Methane flux and regulatory variables in soils of three equal-aged Japanese cypress (Chamaecyparis obtusa) forests in central Japan

To compare factors that control methane flux in forest soils, we studied three equal-aged Japanese cypress (Chamaecyparis obtusa) forests in Chubu district, central Japan. The three sites are located at different altitudes: 630 m (SET), 1010 m (INB), and 1350 m (OSK). Methane was absorbed at every site. The highest uptake rate was observed in the middle-altitude soil (INB, 5.89 mg CH₄ m⁻² d⁻¹), which was the only site where methane uptake rate was correlated with air and soil surface temperatures. Methane flux in the field was not correlated with water content, inorganic N content, or water-soluble organic carbon. C/N ratio was correlated with methane flux (r=0.64,p<0.001). The results suggest that some organic inhibitors might be produced through decomposition of organic matter. There was a negative correlation between methane uptake rate and water-soluble Al (r=−0.63,p<0.001). Inhibition of methane consumption by 1 and 5 mM Al solutions was observed in laboratory incubation. This result suggests that water-soluble Al may be a factor controlling methane uptake. Multiple regression with a backward-elimination procedure identified three variables that were significantly associated with methane flux in the field (p<0.05): air temperature, C/N ratio, and the concentration of water-soluble Al.     

Sulphur immobilization and arylsulphatase activity in two calcareous arable and fallow soils as affected by glucose additions

In this study we examined the effects of glucose-C on the activities of fungi and bacteria determined by the method of substrate-induced respiration (SIR) in combination with the selective inhibition technique, the immobilized-S and the arylsulphatase (ARS) activity in two calcareous arable and fallow soils. The amounts of glucose-C were added at six doses: 0, 125, 250, 500, 750 and 1000 mg kg^− 1 soil to the soils and then incubated for one week with a Na₂³⁵SO₄ solution (518.9 kBq kg^− 1 dry soil and 20 mg S kg^− 1 dry soil) prior to analysis. At the highest dose of 1000 mg kg^− 1 soil, fungal activity increased by 59.1% (of the dose 0) versus 45.5% for bacterial activity in the arable soil, while in the fallow soil the increases were more marked and corresponded to 69.9% and 71.1%, respectively. Largest increase in immobilized-S was observed in the arable soil (300.7%) compared with the fallow soil (153.1%). In contrast, the ARS activity increased by 16.4% in the arable soil versus 32.1% in the fallow soil. These results indicate that glucose proportionately affected more the intensities of immobilized-S than those of ARS. Strong positive correlation coefficients were found between fungal activities and immobilized-S in the arable soil (r = 0.96, P < 0.01) and in the fallow soil (r = 0.98, P < 0.001). However, non-significant correlations were observed between fungal activities and ARS in both studied soils. As to bacterial activities, positive significant correlation coefficients were found with immobilized-S in the arable soil (r = 0.95, P < 0.01) and in the fallow soil (r = 0.90, P < 0.05) as well as with ARS activities in the arable soil (r = 0.83, P < 0.05) and in the fallow soil (r = 0.97, P < 0.01). Overall, we also found positive and significant correlation coefficients of immobilized-S with ARS activities in the arable soil (r = 0.86, P < 0.05) and in the fallow soil (r = 0.83, P < 0.05). Accordingly, the results showed a presence of extracellular arylsulphatase activity of 38.7 mg p-nitrophenol kg^− 1 soil h^− 1 in the arable soil and of 63.5 mg p-nitrophenol kg^− 1 soil h^− 1 in the fallow soil. It was concluded that fallowing maintained larger activities of fungi, bacteria and arylsulphatase compared with the arable soil.     

Controls over pathways of carbon efflux from soils along climate and black spruce productivity gradients in interior Alaska

Small changes in C cycling in boreal forests can change the sign of their C balance, so it is important to gain an understanding of the factors controlling small exports like water-soluble organic carbon (WSOC) fluxes from the soils in these systems. To examine this, we estimated WSOC fluxes based on measured concentrations along four replicate gradients in upland black spruce (Picea mariana [Mill.] BSP) productivity and soil temperature in interior Alaska and compared them to concurrent rates of soil CO₂ efflux. Concentrations of WSOC in organic and mineral horizons ranged from 4.9 to 22.7 g C m⁻² and from 1.4 to 8.4 g C m⁻², respectively. Annual WSOC fluxes (4.5-12.0 g C m⁻² y⁻¹) increased with annual soil CO₂ effluxes (365-739 g C m⁻² y⁻¹) across all sites (R²=0.55, p=0.02), with higher fluxes occurring in warmer, more productive stands. Although annual WSOC flux was relatively small compared to total soil CO₂ efflux across all sites (<3%), its relative contribution was highest in warmer, more productive stands which harbored less soil organic carbon. The proportions of relatively bioavailable organic fractions (hydrophilic organic matter and low molecular weight acids) were highest in WSOC in colder, low-productivity stands whereas the more degraded products of microbial activity (fulvic acids) were highest in warmer, more productive stands. These data suggest that WSOC mineralization may be a mechanism for increased soil C loss if the climate warms and therefore should be accounted for in order to accurately determine the sensitivity of boreal soil organic C balance to climate change.     

Earthworms change the distribution and availability of phosphorous in organic substrates

In laboratory controlled soil microcosms, the distribution and availability of phosphorous (P) were determined in the surface-casts and the burrows-linings of the anecic earthworm L. terrestris and were compared with non-ingested soil. To simulate more realistic earthworm community conditions, a combination of L. terrestris plus the endogeic A. caliginosa was tested. For a 2-month period, the earthworms were given two organic food substrates: rye-grass littered onto the soil surface and sewage sludge mixed with soil. The following treatments were designed: (i) soil alone (S), (ii) soil and sewage sludge (SS), soil and rye-grass litter (SL), and (iv) soil, litter and sludge (SSL). Analyses were performed for P contents (total, available and organic), organic matter content (organic carbon, C_org and total nitrogen, N_tot) and the two acid and alkaline phosphatase activities (AcPA and AkPA). Earthworms enhanced AcPA and were also responsible for additional AkPA in soil. The two AcPA and AkPA increased not only in surface-casts but also in burrows-linings that paralleled with the decrease of organic P in SL and SSL treatments. The stimulation of AcPA began quickly and declined rapidly in casts (from 19 to 8 μmol phenol g⁻¹ dry wt h⁻¹, respectively at week 2 and 8 in the SL treatment) but it was initiated later and maintained at a high level for longer in burrows (more than 10 μmol phenol g⁻¹ dry wt h⁻¹ at week 8 in the SL treatment). Significant positive correlations were found between the AkPA activities and N_tot contents (r=0.95, p=0.001) and to a lesser extend with C_org contents (r=0.76, p=0.05) in casts from the SL treatment, while AcPA significantly correlated with N_tot (r=0.91, p=0.004) but not with C_org (r=0.72, p=0.06). P availability was always highest in casts. However, the available P contents decreased sharply over time in casts and were still low in burrow-linings, suggesting that a large part of inorganic P produced was rapidly immobilized for the microbial growth. Total P content was unchanged except in the SL treatment in which it increased in casts and burrows (ca. 725 μg g⁻¹, at week 4). Organic P was first the highest in casts and then decreased over time (from 168 at week 1 to 140 μg g⁻¹ at week 8 in the SL treatment). This study illustrates that earthworms facilitate P transfer downward increasing a P patchy distribution in the soil, and significantly change the biogeochemical status of P (availability, organic phosphorous pool, AcPA activities) in certain hot spots such as casts and burrow-linings.     

Microbial community response to nitrogen deposition in northern forest ecosystems

The productivity of temperate forests is often limited by soil N availability, suggesting that elevated atmospheric N deposition could increase ecosystem C storage. However, the magnitude of this increase is dependent on rates of soil organic matter formation as well as rates of plant production. Nonetheless, we have a limited understanding of the potential for atmospheric N deposition to alter microbial activity in soil, and hence rates of soil organic matter formation. Because high levels of inorganic N suppress lignin oxidation by white rot basidiomycetes and generally enhance cellulose hydrolysis, we hypothesized that atmospheric N deposition would alter microbial decomposition in a manner that was consistent with changes in enzyme activity and shift decomposition from fungi to less efficient bacteria. To test our idea, we experimentally manipulated atmospheric N deposition (0, 30 and 80 kg NO₃⁻-N) in three northern temperate forests (black oak/white oak (BOWO), sugar maple/red oak (SMRO), and sugar maple/basswood (SMBW)). After one year, we measured the activity of ligninolytic and cellulolytic soil enzymes, and traced the fate of lignin and cellulose breakdown products (¹³C-vanillin, catechol and cellobiose).In the BOWO ecosystem, the highest level of N deposition tended to reduce phenol oxidase activity (131±13 versus 104±5 μmol h⁻¹ g⁻¹) and peroxidase activity (210±26 versus 190±21 μmol h⁻¹ g⁻¹) and it reduced ¹³C-vanillin and ¹³C-catechol degradation and the incorporation of ¹³C into fungal phospholipids (p<0.05). Conversely, in the SMRO and SMBW ecosystems, N deposition tended to increase phenol oxidase and peroxidase activities and increased vanillin and catechol degradation and the incorporation of isotope into fungal phospholipids (p<0.05). We observed no effect of experimental N deposition on the degradation of ¹³C-cellulose, although cellulase activity showed a small and marginally significant increase (p<0.10). The ecosystem-specific response of microbial activity and soil C cycling to experimental N addition indicates that accurate prediction of soil C storage requires a better understanding of the physiological response of microbial communities to atmospheric N deposition.     

Enzyme activities and microbial biomass in coastal soils of India

Soil salinity is a serious problem for agriculture in coastal regions, wherein salinity is temporal in nature. We studied the effect of salinity, in summer, monsoon and winter seasons, on microbial biomass carbon (MBC) and enzyme activities (EAs) of the salt-affected soils of the coastal region of the Bay of Bengal, Sundarbans, India. The average pH of soils collected from different sites, during different seasons varied from 4.8 to 7.8. The average organic C (OC) and total N (TN) content of the soils ranged between 5.2-14.1 and 0.6-1.4 g kg⁻¹, respectively. The electrical conductivity of the saturation extract (ECe) of soils, averaged over season, varied from 2.2 to 16.3 dSm⁻¹. The ECe of the soils increased five fold during the summer season (13.8 dSm⁻¹) than the monsoon season (2.7 dSm⁻¹). The major cation and anion detected were Na⁺ and Cl⁻, respectively. Seasonality exerted considerable effects on MBC and soil EAs, with the lowest values recorded during the summer season. The activities of β-glucosidase, urease, acid phosphatase and alkaline phosphatase were similar during the winter and monsoon season. The dehydrogenase activity of soils was higher in monsoon than in winter. Average MBC, dehydrogenase, β-glucosidase, urease, acid phosphatase and alkaline phosphatase activities of the saline soils ranged from 125 to 346 mg kg⁻¹ oven dry soil, 6-9.9 mg triphenyl formazan (TPF) kg⁻¹ oven dry soil h⁻¹, 18-53 mg p-nitro phenol (PNP) kg⁻¹ oven dry soil h⁻¹, 38-86 mg urea hydrolyzed kg⁻¹ oven dry soil h⁻¹, 213-584 mg PNP kg⁻¹ oven dry soil h⁻¹ and 176-362 mg PNP g⁻¹ oven dry soil h⁻¹, respectively. The same for the non-saline soils were 274-446 mg kg⁻¹ oven dry soil, 8.8-14.4 mg TPF kg⁻¹ oven dry soil h⁻¹, 41-80 mg PNP kg⁻¹ oven dry soil h⁻¹, 89-134 mg urea hydrolyzed kg⁻¹ oven dry soil h⁻¹, 219-287 mg PNP kg⁻¹ oven dry soil h⁻¹ and 407-417 mg PNP kg⁻¹ oven dry soil h⁻¹, respectively. About 48%, 82%, 48%, 63%, 40% and 48% variation in MBC, dehydrogenase activity, β-glucosidase activity, urease activity, acid phosphatase activity and alkaline phosphatase activity, respectively, could be explained by the variation in ECe of saline soils. Suppression of EAs of the coastal soils during summer due to salinity rise is of immense agronomic significance and needs suitable interventions for sustainable crop production.     

Phytase and phosphatase producing fungi in arid and semi-arid soils and their efficiency in hydrolyzing different organic P compounds

Seven most efficient phytase and phosphatases producing fungi were isolated from the soils of arid and semi-arid regions of India and tested for their efficiency on hydrolysis of two important organic P compounds: phytin and glycerophosphate. The native soil organic P may be exploited after using these organisms as seed inoculants, to help attain higher P nutrition of plants. The identified organisms belong to the three genera: Aspergillus, Emmericella and Penicillium. Penicillium rubrum released the most acid into the medium during growth. Aspergillus niger isolates were found to accumulate biomass the fastest. A significant negative correlation (r=−0.593,n=21, p<0.01) was observed between the development of fungal mat and pH of the media. The extracellular (E) phosphatases released by different fungi were less than their intracellular (I) counterpart, but the trend was reversed in case of phytase production. The E:I ratio of different fungi ranged from 0.39 to 0.86 for acid phosphatase, 0.29 to 0.41 for alkaline phosphatases and 9.4 to 19.9 for phytase. The efficiency of hydrolysis of different organic P compounds of different fungi varied from 2.12-4.85 μg min⁻¹ g⁻¹ for glycerophosphate to 0.92-2.10 μg min⁻¹ g⁻¹ for phytin. The trend of efficiency was as follows: Aspergillus sp.>Emmericella sp.>Penicillium sp. The results indicated that the identified fungi have enough potential to exploit native organic phosphorus to benefit plant nutrition.     

Effects of woody debris, microtopography, and organic matter amendments on the biotic community of constructed depressional wetlands

To increase wetland acreage and biodiversity, Delaware agencies constructed >220 depressional wetlands. During construction, agencies included amendments thought to increase biodiversity. Because the efficacy of amendments is unknown, we investigated their effects on macroinvertebrate and vegetative communities. We selected 20 standardized wetlands (five contained coarse woody debris (CWD) and microtopography amendments (land surface ridges and furrows), five had neither, five had CWD only, and five had microtopography only). Additionally, 12 wetlands had received organic matter amendments (i.e., straw). Insect richness (P = 0.010; r² = 0.16), insect biomass (P = 0.023; r² = 0.13), intolerant insect biomass (P = 0.033, r² = 0.03), Ephemeroptera biomass (P = 0.027; r² = 0.12), and Odonata biomass (P = 0.046; r² = 0.10) increased with CWD volume. Obligate plant percent cover increased with microtopographic variation (P = 0.029; r² = 0.120). Although organic matter amendments did not increase percent soil organic matter (t_13.7 = −1.16, P = 0.264), total (P = 0.027; r² = 0.12), native (P = 0.036; r² = 0.11), and facultative (P = 0.001; r² = 0.24) plant richness increased with percent soil organic matter. To enhance biodiversity, constructed wetlands should contain CWD, but additional research is needed to understand the benefits of microtopography and organic matter amendments.     

Unusually high levels of bio-available phosphate in the soils of Ogasawara Islands, Japan: Putative influence of seabirds

Ogasawara Islands are important ecosystems sustaining many indigenous spices. To clarify the indigenous soil environments of Ogasawara Islands, we studied the chemistry of the soils. Many surface soils were low in bio-available P (0 to 0.55 g P₂O₅ kg⁻¹, average: 0.04 g P₂O₅ kg⁻¹ as Bray II P, n = 22), but several soils were found to contain extremely large amounts of bio-available P (1.36 to 6.98 g P₂O₅ kg⁻¹, average: 2.93 g P₂O₅ kg⁻¹, n = 5). From soil profile analyses, the authors concluded that the extremely large amount of bio-available P could not be explained by the effects of parent materials with high P contents nor the effect of fertilizations by human activity, but the effects of natural seabird activities in the past could be the cause. The soil profiles with large amounts of bio-available P indicate deep migration of soil materials from A horizons, which could be a result of intensive mixing of upper horizons by seabird activities. The intensive mixing was supported by the low mechanical impedance of the horizons for the P-accumulating soils (8.17 ± 2.54 kg cm⁻², n = 8) than those for the non-P-accumulating soils (17.46 ± 3.52 kg cm⁻², n = 36). It is likely that in the past seabirds, such as shearwaters, made burrows in the soils for nesting and propagating and inadvertently transported a large amount of P from the sea to the soils, resulting in the extremely large amounts of bio-available P in the present soils.     

Increased degradation of phenanthrene in soil by Pseudomonas sp. GF3 in the presence of wheat

A phenanthrene-degrading bacterial strain Pseudomonas sp. GF3 was examined for plant-growth promoting effects and phenanthrene removal in soil artificially contaminated with low and high levels of phenanthrene (0, 100 and 200 mg kg⁻¹) in pot experiments. Low and high phenanthrene treatments significantly decreased the growth of wheat. Inoculation with bacterial strain Pseudomonas sp. GF3 was found to increase root and shoot growth of wheat. Strain GF3 was able to degrade phenanthrene effectively in the unplanted and planted soils. Over a period of 80 days the concentration of phenanthrene in soil in which wheat was grown was significantly lower than in unplanted soil (p<0.05). At the end of the 80-d experiments, 62.2% and 42.3% of phenanthrene had disappeared from planted soils without Pseudomonas sp. GF3 when the phenanthrene was added at 100 and 200 mg kg⁻¹ soil, respectively, but 84.8% and 70.2% of phenanthrene had disappeared from planted soils with the bacterial inoculation. The presence of vegetation significantly enhances the dissipation of phenanthrene in the soil. There was no significant difference in soil polyphenol oxidase activities among the applications of 0, 100 and 200 mg kg⁻¹ of phenanthrene. However, the enzyme activities in planted and unplanted soils inoculated with the strain Pseudomonas sp. GF3 were significantly higher than those of non-inoculation controls. The bacterial isolate was also able to colonize and develop in the rhizosphere soil of wheat after inoculation.     

Increase in soil pH due to Ca-rich organic matter application causes suppression of the clubroot disease of crucifers

Clubroot disease of cruciferous plants caused by the soil-borne pathogen Plasmodiophora brassicae is difficult to control because the pathogen survives for a long time in soil as resting spores. Disease-suppressive and conducive soils were found during the long-term experiment on the impact of organic matter application to arable fields and have been studied to clarify the biotic and abiotic factors involved in the disease suppression. The fact that a large amount of organic matter, 400 t ha⁻¹ yr⁻¹ farmyard manure (FYM) or 100 t ha⁻¹ yr⁻¹ food factory sludge compost (FSC), had been incorporated for more than 15 yr in the suppressive soils and these soils showed higher pH and Ca concentration than the disease conducive soil led us to hypothesize that an increase in soil pH due to the long-term incorporation of Ca-rich organic matter might be the primary cause of the disease suppression. We have designed a highly reproducible bioassay system to examine this hypothesis. The suppressive and conducive soils were mixed with the resting spores of P. brassicae at a rate of 10⁶ spore g⁻¹ soil, and Brassica campestris was grown in a growth chamber for 8 d. The number of root hair infections was assessed on a microscope. It was found that the incorporation of FYM and FSC at 2.5% (w/w) to the conducive soil suppressed the infection and that the finer particles (?5 mm) of FSC inhibited the infection and increased soil pH more effectively. Neutralization of the conducive soil by Ca(OH)₂, CaCO₃ and KOH suppressed the infection, but the effectiveness of KOH was less than those of Ca(OH)₂ and CaCO₃. Acidification of the suppressive soils by H₂SO₄, promoted the infection. The involvement of soil biota in the disease suppression was investigated using the sterilized (γ-ray irradiation) suppressive soils with respect to soil pH. The γ-ray irradiation promoted the infection at pH 5.5, but no infection was observed at pH 7.4 irrespective of the sterilization status. All these observations suggest that soil pH is a major factor in disease suppression by organic matter application and that Ca and soil biota play certain roles in the suppression under the influence of soil pH.     

Effects of long-term organic and mineral fertilizers on bulk density and penetration resistance in semi-arid Mediterranean soil conditions

Soil aggregation is of great importance in agriculture due to its positive effect on soil physical properties, plant growth and the environment. A long-term (1996-2008) field experiment was performed to investigate the role of mycorrhizal inoculation and organic fertilizers on some of soil properties of Mediterranean soils (Typic Xerofluvent, Menzilat clay-loam soil). We applied a rotation with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) as a second crop during the periods of 1996 and 2008. The study consisted of five experimental treatments; control, mineral fertilizer (300-60-150 kg N-P-K ha⁻¹), manure at 25 t ha⁻¹, compost at 25 t ha⁻¹ and mycorrhiza-inoculated compost at 10 t ha⁻¹ with three replicates. The highest organic matter content both at 0-15 cm and 15-30 cm soil depths were obtained with manure application, whereas mineral fertilizer application had no effect on organic matter accumulation. Manure, compost and mycorrhizal inoculation + compost application had 69%, 32% and 24% higher organic matter contents at 0-30 cm depth as compared to the control application. Organic applications had varying and important effects on aggregation indexes of soils. The greatest mean weight diameters (MWD) at 15-30 cm depth were obtained with manure, mycorrhiza-inoculated compost and compost applications, respectively. The decline in organic matter content of soils in control plots lead disintegration of aggregates demonstrated on significantly lower MWD values. The compost application resulted in occurring the lowest bulk densities at 0-15 and 15-30 cm soil depths, whereas the highest bulk density values were obtained with mineral fertilizer application. Measurements obtained in 2008 indicated that manure and compost applications did not cause any further increase in MWD at manure and compost receiving plots indicated reaching a steady state. However, compost with mycorrhizae application continued to significant increase (P < 0.05) in MWD values of soils. Organic applications significantly lowered the soil bulk density and penetration resistance. The lowest penetration resistance (PR) at 0-50 cm soil depth was obtained with mycorrhizal inoculated compost, and the highest PR was with control and mineral fertilizer applications. The results clearly revealed that mycorrhiza application along with organic fertilizers resulted in decreased bulk density and penetration resistance associated with an increase in organic matter and greater aggregate stability, indicated an improvement in soil structure.     

Gross N transformation rates and net N mineralisation rates related to the C and N contents of soil organic matter fractions in grassland soils of different age

We investigated the relationship between soil organic matter (SOM) content and N dynamics in three grassland soils (0-10 and 10-20 cm depth) of different age (6, 14 and 50 y-old) with sandy loam textures. To study the distribution of the total C and N content the SOM was fractionated into light, intermediate and heavy density fractions of particulate macro-organic matter (150-2000 μm) and the 50-150 μm and <50 μm size fractions. The potential gross N transformation rates (mineralisation, nitrification, NH₄⁺ and NO₃⁻ immobilization) were determined by means of short-term, fully mirrored ¹⁵N isotope dilution experiments (7-d incubations). The long-term potential net N mineralisation and gross N immobilization rates were measured in 70-d incubations. The total C and N contents mainly tended to increase in the 0-10 cm layer with increasing age of the grassland soils. Significant differences in total SOM storage were detected for the long-term (50 y-old) conversion from arable land to permanent grassland. The largest relative increase in C and N contents had occurred in the heavy density fraction of the macro-organic matter, followed by the 50-150 and <50 μm fractions. Our results suggest that the heavy density fraction of the macro-organic matter could serve as a good indicator of early SOM accumulation, induced by converting arable land to permanent grassland. Gross N mineralisation, nitrification, and (long-term) gross N immobilization rates tended to increase with increasing age of the grasslands, and showed strong, positive correlations with the total C and N contents. The calculated gross N mineralisation rates (7-d incubations) and net N mineralisation rates (70-d incubations) corresponded with a gross N mineralisation of 643, 982 and 1876 kg N ha⁻¹ y⁻¹, and a net N mineralisation of 195, 208 and 274 kg N ha⁻¹ y⁻¹ in the upper 20 cm of the 6, 14 and 50 y-old grassland soils, respectively. Linear regression analysis showed that 93% of the variability of the gross N mineralisation rates could be explained by variation in the total N contents, whereas total N contents together with the C-to-N ratios of the <50 μm fraction explained 84% of the variability of the net N mineralisation rates. The relationship between long-term net N mineralisation rates and gross N mineralisation rates could be fitted by means of a logarithmic equation (net m=0.24Ln(gross m)+0.23, R²=0.69, P<0.05), which reflects that the ratio of gross N immobilization-to-gross N mineralisation tended to increase with increasing SOM contents. Microbial demand for N tended to increase with increasing SOM content in the grassland soils, indicating that potential N retention in soils through microbial N immobilization tends to be limited by C availability.     

Surface runoff phosphorus (P) loss in relation to phosphatase activity and soil P fractions in Florida sandy soils under citrus production

Phosphorus losses by surface runoff from agricultural lands have been of public concern due to increasing P contamination to surface waters. Five representative commercial citrus groves (C1-C5) located in South Florida were studied to evaluate the relationships between P fractions in soils, surface runoff P, and soil phosphatase activity. A modified Hedley P sequential fractionation procedure was employed to fractionate soil P. Soil P consisted of mainly organically- and Ca/Mg-bound P fractions. The organically-bound P (biological P, sum of organic P in the water, NaHCO₃ and NaOH extracts) was dominant in the acidic sandy soils from the C2 and C3 sites (18% and 24% of total soil P), whereas the Ca/Mg-bound P (HCl-extractable P) accounted for 45-60% of soil total P in the neutral and alkaline soils (C1, C4 and C5 soils). Plant-available P (sum of water and NaHCO₃ extractable P fractions) ranged from 27 to 61 mg P kg⁻¹ and decreased in the order of C3>C4>C1>C2>C5. The mean total P concentrations (TP) in surface runoff water samples ranged from 0.51 to 2.64 mg L⁻¹. Total P, total dissolved P (TDP), and PO₄³⁻-P in surface runoff were significantly correlated with soil biological P and plant-available P forms (p<0.01), suggesting that surface runoff P was directly derived from soil available P pools, including H₂O- and NaHCO₃- extractable inorganic P, water-soluble organic P, and NaHCO₃- and NaOH-extractable organic P fractions, which are readily mineralized by soil microorganisms and/or enzyme mediated processes. Soil neutral (55-190 mg phenol kg⁻¹ 3 h⁻¹) and natural (measured at soil pH) phosphatase activities (77-295 mg phenol kg⁻¹ 3 h⁻¹) were related to TP, TDP, and PO₄³⁻-P in surface runoff, and plant-available P and biological P forms in soils. These results indicate that there is a potential relationship between soil P availability and phosphatase activities, relating to P loss by surface runoff. Therefore, the neutral and natural phosphatase activities, especially the natural phosphatase activity, may serve as an index of surface runoff P loss potential and soil P availability.     

A protocol for the assay of arylesterase activity in soil

We set up a protocol for the assay of the arylesterase activity, using p-nitrophenyl acetate (p-NPA) as substrate, dimethylsulfoxide as solvent, modified universal buffer at pH 7.5, and determination of the reaction product (p-nitrophenol) after separation of non-hydrolysed p-NPA after reaction, and tested it using eight soils with a wide range of characteristics. Various incubation temperatures and times, pH values and substrate concentrations were also used to find the optimal conditions for the enzyme activity and to determine characteristics and kinetic parameters of soil arylesterase. Arylesterase activity was significantly correlated with total organic C, total N, and soil ATP content. Soil arylesterase activity showed a pH optimum at 7.5, optimal temperature between 55 and 65 °C and linear increase with incubation time. The K_m values ranged from 4.3 to 8.5 mM, the V_max values from 326 to 803 μmol p-NP g⁻¹ h⁻¹, with higher K_m values observed in soils with higher organic matter content. We conclude that the proposed assay protocol is suitable to determine the arylesterase activity in a wide range of soils.     

Long-term effects of metal-containing farmyard manure and sewage sludge on soil organic matter in a fluvisol

Our aim was to establish the long-term effects of repeated applications after 20 y of organic amendments (farmyard manure at 10 t ha⁻¹ y⁻¹, and urban sewage sludge at two different rates, 10 t ha⁻¹ y⁻¹ and 100 t ha⁻¹ every 2 y) on the quality of a sandy and poorly buffered soil (Fluvisol, pH 6). Chemical characteristics and biodegradability of the labile organic matter, which is mainly derived from microbial biomass and biodegradation products of organic residues, were chosen as indicators for soil quality. The organic C content had reached a maximal value (30.6 g C kg⁻¹ in the 100 t sludge-treated soil), i.e. about 2.5 times that in the control. Six years after the last application, the organic C content and the microbial biomass content remained higher in sludge-treated soils than in the control. In contrast, the proportion of labile organic matter was significantly lower in sludge-treated soils than in manure-treated and control soils. The labile organic matter of sludge extracts appeared less humified than that of manure-treated and control soils.     

Chemical composition, or quality, of agroforestry residues influences N2O emissions after their addition to soil

Emissions of N₂O were measured following addition of ¹⁵N-labelled (2.6-4.7 atom% excess ¹⁵N) agroforestry residues (Sesbania sesban, mixed Sesbania/Macroptilium atropurpureum, Crotalaria grahamiana and Calliandra calothyrsus) to a Kenyan oxisol at a rate of 100 mg N kg soil⁻¹ under controlled environment conditions. Emissions were increased following addition of residues, with 22.6 mg N m⁻² (124.4 mg N m⁻² kg biomass⁻¹; 1.1 mg ¹⁵N m⁻²; 1.03% of ¹⁵N applied) emitted as N₂O over 29 d after addition of both Sesbania and Macroptilium residues in the mixed treatment. Fluxes of N₂O were positively correlated with CO₂ fluxes, and N₂O emissions and available soil N were negatively correlated with residue lignin content (r=−0.49;P<0.05), polyphenol content (r=−0.94;P<0.05), protein binding capacity (r=−0.92;P<0.05) and with (lignin+polyphenol)-to-N ratio (r=−0.55;P<0.05). Lower emission (13.6 mg N m⁻² over 29 d; 94.5 mg N m⁻² kg biomass⁻¹; 0.6 mg ¹⁵N m⁻²; 0.29% of ¹⁵N applied) after addition of Calliandra residue was attributed to the high polyphenol content (7.4%) and high polyphenol protein binding capacity (383 μg BSA mg plant⁻¹) of this residue binding to plant protein and reducing its availability for microbial attack, despite the residue having a N content of 2.9%. Our results indicate that residue chemical composition, or quality, needs to be considered when proposing mitigation strategies to reduce N₂O emissions from systems relying on incorporation of plant biomass, e.g. improved-fallow agroforestry systems, and that this consideration should extend beyond the C-to-N ratio of the residue to include polyphenol content and their protein binding capacity.     

Qualitative and quantitative differences in particulate organic matter fractions in organic and conventional farming systems

To quantify functionally important differences in soil organic matter (SOM) that result from use of different farming practices, soils from 9 long-term trials comparing manure+legume-based organic, legume-based organic, and conventional farming systems were collected and particulate organic matter (POM) was fractionated to reflect its position within the soil matrix. The free, light POM (FPOM; <1.6 g cm⁻³) not occluded within aggregates and occluded POM (OPOM; <2.0 g cm⁻³) were compared to an undifferentiated POM fraction (coarse fraction, CF; >53 μm) obtained by wet sieving. Fraction C, N, and hydrolyzable N (quantified using the Illinois test (IL-N)) were determined. Organic farming systems had greater quantities of C and N in the OPOM and CF and, greater IL-N contents in all POM fractions considered. The OPOM's C:N ratio (16-19) and was least in the manure+legume-based organic, intermediate in the legume-based organic, and greatest in the conventional systems (P<0.10). Trends in OPOM C:N and IL-N abundance suggested occluded POM was most decomposed, and possibly a greater N reservoir, in the manured soils. The FPOM quality reflected the residues added to each system and its removal improved resolution of quality-based differences in POM associated with long-term management. Subdivision of POM revealed differences in its quality that were not evident using the undifferentiated CF. Quantification of hydrolysable N (IL-N) in POM did not enhance our understanding of management's affect on SOM quality. This multi-site comparison showed organic management simultaneously increased the size of the labile N reservoir and the amount of POM protected within aggregates; and that, occluded POM is more decomposed in manure+legume- than in legume-based organic systems. The characteristics of POM reveal how organic practices improve SOM and suggest the nutrient and substrate decay dynamics of organic systems may differ as a result of the N fertilization strategies they employ.     

Adenylates as an estimate of microbial biomass C in different soil groups

Adenylate (i.e. adenosine tri- (ATP), di- (ADP) and monophosphates (AMP)) and microbial biomass C data were collected over a wide range of sites including forest floor layers and forest, grassland and arable soils. Microbial biomass C was measured by fumigation extraction and adenylates after alkaline Na₃PO₄/DMSO/EDTA extraction and HPLC detection. Our aims were (1) to test whether the sum of adenylates is a better estimate for microbial biomass than the determination of ATP, (2) to compare our conversion values with those proposed by others, and (3) to analyse whether soil properties or land use form affect the relationships between ATP, adenylates and microbial biomass C. A close relationship was found between microbial biomass C and ATP (r=0.96), but also with the sum of adenylates (r=0.96) within all appropriately conditioned soil samples (n=112). In the mineral soil (n=98), the geometric means of the ATP-to-microbial biomass C ratio and the adenylates-to-microbial biomass C ratio were 7.4 and 11.4 μmol g⁻¹, respectively. The mean ratios did not differ significantly between the different texture classes and land use forms. In the forest floor, the ATP-to-microbial biomass C ratio and the adenylates-to-microbial biomass C ratio were both roughly two-thirds of those of the mineral soil. The average adenylate energy charge (AEC) of all soil samples was 0.79 and showed a strong negative relationship with the soil pH (r=−0.69). However, the AEC is presumably only indirectly affected by the soil pH.