Impact of carbon-rich dairy factory effluent on growth of perennial ryegrass (Lolium perenne) and soil microorganisms

A pot experiment was conducted to investigate the impact of high carbon dairy factory effluent application on the growth of perennial ryegrass (Lolium perenne L.), plant nutrient uptake, soil microbial biomass carbon and nitrogen, populations of soil-microorganisms, root colonising fungi and the microbial functional diversity. The effluent was added at rates of 0, 100,000, 200,000 and 300,000 l ha^–1. These rates are equivalent to 0, × 1, × 2 and × 3 normal field application rates. The added effluent contained (g l^–1), C; 19.42, total P; 0.65; S, 0.75, K; 1.33, Na; 4.55, Mg; 0.11, NH₄; 0.073, total N; 0.073 and had a pH of 4.33. Replicate pots (incubated in a controlled-environment room at 20 °C, with 16 h light/8 h dark) were harvested at 32, 61, and 130 days after setting up of the experiment. In the first sampling, shoot dry matter levels declined significantly (P < 0.01) with increased effluent. By the third sampling the trend was reversed with treated pots having greater amounts of shoot dry matter. The initial depression of growth was possibly due to a combination of factors including excess levels of available carbon (C) for microbes leading to immobilisation of nutrients, particularly nitrogen (N) and sulphur (S). Shoot N and S concentrations were lower (P < 0.001) and the phosphorus concentrations were higher in effluent-treated samples. Soil microbial biomass-C (480 and 770 μg g⁻¹ of biomass C in untreated and treated soil, respectively) and microbial-N (81 and 123 μg g^–1 of microbial-N in untreated and treated soil, respectively) were significantly (P < 0.001) greater in effluent-treated pots at all times. Populations of total culturable bacteria were higher (P < 0.01) in the treated pots in the first sample (log₁₀ populations g^–1 were 7.3 in untreated pots compared to 8.0 averaged across three treatments) but there were no differences in the subsequent two samples. Effluent also increased yeast populations (log₁₀ numbers g^–1 were 0.6 in untreated pots and 3.1 in treated pots averaged across treatments and times P < 0.01) at all three sampling times. The Shannon-Weiner Diversity Index of root fungi decreased with increasing effluent application (P < 0.01) while the species richness decreased with effluent as well as with time (P < 0.1). Potential root pathogens Fusarium oxysporum, total Fusarium spp. and Pythium spp. significantly increased (P < 0.05) in treated samples but in the final sampling, Codinaea fertilis significantly (P < 0.05) decreased with effluent treatment. The microbial functional diversity pattern and the average well colour development (AWCD) in soil were significantly changed by the effluent application but effects were not detectable after 130 days.     

Applying an oxygen-based respiratory assay to assess soil microbial responses to substrate and N availability

Documented approaches for measuring soil microbial activities and their controlling factors under field conditions are needed to advance understanding of soil microbial processes for numerous applications. We manipulated field plots with carbon (C) and nitrogen (N) additions to test the capability of a respiratory assay to: (1) measure respiration of endogenous soil C in comparison to field-measured CO₂ fluxes; (2) determine substrate-induced respiratory (SIR) activities that are consistent with substrate availability in the field; and, (3) report N availability in the field based on assay responses with and without added N. The respiratory assay utilizes a microplate containing an oxygen-sensitive fluorescent ruthenium dye. Respiratory activities measured with this approach have previously been shown to occur within short (6–8 h) incubation periods using low substrate concentrations that minimize enrichment during the assay. Field treatments were conducted in a randomized full-factorial design with C substrate (casamino acids, glucose, or none) and inorganic N (±) as the treatment factors. With one exception, we found that respiration of endogenous soil C in the assay responded to the field treatments in a similar manner to CO₂ fluxes measured in the field. Patterns of SIR with low concentrations of added amino acid or carbohydrate substrate (200 μg C g⁻¹ soil) were consistent with field treatments. The ratio (N_ratio) of carbohydrate respiration with added N (25 μg N g⁻¹ soil) to the same without N in the assay was significantly (P < 0.05) decreased by field N amendment. The carbohydrate N_ratio exhibited a logarithmic relationship (r = 0.64, P < 0.05) with extractable inorganic soil nitrate and ammonium concentrations. These data significantly extend and support the capability of this oxygen-based respiratory assay to evaluate in situ soil activities and examine factors that limit these activities.     

Animal manure and anhydrous ammonia amendment alter microbial carbon use efficiency,microbial biomass,and activities of dehydrogenase and amidohydrolases in semiarid agroecosystems

The potential negative impact of agricultural practices on soil and water quality is of environmental concern. The associated nutrient transformations and movements that lead to environmental concerns are inseparable from microbial and biochemical activities. Therefore, biochemical and microbiological parameters directing nitrogen (N) transformations in soils amended with different animal manures or inorganic N fertilizers were investigated. Soils under continuous corn cultivation were treated with N annually for 5 years at 56, 168, and 504 kg N ha⁻¹ in the form of swine effluent, beef manure, or anhydrous ammonia. Animal manure treatments increased dehydrogenase activity, microbial biomass carbon (C_mic) and N (N_mic) contents, and activities of amidohydrolases, including l-asparaginase, urease, l-glutaminase, amidase, and β-glucosaminidase. Soils receiving anhydrous ammonia demonstrated increased nitrate contents, but reduced microbiological and biochemical activities. All treatments decreased C_mic:organic C (C_org) ratios compared with the control, indicating reduced microbial C use efficiency and disturbance of C equilibrium in these soil environments. Activities of all enzymes tested were significantly correlated with soil C_org contents (P < 0.001, n = 108), but little correlation (r = 0.03, n = 36) was detected between C_mic and C_org. Activities of amidase and β-glucosaminidase were dominated by accumulated enzymes that were free of microbial cells, while activities of asparaginase and glutaminase were originated predominately from intracellular enzymes. Results indicated that soil microbial and biochemical activities are sensitive indicators of processes involved in N flow and C use efficiency in semiarid agroecosystems.     

Impact on soil fauna of sheep faeces containing a range of parasite control agents

To compare the impact of parasite control agents in sheep faeces, 1 kg quantities of fresh faeces were spread uniformly over 1 m² pasture plots in June 2001 (winter; a time of high earthworm activity). Faecal treatments applied to five replicate plots were C− (none), C+ (from untreated sheep), B (from sheep with an intra-ruminal bolus releasing a benzimidazole anthelmintic—‘albendazole’), ML (from sheep with a bolus releasing a macrocyclic lactone anthelmintic—‘ivermectin’), F (from sheep receiving a daily feed supplement containing chlamydospores of the nematophagous fungus, Duddingtonia flagrans). The disappearance of faeces was assessed visually over the 50 days following faecal application, then soil samples were taken to assess: (a) populations of earthworms and other soil macrofauna, (b) nematodes and other soil microfauna, and (c) the presence of D. flagrans in soil. Faecal disappearance was greatest in F and C+ plots and least in ML and B plots at 12 and 23 days (P < 0.05). Earthworm casting after 23 and 50 days was greater (P < 0.05) in plots with faeces (C+, ML, F, but not B) than in plots without faeces (C−). Greater earthworm activity in plots with faeces was reflected in greater numbers of earthworms, cocoons and greater biomass m⁻² than in C− plots. On the basis of faecal dry weight applied, F plots had most earthworms and ML plots the least. After 50 days total nematodes in 0–5 cm soil showed a treatment effect (P < 0.001), being more abundant in F, C+ and B than in C− and ML plots; enchytraeids, rotifers, tardigrades and copepods showed no treatment effects. A few nematode taxa (Acrobeles, Alaimus, Pungentus, Tylencholaimus) showed significant treatment effects. The greatest effect among nematodes was in nematode channel ratio (NCR) (P < 0.008), with a decrease in F plots; changes in NCR may reflect the impact of earthworm activity on soil processes rather than a direct effect of the fungal treatment on nematodes. D. flagrans did not become established in the soil. During the trial conditions were favourable for earthworms and their activity was high in all treatments receiving faeces, with F and ML plots being the extremes. There was an apparent shift towards fungal-mediated decomposition in F plots. At the end of the 50-day trial, in a period when earthworms were active, there was no evidence of differential effects of any of the anthelmintic treatments on environmental indicators.     

Dynamic changes of bacterial community under the influence of bacterial-feeding nematodes grazing in prometryne contaminated soil

Microcosm experiments were carried out to study the effects of bacterial-feeding nematodes and prometryne on soil bacterial communities in contaminated soil. Prometryne (5 or 10 mg kg⁻¹ dry soil, that is, P5 or P10) and bacterial-feeding nematodes (5 or 10 individuals g⁻¹ dry soil, that is, N5 or N10), singly and in combination (P5N5, P5N10, P10N5, P10N10), were added to a nematode-free soil. An uncontaminated nematode-free soil was studied for comparison (Control). Bacterial-feeding nematode grazing boosted soil enzyme activities in contaminated soils, thus speeding up prometryne degradation. In the initial stage of the experiment, prometryne enhanced the soil enzyme activities too, but served the opposite purpose later. Denaturing gradient gel electrophoresis (DGGE) analysis indicated that prometryne contamination and nematode grazing over the incubation period exerted an obvious impact on Species richness (S), Shannon–Wiener index (H′) and Evenness (E_H) of soil bacteria, which increased initially, then decreased and increased again later. The cluster analysis of DGGE profiles showed that the similarity of soil bacterial communities in all treatments with indigenous microbes, P5, P5N5, P5N10, P10, P10N5, and P10N10 and the Control was 75%, 44%, 78% and 49% at Day 0, Day 8, Day 18 and Day 30, respectively. Compared to the Control, DGGE profiles displayed a varying characteristic bands pattern in all treatments over the incubation period with certain bands present in the treatments while not in the Control and vice versa, suggesting that bacterial-feeding nematode grazing and prometryne contamination affected soil bacterial communities evidently. Consequently, when added to contaminated soil, bacterial-feeding nematodes can contribute to restoration of contaminated sites by degrading toxic compounds like prometryne through enhanced microbial activity.     

Accounting for variability in soil microbial communities of temperate upland grassland ecosystems

This study aimed to determine the factors which regulate soil microbial community organisation and function in temperate upland grassland ecosystems. Soil microbial biomass (C_mic), activity (respiration and potential carbon utilisation) and community structure (phospholipid fatty acid (PLFA) analysis, culturing and community level physiological profiles (CLPP) (Biolog^®)) were measured across a gradient of three upland grassland types; Festuca–Agrostis–Galium grassland (unimproved grassland, National Vegetation Classification (NVC) — U4a); Festuca–Agrostis–Galium grassland, Holcus–Trifolium sub-community (semi-improved grassland, NVC — U4b); Lolium–Cynosurus grassland (improved grassland, NVC — MG6) at three sites in different biogeographic areas of the UK over a period of 1 year. Variation in C_mic was mainly due to grassland type and site (accounting for 55% variance, v, in the data). C_mic was significantly (P<0.001) high in the unimproved grassland at Torridon (237.4 g C m⁻² cf. 81.2 g C m⁻² in semi- and 63.8 g C m⁻² in improved grasslands) and Sourhope (114.6 g C m⁻² cf. in 44.8 g C m⁻² semi- and 68.3 g C m⁻² in improved grasslands) and semi-improved grassland at Abergwyngregyn (76.0 g C m⁻² cf. 41.7 g C m⁻² in un- and 58.3 g C m⁻² in improved grasslands). C_mic showed little temporal variation (v=3.7%). Soil microbial activity, measured as basal respiration was also mainly affected by grassland type and site (n=32%). In contrast to C_mic, respiration was significantly (P<0.001) high in the improved grassland at Sourhope (263.4 l h⁻¹m⁻² cf. 79.6 l h⁻¹m⁻² in semi- and 203.9 l h⁻¹m⁻² unimproved grasslands) and Abergwyngregyn (198.8 l h⁻¹m⁻² cf. 173.7 l h⁻¹m⁻² in semi- and 88.2 l h⁻¹m⁻² unimproved grasslands). Microbial activity, measured as potential carbon utilisation, agreed with the respiration measurements and was significantly (P<0.001) high in the improved grassland at all three sites (A₅₉₀ 0.14 cf. 0.09 in semi- and 0.07 in unimproved grassland). However, date of sampling also had a significant (P<0.001) impact on C utilisation potential (v=24.7%) with samples from April 1997 having highest activity at all three sites. Variation in microbial community structure was due, predominantly, to grassland type (average v=23.6% for bacterial and fungal numbers and PLFA) and date of sampling (average v=39.7% for bacterial and fungal numbers and PLFA). Numbers of culturable bacteria and bacterial PLFA were significantly (P<0.001) high in the improved grassland at all three sites. Fungal populations were significantly (P<0.01) high in the unimproved grassland at Sourhope and Abergwyngregyn. The results demonstrate a shift in soil microbial community structure from one favouring fungi to one favouring bacteria as grassland improvement increased. Numbers of bacteria and fungi were also significantly (P<0.001) higher in August than any other sampling date. Canonical variate analysis (CVA) of the carbon utilisation data significantly (P<0.05) differentiated microbial communities from the three grassland types, mainly due to greater utilisation of sugars and citric acid in the improved grasslands compared to greater utilisation of carboxylic acids, phenolics and neutral amino acids in the unimproved grasslands, possibly reflecting substrate availability in these grasslands. Differences in C_mic, activity and community structure between grassland types were robust over time. In addition, broad scale measures of microbial growth and activity (C_mic and respiration) showed little temporal variation compared to measures of soil microbial community structure, which varied quantitatively with respect to environmental variables (temperature, moisture) and plant productivity, hence substrate supply.     

Climatic influences on active fractions of soil organic matter

Biologically active fractions of soil organic matter are important in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28±11 mg g⁻¹ (n=24) in a frigid–dry region (Alberta/British Columbia), 25±5 mg g⁻¹ (n=12) in a frigid–wet region (Maine), 11±4 mg g⁻¹ (n=117) in a thermic–dry region (Texas), and 12±5 mg g⁻¹ (n=131) in a thermic–wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg CO₂–C g⁻¹ SOC d⁻¹ in the thermic compared with the frigid regions, P<0.001), greater net N mineralization (2.8 vs 1.3 mg inorganic N g⁻¹ SOC 24 d⁻¹, P<0.001), and greater SMBC (53 vs 21 mg SMBC g⁻¹ SOC, P<0.001). Specific respiratory activity of SMBC was, however, consistently lower in the thermic than in the frigid regions (29 vs 34 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg CO₂–C g⁻¹ SOC d⁻¹ in the wet compared with the dry regions, P<0.01) and lower SMBC (31 vs 43 mg SMBC g⁻¹ SOC, P<0.001), but had inconsistent effects on net N mineralization that depended upon temperature regime. Specific respiratory activity of SMBC was consistently greater in the wet than the dry regions (≈33 vs 29 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Although the thermic regions were not able to retain as high a level of SOC as the frigid regions, due likely to high annual decomposition rates, biologically active soil fractions were as high per mass of soil and even 2–3-times greater per unit of SOC in the thermic compared with the frigid regions. These results suggest that macroclimate has a large impact on the portion of soil organic matter that is potentially active, but a relatively small impact on the specific respiratory activity of SMBC.     

Relationships between soil pH and microbial properties in a UK arable soil

Effects of changing pH along a natural continuous gradient of a UK silty-loam soil were investigated. The site was a 200 m soil transect of the Hoosfield acid strip (Rothamsted Research, UK) which has grown continuous barley for more than 100 years. This experiment provides a remarkably uniform soil pH gradient, ranging from about pH 8.3 to 3.7. Soil total and organic C and the ratio: (soil organic C)/(soil total N) decreased due to decreasing plant C inputs as the soil pH declined. As expected, the CaCO₃ concentration was greatest at very high pH values (pH > 7.5). In contrast, extractable Al concentrations increased linearly (R² = 0.94, p < 0.001) from below about pH 5.4, while extractable Mn concentrations were largest at pH 4.4 and decreased at lower pHs. Biomass C and biomass ninhydrin-N were greatest above pH 7. There were statistically significant relationships between soil pH and biomass C (R² = 0.80, p < 0.001), biomass ninhydrin-N (R² = 0.90, p < 0.001), organic C (R² = 0.83, p < 0.001) and total N (R² = 0.83, p < 0.001), confirming the importance of soil organic matter and pH in stimulating microbial biomass growth. Soil CO₂ evolution increased as pH increased (R² = 0.97, p < 0.001). In contrast, the respiratory quotient (qCO₂) had the greatest values at either end of the pH range. This is almost certainly a response to stress caused by the low p. At the highest pH, both abiotic (from CaCO₃) and biotic Co₂ will be involved so the effects of high pH on biomass activity are confounded. Microbial biomass and microbial activity tended to stabilise at pH values between about 5 and 7 because the differences in organic C, total N and Al concentrations within this pH range were small. This work has established clear relationships between microbial biomass and microbial activity over an extremely wide soil pH range and within a single soil type. In contrast, most other studies have used soils of both different pH and soil type to make similar comparisons. In the latter case, the effects of soil pH on microbial properties are confounded with effects of different soil types, vegetation cover and local climatic conditions.     

Effects of tillage,organic resources and nitrogen fertiliser on soil carbon dynamics and crop nitrogen uptake in semi-arid West Africa

Tillage, organic resources and fertiliser effects on soil carbon (C) dynamics were investigated in 2000 and 2001 in Burkina Faso (West Africa). A split plot design with four replications was laid-out on a loamy-sand Ferric Lixisol with till and no-till as main treatments and fertiliser types as sub-treatments. Soil was fractionated physically into coarse (0.250–2 mm), medium (0.053–0.250 mm) and fine fractions (< 0.053 mm). Particulate organic carbon (POC) accounted for 47–53% of total soil organic carbon (SOC) concentration and particulate organic nitrogen (PON) for 30–37% of total soil nitrogen concentration. The POC decreased from 53% of total SOC in 2000 to 47% of total SOC in 2001. Tillage increased the contribution of POC to SOC. No-till led to the lowest loss in SOC in the fine fraction compared to tilled plots. Well-decomposed compost and single urea application in tilled as well as in no-till plots induced loss in POC. Crop N uptake was enhanced in tilled plots and may be up to 226 kg N ha⁻¹ against a maximum of 146 kg N ha⁻¹ in no-till plots. Combining crop residues and urea enhanced incorporation of new organic matter in the coarse fraction and the reduction of soil carbon mineralisation from the fine fraction. The PON and crop N uptake are strongly correlated in both till and no-till plots. Mineral-associated N is more correlated to N uptake by crop in tilled than in no-till plots. Combining recalcitrant organic resources and nitrogen fertiliser is the best option for sustaining crop production and reducing soil carbon decline in the more stabilised soil fraction in the semi-arid West Africa.     

Phosphate additions have no effect on microbial biomass and activity in a northern hardwood forest

High rates of atmospheric nitrogen (N) deposition have raised questions about shifting patterns of nutrient limitation in northern hardwood forests. Of particular interest is the idea that increased supply of N may induce phosphorus (P) limitation of plant and microbial processes, especially in acid soils where P sorption by Al is high. In this study, we established field plots and plant-free laboratory mesocosms with P and Ca additions to test the hypotheses that 1) microbial biomass and activity are limited by P in the northern hardwood forest soils at the Hubbard Brook Experimental Forest in NH USA; 2) elevated Ca increases inherent P availability and therefore reduces any effects of added P and 3) P effects are more marked in the more carbon (C) rich Oie compared to the Oa horizon. Treatments included P addition (50 kg P ha⁻¹), Ca addition (850 kg Ca ha⁻¹) and Ca + P addition (850 kg Ca ha⁻¹ and 50 kg P ha⁻¹). The P treatments increased resin-available P levels and reduced phosphatase activity, but had no effect on microbial biomass C, microbial respiration, C metabolizing enzymes, potential net N mineralization and nitrification in the Oie or Oa horizon of either field plots or plant free mesocosms, in either the presence or absence of Ca. Total, prokaryote, and eukaryote PLFA were reduced by P addition, possibly due to reductions in mycorrhizal fungal biomass. These results suggest that increased N deposition and acidification have not created P limitation of microbial biomass and activity in these soils.     

Nitrous oxide production in turfgrass systems: Effects of soil properties and grass clipping recycling

Soil N₂O emissions can affect global environments because N₂O is a potent greenhouse gas and ozone depletion substance. In the context of global warming, there is increasing concern over the emissions of N₂O from turfgrass systems. It is possible that management practices could be tailored to reduce emissions, but this would require a better understanding of factors controlling N₂O production. In the present study we evaluated the spatial variability of soil N₂O production and its correlation with soil physical, chemical and microbial properties. The impacts of grass clipping addition on soil N₂O production were also examined. Soil samples were collected from a chronosequence of three golf courses (10, 30, and 100-year-old) and incubated for 60 days at either 60% or 90% water filled-pore space (WFPS) with or without the addition of grass clippings or wheat straw. Both soil N₂O flux and soil inorganic N were measured periodically throughout the incubation. For unamended soils, cumulative soil N₂O production during the incubation ranged from 75 to 972 ng N g⁻¹ soil at 60% WFPS and from 76 to 8842 ng N g⁻¹ soil at 90% WFPS. Among all the soil physical, chemical and microbial properties examined, soil N₂O production showed the largest spatial variability with the coefficient of variation ~110% and 207% for 60% and 90% WFPS, respectively. At 60% WFPS, soil N₂O production was positively correlated with soil clay fraction (Pearson's r = 0.91, P < 0.01) and soil NH₄⁺–N (Pearson's r = 0.82, P < 0.01). At 90% WFPS, however, soil N₂O production appeared to be positively related to total soil C and N, but negatively related to soil pH. Addition of grass clippings and wheat straw did not consistently affect soil N₂O production across moisture treatments. Soil N₂O production at 60% WFPS was enhanced by the addition of grass clippings and unaffected by wheat straw (P < 0.05). In contrast, soil N₂O production at 90% WFPS was inhibited by the addition of wheat straw and little influenced by glass clippings (P < 0.05), except for soil samples with >2.5% organic C. Net N mineralization in soil samples with >2.5% organic C was similar between the two moisture regimes, suggesting that O₂ availability was greater than expected from 90% WFPS. Nonetheless, small and moderate changes in the percentage of clay fraction, soil organic matter content, and soil pH were found to be associated with large variations in soil N₂O production. Our study suggested that managing soil acidity via liming could substantially control soil N₂O production in turfgrass systems.     

Soil carbon dynamics and crop residue yields of cropping systems in the Northern Guinea Savanna of Burkina Faso

Concerns about effects of increasing atmospheric concentration of carbon dioxide (CO₂) on climate has given rise to the possibility of emission credits for soil organic carbon (SOC) sequestration. The goal of this study was to analyze SOC sequestration options in cropping systems of the Northern Guinea Savanna of West Africa. An 11-year experiment from the region, which consisted of 56 cropping system treatments that combined various crop rotation sequences with various input levels and an additional treatment of native grass fallow, was analyzed. Rotations included one or more of: sorghum (Sorghum bicolor (L.) Moench), cotton (Gossypium hirsutum L.), groundnut (Arachis hypogaea L.), maize (Zea mays L.) and native grass fallow. Inputs were defined by whether or not the plots were plowed and the addition of soil amendments (N, nitrogen; P, phosphorous; K, potassium; D, dolomite; CR, crop residues; CP, compost and ME, manure). Plots were moldboard plowed before seeding, except fallows, which were not plowed. Soil organic carbon in select treatments and residue yields from all cropped treatments were analyzed. The slope parameters from the regression analysis of SOC in the continuous fallow treatment were not significantly different from zero (P > 0.05), suggesting SOC (0.53% after 11 years) was at steady state in this treatment. Rotation and input significantly affected SOC (P < 0.05), but interaction effects were not significant. After 11 years, the cropped rotation with the greatest SOC was sorghum-fallow (0.46%), which was significantly greater (P < 0.05) than SOC in other the rotations measured: continuous cotton (0.36%), continuous sorghum (0.35%), and cotton–maize–sorghum (0.33%). For the input levels, addition of P, K, and ME gave the greatest SOC (0.44%) after 11 years of cropping, which was significantly greater (P < 0.05) than SOC from the N, P, K and D (0.37%), no input (0.32%) and N, P and K (0.34%) treatments. In addition, SOC with inputs of N, P, K and D (0.37%) was significantly greater than SOC with no input (0.32%). Three management practices, which had significantly greater SOC than others and were among the best for yields, were identified as sequestering management options for the region. These were rotating sorghum and fallow, and amending the soil with mineral P, K, and ME or N, P, K and D. However, potential drawbacks, such as a risk of reduced production with increased fallows, must be identified and addressed if the options are to be adopted.     

Influence of organic Crotalaria juncea hay and ammonium nitrate fertilizers on soil nematode communities

Comparisons of organic and inorganic fertilizer effects on nematode communities depend on the specific organic fertilizer used. Field experiments were conducted during 2001 and 2002 in a squash (Cucurbita pepo) agroecosystem to determine if applying sunn hemp (Crotalaria juncea) hay as an organic fertilizer improved nematode communities involved in soil nutrient cycling compared to an equivalent N rate (100 kg N/ha) of ammonium nitrate. Fertilizer source had minimal effect on nematode communities in 2001 when treatments were applied after a winter cover crop of oats (Avena sativa), but differences (P ≤ 0.05) between the fertilizer sources occurred in 2002 when no winter cover cropping preceded squash. Fertilization with sunn hemp hay increased abundance of the bacterivore guilds Ba₁ and Ba₂, and increased fungivores at the end of the experiment. Compared to ammonium nitrate, fertilization with sunn hemp hay resulted in a community with lower maturity index, higher enrichment index, and lower channel index, consistent with a disturbed and nutrient-enriched soil food web undergoing bacterial decomposition. Sunn hemp hay occasionally stimulated omnivorous nematodes, but suppressed plant-parasitic nematodes relative to ammonium nitrate fertilizer. Increasing the sunn hemp hay rate to 200 kg N/ha increased the abundance of bacterivores, fungivores, and predatory nematodes, and total nematode abundance compared to hay at 100 kg N/ha. Fertilization with ammonium nitrate increased the percentage of herbivores, but reduced percentage and abundance of omnivores. In conclusion, sunn hemp fertilizer maintained greater numbers of nematodes involved in nutrient cycling as compared to ammonium nitrate.     

Responses of soil nematode community structure to different long-term fertilizer strategies in the soybean phase of a soybean–wheat–corn rotation

The impact of long-term application of fertilizers in soybean fields on soil nematode community structure was studied. The long-term application model of fertilizers lasted 13 years in a soybean–wheat–corn rotation, and included three treatments: no fertilizer (NF), chemical fertilizer (urea and ammonium phosphate, CF), and pig manure combined with chemical fertilizer (MCF). The soil nematode community structures and ecological indices were determined from soil samples taken at five soybean growth stages from May to October in the soybean phase of the rotation. Fertilizer application had significant effects on abundance of plant parasites, bacterivores and fungivores (P < 0.05), but had no significant effects on total nematodes and omnivores-predators. Abundance of plant parasites was higher in NF than in MCF and CF, and abundance of bacterivores was highest in MCF. Fertilizer application significantly affected Plant-parasitic Nematode Maturity Index (PPI) and Nematode Channel Ratio (NCR) ecological indices (P < 0.05). Shannon–Weaver Index (H′) and Species Richness (SR) indices were higher in MCF than in either NF or CF. The abundances of total nematode and plant parasites showed increasing trend with soybean growth in all three treatments. This is probably due to soil environment being more suitable for soil nematode survival with more food available for plant parasites as the soybean grows. Soybean growth stage significantly affected the H′, Free Living Nematode Maturity Index (MI) and PPI. Bacterivores significantly correlated with soil nutrient status suggesting that they could be used as a potential indicator of soil fertility.     

Soil available phosphorus status determines indigenous mycorrhizal colonization of field and glasshouse-grown spring wheat from Argentina

Wheat production (Triticum aestivum L.) has increased across the world during last century with the intensification of agriculture. Phosphorus (P) fertilization is a common practice to improve wheat growth in Argentina. We investigate whether indigenous arbuscular mycorrhizal colonization (AMC) of hard red spring wheat is controlled by shoot P content (SPc) or by available soil P in an agricultural soil from the southeastern Argentine Pampas. In the field, AMC was monitored four times during two growing seasons of a conventional wheat crop. Treatments were: without P supply, annual supply of 11 and 22 kg P ha⁻¹ during the last 5 years, and 164 kg P ha⁻¹ applied once 5 years before the experiment. In the glasshouse, AMC was assessed three times in wheat growing in pots filled with the soil from unfertilized plots; treatments were: P (0 and 20 mg P pot⁻¹), and nitrogen (N) fertilization (0 and 150 mg N pot⁻¹). A range of soil P between 6 and 60 mg P kg⁻¹ was obtained and the AMC ranged from 1% to 67% of root length colonized under both field and glasshouse conditions. P supplied annually increased growth and SPc but decreased AMC. N fertilization did not affect growth or AMC. Variations in SPc did not account for AMC. Variability in AMC was best accounted for local current soil available P content (r² = 0.59). A linear-plateau relationship between soil P and indigenous AMC was established in wheat plants growing under contrasting environmental and experimental (field and glasshouse) conditions. Indigenous AMC was depressed by available soil P in the range 0–27 mg P kg⁻¹ (a decrease of 2.8% mg P⁻¹ kg⁻¹). Above 27 mg P kg soil⁻¹, AMC was stabilized at about 10%. Grain yield increased with fertilization and the highest relative shoot dry matter in field was obtained at 15.5 mg P kg soil⁻¹. The soil P range that ensures high wheat production without deterring indigenous AMC is discussed.     

Soil microbiological and biochemical properties affected by plant growth and different long-term fertilisation

The purpose of this study was to measure the effects of plant growth on soil microbial biomass C (C_mic) and soil enzyme activities. In a pot experiment using spring barley and sugar beet, we investigated the response of C_mic, hot water extractable C and N fractions (C_hwe, N_hwe), and enzyme activities involved in C, N and P cycling in a loess-derived Chernozem from Bad Lauchstädt (Central Germany). The study site has been receiving the same fertilisation treatments for 100 years. The soil originated from plots fertilised with 15 t ha⁻¹ farmyard manure (FYM) year⁻¹ + mineral fertiliser (NPK), or 15 t ha⁻¹ FYM year⁻¹, or NPK or from an unfertilised control. Pots were sampled monthly, and alkaline phosphatase- (AP), β-glucosidase- and protease-activities were analysed. At the beginning and the end of study, levels of C_mic, C_hwe and N_hwe were also measured. All three enzyme activities and C_mic were significantly and positively correlated with C_org and C_hwe. Results suggest that the enzyme activities measured originated mostly from microorganisms and that C_hwe is an important C source for soil microorganisms. β-Glucosidase and AP activities were higher in summer months than at other times. In contrast, protease activity changed only slightly during the growing period.     

Effect of fertilizer,herbicide and grazing management of pastures on plant and soil communities

Increases in fertilizer inputs and livestock numbers affect plant species composition and richness; this in turn can affect the biodiversity of soil fauna and nutrient cycling in pastures. We selected two adjacent farmlets to study these effects. Since 1980, one farmlet (LF) had not received superphosphate fertilizer (SSP) and has a low stock density of sheep, and the other (HF) had received 37.5 g SSP m⁻² y⁻¹ and has a high stock density. In 2004, at both farmlets, we commenced treatments for 4 years, adding urea to raise N status, and non-residual selective herbicide to remove broadleaf species. Long-term SSP addition and increased sheep numbers, and added urea increased herbage production but reduced plant diversity. The effect of treatments on most of the soil biochemical and biological properties varied between years. This may have partly arisen from an infestation with Wiseana caterpillars in the first winter, causing resources to be low and total soil carbon (C) to be reduced by 4–8%; total C did, however, recover in later years. The urea and herbicide treatments caused greater changes above-ground than below-ground, but they did reduce soil microbial C and N and nematode diversity; urea at LF increased mineralizable N to the levels found at HF. On an area basis, HF generally had higher total C and N, earthworm and nematode numbers (including bacterial feeders, predators and omnivores), and nematode diversity, and greater values for the nematode channel ratio, than did LF. In contrast, the ratios of microbial C/total C and microbial N/total N, total mite numbers (including Oribatida, but not other mite groups), and fungal-feeding nematode numbers were higher at LF than at HF. Canonical correlation analysis suggested the plant and soil nematode communities responded in tandem and in predictable ways to the same environmental factors. Increased quantity and quality of inputs disadvantaged the fungal-based energy channel, with a measurable decline in the quantity of fungal phospholipid fatty acids (PLFAs). While the quantity of bacterial PLFAs appeared to be unaffected by greater plant-derived inputs, the greater numbers of bacterial-feeding nematodes at the HF farmlet suggests the activity and flow of energy and nutrients through the bacterial community would be more important in the HF than the LF farmlet. Overall our results suggest the shift from fungal to bacterial pathways may lead to soil microbial/microfaunal interactions that retain less reactive N within soil biomass, with a consequent greater risk of N loss.     

Changes of microbial properties in (near-) rhizosphere soils after phytoextraction by Sedum alfredii H: A rhizobox approach with an artificial Cd-contaminated soil

The ultimate goal of soil remediation is to restore soil health. Soil microbial parameters are considered to be effective indicators of soil health. The aim of this study was to determine the effects of phytoextraction on microbial properties through the measurement of soil microbial biomass carbon, soil basal respiration and enzyme activities. For this purpose, a pre-stratified rhizobox experiment was conducted with the Cd hyperaccumulator Sedum alfredii H. for phytoextraction Cd from an artificial contaminated soil (15.81 mg kg⁻¹) under greenhouse conditions. The plant and soil samples were collected after growing the plant for three and six months with three replications. The results indicated that the ecotype of S. alfredii H. originating from an ancient silver mining site was a Cd-hyperaccumulator as it showed high tolerance to Cd stress, the shoot Cd concentration were as high as 922.6 mg kg⁻¹ and 581.9 mg kg⁻¹ at the two samplings, and it also showed high BF (58.4 and 36.8 after 3 and 6 months growth), and TF (5.8 and 5.1 after 3 and 6 months growth). The amounts of Cd accumulated in the shoots of S. alfredii reached to an average of 1206 μg plant⁻¹ after 6 months growth. Basal respiration, invertase and acid phosphatase activities of the rhizosphere soil separated by the shaking method were significantly higher (P < 0.01) than that of the near-rhizosphere soil and the unplanted soil after 3 months growth, so were microbial biomass carbon, urease, invertase and acid phosphatase activities of the rhizosphere soil after 6 months growth. Acid phosphatase activity of the 0–2 mm sub-layer rhizosphere soil collected by the pre-stratified method after 3 months growth was significantly higher (P < 0.05) than that of other sub-layer rhizosphere soils and bulk soil, and so were microbial biomass carbon, basal respiration, urease, invertase and acid phosphatase activities of the 0–2 mm sub-layer rhizosphere soil after 6 months growth. It was concluded that phytoextraction by S. alfredii could improve soil microbial properties, especially in rhizosphere, and this plant poses a great potential for the remediation of Cd contaminated soil.     

Long-term management impacts on soil C,N and physical fertility: Part II: Bad Lauchstadt static and extreme FYM experiments

Manure is a source of plant nutrients and can make a valuable contribution to soil organic matter (SOM). Two experimental sites were studied on a Halpic Phaeozem soil near Bad Lauchstadt in Germany. The first experiment, called the static experiment, commenced in 1902. The impact of fresh farmyard manure (FYM) (0, 20 and 30 t ha⁻¹ 2 year⁻¹) combined with P, K and N fertiliser application on total organic C (C_T), labile C (C_L), non-labile C (C_NL), total N (N_T), mean weight diameter (MWD) and unsaturated hydraulic conductivity (K_unsat) was investigated. The second experiment commenced in 1984 and investigated the effect of extreme rates of fresh FYM applications (0, 50, 100 and 200 t ha⁻¹ year⁻¹) and cropping, or a continuous tilled fallow on the same soil properties. At both sites a nearby grassland site served as a reference. On the static experiment, FYM application increased all C fractions, particularly C_L, where application of 30 t ha⁻¹ 2 year⁻¹ increased C_L by 70% compared with no FYM application. Fertiliser additions to the static experiment had a positive influence on C fractions while N_T increased from both FYM and fertiliser application. MWD increased as a result of FYM application, but did not reach that of the grassland site. Both fertiliser and FYM application increased K_unsat (10 mm tension) on the static experiment. In the second experiment application of 200 t ha⁻¹ year⁻¹ of FYM increased concentrations of C_L by 173% and of C_NL by 80%, compared with no FYM application to make them equivalent to, or greater than the grassland site. A continuously tilled fallow resulted in significant decreases in all C fractions, N_T and MWD compared with the cropped site, while K_unsat (10 mm tension) was increased on the 0 and 50 t ha⁻¹ year⁻¹ treatments as a result of a recent tillage. There was no difference in K_unsat between the cropped and the continuous tilled fallow at FYM applications of 100 and 200 t ha⁻¹ year⁻¹. There were similar significant positive correlations of all C fractions and N_T with MWD on both experimental sites but the relationships were much stronger on the extreme FYM experiment. Weaker relationships of C fractions and N_T with K_unsat (10 mm tension) occurred for the static experimental site but these were not significant for the extreme FYM experimental site. The strongest relationship between C fractions and K_unsat was with C_L. This research has shown that applications of FYM can increase SOM and improve soil physical fertility. However, the potential risk of very high rates of FYM on the environment need to be taken into consideration, especially since the application of organic materials to soils is likely to increase in the future.     

Diversity and abundance of earthworms in land use systems in central-western Colombia

The neotropical landscapes of Colombia's Andean region are characterized by a mosaic of agroecosystems presenting a range of vegetational cover for which soil fauna adaptation is still unknown. To analyze the diversity and abundance of earthworm species in relation to changes in selected soil physical and chemical parameters (bulk density, C stock, N stock, %C, %N, ¹³C, ¹⁵N, C:N ratio), these systems were classified into five categories according to land use intensity: non-intensive (NI), low intensity (LI), medium intensity (MI), high intensity (HI), and maximum intensity (IN). The influence of livestock production was confirmed in the significant differences observed in bulk density and ¹⁵N between NI, HI, and IN (P < 0.05). The C and N contents of IN systems (pastures) differed significantly (P < 0.05) in comparison with the other categories. Average δ ¹³C at the three soil depths evaluated (0–10, 10–20, and 20–30 cm) ranged between ?24.9 for LI and ?22.87 for IN, indicating that soil organic carbon was related to C3-type vegetation for all land uses and vegetation covers. Overall, 26 earthworm species were recorded, of which 16 showed a high capacity to adapt to natural, NI, LI, and MI systems. Depending on land use intensity, significant differences were also observed (P < 0.05) in the origin and ecological category of earthworms, mainly in terms of the following variables: diversity of native species, diversity of endogeic species, abundance of native species, biomass of native species, and abundance of exotic species. Based on the results obtained, the most favorable mosaic systems for agrobiodiversity conservation were identified as well as those subsystems that require special management to solve problems of habitat degradation.