Immobilization and mineralization of nitrogen in soils incubated with pig slurry or ammonium sulphate

Nitrogen mineralization and immobilization were investigated in two soils incubated with ammonium sulphate or pig slurry over a range of temperatures and moisture contents. A reduction in the mineralization of soil organic N was observed in soils incubated with 100 μg NH₄⁺-Ng^?1 soil as ammonium sulphate at 30°C but not at lower temperatures. Addition of 100 μg NH₄⁺-N g^?1 soil as pig slurry resulted in a period of nett immobilization lasting up to 30 days at 5°C. Although the length of the immobilization phase was shorter at higher temperatures the total N immobilized was similar. The subsequent rate of mineralization in slurry-treated soils was not significantly greater (P = 0.05) than in untreated soils. There was no evidence of any subsequent increased mineralization arising from the immobilized N or slurry organic N for up to 175 days. The rate of immobilization was found to increase with increasing moisture content, though the period of nett immobilization was shorter, so that the amount of N immobilized was similar over a range of moisture contents from 10 to 40%. Approximately 40% of the NH₄⁺-N in the slurry was immobilized under the incubation conditions used.     

Effects of glucose and rhizodeposits (with or without cysteine-S) on immobilized-35S, microbial biomass-35S and arylsulphatase activity in a calcareous and an acid brown soil

Our aim was to study the effects of C (as glucose and artificial rhizodeposits) on S immobilization, in relation to microbial biomass‐S and soil arylsulphatase (ARS) activity, in contrasting soils (a calcareous and an acid brown soil). The glucose‐C and artificial rhizodeposit‐C with or without cysteine were added at six rates (0, 100, 200, 400, 600 and 800 mg kg^?1 soil) to the two soils and then incubated with Na₂³⁵SO₄ for 1 week prior to analysis. The percentages of ³⁵S immobilized increased when C as glucose and rhizodeposit (without cysteine) were added to both soils. With cysteine‐containing rhizodeposit, the percentages of ³⁵S immobilized remained relatively stable (23.5% to 29.9%) in the calcareous soil, but decreased in the acid brown soil (52.7% to 31.5%). For both soils, cysteine‐containing rhizodeposit additions showed no significant correlation between immobilized‐³⁵S and microbial biomass‐³⁵S, suggesting that microorganisms immobilized cysteine‐S preferentially instead of ³⁵S from the tracer (Na₂³⁵SO₄). In the calcareous soil, a positive and significant correlation was found between ARS activity and microbial biomass‐³⁵S (r = 0.85, P < 0.05) when glucose was added. We also saw this correlation in the acid brown soil when rhizodeposit‐C without cysteine was added (r = 0.90, P < 0.05). Accordingly, the results showed the presence of extracellular arylsulphatase activity of 48.7 mg p‐nitrophenol kg^?1 soil hour^?1 in the calcareous soil and of 27.0 mg p‐nitrophenol kg^?1 soil hour^?1 in the acid brown soil.     

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.     

Gross nitrogen mineralization-, immobilization-, and nitrification rates as a function of soil C/N ratio and microbial activity

A laboratory experiment was designed to challenge the idea that the C/N ratio of forest soils may control gross N immobilization, mineralization, and nitrification rates. Soils were collected from three deciduous forests sites varying in C/N ratio between 15 and 27. They were air-dried and rewetted to induce a burst of microbial activity. The N transformation rates were calculated from an isotope dilution and enrichment procedure, in which ¹⁵NH₄Cl or Na¹⁵NO₃ was repeatedly added to the soils during 7 days of incubation. The experiments suggested that differences in gross nitrogen immobilization and mineralization rates between the soils were more related to the respiration rate and ATP content than to the C/N ratio. Peaks of respiration and ATP content were followed by high rates of mineralization and immobilization, with 1-2 days of delay. The gross immobilization of NH₄⁺ was dependent on the gross mineralization and one to two orders of magnitude larger than the gross NO₃⁻ immobilization. The gross nitrification rates were negatively related to the ATP content and the C/N ratio and greatly exceeding the net nitrification rates. Taken together, the observations suggest that leaching of nitrate from forest soils may be largely dependent on the density and activity of the microbial community.     

Predicting Nitrogen and Carbon Mineralization of Composted Manure and Sewage Sludge in Soil

The capability of organic wastes to release available N in soil varies largely, depending on their source and form of production, or rather on their composition and biodegradability. Our purpose was to predict mineralization rates of different materials using their analyses joined with a simulation model, and to evaluate the influence of soil type and application rate of the organic materials on N and C transformations in soil. Four organic materials, sewage sludge (SS), sewage sludge compost (SSC), cattle manure compost (CMC), hen and cattle manure compost (HCMC), were applied to two soils at rates of 2 and/or 4%. The soils were incubated aerobically for 168 days at 30°C, during which CO₂ evolution rates and mineral-N concentrations were measured periodically. Hot water extractable C and N of all organic amendments correlated well with short term C and N mineralization, except HCMC that immobilized N although its soluble N content was large. NCSOIL, a computer model that simulates C and N cycling in soil with organic amendments, predicted well C and N mineralization of SS, SSC and CMC when considered as three-pool materials that decomposed at specific rates of 0.4, 0.024 and 10^?4 d^?1, using hot water soluble C and N as the labile pool. N immobilization by HCMC could be simulated only if the distribution of N between the labile and resistant pools was derived by optimization of NCSOIL, while hot water soluble C was labile. Laboratory methods to determine an intermediate pool or components that contribute to immobilization are required for improving the predictions of C and N mineralization from organic amendments.     

Effects of soil moisture on gross N transformations and N2O emission in acid subtropical forest soils

Soil moisture changes, arising from seasonal variation or from global climate changes, could influence soil nitrogen (N) transformation rates and N availability in unfertilized subtropical forests. A ¹⁵?N dilution study was carried out to investigate the effects of soil moisture change (30–90 % water-holding capacity (WHC)) on potential gross N transformation rates and N₂O and NO emissions in two contrasting (broad-leaved vs. coniferous) subtropical forest soils. Gross N mineralization rates were more sensitive to soil moisture change than gross NH₄ ⁺ immobilization rates for both forest soils. Gross nitrification rates gradually increased with increasing soil moisture in both forest soils. Thus, enhanced N availability at higher soil moisture values was attributed to increasing gross N mineralization and nitrification rates over the immobilization rate. The natural N enrichment in humid subtropical forest soils may partially be due to fast N mineralization and nitrification under relatively higher soil moisture. In broad-leaved forest soil, the high N₂O and NO emissions occurred at 30 % WHC, while the reverse was true in coniferous forest soil. Therefore, we propose that there are different mechanisms regulating N₂O and NO emissions between broad-leaved and coniferous forest soils. In coniferous forest soil, nitrification may be the primary process responsible for N₂O and NO emissions, while in broad-leaved forest soil, N₂O and NO emissions may originate from the denitrification process.     

Net Nitrogen and Sulfur Mineralization in Mountainous Soils Amended with Indigenous Plant Residues

Abstract

Mineralization of nitrogen (N) and sulfur (S) were examined over a 31‐week incubation period under aerobic conditions in 13 soils selected from the mountainous district of Pertouli, central Greece. The main native plant species are turf, forage crops, forest trees, herbs, and shrubs. Net mineralization and immobilization of N and S by indigenous plants were estimated. Most soils are acidic in this xeric, mesic climatic regime, and organic carbon (C), organic nitrogen (N), and sulfur (S) appreciably varied in soils and plant species. The cumulative net mineralization of N is much higher in comparison to the amount of mineralized S. Release of N is curvilinear with time and associated with a declining mineralization in the later stages of the incubation. Close relationships exist between soil organic S and N and between organic C and N, indicating that these elements are mainly bound in the organic matter. The cumulative net mineralization from soils varies greatly and ranges between 36.6 and 212.8 (average 104.8) mg/kg for N and between 21.4 and 45.2 (average 31.8) mg/kg for S. Immobilization occurs in most soils amended with indigenous plant residues, with the amount of immobilized N varying between 14.8 and 49.5 mg/kg and that of immobilized S ranging from 2.6 to 30.7 mg/kg. However, the estimated rates of N and S mineralization are not negligible and can be taken into account as potential sources in the management of the upland soils.     

Sulphur transformations in soils using sulphur-35 labelling

Mineralization and immobilization of sulphur in two soils was studied by means of incubation experiments involving the labelling of soil with radioactive ³⁵S. The results obtained were consistent with the concept of a continuous, concurrent mineralization and immobilization cycle taking place involving a relatively small proportion (3–6%) of the soil organic S pool. Addition of glucose to the soil increased both the amounts of S mineralized or immobilized and the apparent size of the actively cycling organic S fraction. HI-reducible forms of soil S appeared to be the predominant forms of organic S involved in the S transformations.Re-incubation of the soils after leaching and drying caused a considerable amount of fresh sulphate mineralization to take place. The original actively cycling organic S pool initially contributed an extremely high proportion (40–70%) of the freshly mineralized sulphate. However, as the re-incubation proceeded, a much greater proportion of the soil organic S became involved in S transformations.     

Effect of bone chars on phosphorus‐cadmium‐interactions as evaluated by three extraction procedures

Foreseen P shortage and contamination problems have stimulated the search for renewable and contaminant‐free P‐fertilizers and amendments that immobilize Cd. We investigated the P‐dissolution and Cd‐immobilizing effect of bone char (pyrolyzed de‐fatted bone chips; BC) and bone char with added reduced S compounds (BC^plus). Five soils varying in pH and low to high Cd‐contamination were incubated with slow‐release P‐fertilizers (BC and BC^plus) and the fast P‐release diammonium phosphate (DAP), and extracted with NH₄NO₃‐, NaHCO₃‐solutions, and H₂O. The P‐concentrations obtained by the three extractants were well correlated and NH₄NO₃ well suited to simultaneously assess the P‐ and Cd‐solubility. The addition of BC increased pH in all soils whereas BC^plus and DAP lowered the pH in soils with pH > 5. Similar trends for NH₄NO₃‐P differences between treatments and control were observed for BC and BC^plus during the incubation period, although BC^plus resulted in much larger P‐concentrations. The highest Cd‐immobilization efficiency was obtained in BC‐treated soils. The addition of BC^plus and DAP decreased the Cd‐concentrations until 34 d of incubation in all soils and remained effective in Cd‐immobilizing in soils that showed a pH raise over 145 d of incubation. Thus, the results indicate that surface modification of BC may promote the P‐dissolution along with a concomitant Cd‐immobilization largely through its pH‐effect but this must be confirmed in studies under non‐equilibrium conditions.     

Determination of the fate of regularly applied 13C‐labeled‐artificial‐exudates C in two agricultural soils

Exudates are part of the total rhizodeposition released by plant roots to soil and are considered as a substantial input of soil organic matter. Exact quantitative data concerning the contribution of exudates to soil C pools are still missing. This study was conducted to reveal effects of ¹³C‐labeled exudate (artificial mixture) which was regularly applied to upper soil material from two agricultural soils. The contribution of exudate C to water‐extractable organic C (WEOC), microbial biomass C (MBC), and CO₂‐C evolution was investigated during a 74 d incubation. The WEOC, MBC, and CO₂‐C concentrations and the respective δ¹³C values were determined regularly. In both soils, significant incorporation of artificial‐exudate‐derived C was observed in the WEOC and MBC pool and in CO₂‐C. Up to approx. 50% of the exudate‐C amounts added were recovered in the order WEOC << MBC < CO₂‐C in both soils at the end of the incubation. Newly built microbial biomass consisted mainly of exudates, which substituted soil‐derived C. Correspondingly, the CO₂‐C evolved from exudate‐treated soils relative to the controls was dominated by exudate C, showing a preferential mineralization of this substrate. Our results suggest that the remaining 50% of the exudate C added became stabilized in non‐water‐extractable organic fractions. This assumption was supported by the determination of the total organic C in the soils on the second‐last sampling towards the end of the incubation. In the exudate‐treated soils, significantly more soil‐derived C compared to the controls was found in the WEOC on almost all samplings and in the MBC on the first sampling. This material might have derived from exchange processes between the added exudate and the soil matrix. This study showed that easily available substrates can be stabilized in soil at least in the short term.     

Effect of soil characteristics on N mineralization capacity in 112 native and agricultural soils from the northwest of Spain

N mineralization capacity and its main controlling factors were studied in a large variety (n=112) of native (forest, bush) and agricultural (pasture, cultivated) soils from several climatic zones in Spain. The available inorganic N content, net N mineralization, and net N mineralization rate were determined after 6 weeks of aerobic incubation. NH _inf4 ^sup+ –N largely predominated over NO _inf3 ^sup- -N (ratio near 10:1) except in some agricultural soils. Net N mineralization predominated (83% of soils) over net N immobilization, which was more frequent in agricultural soils (25%) than in native soils (9%). In forest soils, both net N mineralization and the net N mineralization rate were significantly higher than in the other soil groups. The net N mineralization rate of pasture and cultivated soils was similar to that of bush soils, but available inorganic N was lower. The net N mineralization rate decreased in the order: soils over acid rocks>soils over sediments>soils over basic rocks or limestone; moreover, the highest net N mineralization and available inorganic N were found in soils over acid rocks. The highest N mineralization was found in soils with low C and N contents, particularly in the native soils, in which N mineralization increased as the C:N ratio increased. N mineralization was higher in soils with a low pH and base saturation than in soils with high pH and base saturation values, which sometimes favoured N immobilization. Soils with an Al gel content of >1% showed lower net N mineralization rates than soils with Al gel contents of <1%, although net N mineralization and available inorganic N did not differ between these groups. The net N mineralization rate in silty soils was significantly lower than in sandy and clayey soils, although soil texture only explained a low proportion of the differences in N mineralization between soils.     

Root exudate effects on the bacterial communities, CO2 evolution, nitrogen transformations and ATP content of rhizosphere and bulk soils

The ATP content, soil respiration, bacterial community composition, and gross N mineralization and immobilization rates were monitored under laboratory condition at 25 °C for 28 d in a model system where low molecular weight root exudates (glucose and oxalic acid) were released by a filter placed on the surface of a forest soil also treated with ¹⁵N, so as to simulate rhizosphere conditions. Periodically, the soil was sampled from two layers, 0-2 and 6-14 mm below the filter's surface, which were indicated as rhizosphere and bulk soils, respectively. The isotope dilution technique was used to determine the effect of these low molecular weight organic compounds (LMWOCs) on gross N mineralization and immobilization rates. From 0 to 3 d both glucose and oxalic acid amended soils showed a rapid evolution of CO₂, more pronunced in the latter treatment together with a decrease in the amount of mineral N of the rhizosphere soil, probably due to N immobilization. Nevertheless, these changes were accompanied by a very small increase in the net ATP content probably because the low C application rate stimulated microbial activity but microbial growth only slightly. A positive ‘priming effect’ probably developed in the oxalic acid amended soil but not in the glucose amended soil. Gross N mineralization and immobilization rates were only observed in the rhizosphere soil, probably due to the greater C and N concentrations and microbial activity, and were a little higher in both amended soils than in the control soil, only between 1 and 7 d. Both glucose and oxalic acid influenced the bacterial communities of the rhizosphere soil, as new bands in the DGGE profiles appeared at 3 and 7 d. Glucose induced lower changes in the bacterial community than oxalic acid, presumably because the former stimulated a larger proportion of soil microorganisms whereas the latter was decomposed by specialized microorganisms. Peaks of net daily soil respiration and net ATP content and the appearence of new dominant bacterial populations were shifted in time, probably because there was less ATP synthesis and DGGE patterns changed after complete substrate mineralization.     

Sulphur immobilization and availability in soils assessed using isotope dilution

Increasing recognition of S deficiency in soils has raised the need for understanding processes governing S cycling and availability in soils. However, the quantification of the two main processes of S cycling, i.e. mineralization and immobilization, remains difficult as these processes occur simultaneously. A modified isotope ³⁵SO₄ dilution technique was developed and used to measure the effect of sulphate (SO₄) fertilization on S mineralization and immobilization in planted (pot experiment with ryegrass (Lolium multiflorum L.)) and unplanted soils (incubation). The immobilization and mineralization of S was calculated from the dynamics of stable and labelled S in soil KH₂PO₄ extracts containing an anion exchange membrane that concentrates SO₄ and mainly excludes other S species. The mathematical analysis of the isotope dilution data differs from methods proposed earlier. The radiolabile S in unplanted soil (E value) and in ryegrass (L value) were used as a measure of total available S in soils. Sulphate immobilization rate significantly declined during incubation. Sulphate application reduced gross mineralization but surprisingly reduced SO₄ immobilization. The E value significantly increased during the incubation in all soils as a result of gross mineralization, e.g. from 3.8 mg S kg⁻¹ at day 0 to 11.5 mg S kg⁻¹ at day 43 in the sandy soil with no sulphate addition. A full recovery in the E value of S added in (+S) treatments was achieved. Similarly, radiolabile S in the above-ground ryegrass biomass (L value) increased with S addition, with a full recovery of added S. The E and L values nearly fit a 1:1 line suggesting identical S dynamics in a planted and unplanted soil. The method proposed has operational advantages compared to methods used earlier.     

Plant availability of boron in acid soils as assessed by different extractants

Estimation of available‐boron (B) status through conventional methods in B‐deficient acidic Inceptisols and Entisols is often hampered because of their very low B content. In the present study, the extractability of available B by different extractants was tested in relation to soil properties. Plant availability of B was assessed with mustard (Brassica campestris L.) and wheat (Triticum aestivum L.) in pot experiments. Twelve soils with varying characteristics were extracted for available B with hot water (HW), hot CaCl₂ (HCC), KH₂PO₄ (PDP), tartaric acid (TA), and mannitol‐CaCl₂ (MCC). Mustard (cv. B‐9) and wheat (cv. PBW‐343) were grown with four levels of B (0, 0.25, 0.50, and 1.0 mg [kg soil]^–1). Dry‐matter accumulation and B concentrations were determined at pre‐flowering and full‐maturity stages for mustard and at panicle‐initiation and maturity stages for wheat. The extraction of B from the soils ranked HCC > HW > PDP > TA > MCC. The higher extractability with HW and HCC was likely due to higher temperature and that of PDP because of its phosphorus content, which facilitated the desorption of B. The low B extraction with MCC resulted from the poor mannitol‐B complex formation in acidic soils. The application of B increased dry‐matter accumulation, plant B concentration, and uptake at all B levels and growth stages in both crops with the responses being more pronounced during the early developmental stage. Based on linear correlations, Mallow's Cp statistics, and principal‐component analyses, HCC and HW were the best extractants for estimating available B in the acidic experimental soils.     

Effects of carbon and phosphorus addition on microbial respiration,N<Subscript>2</Subscript>O emission,and gross nitrogen mineralization in a phosphorus-limited grassland soil

Soil microbes are frequently limited by carbon (C), but also have a high phosphorus (P) requirement. Little is known about the effect of P availability relative to the availability of C on soil microbial activity. In two separate experiments, we assessed the effect of P addition (20 mg P kg^?1 soil) with and without glucose addition (500 mg C kg^?1 soil) on gross nitrogen (N) mineralization (¹⁵N pool dilution method), microbial respiration, and nitrous oxide (N₂O) emission in a grassland soil. In the first experiment, soils were incubated for 13 days at 90% water holding capacity (WHC) with addition of NO₃^? (99 mg N kg^?1 soil) to support denitrification. Addition of C and P had no effect on gross N mineralization. Initially, N₂O emission significantly increased with glucose, but it decreased at later stages of the incubation, suggesting a shift from C to NO₃^? limitation of denitrifiers. P addition increased the N₂O/CO₂ ratio without glucose but decreased it with glucose addition. Furthermore, the ¹⁵N recovery was lowest with glucose and without P addition, suggesting a glucose by P interaction on the denitrifying community. In the second experiment, soils were incubated for 2 days at 75% WHC without N addition. Glucose addition increased soil ¹⁵N recovery, but had no effect on gross N mineralization. Possibly, glucose addition increased short-term microbial N immobilization, thereby reducing N-substrates for nitrification and denitrification under more aerobic conditions. Our results indicate that both C and P affect N transformations in this grassland soil.     

Effects of glucose,cellulose, and humic acids on soil microbial eco‐physiology

Microbial eco‐physiology in soils is regulated by substrate quality of the organic matter. This regulation was studied for a forest and an agricultural soil by the combination of activity and biomass techniques. Soil respiration was stimulated by the substrate quality in the order, humic acid < cellulose < glucose over 20 days. Concurrently, substrate addition increased the respiratory quotient (RQ), defined as the ratio of mol CO₂ evolution per mol O₂ uptake. Anabolic processes were mainly induced by glucose addition. Soil preconditioned with glucose showed a decrease in the RQ value during glucose‐induced microbial growth in comparison to non‐amended control. The decrease in the RQ value induced by preconditioning with cellulose and humic acid was lower. Glucose, cellulose, and humic acid addition modified the microbial biomass as estimated by fumigation‐extraction (FE), substrate‐induced respiration (SIR), and ATP content. Since each biomass estimate refers to specific microbial components, shifts in microbial eco‐physiology and community structure induced by substrate quality were reflected by SIR : FE and SIR : ATP ratios. The active and glucose‐responsive biomass in the forest soil which was earlier suggested as being dominated by K‐strategists was increased in the order, humic acid < cellulose < glucose.     

Sulfur mineralization in two soils amended with organic manures,crop residues,and green manures

The mineralization of sulfur (S) was investigated in a Vertisol and an Inceptisol amended with organic manures, green manures, and crop residues. Field‐moist soils amended with 10 g kg^—1 of organic materials were mixed with glass beads, placed in pyrex leaching tubes, leached with 0.01 M CaCl₂ to remove the mineral S and incubated at 30 °C. The leachates were collected every fortnight for 16 weeks and analyzed for SO₄‐S. The amount of S mineralized in control and in manure‐amended soils was highest in the first week and decreased steadily thereafter. The total S mineralized in amended soils varied considerably depending on the type of organic materials incorporated and soil used. The cumulative amounts of S mineralized in amended soils ranged from 6.98 mg S (kg soil)^—1 in Inceptisol amended with wheat straw to 34.38 mg S (kg soil)^—1 in Vertisol amended with farmyard manure (FYM). Expressed as a percentage of the S added to soils, the S mineralized was higher in FYM treated soils (63.5 to 67.3 %) as compared to poultry manure amended soils (60.5 to 62.3 %). Similarly the percentage of S mineralization from subabul (Leucaena leucocephala) loppings was higher (53.6 to 55.5 %) than that from gliricidia (Gliricidia sepium) loppings (50.3 to 51.1 %). Regression analysis clearly indicated the dependence of S mineralization on the C : S ratio of the organic materials added to soil. The addition of organic amendments resulted in net immobilization of S when the C : S ratio was above 290:1 in Vertisol and 349:1 in Inceptisol. The mineralizable S pool (S_o) and first‐order rate constant (k) varied considerably among the different types of organic materials added and soil. The S_o values of FYM treated soils were higher than in subabul, gliricidia, and poultry manure treated soils.     

Mineralization of methionine sulphur in soils and forest floor layers

A1-horizon soils and 01, 02 forest floor layers from a mixed mature hardwood forest rapidly converted methionine-S to readily-available (salt-extractable) and less readily-available (acid- and base-extractable) inorganic sulphate (SO^?2₄). It is suggested that this latter conversion represents the incorporation into organic matter of a portion of the (SO^?2₄) released by mineralization. On a dry weight basis, the 02 layer of the forest floor was the most active with respect to both conversions. Moreover, capacities for mineralization and (SO^?2₄) incorporation decreased with increasing sample depth within the mineral horizon. Both conversions were dependent upon temperature and duration of incubation and were absent from samples which had been autoclaved. Sodium azide and the broad-spectrum antibiotic, tetracycline also inhibited each conversion to varying extents depending upon the type of sample incubated with methionine.     

Immobilization of sulphur-35, microbial biomass and arylsulphatase activity in soils from field-grown rape,barley and fallow

Soils under rape, barley and fallow were monitored for S immobilization, microbial biomass and arylsulphatase (ARS) activity. These soils were regularly collected in the field every fortnight from mid-April to the end of June 2001. Then, they were incubated for 1 week at 25°C with carrier-free ³⁵S (Na₂³⁵SO₄). The results showed significant correlation of both immobilized ³⁵S ( r =0.94, P <0.001) and ARS ( r =0.79, P <0.001) with microbial biomass-³⁵S (MB-³⁵S) but not with microbial biomass-C (MB-C). Therefore, only a fraction of MB-C was involved in the immobilization of ³⁵S and hence the release of ARS. In addition, positive correlations appeared between hot water-soluble C (HW-C), ³⁵S immobilization, MB-³⁵S and ARS, indicating the pivotal role of HW-C in regulating S immobilization and turnover. The mean percentage values of immobilized ³⁵S, MB-³⁵S and ARS were higher in soil from the fallow, followed by soils from barley and rape. However, under culture, the mean values of ARS per unit of MB-³⁵S showed the decreasing order: soil from rape (0.58)>soil from barley (0.46)>soil from fallow (0.34). It is concluded that microorganisms under rape were most efficient in producing ARS, a strategy used to cope with the SO₄^2--S limiting conditions.     

Mineralization of sulfur from organic residues assessed by inverse isotope dilution

Sulfur (S) deficiency in soils is increasingly recognized in agricultural systems. The quantification of S mineralization/immobilization processes after incorporation of organic materials into soils is a key factor to predict the availability of S to growing plants. However, immobilization and mineralization occur simultaneously making the quantification of the magnitude of each process difficult. We used the inverse isotope (³⁵SO₄) dilution technique to quantify immobilization and mineralization fluxes after incorporation of two organic residues with contrasting C/S ratio's (cabbage or wheat straw) into a sandy soil in planted and unplanted soils (pot trial with ryegrass and incubation). The soil was labeled with ³⁵SO₄ and incubated for 63 days prior to the application of residues. The specific activity (SA) of soil-extractable SO₄ did not change significantly in the control soil during the subsequent experimental period despite significant net mineralization, illustrating that labile-S in soil was homogenously labeled. Application of residues decreased the SAs during the incubation due to the dilution with unlabeled-S from the residues. A three-compartment dynamic model was fitted to the SA data predicting that gross mineralization of residue-S was almost complete over 43 days incubation although this release was not matched by the increase in soil SO₄ due to immobilization reactions. Soil-extractable SO₄ was significantly increased in the cabbage-treated soil while the reverse was true in the wheat straw amended soil in which the S-immobilization was almost twice the gross mineralization of residue-S. The SA of S in ryegrass were maximally 15% lower than in corresponding soil extracts suggesting that residue mineralization was similar in planted and unplanted soils. The inverse isotope dilution method offers potential for screening S release of different residues; however the details of the dynamics of soil-S isotopes show that the individual fluxes are not constant during the incubation.