Influence of soil compaction on carbon and nitrogen mineralization of soil organic matter and crop residues

 We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop residues. Samples of unamended soil, and soil amended with leek residues, at six bulk densities ranging from 1.2 to 1.6 Mg m^–3 and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studied did not influence any measure of microbial activity significantly. A small (but insignificant) decrease in nitrification rate at the highest bulk density was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of the incubation were equal at all bulk densities, but first-order N mineralization rates tended to increase with increasing compaction, although the increase was not significant. Nitrification in the amended soils was more affected by compaction, and NO₃ ^–-N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m^–3 were significantly lower (by about 8% and 16% of total added N, respectively), than those of the less compacted treatments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m^–3, compared with the other treatments. The depression of C mineralization in compacted soils can lead to higher organic matter accumulation. Since N mineralization was not affected by compaction (within the range used here) the accumulated organic matter would have had higher C : N ratios than in the uncompacted soils, and hence would have been of a lower quality. In general, increasing soil compaction in this soil, starting at a bulk density of 1.5 Mg m^–3, will affect some microbially driven processes. Received: 10 June 1999     

A microcosm study on the influence of pH and the host-plant on the soil persistence of two alfalfa-nodulating rhizobia with different saprophytic and symbiotic characteristics

The acid tolerance of Sinorhizobium meliloti in culture media and in soils is considered a useful criteria to select for strains with improved survival in agricultural acidic soils. Using a glass tube system with gamma-irradiated soil at different pH values, we analysed the survival of two different alfalfa-nodulating rhizobia: S. meliloti (pH_limit for growth 5.6–6.0) and the acid-tolerant Rhizobium sp. LPU83, closely related to the strain Rhizobium sp. Or191 (pH_limit for growth below 5.0). Although the acid-tolerant rhizobia showed a slightly better survival during the first months in acid soil (pH=5.6), none of the strains could be detected 2 months after inoculation (bacterial counts were below 10³ colony-forming units (cfu)/30 g of soil). The inclusion of two alfalfa plants/glass tube with soil, however, supported the persistence of both types of rhizobia at pH 5.6 for over 2 months with counts higher than 9×10⁶ cfu/30 g of soil. Remarkably, in the presence of alfalfa the cell densities reached by S. meliloti were higher than those reached by strain LPU83, which started to decline 1 week after inoculation. Although more acid-sensitive in the culture medium than the Or191-like rhizobia, in the presence of the host plant the S. meliloti strains showed to be better adapted to the free-living condition, irrespective of the pH of the soil.     

Nitrogen mineralization and availability of mixed leguminous and non-leguminous cover crop residues in soil

Whereas non-leguminous cover crops such as cereal rye (Secale cereale) or annual ryegrass (Lolium multiflorium) are capable of reducing nitrogen (N) leaching during wet seasons, leguminous cover crops such as hairy vetch (Vicia villosa) improve soil N fertility for succeeding crops. With mixtures of grasses and legumes as cover crop, the goal of reducing N leaching while increasing soil N availability for crop production could be attainable. This study examined net N mineralization of soil treated with hairy vetch residues mixed with either cereal rye or annual ryegrass and the effect of these mixtures on growth and N uptake by cereal rye. Both cereal rye and annual ryegrass contained low total N, but high water-soluble carbon and carbohydrate, compared with hairy vetch. Decreasing the proportion of hairy vetch in the mixed residues decreased net N mineralization, rye plant growth and N uptake, but increased the crossover time (the time when the amount of net N mineralized in the residue-amended soil equalled that of the non-amended control) required for net N mineralization to occur. When the hairy vetch content was decreased to 40% or lower, net N immobilization in the first week of incubation increased markedly. Residue N was significantly correlated with rye biomass (r=0.81, P<0.01) and N uptake (r=0.83, P<0.001), although the correlation was much higher between residue N and the potential initial N mineralization rate for rye biomass (r=0.93, P<0.001) and N uptake (r=0.99, P<0.001). Judging from the effects of the mixed residues on rye N Concentration and N uptake, the proportion of rye or annual ryegrass when mixed with residues of hairy vetch should not exceed 60% if the residues are to increase N availability. Further study is needed to examine the influence of various mixtures of hairy vetch and rye or annual ryegrass on N leaching in soil. Received: 10 March 1997     

Use of agricultural by-products to study the pH effects in an acid tea garden soil

The amelioration of an acid Alfisol from a tea garden was studied by incorporating various plant materials: canola straw, wheat straw, rice straw, corn straw, soybean straw, peanut straw, faba bean straw, Chinese milk vetch shoot and pea straw prior to incubation for a maximum of 65 days. Soil pH increased after incubation with all the incorporated materials with the legumes causing the largest increases. The final soil pH was correlated with ash alkalinity ( r ² = 0.73), base cations ( r ² = 0.74) and N content ( r ² = 0.93) of the applied materials. It was assumed that the incubation released the base cations in plant materials as they decomposed which ultimately increased the base cation saturation of the soil. Similarly, soil exchangeable Al was also decreased with the incorporation of the legume plant materials and corn straw and rice straw. Our investigation demonstrated that legumes are the preferred choice for controlling the soil acidity and also for reducing the toxicity of Al in acid soils.     

Influence of growth-promoting bacteria on the growth of wheat in different soils and temperatures

Plant-growth-promoting bacteria isolated from the rhizosphere, phyllosphere and soil of the root zone in different climatic regions of Germany and Uzbekistan were analysed for plant-growth-promoting effects and nutrient uptake on winter wheat on different soils and under different temperature regimes. The investigations were carried out in pot experiments using loamy sand and sandy loam soils from Müncheberg, Germany and Calcisol soil from Tashkent, Uzbekistan. The temperature and soil types were found to influence growth-promoting effects. Inoculation with bacterial strains Pseudomonas fluorescens PsIA12, Pantoea agglomerans 050309 and Mycobacterium sp. 44 isolated from Müncheberg (semi-continental climate) was found to significantly increase the root and shoot growth of winter wheat at 16 °C compared to 26 °C in loamy sand. Mycobacterium phlei MbP18 and Mycoplana bullata MpB46 isolated from Tashkent (semi-arid climate) were found to significantly increase the root and shoot growth of winter wheat in nutrient-poor Calcisol at 38 °C as well as in nutrient-rich loamy sand at 16 °C. Bacterial inoculation also resulted in significantly higher N, P, and K contents of plant components. The bacteria isolates were able to survive in the rhizosphere and in the soil of winter wheat after root and shoot inoculation.     

Microbial community composition and substrate use in a highly weathered soil as affected by crop rotation and P fertilization

A better understanding of soil microbial processes is required to improve the synchrony between nutrient release from plant residues and crop demand. Phospholipid fatty acid analysis was used to investigate the effect of two crop rotations (continuous maize and maize-crotalaria rotation) and P fertilization (0 and 50 kg P ha⁻¹ yr⁻¹, applied as triple superphosphate) on microbial community composition in a highly weathered soil from western Kenya. Microbial substrate use in soils from the field experiment was compared in incubation experiments. Higher levels of soil organic matter and microbial biomass in the maize-crotalaria rotation were connected with higher total amounts of phospholipid fatty acids and an increase in the relative abundances of indicators for fungi and gram-negative bacteria. P fertilization changed the community profile only within the continuous maize treatment. The decomposition of glucose, cellulose and three plant residues (all added at 2.5 g C kg⁻¹ soil) proceeded faster in soil from the maize-crotalaria rotation, but differences were mostly transient. Microbial P and N uptake within one week increased with the water-soluble carbon content of added plant residues. More P and N were taken up by the greater microbial biomass in soil from the maize-crotalaria rotation than from continuous maize. Re-mineralization of nutrients during the decline of the microbial biomass increased also with the initial biological activity of the soil, but occurred only for a high quality plant residue within the half year incubation period. Compared to the effect of crop rotation, P fertilization had a minor effect on microbial community composition and substrate use.     

Influence of pH,dispersion methods,and different compounds on adenosine triphosphate recovery from soil

We studied the recovery of ATP from soil. A soil-water suspension was prepared by two different methods (simple stirring or ballottini mill treatment) at different pH levels and in the presence of different chemicals [Na₂SO₄, Na₃PO₄, Na₅P₃O₁₀, adenosine, ethylenediaminetetra-acetic acid (EDTA), TRIS]. The ATP recovery was evaluated by adding [³H]-8 ATP to the solution and comparing the values obtained by radioactivity measurements with those obtained by an enzymatic assay. Strongly acidic (pH lower than 1.0) or alkaline (pH 10.0) extractions yielded the best ATP recoveries compared with intermediate pH values. At pH 10.0, the addition of Na₃PO₄ or Na₅P₃O₁₀ gave a high level of ATP recovery, 68 and 96%, respectively. No ATP hydrolysis occurred under alkaline extraction conditions. Under acidic extraction conditions, the addition of adenosine, EDTA, Na₂SO₄, or Na₅P₃O₁₀ improved ATP recovery but it was never higher than 34%. The results were discussed in terms of the effects of different physical and chemical conditions on cell disruption, ATP stability, ATP interactions with soil components, and ATP solubilization.     

Effect of plant residue return on the development of surface soil pH gradients

 In the field, surface soil pH gradients were observed under senescing plants over late spring and summer. A soil incubation experiment was conducted (119 days, 20  °C) to provide direct evidence of the influence of plant residue incorporation on soil pH. This was investigated in terms of plant residue type (wheat and subterranean clover) and dry matter addition rate (0, 6.25, 12.5 and 25.0 g kg^–1), as well as the soil layer of incorporation (0–2.5 and 7.5–10 cm) and moisture regime (continuously moist and moist-dry cycles). During incubation, moist unamended soils slowly acidified. In contrast, the addition of plant residue resulted in a rapid (day 0–7) increase of soil pH due to the association, and particularly oxidation, of added organic anions. This was followed by a gradual (day 7–119) pH decline attributed to the mineralization and subsequent nitrification of added organic N. The addition of 12.5–25.0 g kg^–1 of cereal crop residues, and 6.25–25.0 g kg^–1 of legume-based pasture residues, resulted in a net alkalization of the surface 2.5 cm of soil. It was therefore concluded that surface soil pH gradients observed in the field were largely attributable to an increase of pH at the surface 2.5 cm in response to plant residue return. The magnitude of such gradients will be particularly large with the return of large quantities of plant residues of high ash alkalinity in soils of relatively low initial pH and biological activity, and when the surface of the soil is exposed to moist-dry cycles. Received: 11 October 1999     

Water and temperature dynamics in a clay soil under winter wheat: influence on straw decomposition and N immobilization

Summary Winter wheat grown on a clay soil was subjected to one of four treatments. The control was not irrigated; the drought treatment had screens to divert rainwater; the irrigation and irrigation/fertilization treatments were irrigated using a drip-tube system with liquid fertilizer (200 kg N ha^-1 year^-1) applied daily in the irrigation/fertilization treatment according to predicted plant uptake. All other treatments also received 200 kg N, but as a single application of bag fertilizer. Soil temperature was monitored. Soil moisture was measured using gravimetric samplings and a capacitance method. Litter bags with barley straw were buried at 10 cm depth in the spring and sampled repeatedly during the growing season. Decomposition rates were calculated assuming exponential decay and that water-soluble components were immediately decomposed or leached from the litter bags. Rates were highly dependent on soil moisture, and the constants ranged from 0.11% day^-1 in the drought treatment to 0.55% day^-1 in the irrigation/fertilization treatment. A simulation model with driving variables based on Q ₁₀ temperature dependence and a log/linear relationship between soil water tension and activity was fitted to the data. The control and drought treatments showed high climate-corrected decomposition constants. The high values were attributed to low and erratic mass loss due to drought, and to low precision in the conversions from water content to tension in the dry range. The irrigated treatments showed good fits, and there was little or no difference in decomposition rates between the two irrigated treatments. The N dynamics of the straw differed considerably between treatments, and the ranking of plots in terms of net immobilization in the straw was control>irrigation/fertilization>irrigation>drought.     

Development and evaluation of a soil water evaporation model to assess the effects of soil texture, tillage and crop residue management under field conditions

Abstract. A number of mathematical models to predict soil water evaporation are available in the literature which generally require complex input data. In the present study, a simple parametric model has been developed by coupling existing and newly developed equations to assess soil water evaporation and drainage under field conditions in relation to potential evaporation rate, soil texture, time and depth of tillage and crop residue management. The model has moderate input data requirements and predicts well the effects of tillage and crop residue management practices on soil water loss (evaporation+drainage) with multi-drying and -wetting cycles prevailing under natural conditions. The root mean squares of deviations between observed and predicted cumulative water loss at different periods of study were 0.82, 2.04, 2.31 and 1.74  cm for untreated, residue-mulch, tillage and residue-incorporated treatments, respectively. Simulation analysis on cumulative evaporation and evaporation rate has shown that the evaporation reduction with different combinations of tillage and crop residue followed the order of residue-undercut>residue-mulch>residue-incorporated>tillage. Thus, the magnitude of beneficial effects of crop residues and tillage on soil water evaporation reduction are associated with amount of residues, mode of residue management (mulched or incorporated in the soil) and time and depth of tillage.     

Influence of long-term conservation tillage on soil and rhizosphere microorganisms

 In long-term field experiments on sandy loam and loamy sand soils, the influence of conservation and conventional tillage on soil and rhizosphere microorganisms was studied. Conservation tillage stimulated rhizosphere bacteria on winter wheat, winter barley, winter rye and maize in different soil layers. Particularly the populations of Agrobacterium spp. and Pseudomonas spp. were increased. On the sandy loam, N₂ fixation and nodulation of pea plants were significantly increased. No influence of different soil tillage was determined on the colonization of the rhizosphere by mycorrhiza and saprophytic fungi. Stubble residues infected with Gaeumanomyces graminis were infectious for a longer time on the soil surface than after incorporation into the soil. Received: 10 March 1998     

Effects of tillage on soil microrelief, surface depression storage and soil water storage

Conservation of soil water is an important management objective for crop production in the semi-arid tropics where droughts are persistent. Identification of the best tillage methods to achieve this objective is thus imperative. The integrated effects of conservation tillage on soil micro topography and soil moisture on a sandy loam soil were evaluated. The field experiment consisted of five tillage treatments, namely tied ridging (TR), no till (NT), disc plough (DP), strip catchment tillage (SCT) and hand hoe (HH). Data measured in the field included soil moisture content, surface roughness, infiltration and sorghum grain yield. A depth storage model was used to estimate depression storage TR treatment and the higher the surface roughness, the greater the depression storage volume. Regression analysis showed that random roughness decreased exponentially with increase in cumulative rainfall. Higher moisture contents were associated with treatments having higher depressional storage. Infiltration rate was significantly higher in the tilled soils than the untilled soils. The DP treatment had the highest cumulative infiltration while NT had the lowest. The Infiltration model which was fitted to the infiltration data gave good fit. Grain yield was highest in TR and least in NT, whereas DP and HH had similar yields.     

Interactions between N fertilization, grass clipping addition and pH in turf ecosystems: Implications for soil enzyme activities and organic matter decomposition

Turf has been acknowledged as an important ecosystem with potential for soil C sequestration. As a major process dictating soil C storage, organic matter decomposition has received little attention in turf systems. Given that soil enzyme-catalyzed biochemical reactions are the rate limiting steps of organic matter decomposition, we examined the activities of oxidative and hydrolytic soil enzymes and their relations with soluble organic compounds and soil C and N mineralization in two turf chronosequences with contrasting soil pH and in response to N fertilization and grass clipping addition. In comparison with turf ecosystems under acidic soil, phenol oxidase activity was about two-fold greater in turf ecosystems under alkaline soil and positively correlated to about two-fold differences in soluble phenolics and dissolved organic C between alkaline and acidic soils. However, the activities of hydrolytic enzymes including cellulase, chitinase, and glucosidase were lower in alkaline soil. It appears that the high concentration of soluble phenolics inhibited the activities of hydrolytic enzymes that in turn limited the decomposition of dissolved organic C and resulted in its accumulation in alkaline soil. Nitrogen mineralization was comparable between alkaline and acidic soils, but CO₂ evolution was about two-fold greater in alkaline soil, possibly due to considerable abiotic carbonate dissolution. We observed that mineral N input at 60 mg N kg⁻¹ soil had very minor negative effects on the activities of both phenol oxidase and hydrolytic enzymes. Grass clipping addition did not affect the activity of phenol oxidase, but increased the activities of soil chitinase, cellulase, glucosidase, and glucosaminidase by up to 20% and also soluble phenolics in soil by about 10%. Our results suggest that soil phenol oxidase might regulate the activities of hydrolytic soil enzymes via its control on soluble phenolics and function as an ‘enzymatic latch’ to hold soil organic C in highly managed turf ecosystems. While soil pH is important to affect phenol oxidase activity and therefore decomposition, management practices, i.e., N fertilization and grass clipping addition may indirectly affect the decomposition through enhancing turfgrass productivity and thus soil C input.     

Effects of two perennials, fallow and millet on distribution of phosphorous in soil and biomass on sloping loess land, China

Soil erosion causes sequential eco-environmental problems in the Loess Plateau of China. Therefore, vegetation restoration measures were adopted in this area by converting steep cropland into alternative land uses in order to remedy the erosion problem and alleviate land degradation. As part of the effort to understand the potential of this management practice in sequestering nutrients, the present study assessed the impact of vegetation restoration on the distribution of phosphorus (P) and aboveground biomass on a sloping land in this area. The study compared four land uses, Korshinsk Peashrub (KOP), alfalfa (ALF), natural fallow (NAF), and millet (MIL). KOP, ALF and NAF were adopted as vegetation restoration patterns, while MIL was an example of crop land. For the sampling time the average aboveground P pools of KOP, ALF, and MIL were 1.118, 0.406, and 0.091 g m^− 2, respectively. The magnitudes of the aboveground biomass and tissue P concentration of the three land uses followed the same order. Both vertical and horizontal distributions of soil Olsen P of KOP were relatively uniform. Soil Olsen P of ALF accumulated significantly at the down-slope position within the surface 60 cm soil profile. The down-slope accumulation of soil Olsen P was significant at the depth of 80 to 100 cm and 0 to 20 cm for NAF and MIL, respectively. The soil Olsen P concentrations in deeper soil layers of KOP and ALF were clearly lower than that of NAF, while MIL had apparent higher concentrations of soil Olsen P in the 0 to 20 cm layer of soil. Generally, the vegetation restoration did not present a significant effect on total soil P level, with the average values of 0.722, 0.751, 0.747, and 0.729 kg m^− 2 for KOP, ALF, NAF, and MIL, respectively. However, an apparent accumulation of total soil P at the down-slope position was observed for NAF and MIL treatments. For ALF and KOP, the soil Olsen P and the soil moisture content were correlated positively to the aboveground P pool. For MIL, however, the higher aboveground P corresponded to a higher soil Olsen P but to a lower soil moisture content. Results from the four years of data suggest that ALF is the fastest absorber of soil P, while KOP is the best vegetation to sequester soil P of the soil–plant systems in this study.     

不同氮水平对甘蔗氮素利用及土壤氮素残留的影响

以新台糖22号(ROC22)为试材,设施用15N标记的尿素2.5 g pot-1、5.0 g pot-1及7.5 g pot-13个处理,研究施氮水平对甘蔗氮素利用效率及土壤氮素的影响。结果表明,随氮肥施用水平的提高,甘蔗干物质积累、氮素积累、来源于肥料氮素的比率不同程度增加,同时土壤碱解氮含量、硝态氮含量、肥料氮素残留量及肥料氮素残留率显著增加,但甘蔗氮肥利用率显著下降;施氮水平还明显影响甘蔗各器官的干物质分配、氮素分配、氮肥利用率及氮素在不同土层的分布。本试验条件下,甘蔗氮肥施用土层深度为20 cm、尿素施用量为5.0 g pot-1较为适宜。     

Diversity of proteolytic community encoding for subtilisin in an arable field: spatial and temporal variability

The diversity of soil proteolytic community encoding for subtilisin (sub) was investigated at a cultivated field site with four different soil types and at three different depths in April, July, and October. A terminal restriction fragment length polymorphism (T-RFLP) analysis of subtilisin gene (sub) was applied to study dynamic of the sub gene pool. The aim of the present study was to relate differences in sub community structure to the vertical, site, and seasonal variations naturally occurring at the field site under investigation. A significant spatial variability in the community structure of sub-containing bacteria was observed. The richness of sub proteolytic bacterial population decreased with increasing soil depth, revealing the highest values in upper layers. A similar trend was observed among the different sites; the highest diversity was noticed at the site with the highest silt and nutrient content. The reasons behind the observed patterns in the community structure might be varying water amount and spatial isolation along the soil profile as well as variability of the quantity and quality of available substrates among different depths and different sites.     

Factors controlling decomposition of soil organic matter in fallow systems of the high tropical Andes: A field simulation approach using C- and N-labelled plant material

N-rich (C:N=27) and N-poor (C:N=130) wheat straw, labelled with ¹⁴C and ¹⁵N, was incubated for 2 yr in two major ecosystems of the upper elevation belt of cultivation in the high Andes: the moist Paramo (precipitation=1329 mm, altitude=3400 m asl, Andes of Merida, Venezuela) and the dry Puna (precipitation=370 mm, altitude=3800 m asl, Central Altiplano, Bolivia). The experiment was installed in young (2 yr) and old (7 yr) fallow plots. The following soil analyses were performed at nine sampling occasions: soil moisture, total-¹⁴C and -¹⁵N, and Microbial Biomass (MB)-¹⁴C and -¹⁵N. The measured data were fitted by the MOMOS-6 model (a process based model, with five compartments: labile and stable plant material, MB, and labile (HL) and stable humus (HS)) coupled with the SAHEL model (soil moisture prediction) using daily measured and/or predicted meteorological data. The aim was to understand how (1) the climatic conditions, (2) the quality of plant material, (3) the fallow age and (4) the soil properties affect the cycling of C and N within the soil organic matter system.The fallow age (2 and 7 yr) did not affect the measured data or the model predictions, indicating that in these systems the decomposition potential is not affected by fallow length. During the short initial active decomposition phase, the labile plant material was quickly exhausted, enabling a build up of MB and of HL. During the low activity phase, that covered 4/5 of the time of exposure, the MB size decreased slowly and the HL pool was progressively exhausted as it was reused by the MB as substrate. The HL compartment was directly or indirectly the major source for the inorganic ¹⁵N production. If the C:N ratio of the added plant material increased, the model predicted (1) a reduction of the decomposition rates of the plant material (essentially the stable plant material) and (2) an increased mortality of the MB which increased the production of HL (microbial cadavers and metabolites). Thus the essential effect of the slower decomposition due to the N-poor plant material was a higher accumulation of C and N in the HL and its slower recycling by the MB during the low activity phase. The labelling experiment allows to understand the higher soil native organic matter content in Paramo soils compared to Puna. The large sequestration of organic matter generally observed in the Paramo soils can be explained by two abiotic factors: the unfavourable soil microstructure and the accumulation of free aluminium linked to the climatic and acid soil conditions, inhibiting the microbial activity physically and chemically.     

Spatial heterogeneity in the relocation of added C within the structure of an upland grassland soil

A pulse of ¹³CO₂ was added to the above ground vegetation in an upland grassland to determine the effects of faunal diversity on the flux of carbon to the surface horizons of the soil. Faunal diversity was manipulated by liming and biocide treatments for three years prior to the pulse addition. The relocation of ¹³C within roots and rhizosphere soil was determined by analysis of samples of bulk soil and of specific features identified on soil thin sections on four dates after the addition of the ¹³CO₂ pulse. Analysis of bulk soils showed only a small enrichment in ¹³C and no significant effects of the treatments. Analysis by isotope ratio mass spectrometry of the products of in situ laser combustion of root material and aggregates formed from faunal excrement showed that the distribution of the newly photosynthesised ¹³C is very localised, with large spatial variability in soil and root δ¹³C at scales of less than 1 mm. δ¹³C values ranged from the natural abundance level of around −28‰ to −4.9‰ in roots and to −8.4‰ in aggregates. The small pulse and large spatial variability masked any effects of the liming and biocide treatments in these soils. However, the variability in the relocation of newly photosynthesised carbon may help to explain the large spatial variability found in bacterial numbers at the sub-mm scale within soils and emphasises the importance of the accessibility of substrates to decomposers in undisturbed structured soils.     

Simultaneous effects of plants and earthworms on mineralisation of 15N-labelled organic compounds adsorbed onto soil size fractions

 The simultaneous impact of three successive crops of wheat (Triticum aestivum L.) and of the earthworm (Lumbricus terrestris L.) on the mineralisation of ¹⁵N-labelled organic compounds adsorbed to different soil size fractions (sand and organic residues >50 μm; silt 50–2 μm; coarse clay 2–0.2 μm and fine clay <0.2 μm) was studied under controlled conditions in the greenhouse. Unplanted soils (UPS) were used as controls. In planted soils without earthworm (PS) total plant biomass decreased with each cropping by up to 50%. However, in planted soils with earthworms (PES) the total plant biomass loss was only 17%. This pattern was explained by the earthworm effect. Compared to the unplanted soils, the planted soils had an increased (mean +37%) mineralisation of ¹⁵N adsorbed onto fine clays and a partial transfer of ¹⁵N to silt and coarse clay. The quantities of ¹⁵N mineralised and transferred were higher in the planted soils with earthworms, indicating an amplification of the phenomenon in the presence of earthworms. The simultaneous effect of the rhizosphere and the drilosphere did not lead to increased mineralisation of N adsorbed onto coarse clays and silts but instead a greater transfer of N associated with the fine fractions towards the coarser fractions. Received: 25 April 2000