Increased Aggregate Stability Research Articles

Agricultural management practices impacted carbon and nutrient concentrations in soil aggregates, with minimal influence on aggregate stability and total carbon and nutrient stocks in contrasting soils

Agricultural management practices can affect soil structure and soil organic carbon (SOC) and nutrient stocks, which are important for sustainable agriculture. There is however limited understanding of the long-term impact of management practices on SOC and total nitrogen (N), sulphur (S) and phosphorus (P) concentrations in aggregates from different soils, and consequent effects on SOC and nutrient storage in agro-ecosystems. Soils from long-term (16–46 years) management systems in semi-arid (Luvisol, at Condobolin, NSW), Mediterranean (Luvisol, at Merredin, WA) and sub-tropical (Vertisol, at Hermitage, QLD) environments in Australia were collected from 0 to 10 cm, 10 to 20 cm and 20 to 30 cm depths. Dry- and wet-sieving techniques were used to fractionate the soils into mega-aggregates (>2 mm), macro-aggregates (2–0.25 mm), micro-aggregates (0.25–0.053 mm), and silt-plus-clay particles, including micro-structures (<0.053 mm) i.e. “silt-plus-clay fractions”. Management practices in the Luvisols comprised conventional (CT) and reduced tillage (RT) under mixed crop–pasture rotation, no-till (NT) under continuous cereal–cover crop rotation, and perennial pasture (PP) at Condobolin, and stubble either retained (SR) or burnt (SB) under direct-drilled continuous wheat–legume rotation at Merredin. The practices in the Vertisol comprised a factorial combination of CT, NT, SR, SB, with either 0 (0N) or 90 kg urea-N ha−1 (90N) under continuous wheat–wheat rotation.In the Luvisol at Condobolin, the PP and NT had significantly (p < 0.05) higher soil aggregate stability than the CT and RT, with no impacts of management on SOC and total N, S and P stocks at all depths. The practices in the Luvisol at Merredin and Vertisol at Hermitage had no impact on soil aggregate stability, or on SOC and nutrient stocks at all depths, except the NT-SR-90N at Hermitage showed higher SOC (p < 0.10) and nutrient (p < 0.05) stocks than the other treatments at 0–10 cm only. The SOC and N concentrations were higher (p < 0.05) in the wet-sieved silt-plus-clay fractions and mega-aggregates than macro- and micro-aggregates in the PP and NT at Condobolin, and SR at Merredin, but were similar across aggregates in the CT and RT at Condobolin and SB at Merredin at 0–10 cm depth. Further, at Hermitage, SOC and N concentrations were similar among the aggregate-sizes across different treatments and depths. The only exception was the NT-SR-90N treatment, where SOC and N concentrations were higher (p < 0.05) in the silt-plus-clay fractions or micro-aggregates than in mega- and macro-aggregates, obtained by either dry- or wet-sieving. Total S concentration was in the order of macro- ≥ micro- > mega-aggregates across all the treatments and was higher in the PP at Condobolin (0–10 cm depth), and in the SR at Merredin (all soil depths) than the other corresponding treatments. Further, at Merredin, both SR and SB had higher P concentration in macro- and micro- than mega-aggregates. Across all the practices, SOC and N concentrations were higher in the dry- and wet-sieved silt-plus-clay fractions or micro- than mega- and macro-aggregates in both Luvisols, with no differences in the Vertisol. In summary, although the PP, NT, and SR (compared with other corresponding treatments at each site) had minimal impact on total SOC and nutrient stocks in bulk soils, these practices increased aggregate stability in some systems (i.e. Condobolin), and SOC and nutrient concentrations in the silt-plus-clay fractions or micro-aggregates in both Luvisols. These findings suggest that reducing soil disturbance and enhancing crop residue input in farming systems are important for SOC and nutrient storage, particularly in finer aggregate fractions.

Effect of phenolic acids on the formation and stabilization of soil aggregates

ABSTRACTTo examine the effects of phenolic acids, which are generated by the decomposition of cell walls in plant residues, and other constituents on the stability of soil aggregates, phenolic acids and carbohydrates were mixed into three different types of soil. After a 1-month incubation, the plot containing soil mixed with phenolic acids showed the greatest mean weight diameter of all the soils. In the treated soils, before incubation, the decline of saturated water permeability during continuous water percolation was mitigated in the plot containing soil mixed with phenolic acids compared with that in the other plots. Soil aggregates were synthesized with the addition of phenolic acids and carbohydrates using two methods (mixing and surface brushing) and were incubated for 153 days. The aggregate stability was greatest in the plots surface-brushed with phenolic acids for Andosol and gray lowland soil, whereas the aggregate stability was most stable in the plots mixed with phenolic acids for yellow soil. This difference in the effectiveness of application methods is rationalized by the densities of the active Al and Fe contents, the carbon content, and the specific surface area of the soils. The phenolic acids also affected sandy soil. In a similar experiment using a gray lowland soil, mixing a portion of p-coumaric acid into synthetic aggregates was found to shift the molecular weight distribution of substances to larger molecular weights, as determined by size exclusion chromatography of the liquid extracted from the aggregates, which was likely accompanied by an increase in aggregate stability. The effects of fungi and bacteria on soil long-term stability were not greater than those of phenolic acids. Our findings and previous results show that microorganisms aid in soil-aggregate formation during the early stages, and phenolic acids not only aid in the formation of aggregates but also strongly stabilize them.

Effect of Jatropha curcas Seed Cake on Soil Health Parameters and Growth of Wheat Plant (Triticum aestivum L.) Grown In Sandy and Calcareous Soils

Jatropha curcas seed cake (J.S.C.) is a useful organic byproduct containing considerable amounts of nitrogen, phosphrous, potassium and micronutrients. It can be considered as a green economy soil amendment. Jatropha curcas seed cake was used as an attempt to enhance soil health and to sustain soil productivity. The objective of this study, therefore, was to investigate the effects of Jatropha curcas seed cake (J.S.C.) as an organic fertilizer and NPK as chemical fertilizers on the health parameters of sandy and calcareous soils cultivated with wheat (Triticum aestivum L .). The results indicated that the percentages of N, P and K in J.S.C. were 2.63%, 0.23% and 3.1% respectively. The results assured that application of the organic fertilizer (J.S.C.) and chemical fertilizers (NPK) had positively affected soil health parameters. Using organic and chemical fertilizers decresed bulk density of sandy and calcereous soils compared with the control treatment. And increased aggregate stability index. Soil organic carbon was increased by using Jatropha seed cake as organic fertilizer. In addition, the differences in dehydrogenase activity were highly significant when combined with different levels of organic and chemical fertilization with the two used soils. The soil pH decreased as the levels of Jatropha seed cake increased, while the EC was increased. The increasing application of jatropha seed cake and mineral fertilizationincreased the concentration of macro and micronutrients in plant and soil. The results indicated that the decomposition of jatropha seed cake is a microbiological process, which release inorganic forms of nutrients (N, P and K). It is also that addition of jatropha seed cake to sandy and calcareous soils positively affected the physical, chemical and biological indicators of the soil health.

Does arbuscular and ectomycorrhizal fungal inoculation improve soil aggregate stability? A case study on three tropical species growing in ultramafic Ferralsols

In many tropical regions, soil erosion from anthropogenic activities and subsequent ecological restoration are major issues. Revegetation would be an efficient way to mitigate soil degradation. Some plant and mycorrhizal species are more useful than others for stabilizing soil aggregates and thus reducing erodibility. However, the utility of inoculating plants with mycorrhizas in tropical regions is not clear with regard to the effects on soil aggregate stability. We investigated how plant inoculation with arbuscular (AMF) and ectomycorrhizal (ECM) fungi could improve aggregate stability of ultramafic Ferralsols in New Caledonia. We performed inoculations with local AMF and ECM on three endemic plant species grown in a glasshouse: the sedge Costularia arundinacea (Sol. Ex Vahl) Kük., an AMF host, the shrub Tristaniopsis glauca Brongn. & Gris and the tree Arillastrum gummiferum (Pancher ex Brongn. & Gris) Baill., the latter species are both ECM hosts. We carried out three treatments for each plant species growing in an ultramafic topsoil, (i) control soil with no plants, (ii) non-inoculated plants and (iii) inoculated plants. Inoculation of C. arundinacea was carried out by adding to the topsoil a suspension of mixed AMF spores. T. glauca and A. gummiferum were inoculated with the ECM fungus Pisolithus microcarpus. After 9–10 months, aggregate stability, soil organic carbon, above- and below-ground dry biomass and root traits were measured. Results showed that on ultramafic topsoil, AMF and ECM inoculations significantly increased plant growth, modified roots (especially root length density) and consequently enhanced aggregate stability. Out of the non-inoculated plant species, C. arundinacea had the greatest positive effect on aggregate stability. The association of T. glauca and P. microcarpus was also found to significantly increase aggregate stability. Thus, the inoculation of native plant species with carefully selected mycorrhizal fungi could be an efficient technique to aid in the revegetation of bare slopes.

Ameliorating soil acidity and physical properties of two contrasting texture Ultisols with wastewater sludge biochar.

The production of biochar is a safe and beneficial disposal way for wastewater sludge. The biochar produced from wastewater sludge can be used as soil amendments for improving soil properties and for increasing crop yield. This work investigated the influences of wastewater sludge biochar (WSB) on the pH, exchangeable acidity, and physical properties of strongly acidic Ultisols with contrasting texture (clayey soil and sandy loam). Two soils were mixed with WSB at the rate of 0, 10, 20, and 40g biochar kg-1 soil and incubated for 240days at 75% field water capacity. Incubation experimental results indicated that WSB significantly increased soil pH and exchangeable Ca2+ and Mg2+ contents, and decreased soil exchangeable H+ and Al3+, compared with the control. The application of WSB enhanced the formation of 5-2-mm macroaggregate, and decreased the content of <0.25-mm microaggregate. WSB application significantly increased aggregate stability of soils, determined by mean weight diameter (MWD) of aggregate. WSB increased the field water capacity and available water content (AWC) of sandy loam while WSB was not found to increase significantly water-holding capacity and AWC of clayey soil. WSB significantly reduced plastic index and tensile strength (TS) of clayey soil and did not alter the TS of sandy loam. Overall results suggest that WSB is a suitable amendment for strongly acidic Ultisols with poor physical properties. However, the soil texture affected greatly the improvement effect of WSB on poor physical properties in soils.

Biochar additions can enhance soil structure and the physical stabilization of C in aggregates

Biochar soil amendments are often considered as a soil carbon (C) sequestration strategy that can have beneficial impacts on a range of soil properties and plant production. We investigated the impact of two distinct types of biochar on soil chemical properties, microbial communities, soil aggregation and aggregate-associated C within two California agricultural soils in a laboratory incubation study (60weeks). Water stable aggregation and associated C were examined via wet-sieving to obtain four aggregate size classes: large macroaggregates (2000-8000μm), small macroaggregates (250-2000μm), microaggregates (53-250μm) and silt and clay fraction (<53μm). Biochars enhanced aggregation in the finer textured Yolo soil, with 217% and 126% average increases in mean weight diameter for a softwood biochar (pyrolyzed at 600-700°C with algal digestate) and a walnut shell biochar gasified at 900°C), respectively. The increase in aggregate stability was associated with an increase in physically-protected C incorporated into macroaggregates. Both biochars had substantial impacts on microbial community composition in both soils, but only increased microbial biomass in Yolo soil. In the coarser textured Vina soil, neither biochar had an effect on aggregation and the subsequent lack of soil organic matter (SOM) stabilization in macroaggregates was associated with a significant loss of soil C in both biochar treatments over the course of the incubation. Our results suggest that biochar can increase the physical-protection of SOM in Yolo soil by enhancing the proportion of C stored within macroaggregates and thus offers a novel mechanism by which biochar may contribute to soil C sequestration. Better understanding of these drivers and identifying soil conditions that determine whether biochar will physically protect SOM vs. stimulate soil C loss must be considered in managing agroecosystems for both mitigation of, and adaptation to, climate change.

Organic matter dynamics, soil aggregation and microbial biomass and activity in Technosols created with metalliferous mine residues, biochar and marble waste

Creation of Technosols by use of different materials can be a sustainable strategy to reclaim mine tailings spread on the environment. A proper selection of materials is critical to efficiently contribute to soil creation, with development of soil structure, organic matter stabilization and stimulation of microbial growth. For this purpose, a 90days incubation experiment was designed with biochars derived from different feedstocks, added to tailings alone or in combination with marble waste (MaW). We aimed to assess the effects of the different materials on the evolution of C and N contents and pools, greenhouse gas (GHG) emissions, aggregate stability, and microbial biomass and activity. Results showed that carbonates provided by MaW increased pH around the target value of 8, with significant decrease in salinity by precipitation of soluble salts. Organic C and total N remained stable during the incubation, with high recalcitrant indices. Labile and soluble C and N pools were low in Technosols, with no differences with unamended tailings at the end of incubation. All biochars increased aggregate stability with regard to control by ~40%, with no effect of addition of MaW. Biochars significantly increased microbial biomass C during the first 7days of incubation; however, from this date, there were no significant differences with unamended tailings. The β-glucosidase activity was below detection limit in all samples, while arylesterase activity increased in biochar-amended samples favored by increases in pH. CO2 emissions were not significantly affected by any amendment, while N2O emissions increased with the addition of biochars with lower recalcitrance. CH4 emissions decreased in all Technosols receiving biochar. Thus, the combined use of biochar and MaW contributed to soil C sequestration and improved soil structure. However, labile sources of organic compounds would be needed to stimulate microbial populations in the Technosols.

Elevated CO2 and Tree Species Affect Microbial Activity and Associated Aggregate Stability in Soil Amended with Litter

(1) Elevated atmospheric CO2 (eCO2) may affect organic inputs to woodland soils with potential consequences for C dynamics and associated aggregation; (2) The Bangor Free Air Concentration Enrichment experiment compared ambient (330 ppmv) and elevated (550 ppmv) CO2 regimes over four growing seasons (2005–2008) under Alnus glutinosa, Betula pendula and Fagus sylvatica. Litter from the experiment (autumn 2008) and Lumbricus terrestris were added to mesocosm soils. Microbial properties and aggregate stability were investigated in soil and earthworm casts. Soils taken from the field experiment in spring 2009 were also investigated; (3) eCO2 litter had lower N and higher C:N ratios. F. sylvatica and B. pendula litter had lower N and P than A. glutinosa; F. sylvatica had higher cellulose. In mesocosms, eCO2 litter decreased respiration, mineralization constant (respired C:total organic C) and soluble carbon in soil but not earthworm casts; microbial‐C and fungal hyphal length differed by species (A. glutinosa = B. pendula > F. sylvatica) not CO2 regime. eCO2 increased respiration in field aggregates but increased stability only under F. sylvatica; (4) Lower litter quality under eCO2 may restrict its initial decomposition, affecting C stabilization in aggregates. Later resistant materials may support microbial activity and increase aggregate stability. In woodland, C and soil aggregation dynamics may alter under eCO2, but outcomes may be influenced by tree species and earthworm activity.

Management Options for Contaminated Urban Soils to Reduce Public Exposure and Maintain Soil Health.

Soil management in urban areas faces dual challenges of reducing public exposure to soil contaminants, such as lead (Pb) and polycyclic aromatic hydrocarbons, and maintaining soil function. This study evaluated three management options for an urban lot in Cleveland, OH, containing 185 to 5197 mg Pb kg and 0.28 to 5.50 mg benzo(a)pyrene kg. Treatment options included: (i) cap the site with a soil blend containing compost and beneficially reused dredged sediments, (ii) mix compost with the soil, and (iii) mix compost and sediments with the soil. The soil blend cap reduced surface soil Pb to 12.4 mg Pb kg and benzo(a)pyrene content to 0.99 ± 0.41 mg kg. Aggregate stability for 2- to 0.25-mm aggregates in the soil blend cap was 13% compared with the 38% aggregate stability in the urban soil. Mixing compost with the soil reduced benzo(a)pyrene content, but sample variability indicated that elevated spots likely remained exposed at the surface. Compost addition diluted soil Pb and increased aggregate stability to 60%. Mixing compost and sediments with the soil was the only management option accomplishing both management goals of reducing surface soil contaminants and maintaining soil health. For this combined mixing option, aggregate stability was 37%, soil Pb was 15 mg kg, and benzo(a)pyrene was 0.99 ± 0.09 mg kg. Food-grade oil addition did not increase benzo(a)pyrene degradation. Future studies should evaluate how incorporating soil blends in different soil types with a range of contaminants may offer a suitable long-term management option for urban soil contaminants.

Sewage Sludge Amendment Combined with Green Manuring to a Coastal Mudflat Salt-Soil in Eastern China: Effects on Soil Physicochemical Properties and Maize Yield

Sewage sludge and green manure have become widely used organic amendments to croplands in many regions of the world. However, the amending effect of the combination of sewage sludge with green manuring in reclaimed coastal mudflat salt-soil has been unclear yet. This paper was one of earlier studies to investigate and evaluate the effects of sewage sludge amendment combined with green manuring on selected soil physicochemical properties of the mudflat soil in a rain-fed agroecosystem. The mudflat salt-soil was amended by one-time input of sewage sludge at the rates of 0, 30, 75, 150, and 300 t ha−1. After green manuring for three consecutive seasons, maize (Zea maysL.) was planted in 2013 and 2014. The results showed that SSA combined with green manuring decreased bulk density, pH, salinity, and exchangeable sodium percentage of the topsoil (0–20 cm soil layer) and increased aggregate stability, cation exchange capacity, and N and P concentration of the topsoil. As a result, the maize yield increased with the increase of SSA rates. Sewage sludge combined with green manuring can be applied in coastal mudflat salt-soil amendment, which provides an innovative way to create arable land resources and safe disposal of sewage sludge.

Can soil crusting be reduced through application of gypsum, organic waste, and phosphoric acid?

Soil crusting is a form of land degradation in which the breakdown of aggregates results in the formation of a thin impermeable layer on the soil surface. An earlier pilot trial indicated that application of paper waste, gypsum, phosphoric acid, and covering the soil surface with wire mesh showed potential for reducing soil crusting. This study was established to evaluate the use of products for reducing the severity of soil crusting, while also testing different approaches for measuring the severity and likelihood of soil crust formation. Gypsum was applied at 0.25 and 0.50 kg m–2 (low rate [LG] and high rate [HG], respectively), paper waste was applied at 1, 2.5, and 7.5 kg m–2 (low [LPW], moderate [MPW], and high [HPW] rates, respectively), and phosphoric acid was applied at 80 and 160 mL m–2 (low rate [LP] and high rate [HP], respectively). Combinations of these products were made including (1) wire mesh (WM) and 0.50 kg m–2 gypsum (WM + HG); (2) 0.50 kg m–2 gypsum and 80 mL m–2 phosphoric acid (HG + LP); (3) 2.5 kg m–2 paper waste, 0.50 kg m–2 gypsum, and 80 mL m–2 phosphoric acid (MPW + HG + LP); and (4) 7.5 kg m–2 paper waste and 160 mL m–2 phosphoric acid (HPW + HP). The likelihood of crust formation was inferred from aggregate stability determined by rainfall simulation and wet sieving, while the severity of soil crusting was inferred from crust density, hydraulic conductivity, and penetration resistance. The four measures of crust severity/likelihood were highly correlated with each other (R2 = 0.57 to 0.80). The HPW + HP, MPW + HG + LP, and MPW treatments increased hydraulic conductivity by 72%, 66%, and 45%, respectively; increased aggregate stability determined by rainfall simulation by 28%, 37%, and 39%, respectively; reduced surface density by 10%, 7%, and 6%, respectively; and reduced penetration resistance by 33%, 37%, and 34% as average at all five sampling dates (days 8, 14, 28, 71, and 197). Moreover, the high rate of gypsum significantly reduced bulk density by 7% and penetration resistance by 26%, yet had no effect on any other measure of crusting. Phosphoric acid (HP) significantly increased aggregate stability determined by rainfall simulation by 29% (days 8, 14, 28, and 71), reduced bulk density by 6% (days 8 and 14), and increased hydraulic conductivity at day 8 by 110%. Reduced severity and or likelihood of crust formation following application of gypsum and paper waste were attributed to the increase in calcium cations (Ca+2) and soil organic carbon (C). The paper waste and gypsum were the most effective amendments over the duration of the trial, while phosphoric acid reduced the severity of crust formation in the 14 days after application. Recommendations are provided on the efficiency of different approaches for measuring soil crusting, in which penetration resistance is preferable because of its high correlation with other measurements and being the least time consuming.

Alteration of physical and chemical characteristics of clayey soils by irrigation with treated waste water

The effect of irrigation with treated wastewater (TWW) on soil physico-chemical and hydraulic properties was evaluated in this study. Field treatments were: non-irrigated (rain-fed) plot (control), rain-fed plot for the first three years and irrigated with TWW for the last two years (2yr) and plot irrigated with TWW for five years (5yr). Soil samples were collected from two depth intervals (0–15 and 15–30cm) in five replicates. Irrigation with TWW significantly increased aggregate stability (AS), exchangeable sodium percentage (ESP), organic matter (OM), and electrical conductivity (EC). Both hydraulic conductivity (HC) and cumulative infiltration (F(t)) were decreased significantly with TWW use and period of application. Moreover, reduction of HC at different tension revealed that pore clogging occurred at both, macro and micro scale. Scanning electron microscopy (SEM) images showed that soil pores were clogged partially and/or fully as a result of suspended particulates and organic matter. Enhanced AS of treated areas indicated that infiltration was more affected by pore clogging than soil dispersion and swelling.

Crop residue retention enhances soil properties and nitrogen cycling in smallholder maize systems of Chiapas, Mexico

Conservation agriculture (CA) encompasses three principles: reduced tillage, residue retention, and crop rotation, and is thought to hold great potential for improving soil function and maintaining long-term productivity in agricultural systems. However, the benefits of CA likely depend on the biophysical and socioeconomic context in which CA technologies are implemented and whether or not all components of CA are in place. To better understand the effect of key CA components on soil quality and nutrient use efficiency, an on-farm field trial was conducted during the second year of an irrigated field experiment in Chiapas, Mexico. The experiment included two levels each of tillage and residue management in a full factorial design (three replicates each) with the following treatments: 1) conventional tillage with crop residue; 2) conventional tillage without crop residue; 3) zero tillage with crop residue; and 4) zero tillage without crop residue. In each replicate plot (8×20m) a 15N labeled microplot (0.95×0.93m) was established and fertilized according to standard rates. Mid-season soil samples were analyzed for total and available (Bray) P, as well as microbial community composition using phospholipid fatty acid (PLFA) analysis. Crop yield, nutrient uptake, and aggregate-associated soil organic matter were evaluated at harvest. Crop residue retention led to small increases in aggregate stability and greater fungal PLFA. The retention of crop residues also resulted in a 110% increase in the recovery of fertilizer-derived N in vegetative biomass, leading to a marginally significant increase of 41% for total recovery of fertilizer N in the soil-crop system. Effects of tillage were less prominent, but conventional tillage was found to improve mid-season available P by roughly 60%. These results indicate that CA technologies, residue retention in particular, offer a promising set of options for farmers to improve soil functioning and reduce fertilizer-N losses, particularly for sandy soils within sub-humid tropical regions.

Dairy Cattle Manure Effects on Soil Quality: Porosity, Earthworms, Aggregates and Soil Organic Carbon Fractions

AbstractIn the European Union, the maintenance of soil quality is a key point in agricultural policy. The effect of additions of dairy cattle (Bos taurus) manure (DCM) during a period of 11 years were evaluated in a soil under irrigated maize (Zea mays L.) monoculture. DCM was applied at sowing, at wet‐weight rates of 30 or 60 Mg ha−1yr−1 (30DCM or 60DCM). These were compared with a mineral‐N treatment (300 kg N ha−1, MNF), applied at six to eight emerged leaves and with a control (no N, no manure). Treatments were distributed in a randomized block design. Factors analysed were stability against wetting stress disaggregation, porosity, soil organic carbon (SOC) fractions and earthworm abundance, studied eight months after the last manure application. The application rate of 30DCM increased aggregate stability and the light SOC fraction, but not the pore volume, nor the earthworm abundance, compared with MNF. The DCM rates did not result in unbalanced agronomic advantages versus MNF, as high yields (12–16 Mg ha−1 yr−1) were obtained. In Mediterranean environments, the use of DCM should be encouraged mainly because of its contribution to the light SOC fraction which protects dry macro‐aggregates from implosion (slaking) during the wetting process. Thus, in intensive agricultural systems, it protects soil from physical degradation. Copyright © 2016 John Wiley &amp; Sons, Ltd.

The effect of straw and wood gasification biochar on carbon sequestration, selected soil fertility indicators and functional groups in soil: An incubation study

Annual removal of crop residues may lead to depletion of soil organic carbon and soil degradation. Gasification biochar (GB), the carbon-rich byproduct of gasification of biomass such as straw and wood chips, may be used for maintaining the soil organic carbon content and counteract soil degradation if applied to soil. This study investigated the effect of straw removal and GB addition on soil biological, chemical and physical properties in a 22-months soil incubation study with a temperate sandy loam soil.Soil application of wood and straw GB (WGB and SGB) resulted in very low CO2 emissions, confirming the stability of the material against microbial degradation. Both GBs increased total organic carbon, cation exchange capacity and pH of the soil. The application of SGB and WGB did not affect aggregate stability, whereas SGB did not affect and WGB decreased clay dispersibility. In contrast, the addition of straw resulted in a high soil respiration rate, and about 80% of the added carbon was respired at the end of the incubation. However, the addition of straw increased aggregate stability and decreased clay dispersibility. Results from Fourier transformed infrared photoacoustic spectroscopy revealed a lower content of O–H and aliphatic C–H together with a higher content of aromatic groups in soils amended with GB compared to soils amended with straw. This suggested that the improvement in aggregate stability in straw treatments could be related to microbial derived aliphatics and simple sugars, and that increased stability against microbial degradation in biochar amended soil was related to highly condensed aromatic groups. Addition of nutrients (N, P and S) together with straw resulted in higher soil respiration compared to the straw treatment, but did not cause differences in other soil processes.Results from this study suggest that GB has a potential for increasing soil carbon sequestration, CEC and pH. However, the straw turnover process plays a vital role for aggregate stability and clay dispersibility. Thus, annual straw removal on loamy soil needs to be considered with care in order to avoid soil degradation and risk of soil compaction or erosion.

Effect of Irrigation by Waste Water on Accumulation of Heavy Metals in Soil and Plant

A pot experiment was carried out to determine the effect of using waste water irrigation on the concentrations of (Pb, Cd, Cu and Zn) in soil and their subsequent accumulation in plant. Waste water was diluted to give four treatments: T1 (waste water), T2 (1:1), T3 (2:1) well water: waste water and T4 (well water) as control. Higher concentrations of Pb, Cd, Cu and Zn were observed in soil of T1 compared toT4. Pb and Cd in (tops and roots) of chard were above the maximum permissible level in vegetables (WHO,1984). The results also showed a decrease in microbial density caused by high level of heavy metals concentration and an increase in aggregate stability.

Relative importance of organic carbon, land use and moisture conditions for the aggregate stability of post-glacial clay soils

Erosion and colloidal particle loss from cultivated clay soils in cool humid climate pose a risk to soil productivity and surface water quality. To enhance soil management practices that effectively reduce clay soil erosion, we need to understand the interrelations between the key variables on which clay soil aggregate structure depend. The objective of our study was to quantify the relative importance of OC content, land use and soil wetness, for the aggregate stability of post-glacial clay soils. We estimated the potential effect of five different land uses (cereal cropping with variable tillage and crop rotation, permanent grassland and forest as a control) on clay soil aggregate stability as measured by the percentage of water-stable aggregates (WSA% of 2–5mm fraction) and colloid detachment (turbidity, NTU%), after controlling for the differences in organic carbon content (OC%) and Clay%/OC% ratio between the land uses through covariance analysis. Further, we assessed how aggregate structure in differently managed soils with different OC% tolerates the varying moisture conditions. The higher OC% was reflected in all cultivated soils by increased aggregate stability, implying that OC inputs may positively affect soil structure even in low additions. When compared at equal OC%, the mean WSA% was lower in cereal cropping than in soils under forest or permanent grass, clearly showing the negative effect of tillage frequency on the aggregate stability. The detachment of colloidal particles from aggregates was higher in soils with a high Clay%/OC% ratio, and thus the Clay%/OC% ratio may offer an easily measurable indicator for the risk of clay dispersion, and thereby soil erosion and the export of particle-bound nutrients from cultivated fields. When compared at an equal Clay%/OC% ratio, colloid detachment from aggregates representing cereal fields was two- to threefold as compared to permanent grassland. Cultivated soils with less than 4–6% OC were very sensitive to waterlogging, highlighting the role of proper drainage in erosion control. The results demonstrate that an increase in OC content and proper drainage would help to improve the aggregate stability of cultivated clay soils in cool humid climate, but the effective improvement of the present situation would require drastically reduced soil disturbances for areas with the highest erosion risk.

In situ effects of biochar on aggregation, water retention and porosity in light-textured tropical soils

Biochar (BC) has been reported to improve soil physical properties mainly in laboratory and greenhouse pot experiments. Here we study, under field conditions, the effect of BC and its particle sizes on soil aggregate stability, bulk density (BD), water retention, and pore size distribution in two experiments in Zambia. A) Farmer practice experiment in sandy loam with maize cob BC in conservation farming planting basins under maize and soybeans crops. B) Maize cob and rice husk BC particle size experiments (≤0.5, 0.5–1 and 1–5mm particle sizes) in loamy sand and sand. In the farmer practice experiment, BC increased aggregate stability by 7–9% and 17–20% per percent BC added under maize and soybeans crops respectively (p<0.05) after two growing seasons. Total porosity and available water capacity (AWC) increased by 2 and 3% respectively per percent BC added (p<0.05) under both crops, whereas BD decreased by 3–5% per percent BC added (p≤0.01). In the maize cob BC particle size experiment after one growing season, dose was a more important factor than particle size across the soils tested. Particle size of BC was more important in loamy sand than in sand, with ≤0.5 and 1–5mm sizes producing the strongest effects on the measured properties. For example, BD decreased while total porosity increased (p<0.01) for all BC particle sizes in sand whereas only 1–5mm BC significantly decreased BD and increased total porosity in loamy sand (p<0.05). However, AWC was significantly increased by only ≤0.5 and 1–5mm BCs by 7–9% per percent BC added in both loamy sand and sand. Rice husk BC effect after one year followed similar pattern as maize cob BC but less effective in affecting soil physical properties. Overall, reduced density of soil due to BC-induced soil aggregation may aid root growth and with more water available, can increase crop growth and yields.

Impact of Long-Term Application of Domestic Sewage Water on Soil Properties Around Hubli City in Karnataka, India

The effects of application of domestic sewage water for over four decades on physical, chemical and biological properties of soils were studied at three sites in Gabbur, Mavanur and Katnur villages near Hubli city in North Karnataka, India. Long-term use of sewage irrigation improved soil physical properties in terms of decrease in bulk density and dispersion index and increase in aggregate stability and water holding capacity compared to the unirrigated check. Despite long-term irrigation with sewage water with an EC of >1 dS m−1, the EC of soils was low (0.20–0.45 dS m−1). An increase of one-and-half to two times in organic carbon content, available N, P, K and S, in the sewage irrigated soils was observed compared to soils not irrigated with sewage. However, the available Zn, Fe, Cu and B increased only slightly with the exception of Mn which increased substantially. In general there was a decreasing trend of organic carbon, available N, P, K and S with distance away from the stream course. Despite no heavy metals in sewage water, they were detected in soils but not in toxic levels. There were 94, 80 and 60 % more bacteria, fungi and actinomycetes, respectively, in sewage irrigated soils over the non-irrigated soils. Further, the coliform bacteria (E. coli) were observed in soil samples, suggesting fecal contamination.

Soil aggregate stability to predict organic carbon outputs from soils

Soil structure (e.g. aggregation) has been recognized as a key element in the stabilization of soil organic matter. While aggregate bre akdown is assumed to expose the enclosed soil organic carbon (SOC) to preferential erosion and to accelerated decomposition, the link between the stability of soil aggregates and SOC exports from soils, has either been overlooked or unaccounted for, especially when developing carbon cycle models. This study compared SOC losses in particulate (POC), dissolved (DOC) and gaseous (GOC) forms to an indicator of the soil aggregate stability, the mean weight diameter of aggregates (MWD). SOC outputs were considered at 24 locations of a typical hillslope of the South African Highveld showing clayey to sandy soils. Both POC and DOC were evaluated in-situ under natural rains using 1×1m2 runoff plots while soil CO2 emissions were assessed in the laboratory from undisturbed 0–0.05m soil samples. MWD was finally compared to selected soil and terrain attributes for predictive purpose and as a means to further the understanding of SOC outputs from soils. MWD ranged between 1.4mm for unstable aggregates and 3.4mm for stable aggregates. The increase in aggregate stability resulted in a significant increase in POC and DOC concentrations in the eroded sediments (r=0.76) and in GOC losses from soils (r=0.91 when expressed as g C-CO2 per gram of soil; r=0.95 when as g C-CO2 per gram of soil carbon). In contrast, high aggregate stability induced low total DOC and POC losses (r=−0.81 and −0.77, respectively). The lower POC and DOC losses in the most stable soil aggregates were explained by increased soil infiltration by water and reduced transport by runoff, while the greater CO2 emissions correlated with high SOC concentration. Furthermore, there was a tendency for clayey soils which were fully covered by grass to present stable aggregates and thus to yield greater CO2 emissions but lower POC and DOC outputs than degraded sandy soils of low aggregate stability. Such a quantitative assessment of the role of soil aggregation on SOC outputs might enhance knowledge on organic matter persistence in soils, a prerequisite for developing more accurate global carbon cycle models. Finally further research is required to investigate the downslope to downstream fate of the eroded SOC and to develop land management strategies that aim at lessening carbon losses from soils while enhancing adaptation to climate change.