Karel Vlassak

Long-term kinetics of phosphate release from soil.

Long-term phosphate (P) desorption from soil is described using two discrete P pools in the soil : one available and one strongly fixed pool. The P release kinetics for each pool are described with a first-order rate equation. A new desorption method is used with hydrous iron oxide inside dialysis tubing acting as a P sink. The widely used iron-impregnated filter paper desorption method overestimates initial P desorption by a factor of up to 4 and underestimates the quantitative progression of desorption as a function of time. P desorption continued with substantial rates for periods longer than 1600 h. A wide range in P desorbability was observed : 15-70% of oxalate-extractable P (P ox ) desorbed after 1600 h. P desorbability decreased with increasing Fe ox +Al ox content of the sample. The relative size of the quickly desorbing pool increased with increasing initial degree of P saturation α 0 =P 0X [Fe 0X + Al 0X ] of the soils. This fact is of direct importance for the estimation of P losses from phosphate-rich soils. This study furthermore provides evidence that all oxalate-extractable P potentially is desorbable : no irreversibly fixed P OX exists.

Survival of E. coli and Enterococcus spp. derived from pig slurry in soils of different texture

Abstract Survival in soil of E. coli and Enterococcus spp. derived from pig slurry was examined in three soils of different texture (a sandy soil, a loamy soil and a loamy sand), at three different incubation temperatures (5, 15 and 25°C) and three different soil moisture contents (60, 80 and 100% of field capacity). Bacterial survival was quantified by dilution plating on selective media. Both species survived remarkably better at 5°C than at 25°C. Numbers of Enterococcus spp. remained constant at 5°C in all soil textures, while numbers of E. coli declined gradually and reached the detection limit at day 68. At 25°C, both species declined rapidly and reached the detection limit at day 54 ( Enterococcus spp.) and 26 ( E. coli ) after inoculation. In general, Enterococcus spp. were more persistent than E. coli , except for survival in the sandy soil at 25°C. Amongst the investigated soil types, the sandy soil proved to be the best for E. coli survival and the worst for Enterococcus spp. survival. On the other hand, Enterococcus spp. survived best in the loamy soil. For both bacterial species, a lower incubation temperature and a higher soil moisture content favored survival. At 100% FC and 5°C, numbers of E. coli reached the detection limit at day 80, while the number of Enterococcus spp. remained almost constant until day 80. On the other hand, numbers of Enterococcus spp. and E. coli decreased rapidly at 25°C and 60% FC, and reached the detection limit at day 17 and 38, respectively. The risk of further dispersion of antibiotic resistance through species survival was confirmed.

Spatial distribution of microbial biomass in microaggregates of a silty-loam soil and the relation with the resistance of microorganisms to soil drying

To indicate factors determining the location of microorganisms in the soil structure, moist and dried-remoistened samples of a silty-loam soil were separated into particle-size fractions after gentle dispersion (shaking in water for 16 h). Microbial biomass C and total organic C and N contents of soil fractions were measured. The presence of microaggregates in each size fraction was determined by texture analysis of fractions. More than 50% of the microbial biomass C content of the total soil and 37% of the soil organic C content were associated with the aggregated clay (20-2 μm) and the clay-size (<2 μm) fraction. These represented respectively 19 and 5% of the total soil weight and contained respectively 40 and 23% of clay-size particles present in the whole soil. The coarse silt-size (50-20 μm) fraction, consisting mainly of non-aggregated mineral particles, contributed 38% of the total soil weight and contained only 7% of both total soil microbial biomass C and of total soil organic C. Multiple linear regression showed that the variation in biomass C content between size fractions was almost completely (R2 = 0.99) explained by its clay and organic C content. Such a close relationship would not have been observed if microporosity had determined the location of microorganisms in the soil structure, as fractions were very heterogeneous in respect to microaggregation. Thus, the results indicate that microaggregation per se had no effect in this soil on the microbial distribution in size fractions. The effects of other factors, such as substrate availability, are discussed. Soil drying and rewetting reduced the microbial biomass C content of unfractionated soil by 36% compared to moist soil. Microbial biomass concentrations of size fractions from dried soil (expressed per unit of soil weight, fraction weight, organic C and clays) were compared with those of moist soil. The results indicated that microbial cells associated with the 20-2 μm size fraction were more susceptible to desiccation than those residing in other size fractions. This fraction contained relatively the most clay, suggesting that the presence of clay does not provide extra protection for soil organisms against severe soil drying.

Use ofAzospirillum brasilense as biofertilizer in intensive wheat cropping

SummaryThree field experiments were conducted on ten cultivars of winterwheat and four cultivars of springwheat to estimate the growth promoting effect ofAzospirillum brasilense under varying nitrogen doses. Independent of cultivar selection or nitrogen dose a highly significant yield increase could be observed in winterwheat: strains S631 and SpBr14 increased the average grain yield with 9.14% and 14.82% respectively. When the yield components were studied a coinciding increase in ear density could be demonstrated of resp. 10.57% and 13.55%. Less significant results were obtained with springwheat although in one experiment strain SpBr14 significantly increased grain yield. As with winterwheat tillering of the plant was markedly affected by inoculation with both strains. In a companion greenhouse experiment it was found that inoculation with Azospirillum can cause a decrease in the root mass of wheatplants except when strain SpBr14 is used. Therefore it is suggested that the presence of a higher tillering together with an undisturbed nutrient uptake capacity can result in yield increases after inoculation withAzospirillum brasilense.

Assessment of tillage erosion rates on steep slopes in northern Thailand

Abstract The switch from shifting cultivation to more permanent highland cropping systems in northern Thailand. led to an increase in soil tillage intensity. In order to quantify soil losses by tillage erosion, a tillage experiment was set up and an on-farm survey was conducted. Soil fluxes due to manual tillage on five slopes (32–82%) were measured by monitoring tracers, by measuring tillage step characteristics and by collecting soil material in a trench. The trench method yielded soil flux. values that were significantly smaller than those obtained by the tracer or the step method. Soil fluxes resulting from one manual tillage pass ranged between 39 and 87 kg/m on the tested slopes. On slopes up to 60%, there were no significant differences in soil fluxes. However, on slopes steeper than 70%, soil fluxes increased significantly because the angle of repose for soil clods was exceeded. The soil fluxes are used to construct a nomogram for estimating soil loss rates resulting from manual tillage erosion as a function of slope and plot length. Rates on a typical upland field (slope 30%–50%, slope length 30–50 m) range from 8 to 18 t/ha · tillage pass, so tillage erosion is a significant contributor to the total soil loss. It dominates on short fields and fields with buffer-strips, whereas water erosion is the more important form of soil loss on middle size and long fields. Increasing land pressure will result in increasing tillage erosion rates, and these need to be considered when assessing soil degradation rates or when studying hillslope evolution.

Relationship between soil properties and phosphate saturation parameters a transect study in northern Belgium

We investigated the relations between soil properties and the variability at field-scale of soil P sorption capacity (PSC) and degree of P saturation (DPS), and the relationship between DPS and P concentrations in soil solution. Soil samples of 0–30 cm, 30–60 cm and 60–90 cm depths were collected every 10 meters on a transect of a total length of 539 meters in the northeastern part of the province Antwerp, Belgium. Oxalate extractable iron, aluminium and phosphate content (FeOX, AlOX, and POX) were determined, and the PSC and DPS calculated. Phosphate concentrations measured in 100 mM KCl extracts of the soil samples of the 0–30 cm layer were closely related to the DPS of the samples. The spatial distribution of FeOX, was rather homogeneous, except in the vicinity of a “discontinuity” such as a drainage ditch, where much higher FeOX values were measured. The spatial variability of the AlOX parameter was closely related to the spatial variability of the amount of oxalate extractable carbon and total carbon, but this was not the case for FeOX. Detailed textural information for the transect did not correlate to PSC parameters at this scale; pHKCL or pHH2O showed little variability on the transect scale. Variograms of the PSC parameters were obtained for three separate data-sets: one for the total transect, one for the total transect but excluding ditch data, and for part of the transect situated within one field (the first 410 m of the total transect).

Geostatistical assessment of the regional distribution of phosphate sorption capacity parameters (FeOX and AlOX) in northern Belgium

Abstract Soil samples were analysed for oxalate extractable iron, aluminium and phosphate content (Feox, AlOX and POX) within a 700 km2 area in northern Belgium, using a regular grid of 301 sampling locations with basic distances in between of 1800 m, and local densifications of 900 and 450 m. Variograms of the specific parameters indicated spatial correlation between neighbouring points with range upto 6000 m for FeOX and AlOX. The analysis data for each sampling location were geostatistically interpolated (block-kriging). The spatial distribution of FeOX was closely related to the origin of the specific soil: (light) alluvial regions coincided with (extremely) high FeOX contents, where coarse sandy soils coincided with very low FeOX contents. Regions that are classified as loamy sand or sandy loam contained much more AL. than did coarse or fine sandy soils within the area. The average kriging standard deviations obtained were 20–25% for Alox and 40–50% for FeOX. The phosphate sorption capacity (PSC) was calculated as PSC = 0.5(Al{onOX}+FeOX). The kriged PSC estimates were displayed as maps. The average kriging standard deviation (KSD) for the PSC kriged estimates was about 25%. Within one area, where the sampling grid was denser (900 m), lower kriging standard deviations of about 15 % were obtained for the PSC. Pox and the degree of P saturation (DPS) showed some spatial correlation but with low ranges (750–1000 m). A (large) nugget effect was also encountered for these parameters, that can be explained by taking the large human factor determining Pox and DPS into account. These findings can be used to provide indicative maps on regional scale indicating the sensitivity of certain areas for rapid P saturation and resulting eutrophication problems, especially in intensive animal husbandry regions.

Injection of pig slurry and its effects on dynamics of nitrogen and carbon in a loamy soil unter laboratory conditions

Abstract Dynamics of nitrogen (N) and carbon (C) were investigated in a loamy soil amended or injected with pig slurry. Treatments were with or without acetylene C2H2 (which is assumed to inhibit reduction of nitrous oxide (N2O) to dinitrogen (N2), and soil cores were conditioned for 15 days at 25°C while pH, production of CO2 and N2O, ammonia (NH3) emission and (nitrate) (NO3–) and (ammonium) (NH4+) concentrations were monitored. There was no significant difference in CO2 production between the injected and surface applied pig slurry treatments, and within 15 days ca. 5% of the C applied had been mineralized, if no priming effect was assumed. Neither the production of N2O nor the total gaseous production of the denitrification process (N2O plus N2) were affected by the way the pig slurry was added to the soil. NH3 volatilization, however, decreased by 90% when pig slurry was injected. The addition of C2H2 significantly increased the CO2 production and the concentration of NH4+, but significantly decreased the concentration of NO3–. It was concluded that the injection of pig slurry to a dry soil was an acceptable alternative to its application to the soil surface, as not only was NH3 volatilization reduced, but the production of N2O and N2 through denitrification was not stimulated. It is also suggested that the composition of the organic C fraction in the pig slurry, most likely the concentration of fatty acids, had an important effect on the dynamics of N and C in the soil.

Soil drying and rewetting and the turnover of 14C-labelled plant residues : first order decay rates of biomass and non-biomass 14C

The effects of drying and rewetting on soil organic C derived from added plant material were determined. Three soils, one silty-loam and two loamy sands, were incubated with 14C-labelled plant material for 27 days. Each soil was then subjected (1) to drying at 40°C for 3 days, remoistening and incubation at 25°C for 10 days, and (2) to storage at 4 °C for 3 days and incubation. The silty-loam soil was also treated after 7 days of incubation with 14C-labelled plant material. Residual 14C concentrations were determined at the beginning and at the end of the 10 day incubation. During drying or storage and subsequent incubation of soils, biomass C and 14C were measured, and also the amounts of CO2 and 14CO2 released during the 10 day incubation. Residual 14C concentrations at the end of the incubation were not significantly affected by soil desiccation and remoistening, but the percentages of residual 14C due to biomass 14C were greatly reduced in dried, rewetted and incubated soils. The effect was largest for the soil which had been incubated for the shortest time with 14C-labelled plant material. Average first-order gross decay rates were calculated for biomass 14C and non-biomass 14C, and for different efficiencies of substrate utilization (viz. 20, 40 and 60%). Two time intervals were chosen: with and without inclusion of the drying period. Drying and rewetting of soils enhanced first-order gross decay rates of the two carbon pools. The relative increases were larger for decay rates of biomass 14C than for those of non-biomass 14C. When decay rates were averaged over a time interval that did not include the drying period, observed effects of soil desiccation and remoistening were less pronounced, but still apparent. This suggests that a previous drying-rewetting cycle has an appreciable influence on decomposition processes during later incubation of soils. It was concluded that soil drying and rewetting promoted the turnover of C derived from added plant material, and that this increase in C cycling was mainly due to enhanced turnover of microbial products. This may finally result in a change of quality of the organic C pool coming from added plant residues.

Denitrification, N2-fixation and fermentation during anaerobic incubation of soils amended with glucose and nitrate

Nitrate and glucose additions were investigated for their role in the C and N dynamics during anaerobic incubation of soil. A gas-flow soil core method was used, in which the net production of N2, N2O, NO, CO2, and CH4 under a He atmosphere could be monitored both accurately and frequently. In all experiments clayey silt loam soil samples were incubated for 9 days at 25 °C. Addition of nitrate (50 mg KNO3-N kg-1 soil) had no effect on total denitrification and CO2 production rates, while the N2O/N2 ratio was affected considerably. The cumulative N2O production exceeded the cumulative N2 production for 6 days in the treatment with nitrate addition, compared to 1.2 days in the unamended treatment. Glucose addition stimulated the microbial activity considerably. The denitrification rates were limited by the growth rate of the denitrifying population. During denitrification no significant differences were observed between the treatments with 700 mg glucose-C kg-1 and 4200 mg glucose-C kg-1, both in combination with 50 mg KNO3-N kg-1. The N2 production rates were remarkably low, until NOinf3sup-exhaustion caused rapid reduction of N2O to N2 at day 2. During the denitrification period 15–18 mg N kg-1 was immobilised in the growing biomass. After NOinf3sup-shortage, a second microbial population, capable of N2-fixation, became increasingly important. This change was clearly reflected in the CO2 production rates. Net volatile fatty acid (VFA) production was monitored during the net N2-fixation period with acetate as the dominant product. N2-fixation faded out, probably due to N2 shortage, followed by increased VFA production. In the high C treatment butyrate became the most important VFA, while in the low C treatment acetate and butyrate were produced at equal rates. During denitrification no VFA accumulation occurred; this does not prove, however, that denitrification and fermentation appeared sequentially. The experiments illustrate clearly the interactions of C-availability, microbial population and nitrate availability as influencing factors on denitrification and fermentation.