Microbial biomass phosphorus in soils of beech (Fagus sylvatica L.) forests

Thirty-eight soils from forest sites in central Germany dominated by beech trees (Fagus sylvatica L.) were sampled to a depth of about 10 cm after careful removal of the overlying organic layers. Microbial biomass P was estimated by the fumigation — extraction method, measuring the increase in NaHCO₃-extractable phosphate. The size of the microbial P pool varied between 17.7 and 174.3 g P g^-1 soil and was on average more than seven times larger than NaHCO₃-extractable phosphate. Microbial P was positively correlated with soil organic C and total P, reflecting the importance of soil organic matter as a P source. The mean microbial P concentration was 13.1% of total P, varying in most soils between 6 and 18. Microbial P and microbial C were significantly correlated with each other and had a mean ratio of 14.3. A wide (5.1–26.3) microbial C: P ratio indicates that there is no simple relatinship between these two parameters. The microbial C: P ratio showed strong and positive correlations with soil pH and cation exchange capacity.     

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.     

Establishment and effectiveness of added pigeonpea (Cajanus cajan) rhizobia in different soils of narrow abiotic variability

Summary Pot and laboratory experiments were conducted to study the establishment and effectiveness of a streptomycin-sulphate-resistant (1 mg/ml of medium) pigeonpea rhizobia strain (RM₇) in sterile sand and non-sterile soils. Strain RM₇ increased the drymatter yield of pigeonpea plants (Cajanus cajan) by 106% over control plants under sterile conditions. However, when the rhizobia strain was introduced into 14 different non-sterile soils with a narrow abiotic variability, the comparable beneficial effect was observed only in one soil inoculated with log 6.70 cells/pot. At this inoculum rate, the percentage increase in yield over control plants varied from –1 to 140 in different soils. Rhizobium (RM₇), applied at log 3.70 cells/pot (3 kg soil), showed less than 5% establishment in four soils. However, establishment varied from 8% to 72% at a higher level of inoculation (log 6.70 cells/pot). Displacement of native rhizobia and creation of new sites for nodulation by the introduced rhizobia were also affected by soil properties. The increase in shoot dry-matter yield compared with control plants was positively correlated with the percent establishment of RM₇ (r = 0.60^*) in these soils. Experiments showed that some biotic stresses led to poor survival, proliferation and establishment of the added alien in the soil. Therefore, any culture that is efficient in one soil may not produce similar results under all situations.     

Phosphatase activity and phosphorus fractions in Karri (Eucalyptus diversicolor F. Muell.) forest soils

Summary Karri forest soils contain negligible concentrations of labile-P, low concentrations of total P and more P in organic forms than inorganic. The ratio of organic P to inorganic P was lowest (1:2) in recently burnt surface soils and greatest (7:1) at depth in soil that had been undisturbed for long periods of time. Phosphomonoesterase and phosphodiesterase activities (to 10 cm depth, phosphomonoesterase 700–1300; phosphodiesterase 2000–2400 g nitrophenol released h^-1 g^-1 fresh weight) were comparable to those in other, organically rich forest soils. The optimum pH for phosphatase activities were within 1–2 units of soil pH (6) and little reduction in activity was observed over the pH range 4–8. Phosphatase activity was reduced by air-drying (up to 20-fold reduction) and was almost entirely absent in soils that were heat-affected as a result of logging/burning operations. Neither phosphomonoesterase nor phosphodiesterase were directly related to soil P fractions or total P. A reduction in P demand is postulated as the cause of reduced phosphatase activity and the increased concentration of organic P with increasing soil depth.     

Effects of iron-based amendments on soil structure: a lab experiment using soil micromorphology and image analysis of pores

Purpose

Recent trends in soil green and sustainable remediation require an increased attention on environmental effects. The physical consequences of remediation practices on soil structure are very rarely investigated.

Material and methods

A laboratory experiment was carried out by adding iron grit to a sand (S), a silt loam (L), and a clay (C) soil subjected to several wetting-drying cycles. The physical effects of the treatment on soil pore system were identified and quantified combining physical measurements on repacked samples with image analysis of pores on resin-impregnated soil blocks and micromorphological analysis on thin sections.

Results and discussion

A negligible reduction of total porosity (P) resulted in S, and a slight increase was observed in the L and C soils. However, an important impact on soil structure was identified in pore size range >10 μm for the L and C soils, with the formation of new pores related to the differential shrink-swell behavior between soil matrix and added iron grains. Different plasticity of these soils also played a role in planar pore formation.

Conclusions

Effects of the addition of iron grit on soil pore system are strongly dependent on soil physical properties. The performed experiment showed that iron-based amendments can improve soil structure in low-plastic shrink-swell soil increasing porosity in the range of transmission pores (50–500 μm). This study showed the high potential of soil micromorphology and pore image analysis in order to evaluate the environmental impact of soil remediation practices.     

Use of soil solarization to control root rots in gerberas (Gerbera jamesonii)

Summary An investigation was conducted during the summer months of 1986–1987 and 1987–1988 in Western Australia to evaluate the effect of soil solarization on the control of root rot of gerbera an also on the microbial and nutrient status of the soil. Infested soil cores were sampled from a site where root-rot was a severe problem and were removed to a non-infested site where they were subjected to soil solarization or fumigation. Soil solarization resulted in reduced root rot (root disease index 28.6%) in comparison to the untreated control (52.0%) 8 months after planting. Plants in the fumigated plots had 15.8% less disease than those in solarized plots. Solarization increased the total numbers of bacteria and actinomycetes, and the proportion of bacteria and fungi antogonistic to Fusarium oxysporum, F. solani and Rhizoctonia solani. The proportion of actinomycetes antagonistic to these fungi, however, did not differ between solarized and control soil treatments. There was a significant reduction in disease in plants grown in infested fumigated soil to which a 10% concentration of solarized soil had been added, suggesting the development of microbial suppression in solarized soil. Phytophthora cryptogea was eradicated to 30 cm by solarization as well as by fumigation. Solarization eliminated R. solani but not F. oxysporum to a soil depth of 10 cm. Solarization increased the levels of NO _n3 ^– -N and NH₄ ⁺-N in soil, but did not affect the concentrations of PO₄ ^3–, K⁺, Fe²⁺, organic C and pH. Yield (as number of flowers per plant) was increased by soil solarization and by fumigation.Increased yields and decreased disease severity in the solarized plots could have been caused by (1) a reduction in the infectivity of the infested soils, (2) an increase in the suppressiveness of the soil, and (3) an increased available of plant nutrients.     

Effects of phosphoroamides on ammonia volatilization and nitrite accumulation in soils treated with urea

Summary We compared the effects of N-(n-butyl) thiophosphoric triamide (NBPT), N-(diaminophosphinyl)-cyclohexylamine (DPCA), phenylphosphorodiamidate (PPD), and hydroquinone on transformations of urea N in soils. The ability of these urease inhibitors to retard urea hydrolysis, ammonia volatilization, and nitrite accumulation in soils treated with urea-decreased in the order NBPT > DPCA PPD > HQ. When five soils were incubated at 30°C for 14 days after treatment with urea (1 mg urea N g^–1 soil), on average, the gaseous loss of urea N as ammonia and the accumulation of urea N as nitrite were decreased from 52 to 5 % and from 11 to 1%, respectively, by addition of NBPT at the rate of 10 g g^–1 soil (0.47 parts of NBPT per 100 parts of urea). The data obtained support previous evidence that NBPT is more effective than PPD for reduction of the problems encountered in using urea as a fertilizer and deserves consideration as a fertilizer amendment for retarding hydrolysis of urea fertilizer in soil.     

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.     

Intrinsic antibiotic resistance and competition in fast- and slow-growing soybean rhizobia on a hybrid of Asian and US cultivars

Summary Six fast-growing soybean rhizobia (Rhizobium fredii) and thirteen slow-growing soybean rhizobia (Bradyrhizobium japonicum) were examined for resistance to 10 antibiotics. Axenic studies were carried out to determine the competitiveness of dual-strain inocula consisting of fast- and slow-growing rhizobia isolated from subtropical-tropical soils for nodule occupancy on a hybrid of Asian and US soybean cultivars. Nodule occupancy was determined by intrinsic resistance to erythromycin and neomycin. The results showed wide variability in resistance to 10 antibiotics for fast- and slow-growing rhizobia. The intrinsic antibiotic resistance of fast- and slow-growing rhizobia was extremely high against nalidixic acid (400 g ml^–1) and penicillin (200 g ml^–1). The competitive ability of inoculant strains for nodule occupancy varied for different combination sets and with the plant growing media. Our results show that fast-growing rhizobia nodulate a hybrid of Asian and US soybean cultivars. Fast-growing soybean rhizobia did not completely exclude nodulation by the slow-growing strains, which formed 0–79% nodules, depending on the strain used in the inoculum.     

Using the spermosphere model technique to describe the dominant nitrogen-fixing microflora associated with wetland rice in two Egyptian soils

Summary This study is an attempt to describe the dominant N₂-fixing microflora associated with the roots of wetland rice. Rice cultivar Giza 171 was grown in a phytotron on two alluvial Egyptian soils for 8 days, a stage when the nitrogenase activity of undisturbed plants reached a level of 245 × 10^–6 mol C₂H₄ h^–1 g^–1 dry weight of leaf. The roots and rhizosphere soils were then used for counting and isolating dominant diazotrophs. Counts and initial enrichment steps were carried out on a selective medium made of an axenic rice plantlet, the spermosphere model, incubated under 1 % acetylene. The counts were very high, exceeding 10⁸ bacteria g^–1 dry weight of rhizosphere soil. Enterobacteriaceae were dominant; most isolates were Enterobacter cloacae belonging to different biotypes in the two soils. Enterobacter agglomerans, Citrobacter freundii and Klebsiella planticola were also present as members of the dominant microflora. Azospirillum brasilense and Azospirillum lipoferum were present as well, but less abundant.     

Phosphatase activity in Nasutitermes ephratae termite nests

Summary The relationship between phosphatase activity and soil was studied in 14 mounds and adjacent control soils of plant debris-feeding termites from a Venezuelan savanna. The soils were assayed for acid phosphatase activity with p-nitrophenyl phosphate as substrate and for the effect of inorganic P (300 g P g^–1). The proportion of organic matter in the mounds was four times that found in topsoils, indicating strong selection by the termites for organic-rich soil fractions. A comparison of phosphatase activities found no difference between mounds and adjacent soils. It seems possible that the expected increase of enzyme activity in mounds, due to a higher C content, was counteracted by enzyme inhibition due to higher levels of available inorganic P in the mounds. Addition of inorganic P to soil and mound material reduced enzyme activities by 10%–45%, but after a 2-day incubation period differences between the treated soil and the control tended to disappear.     

Mineral-fixed ammonium in clay- and silt-size fractions of soils incubated with 15N-ammonium sulphate for five years

Summary Four soils with 6, 12, 23, and 47% of clay were incubated for 5 years with ¹⁵N-labeled (NH_{4 2}SO₄ and hemicellulose. The incubations took place at 20°C and 55% water-holding capacity. Samples of whole soils, and clay- (<2 m) and silt-(2–20 m) size fractions (isolated by ultrasonic dispersion and gravity sedimentation) were analysed for labeled and native mineral-fixed ammonium. Mineral-fixed ammonium in non-incubated soil samples accounted for 3.4%–8.3% of the total N and showed a close positive correlation with the soil clay content (r ² = 0.997). After 5 years of incubation, the content of mineral-fixed ammonium in the clay fraction was 255–430 g N g^–1, corresponding to 71%–82% of the mineral-fixed ammonium in whole soils. Values for silt were 72–166 g N g^–1 (14%–33% of whole soil content). In the soils with 6% and 12% clay, less than 1 % of the labeled clay N was present as mineral-fixed ammonium. In the soil with 23% clay, 3% of the labeled N in the clay was mineral-fixed ammonium. Labeled mineral-fixed ammonium was not detected in the silt fractions. For whole soils, and clay and silt fractions, the proportion of native N present as mineral-fixed ammonium varied between 3% and 6%. In contrast, the proportion of labeled N found as mineral-fixed ammonium in the soil with 4701o clay was 23%, 38% and 31% for clay, silt, and whole-soil samples, respectively. Corresponding values for native mineral-fixed ammonium were 12%, 16%, and 10%. Consequently, studies based on soil particle-size fractions and addressing the N turnover in clay-rich soils should consider the pool of mineral-fixed ammonium, especially when comparing results from different size fractions with those from fractions isolated from soils of a widely different textural composition.     

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.     

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).     

Evaluation of some phosphoroamides as soil urease inhibitors

Summary The effectiveness of six phosphoroamides for retardation of urea hydrolysis in soils was studied by determining the effects of 10 g g^–1 soil of each compound on the amounts of urea hydrolyzed when soils treated with urea were incubated at 10°, 20°, 30°, and 40°C for 3, 7, and 14 days. The phosphoroamides used wereN-(diaminophosphinyl)-cyclohexylamine,N-benzyl-N-methyl phosphoric triamide, diethyl phosphoric triamide, trichloroethyl phosphorodiamidate, dimethyl phosphoric triamide, andN-butyl phosphorothioic triamide [N-(n-butyl) thiophosphoric triamide]. The soils used were selected to obtain a range in properties, and the effects of the six phosphoroamides studied were compared with those of two compounds known to be among the most effective compounds thus far proposed for retardation of urea hydrolysis in soils (phenylphosphorodiamidate and hydroquinone). The data obtained showed that all six of the phosphoroamides evaluated compared favorably with hydroquinone as soil urease inhibitors and that two of them [N-butyl phosphorothioic triamide andN-(diaminophosphinyl)-cyclohexylamine] were superior to phenylphosphorodiamidate for retardation of urea hydrolysis in soils at 20°, 30°, or 40°C.     

The role of tephra covers on soil moisture conservation at Haleakala's crater (Maui, Hawai'i)

The influence of tephra covers on soil water was studied in Haleakala (Maui, Hawai'i) during two summers; eight sites with tephra layers and silverswords (Argyroxiphium sandwicense DC.) were sampled at 2415–2755 m. At each site, eight paired-sample sets were obtained in bare soils and under adjacent tephra, at three depths. Tephra were sharply separated from underlying soils and showed prominent vertical stratification. Tephra clast size-distribution was assessed by photosieving and on interstitial-gravel samples; stones included 45.6% cobbles, 29.4% pebbles, and 25% blocks.Moisture content increased with depth in both positions, but soils below tephra had more water at all depths than exposed areas. Surface soils beneath tephra contained 83% more water than bare ground. Soils at 5–10 cm had  106% greater moisture under rocks, but only  70% at 10–15 cm. Differences between plots were statistically significant ( p < 0.001) for surface soils, but less pronounced for subsoils. Soils above 2650 m had greater water content than at lower elevations, and moisture disparity between sample pairs increased with altitude.All soils were coarse, with  20% gravel and  94% sand; most fine material (≤ 0.063 mm) was silt, as clay content was negligible. Organic-matter percentage was low (1.65%). Bulk density and porosity were associated with moisture variation both in tephra-insulated and bare soils; 80% of field moisture was statistically (p < 0.001) accounted for by pore space. Air and soil temperatures were recorded at three sites during  one-week periods prior to moisture sampling. Tephra substantially decreased soil maxima and daily thermal amplitude in underlying soils, but did not noticeably affect nightly minima. Thin (5–6 cm) tephra layers were nearly as effective as thicker (9–15 cm) deposits in depressing soil maxima. Possible water-conservation mechanisms under tephra include: decreased evaporation due to ground shielding and lower maxima; reduced capillary flow; greater infiltration depth; nocturnal dew condensation; and fog interception by blocks.     

Effect of cultivation on microbial carbon and nitrogen in dry tropical forest soil

Summary Fifteen- and forty-year-old cropfields developed from a dry tropical forest were examined for soil organic C and total N and soil microbial C and N. The 15-year-old field had never been manured while the 40-year-old field had been fertilized with farmyard manure every year. The native forest soil was also examined. The results indicated that the native forest soil lost about 57% and 62% organic C and total N, respectively, in the 0–10 cm layer after 15 years of cultivation. The microbial C and N contents of the forest soil were greater than those of the cultivated soils. Application of farmyard manure increased the biomass-C and -N levels in the cultivated soil but the values were still markedly lower than in the forest soil. There was an appreciable seasonal variation in biomass C and N, the values being highest in summer and lowest in the rainy season. During an annual cycle, biomass-C contents varied from 180 to 727 g g^–1 and N from 20 to 80 g g^–1 dry soil, and both were linearly related. Microbial biomass C represented 1.6%–3.6% of total soil organic C and microbial biomass N represented 1.7% 1–4.4% of soil organic N.     

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.