Nitrous oxide emissions from soil during soybean [(Glycine max (L.) Merrill] crop phenological stages and stubbles decomposition period

The purpose of this study was to evaluate, during the phenological stages of inoculated soybean crop [Glycine max (L.) Merrill], the effect of different N fertilization levels and inoculation with Bradyrhizobium japonicum on N₂O emissions from the soil. Gas emissions were evaluated at field conditions by the static-chamber method. Nitrogen fertilization increased N₂O emissions significantly (P < 0.05). The variable that best explained cumulative N₂O emissions during the whole soybean growing season was the soil nitrate level (r ² = 0.1899; P = 0.0231). Soil moisture presented a greater control on N₂O emissions between the grain-filling period and the crop commercial maturity (r ² = 0.5361; P < 0.0001), which coincided with a positive balance of the available soil N, as a consequence of the decrease in crop requirements and root and nodular decomposition. Only soil soluble carbon (r ² = 0.29; P = 0.019) and moisture (r ² = 0.24; P = 0.039) were correlated with N₂O emissions during the residue decomposition period. The relationship between soil variables and N₂O emissions depended on crop phenological or stubbles decomposition stages.     

Gross nitrogen mineralisation and fungi-to-bacteria ratios are negatively correlated in boreal forests

In terrestrial ecosystems, gross nitrogen mineralisation is positively correlated to microbial biomass but negatively to soil organic matter C-to-N ratios; the influence of the microbial community structure is less well known. Here, we relate rates of gross N mineralisation to fungi-to-bacteria ratios in three natural forest types of contrasting N availability and in a long-term N-loading experiment in a boreal forest. We report, for the first time, a strong negative correlation between gross N mineralisation and the fungi-to-bacteria ratio ( = 0.91, P = 0.0005, N = 7). There was also a negative correlation between gross N mineralisation and the C-to-N ratio ( = 0.89, P = 0.001, N = 7), but a weaker positive correlation between gross N mineralisation and soil pH ( = 0.64, P = 0.019, N = 7). Our analysis suggests that soil fungi-to-bacteria and C-to-N ratios are interrelated and that they exert strong influences on soil N cycling in boreal forests.     

Mineralization and denitrification in upland, nearly saturated and flooded subtropical soil II. Effect of organic manures varying in N content and C:N ratio

 Nitrogen and carbon mineralization of cattle manure (N=6 g kg^–1; C:N=35), pressmud (N=17.4 g kg^–1; C:N=22), green manure (N=26.8 g kg^–1; C:N=14) and poultry manure (N=19.5 g kg^–1; C:N=12) and their influence on gaseous N losses via denitrification (using the acetylene inhibition technique) in a semiarid subtropical soil (Typic Ustochrepts) were investigated in a growth chamber simulating upland, nearly saturated, and flooded conditions. Mineralization of N started quickly in all manures, except pressmud where immobilization of soil mineral N was observed for an initial 4 days. Accumulation of mineral N in upland soil plus denitrified N revealed that mineralization of cattle manure-, pressmud-, poultry manure- and green manure-N over 16 days was 12, 20, 29 and 44%, respectively, and was inversely related to C:N ratio (R ²=0.703, P=0.05) and directly to N content of organic manure (R ²=0.964, P=0.01). Manure-C mineralized over 16 days ranged from 6% to 50% in different manures added to soil under different moisture regimes and was, in general, inversely related to initial C:N ratio of manure (R ²=0.690, P=0.05). Cumulative denitrification losses over 16 days in control soils (without manure) under upland, nearly saturated, and flooded conditions were 5, 23, and 24 mg N kg^–1, respectively. Incorporation of manures enhanced denitrification losses by 60-82% in upland, 52–163% in nearly saturated, and 26–107% in flooded soil conditions over a 16-day period, demonstrating that mineralized N and C from added manures could result in 2- to 3-fold higher rate of denitrification. Cumulative denitrification losses were maximal with green manure, followed by poultry manure, pressmud and cattle manure showing an increase in denitrification with increasing N content and decreasing C:N ratio of manure. Manure-amended nearly saturated soils supported 14–35% greater denitrification than flooded soils due to greater mineralization and supply of C.     

Impact of coastal wetland cultivation on microbial biomass, ammonia-oxidizing bacteria, gross N transformation and N2O and NO potential production

A ¹⁵N dilution experiment was carried out to investigate effects of cultivation on the gross N transformation rate in coastal wetland zone. Microbial community composition was estimated by phospholipid fatty acid (PLFA) analysis and abundance of soil ammonia-oxidizing bacteria (AOB) was quantified by real-time polymerase chain reaction (PCR). Soil salinity decreased significantly, while total N increased after coastal wetland was cultivated. Microbial biomass (total PLFA), bacterial biomass, fungal biomass, and actinomycete biomass of the native coastal wetland soils were significantly (p < 0.05) lower than those of the cultivated soils whereas AOB population size also significantly increased after coastal wetland cultivation. Multiple regression analysis showed that total PLFA biomass and soil total N (TN) explained 97% of the variation of gross N mineralization rate in the studied soils (gross mineralization rate = 0.179 total PLFA biomass + 5.828TN − 2.505, n = 16, p < 0.01). Gross nitrification rate increased by increasing the soil AOB population size and gross mineralization rate (M) (gross nitrification rate = 3.39AOB + 0.18 M − 0.075, R ² = 0.98, n = 16, p < 0.01). Management of salt discharge and mineral N fertilization during the cultivation of wetland soils might have changed composition of soil microflora and AOB population size, thus influencing mineralization and nitrification. Probably, the cultivation of coastal wetland soils increased the risk of N losses from soil through nitrate leaching and gas emission (e.g., N₂O and NO).     

Leaf litter ingestion and assimilation by two endemic pill millipedes (<Emphasis Type="Italic">Arthrosphaera</Emphasis>)

This study aims to determine leaf litter preference, consumption rate, growth rate, food conversion efficiency, and quality of fecal pellets of two endemic pill millipedes (Arthrosphaera dalyi and Arthrosphaera davisoni) of the Western Ghats of India by laboratory microcosm experiments. Among seven combinations of three plantation leaf litters offered in 4-day trial, top three preferred combinations were selected for 4-week trial. In 4-week trial, preference of mixed litter diet was higher than single litter diet, which resulted in enhanced growth as well as food conversion efficiency of millipedes. Among Hopea , Pongamia , and Areca litters, A. dalyi preferred Hopea + Pongamia, and its consumption was significantly correlated with contents of organic carbon (P < 0.05; r = –0.97) and nitrogen (P < 0.01; r = 0.99), while growth rate with phosphorus content (P < 0.05; r = 0.97) and food conversion efficiency with contents of organic carbon (P < 0.05; r = 0.98) and calcium (P < 0.01; r = –0.99). Among Areca , Elettaria , and Coffea litters, Areca + Elettaria  + Coffea was most preferred by A. davisoni, which was significantly correlated with organic carbon content (P < 0.05; r = 0.98) and food conversion efficiency with calcium content (P < 0.0001; r = 0.99). The food conversion efficiency, however, was the highest in millipedes fed with Areca  +  Elettaria. The present study demonstrated increased nitrogen and phosphorus contents and decreased phenolic content and C/N ratio in fecal pellets of pill millipedes fed with plantation litter, and thus, these millipedes play an important role in leaf litter mineralization and soil enrichment in plantations Western Ghats.     

Upland rice seedling wilt and microbial biomass and enzyme activities of compost-treated soils

Rice seedling wilt frequently occurs in upland nurseries under well-aerated conditions and causes considerable economic loss. Whether the wilt is pathogenic or edaphic is not known. We hypothesize the use of composts to alleviate seedling wilt. The severity level of upland rice seedling wilt was significantly (p < 0.05) positively correlated with soil pH (r = 0.499; n = 19), but negatively correlated with soil organic matter (r = −0.745), microbial biomass C (r = −0.669), activities of dehydrogenase (r = −0.589), arylsulfatase (r = −0.272), fluorescein diacetate hydrolysis (r = −0.466), and β-glucosidase (r = −0.280). Correlations between severity level and soil inorganic N and exchangeable potassium K were not significant. Contents of Fe, Zn, Cu, and Mn in healthy seedlings were not significantly (p < 0.05) different from those in infected seedlings. These data suggest that seedling wilts are not associated with nutrient constraints. Compost amendment at the rate of 3% or above in pot experiments significantly improved seedling growth and reduced the wilt symptoms. Field trials further showed that aboveground weight of seedlings in compost-amended treatment ranged from 11.5 to 14.9 mg per plant, significantly higher than the range from 6.38 to 12.1 mg per plant in the control treatment; in addition to rice growth compost significantly increased microbial biomass and enzyme activities of soils. Soil fumigation significantly increased rice growth and alleviation symptoms in 11 out of 19 soils, suggesting the involvement of pathogens. It is concluded that upland seedling wilt is a pathogen-associated disease. Probably high soil pH and low soil biochemical activities may favor pathogen activities.     

Spatial changes of soil fungal and bacterial biomass from a sub-alpine coniferous forest to grassland in a humid, sub-tropical region

 Fungal and bacterial biomass were determined across a gradient from a forest to grassland in a sub-alpine region in central Taiwan. The respiration-inhibition and ergosterol methods for the evaluation of the microbial biomass were compared. Soil fungal and bacterial biomass both significantly decreased (P<0.05) with the shift of vegetation from forest to grassland. Fungal and bacterial respiration rates (evolved CO₂) were, respectively, 89.1 μl CO₂ g^–1 soil h^–1 and 55.1 μl CO₂ g^–1 soil h^–1 in the forest and 36.7 μl CO₂ g^–1 soil h^–1 and 35.7 μl CO₂ g^–1 soil h^–1 in the grassland surface soils (0–10 cm). The fungal ergosterol content in the surface soil decreased from the forest zone (108 μg g^–1) to the grassland zone (15.9 μg g^–1). A good correlation (R ²=0.90) was exhibited between the soil fungal ergosterol content and soil fungal CO₂ production (respiration) for all sampling sites. For the forest and grassland soil profiles, microbial biomass (respiration and ergosterol) declined dramatically with depth, ten- to 100-fold from the surface organic horizon to the deepest mineral horizon. With respect to fungal to bacterial ratios for the surface soil (0–10 cm), the forest zone had a significantly (P<0.05) higher ratio (1.65) than the grassland zone (1.05). However, there was no fungal to bacterial ratio trend from the surface horizon to the deeper mineral horizons of the soil profiles. Received: 30 March 2000     

The influence of soil compaction and the removal of organic matter on two native earthworms and soil properties in an oak-hickory forest

 Earthworms may alter the physical, chemical, and biological properties of a forest soil ecosystem. Any physical manipulation of the soil ecosystem may, in turn, affect the activities and ecology of earthworms. The effects of removing organic matter (logs and forest litter) and severely compacting the soil on native earthworm species were measured in a central USA hardwood region (oak-hickory) forest in the Missouri Ozarks (USA). Soils in this region are characterized by a cherty residuum that is primarily of the Clarksville series (Loamy-skeletal, mixed, mesic Typic Paledults). Earthworms were collected from 0–15 cm depth each spring and fall for 2 years by handsorting, and densities were determined on a per meter square basis. Two native earthworm species, Diplocardia ornata and Diplocardia smithii, were dominant on this site. Organic matter removal decreased the average individual biomass of both species. However, both species responded differently to soil compaction. Soil compaction affected D. ornata adversely and D. smithii favorably. This suggested that the degree of soil compaction was not as restrictive with respect to D. smithii (2 mm diameter) as to D. ornata (5 mm diameter). Moreover, the apparently improved soil environmental conditions resulting from the remaining organic matter in compacted soil enhanced the population and growth of D. smithii. Sampling position on the landscape affected D. ornata but not D. smithii. Soil microbial biomass C and soil microbial biomass N were decreased under soil compaction when the organic matter was removed. Other factors influencing the ecology and activity of these two species will require further study. Received: 6 January 1999     

Effects of liming on survival and reproduction of two potentially invasive earthworm species in a northern forest Podzol

During the last several decades, colonization of soil by exotic earthworms and their effects on soil properties and biodiversity have been reported in forests of North America. In some northern hardwood stands, acid soils or harsh climate may have prevented earthworm colonization. However, climatic change and the increasing use of liming to restore the vigor of declining sugar maple (Acer saccharum Marsh.) stands, situated on base-poor soils in USA and Canada, could make many of these sites more suitable for earthworm colonization. We tested survival and reproduction of two exotic earthworm species (Lumbricus terrestris and Amynthas hawayanus) in unlimed and limed soils at the northern limit of the northern hardwood forest distribution in Canada. Improving soil parameters of base-poor, acidic soils by liming positively influenced activity, survivability and reproductive output of L. terrestris in this northern hardwood forest. In contrast, the high mortality and low vigor of L. terrestris observed in the unlimed plots show that soils in this area with a pH of 4.3 are not favorable to this species. Our results suggest that A. hawayanus was very active prior to winter at both soil pHs, but was not able to complete its life cycle during one year at this latitude. Both earthworm species significantly reduced organic C and total N, and increased the C/N ratio of the forest floor. Given that forest liming activities are increasing in proximity to human activities, there is high probability that some earthworm species, such as L. terrestris, will invade limed northern hardwood forests in the next decades, with possible consequences for soil organic matter turnover, nutrient cycling and forest biodiversity and dynamics.     

Long-term winter cover cropping effects on corn (Zea mays L.) production and soil nitrogen availability

 This study was conducted to determine effects of long-term winter cover cropping with hairy vetch, cereal rye and annual ryegrass on soil N availability and corn productivity. From 1987 to 1995, with the exception of the first year of the study, the cover crops were seeded each year in late September or early October after the corn harvest and incorporated into the soil in late April or early May. Corn was seeded 10 days to 2 weeks after the cover crop residues had been incorporated, and N fertilizer was applied as a side-dressing at rates of 0, 67, 134, or 201 kg N ha^–1 each year. While the average annual total N input from the above-ground biomass of the cover crops was highest for hairy vetch (72.4 kg N ha^–1), the average annual total C input was highest for cereal rye (1043 kg C ha^–1) compared with the other cover crops. Hairy vetch was the only cover crop that significantly increased pre-side-dressed NO₃ ^–-N (N_i) corn biomass and N uptake at 0 N. At an N fertilizer rate of 134 kg N ha^–1 or higher, the cover crops had a minimal effect on corn biomass. This indicated that even after 9 years of winter cover cropping, the effect of the cover crops on corn growth resulted primarily from their influence on soil N availability. The amount of available N estimated from the cover crops (N_ac) was significantly correlated with relative corn biomass production (r ²=0.707, P<0.001). The total amount of available N, comprising N_ac and N added from fertilizer (N_f), was strongly correlated (r ²=0.820, P<0.001)) with relative corn biomass production. The correlation was also high for the available N comprising N_i and N_f (r ²=0.775, P<0.001). Although cereal rye and annual ryegrass did not improve corn biomass production in the short term, they benefited soil organic N accumulation and gradually improved corn biomass production compared with the control over the long term. Received: 10 August 1999     

Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N₂O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N₂O emission. Residues were added at the rate of 3 t C ha⁻¹ to soil with, and without, 150 kg urea N ha⁻¹. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO₃⁻–N, which reached minimum values of 2.8 mg N kg⁻¹ for sugarcane (at day 28), 10.3 mg N kg⁻¹ for maize (day 7), and 5.9 mg N kg⁻¹ for sorghum (day 7), compared to 22.7 mg N kg⁻¹ for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg⁻¹ in sugarcane, 34 mg kg⁻¹ in maize, 29 mg kg⁻¹ in sorghum, and 16 mg kg⁻¹ in cotton amended soil compared to soil + N fertilizer, although soil NO₃⁻–N increased by 7 mg kg⁻¹ in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N₂O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N₂O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N₂O emission was significantly and positively correlated with NO₃⁻–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO₂ in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).     

An examination of potential extraction methods to assess plant-available organic phosphorus in soil

The role of soil organic phosphorus (P) in plant nutrition was assessed using data from a glasshouse pot experiment carried out on seven soil types using two contrasting plant species (Lolium perenne, Pinus radiata) and 12 different extractants (five salts (0.025 M ethylenediaminetetraacetic acid (EDTA), 0.025 M EDTA pH 7, Olsen, Mehlich-III, and 6% NaOCl pH 7.5) and seven exchange resins (Hampton chelating resin, Bio-Rad Chelex-100, Dow MAC-3, Amberlite IRC76, Diaion WT01S, Lewatit MP500A, Diaion WA30)). The contribution from mineralization of soil organic P was inferred by consistent increases in correlation coefficients between extractable P and plant P uptake when organic P was considered in addition to inorganic P. The best correlated extractants for combined inorganic and organic P were NaOCl (r = 0.84), Hampton chelating resin (r = 0.78), and MP500A resin (r = 0.73), which compared favorably with Olsen P (r = 0.66) and EDTA (r = 0.72). ³¹P nuclear magnetic resonance analysis of selected extracts from two soils confirmed that the Hampton-chelating-resin-extractable P was mainly monoester and diester forms of organic P, while there was no monoester or diester organic P in the IRC76 resin extract—poorly correlated with plant uptake. The findings of this study suggest that readily extractable forms of organic P in soil contribute to short-term plant P uptake, and this P should be considered for inclusion in routine tests for soil P availability.     

Relationship between nitrous oxide emission and winter wheat production

A 3-year field study in southeast China was performed to examine the relationship between N₂O emission and winter wheat production. Over the 2002–03, 2003–04 and 2004–05 wheat-cropping seasons, N₂O emissions depended on nitrogen addition, plowing practice, and preceding crop type treatments, and showed a pronounced inter-annual variation. N₂O–N emission factor, the proportion of fertilizer N released as N₂O–N from the wheat field, varied from 1.33% to 2.97%. The relationship between N₂O emission (y) and crop yield (x) was well explained by the function y = 3.773Ln(x) + 1.46. Similarly, the function y = 4.445Ln(x) − 3.52 can be employed to address the relationship between N₂O emission (y) and above ground biomass (x). About 84% and 87% of variation in seasonal N₂O emission were explained by the two functions, while only 66% of the variation was represented by the N input with a linear relationship. The results of this study suggest that seasonal N₂O emission of soil under winter wheat could be better predicted by crop yield and biomass than by N input. Submitted to Biology and Fertility of Soils.     

Soil fauna and organic amendment interactions affect soil carbon and crop performance in semi-arid West Africa

A field experiment was conducted at Kaibo in southern Burkina Faso on an Eutric Cambisol during the 2000 rainy season to assess the interaction of organic amendment quality and soil fauna, affecting soil organic carbon and sorghum (Sorghum bicolor L. Moench) performance. Plots were treated with the pesticides Dursban and Endosulfan to exclude soil fauna or left untreated. Sub-treatments consisted of surface-placed maize straw (C/N ratio = 58), Andropogon straw (C/N ratio = 153), cattle dung (C/N ratio = 40), sheep dung (C/N ratio = 17) or compost (C/N ratio = 10) and the control. Organic amendments were applied at a dose equivalent to the application of 40 kg N ha⁻¹. The presence of soil fauna increased soil total carbon by 32% and grain yield production by 50%. The interaction between high C/N ratio organic amendment, Andropogon straw (SA), and soil fauna reduced soil carbon build-up. We suggest that this is due to a priming effect of SA on soil organic matter in the presence of soil fauna. We also suggest that the interaction between soil fauna and easily decomposable organic amendment led to the smallest decrease in soil carbon build-up. It is concluded that in semi-arid West Africa, in the presence of soil fauna, soil carbon build-up is more affected by the quality of organic amendments than the quantity of carbon inputs. Sorghum grain yield production was significantly reduced in the absence of soil fauna. High C/N ratio organic amendment interacted negatively with soil fauna in its effects on crop performance. We propose that the effect of soil fauna on soil carbon build-up and crop performance can be optimised by using high quality organic matter or supplementing low-quality organic matter with inorganic nitrogen in semi-arid West Africa.     

Effects of cow manure and sewage sludge on the activity and kinetics of <Emphasis Type="SmallCaps">l</Emphasis>-glutaminase in soil

A study was conducted to investigate the effects of cow manure and sewage sludge application on the activity and kinetics of soil l-glutaminase. Soil samples were collected from a farm experiment in which 0, 25, and 100 Mg ha⁻¹ of either cow manure or sewage sludge had been applied annually for 4 consecutive years to a clay loam soil (Typic Haplargid). A chemical fertilizer treatment had also been applied. Results indicated that the effects of chemical fertilizer and the solid waste application on pH in the 18 surface soil (0–15 cm) samples were not significant. The organic C content, however, was affected significantly by the different treatments, being the greatest in soils treated with 100 Mg ha⁻¹ cow manure, and the least in the control treatment. l-Glutaminase activity was generally greater in solid-waste applied soils and was significantly correlated (r = 0.939, P < 0.001) with organic C content of soils. The values of l-glutaminase maximum velocity (Vmax) ranged from 331 to 1,389 mg NH₄ ⁺–N kg⁻¹ 2 h⁻¹. Values of the Michaelis constant (K _m) ranged from 35.1 to 71.7 mM. Organic C content of the soils were significantly correlated with V _max (r = 0.919, P < 0.001) and K _m (r = 0.763, P < 0.001) values. These results demonstrate the considerable influence that solid waste application has on this enzymatic reaction involved in N mineralization in soil.     

Soil organic carbon stocks,distribution, and composition affected by historic land use changes on adjacent sites

Historic alterations in land use from forest to grassland and cropland to forest were used to determine impacts on carbon (C) stocks and distribution and soil organic matter (SOM) characteristics on adjacent Cambisols in Eastern Germany. We investigated a continuous Norway spruce forest (F-F), a former cropland afforested in 1930 (C-F), and a grassland deforested in 1953 (F-G). For C and N stocks, we sampled the A and B horizons of nine soil pits per site. Additionally, we separated SOM fractions of A and B horizons by physical means from one central soil pit per pedon. To unravel differences of SOM composition, we analyzed SOM fractions by ¹³C-CPMAS NMR spectroscopy and radiocarbon analysis. For the mineral soils, differences in total C stocks between the sites were low (F-F = 8.3 kg m⁻²; C-F = 7.3 kg m⁻²; F-G = 8.2 kg m⁻²). Larger total C stocks (+25%) were found under continuous forest compared with grassland, due to the C stored within the organic horizons. Due to a faster turnover, the contents of free particulate organic matter (POM) were lower under grassland. High alkyl C/O/N-alkyl C ratios of free POM fractions indicated higher decomposition stages under forest (1.16) in relation to former cropland (0.48) and grassland (0.33). Historic management, such as burning of tree residues, was still identifiable in the subsoils by the composition and ¹⁴C activity of occluded POM fractions. The high potential of longer lasting C sequestration within fractions of slower turnover was indicated by the larger amounts of claybound C per square meter found under continuous forest in contrast to grassland.     

Components of organic phosphorus in soil extracts that are hydrolysed by phytase and acid phosphatase

 Extracts were prepared from soil using water, 50 mM citric acid (pH ∼2.3) or 0.5 M NaHCO₃ (pH 8.5), and were incubated with excess phytase from Aspergillus niger to determine the amounts of labile P. Two A. niger phytase preparations were used: (1) a purified form which exhibited a narrow substrate specificity and high specific activity against phytate; and (2) a commercial preparation (Sigma) with activity against a broad range of P compounds. A comparatively large proportion (up to 79%, or 5.7 μg g^–1 soil) of the organic P (P_o) extracted with citric acid was hydrolysed by the commercial phytase, while between 28% and 40% (up to 3.1 μg g^–1 soil) was hydrolysed using purified phytase. By comparison, only small quantities of the P_o in water and NaHCO₃ soil extracts were enzyme labile. While extractable P_o was increased both with increasing concentrations of citric acid (up to 50 mM) and increasing pH (pH 2.3–6.0), enzyme-labile P increased only with citric acid concentration. The labile component of P_o in citric acid extracts from soils with contrasting fertiliser histories indicated that enzyme-labile P_o is a relatively large soil P pool and is potentially an important source of P for plants. Received: 29 October 1999     

Evaluation of biological and chemical nitrogen indices for predicting nitrogen-supplying capacity of paddy soils in the Taihu Lake region,China

Simple and rapid chemical indices of soil nitrogen (N)-supplying capacity are necessary for fertilizer recommendations. In this study, pot experiment involving rice, anaerobic incubation, and chemical analysis were conducted for paddy soils collected from nine locations in the Taihu Lake region of China. The paddy soils showed large variability in N-supplying capacity as indicated by the total N uptake (TNU) by rice plants in a pot experiment, which ranged from 639.7 to 1,046.2 mg N pot⁻¹ at maturity stage, representing 5.8% of the total soil N on average. Anaerobic incubation for 3, 14, 28, and 112 days all resulted in a significant (P < 0.01) correlation between cumulative mineral NH₄⁺-N and TNU, but generally better correlations were obtained with increasing incubation time. Soil organic C, total soil N, microbial C, and ultraviolet absorbance of NaHCO₃ extract at 205 and 260 nm revealed no clear relationship with TNU or cumulative mineral NH₄⁺-N. Soil C/N ratio, acid KMnO₄-NH₄⁺-N, alkaline KMnO₄-NH₄⁺-N, phosphate–borate buffer extractable NH₄⁺-N (PB-NH₄⁺-N), phosphate–borate buffer hydrolyzable NH₄⁺-N (PB_HYDR-NH₄⁺-N) and hot KCl extractable NH₄⁺-N (H_KCl−NH₄⁺-N) were all significantly (P < 0.05) related to TNU and cumulative mineral NH₄⁺-N of long-term incubation (>28 days). However, the best chemical index of soil N-supplying capacity was the soil C/N ratio, which showed the highest correlation with TNU at maturity stage (R = −0.929, P < 0.001) and cumulative mineral NH₄⁺-N (R = −0.971, P < 0.001). Acid KMnO₄-NH₄⁺-N plus native soil NH₄⁺-N produced similar, but slightly worse predictions of soil N-supplying capacity than the soil C/N ratio.     

Collembolans feeding on soil affect carbon and nitrogen mineralization by their influence on microbial and nematode activities

We manipulated Collembola Folsomia candida Willem density and observed the density effect on carbon and nitrogen mineralization and on nematodes in microcosms filled with mineral soil. Collembolan densities were 0 (control), 25 (low), 100 (medium), and 400 (high) individuals per microcosm. The Collembola enhanced soil respiration and nitrogen mineralization rate in a density-dependent manner (P < 0.05). The correlation between collembolan density and the metabolic quotient of microbes, qCO₂, was weakly positive (r = 0.44, P < 0.05). Collembola did not affect microbial biomass. These results suggested that enhanced carbon and nitrogen mineralization was an indirect effect of Collembola mediated by increased microbial activity. Collembola changed the C_nema/C_mic ratio, but only when present at the low density. Thus, Collembola had both positive and negative effects on the nematode population. The positive impact probably depends on the enhancement of microbial activity due to Collembola grazing behavior, while the negative effect appears to result from predation of nematodes.     

Effects of deforestation on phosphorus pools in mountain soils of the Alay Range, Khyrgyzia

 The amount, quality and turnover of soil P is heavily influenced by changes in soil management. The objective of this study was to investigate the effects of deforestation and pasture establishment on the concentrations, forms and turnover rate of soil P in mountain soils of the Alay Range, Khyrgyzia. A sequential extraction was applied to distinguish soil P pools. We used particle-size fractionation to follow the dynamics of different P pools in soils under forest and pasture and ³¹P-NMR spectroscopy to investigate the structure of alkali-soluble P forms. In the A horizons of the forest soils, total soil P concentration was 1093 mg kg^–1, organic P (P_o) representing 46% of the total P. Deforestation followed by pasture establishment not only increased significantly (P<0.01) the total P concentration (1560 mg kg^–1) but also the contribution of P_o to total P was increased by 17%. Pasture soils had significantly higher P pools than forest soils except highly labile inorganic P (P_i NaHCO₃) and primary P_i (P_i HCl_dil). Both in forest and pasture soils stable P increased with decreasing particle size (coarse sand 50%, clay 80% of total P) and primary P decreased with decreasing particle size. Phosphate monoesters and diesters represented 80% of P identified by ³¹P NMR. Low monoester to diester ratios in the alkali extracts of forest and pasture soils indicate low microbial activity. This is consistent with high C/P_o ratios and high stable P_o concentrations in the fine earth of forest and pasture. Received: 10 March 1999