Sulphur mineralization and release of soluble organic sulphur from camp and non-camp soils of grazed pastures receiving long-term superphosphate applications

Summary Topsoils (0–75 mm) from four soil types with different sulphate retention capacities were collected from stock camp and non-camp (main grazing area) sites of grazed pastures in New Zealand which had been annually fertilized with superphosphate for more than 15 years. These soils were analysed for different S fractions and incubated at 30°C for 10 weeks using an open incubation technique in order to assess the extent of S mineralization and the release of soluble soil organic S from camp and non-camp soils during incubation. The soils were preleached with 0.01 M KCl, followed by 0.04 M Ca(H₂PO₄)₂ before being incubated. Pre-incubation leachates and weekly 0.01 M KCl leachates were analysed for mineralized S (i.e., hydriodic acid-reducible S) and total S. Soluble organic S was estimated as the difference between these two S fractions. Results obtained show higher cumulative amounts of all three S fractions in leachates over a 10-week incubation period in camp than in non-camp soils, suggesting that higher mineralization occurred in camp soils. Cumulative amounts of mineralized S from camp and non-camp soils showed a linear relationship with duration of incubation (R ²0.985^***), while the cumulative release of soluble organic S followed a quadratic relationship (R ²0.975^***). A significant proportion (14.6%–40.8%) of total S release in KCl leachates was soluble organic S, indcating that organic S should be taken into account when assessing S mineralization. Mineralized S and soluble organic S were best correlated with 0.01 M CaCl₂-extractable soil inorganic S (R ²=0.767^***) and 0.04 M Ca(H₂PO₄)₂-extractable soil inorganic S(R ²=0.823^***), respectively. Soil sulphate retention capacity was found to influence amounts of mineralized S and soluble organic S, and thus periodic leaching with KCl to remove mineralized S from soils may not adequately reflect the extent of soil S mineralization in high sulphate-retentive soils. In low (<10%) sulphateretentive soils, increasing the superphosphate applications from 188 to 376 kg ha^–1 year^–1 increased S mineralization but not amounts of C-bonded and hydriodic acid-reducible soil S fractions.     

Arylamidase and amidohydrolases in soils as affected by liming and tillage systems

This study investigated the long-term effect of lime application and tillage systems (no-till, ridge-till and chisel plow) on the activities of arylamidase and amidohydrolases involved in N cycling in soils at four long-term research sites in Iowa, USA. The activities of the following enzymes were assayed: arylamidase, -asparaginase, -glutaminase, amidase, urease, and -aspartase at their optimal pH values. The activities of the enzymes were significantly (P<0.001) and positively correlated with soil pH, with r values ranging from 0.42^* to 0.99^*** for arylamidase, 0.81^*** to 0.97^*** for -asparaginase, 0.62^*** to 0.97^*** for -glutaminase, 0.61^*** to 0.98^*** for amidase, 0.66^** to 0.96^*** for urease, and 0.80^*** to 0.99^*** for -aspartase. The Δactivity/ΔpH values were calculated to assess the sensitivity of the enzymes to changes in soil pH. The order of the sensitivity of enzymes was as follows: -

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-aspartase. The enzyme activities were greater in the samples of the 0–5 cm depth than those of the 0–15 cm samples under no-till treatment. Most of the enzyme activities were significantly (P<0.001) and positively correlated with microbial biomass C (C_mic) and N (N_mic). Lime application significantly affected the specific activities of the six enzymes studied. Results showed that soil management practices, including liming and type of tillage significantly affect soil biological and biochemical properties, which may lead to changes in nitrogen cycling, including N mineralization in soils.     

Effect of freeze-thaw events on mineralization of soil nitrogen

Summary In humid regions of the United States there is considerable interest in the use of late spring (April–June) soil NO ₃ ^– concentrations to estimate fertilizer N requirements. However, little information is available on the environmental factors that influence soil NO ₃ ^– concentrations in late winter/early spring. The influence of freeze-thaw treatments on N mineralization was studied on several central Iowa soils. The soils were subjected to temperatures of-20°C or 5°C for 1 week followed by 0–20 days of incubation at various temperatures. The release of soluble ninhydrin-reactive N, the N mineralization rate, and net N mineralization (mineral N flush) were observed. The freeze-thaw treatment resulted in a significant increase in the N mineralization rate and mineral N flush. The N mineralization rate in the freeze-thaw treated soils remained higher than in non-frozen soils for 3–6 days when thawed soils were incubated at 25°C and for up to 20 days in thawed soils incubated at 5°C. The freeze-thaw treatments resulted in a significant release of ninhydrin-reactive N. These values were closely correlated with the mineral N flush (r ²=0.84). The release of ninhydrin-reactive N was more closely correlated with biomass N (r ²=0.80) than total N (r ²=0.65). Our results suggest that freeze-thaw events in soil disrupt microbial tissues in a similar way to drying and re-wetting or chloroform fumigation. Thus the level of mineral N released was directly related to the soil microbial biomass. We conclude that net N mineralization following a spring thaw may provide a significant portion of the total NO ₃ ^– present in the soil profile.     

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.     

Effect of pH on nitrogen mineralization in crop-residue-treated soils

Summary This study compares N mineralization in soils treated with crop residues [corn (Zea mays L.), soybean (Glycine max (L.) Merr.), sorghum (Sorghum vulgare Pers.)] or alfalfa (Medicago sativa L.) at three adjusted soil pH values (4, 6, and 8); pH was adjusted with dilute H₂SO₄ or KOH. A sample of soil (20 g) was treated with 0.448 g plant material (equivalent to 50t ha^–1), mixed with 20 g silica sand adjusted to the pH of the soil, and packed in a leaching tube. The soil-sand mixture was leached with 100 ml 5 mM CaCl₂ adjusted to the same pH as that of the treated soil to remove the initial mineral N, and incubated at 30°C. The leaching procedure was repeated every 2 weeks for 20 weeks. Results from three soils showed that N mineralization increased as the soil pH increased. In one soil (Lester soil), significant amounts of NH ₄ ⁺ -N accumulated at pH 4 during the first 12 weeks. Treatment with corn and soybean residues resulted in a marked reduction in N mineralization, especially at pH 4. The percentage of organic N mineralized from sorghum residue and alfalfa added to soils increased as the soil pH increased; the values ranged from 7.7% to 37.0% for sorghum and from 17.2% to 30.1% for alfalfa.     

Mineralization of organic sulfur and its importance as a reservoir of plant-available sulfur in upland soils of north China

 An open incubation technique was used to measure S mineralization in a range of upland soils of north China. Six mineralization patterns were examined, and a soil S-exhaustion experiment with ryegrass (Lolium multiflorum L.) was conducted to investigate the availability of various organic S pools to plants. For all of the 12 soils tested, the release of S as SO₄ ^2– was curvilinear with time, and during a 28-week incubation at 30  °C the amount of S mineralized ranged from 14.0 mg S kg^–1 soil to 37.4 mg S kg^–1 soil. A first-order model and Gompertz model appeared to best describe S mineralization. Examination of the soils after incubation revealed the bulk of the mineralized S was mainly derived from the C-bonded S pool, while the majority of mineralized S under soil S exhaustion by ryegrass was derived from the HI-reducible S pool. Received: 9 July 1998     

Phosphorus mineralization in subarctic agricultural and forest soils

Summary Potential P and C mineralization rates were determined in a 12-week laboratory incubation study on subarctic forest and agricultural soil samples with and without N fertilizer added. There was no significant difference in net inorganic P produced between N fertilized and unfertilized soils. The forest soil surface horizons had the highest net inorganic P mineralized, 32 mg P kg^-1 soil for the Oie and 17 mg P kg^-1 soil for the Oa. In the cropped soils net inorganic P immobilization started after 4 weeks and lasted through 12 weeks of incubation. Cumulative CO₂–C evolution rates differed significantly among soils, and between fertilizer treatments, with the N-fertilized soils evolving lower rates of CO₂–C than the unfertilized soils. Soils from the surface horizons in the forest evolved the highest rates of CO₂–C (127.6 and 89.4 mg g^-1 soil for the Oie and Oa horizons, respectively) followed by the cleared uncropped soil (42.8 mg g^-1 soil C), and the cropped soils (25.4 and 29.0 mg g^-1 soil C). In vitro soil respiration rates, or potential soil organic matter decomposition rates, decreased with increasing time after clearing and in accord with the degree of disturbance. Only soils with high potential C mineralization rates and high organic P to total P ratios, mineralized P by the end of the study. Mineralizable P appeared to be associated with readily mineralizable organic C.     

Carbon mineralization in soils of different textures as affected by water-soluble organic carbon extracted from composted dairy manure

The water-soluble organic C in composted manure contains a portion of labile C which can stimulate soil microbial activity. The objective of this experiment was to evaluate the effects of water-soluble organic C extracted from composted dairy manure on C mineralization in soil with different textures. Three soils with textures varying from 3 to 54% clay were amended with 0 to 80 mg water-soluble organic C kg^–1 soil extracted from a composted dairy manure and incubated for 16 weeks at 23°C. The total amount of C mineralized was greater than the amount of C added in the three soils. Differences in mineralizable C with and without added water-soluble organic C were approximately 13–16 times, 4.8–8 times, and 7.5–8 times greater than the amount of C added to clay, loam, and sand soils, respectively. The results of this experiment suggest that immediately following composted manure applications, C mineralization rates increase, and that most of the C mineralized comes mainly from the indigenous soil organic C pool.CLBRR contribution No. 94-71     

Impacts of elemental S applied under various temperature and moisture regimes on pH and available P in acidic, neutral and alkaline soils

We evaluated the effect of elemental S (S⁰) under three moisture (40, 60, 120% water-filled pore space; WFPS) and three temperature regimes (12, 24, 36°C) on changes in pH and available P (0.5 N NaHCO₃-extractable P) concentrations in acidic (pH 4.9), neutral (pH 7.1) and alkaline (pH 10.2) soils. Repacked soil cores were incubated for 0, 14, 28 and 42 days. Application of S⁰ did not alter the trends of pH in acidic and neutral soils at all moisture regimes but promoted a decrease in the pH of alkaline soil under aerobic conditions (40%, 60% WFPS). Moisture and temperature had profound effects on the available P concentrations in all three soils, accumulation of available P being greatest under flooded conditions (120% WFPS) at 36°C. Application of S⁰ in acidic, neutral and alkaline soils resulted in the net accumulation of 16.5, 14.5 and 13 g P g^–1 soil after 42 days at 60% WFPS, but had no effect under flooded conditions. The greatest available P accumulations in the respective soils were 19, 19.5 and 20 g P g^–1 soil (equivalent to 38, 41, 45 kg P ha^–1) with the combined effects of 36°C, 60% WFPS and applied S⁰. The results of our study revealed that oxidation of S⁰ lowered the pH of alkaline soil (r=–0.88, P<0.01), which in turn enhanced available P concentrations. Also, considering the significant relationship between the release of sulphate and accumulation of P, even in acidic soil (r=0.92, P<0.01) and neutral soil (r=0.85, P<0.01) where the decrease in pH was smaller, it is possible that the stimulatory effect of sulphate on the availability of P was due to its concurrent desorption from the colloidal surface, release from fixation sites and/or mineralization of organic P. Thus, in the humid tropics and irrigated subtropics where high moisture and temperature regimes are prevalent, the application of S⁰ could be beneficial not only in alleviating S deficiency in soils but also for enhancing the availability of P in arable soils, irrespective of their initial pH.     

Effect of cropping systems on nitrogen mineralization in soils

 Understanding the effect of cropping systems on N mineralization in soils is crucial for a better assessment of N fertilizer requirements of crops in order to minimize nitrate contamination of surface and groundwater resources. The effects of crop rotations and N fertilization on N mineralization were studied in soils from two long-term field experiments at the Northeast Research Center and the Clarion-Webster Research Center in Iowa that were initiated in 1979 and 1954, respectively. Surface soil samples were taken in 1996 from plots of corn (Zea mays L.), soybean (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha^–1 before corn and an annual application of 20 kg P and 56 kg K ha^–1. N mineralization was studied in leaching columns under aerobic conditions at 30  °C for 24 weeks. The results showed that N mineralization was affected by cover crop at the time of sampling. Continuous soybean decreased, whereas inclusion of meadow increased, the amount of cumulative N mineralized. The mineralizable N pool (N _o) varied considerably among the soil samples studied, ranging from 137 mg N kg^–1 soil under continuous soybean to >500 mg N kg^–1 soil under meadow-based rotations, sampled in meadow. The results suggest that the N _o and/or organic N in soils under meadow-based cropping systems contained a higher proportion of active N fractions. Received: 10 February 1999     

Temporal and spatial patterns of denitrification enzyme activity and nitrous oxide fluxes in three adjacent vegetated riparian buffer zones

The aim of this study was to investigate temporal and spatial patterns of denitrification enzyme activity (DEA) and nitrous oxide (N₂O) fluxes in three adjacent riparian sites (mixed vegetation, forest and grass). The highest DEA was found in the surface (0–30 cm) soil and varied between 0.7±0.1 mg N kg^–1 day^–1 at 5°C and 5.9±0.4 mg N kg^–1 day^–1 at 15°C. There was no significant difference (P >0.05) between the DEA in the uppermost (0–30 cm and 60–90 cm) soil depths under different vegetation covers. In the two deepest (120–150 cm and 180–210 cm) soil depths the DEA varied between 0.0±0.0 mg N kg^–1 day^–1 at 5°C and 4.4±0.9 mg N kg^–1 day^–1 at 15°C and was clearly associated with the accumulation of buried organic carbon (OC). Two threshold values of OC were observed before DEA started to increase significantly, namely 5 and 25 g OC kg^–1 soil at 10–15°C and 5°C, respectively. In the three riparian sites N₂O fluxes varied between a net N₂O uptake of –0.6±0.4 mg N₂O-N m^–2 day^–1 and a net N₂O emission of 2.5±0.3 mg N₂O-N m^–2 day^–1. The observed N₂O emission did not lead to an important pollution swapping (from water pollution to greenhouse gas emission). Especially in the mixed vegetation and forest riparian site highest N₂O fluxes were observed upslope of the riparian site. The N₂O fluxes showed no clear temporal trend.     

Activity and biomass of soil microorganisms at different depths

We measured microbial biomass C and soil organic C in soils from one grassland and two arable sites at depths of between 0 and 90 cm. The microbial biomass C content decreased from a maximum of 1147 (0–10 cm layer) to 24 g g^-1 soil (70–90 cm layer) at the grassland site, from 178 (acidic site) and 264 g g^-1 soil (neutral site) at 10–20 cm to values of between 13 and 12 g g^-1 soil (70–90 cm layer) at the two arable sites. No significant depth gradient was observed within the plough layer (0–30 cm depth) for biomass C and soil organic C contents. In general, the microbial biomass C to soil organic C ratio decreased with depth from a maximum of between 1.4 and 2.6% to a minimum of between 0.5 and 0.7% at 70–90 cm in the three soils. Over a 24-week incubation period at 25°C, we examined the survival of microbial biomass in our three soils at depths of between 0 and 90 cm without external substrate. At the end of the incubation experiment, the contents of microbial biomass C at 0–30 cm were significantly lower than the initial values. At depths of between 30 and 90 cm, the microbial biomass C content showed no significant decline in any of the four soils and remained constant up to the end of the experiment. On average, 5.8% of soil organic C was mineralized at 0–30 cm in the three soils and 4.8% at 30–90 cm. Generally, the metabolic quotient qCO₂ values increased with depth and were especially large at 70–90 cm in depth.     

Status and distribution of soil sulphur fractions,total nitrogen and organic carbon in camp and non-camp soils of grazed pastures supplied with long-term superphosphate

Summary Topsoils (0–75 mm) from four different soil types were collected from stock camp and non-camp (main grazing area) areas of grazed pastures in New Zealand, which had been fertilised annually with superphosphate for more than 15 years, in order to assess the effects of grazing animals on the status and distribution of soil S fractions and organic matter. These soils were analysed for organic C, total N, total S, C-bonded S, hydriodic acid-reducible S, 0.01 M CaCl₂, and 0.04 M Ca(H₂PO₄)₂-extractable S fractions, and soil pH. Soil inorganic and organic S fractions extracted by NaHCO₃ and NaOH extractants were also determined. The results obtained showed that camp soils contain higher soil pH, organic C, total N, total S, organic (C-bonded S and hydriodic acid-reducible S) and inorganic S fractions, NaHCO₃-and NaOH-extractable soil S fractions but a lower anion retention capacity than non-camp soils, attributed to a higher return of plant litter and animal excreta to camp soils. In both soils, total S, organic S, C-bonded S, and hydriodic acid-reducible S were significantly correlated with organic C (r0.90***, ***P0.001) and total N (r0.95***), suggesting that C, N, and S are integral components of soil organic matter. However, C: N : S ratios tended to be lower in camp (60: 5.6: 1–103: 7.2: 1) than in non-camp soils (60:6.1:1–117:8.3:1). Most (>95%) of the total soil S in camp and non-camp soils is present as organic S, while the remainder is readily soluble and adsorbed S (i.e. Ca(H₂PO₄)₂-extractable S). C-bonded S and hydriodic acid-reducible S constituted 55%–74% and 26%–45% of total S, respectively, reflecting a regular return of plant litter and animal excreta to the grazed pastures. NaHCO₃, and especially NaOH, extracted significantly higher amounts of total soil S (13%–22% and 49%–75%, respectively) than Ca(H₂PO₄)₂ or CaCl₂ (<5%). In addition, NaHCO₃ and NaOH-extractable soil S fractions were significantly rorrelated with soil organic S (r0.94***), C-bonded S (r0.90***) and hydriodic acid-reducible soil S (r0.93***). Differences between soils in either camp or non-camp areas were related to their sulphate retention capacities, as soils with high sulphate retention capacities (>45%) contain higher levels of C-bonded and hydriodic acid-reducible S fractions than those of low sulphate retention soils (<10%). Long-term annual superphosphate applications significantly increased the accumulation of soil organic and inorganic S fractions, and organic C and total N in the topsoil, although this accumulation did not occur when the superphosphate application rates were increased from 188 to 376 kg ha^-1 year^-1.     

Transformation of 15N-labelled leguminous plant material in three contrasting soils

Summary Two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) were incubated with ¹⁵N-labelled soybean tops for up to 20 weeks at 30°C. Mineralization of soybean ¹⁵N was slightly more rapid in the Pakistani soils, and after 20 weeks of incubation, 50%, 53%, and 56% of the applied ¹⁵N was accounted for as (NH₄ ⁺+NO₃ ^–)-N in Drummer, Hafizabad, and Khurrarianwala soils, respectively. Potentially mineralizable N (determined by anaerobic incubation) varied between 1.5% and 10% of the applied ¹⁵N in the three soils at different stages of incubation; somewhat higher percentages were mineralizable in the Pakistani soils than in the Drummer soil. From 3.7% to 9% of the applied ¹⁵N was accounted for in the microbial biomass. From 10% to 32% of the applied N was recovered in the humic acid and fulvic acid fractions of the organic matter by sequential extraction with Na₄P₂O₇ and NaOH; from 12% to 49% was recovered in the humin fraction. Of the three soils, Drummer soil contained more ¹⁵N as humic and fulvic acids. In all cases, the ¹⁵N was approximately equally distributed between the humic and fulvic acid fractions. A significant percentage of the humin ¹⁵N (52%–78%, equivalent to 8%–34% of the applied ¹⁵N) occurred in non-hydrolyzable (6 N HCl) forms. Of the hydrolyzable ¹⁵N, 42%–51% was accounted for as amino acid-N followed in order by NH₃ (17%–30%), hydrolyzable unknown forms (20%–22%), and amino sugars (6%–2%). The recovery of applied ¹⁵N for the different incubation stages was 87±22%. Recovery was lowest with the Khurrarianwala soil, presumably because of NH₃ volatilization losses caused by the high pH of this soil.     

Nitrogen mineralization in relation to site history and soil properties for a range of Australian forest soils

Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha^-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha^-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.     

Estimation of sulfur mineralization and relationships with nitrogen and carbon in soils

A 25-week laboratory study was carried out to determine sulfur, carbon, and nitrogen mineralization rates in soil samples obtained from representative soils in France. Their relationship with some of the soil properties was investigated to find a predictor of mineralized S in soils. At 20°C and 80% water-holding capacity, the S mineralization rate ranged from 0.02 to 0.16 mg kg⁻¹ day⁻¹. It was significantly positively related to soil organic C and N and to C and N mineralization rates. It was weakly related to total soil S. The results suggest that the S mineralization is predominantly driven by heterotrophic microbial activity. A predictive equation for S mineralization based on soil C content, soil pH, and clay content is proposed.     

Effect of bamboo harvest on dynamics of nutrient pools,N mineralization,and microbial biomass in soil

The effect of harvesting bamboo savanna on the dynamics of soil nutrient pools, N mineralization, and microbial biomass was examined. In the unharvested bamboo site NO _inf3 ^sup- -N in soil ranged from 0.37 to 3.11 mg kg^-1 soil and in the harvested site from 0.43 to 3.67 mg kg^-1. NaHCO₃-extractable inorganic P ranged from 0.55 to 3.58 mg kg^-1 in the unharvested site and from 1.01 to 4.22 mg kg^-1 in the harvested site. Over two annual cycles, the N mineralization range in the unharvested and harvested sites was 0–19.28 and 0–24.0 mg kg^-1 soil month^-1, respectively. The microbial C, N, and P ranges were 278–587, 28–64, and 12–26 mg kg^-1 soil, respectively, with the harvested site exhibiting higher values. Bamboo harvesting depleted soil organic C by 13% and total N by 20%. Harvesting increased N mineralization, resulting in 10 kg ha^-1 additional mineral N in the first 1st year and 5 kg ha^-1 in the 2nd year following the harvest. Microbial biomass C, N and P increased respectively by 10, 18, and 5% as a result of bamboo harvesting.     

Decomposition of different organic materials in soils

Laboratory experiments were conducted to evaluate organic C mineralization of various organic materials added to soils. A soil sample was mixed with organic material to approximate a field application of 9 g organic C kg^-1 soil (0.9% or 50 Mg ha^-1). The organic materials used were four crop residues [corn (Zea mays L.), soybean (Glycine max L. Merr.), sorghum (Sorghum vulgare Pers.), and alfalfa (Medicago sativa L.)], four animal manures [chicken (Gallus domesticus), pig (Sus scrofa), horse (Equus caballus), and cow (Bos taurus)] and four sewage sludges [Correctionville (Imhoff tank), Charles City (holding tank), Davenport (secondary digester), and Keokuk (primary digester)]. The soil-organic material mixture was incubated under aerobic conditions at room temperature (20±2°C) for 30 days. The CO₂ evolved was collected in standard KOH solution by continuously passing CO₂-free air over the soil. Results showed that, in general, the amounts of CO₂-C released mereased rapidly initially, but the pattern differed among the organic materials used. More than 50% of the total CO₂ produced in 30 days of incubation was evolved in the first 6 days. Expressed as percentages of organic C added, the amounts of CO₂ evolved ranged from 27% with corn to 58% with alfalfa. The corresponding percentages for animal manures ranged from 21 to 62% with horse and pig manures, respectively, and for sewage sludges they ranged from 10 to 39% for Charles City and Keokuk sludges. All CO₂ evolution data conformed well to a first-order kinetic model. Potentially, readily mineralizable organic C values and first-order rate constants (k) of the organic matter-treated soils ranged from 1.422 g C kg^-1 soil with ak value of 0.0784 day^-1 to 6.253 g C kg^-1 soil with ak value of 0.0300 day^-1. The half-lives of the C remaining in soils ranged from 39 to 54 days for plant materials. The corresponding half-lives for the C remaining from animal manures and sewage sludges ranged from 37 to 169 days and from 39 to 330 days, respectively.     

Lethal and sublethal effects of imidacloprid on two earthworm species (Aporrectodea nocturna and Allolobophora icterica)

The chemical imidacloprid is the major component of many widely used insecticides and is relatively persistent in soils. A set of experiments was carried out to estimate the lethal (mortality) and sublethal (weight loss) effects of one of these insecticides, Confidor, on two earthworm species commonly found in agricultural soils. A preliminary experiment in the absence of earthworms showed that imidacloprid was not rapidly degraded, with a decrease of less than 10% after 2 weeks, and that it was distributed in a reasonably homogeneous manner throughout the soil (less than 10% of variation between samples). The LC₅₀ of imidacloprid for the anecic species Aporrectodea nocturna and the endogeic species Allolobophora icterica was between 2 and 4 mg kg^–1 dry soil. This result is consistent with previous findings obtained with other earthworm species and natural soils. When sublethal effects were examined, significant decreases in weight were observed at concentrations of 0.5 and 1 mg kg^–1 dry soil for the two earthworm species whereas no effect was observed at a concentration of 0.1 mg kg^–1 dry soil (NOEC value). These concentrations are close to 0.33 mg kg^–1 which is the Predictive Environmental Concentration. Weight loss appears to be a valuable endpoint that can be used with worms freshly collected in the field as long as variability in the response of a control is taken into account.     

Bioavailability of stabilized oily waste organics in ultrasonified soil aggregates

Stabilization of oily waste organics (OWO) in soils of land treatment farms (LTF) can limit the availability of the OWO for biodegradation. The effect of physical dispersion on bioavailability and biodegradation of OWO in a soil from a LTF was investigated. Soil samples from the LTF were ultrasonically dispersed at increasing energy levels (EL), ranging from 0 to 30 kJ kg^?1 at 5 kJ kg^?1 increments, and incubated in glass jars for 12 weeks. The headspace CO₂ concentration in the jars was monitored weekly by gas chromatography and expressed as C mineralization rates (CMR). The CMR and the cumulative C mineralized (CCM) increased with increasing EL. The CCM from the oily waste treated soil dispersed at 30 kJ kg^?1 was 710% higher compared to that from the untreated soil dispersed at the same EL. Compared to the treated soil dispersed at zero EL, 48% more C mineralized from that dispersed at 30 kJ kg^?1. The size of the potentially mineralizable C (C₀) increased with increasing EL. The C₀ for the treated soil dispersed at 30 kJ kg^?1 was 66% larger compared to that at zero EL. The C₀'s for the treated soil dispersed at different EL were similar, suggesting the chemistry of the C₀ exposed by dispersion were similar. The results show that substantial amounts of potentially biodegradable OWO were physically protected in soil aggregates.