Search

Summary: The reduced use of phosphorus (P) fertilizer in fertile soil has reverted the P balance to negative in some regions. It is unclear how long current soil P stocks will ensure adequate P supply to crops. In addition, it is unknown if current soil tests for available P describe bioavailable P adequately in soil where P is becoming depleted. We set up an accelerated soil P mining test to address these questions. Perennial ryegrass (Lolium perenne, Melpetra tetra) was grown for 2 years in a greenhouse on 5‐cm‐deep soil layers of eight contrasting soils with periodic grass clipping. Each soil was split into four fertilizer treatments (i.e. no P (–P) and adequate P (+P)) and two nitrogen levels, the latter to alter the rate of P uptake. The long‐term P mining induced P‐related yield losses in seven of the 16 soil treatments. The cumulative uptake of shoot P at which yield loss started to exceed 10% (–P versus +P) varied over a small range of 37–74 mg P kg−1 soil among the soils. This critical cumulative P uptake (CCP) was related to the soil P content prior to mining measured by five soil P tests (ammonium oxalate, ammonium lactate (AL), Olsen P, 0.01  m CaCl2 and the diffusive gradient in thin film technique (DGT)); the largest R2 values were observed for P‐AL (R2 = 0.72) and P‐DGT (R2 = 0.73). However, none of the tests was diagnostic for yield loss during the depletion period. Increased N supply accelerated growth and rates of P uptake and decreased the CCP by a factor of 1.7 on average, illustrating the effect of the rate of biomass production. The CCP values obtained in the treatment with reduced N fertilizer application are likely to be the most relevant for the field and suggest that current stocks allow adequate P supply for arable crops for 3–8 years under zero P application (0–23 cm) in soils similar to those tested. The lack of a successful diagnosis for P deficiency during this depletion experiment calls for further calibration of soil tests for available P in the field. Highlights: The availability of legacy P in well‐fertilized soil was evaluated with a P mining pot trial 10% loss of crop growth occurred when soil P was depleted by 37–74 mg P kg−1 soil Accelerated plant growth with increased N supply decreased total P uptake beyond which P deficiency occurs In a depletion scenario, current soil P tests are not diagnostic but they can be used for prediction [ABSTRACT FROM AUTHOR]

Most soil tests for available phosphorus (P) perform rather poorly in predicting crop response. This study was set up to compare different established soil tests in their capacity to predict crop response across contrasting types of soil. Soil samples from long-term field experiments, the oldest >100 years old, were collected in five European countries. The total number of soil samples ( n = 218), which differed in cropping and P treatment, and originated from 11 different soil types, were analysed with five tests: ammonium oxalate (Ox), ammonium lactate (AL), Olsen P, 0.01 m CaCl2 and the diffusive gradient in thin film (DGT). The first three tests denote available P quantity (Q), whereas the last two indicate P intensity (I) of the soil solution. All five tests were positively related to the crop yield data ( n = 317). The Q-tests generally outperformed I-tests when evaluated with goodness of fit in Mitscherlich models, but critical P values of the I-tests varied the least among different types of soil. No test was clearly superior to the others, except for the oxalate extraction, which was generally poor. The combination of Q- and I-tests performed slightly better for predicting crop yield than any single soil P test. This Q + I analysis explains why recent successes with I-tests (e.g. DGT) were found for soils with larger P sorption than for those in the present study. This systematic evaluation of soil tests using a unique compilation of established field trials provides critical soil P values that are valid across Europe. Highlights We compared soil P tests for predicting crop response across contrasting soil types., No test was clearly superior to the others except for the oxalate extraction, which was generally poor., This study suggests that intensity tests do not perform markedly better than quantity tests., The evaluation of soil P tests on this unique dataset provided critical soil P values across Europe. [ABSTRACT FROM AUTHOR]

Soil organic carbon ( SOC) stocks and their changes are important indicators in ecosystem service assessments. Routine soil inventories are often limited to the topsoil, even though a non-negligible fraction of SOC is known to be stored in deeper horizons. To assess SOC stocks and their changes in the upper metre of the soil profile, vertical extrapolation of topsoil SOC measurements is necessary. The commonly used exponential decline function is not valid, however, for soil types in which subsurface horizons with a larger SOC content ('anomalies') occur. Here, we propose an exponential change decline function to account for these profile anomalies. Therefore, we applied the exponential decline function to the difference between the recent (2008-11) and historical (1947-74) SOC contents in the topsoil and compared the results with those derived by the original method. We applied the exponential change decline function to 54 041 agricultural land units (7159 km2) in Flanders ( Belgium) and were able to model specific profile characteristics such as spodic horizons, plaggic topsoil and peat substrates. For these particular land units, the exponential decline function underestimated SOC stocks; therefore, it compromised an in-depth assessment of changes in SOC stocks over time. This study shows that the exponential change decline function is promising for certain soil types and will contribute to the more accurate assessment of ecosystem service indicators. In addition, we emphasize the need for more detailed descriptions of subsoil reference profiles, sampled by pedogenetic horizon rather than by fixed depth interval to optimize calibration of the decline functions. Highlights When recent soil sampling is limited to the topsoil, extrapolation is needed to assess subsoil SOC stocks., We modified the exponential decline function to model SOC-rich subsurface horizons with the integration of legacy data., An appropriate extrapolation approach is essential for in-depth SOC assessments., Detailed subsoil data are needed to optimize the calibration of the decline functions. [ABSTRACT FROM AUTHOR]

The management of crop residues affects carbon (C)-sequestration. This study aimed to identify the interaction between residue management and soil properties on C-sequestration. The hypothesis was that larger silt and clay contents and larger residue inputs enhance C-sequestration. The soil was sampled in Belgium in long-term (≥ 15 years) cropping systems with grain maize, Zea mays L. (all stover (leaves and stalks) returned), silage maize (all stover removed) or permanent grass. The fields sampled were distributed over two adjacent regions; one with sandy soil (33% silt + clay) and one with silty loam soil (71% silt + clay). The 13C abundance of the soil organic carbon ( SOC) revealed that topsoil (0-30 cm) under grain maize contained more maize-derived SOC than that under silage maize (14 ± 1 and 9 ± 1 Mg C ha−1, respectively, P < 0.001, in sand, and 17 ± 1 and 14 ± 1 Mg C ha−1, respectively, P < 0.001, in silty loam). Total SOC stocks were unaffected by crop management, however, which suggests substitution of native SOC by fresh SOC derived from residues, especially in sandy soil where the silt and clay fraction is saturated with SOC. The maize-derived SOC stocks of the silty loam soil are 3.5-5.5 Mg C ha−1 larger than those of the sandy soil, which confirms the larger potential of the former to sequester C. Surprisingly, the native C3-SOC stocks in the sandy soil were 25-30 Mg C ha−1 larger than those of the silty loam soil. The concentration of SOC in the silt and clay fraction was on average three times larger for the sandy than for the silty loam soil. The concentration of oxalate-extractable Al and Fe in the same fraction was on average 2.5 times larger for sandy than for silty loam soil. Silt and clay-associated SOC correlated with the Fe + Al concentration across all regions and treatments ( R2 = 0.62). This suggests that organo-mineral associations and the formation of organo-metal complexes promote C-sequestration in this temperate region. Highlights Different factors affect carbon stocks under two contrasting soils and four cropping systems., Returning maize residues promotes substitution of native soil organic carbon by maize-derived carbon., Organo-mineral interactions in the silt and clay fraction enhance carbon sequestration., The legacy of historical plaggen manuring has a strong effect on native soil carbon stocks. [ABSTRACT FROM AUTHOR]

Some studies suggest that incorporation of catch crop residues leads to increased availability of P to plants. However, little information is available on how this affects P leaching in soils with a high P load. We tested the effect of catch-crop residue incorporation at the end of winter on the P leaching potential in a soil column experiment under unsaturated conditions using a typical sandy loam soil of NW Europe characterized by a high P load. We sampled the catch crops white mustard ( Sinapis alba L.), Italian ryegrass ( Lolium multiflorum L.), black oats ( Avena strigosa L.) and a perennial ryegrass-white clover mix ( Lolium perenne L.- Trifolium repens L.) from a field trial on catch crops and soil from the plots where they were grown. Plant biomass was incorporated taking account of the differences in conditions of the plant material at the end of winter and the biomass yield of each catch crop. Incorporation of catch-crop residues decreased P leaching compared to the fallow treatment probably through immobilization of soil P during catch crop residue decomposition. The exception was black oats, where the leaching of P was the same as for fallow soil. We observed clear differences in C/N, C/P, water soluble and total P concentration, and biodegradability between the tested catch crops, which seemed to affect the P leaching. We conclude that the incorporation of catch crop residues under typical soil and weather conditions and agricultural practices of NW Europe does not increase the potential P leaching losses. [ABSTRACT FROM AUTHOR]

Accelerated soil erosion leads to sedimentation in reservoirs and a decline in their life span. As many reservoirs in northern Ethiopia are dry at the end of the dry season, we were able to evaluate the potential of using reservoir sediments for land reclamation. Stripped land from which construction material for the reservoirs had been excavated was covered with 0, 15 and 30 cm of sediment and planted with a local garlic cultivar ( Allium sativum). The applied reservoir sediments had low to medium organic C and total N contents and were high in available P and exchangeable cations. The yield of garlic increased with additional available water and the application of sediments. The results show that total biomass and bulb yield were three times higher on the reclaimed plots than on the control ones (11.7 t/ha vs. 3.6 t/ha for the biomass; 7.7 t/ha vs. 2.0 t/ha for the yield). When sediment transport and labour costs were taken into account, plots with 15 cm of sediments had in the first cropping season a cost-benefit ratio of 3, whilst those with 30 cm had a cost-benefit ratio of 0.9. This study demonstrates that the use of relatively small quantities of reservoir sediments is an economically viable strategy for land reclamation. The result can be improvement in income for resource-poor farmers by as much as 76%, and the life expectancy of the reservoirs is also increased. [ABSTRACT FROM AUTHOR]

It is well established that dissolved organic matter (DOM) mobilizes copper (Cu) in soils but it is unknown to what extent variable DOM quality affects this. During a 5 month period, 250 leachates of an uncontaminated agricultural soil were sampled at 45 cm depth using passive capillary wick samplers. The dissolved Cu and organic carbon (DOC) concentrations varied sevenfold and were weakly correlated ( r = 0.56). The [Cu] : [DOC] ratio varied fivefold and exhibited a significant positive correlation ( r = 0.77) with the specific UV-absorbance of DOM at 254 nm (SUVA), indicating that the more aromatic DOM had higher Cu affinity. The dissolved Cu concentrations were predicted by an assemblage model in WHAM6 using the composition of the solid phase above the wick samplers and that of the solution, including DOC. The predicted [Cu] : [DOC] ratio was almost constant when assuming default DOM properties with 65% of all DOM active as fulvic acid (%AFA). The %AFA was subsequently varied proportionally to the SUVA of DOM and using the SUVA of pure FA (SUVAFA) as a fitting parameter. In that case, the variation in the predicted [Cu] : [DOC] ratio was much larger and the predicted Cu concentrations were within a factor of 1.4 of the measured values for 90% of the samples. The fitted SUVAFA was 38 l g−1 cm−1, in excellent agreement with that of Suwannee River FA (SUVAFA = 37 l g−1 cm−1). It is concluded that the DOM quality, e.g. the aromaticity, should be taken into account when estimating Cu mobility in soils. [ABSTRACT FROM AUTHOR]

Wind and water erosion induce breakdown of soil aggregates and loss of soil organic matter. Whereas most of the relations between aggregation and its driving factors have been established on a plot scale, these relations might be very different within an undulating landscape where both erosion (by wind or water) and deposition occur. The aim of this study was to investigate to what degree spatial patterns in soil variables influence spatial patterns in aggregation under different tillage intensities. We studied an agricultural field of about 3 ha in the silty region of Belgium. The site was split into a conventional tillage (CT) and a minimum tillage (MT) system. Within the field, 396 geo-referenced surface soil samples (0–5 cm) were taken and analyzed for organic matter content, quantity of aggregates and a number of other soil properties. Under CT, 28.5% of the total sample variation was explained by the occurrence of depositional areas, 20.8% by the amount of soil organic matter, and 13.8% by the presence of a clay-rich B horizon which surfaced due to progressive water and tillage erosion. Regression analysis revealed that 27% of the variation in the quantity of macroaggregates (>0.25 mm) was accounted for by these three factors. Under MT, 27.1% of the total sample variation was related to the surface cover of Tertiary sand, 22.6% to the amount of soil organic matter, and 13% to erodibility. These three factors explained 53% of the variation in the quantity of macroaggregates. In the CT system, the correlation between grass- or maize- carbon and the quantity of macroaggregates was strongly linked to erodibility, while this was not the case in the MT system. We concluded that at this site, macroaggregation is dominated by landscape-scale processes (such as water or tillage erosion) rather than determined by the commonly considered local variables (such as small variations in texture or organic matter content). [ABSTRACT FROM AUTHOR]

The potential effect of acidification of contaminated sandy soils on Cd transport in the unsaturated zone was assessed. Forty-eight soil profiles were sampled at five depths in a polluted field that was set aside in 1992. The Cd concentration in the top 30 cm of this field was, on average, 10 mg kg−1. A column experiment was carried out with one of the topsoil samples. Homogeneously packed columns were leached with 0.001m CaCl2, adjusted to pH 3 or pH 5.7, at a pore water velocity of 6 cm day−1. The Cd and proton transport was predicted with coupled transport equations. The Cd transport was modelled by assuming local equilibrium and by using sorption parameters derived from batch experiments, while acidification was modelled with a kinetic approach, on the assumption that proton buffering was due to cation exchange and mineral weathering. Organic matter was the main contributor to the cation exchange capacity of these soils. Observed and predicted pH and Cd profiles in the columns agreed well. With the same model, the proton and Cd transport at field scale was calculated for each of the 48 profiles sampled (‘grid model’). It was predicted that the field-averaged Cd concentration in the seepage water will increase from 6 μg litre−1 at present to 200 μg litre−1 over 260 years, which greatly exceeds the maximum permissible concentration (MPC) in groundwater of 5 μg litre−1. Predictions of Cd transport using field-averaged soil properties yielded a later breakthrough time and a larger peak Cd concentration than predicted with the grid model, which illustrates the impact of spatial variability on solute transport. Continuation of liming practices is a possible solution to prevent breakthrough of Cd at concentrations far in excess of the MPC. [ABSTRACT FROM AUTHOR]

The magnitude of and mechanisms for long-term differences in soil organic matter stocks under no-tillage and conventional tillage are still relatively poorly known. We quantified differences in total C and N stocks after 32 years of no-tillage (NT) and conventional tillage (CT) in plots with a long-term cultivation history before differentiation and the same annual C and N returns to the soil. The role of physical protection of organic matter (OM) in these stock differences was further investigated by examining the changes at different levels of structural complexity, i.e. organic matter fractions, aggregation and pore-size distribution. Four structural zones were sampled: loose and dense soil zones under CT and the 0–5 cm (rich in OM) and 5–20 cm (massive structure) soil layers under NT. The C and N stocks, calculated for an equivalent mass of dry soil, were only 10–15% larger under NT than under CT. Mineral-associated N and particulate organic matter accounted for about 50% of the difference in N stocks. However, 66% of the total difference in C stocks was due to differences in the particulate organic matter (58%) and free residues (8%) fractions. The additional C and N under NT were almost exclusively situated in aggregates larger than 250 μm in diameter. Our results suggest that physical protection of OM under NT contributes significantly to the differences in C and N stocks between NT and CT by (i) enhanced macroaggregate formation in the 0–5 cm layer due to greater microbial activity and OM content and (ii) a better protection of soil organic matter in the 5–20 cm layer due to the presence of small pores and lack of soil disruption by tillage or climate. [ABSTRACT FROM AUTHOR]

One way to increase the amount of carbon sequestered in agricultural land is to convert conventional tillage into no-tillage systems. This greatly affects the location of crop residues in soil. To investigate the impact of the location of residues on soil physical and biological properties and how the interactions between those properties influence the fate of carbon and nitrogen in soil, we did a laboratory experiment with repacked soil in columns. Doubly labelled 13C15N oilseed rape residues were incorporated in the 0–10 cm layer or left on the soil surface. The columns were incubated for 9 weeks at 20°C and were submitted to three cycles of drying and wetting, each of them induced by a rain simulator. The location of the residues affected the water dynamics and the distribution of C and N in the soil, which in turn influenced microbial activity and the decomposition rate of the added residues. After 9 weeks of’incubation, 18.4 ± 1.5% of the surface applied residue-C and 54.7 ± 1.3% of the incorporated residue-C was mineralized. We observed a nitrate accumulation of 10.7 mg N kg−1 with residues at the soil surface, 3.6 mg N kg−1 with incorporated residues and 6.3 mg N kg−1 without addition of fresh organic matter, which entailed net N mineralization in soil under mulch and immobilization of N with residue incorporation compared with the control soil. We concluded that application of oilseed rape residues at the soil surface increased the storage of fresh organic C in soil in the short term, compared with the incorporation treatment, but increased the risk of nitrate leaching. [ABSTRACT FROM AUTHOR]

Recent studies have concluded that the dynamics of soil structure are central to the understanding of soil organic matter (SOM) cycling and the ensuing soil-water–nutrient relationships. Aggregate turnover directly controls the stabilization and physical protection of SOM. Therefore, quantifying aggregate dynamics will improve our ability to predict SOM behaviour as affected by ecosystem management and global change. We present an approach to directly quantify aggregate dynamics using rare-earth oxides as tracers. A 6-week laboratory incubation was set up to measure aggregate dynamics at different times. We made samples in which each different aggregate size-fraction contained a different tracer. By following the redistribution of these tracers into the other aggregate size-fractions, we could quantify all soil mass transfers between aggregate size-fractions. A comparison with a control soil showed that the tracer did not affect soil respiration or the aggregation process itself. Tracer mixing homogeneity, recovery and immobility were tested and validated. While initially macroaggregate formation occurred rapidly, microaggregate formation occurred more slowly during the experiment. Subsequent aggregate stabilization was more pronounced for the newly formed microaggregates than for the newly formed macroaggregates. Calculated turnover times were smaller for macroaggregates than for microaggregates (i.e. 30 vs. 88 days). Further research is needed to investigate to what extent these results can be extrapolated to the field. Our results confirmed existing qualitative views and concepts on aggregate dynamics in a quantitative way and will be valuable in directly linking aggregate turnover to the stabilization and protection of SOM. [ABSTRACT FROM AUTHOR]

One of the main effects of soil tillage is that it determines the distribution of crop residues in the soil profile. Little information is available on how this affects the distribution of carbon (C) and nitrogen (N) in the soil and more specifically in soil aggregates, as in general the contribution of other effects of tillage (i.e. mechanical disturbance) cannot be distinguished. A laboratory experiment was conducted with repacked soil columns to improve our understanding of the mechanisms of soil aggregate stabilization and storage of organic C in relation to residue location. Double-labelled oilseed rape residues (13C and 15N) were incorporated in the 0–10 cm soil layer or left on the soil surface. The soil columns were incubated for 33 weeks at 20°C. Artificial rain events followed by evaporation imposed several dry-wet cycles on the soil-residue system. Spatial separation of surface-placed residues from the soil slowed down the decomposition rate compared with incorporated residues. This resulted, in the short term, in a large fraction of residue fragments at the soil surface (equivalent to 52% of carbon applied). In contrast, initial crop residue location did not significantly affect incorporation of residue-C in soil aggregates (only 10–12% of carbon applied), as new C entered the aggregate fractions mainly as soluble C. However, concentrations of residue-derived C and N were greater and aggregation increased near the soil surface with mulch, which in the longer term can modify physical and biological properties of the upper cm of the soil. [ABSTRACT FROM AUTHOR]

It has been hypothesized that aggregate stability is partly caused by subcritical (i.e. mild) water repellence. We conducted both a field study at a pasture site and an incubation experiment in the laboratory to characterize the relationship between aggregate dynamics and water repellence. In the field study, we investigated 53 samples from a natural texture gradient for water repellence and aggregation. In the 3-week incubation experiment, we added wheat straw to crushed soil material from five of the 53 soils representative of the texture gradient (a sandy loam, two loams, a silt loam and a clay loam), and followed changes in aggregate formation and water repellence. Although there was a dramatic increase ( P < 0.0001) in aggregation in all the soils during incubation (the mean weight diameter increased from about 300 μm to at least 900 μm), we observed a concomitant increase in water repellence only in the clay loam soil ( P = 0.0003). We found no significant correlations between water repellence and aggregation ( n = 53, P > 0.05) in the field. Whereas correlation between aggregate formation and texture was weak in the incubation experiment, we found a significant correlation between aggregation and textural parameters in the field. The amount of large macro-aggregates (> 2000 μm) was positively correlated with clay content ( n = 53, r = 0.53, P < 0.001), and negatively with sand content ( n = 53, r = −0.46, P < 0.001). These results indicate that (i) microbially induced water repellence might only become apparent when enough easily decomposable substrate is available, (ii) soil texture affects aggregate stabilization rather than aggregate formation, and (iii) aggregate formation is not necessarily associated with an increase in water repellence. [ABSTRACT FROM AUTHOR]

We report a case of cryptococcal meningitis in an immunocompetent female patient with exposure to a pet magpie (Pica pica). Genetically indistinguishable isolates were cultured from the cerebrospinal fluid of the patient and excreta of the bird. Our data strongly suggest zoonotic transmission of Cryptococcus neoformans var. grubii from a magpie to this patient. [ABSTRACT FROM AUTHOR]

Lagrou K, Van Eldere J, Keuleers S, Hagen F, Merckx R, Verhaegen J, Peetermans WE, Boekhout T (University Hospital Leuven, Leuven, Belgium; and Centraalbureau voor Schimmelcultures, CT Utrecht, the Netherlands) Zoonotic transmission ofCryptococcus neoformansfrom a magpie to an immunocompetent patient (Case report).J Intern Med2005;257:385–388.We report a case of cryptococcal meningitis in an immunocompetent female patient with exposure to a pet magpie (Pica pica). Genetically indistinguishable isolates were cultured from the cerebrospinal fluid of the patient and excreta of the bird. Our data strongly suggest zoonotic transmission ofCryptococcus neoformansvar.grubiifrom a magpie to this patient. [ABSTRACT FROM AUTHOR]

The use of ultrasonic energy for the dispersion of aggregates in studies of soil organic matter (SOM) fractionation entails a risk of redistribution of particulate organic matter (POM) to smaller particle-size fractions. As the mechanical strength of straw also decreases with increasing state of decomposition, it can be expected that not all POM will be redistributed to the same extent during such dispersion. Therefore, we studied the redistribution of POM during ultrasonic dispersion and fractionation as a function of (i) dispersion energy applied and (ii) its state of decomposition. Three soils were dispersed at different ultrasonic energies (750, 1500 and 2250 J g−1 soil) or with sodium carbonate and were fractionated by particle size. Fraction yields were compared with those obtained with a standard particle-size analysis. Undecomposed or incubated (for 2, 4 or 6 months)13C-enriched wheat straw was added to the POM fraction (0.25–2 mm) of one of the soils before dispersion and fractionation. Dispersion with sodium carbonate resulted in the weakest dispersion and affected the chemical properties of the fractions obtained through its high pH and the introduction of carbonate. The mildest ultrasonic dispersion treatment (750 J g−1) did not result in adequate soil dispersion as too much clay was still recovered in the larger fractions. Ultrasonic dispersion at 1500 J g−1 soil obtained a nearly complete dispersion down to the clay level (0.002 mm), and it did not have a significant effect on the total amount of carbon and nitrogen in the POM fractions. The 2250 J g−1 treatment was too destructive for the POM fractions since it redistributed up to 31 and 37%, respectively, of the total amount of carbon and nitrogen in these POM fractions to smaller particle-size fractions. The amount of13C-enriched wheat straw that was redistributed to smaller particle-size fractions during ultrasonic dispersion at 1500 J g−1 increased with increasing incubation time of this straw. Straw particles incubated for 6 months were completely transferred to smaller particle-size fractions. Therefore, ultrasonic dispersion resulted in fractionation of POM, leaving only the less decomposed particles in this fraction. The amounts of carbon and nitrogen transferred to the silt and clay fractions were, however, negligible compared with the total amounts of carbon and nitrogen in these fractions. It is concluded that ultrasonic dispersion seriously affects the amount and properties of POM fractions. However, it is still considered as an acceptable and appropriate method for the isolation and study of SOM associated with silt and clay fractions. [ABSTRACT FROM AUTHOR]

We sought to examine the distribution of carbon (C) decomposition within the framework of the soil pore system. Soils were sampled from a transect having a natural gradient in pore-size distribution. After the addition of labelled wheat straw (13C) the repacked soil columns were incubated (25°C) at soil water matric potentials of either−75 kPa or−5 kPa and for either 4 or 90 days. Pore-size distribution was determined for each soil column after incubation and soils were then analysed for soluble C, label-derived residual C, label-derived and native biomass C, nematode abundance, and ergosterol concentration as an indicator of fungal biomass. Overall, the data suggested that pore-size distribution and its interaction with soil water give rise to a highly stratified biogeography of organisms through the pore system. This results in different rates of decomposition in pores of different size. Added plant material seemed to decompose most rapidly in soils with a relatively large volume of pores with neck diametersc. 15–60 µm and most slowly in soils with large volumes of pores with neck diameters< 4 µm. Regression analysis suggested that at matric potentials of both−75 kPa and−5 kPa the fastest decomposition of organic substrate occurred close to the gas–water interface. This analysis also implied that slower rates of decomposition occur in the pore class 60–300 µm. Correlations between the mass of soil biota and the pore volume of each pore class point to the importance of fungi and possibly nematodes in the rapid decomposition of C in the poresc. 15–60 µm during the early stages of decomposition. [ABSTRACT FROM AUTHOR]

The pH dependent cation exchange capacity (CEC) and pH buffering capacity (pH BC) are two crucial properties in soil fertility management of variable charge soils. However, they are rarely measured, since most of the existing methods are cumbersome and time-consuming. We propose a new method, based on the silver-thiourea (AgTU) method, for the simultaneous measurement of pH dependent CEC and the pH BC. In general, this method consists of first increasing the pH of the soil to about pH 7 and then gradually acidifying the soil by titrating with HNO3, while measuring pH and CEC after each equilibration. This way, both CEC at different pH values and the pH BC are measured on the same sample, without the multiple washing steps needed in other procedures. Various aspects of the proposed method were tested and the results obtained were compared with the ion adsorption method. The modified AgTU method obtains field relevant results and is suitable for the routine analysis of large numbers of samples. [ABSTRACT FROM AUTHOR]

The prediction of the mobility of arsenic (As) is crucial for predicting risks in soils contaminated with As. The objective of this study is to predict the distribution of As between solid and solution in soils based on soil properties and the fraction of As in soil that is reversibly adsorbed. We studied adsorption of As(V) in suspensions at radiotrace concentrations for 30 uncontaminated soils (pH 4.4–6.6). The solid–liquid distribution coefficient of As ( Kd) varied from 14 to 4430 l kg−1. The logarithm of the concentration of oxalate-extractable Fe explained 63% of the variation in log Kd; by introducing the logarithm of the concentration of oxalate-extractable P in the regression model, 85% of the variation in log Kd is explained. Double labelling experiments with 73As(V) and 32P(V) showed that the As to P adsorption selectivity coefficient decreased from 3.1 to 0.2 with increasing degree of P saturation of the amorphous oxides. The addition of As(V) (0–6 mmol kg−1) reduced the Kd of 73As up to 17-fold, whereas corresponding additions of P(V) had smaller effects. These studies suggest that As(V) is adsorbed to amorphous oxides in soils and that sites of adsorption vary in their selectivity in respect of As and P. The concentration of isotopically exchangeable As in 27 contaminated soils (total As 13–1080 mg kg−1) was between 1.2 and 19% (mean 8.2%) of its total concentration, illustrating that a major fraction of As is fixed. We propose a two-site model of competitive As(V)–P(V) sorption in which amorphous Fe and Al oxides represent the site capacity and the isotopically exchangeable As represents the adsorbed phase. This model is fitted to 73As adsorption data of uncontaminated soils and explains 69% of the variation of log Kd in these soils. The log Kd in contaminated soils predicted using this two-site model correlated well with the observed log Kd ( r = 0.75). We conclude that solubility of As is related to the available binding sites on amorphous oxides and to the fraction of As that is fixed. [ABSTRACT FROM AUTHOR]

Isotopically exchanged phosphorus is difficult to determine in soils that strongly sorb P (so that there is little P in solution) and in soils with large concentrations of colloidal P in soil suspensions. A method is proposed in which anion exchange membranes (AEM) are added to the soil suspension after an initial period of isotopic exchange with 32P-labelled phosphate ions. Isotopically exchanged P, termed EAEM, is calculated from the ratio of labelled phosphate ions to the total phosphate ions on the membrane. The EAEM was compared with the E value measured in an aqueous soil extract ( EWater extract) for 14 soils with different degrees of P sorption. The two methods gave similar results in soils with large P concentrations in an aqueous soil extract. However, EWater extract values significantly exceeded the EAEM values by up to 18-fold when soluble P was near the determination limit (0.008 mg P l−1). In a second experiment, two Ferralsols received further P from inorganic and plant sources and were incubated for 7 days. Treatment effects on labile P were erroneous as detected by the EWater extract but were significant as detected with the AEM method. Third, EAEM values were followed in a Lixisol and a Ferralsol which received labelled phosphate ions with carrier just before the beginning of a 23-day incubation. The approximate recovery of added inorganic P in the EAEM value suggested that this method adequately samples labile P in P-sorbing soils. All these results showed that errors in the determination of E values for soils with very small concentrations of P in the soil solution are reduced using the proposed method. [ABSTRACT FROM AUTHOR]

Summary Risk assessment of heavy metals in soil requires an estimate of the concentrations in the soil solution. In spite of the numerous studies on the distribution of Cd and Zn in soil, few measurements of the distribution coefficient in situ , K d , have been reported. We determined the K d of soils contaminated with Cd and Zn by measuring metal concentrations in the soil and in the soil solution and attempted to predict them from other soil variables by regression. Soil pH explained most of the variation in logK d (R 2 = 0.55 for Cd and 0.70 for Zn). Introducing organic carbon content or cation exchange capacity (CEC) as second explanatory variable improved the prediction (R 2 = 0.67 for Cd and 0.72 for Zn), but these regression models, however, left more than a factor of 10 of uncertainty in the predicted K d . This large degree of uncertainty may partly be due to the variable degree of metal fixation in contaminated soils. The labile metal content was measured by isotopic dilution (E value). The E value ranged from 18 to 92% of the total metal content for Cd and from 5 to 68% for Zn. The prediction of K d improved when metals in solution were assumed to be in equilibrium with the labile metal pool instead of the total metal pool. It seems necessary therefore to discriminate between ‘labile’ and ‘fixed’ pools to predict K d for Cd and Zn in field contaminated soils accurately. Dilute salt extracts (e.g. 0.01 m CaCl2 ) can mimic soil solution and are unlikely to extract metals from the fixed pool. Concentrations of Cd and Zn in the soil solution were predicted from the concentrations of Cd and Zn in a 0.01 m CaCl2 extract. These predictions were better correlated with the observations for field contaminated soils than the predictions based on the regression equations relating logK d to soil properties (pH, CEC... [ABSTRACT FROM AUTHOR]

Sediment and total phosphorus (TP) export vary through space and time. This study was conducted to determine the factors controlling sediment and TP export in two agricultural catchments situated in the Belgian Loess Belt. At the outlet of these catchments runoff discharge was continuously measured and suspended sediment samples were taken during rainfall events. Within the catchments vegetation type and cover, soil surface parameters, erosion features, sediment pathways, and rainfall characteristics were monitored. Total P content and sediment characteristics such as clay, organic carbon, and suspended sediment concentration were correlated. Total sediment and TP export differ significantly between the monitored catchments. Much of the difference is due to the occurrence of an extreme event in one catchment and the morphology and spatial organization of land use in the catchments. In one catchment, the direct connection between erosive areas and the catchment outlet by means of a road system contributed to a high sediment delivery ratio (SDR) at the outlet. In the other catchment, the presence of a wide valley in the center of the catchment caused sediment deposition. Vegetation also had an effect on sediment production and deposition. Thus, many factors control sediment and TP export from small agricultural catchments; some of these factors are related to the physical catchment characterisics such as morphology and landscape structure and are (semi)permanent, while others, such as vegetation cover and land use, are time dependent. [ABSTRACT FROM AUTHOR]

Identifying ‘functional' pools of soil organic matter and understanding their response to tillage remains elusive. We have studied the effect of tillage on the enriched labile fraction, thought to derive from microbes and having an intermediate turnover time. Four soils, each under three regimes, long-term arable use without tillage (NT), long-term arable under conventional tillage (CT), and native vegetation (NV), were separated into four aggregate size classes. Particle size fractions of macro- (250–2000 μm) and microaggregates (53–250 μm) were isolated by sonication and sieving. Subsequently, densiometric and chemical analyses were made on fine-silt-sized (2–20 μm) particles to isolate and identify the enriched labile fraction. Across soils, the amounts of C and N in the particle size fractions were highly variable and were strongly influenced by mineralogy, specifically by the contents of Fe and Al oxides. This evidence indicates that the fractionation procedure cannot be standardized across soils. In one soil, C associated with fine-silt-sized particles derived from macroaggregates was 567 g C m-2 under NV, 541 g C m-2 under NT, and 135 g C m-2 under CT, whereas C associated with fine-silt-sized particles derived from microaggregates was 552, 1018, 1302 g C m-2 in NV, NT and CT, respectively. These and other data indicate that carbon associated with fine-silt-sized particles is not significantly affected by tillage. Its location is simply shifted from macroaggregates to microaggregates with increasing tillage intensity. Natural abundance 13C analyses indicated that the enriched labile fraction was the oldest fraction isolated from both macro- and microaggregates. We conclude that the enriched labile fraction is a ‘passive' pool of soil organic matter in the soil and is not derived from microbes nor sensitive to cultivation. [ABSTRACT FROM AUTHOR]

Spinach plants were grown in hydroponic culture provided with variable limiting amounts of N. During a complete diurnal cycle, growth of the root and shoot parts, as well as levels of soluble and insoluble sugars and of free amino acids, were monitored. No clear relationship could be detected between the level of N feeding and the levels of free sugars and amino acids. Analysis of variance revealed that the variances in the relative growth rates of plant root and shoot could be correlated with the levels of sugars and amino acids. Root amino acid concentration could be correlated with shoot amino acid concentration and root sugar concentration. No relationship was found between the variances in root and shoot free sugar concentrations. [ABSTRACT FROM AUTHOR]

spinach plants (Spinacia oleracea L) were grown hydroponically in fixed environmental conditions either at full nitrate availability (11.8mol m-3) or at a suboptirnum relative nitrate addition rate of 0.20 d-1, 0.15d-1 or 0.10d-1, respectively, the other nutrients being adequately provided. The relative growth rate (RGR) of the plants varied significantly with the nutrition treatment and decreased during development in all treatments. The concentration of reduced nitrogen in the plants grown at full nitrate availability did not change significantly during the experimental growth period and nitrate accumulation was substantial. After an adaptation period, the concentration of reduced nitrogen in the plants at the suboptimum nitrate addition rates increased during growth and was lowest at the lowest relative nitrate addition rate. Nitrate uptake was almost complete in the suboptimum treatments and nitrate accumulation was negligible as long as the concentration of reduced nitrogen was below 2.0 mmol (g dry weight)-1. The RGR of all plants was proportional to the concentration of reduced nitrogen in the plant minus a minimal tissue concentration required for growth. However, the proportionality factor was inversely related to the plant mass. This relationship was summarized in an empirical model which explained 98.7% of the variance of the dry weight (log scale) data of all treatments at all harvests. The model was compared with other growth models found in the literature. The shoot/root weight ratio increased from 2 to 4 if nitrate provision was not limiting and initially, this ratio decreased at suboptimum nitrate provision but increased at higher growth stages. Possible explanations of the dynamics of dry matter partitioning are discussed in relation to models. [ABSTRACT FROM AUTHOR]

Investigates the impact of fleshly added nitrogen (N)-15-labeled leucaena and dactyladenia residues on the C and N dynamics of soil organic matter (SOM) fractions of varying sizes. Materials and methods used; Importance of SOM in low-input tropical cropping systems; Results and discussion.

Focuses on measuring the recovery of nitrogen (N) released from N-15-labeled residues with contrasting quality (leucaena) and (dactyladenia)) by maize and cowpeas crops and soil during growing season. Materials and methods used; Reference to studies reporting residue N recovery in alley cropping systems; Results and discussion.

Examines the dynamics of mineral nitrogen (N) in Leucaena leucocephala and Dactyladenia barteri alley cropping systems in Europe. Application of N-labelled leaf residues on bare and cropped microplots; Differences in the nitrate in the profile between the bare and the cropped microplots; Strategies for reducing nitrate losses.

Cashew soils of South Eastern Tanzania become acidified due to sulphur used for controlling powdery mildew disease (Oidium anacardii Noack). The buffering capacity of surface and subsurface horizons of 35 soil profiles of major cashew growing areas the Makonde plateau, its piedmont and inland plains was studied. The buffering capacity of surface and subsurface horizons was strongly correlated with clay contentand weakly with organic carbon content. In addition, it was only weakly correlated with total exchangeable bases and available P of the surface horizon, but strongly with soil pH, base saturation and cationexchange capacity of the clay fraction of the subsurface horizon. Highly weathered sandy soils, dominant on the Makonde plateau and common on the Piedmont, had the lowest buffering capacity. Soils from the inland plains had better buffering capacities as they are generally more clayey or are less weathered. The risk of severe acidification and of a decline in productivity of cashew and of food crops is higheston the Makonde plateau. Further development and dissemination of methods which can reduce the use of sulfur are required. [ABSTRACT FROM AUTHOR]