crop production, but the energy‐efficient pyrolysis technique does matter.     

Soil resource heterogeneity in the form of aggregated litter alters maize productivity

Conservation agriculture in its version of permanent raised bed planting with crop residue retention increases yields and improves soil characteristics, e.g. aggregate distribution, organic matter content, so it remained to be seen how greenhouse gas emissions and dynamics of C and N might be altered. The objective of this study was to investigate how conservation agriculture with permanent raised beds, tied ridges, i.e. dykes within the furrows to prevent water run-off, and residue retention affected greenhouse gas emissions. A field experiment was started in 1999 comparing permanent and conventionally tilled raised beds with different residue management under rain fed conditions. Soil was characterized and emissions of CH₄, N₂O and CO₂ and dynamics of NH₄ ⁺, NO₂ ^? and NO₃ ^? were monitored in a laboratory experiment. The crop and tied ridges had no effect on soil characteristics and dynamics of C and N. Tilled beds reduced the water holding capacity (WHC) 1.1 times and increased conductivity 1.3 times compared to soil under nontilled beds with retention of all crop residues. The WHC, organic C, soil microbial biomass and total N were ≥1.1 larger in soil from nontilled beds where the crop residue was retained compared to where it was removed after only 6 years. The emission of CO₂ was 1.2 times and production of NO₃ ^? 1.8 times larger in nontilled beds where the crop residue was retained compared to where it was removed. The CO₂ emission was 1.2 times and the emission of N₂O after 1 day 2.3 times larger in soil under tilled beds compared to nontilled beds with full residue retention, while the increase in concentration of NO₃ ^? was 0.05 mg N kg^?1 soil in the former and 2.38 in the latter. We found that permanent raised bed planting with crop residue retention decreased emissions of N₂O and CO₂ compared to soil under conventionally tilled raised beds. Production of NO₃ ^? is larger in soil with permanent raised bed planting with crop residue retention compared to conventionally tilled raised beds.     

The legacy of stockless organic conversion strategies

Legume‐containing leys are commonly used to improve soil fertility in the 2‐year conversion period from conventional to organic production. While in‐conversion land may be grazed, in stockless farming systems, land is effectively out of production, leading to a reduction in income and pressure on cash flow. The impacts of seven organic conversion strategies on the first organic crop (winter wheat) were previously reported. This study investigates the effect of the conversion strategies on the second (winter beans) and third (winter oats) organic crops, thereby extending the analysis throughout the first complete rotation. The strategies were (a) 2‐years’ red clover–ryegrass green manure, (b) 2‐years’ hairy vetch green manure, (c) red clover for seed production then a red clover–ryegrass green manure, (d) spring wheat undersown with red clover, then a red clover green manure, (e) spring oats, then winter beans, (f) spring wheat, then winter beans and (g) spring wheat undersown with red clover, then a barley–pea intercrop. Conversion strategy had a significant impact on organic bean yield, which ranged from 2.78 to 3.62 t ha^?1, and organic oat yield, which ranged from 3.24 to 4.17 t ha^?1. In the organic bean crop, weed abundance prior to harvest, along with soil texture, accounted for 70% of yield variation. For the oats, soil mineral nitrogen in November together with weed abundance in April accounted for 72% of the variation in yield. The impacts of conversion strategies on soil mineral nitrogen levels were still detectable 3 years after conversion. The results from this study indicate that the choice of conversion crop has important long‐term implications. More exploitative conversion strategies, that is those with a higher proportion of cash cropping, had an increased weed burden and decreased levels of soil mineral nitrogen, leading to reduced yields of beans and oats, 2 and 3 years after conversion.     

Carbon emission and sequestration by agricultural land use: a model study for Europe

A model was developed to calculate carbon fluxes from agricultural soils. The model includes the effects of crop (species, yield and rotation), climate (temperature, rainfall and evapotranspiration) and soil (carbon content and water retention capacity) on the carbon budget of agricultural land. The changes in quality of crop residues and organic material as a result of changes in CO₂ concentration and changed management were not considered in this model. The model was parameterized for several arable crops and grassland. Data from agricultural, meteorological, soil, and land use databases were input to the model, and the model was used to evaluate the effects of different carbon dioxide mitigation measures on soil organic carbon in agricultural areas in Europe. Average carbon fluxes under the business as usual scenario in the 2008–2012 commitment period were estimated at 0.52 tC ha^?1 y^?1 in grassland and ?0.84 tC ha^?1 y^?1 in arable land. Conversion of arable land to grassland yielded a flux of 1.44 tC ha^?1 y^?1. Farm management related activities aiming at carbon sequestration ranged from 0.15 tC ha^?1 y^?1 for the incorporating of straw to 1.50 tC ha^?1 y^?1 for the application of farmyard manure. Reduced tillage yields a positive flux of 0.25 tC ha^?1 y^?1. The indirect effect associated with climate was an order of magnitude lower. A temperature rise of 1 °C resulted in a ?0.05 tC ha^?1 y^?1 change whereas the rising CO₂ concentrations gave a 0.01 tC ha^?1 y^?1 change. Estimates are rendered on a 0.5 × 0.5° grid for the commitment period 2008–2012. The study reveals considerable regional differences in the effectiveness of carbon dioxide abatement measures, resulting from the interaction between crop, soil and climate. Besides, there are substantial differences between the spatial patterns of carbon fluxes that result from different measures.     

Simple models and concepts as tools for the study of sustained soil productivity in long-term experiments. I. New soil organic matter and residual effect of P from fertilizers and farmyard manure in Kabete, Kenya

The rough outcomes of a long-term experiment in Kenya were (re-)interpreted using simple models to find causes of success or failure with regard to sustained soil productivity. A two- pools model calculated the development of soil organic matter, and a practical equation estimated the residual effect of fertilizer P. Relative mineralization rate was 4 and 8%?y^?1 for original and newly formed soil organic carbon (SOC). Maize yielded 0.25 and 1.1 t?ha^?1 per g?kg^?1 of original and new SOC, respectively. Yields of fertilized maize increased initially as a result of increasing residual effects of applied P, but decreased later presumably because SOC declined to below a critical level of 16 g?kg^?1. To maintain SOC above this level, about 10 tons of farmyard manure (dry matter) must be applied annually. Agronomic nutrient use efficiencies for fertilizer N and P were low, but the residual effect of P was high. The simple model outlined half a century ago adequately calculated build-up of new soil organic matter. The estimated residual effect of fertilizer P explained increasing crop responses to repeated P applications. The absence of data on nutrient uptake by the crop strongly limited the understanding of the experimental results.     

Selenium Accumulation by Brassica Napus Grown in Se-Laden Soil From Different Depths of Kesterson Reservoir

Selenium (Se) may be present in soils and sediments in high concentrations and yet not be mobile or available for plant uptake. Phytoremediation of Se by canola (Brassica napus) was evaluated in sediment from Kesterson Reservoir at three different depths (0 to 30, 30 to 60, 60 to 90?cm) under greenhouse and field conditions. In the greenhouse study, total soil Se concentrations at preplant ranged from 10 to 112?mg kg^?1. Shoot Se concentrations of canola were 182, 53, and 19?mg kg^?1 DM in the 0 to 30, 30 to 60, and 60 to 90?cm depths, respectively. Percentages of Se accumulated by canola relative to total Se loss in the soil at postharvest were as high as 24%. In the field study, total soil Se concentrations were as high as 26?mg kg^?1 soil. Field-grown canola accumulated approximately 50?mg kg^?1 DM, which accounted for less than 10% of total Se lost in the soil at postharvest. Phytoremediation of Se-laden soils under field conditions was about 50% of that observed under controlled greenhouse conditions. This relationship may be useful for prediction of field remediation operation using greenhouse generated data.     

Foliar maintenance respiration of subalpine and boreal trees and shrubs in relation to nitrogen content

A nitrogen-based model of maintenance respiration (R_m) would link R_m with nitrogen-based photosynthesis models and enable simpler estimation of dark respiration flux from forest canopies. To test whether an N-based model of R_m would apply generally to foliage of boreal and subalpine woody plants, I measured R_m (CO₂ efflux at night from fully expanded foliage) for foliage of seven species of trees and shrubs in the northern boreal forest (near Thompson, Manitoba, Canada) and seven species in the subalpine montane forest (near Fraser, Colorado, USA). At 10°C, average R_m for boreal foliage ranged from 0.94 to 6.8μmol kg^?1 s^?1 (0.18–0.58 μmol m^?2 s^?1) and for subalpine foliage it ranged from 0.99 to 7.6 μmol kg^?1 s^?1 (0.28–0.64μmol m^?2 s^?1). CO₂ efflux at 10°C for the samples was only weakly correlated with sample weight (r = 0.11) and leaf area (r = 0.58). However, CO₂ efflux per unit foliage weight was highly correlated with foliage N concentration [r = 0.83, CO₂ flux at 10°C (mol kg^?1 s^?1) = 2.62 × foliage N (mol kg^?1)J, and slopes were statistically similar for the boreal and subalpine sites (P=0.28). CO₂ efflux per unit of foliar N was 1.8 times that reported for a variety of crop and wildland species growing in warmer climates.