Substituting energy crops with organic wastes and agro-industrial residues for biogas production

In this study, industrial and agro-industrial by-products and residues (BRs), animal manures (AMs), and various types of organic wastes (OWs) were analyzed to evaluate their suitability as substitutes for energy crops (ECs) in biogas production. A comparison between the costs of the volume of biogas that can be produced from each substrate was presented with respect to the prices of the substrates in the Italian market. Furthermore, four different feeding mixtures were compared with a mixture of EC and swine manure (Mixture A) used in a full-scale plant in Italy. Swine manure is always included as a basic substrate in the feeding mixtures, because many of the Italian biogas plants are connected to farms. When EC were partially substituted with BR (Mixture B), the cost (0.28 € Nm⁻³) of the volume of biogas of Mixture A dropped to 0.18 € Nm⁻³. Furthermore, when the organic fraction of municipal solid waste (OFMSW) and olive oil sludge (OS) were used as possible solutions (Mixtures C and D), the costs of the volume of biogas were −0.20 and 0.11 € Nm⁻³, respectively. The negative price signifies that operators earn money for treating the waste. For the fifth mix (Mixture E) of the OFMSW with a high solid substrate, such as glycerin from biodiesel production, the resulting cost of the volume of biogas produced was −0.09 € Nm⁻³. By comparing these figures, it is evident that the biogas plants at farm level are good candidates for treating organic residues of both municipalities and the agro-industrial sector in a cost-effective way, and in providing territorially diffused electric and thermal power. This may represent a potential development for agrarian economy.     

Olive oil mill wastewater for soil nitrogen and carbon conservation

In this work the application of two levels of N fertilizer (NH₄NO₃) dissolved in water or olive oil mill wastewater (OOMW) diluted 10 or 20 times in water, has been studied in relation to the properties of two soils (Loam and Silt-Clay-Loam). Also, the effect of irrigation water bubbled with CO₂ (Dissolved Inorganic Carbon, DIC) was studied. Nitrate N, ammonium N, total N, organic C (OC), and CaCO₃ contents were determined in the soil as well as pH, electrical conductivity (EC), oxidation–reduction potential (ORP), and absorbance at 250 and 360 nm.     

Household organic waste composting using bins with different types of passive aeration

The objective of this study was to study the performances of six 200-L polyethylene bins, each with different design for passive aeration to organic wastes composting. Food scraps and dry leaves (1.6 kg) were added to each bin once a day until the bin was full. Temperatures at the middle portion were measured daily. The compost from each bin was taken once a week for 120 days for analysis of C, N, volatile solids, and a germination index once a week for 120 days. After 120 days, the compost sample from each bin was taken to determine the mass reduction, size distribution, CEC, N, P and K values. The results showed that the temperatures inside the bins were in the ranges of 24 °C-57 °C. The composts in all bins were found to be stable at around 56-91 days. The wastes decayed fastest in bins with lateral and vertical systems of natural ventilation. It took about two months to stabilize the organic wastes, with a 59-62% decrease of mass. The C/N ratio, CEC, N, P, and K values of the final composts were 14.8-16.0, 66-68 cmol/kg, and 1.26-1.50% N, 0.52-0.56% P₂O₅ and 1.66-1.92% K₂O, respectively.     

Humic acid formation in artificial soils amended with compost at different stages of organic matter evolution

A composting process was conducted under optimal conditions for 150 d, obtaining three biomasses at different levels of maturity: raw material (RM), fresh compost obtained after 11 d of composting (FC), and evolved compost (EC) obtained after 150 d of composting. During the composting process, HAs were extracted and fully characterized by mass balance, DRIFT, and 1H and 13C-nuclear magnetic resonance spectroscopy. Each compost sample was incubated for 180 d in an artificial soil, after which HA extraction was repeated and characterized. To compare composts containing different amounts of labile organic matter (OM), an equal amount of unhydrolyzable OM was added to the soils. Our results indicated that compost HAs consist of a biologically and chemically stable fraction (i.e., the unhydrolyzable HA [U-HA]) and a labile fraction, whose relative contents depended on the composting duration. Humic acid from more EC contained a higher amount of recalcitrant fraction (aromatic carbon) and a lesser amount of labile fraction (aliphatic carbon) than HA from RM and FC. These results suggest that the humification process during composting preserves the more recalcitrant fraction of the compost-alkali soluble/acid insoluble fraction (HA-fraction). Incubation of composts in soil showed that due to the higher labile fraction content, HAs from raw material were more degraded than those from EC. The abundance of labile carbon of soil amended with less-evolved compost (RM and FC) allowed the more recalcitrant fractions of U-HA to be more preserved than in EC. These results suggest that less-evolved compost could contribute more than well evolved compost to the stable soil OM.     

Nitrogen, phosphorus, and potassium uptake by wheat and their distribution in soil following successive, annual compost applications

The overall objective of the present study was to determine the loading limits of composts that should be applied annually to irrigated wheat. We conducted a container experiment in a greenhouse during four years. It included eight treatments: sewage sludge compost (SSC) and cattle manure compost (CMC), each applied annually to a sandy soil, at rates equivalent to 3, 6, and 12 kg m(-2), and two controls, one fertilized and one unfertilized. Total dry matter (DM), grain production, and the amount of N, P, and K taken up by plants increased with increasing compost rate. Nitrogen uptake by the plants of the fertilized control was much higher than by the plants of the highest compost rate. Phosphorus and K uptake by the plants amended with the highest compost rate was much higher than by the fertilized control plants. Inorganic N quantity in the soil increased with increasing compost rate and with successive applications. The net N mineralization during the first year of wheat growth was very low, less than 3.5% of the applied organic N under all compost application rates. The contribution of the organic N mineralization increased during the second and third years. Most of the N increase in the compost treatment was found in the upper layer of 0 to 15 cm, whereas in the fertilized treatment N accumulated from the surface to the bottom of the container, 0 to 55 cm. The successive application of high rates of composts resulted in P and K accumulation in the soil profile.     

Copper and zinc bioavailabilities to ryegrass (Lolium perenne L.) and subterranean clover (Trifolium subterraneum L.) grown in biosolid treated Chilean soils

The purpose of this study was assessing Cu and Zn availabilities in soils amended with a biosolid through the determination of their sequentially extracted chemical forms and their relationship with the contents of these metals in ryegrass (Lolium perenne L.) and subterranean clover (Trifolium subterraneum L.) plant tissues cultivated in a greenhouse using four soils classified as Aquic Xerochrepts and Ultic Haploxeralfs representatives of potential areas for biosolids application in the central zone of Chile. The soils were treated with sewage sludge at a rate of 0 and 30 Mg ha⁻¹. The greenhouse experiment was carried out through a completely randomized block design in a 2 × 4 (biosolid rate × soil) arrangement, considering three repetitions per treatment. The soils used in the greenhouse experiment before and after cultivation, were sequentially extracted with specific reagents and conditions in order to obtain the following fractions: exchangeable, sodium acetate-soluble, soluble in moderately reducing condition, K₄P₂O₇-soluble, soluble in reducing condition, and soluble in strongly acid and oxidizing condition. It was established that Cu and Zn were predominantly found in soils in less available forms, associated to organic matter, oxides and clay minerals. Zinc concentration in ryegrass plants was higher than that found in subterranean clover plants in biosolid-amended soils. Zinc contents in ryegrass shoot and root correlated with the exchangeable, bound-to-carbonate, and bound-to-FeOx metal forms in control soil. Copper and Zn bioavailabilities were estimated through satisfactorily fitted multiple linear regression models, with determination coefficients from 0.77 to 0.99, which showed a positive contribution of the labile metal forms in soils, especially in relation to Zn in both plant species.     

Changes in microbial biomass parameters of a heavy metal-contaminated calcareous soil during a field remediation experiment

Soil microbial biomass parameters give useful information about the restoration degree and quality of contaminated soils. These parameters were studied in a field experiment where the effect of two organic amendments on the bioavailability of heavy metals in an agricultural soil and on their accumulation in Beta vulgaris and Beta maritima was assessed. The soil was a calcareous Xeric Torriorthent and the total metal levels were (mg kg(-1)): 2706 Zn, 3235 Pb, and 39 Cu. The treatments were: fresh cow manure, olive husk, and inorganic fertilizer as a control. Two successive crops (B. vulgaris and B. maritima) were grown on the treated and untreated plots. The soil was sampled before each planting and after each harvest over a 15-mo period. Biomass C and N increased in all plots, especially in the organically amended ones. The ratio CO(2)-C/biomass C decreased in olive husk and manure-treated plots, in comparison with the control, and also during the experiment, suggesting a beneficial effect of the organic amendments. In olive husk-treated plots a significant increase in the ratio of biomass C/total organic carbon (TOC) with time was observed. This indicated a reduction of heavy metal stress on the microbial population. The amendments showed, in general, a beneficial effect on soil quality and fertility, while microbial biomass parameters were found to be useful indicators of the evolution of the remediation processes.     

Cattle feedlot soil moisture and manure content: I. Impacts on greenhouse gases, odor compounds, nitrogen losses, and dust

Beef cattle feedlots face serious environmental challenges associated with manure management, including greenhouse gas, odor, NH3, and dust emissions. Conditions affecting emissions are poorly characterized, but likely relate to the variability of feedlot surface moisture and manure contents, which affect microbial processes. Odor compounds, greenhouse gases, nitrogen losses, and dust potential were monitored at six moisture contents (0.11, 0.25, 0.43, 0.67, 1.00, and 1.50 g H2O g(-1) dry matter [DM]) in three artificial feedlot soil mixtures containing 50, 250, and 750 g manure kg(-1) total (manure + soil) DM over a two-week period. Moisture addition produced three microbial metabolisms: inactive, aerobic, and fermentative at low, moderate, and high moisture, respectively. Manure content acted to modulate the effect of moisture and enhanced some microbial processes. Greenhouse gas (CO2, N2O, and CH4) emissions were dynamic at moderate to high moisture. Malodorous volatile fatty acid (VFA) compounds did not accumulate in any treatments, but their persistence and volatility varied depending on pH and aerobic metabolism. Starch was the dominant substrate fueling both aerobic and fermentative metabolism. Nitrogen losses were observed in all metabolically active treatments; however, there was evidence for limited microbial nitrogen uptake. Finally, potential dust production was observed below defined moisture thresholds, which were related to manure content of the soil. Managing feedlot surface moisture within a narrow moisture range (0.2-0.4 g H2O g(-1) DM) and minimizing the accumulation of manure produced the optimum conditions that minimized the environmental impact from cattle feedlot production.     

Assessment of soil erosion under rainfed sugarcane in KwaZulu‐Natal,South Africa

Soil erosion from agricultural land use runoff is a major threat to the sustainability of soil composition and water resource integrity. Sugarcane is an important cash and food security crop in South Africa, subjected to an intensive soil erosion, and consequently, severe land degradation. This study aimed to investigate soil erosion and associated soil and cover factors under rainfed sugarcane, in a small catchment, KwaZulu‐Natal, South Africa. Three replicated runoff plots were installed at different slope positions (down, mid and upslope) within cultivated sugarcane fields to monitor soil erosion during the 2016–2017 rainy season. On average, annual runoff (RF) was significantly greater from 10 m² plots with 1163.77 ± 2.63 l/m/year compared to 1 m² plots. However, sediment concentration (SC) was significantly lower in 10 m² (0.34 ± 0.04 g/l) compared to 1 m² (6.94 ± 0.24 g/l) plots. The annual soil losses (SL) calculated from 12 rainfall events was 58.36 ± 0.77 and 8.84 ± 0.20 t/ha from 1 m² and 10 m² plots, respectively. The 1 m² plot, SL (2.4 ± 1.41 ton/ha/year) in the upslope experienced 33% more loss than the midslope and 50% more loss than the downslope position. SL was relatively lower from the 10 m² plots than the 1 m² plots, which is explained by high sediment deposition at the greater plot scale. SL was negatively correlated with the soil organic carbon stocks (r = ?0.82) and soil surface cover (r = ?0.55). RF decreased with the increase of slope gradient (r = ?0.88) and soil infiltration rate (r = ?0.87). There were considerable soil losses from cultivated sugarcane fields with low organic matter. These findings suggest that to mitigate soil erosion, soil organic carbon stocks and vegetation cover needs to be increased through appropriate land management practices, particularly in cultivated areas with steep gradients.     

Modeling carbon and nitrogen transformations for adjustment of compost application with nitrogen uptake by wheat

Environmentally sound management of the use of composts in agriculture relies on matching the rate of release of available N from compost-amended soils to the crop demand. To develop such management it is necessary to (i) characterize the properties of composts that control their rates of decomposition and release of N and (ii) determine the optimal amount of composts that should be applied annually to wheat (Triticum aestivum L.). Carbon and N mineralization were measured under controlled conditions to determine compost decomposition rate parameters, and the NCSOIL model was used to derive the organic wastes parameters that control the rates of N and C transformations in the soil. We also characterized the effect of a drying period to estimate the effects of the dry season on C and N dynamics in the soil. The optimized compost parameters were then used to predict mineral N concentration dynamics in a soil-wheat system after successive annual applications of compost. Sewage sludge compost (SSC) and cattle manure compost (CMC) mineralization characteristics showed similar partitioning into two components of differing ease of decomposition. The labile component accounted for 16 to 20% of total C and 11 to 14% of total N, and it decomposed at a rate of 2.4 x 10(-2) d(-1), whereas the resistant pool had a decomposition rate constant of 1.2 to 1.4 x 10(-4) d(-1). The main differences between the two composts resulted from their total C and N and inorganic N contents, which were determined analytically. The long-term effect of a drying period on C and N mineralization was negligible. Use of these optimization results in a simulation of compost mineralization under a wheat crop, with a modified plant-effect version of the NCSOIL model, enabled us to evaluate the effects of the following factors on the C and N dynamics in soil: (i) soil temperature, (ii) mineral N uptake by plants, and (iii) release of very labile organic C in root exudates. This labile organic C enhanced N immobilization following application, and so decreased the N available for uptake by plants.     

Removal of organic pollutants and nutrients from olive mill wastewater by a sand filter

The aim of this work was to examine the performance of a sand filter in treating modern olive mill (OMW) effluents after dilution with domestic wastewater on a one-to-one basis. The experimental pilot consisted of a column of opaque PVC, and the sand filter was filled with 50 cm of sand and 10 cm of gravel in the top and the bottom of the filter. The alimentation (4 cm/day) was done sequentially following a 1 day wet/3 days dry cycle. The OMW effluent was very acidic with a pH of 4.12, and had high concentrations of phenolic compounds (7.2 g/L) and total chemical oxygen demand (65 g/L). The percolation of the diluted OMW through the sand filters caused an increase in pH from 4.84 to 8.25 and a 90% removal of total suspended solids. The sand filter treatment also led to important reductions in organic matter (90% of total COD, 83% of dissolved COD and 92% of phenolic compounds) and nutrients (91% of Kjeldahl-nitrogen, 97% of ammonia-nitrogen, 99% of nitrate-nitrogen and 99% of phosphates). The flow rate became very low indicating clogging of the sand pores after 10 weeks. HPLC analysis of the diluted OMW before and after passage through the sand filter showed an important reduction in the toxic monomeric compounds after the treatment.     

Nitrogen availability from composts for humid region perennial grass and legume-grass forage production

Perennial forages may be ideally suited for fertilization with slow N release amendments such as composts, but difficulties in predicting N supply from composts have limited their routine use in forage production. A field study was conducted to compare the yield and protein content of a binary legume-grass forage mixture and a grass monocrop cut twice annually, when fertilized with diverse composts. In all three years from 1998-2000, timothy (Phleum pratense L.)-red clover (Trifolium pratense L.) and timothy swards were fertilized with ammonium nitrate (AN) at up to 150 and 300 kg N ha(-1) yr(-1), respectively. Organic amendments, applied at up to 600 kg N ha(-1) yr(-1) in the first two years only, included composts derived from crop residue (CSC), dairy manure (DMC), or sewage sludge (SSLC), plus liquid dairy manure (DM). Treatments DM at 150 kg N ha(-1) yr(-1) and CSC at 600 kg N ha(-1) yr(-1) produced cumulative timothy yields matching those obtained for inorganic fertilizer. Apparent nitrogen recovery (ANR) ranged from 0.65% (SSLC) to 15.1% (DMC) for composts, compared with 29.4% (DM) and 36.5% (AN). The legume component (approximately 30%) of the binary mixture acted as an effective "N buffer" maintaining forage yield and protein content consistently higher, and within a narrower range, across all treatments. Integrating compost utilization into livestock systems that use legume-grass mixtures may reduce the risk of large excesses or deficits of N, moderate against potential losses in crop yield and quality, and by accommodating lower application rates of composts, reduce soil P and K accumulation.     

Sewage sludge as organic ameliorant for revegetation of fine bauxite refining residue

The rise in aluminium demand in the world has significantly increased the generation of bauxite residue which occupies huge areas of land worldwide. Direct revegetation of residue storage areas has been unsuccessful because of the high alkalinity and salinity, and poor nutrient contents of the fine residue (red mud). This paper describes glasshouse and field experiments evaluating the potential use of sewage sludge as an organic ameliorant for gypsum amended red mud. The growth of Agropyron elongatum in red mud receiving gypsum (0 and 38.5 t ha⁻¹) and sewage sludge (0, 38.5 and 77 t ha⁻¹) amendment was assessed in a glasshouse study. Leachate and soil analyses revealed that gypsum was effective in reducing the pH, EC and ESP of red mud, while sewage sludge gave additional reductions in EC, Na and ESP. No evidence of any significant increases in heavy metal contents were observed in the leachates following sewage sludge amendment. However, soil Al contents were more available in red mud receiving only sewage sludge treatment. Sewage sludge amendment significantly increased dry weight yield and provided sufficient nutrients for plant growth except K which was marginal. No heavy metal accumulation was observed in Agropyron. Following that, a field experiment was performed having red mud amended with sewage sludge (38.5, 77 and 154 t ha⁻¹) and gypsum (38.5 and 77 t ha⁻¹) to evaluate their effects on soil physical properties of red mud. Sewage sludge significantly reduced soil bulk density (25%) and particle density (9%) and increased the total porosity of red mud (8%). Hydraulic conductivity also increased from 1.5 to 23 × 10⁻⁵ m s⁻¹. Plant cover percentage and dry weight yield of Agropyron increased with an increase in gypsum and sewage sludge amendment. The results confirm that sewage sludge is effective in improving both soil structure and nutrient status of gypsum amended red mud. The use of sewage sludge for red mud revegetation provides not just an option for sludge disposal, but also a cost effective revegetation strategy for bauxite refining industry.     

Predicting phosphorus availability from soil-applied composted and non-composted cattle feedlot manure

Prediction of phosphorus (P) availability from soil-applied composts and manure is important for agronomic and environmental reasons. This study utilized chemical properties of eight composted and two non-composted beef cattle (Bos taurus) manures to predict cumulative phosphorus uptake (CPU) during a 363-d controlled environment chamber bioassay. Ten growth cycles of canola (Brassica napus L.) were raised in pots containing 2 kg of a Dark Brown Chernozemic clay loam soil (fine-loamy, mixed, Typic Haploboroll) mixed with 0.04 kg of the amendments. Inorganic P fertilizer (KH2PO4) and an unamended control were included for comparison. All treatments received a nutrient solution containing an adequate supply of all essential nutrients, except P, which was supplied by the amendments. Cumulative P uptake was similar for composted (74 mg kg-1 soil) and non-composted manures (60 mg kg-1 soil) and for the latter and the fertilizer (40 mg kg-1 soil). However, the CPU was significantly higher for organic amendments than the control (24 mg kg-1 soil) and for composted manure than the fertilizer. Apparent phosphorus recovery (APR) from composted manure (24%) was significantly lower than that from non-composted manure (33%), but there was no significant difference in APR between the organic amendments and the fertilizer (27%). Partial least squares (PLS) regression indicated that only two parameters [total water-extractable phosphorus (TPH2O) and total phosphorus (TP) concentration of amendments] were adequate to model amendment-derived cumulative phosphorus uptake (ACPU), explaining 81% of the variation in ACPU. These results suggest that P availability from soil-applied composted and non-composted manures can be adequately predicted from a few simple amendment chemical measurements. Accurate prediction of P availability and plant P recovery may help tailor manure and compost applications to plant needs and minimize the buildup of bioavailable P, which can contribute to eutrophication of sensitive aquatic systems.     

Increasing exfiltration from pervious concrete and temperature monitoring

Pervious concrete typically has an infiltration rate far exceeding any expectation of precipitation rate. The limiting factor of a retention based pervious concrete system is often defined by how quickly the underlying soil subgrade will infiltrate the water temporarily stored within the concrete and/or aggregate base. This issue is of particular importance when placing a pervious concrete system on compacted fine textured soils. This research describes the exfiltration from twelve pervious concrete plots constructed on a compacted clay soil in eastern Tennessee, USA. Several types of treatments were applied to the clay soil prior to placement of the stone aggregate base and pervious concrete in an attempt to increase the exfiltration rate, including: 1) control – no treatment; 2) trenched – soil trenched and backfilled with stone aggregate; 3) ripped – soil ripped with a subsoiler; and 4) boreholes – placement of shallow boreholes backfilled with sand. The average exfiltration rates were 0.8 cm d⁻¹ (control), 4.6 cm d⁻¹ (borehole), 10.0 cm d⁻¹ (ripped), and 25.8 cm d⁻¹ (trenched). The trenched treatment exfiltrated fastest, followed by the ripped and then the borehole treatments, although the ripped and borehole treatments were not different from one another at the 5% level of significance. The internal temperature of the pervious concrete and aggregate base was monitored throughout the winter of 2006–2007. Although the temperature of the pervious concrete dropped below freezing 24 times, freezing concrete temperatures never coincided with free water being present in the large pervious concrete pores. The coldest recorded air temperature was −9.9 °C, and the corresponding coldest recorded pervious concrete temperature was −7.1 °C. The temperature of the pervious concrete lagged diurnal air temperature changes and was generally buffered in amplitude, particularly when free water was present since the addition of water increases the thermal capacity of the pervious concrete greatly. The temperature of the aggregate base was further buffered to diurnal changes, and no freezing temperatures were recorded.     

Production of lightweight aggregates from mining and industrial wastes

Washing aggregate sludge from a gravel pit, sewage sludge from a wastewater treatment plant (WWTP) and a clay-rich sediment have been physically, chemically and mineralogically characterized. They were mixed, milled and formed into pellets, pre-heated for 5 min and sintered in a rotary kiln at 1150 °C, 1175 °C, 1200 °C and 1225 °C for 10 and 15 min at each temperature. The effects of the raw material characteristics, heating temperatures and dwell times on the loss on ignition (LOI), bloating index (BI), bulk density (ρ_b), apparent and dry particle densities (ρ_a, ρ_d), voids (H), water absorption (WA_24h) and compressive strength (S) were determined. All the mixtures presented a bloating potential taking into consideration the gases released at high temperatures. The products obtained were lightweight aggregates (LWAs) in accordance with Standard UNE-EN-13055-1 (ρ_b ≤ 1.20 g/cm³ or particle density ≤ 2.00 g/cm³). LWAs manufactured with 50% washing aggregate sludge and 50% clay-rich sediment were expanded LWAs (BI > 0) and showed the lowest apparent particle density, the lowest water absorption and the highest compressive strength. It was possible to establish three groups of LWAs on the basis of their properties in comparison to Arlita G3, F3 and F5, commercially available lightweight aggregates manufactured in Spain. Our LWAs may have the same or similar applications as these commercial products, such as horticulture, prefabricated lightweight structures and building structures.     

Evaluation of the efficiency of some sediment trapping methods after a Mediterranean forest fire

Forest fires are common in Mediterranean environments and may become increasingly more frequent as the climate changes. Destruction of the forest cover and litter layer leads to greater overland flow and increased erosion rates. The greatest risk occurs during the first rainstorms following a major fire, so local authorities must act quickly to put erosion control methods in place in order to avoid excessive post-fire sediment loads in river channels. Deciding on which methods to use requires accurate knowledge of their impact on sediment load and an estimate of their cost efficiency. The objective of this study was to evaluate the efficiency of Log Debris Dams (LDDs) and a sedimentation basin for their effectiveness in trapping sediments. Paired sub-catchments were studied to quantify the amount of sediments trapped in stream channels by a series of LDDs and a sedimentation basin. Cost efficiency was evaluated for each of the measures as a function of the cost per unit volume of sediments trapped. In addition, grain size analyses were performed to characterise the nature of the sediments trapped. A third sediment trapping method, Log Erosion Barriers (LEBs) was evaluated more superficially than the first two and conclusions regarding this method are tentative. LDDs trapped a mean volume of 1.57 m³ per unit (median = 1.28 m³); mean LDD height was 105.4 cm (std. dev. = 21.9 cm), and mean height of trapped sediments was only 50.0 cm (std. dev. = 22.9 cm), showing that the traps were only half filled. Sediment height was limited by the presence of gaps between logs or branches that allowed runoff to flow through. Comparison of the textural characteristics of slope and trapped sediments showed distinct sorting: particles greater than 20 mm were not mobilised from the slopes during the study period, sediments in the medium to coarse sand size fractions were trapped preferentially by the LDDs, and sediments in the sedimentation basin were enriched by clay and silt sized (<0.050 mm) particles as coarser sediments were trapped upstream by the LDDs. Cost efficiency of LDDs was estimated at about 143 € m⁻³ for the LDDs and 217 € m⁻³ for the sedimentation basin at the time of sampling. The LDDs are therefore a cost effective method of trapping sediments, but they can only be used when pine trees or straight-trunked trees are locally available. In this case, they should be combined with LEBs, which had a cost efficiency estimated at about 250 € m⁻³. Installation of the LEBs had not been optimised and they have the advantage of trapping sediments on the slopes where they can continue to play an ecological role, so this method can give better results with more care. Sedimentation basins can be emptied if necessary and are useful in areas where pine trees are not available and where the site can be secured.     

Effects of organic amendments on the reduction and phytoavailability of chromate in mineral soil

In this study, seven organic amendments (biosolid compost, farm yard manure, fish manure, horse manure, spent mushroom, pig manure, and poultry manure) were investigated for their effects on the reduction of hexavalent chromium [chromate, Cr(VI)] in a mineral soil (Manawatu sandy soil) low in organic matter content. Addition of organic amendments enhanced the rate of reduction of Cr(VI) to Cr(III) in the soil. At the same level of total organic carbon addition, there was a significant difference in the extent of Cr(VI) reduction among the soils treated with organic amendments. There was, however, a significant positive linear relationship between the extent of Cr(VI) reduction and the amount of dissolved organic carbon in the soil. The effect of biosolid compost on the uptake of Cr(VI) from the soil, treated with various levels of Cr(VI) (0-1200 mg Cr kg(-1) soil), was examined with mustard (Brassica juncea L.) plants. Increasing addition of Cr(VI) increased Cr concentration in plants, resulting in decreased plant growth (i.e., phytotoxicity). Addition of the biosolid compost was effective in reducing the phytotoxicity of Cr(VI). The redistribution of Cr(VI) in various soil components was evaluated by a sequential fractionation scheme. In the unamended soil, the concentration of Cr was higher in the organic-bound, oxide-bound, and residual fractions than in the soluble and exchangeable fractions. Addition of organic amendments also decreased the concentration of the soluble and exchangeable fractions but especially increased the organic-bound fraction in soil.     

Soil Organic Carbon Stock and Distribution in Cultivated Land Converted to Grassland in a Subtropical Region of China

Land-use change from one type to another affects soil carbon (C) stocks which is associated with fluxes of CO₂ to the atmosphere. The 10-years converted land selected from previously cultivated land in hilly areas of Sichuan, China was studied to understand the effects of land-use conversion on soil organic casrbon (SOC) sequestration under landscape position influences in a subtropical region of China. The SOC concentrations of the surface soil were greater (P < 0.001) for converted soils than those for cultivated soils but lower (P < 0.001) than those for original uncultivated soils. The SOC inventories (1.90–1.95 kg m^?2) in the 0–15 cm surface soils were similar among upper, middle, and lower slope positions on the converted land, while the SOC inventories (1.41–1.65 kg m^?2) in this soil layer tended to increase from upper to lower slope positions on the cultivated slope. On the whole, SOC inventories in this soil layer significantly increased following the conversion from cultivated land to grassland (P < 0.001). In the upper slope positions, converted soils (especially in 0–5 cm surface soil) exhibited a higher C/N ratio than cultivated soils (P = 0.012), implying that strong SOC sequestration characteristics exist in upper slope areas where severe soil erosion occurred before land conversion. It is suggested that landscape position impacts on the SOC spatial distribution become insignificant after the conversion of cultivated land to grassland, which is conducive to the immobilization of organic C. We speculate that the conversion of cultivated land to grassland would markedly increase SOC stocks in soil and would especially improve the potential for SOC sequestration in the surface soil over a moderate period of time (10 years).     

Simulating urban waste compost effects on carbon and nitrogen dynamics using a biochemical index

Composting has emerged as a valuable route for the disposal of urban waste, with the prospect of applying composts on arable fields as organic amendments. Proper management of urban waste composts (UWCs) requires a capacity to predict their effects on carbon and nitrogen dynamics in the field, an issue in which simulation models are expected to play a prominent role. However, the parameterization of soil organic amendments within such models generally requires laboratory incubation data. Here, we evaluated the benefit of using a biochemical index based on Van Soest organic matter fractions to parameterize a deterministic model of soil C and N dynamics, NCSOIL, as compared with a standard alternative based on laboratory incubation data. The data included C mineralization and inorganic N dynamics in samples of a silt loam soil (Typic Hapludalf) mixed with various types of UWC and farmyard manure. NCSOIL successfully predicted the various nitrogen mineralization-immobilization patterns observed, but underestimated CO(2) release by 10 to 30% with the less stable amendments. The parameterization based on the biochemical index achieved a prediction error significantly larger than the standard parameterization in only 10% of the tested cases, and provided an acceptable fit to experimental data. The decomposition rates and C to N ratios of compost organic matter varied chiefly according to the type of waste processed. However, 62 to 66% of their variance could be explained by the biochemical index. We thus suggest using the latter to parameterize organic amendments in C and N models as a substitute for time-consuming laboratory incubations.