A field study of gross rates of N mineralization and nitrification and their relationships to microbial biomass and enzyme activities in soils treated with dairy effluent and ammonium fertilizer

Abstract. Gross N mineralization and nitrification rates were measured in soils treated with dairy shed effluent (DSE) (i.e. effluent from the dairy milking shed, comprising dung, urine and water) or ammonium fertilizer (NH₄Cl) under field conditions, by injecting ¹⁵N-solution into intact soil cores. The relationships between gross mineralization rate, microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) as affected by the application of DSE and NH₄Cl were also determined. During the first 16 days, gross mineralization rate in the DSE treated soil (4.3–6.1 μg N g^?1 soil day^?1) were significantly (P 14;< 14;0.05) higher than those in the NH₄Cl treated soil (2.6–3.4 μg N g^?1 soil day^?1). The higher mineralization rate was probably due to the presence of readily mineralizable organic substrates in the DSE, accompanied by stimulated microbial and extracellular enzyme activities. The stable organic N compounds in the DSE were slow to mineralize and contributed little to the mineral N pool during the period of the experiment. Nitrification rates during the first 16 days were higher in the NH₄Cl treated soil (1.7–1.2 μg N g^?1 soil day^?1) compared to the DSE treated soil (0.97–1.5 μg N g^?1 soil day^?1). Soil microbial biomass C and N and extracellular enzyme activities (protease, deaminase and urease) increased after the application of the DSE due to the organic substrates and nutrients applied, but declined with time, probably because of the exhaustion of the readily available substrates. The NH₄Cl application did not result in any significant increases in microbial biomass C, protease or urease activities due to the lack of carbonaceous materials in the ammonium fertilizer. However, it did increase microbial biomass N and deaminase activity. Significant positive correlations were found between gross N mineralization rate and soil microbial biomass, protease, deaminase and urease activities. Nitrification rate was significantly correlated to biomass N but not to the microbial biomass C or the enzyme activities. Stepwise regression analysis showed that the variations of gross N mineralization rate was best described by the microbial biomass C and N.     

Combining field incubation with nitrogen-15 labelling to examine nitrogen transformations in low to high intensity grassland management systems

 The ¹⁵N isotope dilution method was combined with a field incubation technique to provide simultaneous measurements of gross and net rates of N turnover in three long-term swards: unfertilized (Z) or receiving N either from N fixation as clover (C), or as 200 kg fertilizer N ha^–1 year^–1 (F). Uniform N enrichment of soil microplots was achieved with a multi-point soil injector to measure mineralization/immobilization turnover and nitrification over a 4-day incubation. Net rates of mineralization ranged between 0.6 and 2.9 μg N g^–1 day^–1 and in all three treatments were approximately half the gross rates. Nitrification rates (gross) were between 1.0 and 1.6 μg N g^–1 day^–1. In the F treatment, the turnover of NH₄ ⁺-N and NO₃ ^–-N pools was on a 2- and 4-day cycle, respectively, whereas in the N-limited treatments (C and Z) turnover rates were faster, with the NO₃ ^–-N pools turning over twice as fast as the NH₄ ⁺-N pools. Therefore, available N was recycled more efficiently in the C and Z treatments, whereas in the F treatment a higher N pool size was maintained which would be more vulnerable to leakage. A large proportion of the added ¹⁵N was recovered in the soil microbial biomass (SMB), which represented a 4–5 times larger sink for N than the plant biomass. Although the C treatment had a significantly lower SMB than the grass-only treatments, there were no differences in microbial activity. Gross rates of nitrification increased along the gradient of N input intensity (i.e. Z<C<F), and the addition of a nitrification inhibitor (C₂H₂) tended to increase microbial immobilization, but did not influence plant N uptake. In this study, the value of combining different techniques to verify net rates was demonstrated and the improved methodology for ¹⁵N labelling of soil enabled measurements to be obtained from relatively undisturbed soil under natural field conditions. Received: 25 May 1999     

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     

Effects of storage time and straw content of cattle slurry on the mineralization of nitrogen and carbon in soil

 Animal slurries are stored for a variable period of time before application in the field. The effect of cattle slurry storage time and temperature on the subsequent mineralization of C and N in soil was studied under laboratory conditions. Urine and faeces from a dairy cow were sampled separately and mixed to a slurry. After 4 weeks of storage under anaerobic conditions at 15  °C, the NH₄ ⁺ N content exceeded the original urinary N content of the slurry; the NH₄ ⁺ content increased only slightly during the following 16 weeks of storage. After 4 weeks of storage, the proportion of slurry C in volatile fatty acids (VFA) amounted to 10% and increased to 15% after 20 weeks. Straw addition to the slurry caused an increase of VFA-C in stored slurry, but had a negligible influence on the proportion of slurry N in the form of NH₄ ⁺. Slurries subjected to different storage conditions were added to a sandy and a sandy loam soil. After 1 week, the preceding storage period (0–20 weeks) and temperature (5  °C or 15  °C) had no significant effect on the net release of inorganic N from the slurry in soil. Thus, the increased NH₄ ⁺ content in the slurry after storage was followed by increased net N immobilization in soil. Additional straw in the slurry caused increased net N immobilization only in the sandy loam soil. Following anaerobic storage, 8–14% of slurry C was released in gaseous form, and the net mineralization of slurry C after 12 weeks in soil amounted to 54–63%. The extra net mineralization of C in soil due to straw in slurry was equivalent to 76% of straw C, suggesting that the straw accelerated the mineralization of C derived from faeces, urine and/or soil. Received: 25 August 1997     

Impact of land-use change on nitrogen mineralization in subalpine grasslands in the Southern Alps

 A field study was conducted to investigate the effect of abandonment of management on net N mineralization (NNM) in subalpine meadows. NNM, soil microbial biomass N (SMBN), fungal biomass and physicochemical characteristics (total C, total N, dissolved organic carbon (DOC) and pH) were determined in surface (0–10 cm) soil from May to October 1997 in an intensively managed and an abandoned meadow at 1770 m a.s.l.. The cumulative NNM was lower in the abandoned area and ranged from 150 to 373 and from 25 to 85 μg N g^–1 soil in the intensively managed and the abandoned areas, respectively. The total organic C increased in the abandoned area, while total N showed no difference between abandoned and managed meadow. SMBN showed no difference between the investigated sites, whereas ergosterol, a measure of fungal biomass, increased significantly at the abandoned site. The cumulative NNM was negatively correlated with total organic C, C : N ratio, DOC and ergosterol content, and positively correlated with the NH₄ ⁺-N content of soil. The decrease in NNM at the abandoned site may be explained by the lower availability of N in substrates characterized by a high C : N ratio which, together with a decrease in pH in the litter layer, may have increased fungal biomass. Received: 8 January 1999     

Influence of soil phosphorus status and nitrogen addition on carbon mineralization from 14C-labelled glucose in pasture soils

 This study examines the effect of soil P status and N addition on the decomposition of ¹⁴C-labelled glucose to assess the consequences of reduced fertilizer inputs on the functioning of pastoral systems. A contrast in soil P fertility was obtained by selecting two hill pasture soils with different fertilizer history. At the two selected sites, representing low (LF) and high (HF) fertility status, total P concentrations were 640 and 820 mg kg^–1 and annual pasture production was 4,868 and 14,120 kg DM ha^–1 respectively. Soils were amended with ¹⁴C-labelled glucose (2,076 mg C kg^–1 soil), with and without the addition of N (207 mg kg^–1 soil), and incubated for 168 days. During incubation, the amounts of ¹⁴CO₂ respired, microbial biomass C and ¹⁴C, microbial biomass P, extractable inorganic P (P_i) and net N mineralization were determined periodically. Carbon turnover was greatly influenced by nutrient P availability. The amount of glucose-derived ¹⁴CO₂ production was high (72%) in the HF and low (67%) in the LF soil, as were microbial biomass C and P concentrations. The ¹⁴C that remained in the microbial biomass at the end of the 6-month incubation was higher in the LF soil (15%) than in the HF soil (11%). Fluctuations in P_i in the LF soil during incubation were small compared with those in HF soil, suggesting that P was cycling through microbial biomass. The concentrations of P_i were significantly greater in the HF samples throughout the incubation than in the LF samples. Net N mineralization and nitrification rates were also low in the LF soils, indicating a slow turnover of microorganisms under limited nutrient supply. Addition of N had little effect on biomass ¹⁴C and glucose utilization. This suggests that, at limiting P fertility, C turnover is retarded because microbial biomass becomes less efficient in the utilization of substrates. Received: 18 October 1999     

Critical sulphur concentration and sulphur requirement of microbial biomass in a glucose and cellulose-amended regosol

 The critical S concentration and S requirement of the soil microbial biomass of a granitic regosol was examined. S was applied at the rate of 0, 5, 10, 20, 30 and 50 μg S as MgSO₄·7H₂O, together with either 3000 μg glucose-C or 3333 μg cellulose-C, 400 μg N, and 200 μg P g ^–1 soil and 200 μg K g^–1 soil. Microbial biomass, inorganic SO₄ ^2–-S, and CO₂ emission were monitored over 30 days during incubation at 25  °C. Both glucose and cellulose decomposition rates responded positively to the S made available for microbial cell synthesis. The amounts of microbial biomass C and S increased with the level of applied S up to 10 μg S g^–1 soil and 30 μg S g^–1 soil in the glucose- and cellulose-amended soil, respectively, and then declined. Incorporated S was found to be concentrated within the microbial biomass or partially transformed into soil organic matter. The concentration of S in the microbial biomass was higher in the cellulose- (4.8–14.2 mg g^–1) than in the glucose-amended soil (3.7–10.9 mg g^–1). The microbial biomass C:S ratio was higher in the glucose- (46–142 : 1) than in the cellulose-amended soil (36–115 : 1). The critical S concentration in the microbial biomass (defined as that required to achieve 80% of the maximum synthesis of microbial biomass C) was estimated to be 5.1 mg g^–1 in the glucose- and 10.9 mg g^–1 in the cellulose-amended soil. The minimum requirement of SO₄ ^2–-S for microbial biomass formation was estimated to be 11 μg S g^–1 soil and 21 μg S g^–1 soil for glucose- and cellulose-amended soil, respectively. The highest levels of activity of the microbial biomass were observed at the SO₄ ^2–-S concentrations of 14 μg S g^–1 soil and 17 μg S g^–1 soil, for the glucose and cellulose amendments, respectively, and were approximately 31–54% higher during glucose than cellulose decomposition. Received: 20 October 1999     

Substrate type,temperature, and moisture content affect gross and net N mineralization and nitrification rates in agroforestry systems

Accurate prediction of soil N availability requires a sound understanding of the effects of environmental conditions and management practices on the microbial activities involved in N mineralization. We determined the effects of soil temperature and moisture content and substrate type and quality (resulting from long-term pasture management) on soluble organic C content, microbial biomass C and N contents, and the gross and net rates of soil N mineralization and nitrification. Soil samples were collected at 0–10 cm from two radiata pine (Pinus radiata D. Don) silvopastoral treatments (with an understorey pasture of lucerne, Medicago sativa L., or ryegrass, Lolium perenne L.) and bare ground (control) in an agroforestry field experiment and were incubated under three moisture contents (100, 75, 50% field capacity) and three temperatures (5, 25, 40 °C) in the laboratory. The amount of soluble organic C released at 40 °C was 2.6- and 2.7-fold higher than the amounts released at 25 °C and 5 °C, respectively, indicating an enhanced substrate decomposition rate at elevated temperature. Microbial biomass C:N ratios varied from 4.6 to 13.0 and generally increased with decreasing water content. Gross N mineralization rates were significantly higher at 40 °C (12.9 g) than at 25 °C (3.9 g) and 5 °C (1.5 g g^–1 soil day^–1); and net N mineralization rates were also higher at 40 °C than at 25 °C and 5 °C. The former was 7.5-, 34-, and 29-fold higher than the latter at the corresponding temperature treatments. Gross nitrification rates among the temperature treatments were in the order 25 °C >40 °C >5 °C, whilst net nitrification rates were little affected by temperature. Temperature and substrate type appeared to be the most critical factors affecting the gross rates of N mineralization and nitrification, soluble organic C, and microbial biomass C and N contents. Soils from the lucerne and ryegrass plots mostly had significantly higher gross and net mineralization and nitrification rates, soluble organic C, and microbial biomass C and N contents than those from the bare ground, because of the higher soil C and N status in the pasture soils. Strong positive correlations were obtained between gross and net rates of N mineralization, between soluble organic C content and the net and gross N mineralization rates, and between microbial biomass N and C contents.     

Short‐term effects of polyacrylamide and dicyandiamide on C and N mineralization in a sandy loam soil

The soil conditioners anionic polyacrylamide (PAM) and dicyandiamide (DCD) are frequently applied to soils to reduce soil erosion and nitrogen loss, respectively. A 27‐day incubation study was set up to gauge their interactive effects on the microbial biomass, carbon (C) mineralization and nitrification activity of a sandy loam soil in the presence or absence of maize straw. PAM‐amended soils received 308 or 615 mg PAM/kg. Nitrogen (N)‐fertilized soils were amended with 1800 mg/kg ammonium sulphate [(NH₄)₂SO₄], with or without 70 mg DCD/kg. Maize straw was added to soil at the rate of 4500 mg/kg. Maize straw application increased soil microbial biomass and respiration. PAM stimulated nitrification and C mineralization, as evidenced by significant increases in extractable nitrate and evolved carbon dioxide (CO₂) concentrations. This is likely to have been effected by the PAM improving microbial conditions and partially being utilized as a substrate, with the latter being indicated by a PAM‐induced significant increase in the metabolic quotient. PAM did not reduce the microbial biomass except in one treatment at the highest application rate. Ammonium sulphate stimulated nitrification and reduced microbial biomass; the resultant acidification of the former is likely to have caused these effects. N fertilizer application may also have induced short‐term C‐limitation in the soil with impacts on microbial growth and respiration. The nitrification inhibitor DCD reduced the negative impacts on microbial biomass of (NH₄)₂SO₄ and proved to be an effective soil amendment to reduce nitrification under conditions where mineralization was increased by addition of PAM.     

Effects of salts and moisture content on N2O emission and nitrogen dynamics in Yellow soil and Andosol in model experiments

 The effects of salt type and its concentration on nitrification, N mineralization and N₂O emission were examined under two levels of moisture content in Yellow soil and Andosol samples as simulated to agriculture under arid/semi-arid conditions and under heavy application of fertilizer in a glass-house, respectively. The salt mixtures were composed of chlorides (NaCl and NH₄Cl) or sulphates [Na₂SO₄ and (NH₄)₂SO₄] and were added at various concentrations (0, 0.1, 0.2, 0.4 and 0.6 M as in the soil solution). These salts were added to non-saline Yellow soil at different moisture contents (45 or 40 and 65% of maximum water-holding capacity; WHC) and their effects on the changes in mineral N (NH₄ ⁺-N and NO₃ ^–-N) concentration as well as N₂O emission were examined periodically during laboratory incubation. We also measured urease activities to know the effect of salts on N mineralization. Furthermore, Ca(NO₃)₂ solution was added at various concentrations (0, 0.1, 0.3, 0.5 and 0.8 M as in the soil solution) to a non-saline Andosol taken from the subsurface layer in a glass-house and incubated at different moisture contents (50% and 70% of WHC) to examine their effects on changes in mineral N. Nitrification was inhibited by high, but remained unaffected by low, salt concentrations. These phenomena were shown in both the model experiments. It was considered that the salinity level for inhibition of nitrification was an electric conductivity (1 : 5) of 1 dS m^–1. This level was independent of the type of salts or soil, and was not affected by soil moisture content. The critical level of salts for urease activities was about 2 dS m^–1. The emission rate of N₂O was maximum at the beginning of the incubation period and stabilized at a low level after an initial peak. There was no significant difference in N₂O emission among the treatments at different salt concentrations, while higher moisture level enhanced N₂O emission remarkably. Received: 29 July 1998     

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.     

Conversion of grassland to coniferous woodland has limited effects on soil nitrogen cycle processes

In the last century, conversion of native North American grasslands to Juniperus virginiana forests or woodlands has dramatically altered ecosystem structure and significantly increased ecosystem carbon (C) stocks. We compared soils under recently established J. virginiana forests and adjacent native C₄-dominated grassland to assess changes in potential soil nitrogen (N) transformations and plant available N. Over a 2-year period, concentrations of extractable inorganic N were measured in soils from forest and grassland sites. Potential gross N ammonification, nitrification, and consumption rates were determined using ¹⁵N isotope-dilution under laboratory conditions, controlling for soil temperature and moisture content. Potential nitrification rates (V_max) and microbial biomass, as well as soil physical and chemical properties were also assessed. Extractable NH₄⁺ concentrations were significantly greater in grassland soils across the study period (P ≤ 0.01), but analysis by date indicated that differences in extractable inorganic N occurred more frequently in fall and winter, when grasses were senescent but J. virginiana was still active. Laboratory-based rates of gross N mineralization (ammonification) and nitrification were greater in grassland soils (P ≤ 0.05), but only on one of four dates. Potential nitrification rates (V_max) were an order of magnitude greater than gross nitrification rates in both ecosystems, suggesting that nitrification is highly constrained by NH₄⁺ availability. Differences in plant uptake of N, C inputs, and soil microclimate as forests replace grasslands may influence plant available N in the field, as evidenced by seasonal differences in soil extractable NH₄⁺, and total soil C and N accumulation. However, we found few differences in potential soil N transformations under laboratory conditions, suggesting that this grassland-to-forest conversion caused little change in mineralizable organic N pools or potential microbial activity.     

Phosphorus mineralization and microbial biomass in a Florida Spodosol: effects of water potential, temperature and fertilizer application

 Phosphorus mineralization and microbial biomass were measured in the surface 5 cm of a Spodosol (sandy, siliceous hyperthermic Ultic Alaquod) from north-central Florida. Soils from fertilized and unfertilized plantations of loblolly pine (Pinus taeda L.) were incubated at a range of water potentials (∼0, –3, –8, –10 and –1500 kPa) and temperatures (15  °C, 25  °C and 38  °C) for 14 days and 42 days. Increasing water potential and temperature increased specific P mineralization (mineralization expressed as a percentage of total P) regardless of fertilizer treatment. An increase in water potential from –10 kPa to –0.1 kPa resulted in an increase of between 38% and 239% in the concentration of KCl-extractable inorganic P, depending on incubation temperature and time. An increase in incubation temperature from 15  °C to 38  °C resulted in an increase of between 13% and 53% in KCl-extractable inorganic P. Changes in specific P mineralization with change in water potential or temperature were not affected by fertilizer application. This suggests that, although specific P mineralization was greater in the fertilized soils, environmental control of P mineralization was the same for both treatments. Specific P mineralization was most sensitive when soils were at higher water potentials, and decreased logarithmically to water potentials of between –3 kPa and –8 kPa. Specific P mineralization was relatively insensitive to changes in water potential when water potential was lower than –8 kPa. Microbial biomass C showed no consistent responses to changes of temperature or water potential and was not significantly correlated with specific P mineralization. Our results suggest that field estimates of P mineralization in these Spodosols may be improved by accounting for changes in soil water potential and temperature. Received: 30 October 1997     

Seasonal dynamics of carbon and nitrogen pools and fluxes under continuous arable and ley-arable rotations in a temperate environment

Improved understanding of the seasonal dynamics of C and N cycling in soils, and the main controls on these fluctuations, is needed to improve management strategies and to better match soil N supply to crop N demand. Although the C and N cycles in soil are usually considered to be closely linked, few data exist where both C and N pools and gross N fluxes have been measured seasonally. Here we present measurements of inorganic N, extracted soluble organic N, microbial biomass C and N, gross N fluxes and CO₂ production from soil collected under wheat in a ley‐arable and continuous arable rotation within a long‐term experiment. The amounts of inorganic N and extracted soluble organic N were similar (range 5–35 kg N ha⁻¹; 0–23 cm) but had different seasonal patterns: whilst inorganic N declined during wheat growth, extracted soluble organic N peaked after cultivation and also during maximal stem elongation. The microbial biomass was significantly larger in the ley‐arable (964 kg C ha⁻¹; 0–23 cm) than the continuous arable rotation (518 kg C ha⁻¹; 0–23 cm) but with no clear seasonal pattern. In contrast, CO₂ produced from soil and gross N mineralization showed strong seasonality linked to soil temperature and moisture content. Normalization of soil CO₂ production and gross N mineralization with respect to these environmental regulators enabled us to study the underlying influence of the incorporation of fresh plant material into soil on these processes. The average normalized gross rates of N mineralized during the growing season were 1.74 and 2.55 kg N ha⁻¹ nday⁻¹ in continuous arable and ley‐arable rotations respectively. Production rates (gross N mineralization, gross nitrification) were similar in both land uses and matched rates of NH₄⁺ and NO₃⁻ consumption, resulting in periods of net N mineralization and immobilization. There was no simple relationship between soil CO₂ production and gross N mineralization, which we attributed to changes in the C : N ratio of the mineralizing pool(s).     

Microbial biomass and C and N transformations in forest floors under European beech, sessile oak, Norway spruce and Douglas-fir at four temperate forest sites

The purpose of this research was to compare soil chemistry, microbially mediated carbon (C) and nitrogen (N) transformations and microbial biomass in forest floors under European beech (Fagus sylvatica L.), sessile oak (Quercus petraea (Mattuschka) Lieblein), Norway spruce (Picea abies (L.) Karst) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) at four study sites. We measured soil chemical characteristics, net N mineralization, potential and relative nitrification, basal respiration, microbial and metabolic quotient and microbial biomass C and N under monoculture stands at all sites (one mixed stand). Tree species affected soil chemistry, microbial activities and biomass, but these effects varied between sites. Our results indicated that the effect of tree species on net N mineralization was likely to be mediated through their effect on soil microbial biomass, reflecting their influence on organic matter content and carbon availability. Differences in potential nitrification and relative nitrification might be related to the presence of ground vegetation through its influence on soil NH₄ and labile C availability. Our findings highlight the need to study the effects of tree species on microbial activities at several sites to elucidate complex N cycle interactions between tree species, ground vegetation, soil characteristics and microbial processes.     

Nitrogen mineralization,N<Subscript>2</Subscript>O production and soil microbiological properties as affected by long-term applications of sewage sludge composts

A field study was conducted to investigate the long-term effect of surface application of sewage sludge composts vs chemical N fertilizer on total N, total C, soluble organic C, pH, EC, microbial biomass C and N, protease activity, deaminase activity, urease activity, gross and net rates of N mineralization and nitrification, CO₂ evolution, and N₂O production. Soil samples were taken from five depths (0–15, 15–20, 20–30, 30–40, and 40–50 cm) of a long-term experiment at the University of Tokyo, Japan. Three fields have been receiving sewage sludge composted with rice husk (RH), sawdust (SD), or mixed chemical fertilizer NPK (CF), applied at the rate of 240 kg N ha^–1 each in split applications in summer and autumn since 1978. Significantly higher amounts of total N and C and soluble organic C were found in the compost than in the CF treatments up to the 40-cm soil depth, indicating improved soil quality in the former. In the CF treatment, soil pH values were significantly lower and electrical conductivity values were significantly higher than those of compost-treated soils of up to 50 cm depth. Soil microbial biomass C and N, CO₂ evolution, protease, deaminase, and urease activities were significantly higher in the compost than in the CF treatments due to greater availability of organic substrates that stimulated microbial activity. Gross N mineralization rates determined by ¹⁵N dilution technique were eight and five times higher in the SD and RH treatments than in the CF treatment, respectively, probably due to high levels of microbial and enzyme activities. Net N mineralization rates were also significantly higher in the compost treatments and were negative in the CF treatment indicating immobilization. Net nitrification rates were higher in compost treatments and negative in the CF treatment. Nitrous oxide productions from compost treatments were higher than the CF treatment due to the greater availability of mineral N as a result of higher mineralization and nitrification rates and soluble organic C in the former. Most of the measured parameters were highest in the surface soil (0–15 cm) and were significantly higher in the SD treatment than in the RH treatment.     

Response of the soil microbial biomass and nematode population to a wetting event in nitrogen-amended Negev desert plots

 Water and N availability are the major limiting factors of primary production in desert ecosystems, and the response of soil biota to these two factors is of great importance. We examined the immediate response of soil nematodes and the microbial biomass to a single pulse of water amendment in N-treated plots in the Israeli Negev desert. Plots were treated with 0, 50 and 100 kg NH₄NO₃ ha^–1 in December 1992, and at the end of the summer period (August 1993) the plots were exposed to a 15 mm water. Soil samples from the 0–10 cm layer were collected daily and analysed soil moisture, total soluble N, nematode populations and microbial biomass. Soil moisture increased to 8.5%, then gradually decreased to 2% during the 11 days of the study. Microbial biomass, soil respiration and metabolic quotient values did not exhibit any significant correlation with soil N levels. Free-living nematode population levels in the different plots were found to increase from a mean level of 45 500 to a mean level of 92 300 individuals m^–2. N treatment was found to affect the patterns of free-living nematode population dynamics. The results of this study demonstrated the importance of moisture availability levels and the ability to mobilize previous N inputs into available N which, occurring in pulses, can affect the microbial ecophysiological status, nematode population dynamics and the interrelationship between these two important components in the desert soil milieu. Received: 5 November 1998     

Quantification of nitrogen excretion rates for three lumbricid earthworms using 15N

 Nitrogen excretion rates of ¹⁵N-labeled earthworms and contributions of ¹⁵N excretion products to organic (dissolved organic N) and inorganic (NH₄-N, NO₃-N) soil N pools were determined at 10  °C and 18  °C under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clitellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rubellus (Hoffmeister) were labeled with ¹⁵N by providing earthworms with ¹⁵N-labeled organic substrates for 5–6 weeks. The quantity of ¹⁵N excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were calculated. N excretion rates ranged from 274.4 to 744 μg N g^–1 earthworm fresh weight day^–1, with a daily turnover of 0.3–0.9% of earthworm tissue N. The N excretion rates of juvenile L. terrestris were significantly lower than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, particularly NH₄-N, were greater in soils incubated with earthworms for 48 h than soils incubated without earthworms. Between 13 and 40% of excreted ¹⁵N was found in the ¹⁵N-mineral N (NH₄-N+NO₃-N) pool, and 13–23% was in the ¹⁵N-DON pool. Other fates of excreted ¹⁵N may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N forms, or gaseous N losses. Earthworm excretion rates were combined with earthworm biomass measurements to estimate N flux from earthworm populations through excretion. Annual earthworm excretion was estimated at 41.5 kg N ha^–1 in an inorganically-fertilized corn agroecosystem, and was equivalent to 22% of crop N uptake. Our results suggest that the earthworms could contribute significantly to N cycling in corn agroecosystems through excretion processes. Received: 12 April 1999     

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.