Response of maize and soil microorganisms to decomposing poplar root residues after shallow or homogenous mixing into soil

During re‐conversion of short‐rotation poplar tree plantations back to arable land use, large amounts of tree residues must be incorporated into soil. A 90‐d pot experiment with and without N addition was carried out after mixing the same amounts of chaffed poplar root residues into the pots at 0–5 cm or at 0–20 cm depth. The objective was to investigate whether shallow mixing has positive effects on maize growth, reduces poplar root residue decomposition, and changes the microbial community structure towards fungi. Aboveground maize yield was strongly reduced after mixing of poplar root residues at 0–20 cm depth without N fertilization, but was not affected if mixed at 0–5 cm depth. Neither the mixing nor N fertilization had significant effects on root residue decomposition, estimated as recovered particulate organic matter. The total increase in microbial biomass C and biomass N was strongest after homogenous mixing of root residues at 0–20 cm, but remained unaffected by N fertilization. In contrast, the total amount of ergosterol remained unaffected by the mixing treatments, but responded positively to N fertilization. Shallow incorporation of poplar root residues did not affect the microbial biomass C/N ratio but disproportionately increased the fungal ergosterol to microbial biomass C ratio. Shallow incorporation of poplar root residues seems to reduce the demand for N fertilization of following crops, which should be further tested in field experiments.     

Nitrogen immobilization and mineralization during initial decomposition of15N-labelled pea and barley residues

The immobilization and mineralization of N following plant residue incorporation were studied in a sandy loam soil using¹⁵N-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorganic N during the complete incubation period of 84 days. The maximum rate of N immobilization was found to 12 and 18 mg soil-derived N g^?1 added C after incorporation of pea and barley residues, respectively. After 7 days of incubation, 21% of the pea and 17% of the barley residue N were assimilated by the soil microbial biomass. A comparison of the¹⁵N enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated directly by the microbial biomass without entering the soil inorganic N pool or the biomass had a higher preference for mineralized ammonium than for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized to a higher degree than the biomass N in the pea residue treatment, which declined during the incubation period. This was probably due to N-deficiency delaying the decomposition of the barley residue. The net mineralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonstrated that even if crop residues have a relative low C/N ratio (15), transient immobilization of soil N in the microbial biomass may contribute to improved conservation of soil N sources.     

Nitrogen immobilization and mineralization during initial decomposition of15N-labelled pea and barley residues

The immobilization and mineralization of N following plant residue incorporation were studied in a sandy loam soil using¹⁵N-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorganic N during the complete incubation period of 84 days. The maximum rate of N immobilization was found to 12 and 18 mg soil-derived N g^–1 added C after incorporation of pea and barley residues, respectively. After 7 days of incubation, 21% of the pea and 17% of the barley residue N were assimilated by the soil microbial biomass. A comparison of the¹⁵N enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated directly by the microbial biomass without entering the soil inorganic N pool or the biomass had a higher preference for mineralized ammonium than for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized to a higher degree than the biomass N in the pea residue treatment, which declined during the incubation period. This was probably due to N-deficiency delaying the decomposition of the barley residue. The net mineralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonstrated that even if crop residues have a relative low C/N ratio (15), transient immobilization of soil N in the microbial biomass may contribute to improved conservation of soil N sources.     

Long-term effect of chemical fertilizer,straw, and manure on labile organic matter fractions in a paddy soil

To assess the effect of long-term fertilization on labile organic matter fractions, we analyzed the C and N mineralization and C and N content in soil, particulate organic matter (POM), light fraction organic matter (LFOM), and microbial biomass. Results showed that fertilizer N decreased or did not affect the C and N amounts in soil fractions, except N mineralization and soil total N. The C and N amounts in soil and its fractions increased with the application of fertilizer PK and rice straw. Generally, there was no significant difference between fertilizer PK and rice straw. Furthermore, application of manure was most effective in maintaining soil organic matter and labile organic matter fractions. Soils treated with manure alone had the highest microbial biomass C and C and N mineralization. A significant correlation was observed between the C content and N content in soil, POM, LFOM, microbial biomass, or the readily mineralized organic matter. The amounts of POM–N, LFOM–N, POM–C, and LFOM–C closely correlated with soil organic C or total N content. Microbial biomass N was closely related to the amounts of POM–N, LFOM–N, POM–C, and LFOM–C, while microbial biomass C was closely related to the amounts of POM–N, POM–C, and soil total N. These results suggested that microbial biomass C and N closely correlated with POM rather than SOM. Carbon mineralization was closely related to the amounts of POM–N, POM–C, microbial biomass C, and soil organic C, but no significant correlation was detected between N mineralization with C or N amounts in soil and its fractions.     

Specific response of fungal and bacterial residues to one-season tillage and repeated slurry application in a permanent grassland soil

The dynamics of fungal and bacterial residues to a one-season tillage event in combination with manure application in a grassland soil are unknown. The objectives of this study were (1) to assess the effects of one-season tillage event in two field trials on the stocks of microbial biomass, fungal biomass, microbial residues, soil organic C (SOC) and total N in comparison with permanent grassland; (2) to determine the effects of repeated manure application to restore negative tillage effects on soil microbial biomass and residues. One trial was started 2 years before sampling and the other 5 years before sampling. Mouldboard ploughing decreased the stocks of SOC, total N, microbial biomass C, and microbial residues (muramic acid and glucosamine), but increased those of the fungal biomarker ergosterol in both trials. Slurry application increased stocks of SOC and total N only in the short-term, whereas the stocks of microbial biomass C, ergosterol and microbial residues were generally increased in both trials, especially in combination with tillage. The ergosterol to microbial biomass C ratio was increased by tillage, and decreased by slurry application in both trials. The fungal C to bacterial C ratio was generally decreased by these two treatments. The metabolic quotient qCO₂ showed a significant negative linear relationship with the microbial biomass C to SOC ratio and a significant positive relationship with the soil C/N ratio. The ergosterol to microbial biomass C ratio revealed a significant positive linear relationship with the fungal C to bacterial C ratio, but a negative one with the SOC content. Our results suggest that slurry application in grassland soil may promote SOC storage without increasing the role of saprotrophic fungi in soil organic matter dynamics relative to that of bacteria.     

Carbon and nitrogen dynamics in a phosphorus-deficient soil amended with organic residues and fertilizers in western Kenya

The contribution of organic resources to the restoration of soil fertility in smallholder farming systems in East Africa is being tested as an alternative to costly fertilizers. Organic inputs are expected to have advantages over fertilizers by affecting many biochemical properties controlling nutrient cycling. Our study examined changes in soil C and N, C and N mineralization, microbial biomass C (MBC) and N (MBN), and particulate organic matter (POM) in a P-limiting soil in western Kenya after applications of organic residues and fertilizers to overcome P limitation to crops. Leaf biomass from six different tree (shrub) species was incorporated into the soil at 5 Mg ha^–1 for five consecutive maize growing seasons, over 2.5 years. Triple superphosphate was applied separately at 0, 10, 25, 50, and 150 kg P ha^–1 in combination with 120 kg N ha^–1 as urea. Soil inorganic N, soil organic C, mineralizable N, and total C in all POM fractions and total N in the 53- to 250-m POM fraction increased following addition of all organic residues compared to the control. Whether there was an advantage of organic residue incorporation over inorganic fertilizer use depended on the soil parameter studied, the organic residue and the rate of fertilization. Most differences were found in N mineralization where 14.4–21.6 mg N kg^–1 was mineralized in fertilizer treatments compared to 25.2–30.5 mg N kg^–1 in organic residue treatments. C and N mineralization and the 53- to 250-m POM fractions were the most sensitive parameters, correlating with most of the studied parameters. Organic residues can contribute to improved soil nutrient cycling while the magnitude of their contribution depends on the biochemical properties of the residues.     

Carbon and nitrogen mineralization of added particulate and macroorganic matter

Particulate organic matter (POM) is more sensitive than total SOM to changes in management practices and, accordingly, may indicate changes in soil quality. A soil incubation study was conducted to determine the effects of added POM (75 to 250 μm size fraction), or macroorganic matter (MOM, 250 to 2000 μm size fraction) on C and N mineralization and microbial C and N content. A 1 kg composite made from 16 predominantly silt loam soils was amended with 10 g of POM, MOM or MOM ground to a reduced size of 75 to 250 μm (GMOM). The MOM amendment equaled 4.55-fold and POM equaled 1.60-fold of total MOM and POM found in the composite soil. Carbon mineralization of MOM and POM after 8 weeks was approximately 9 and 4%, respectively of the total MOM and POM-C added. Reducing the size of MOM to 75 to 250 μm did not affect mineralization. Nitrogen mineralization was slightly greater in the amended soils after 8 weeks and equaled 5 to 6% of the MOM or POM-total N added. Contribution of POM to total mineralized N from soil organic matter (SOM) in the composite soil was proportional to the POM content in SOM or approximately 12%. Amended soils had 25 to 42% more biomass-C than the control soil 2 weeks after amendment application. After 8 weeks, the amended soils contained about 32% more biomass-C. This increase in biomass-C at 8 weeks accounted for approximately 2% of the added C. At 8 weeks, microbial biomass-N in GMOM-, MOM- and POM-amended soils was about 56, 46 and 14% higher, respectively, than in the control soil. These increases were approximately 8% of the MOM-N added and 2% of the POM-N added. Increases in POM resulted in increases in soil respiration and microbial biomass-C and N, which also are suggested indicators of soil quality. Therefore, POM may be a suitable soil quality indicator that provides similar information as soil respiration or microbial biomass determinations.     

Changes in microbial biomass after continuous application of azolla and rice straw in soil

A model experiment was conducted under tropical conditions with a view to evaluating the changes in microbial biomass and nutrient dynamics in upland soil through the continuous application of azolla and rice straw (2 g C kg^-1 soil per each application). Flush decomposition of C was observed immediately after each application and the rate of mineralization did not change appreciably during this period. After flush decomposition, the rate of C mineralization from azolla was higher than that from rice straw until 9 to 13 weeks after each application and thereafter the mineralization rate was similar. The amount of inorganic N released from azolla increased following each application, whereas inorganic N in rice straw plot was immediately immobilized and the rate of immobilization increased until the 3rd application and did not increase further after the 4th application. The amounts of biomass C and N increased immediately after residue incorporation, reached the maximum level one week after each application and declined thereafter. Maximum biomass formation increased until the 2nd application and then the level remained constant. Maximum biomass N formation was higher in azolla than in rice straw after the 1st application, but after repeated applications, the difference became less pronounced: Continuous increase in biomass in a certain week after each application was observed, probably because of the cumulative effects of the previous applications. The increase suggests that continuous application of organic materials may enable to improve the amount of soil microbial biomass.     

Interaction of biochemical quality and particle size of crop residues and its effect on the microbial biomass and nitrogen dynamics following incorporation into soil

 Mineralization of N from organic materials added to soil depends on the quality of the substrate as a carbon, energy and nutrient source for the saprophytic microflora. Quality reflects a combination of biochemical and physical attributes. We investigated how biochemical composition interacts with particle size to affect the soil microflora and N dynamics following incorporation of crop residues into soil. Four fresh shoot and root crop residues were cut into coarse and fine particle sizes, and incorporated into sandy-loam soil which was incubated under controlled environment conditions for 6 months. In the case of the highest biochemical quality material, potato shoot (C/N ratio of 10 : 1), particle size had no effect on microbial respiration or net N mineralization. For lower biochemical quality Brussels sprout shoot (C/N ratio of 15 : 1), reducing particle size caused microbial respiration to peak earlier and increased net mineralization of N during the early stages of decomposition, but reduced net N mineralization at later stages. However, for the lowest biochemical quality residues, rye grass roots (C/N ratio of 38 : 1) and straw (C/N ratio of 91 : 1) reducing particle size caused microbial respiration to peak later and increased net immobilization of N. For Brussels sprout shoot, reducing particle size decreased the C content and the C/N ratio of residue-derived light fraction organic matter (LFOM) 2 months following incorporation. However C and N content of LFOM derived from the other materials was not affected by particle size. For materials of all qualities, particle size had little effect on biomass N. We conclude that the impact of particle size on soil microbial activities, and the protection of senescent microbial tissues from microbial attack, is dependant on the biochemical quality of the substrate. Received: 3 July 1998     

Depressed gross mineralization and immobilization of nitrogen and changes in microbial population in soil after heat treatment

Partial sterilization causes a change in N mineralization in soil. An increase in the net rate of N mineralization was reported in soil with chloropicrin applied to it (Rovira 1976), and has been well known in soil fumigated with chloroform to measure the microbial biomass N (Jenkinson and Ladd 1981). The gross rate of N mineralization increased in soil inoculated with fresh soil following fumigation with chloroform (Shen et al. 1984). The increased rate of N mineralization has been attributed to the rapid decomposition of organisms killed by partial sterilization (Jenkinson 1966). On the other hand, Nira et al. (1996) reported that the application of a fumigant in a field depressed the gross rates of N mineralization and immobilization in spite of the increase in the net rate of N mineralization. These results suggested that the increase in the net rate of N mineralization by partial sterilization is presumably due to the change in the ratio of N mineralization to immobilization. However, the residues of a fumigant may depress gross N transformation in the field, because the residues may continue to influence microbial activity long after the original treatment (Jenkinson 1966). Some effects of partial sterilization without residues on gross N mineralization remain to be determined.     

Nitrogen Mineralization Potential as Influenced by Microbial Biomass,Cotton Residues and Temperature

Integrating information on nitrogen (N) mineralization potentials into a fertilization plan could lead to improved N use efficiency. A controlled incubation mineralization study examined microbial biomass dynamics and N mineralization rates for two soils receiving 56 and 168 kg N ha^?1 in a Panoche clay loam (Typic Haplocambid) and a Wasco sandy loam (Typic Torriorthent), incubated with and without cotton (Gossypium hirsutum L.) residues at 10 and 25°C for 203 days. Microbial biomass activity determined from mineralized carbon dioxide (CO₂) was higher in the sandy loam than in clay loam independent of incubation temperature, cotton residue addition and N treatment. In the absence of added cotton residue, N mineralization rates were higher in the sandy loam. Residue additions increased N immobilization in both soils, but were greater in clay loam. Microbial biomass and mineralization were significantly affected by soil type, residue addition and temperature but not by N level.     

Development of ergosterol, microbial biomass C, N, and P after steaming as a result of sucrose addition, and Sinapis alba cultivation

A pot experiment was carried out to monitor the recovery of a steaming-reduced microbial biomass (C, N, and P) and fungal ergosterol by sucrose addition. The second objective was to investigate the recovery of a steaming-reduced microbial biomass by white mustard (Sinapis alba) cultivation and its interactions with microbial residues, freshly formed from sucrose addition. Thirty days after steaming, the soil microbial biomass C and N was still significantly reduced by 80%, leading to a rather constant microbial biomass C/N ratio around 7 throughout the experiment. The steaming-induced decreases of microbial biomass P and ergosterol were only roughly 50%, leading to a decrease in the microbial biomass C/P ratio and an increase in the ergosterol-to-microbial biomass C ratio. Sucrose addition led to a 25% reduction in the ergosterol-to-microbial biomass C ratio. Mustard cultivation had significant positive effects on microbial biomass C, N, P, and ergosterol, but the effects were smaller than those of sucrose addition. Cultivating mustard had no significant effects on the C loss or on the incorporation of sucrose C into the microbial biomass. In contrast, the application of sucrose led to a significant decrease in the mustard shoot biomass and especially in the mustard root biomass.     

Tillage effects on microbial biomass and nutrient distribution in soils under rain-fed corn production in central-western Mexico

Quantifying how tillage systems affect soil microbial biomass and nutrient cycling by manipulating crop residue placement is important for understanding how production systems can be managed to sustain long-term soil productivity. Our objective was to characterize soil microbial biomass, potential N mineralization and nutrient distribution in soils (Vertisols, Andisols, and Alfisols) under rain-fed corn (Zea mays L.) production from four mid-term (6 years) tillage experiments located in central-western, Mexico. Treatments were three tillage systems: conventional tillage (CT), minimum tillage (MT) and no tillage (NT). Soil was collected at four locations (Casas Blancas, Morelia, Apatzingán and Tepatitlán) before corn planting, at depths of 0–50, 50–100 and 100–150 mm. Conservation tillage treatments (MT and NT) significantly increased crop residue accumulation on the soil surface. Soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were highest in the surface layer of NT and decreased with depth. Soil organic C, microbial biomass C and N, total N and extractable P of plowed soil were generally more evenly distributed throughout the 0–150 mm depth. Potential N mineralization was closely associated with organic C and microbial biomass. Higher levels of soil organic C, microbial biomass C and N, potential N mineralization, total N, and extractable P were directly related to surface accumulation of crop residues promoted by conservation tillage management. Quality and productivity of soils could be maintained or improved with the use of conservation tillage.     

Impact of activated charcoal and tannin amendments on microbial biomass and residues in an irrigated sandy soil under arid subtropical conditions

Effects of goat manure application combined with charcoal and tannins, added as feed additives or mixed directly, on microbial biomass, microbial residues and soil organic matter were tested in a 2-year field trial on a sandy soil under Omani irrigated subtropical conditions. Soil microbial biomass C revealed the fastest response to manure application, followed by microbial residue C, estimated on the basis of fungal glucosamine and bacterial muramic acid, and finally soil organic C (SOC), showing the slowest, but still significant response. At the end of the trial, microbial biomass C reached 220 μg g^?1 soil, i.e. contents similar to sandy soils in temperate humid climate, and showed a relatively high contribution of saprotrophic fungi, as indicated by an average ergosterol to microbial biomass C ratio of 0.35 % in the manure treatments. The mean fungal C to bacterial C ratio was 0.55, indicating bacterial dominance of microbial residues. This fraction contributed relatively low concentrations of between 20 and 35 % to SOC. Charcoal added to manure increased the SOC content and the soil C/N ratio, but did not affect any of the soil microbial properties analysed. Tannins added to manure reduce the 0.5 M K₂SO₄-extractable N to N total ratio compared to manure control. These effects occurred regardless of whether charcoal or tannins were supplied as feed additive or directly mixed to the manure.     

Fate of nitrogen from crop residues as affected by biochemical quality and the microbial biomass

Net mineralization of N from a range of shoot and root materials was determined over a period of 6 months following incorporation into a sandy-loam soil under controlled environment conditions. Biochemical “quality” components of the materials showed better correlation with net N mineralization than did gross measures of the respiration and N content of the soil microbial community during decomposition. The quality components controlling net N mineralization changed during decomposition, with water-soluble phenolic content significantly correlated with net N mineralization at early stages, and water-soluble N, followed by cellulose at later stages. C-to-N and total N were correlated with net N mineralization towards the end of the incubation only. Cumulative microbial respiration during the early stages of decomposition was correlated with net N mineralization measured after 2 months, at which time maximum net N mineralization was recorded for most residues. However, there was no relationship between microbial-N and net N mineralization. Biochemical quality factors controlling the C and N content of the residue remaining at the end of the incubation as light fraction organic matter (LFOM) were also investigated. Both C and N content of LFOM derived from the residues were correlated with residue cellulose content, and the chemical characteristics of LFOM were highly correlated with those of the original plant material. Incorporation of low cellulose, high water-soluble N-containing shoot residues resulted in more N becoming mineralized than had been added in the residues, demonstrating that net mineralization of native soil organic matter had occurred. Large amounts of N were lost from the mineral-N pool during the incubation, which could not be accounted for by microbial immobilization.     

Effect of litter quality and soil fungi on macroaggregate dynamics and associated partitioning of litter carbon and nitrogen

We investigated the effect of plant residue decomposability and fungal biomass on the dynamics of macroaggregate (250–2000 μm) formation in a three months' incubation experiment and determined the distribution of residue-derived C and N in the microbial biomass and in aggregate size fractions (250–2000 μm, 53–250 μm and <53 μm) using ¹³C and ¹⁵N data. A silty loam soil (sieved <250 μm) was incubated with and without addition of ¹⁵N labelled maize leaves (C/N = 27.4) and roots (C/N = 86.4). Each treatment was carried out with and without fungicide application. The addition of maize residues enhanced soil respiration and microbial biomass C and N and resulted in increased macroaggregate formation with a higher and more rapid maximum macroaggregation in the soil amended with maize leaves than in that with addition of roots. Fungicide application led to a significant decline of microbial biomass C and mineralization of the added residues compared to untreated soils, which demonstrates a successful suppression of part of the active microbial biomass by the fungicide. However, this was not confirmed by a generally lower ergosterol concentration. Consequently, ergosterol was no reliable fungal biomarker in periods of rapid decline of the fungal biomass. A single addition of fungicide was insufficient for continued inhibition of the fungal biomass. Yet, a significant delay (28–42 days) in macroaggregation in fungicide treated compared to untreated samples highlighted the importance of the fungal biomass in macroaggregate formation. Macroaggregates were enriched in maize-derived ¹³C and ¹⁵N compared to microaggregates or the fraction < 53 μm. They turned over rapidly with decreasing substrate availability, which entailed a transfer of maize-derived C and N stored within macroaggregates during the first weeks of incubation to microaggregates with proceeding incubation time. Our results indicate that this transfer happened within macroaggregates, because no considerable amount of free particulate organic matter (POM) was released upon macroaggregate breakdown. We conclude that substrate decomposability and fungal activity are key factors determining extent and dynamics of macroaggregation during decomposition processes. Macroaggregate formation implied rapid incorporation and thereby short-term protection of maize-derived C and N. Moreover, macroaggregates allowed a transfer of maize-derived organic matter into microaggregates within macroaggregates, which prevented the release of significant amounts of free POM upon macroaggregate breakdown. Consequently, macroaggregates constitute to the transfer of recently added C into more stable soil organic matter fractions.     

Response of soil chemical and biological variables to small and large scale changes in climatic factors

This paper studies the effect of large- and small-scale changes of soil temperature and humidity on soil microbial biomass C and N, ergosterol, carbon utilization potential, organic and inorganic N and rate of C and N mineralization at 25°C. Large-scale variations are identified with seasonal changes in temperature and humidity. To simulate small-scale changes, soil temperature and humidity were manipulated in the field. The treatment resulted in damping of temperature fluctuations and a decrease of soil humidity.The majority of the studied variables exhibit pronounced seasonality, showing a clear-cut distinction between summer (July–August) and winter (December). In summer, C mineralization rate and carbon utilization potential was high but microbial and fungal biomass (ergosterol) was low.C and N mineralization rate and microbial and fungal biomass were only affected by sampling date, demonstrating that gross parameters of biomass and activity of microorganisms are not affected by small-scale changes in temperature and humidity. In contrast, variables relating to N availability (organic N, NH₄⁺ and NO₃⁻, microbial biomass N) and carbon utilization potential of the microbial community were highly affected by small-scale changes in soil abiotic conditions. The results suggest that changes in N dynamics induced by small-scale changes of temperature and humidity are caused by shifts in the structure of the microbial community rather than by variations in microbial biomass.     

Climatic conditions and crop‐residue quality differentially affect N,P, and S mineralization in soils with contrasting P status

The substitution of the widely practiced crop‐residue burning by residue incorporation in the subtropical zone requires a better understanding of factors determining nutrient mineralization. We examined the effect of three temperature (15°C, 30°C, and 45°C) and two moisture regimes (60% and 90% water‐filled pore space (WFPS)) on the mineralization‐immobilization of N, P, and S from groundnut (Arachis hypogae) and rapeseed (Brassica napus) residues (4 t ha^–1) in two soils with contrasting P fertility. Crop‐residue mineralization was differentially affected by incubation temperature, soil aeration status, and residue quality. Only the application of groundnut residues (low C : nutrient ratios) resulted in a positive net N and P mineralization within 30 days of incubation, while net N and P immobilization was observed with rapeseed residues. Highest N and P mineralization and lowest N and P immobilization occurred at 45°C under nearly saturated soil conditions. Especially net P mineralization was significantly higher in nearly saturated than in aerobic soils. In contrast, S mineralization was more from rapeseed than from groundnut residues and higher in aerobic than in nearly saturated soil. The initial soil P content influenced the mineralization of N and P, which was significantly higher in the soil with a high initial P fertility (18 mg P (kg soil)^–1) than in the soil with low P status (8 mg P (kg soil)^–1). Residue‐S mineralization was not affected by soil P fertility. The findings suggest that climatic conditions (temperature and rainfall‐induced changes in soil aeration status) and residue quality determine N‐ and S‐mineralization rates, while the initial soil P content affects the mineralization of added residue N and P. While the application of high‐quality groundnut residues is likely to improve the N supply to a subsequent summer crop (high temperature) under aerobic and the P supply under anaerobic soil condition, low‐quality residues (rapeseed) may show short‐term benefits only for the S nutrition of a following crop grown in aerobic soil.     

Nitrogen mineralization and microbial activity in permanent pastures amended with nitrogen fertilizer or dung

 Gross rates of soil processes and microbial activity were measured in two grazed permanent pasture soils which had recently been amended with N fertilizer or dung. ¹⁵N studies of rates of soil organic matter turnover showed gross N mineralization was higher, and gross N immobilization was lower, in a long-term fertilized soil than in a soil which had never received fertilizer N. Net mineralization was also found to be higher in the fertilized soil: a consequence of the difference between the opposing N turnover processes of N mineralization and immobilization. In both soils without amendments the soil microbial biomass contents were similar, but biomass activity (specific respiration) was higher in the fertilized soil. Short-term manipulation of fertilizer N input, i.e. adding N to unfertilized soil, or witholding N from previously fertilized soil, for one growing season, did not affect gross mineralization, immobilization or biomass size and activity. Amendments of dung had little effect on gross mineralization, but there was an increase in immobilization in both soils. Total biomass also increased under dung in the unfertilized soil, but specific respiration was reduced, suggesting changes in the composition of the biomass. Dung had a direct effect on the microbial biomass by temporarily increasing available soil C. Prolonged input of fertilizer N increases soil C indirectly as a result of enhanced plant growth, the effect of which may not become evident within one seasonal cycle. Received: 18 December 1998     

Short-term effects of earthworm activity and straw amendment on the microbial C and N turnover in a remoistened arable soil after summer drought

Short-term effects of actively burrowing Octolasion lacteum (Örl.) (Lumbricidae) on the microbial C and N turnover in an arable soil with a high clay content were studied in a microcosm experiment throughout a 16 day incubation. Treatments with or without amendment of winter wheat straw were compared under conditions of a moistening period after summer drought. The use of ¹⁴C labeled straw allowed for analyzing the microbial use of different C components. Microbial biomass C, biomass N and ergosterol were only slightly affected by rewetting and not by O. lacteum in both cases. Increased values of soil microbial biomass were determined in the straw treatments even after 24 h of incubation. This extra biomass corresponded to the initial microbial colonization of the added straw. O. lacteum significantly increased CO₂ production from soil organic matter and from the ¹⁴C-labeled straw. Higher release rates of ¹⁴C-CO₂ were recorded shortly after insertion of earthworms. This effect remained until the end of the experiment. O. lacteum enhanced N mineralization. Earthworms significantly increased both mineral N content of soil and N leaching in the treatments without straw addition. Moreover, earthworms slightly reduced N immobilization in the treatments with straw addition. The immediate increase in microbial activity suggests that perturbation of soil is more important than substrate consumption for the effect of earthworms on C and N turnover in moistening periods after drought.