Short-term nitrogen transformation rates in riparian wetland soil determined with nitrogen-15

N transformation rates in soil from a riparian wetland that receives runoff from adjacent pastoral land were investigated in a short-term (250 min), anaerobic laboratory incubation (20°C). A joint ¹⁵N tracing-isotope dilution technique was employed that used paired incubations of labelled (99 atom % ¹⁵N) NO₃^–-unlabelled NH₄⁺ and unlabelled NO₃^–-labelled (99 atom % ¹⁵N) NH₄⁺ at three N input levels (0.4, 4 and 24 g N g^–1 soil). At each N input level NO₃^– and NH₄⁺ were added in equal proportions (0.2, 2 and 12 g N g^–1 soil). Soil and gas samples were analysed after 10, 70 and 250 min, and the fate of ¹⁵N and N transformation rates were determined for each time period; 0–10 min (phase 1), 10–70 min (phase 2) and 70–250 min (phase 3). N transformation rates for all processes except gross NH₄⁺ mineralisation were very high during phase 1. Processes favoured by aerobic conditions, NO₃^– immobilisation (0–17% NO₃^– removal, 0–8.2 g N g^–1 soil h^–1), autotrophic nitrification (~2% NH₄⁺ removal, 0.58–0.88 g N g^–1 soil h^–1) and heterotrophic nitrification (11–35 g N g^–1 soil h^–1) increased with increased N input while the anaerobic dissimilatory NO₃^– reduction to NH₄⁺ process (1–6% NO₃^– removal, 0.48–0.62 g N g^–1 soil h^–1) decreased, presumably due to the oxidising effect of higher NO₃^– inputs. Denitrification (8–78% NO₃^– removal, 3.8–9.6 g N g^–1 soil h^–1) exhibited no clear trend related to N input levels. NH₄⁺ immobilisation (39–72% NH₄⁺ removal, 15–19 g N g^–1 soil h^–1) was higher than NO₃^– immobilisation. Gross NH₄⁺ mineralisation (0.27–0.80 g N g^–1 soil h^–1) was the only process not detected in phase 1 and one of few processes measurable in phases 2 or 3.     

Effects of nitrification inhibitors on denitrification of nitrate in soil

Summary The influence of 28 nitrification inhibitors on denitrification of nitrate in soil was studied by determining the effects of different amounts of each inhibitor on the amounts of nitrate lost and the amounts of nitrite, N₂O and N₂ produced when soil samples were incubated anaerobically after treatment with nitrate or with nitrate and mannitol. The inhibitors used included nitrapyrin (N-Serve), etridiazole (Dwell), potassium azide, 2-amino-4-chloro-6-methylpyrimidine (AM), sulfathiazole (ST), 4-amino-1,2,4-triazole(ATC),2,4-diamino-6-trichloromethyl-s-triazine (CL-1580), potassium ethylxanthate, guanylthiourea (ASU), 4-nitrobenzotrichloride, 4-mesylbenzotrichloride, sodium thiocarbonate (STC), phenylmercuric acetate (PMA), and dicyandiamide (DCD).Only one of the nitrification inhibitors studied (potassium azide) retarded denitrification when applied at the rate of 10 g g^–1 soil, and only two (potassium azide and 2,4-diamino-6-trichloromethyl-s-triazine) inhibited denitrification when applied at the rate of 50 g g^–1 soil. The other inhibitors either had no appreciable effect on denitrification, or enhanced denitrification, when applied at the rate of 10 or 50 g g^–1 soil, enhancement being most marked with 3-mercapto-1,2,4-triazole. Seven of the inhibitors (potassium azide, sulfathiazole, potassium ethylxanthate, sodium isopropylxanthate, 4-nitrobenzotrichloride, sodium thiocarbonate, and phenylmercuric acetate) retarded denitrification when applied at the rate of 50 g g^–1 soil to soil that had been amended with mannitol to promote microbial activity.Reports that nitrapyrin (N-Serve) and etridiazole (Dwell) inhibit denitrification when applied at rates as low as 0.5 g g^–1 soil could not be confirmed. No inhibition of denitrification was observed when these compounds were applied at the rate of 10 g g^–1 soil, and enhancement of denitrification was observed when they were applied at the rate of 50 or 100 g g^–1 soil.     

Disturbance effects on soil solution chemistry due to heating cable installation

Installation of heating cables for warming soil was used to evaluate the effect of disturbance on soil solution chemistry within a northern hardwood forest (Adirondack Mountains, New York). Differences in response among treatments suggested the importance of both the depth and timing of cable installation. There were increases (p>0.05) in many solutes within pilot study plots in which surrogate cable was installed at 15 cm depth. Most notably, mean nitrate concentrations for the 1st year following disturbance were 744 eq l^-1 at 15 cm depth compared to 7 eq l^-1 for the non-disturbed control. A comparison of pilot plots with 5 cm cable depth and an unheated soil-warming control plot with the same cable disturbance showed that the seasonality of soil disturbance may have a key role in response to disturbance. The soil solution response was diminished if installation occurred during the spring, a period of rapid uptake of nitrogen by vegetation. Mean nitrate concentrations were 176 eq l^-1 for 5-cm pilot plots (installed in fall 1991) versus 6 eq l^-1 for 5-cm, unheated soil-warming control plots (installed in spring 1992). Disturbance effects were attenuated over time and not generally apparent 1 year after installation.     

Identification of ergosterol in vesicular-arbuscular mycorrhizae

Summary Ergosta-5,7,22-tri-3-enol (ergosterol) was identified by gas chromatography-mass spectrometry in roots of berseem (Trifolium alexandrinum L., cv. Landsorte) and sweet corn (Zea mays L., cv. Honeycomb-F1) infected with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices. The fungal-derived compound ergosterol was determined quantitatively in root extracts using reverse-phase high performance liquid chromatography. The concentrations of ergosterol in VAM-infected roots reached 72 g^-1 dry material in berseem and 52 g^-1 in sweet corn after 80 days of growth, whereas concentrations in non-infected roots remained below 8 g^-1 dry weight. Additionally, phytosterols such as -sitosterol, campesterol, and stigmasterol were detected in both infected and non-infected roots. Ergosterol, as a characteristic fungal substance, is proposed as an indicator of fungal biomass in the early stages of VAM infection.     

Seasonal changes and the effects of fertiliser on some chemical,biochemical and microbiological characteristics of high-producing pastoral soil

Summary A 2-year study (1983–1984 to 1984–1985) was conducted to estimate temporal and seasonal changes and the effects of fertiliser on some soil chemical, biochemical and microbiological characteristics. The soil used was a Typic Vitrandept under grazed pasture. Soil samples were taken regularly to a depth of 75 mm from paired unfertilised and fertilised (500 kg ha^– 30% potassic superphosphate) plots. Except for organic C, fertiliser had little or no effect on the characteristics measured. Organic C averaged about 9.2% in unfertilised soil and was about 0.3% higher in the fertilised soil. The size of the microbial biomass fluctuated widely in the 1st year (3000 g C g^–1 in February to 1300 g C g^–1 in September) but there was less variation in the 2nd year (range 1900 g C g^–1 to 2500 g C g^–1 soil). CO₂ production values (10- to 20-day estimates averaged 600 g of CO₂-C g^–1 soil) were generally higher in spring compared to the rest of the year. Water extractable C increased over winter and declined through spring in both years (range 50 g C g^–1 soil to 150 g C g^–1 soil). Mineral-N flush values were higher in summer (300 g N g^–1 soil) and lower in winter months (200 g N g^–1 soil). The pattern of variation of microbial N values was one of gradual accumulation followed by rapid decline. This rapid decline in values occurred in spring and autumn (range 130–220 g N g^–1 soil). N mineralisation and bicarbonate-extractable N showed no clear trend; these values ranged from 100–200 and 122–190 g N g^–1 soil, respectively. There was a significant correlation (0.1%) between N mineralisation and bicarbonate-extractable N in the late summer-autumn-early winter period (February–August) in both years but not in spring. These results and their relationships to climatic factors and rates of pasture production are discussed.     

Seasonal changes in microbial biomass and nutrient flush in forest soils

Microbial biomass and N, P, K, and Mg flushes were estimated in spring, summer, autumn, and winter samples of different forest soils. The microbial biomass showed significant seasonal fluctuations with an average distribution of 880±270 g C g^-1 soil in spring, 787±356 g C g^-1 soil in winter, 589±295 g C g^-1 soil in summer, and 560±318 g C g^-1 soil in autumn. The average annual concentrations of C, N, P, K, and Ca in the microbial biomass were 704, 106, 82, 69 and 10 g g^-1 soil, respectively. Microbial C represented between 0.5 and 2% of the organic soil C whereas the percentage of microbial N with respect to the total soil N was two-to threefold higher than that of C; the annual fluctuations in these percentages followed a similar trend to that of the microbial biomass. Microbial biomass was positively correlated with soil pH, moisture, organic C, and total N. The mean nutrient flush was 31, 15, 7, and 4 g g^-1 soil for N, K, P, and Mg, respectively, and except for K, the seasonal distribution was autumn spring winter summer. The average increase in available nutrient due to the mineralization of dead microbial cells was 240% for N, and 30, 26, and 14% for P, K, and Mg, respectively. There was a positive relationship between microbial biomass and the N, P, K, and Mg flushes. All the variables studied were significantly affected by the season, the type of soil, and the interaction between type of soil and season, but soil type often explained most of the variance.     

Effect of the insecticide methidathion on agricultural soil microflora

Summary We studied the effects of the organophosphorus insecticide methidathion, at concentrations of 10, 50, 100, 200 and 300 g g^-1 in an agricultural soil, on fungi, total bacterial populations, aerobic N₂-fixing bacteria, denitrifying bacteria, nitrifying bacteria (phases I and II), and nitrogenase activity (acetylene reduction assay). The presence of 10–300 g g^-1 of methidathion significantly increased fungal populations (colony-forming units). Denitrifying bacteria, aerobic N₂-fixing bacteria and N₂ fixation were significantly increased at concentrations of 50–300 g g^-1. The total number of bacteria increased significantly at concentrations of 100–300 g g^-1. Nitrifying bacteria decreased initially at concentrations of 300 g g^-1, but recovered rapidly to levels similar to those in the control soil without the insecticide.     

The spatial distribution of soil microbial biomass in a permanent row crop

The effects of soil texture (silt loam or sandy loam) and cultivation practice (green manure) on the size and spatial distribution of the microbial biomass and its metabolic quotient were investigated in soils planted with a permanent row crop of hops (Humulus lupulus). The soil both between and in the plant rows was sampled at three different depths (0–10, 10–20, and 20–30 cm). The silt loam had a higher overall microbial biomass C concentration (260 g g^-1) than the sandy loam (185 g g^-1), whereas the sandy loam had a higher (3.1 g CO₂-C mg^-1 microbial Ch^-1) metabolic quotient than the silt loam (2.6 g CO₂-C mg^-1 microbial C h^-1), on average over depth (0–30 cm) and over all treatments. There was a sharp decrease in the microbial biomass with increasing depth for all plots. However, this was more pronounced in the silt loam than in the sandy loam. There was no distinct influence of sampling depth on the metabolic quotient. The microbial biomass was considerably higher in the rows than between the rows, especially in the silt loam plots. There was no significant difference between plots without green manure and plots with green manure for either the microbial biomass or the metabolic quotient.     

Effect of fractionation on location of enzyme activities in soil structural units

Summary Two different soils were fractionated with three granulometric procedures. Enzyme activity measurements were carried out in four fractions of real (undisturbed), stable (water-dispersed) and structural (water-dispersed and physically disrupted) soil units.A predominance of catalase (67.3%), dehydrogenase (57.6%) and urease (65.3%) activities was observed in soil structural microunits with diameters of less than 50 m. Highest protease activity was found in fractions with a diameter of less than 50 m (66.3%) and in fractions with a diameter ranging from 200 to 2000 m (18.5%) which contain organic matter of a higher and lower grade of humification, respectively.All fractionation procedures result in a loss of total enzyme activities, which was more clear in the physically disrupted soil than in the sieved soil (e.g. 38.4% vs. 5.9% loss of urease activity).     

Ergosterol and microbial biomass relationship in soil

Ergosterol and microbial biomass C were measured in 26 arable, 16 grassland and 30 forest soils. The ergosterol content ranged from 0.75 to 12.94 g g^-1 soil. The geometric mean ergosterol content of grassland and forest soils was around 5.5 g g^-1, that of the arable soils 2.14 g g^-1. The ergosterol was significantly correlated with biomass C in the entire group of soils, but not in the subgroups of grassland and forest soils. The geometric mean of the ergosterol: microbial biomass C ratio was 6.0 mg g^-1, increasing in the order grassland (5.1), arable land (5.4) and woodland (7.2). The ergosterol:microbial biomass C ratio had a strong negative relationship with the decreasing cation exchange capacity and soil pH, indicating that the fungal part of the total microbial biomass in soils increased when the buffer capacity decreased. The average ergosterol concentration calculated from literature data was 5.1 mg g^-1 fungal dry weight. Assuming that fungi contain 46% C, the conversion factor from micrograms ergosterol to micrograms fungal biomass C is 90. For soil samples, neither saponification of the extract nor the more effective direct saponification during extraction seems to be really necessary.     

Effects of the herbicide Topogard on soil respiration, nitrification, and denitrification in potato-cultivated soils differing in pH

We investigated the effects of Topogard 50 WP (3 kg ha^–1) on soil respiration, mineral N content, and number of denitrifying and total bacteria in four coarse-textured volcanic soils for 91 days. Topogard application decreased CO₂ evolution in acid soils (Tepedibi and Karaçakl) whereas soil respiration was initially increased in neutral and alkaline soils (Kaba and Balar). The herbicide application significantly stimulated ammonification in Kaba and Balar soils, while Tepedibi and Karaçakl soils showed significantly lower NH₄⁺-N contents than the control. The treatment inhibited the activity of nitrifying microorganisms and, thus it decreased the NO₃^–-N content in Tepedibi, Karaçakl, and Kaba soils, whereas the NO₃^–-N content was increased in Balar soil. The NO₂^–-N content of soils was not affected by the treatment. The activity of denitrifying bacteria was stimulated by the addition of herbicide in all soils, whereas the total number of bacteria was not influenced. It may be concluded that the effects of Topogard on the microbiological characteristics of coarse-textured soils are likely to be dependent on soil pH.     

Carbon and nitrogen relationships in the microbial biomass of soils in beech (Fagus sylvatica L.) forests

Soils from 38 German forest sites, dominated by beech trees (Fagus sylvatica L.) were sampled to a depth of about 10 cm after careful removal of overlying organic layers. Microbial biomass N and C were measured by fumigation-extraction. The pH of the soils varied between 3.5 and 8.3, covering a wide range of cation exchange capacity, organic C, total N, and soil C:N values. Maximum biomass C and biomass N contents were 2116 g C m^-2 and 347 g N m^-2, while minimum contents were 317 and 30 g m^-2, respectively. Microbial biomass N and C were closely correlated. Large variations in microbial biomass C:N ratios were observed (between 5.4 and 17.3, mean 7.7), indicating that no simple relationship exists between these two parameters. The frequency distribution of the parameters for C and N availability to the microflora divided the soils into two subgroups (with the exception of one soil): (1) microbial: organic C>12 mg g^-1, microbial:total N>28 mg g^-1 (n=23), a group with high C and N availability, and (2) microbial:organic C12 mg g^-1, microbial:total N28 mg g^-1 (n=14), a group with low C and N availability. With the exception of a periodically waterlogged soil, the pH of all soils belonging to subgroup 2 was below 5.0 and the soil C:N ratios were comparatively high. Within these two subgroups no significant correlation between the microbial C:N ratio and soil pH or any other parameter measured was found. The data suggest that above a certain threshold (pH 5.0) microbial C:N values vary within a very small range over a wide range of pH values. Below this threshold, in contrast, the range of microbial C:N values becomes very large.     

Effects of nitrification inhibitors on germination of various seeds in soil

Summary The effects of 19 nitrificiation inhibitors on germination of seeds in soil were investigated. The nitrification inhibitors tested were sodium azide, potassium azide, potassium ethyl xanthate, nitrapyrin (N-Serve), etridiazole (Dwell), 3-mercapto-1,2,4-triazole (MT), 2-amino-4-chloro-6-methylpyrimidine (AM), 2,4-diamino-6-trichloromethyl-s-triazine, 2-mercaptobenzothiazole (MBT), 4-amino-1,2,4-triazole (ATC), sodium thiocarbonate (STC), guanylthiourea (ASU), thiourea (TU), dicyandiamide (DCD), sulfathiazole (STC), phenylacetylene, 2-ethynyl-pyridine, 3-methylpyrazole-l-carboxamide (MPC), and ammonium thiosulfate (ATS). Germination tests were performed with seeds of alfalfa (Medicago sativa L.), wheat (Triticum aestivum L.), rye (Secale cereale L.), barley (Hordeum vulgare L.), sorghum [Sorghum bicolor (L.) Moench], oats (Avena sativa L.), and corn (Zea mays). Only 2 of the 19 nitrification inhibitors studied (potassium azide and sodium azide) reduced germination of the seeds tested when applied at the rate of 12.5 g g^–1. The other inhibitors studied had no effect on the germination of wheat, alfalfa, barley, corn, oat, rye, or sorghum seeds when they were applied at the rate of 125 g g^–1 soil, and most of them had no effect on seed germination when applied at the rate of 625 g g^–1 soil.     

Application of the selective inhibition method to determine bacterial: fungal ratios in three beechwood soils rich in carbon — Optimization of inhibitor concentrations

Bacterial and fungal contributions to microbial respiration in three beechwood soils rich in C (two basalt soils and one limestone soil) were investigated by using streptomycin and cycloheximide to inhibit substrate-induced respiration after glucose (8000 g g^-1), N, and P addition to soil samples. The inhibitors were added as solutions (2000, 8000, and 16000 g g^-1) and the reduction in substrate-induced respiration after separate and combined inhibitor addition was measured in an automated electrolytic microrespirometer. Bacterial and fungal contributions to microbial respiration were calculated using the interval 6–10 h after inhibitor application. The microbial biomas was smaller in the two basalt soils (Oberhang and Mittelhang) than in the limestone soil (Unterhang). In the presence of both inhibitors, microbial respiration was inhibited by a maximum of 45, 45, and 25% in the two basalt soils and the limestone soil, respectively. Inhibition of microbial respiration was at a maximum at streptomycin and cycloheximide concentrations of 16000 g g^-1. The inhibitor additivity ratio approached 1.0 even at high inhibitor concentrations, indicating high inhibitor selectivity. Calculated prokaryote: eukaryote ratios indicated lower bacterial contributions to the microbial biomass in the Mettelhang (0.74) and Unterhang (0.73) than in the Oberhang (0.88) soil.     

Influence of oxygen on denitrification and aerobic respiration in soil

Summary The influence of the partial pressure of oxygen on denitrification and aerobic respiration was investigated at defined P02 values in a mull rendzina soil. The highest denitrification and respiration rates obtained in remoistened, glucose- and nitrate-amended soil were 43 1 N₂0 h^–1g^–1 soil and 130 1 O₂ h^–1g^–1 soil, respectively. At -55 kPa matric water potential, corresponding to 40% water saturation, N₂0 was produced only below P0₂ 40 hPa. The K _m, for O₂ was 3.0 x 10^–6 M. Formation of N₂O and consumption of O₂ occurred simultaneously with half maximum rates at P0₂ 6.7–13.3 hPa. Nitrite accumulated in soil below 40 hPa and increased with decreasing pO₂. The upper threshold for N₂0 formation in amended soil was P0₂ 33–40 hPa (39-47 M O₂).     

Distribution of two C cycle enzymes in soil aggregates of a prairie chronosequence

Knowledge of the cycling and compartmentalization of soil C that influence C storage may lead to the development of strategies to increase soil C storage potentials. The objective of this study was to use soil hydrolases and soil aggregate fractionation to explore the relationship between C cycling activity and soil aggregate structure. The prairie chronosequence soils were native prairie (NP) and agricultural (AG) and tallgrass prairies restored from agriculture in 1979 (RP-79) and 1993 (RP-93). Assays for -glucosidase (E.C. 3.2.1.21) and N-acetyl--glucosaminidase (NAGase, EC 3.2.1.30) activities were conducted on four aggregate size fractions (>2 mm, 1–2 mm, 250 m–1 mm, and 2–250 m) from each soil. There were significantly greater amounts of >2-mm aggregates in the RP-79 and RP-93 soils compared to the NP and AG soils due to rapid C accumulation from native plant establishment. Activities for both enzymes (g PNP g^–1 soil h^–1) were greatest in the microaggregate (2–250 m) compared to the macroaggregate (>2 mm) fraction; however, microaggregates are a small proportion of each soil (<12%) compared to the macroaggregates (75%). The RP soils have a hierarchical aggregate system with most of the enzyme activity in the largest aggregate fractions. The NP and AG soils show no hierarchical structure based on aggregate C accretion and significant C enzyme activity in smaller aggregates. The distribution of enzyme activity may play a role in the storage of C whereby the aggrading restored soils may be more susceptible to C loss during turnover of macroaggregates compared to the AG and NP soils with less macroaggregates.     

Potential for organic sulfur accumulation in a variety of forest soils at saturating sulfate concentrations

Summary Increasing the sulfate concentration and concomitant increases in the organic S concentration failed to exert any effect on organic S mobilization in samples collected from all depths within the mineral soil profile, from 15 sites differing in soil type, vegetation, and geographic location. Mobilization capacities at saturating concentrations of sulfate for organic S formation generally tended to increase with increasing depth. The potentials for the accumulation of organic S with various sulfate inputs exhibited saturation kinetics similar to those observed for organic S formation; values for the former parameter ranged from 3×10^-3 to 12.6 mol S g^–1 dry weight 24 h^-1 for the uppermost (A, E) soil horizons, 3 nmol to 10 mol S g^-1 dry weight 24 h^–1 for intermediate (primarily AB) soil horizons, and from 3 nmol to 13.4 mol S g^-1 dry weight 24 h^–1 for the lowermost (B, C) soil horizons. Irrespective of depth, the Fullerton, Tarklin, and Loblolly sites in Tennessee and the Florida site showed the least net accumulation of organic S at saturation (<0.2 mol S g^-1 dry weight 24 h^–1 for all horizons examined), while the Duke Forest (North Carolina), Douglas Fir (Washington), Whiteface (New York) and the Howland (Maine) sites had the highest potential net accumulation of organic S at saturation (>1.0 mol S g^-1 dry weight 24 h^-1 for most horizons examined).     

Uptake and toxicity of copper and zinc for the African earthworm,Eudrilus eugeniae (Oligochaeta)

Growth rate change in earthworms is considered to be a suitable endpoint when determining sublethal effects. In this study we evaluated growth and maturation in the vermicomposting earthworm speciesEudrilus eugeniae as marker of sublethal toxicity of copper and zinc. We also compared routes of uptake. Apart from exposing worms experimentally for 73 days to contaminated food, a series of contact filter paper tests was also performed to determine LD₅₀ for copper and zinc. Both copper and zinc at sublethal concentrations affected growth and maturation in worms exposed to contaminated food. These worms had a copper content of 34.5 g g^–1 after 73 days and a zinc content of 184.9 g g^–1, showing a differential uptake. Copper was more toxic than zinc. Also in the contact test worms did take up more zinc than copper and the LD₅₀ (48 h) for copper was 0.011 mg cm^–2 and for zinc 0.066 mg cm^–2, which translated to body burdens of 6 g g^–1 for copper and 131 g g^–1 for zinc. Indications were that a regulatory mechanism existed for both metals. Both metals were taken up through the body wall at a relatively fast rate. This study indicated that the skin was the major route of metal uptake. This study also showed a poor relation between the two types of tests for purposes of evaluating lethality of zinc and copper.     

Influence of soil properties on microbial C,N, and P in dry tropical ecosystems

Summary Relationships between soil physicochemical characteristics and soil microbial C, N, and P in Indian dry tropical ecosystems are discussed. The major ecosystem studies were on forest, savanna, cropped fields, and mine spoils. The highest microbial C, N, and P levels were recorded from the mixed forest and the lowest levels in 5-year-old mine spoil. Across the sites, microbial C ranged from 226 to 643 g g^-1, microbial N from 19 to 71 g g^-1, and microbial P from 9 to 28 g g^-1 soil. The proportion of soil organic C contained in the microbial biomass ranged from 2.2 to 5.0%. The microbial C: N ratio in these soils ranged from 7.4. to 13.1 and the microbial C: P ratio from 16.6 to 30.6. The concentrations of microbial C, N, and P were correlated with several soil properties and among themselves. The soil properties, in various linear combinations, explained 90–99% of the variability in the microbial nutrients. Grazing of the savanna had some effect on the level of microbial biomass, and as the mine spoil aged, the level of microbial C, N, and P also increased.     

Effects of liming and tillage systems on microbial biomass and glycosidases in soils

This study was undertaken to investigate the long-term influence of lime application and tillage systems (no-till, ridge-till and chisel plow) on soil microbial biomass C (C_mic) and N (N_mic) and the activities of glycosidases (- and -glucosidases, - and -galactosidases and -glucosaminidase) at their optimal pH values in soils at four agroecosystem sites [Southeast Research Center (SERC), Southwest Research Center (SWRC), Northwest Research Center (NWRC), and Northeast Research Center (NERC)] in Iowa, USA. Results showed that, in general, the C_mic and N_mic values were significantly (P <0.001) and positively correlated with soil pH. Each lime application and tillage system significantly (P <0.001) affected activities of the glycosidases. With the exception of -glucosidase activity, there was no lime×tillage interaction effect. Simple correlation coefficients between the enzyme activities and soil pH values ranged from 0.51 (P <0.05) for the activity of -glucosidase at the NWRC site (surface of the no-till) to 0.98 (P <0.001) at the SWRC site. To assess the sensitivity of the enzymes to changes in soil pH, the linear regression lines were expressed in activity/pH values. In general, their order of sensitivity to changes in soil pH was consistent across the study sites as follow: -glucosidase>-glucosaminidase>-galactosidase>-galactosidase>-glucosidase. Lime application did not significantly affect the specific activities (g p -nitrophenol released kg^–1 soil organic C h^–1) of the enzymes. Among the glycosidases studied, -glucosidase and -glucosaminidase were the most sensitive to soil management practices. Therefore, the activities of these enzymes may provide reliable long-term monitoring tools as early indicators of changes in soil properties induced by liming and tillage systems.