Margaret M. Roper

Biological N2 fixation by heterotrophic and phototrophic bacteria in association with straw

Much of the crop residues, including cereal straw, that are produced worldwide are lost by burning. Plant residues, and in particular straw, contain large amounts of carbon (cellulose and hemicellulose) which can serve as substrates for the production of microbial biomass and for biological N2 fixation by a range of free-living, diazotrophic bacteria. Microorganisms with the dual ability to utilise cellulose and fix N2 are rate, but some strains that utilize hemicellulose and fix N2 have been found. Generally, cellulolysis and diazotrophy are carried out by a mixed microbial community in which N2-fixing bacteria utilise cellobiose and glucose produced from straw by cellulolytic microorganisms. N2-fixing bacteria include heterotrophic and phototrophic organisms and the latter are apparently more prominent in flooded soils such as rice paddies than in dryland soils. The relative contributions of N2 fixed by heterotrophic diazotrophic bacteria compared with cyanobacteria and other phototrophic bacteria depend on the availability of substrates from straw decomposition and on environmental pressures. Measurements of asymbiotic N2 fixation are limited and variable but, in rice paddy systems, rates of 25 kg N ha-1 over 30 days have been found, whereas in dryland systems with wheat straw, in situ measurements have indicated up to 12 kg N ha-1 over 22 days. Straw-associated N2 fixation is directly affected by environmental factors such as temperature, moisture, oxygen concentration, soil pH and clay content as well as farm management practices. Modification of managements and use of inoculants offer ways of improving asymbiotic N2 fixation.In laboratory culture systems, inoculation of straws with cellulolytic and diazotrophic microorganisms has resulted in significant increases in N2 fixation in comparison to uninoculated controls and gains of N of up to 72 mg N fixed g-1 straw consumed have been obtained, indicating the potential of inoculation to improve N gains in composts that can then be used as biofertilisers. Soils, on the other hand, contain established, indigenous microbial populations which tend to exclude inoculant microorganisms by competition. As a consequence, improvements in straw-associated N2 fixation in soils have been achieved mostly by specific straw-management practices which encourage microbial activity by straw-decomposing and N2-fixing microorganisms.Further research is needed to quantify more accurately the contribution of asymbiotic N2 fixation to cropping systems. New strains of inoculants, including those capable of both cellulolytic and N2-fixing activity, are needed to improve the N content of biofertilisers produced from composts. Developments of management practices in farming systems may result in further improvements in N2 fixation in the field.

Straw decomposition and nitrogenase activity (C2H2 reduction): Effects of soil moisture and temperature

Abstract The effects of moisture and temperature on straw decomposition (CO 2 production) and nitrogenase activity (C 2 H 2 reduction) were measured in laboratory experiments to evaluate the potential for nitrogenase activity at various times of the year in soils with added wheat straw. Soils collected from two areas (Gunnedah and Cowra) representative of large areas of the wheat belt of New South Wales, Australia, were examined. Straw decomposed over a wide range of temperatures (20–50°C, Gunnedah; 15–45°C, Cowra) and moistures (0.3–2.0 times −10 kPa water content). Microbial populations that fix nitrogen were adapted to a broad range of temperatures (10–45°C, Gunnedah; 4–40°C; Cowra). However, nitrogenase activity with added glucose occurred at much lower soil moisture contents in the Gunnedah soil (0.5–1.75 times −10 kPa water content) than in the Cowra soil (1.0–2.5 times −10 kPa water content). Despite the differences between the soils the results show that there is potential for straw decomposition and nitrogenase activity throughout most of the year.

Straw decomposition and nitrogenase activity (C2H2 reduction) by free-living microorganisms from soil: Effects of pH and clay content

Abstract The effects of pH and clay content on decomposition of straw (CO2 production) and nitrogenase activity (C2H2 reduction) were measured in laboratory experiments using microorganisms extracted from soils collected from two areas (Gunnedah and Cowra) of the wheat belt of New South Wales, Australia. Assay systems containing sand and various substrates or finely-ground straw were inoculated with soil:water (1:5) extracts prepared from these soils. Straw decomposed at all the levels of pH tested (initial pH 5.5–9.6) although decomposition was inhibited partially in the presence of added clay (5% montmorillonite). Nitrogenase activity by free-living bacteria, extracted from both soils, was best when the initial pH was 7–7.5, regardless of the pH of the original soil (Gunnedah, pH 7.4; Cowra pH, 5.6). At pH-values outside the 7–7.5 range, nitrogenase activity decreased. The effect of clay on nitrogenase activity by bacteria extracted from each of the soils was complex. Montmorillonite enhanced nitrogense activity of bacteria from both soil extracts when (1% w/w) glucose, xylan, fructose, cellobiose or K-malate were provided as energy sources. For the Cowra soil extract, the pH range for nitrogenase activity increased from pH 5.5–9, in systems without clay, to pH 3.6–10 in the presence of 5% montmorillonite. Organisms from the high clay-containing Gunnedah soil used straw more efficiently to support nitrogenase activity with 5% montmorillonite than without. However, with extracts from the low clay-containing Cowra soil, the effect was reversed. The results show that, despite uniform conditions of moisture and temperature, soils with similar histories of straw retention may differ in their potential for straw-associated N2 fixation because of limitations imposed by soil characteristics such us pH and clay content.

Managing soils to enhance the potential for bioremediation of water repellency

Water repellency can significantly reduce crop and pasture establishment and production in sandy soils. Management practices that increase the rate of water infiltration into dry soils following the first rains at the end of the dry season were investigated. In the laboratory, addition of water to water repellent soil and maintenance of warm moist conditions produced a gradual decline in water repellency. This was supported by results in the field which showed that under daily irrigation there was a gradual decline in water repellency over time. However, under dryland conditions, other mechanisms to increase water infiltration had to be found. In the laboratory, after the addition of lime and kaolinite clay, there was an initial rapid decline in repellency, indicative of a physical mechanism, followed by a more gradual decline suggesting a biological response. In the field, under dryland conditions, the addition of lime and kaolinite clay resulted in a reduction in water repellency, and in the case of lime, this effect increased with the size of application. Estimates of the numbers of wax-degrading bacteria in the treated soils, using a most-probable-number assay, showed at least a 10-fold increase in lime-treated sands, but not in the clay-treated sands. The results suggest that lime may provide a viable alternative for increasing the wettability of soils by physical mechanisms and by promoting microbial activity by bacteria responsible for wax degradation, resulting in more consistent plant germination and establishment, and increased crop yields.

The isolation and characterisation of bacteria with the potential to degrade waxes that cause water repellency in sandy soils

Water repellency in soils is caused by waxy coatings on particles and can seriously limit agricultural production. Bioremediation of these soils, using wax-degrading bacteria isolated from soils and other sources rich in microorganisms, was investigated. Wool wax, a complex mixture of fatty acids and alcohols, was used to select bacteria capable of metabolising hydrophobic compounds. Of the 37 stable isolates, two-thirds were actinomycetes. These organisms are known for their ability to metabolise a wide range of organic compounds. Degradation of waxes associated with soil particles is facilitated by the production of biosurfactants that emulsify hydrophobic compounds. Measurement of biosurfactant production indicated that those isolates that grew best on hydrocarbon were also the most prolific biosurfactant producers. Inoculation of water-repellent soils, under controlled conditions, with the most efficient wax-degrading bacterial isolates resulted in significant improvements in soil wettability.

Potential for non-symbiotic N2-fixation in different agroecological zones of southern Australia

Nitrogen fixation by symbiotic and non-symbiotic bacteria can be a significant source of nitrogen in cropping systems. However, contributions from non-symbiotic nitrogen fixation (NSNF) are dependent on available carbon in the soil and environmental conditions (soil moisture and temperature). In Australia, measurements of NSNF have been made in the field by quantifying nitrogenase activity. These studies have included determinations of the moisture and temperature requirements for NSNF and for crop residue decomposition that supplies carbon to NSNF bacteria. Other studies have determined the N input by NSNF using N budget calculations. These data together with information about carbon supply and environmental conditions were used to estimate potential NSNF in the cropping zones of southern Australia. Using the ArcviewGIS Spatial Analyst (v3.1), maps of Australia showing estimates of NSNF in different cropping zones as determined by rainfall and temperature or carbon availability were generated. In Western Australia (represented by Wongan Hills) and South Australia (represented by Avon), where summers are dry, estimates of NSNF were generally low (10–15 kg N/ha from January to June) due to limitations of soil moisture. In New South Wales, particularly in the north where summer rainfall patterns develop (represented by Gunnedah), the warm, moist conditions produced higher estimates of NSNF (totaling 32–38 kg N/ha from January to June). In this region, the majority of estimated NSNF occurred in January and February leading to the depletion of carbon supplies and reduced NSNF in autumn (March–June). Information about potential supplies of N from NSNF across the cropping zones should be useful for researchers to select and study areas that are most likely to benefit from NSNF. It should also help agronomists and extension officers explain changes in N status within paddocks or within specific farming systems and to provide more accurate advice on N fertiliser requirements, particularly in low-input farming systems.

Factors affecting ammonia-oxidising microorganisms and potential nitrification rates in southern Australian agricultural soils

Ammonia-oxidising archaea (AOA) have recently been described as having an important role in soil nitrification. However, published data on factors which influence their distribution and their impact on a soil’s potential nitrification rates (PNR) are sparse, particularly compared with the amount of information available regarding ammonia-oxidising bacteria (AOB). This study had two aims. First, to investigate which environmental factors affect the AOA : AOB ratio in soils from two agricultural regions, and second, to explore whether the abundance of either AOA or AOB correlated with PNR. Samples were collected from 45 sites within the cropping regions of Western Australia and South Australia. Soils were tested for pH, NH4+/NO3–, organic carbon (C), total nitrogen (N), C : N ratio, PNR, and electrical conductivity. Climate data were obtained from the Queensland Climate Change Centre for Excellence SILO website. Abundances of AOA and AOB were measured using real-time PCR quantification of the gene encoding the ammonia monooxygenase enzyme (amoA). Multivariate statistical analysis was applied to assess correlations between PNR, soil properties, and abundance of AOA or AOB. In the majority samples AOA were present, but their abundance, and the AOA : AOB ratio, varied considerably between sites. Multivariate analysis showed that the distribution of AOA and AOB and the AOA : AOB ratio were strongly correlated with climatic and seasonal factors. Sites where samples were collected during dry, hot periods tended to be AOA-dominated, whereas samples collected during cool, wet periods tended to be AOB-dominated or have equal abundances of AOA and AOB. The PNRs were correlated with total N content, organic C content, and soil pH. There was no clear correlation between AOA or AOB and PNR. This study shows that both AOA and AOB are widespread in Western Australian and South Australian soils and their abundance and ratio are affected by climate and season. It also shows that PNR is more strongly influenced by soil fertility factors than by the AOA : AOB ratio.

Tillage practices altered labile soil organic carbon and microbial function without affecting crop yields

A 7-year tillage experiment was conducted on a deep sand in the central wheat belt of Western Australia between 1998 and 2004 to evaluate the impact of tillage intensity [no-tillage (NT), conservation tillage (CT), and rotary tillage (RT)] on soil organic matter, microbial biomass and function, and crop yields in a wheat–lupin rotation. A fourth treatment (subterranean clover pasture, Pasture) with least soil disturbance was included as a comparison. By March 2004, total soil carbon (C) in NT and CT increased by 4.4 and 2.6 t/ha, respectively, to an average of 17.6 t/ha in the top 0.1 m of the soil profile. There was a loss of total soil C in RT (–0.5 t/ha), which was significant compared with the other 2 tillage treatments. Total soil C and nitrogen (N) contents in the pasture treatment were similar to those in NT and CT at the end of the experiment. Labile fractions of soil C responded more rapidly to tillage practice, with significant reductions by 2001 in light fraction C and dissolved organic C in the RT treatment compared with the other 3 treatments. The effect of RT on biology and function was seen early in the experiment and, compared with Pasture, NT, and CT, intense tillage in RT significantly reduced microbial biomass and cellulase activity in the surface 0.05 m by the third year of the experiment. However, at a depth of 0.05–0.10 m there were no significant differences between treatments. Grain yields in NT, CT, and RT were unaffected by tillage except in 2003, when lupin yield under RT (1.6 t/ha) was significantly less than under NT (2.0 t/ha) and CT (1.9 t/ha). Minimal differences between NT and CT are a reflection of the minimum disturbance in the CT treatment, although there were significant differences between CT and NT in microbial indices such as microbial quotient and metabolic quotient, suggesting a future divergence of these treatments.

Nitrogenase activity (C2H2 reduction) by free-living bacteria in soil in a long-term tillage and stubble management experiment on a vertisol

Abstract The effects on nitrogenase activity (C2H2 reduction) of cereal stubble management, tillage and N fertilizer practices were studied in a long-term experiment on a vertisol at the Hermitage Research Station in southern Queensland, Australia. The experiment compared 8 treatments comprising 3 factors (1) tillage (zero, mechanical cultivation), (2) stubble (burnt, retained) and (3) N fertilizer applied at sowing (0, 69 kg Nha−1 yr−1 as urea) in four randomised blocks. Measurements of nitrogenase activity, using in situ C2H2 reduction assays, began on 1 March 1991 following stubble treatment and continued for 4.5 weeks. Following the application of water to wet the soil to field capacity, nitrogenase activity was observed in all treatments. Nitrogenase activity was greater in the stubble-retained treatments than in the stubble-burnt treatments although this was significant at only 5 of the 8 sampling times. The results suggested that the application of N fertilizer depressed nitrogenase activity and that cultivation encouraged activity compared with the zero-tillage treatments, but these effects were only significant at one each of the 8 sampling times. By using an appropriate mix of management practices it may be possible to promote N2 fixation by free-living bacteria using cereal stubble for energy.

Carbon sequestration under subtropical perennial pastures I: Overall trends

The use of subtropical perennial grasses in temperate grazing systems is increasingly being promoted for production and environmental benefits. This study employed a combination of elemental and stable isotope analyses to explore whether pastures sown to either kikuyu (Pennisetum clandestinum) or a combination of panic (Panicum maximum) and Rhodes grass (Chloris gayana) could increase soil organic carbon (SOC) levels in five regions across southern Australia. Carbon was sequestered under kikuyu at a rate of 0.90 ± 0.25 Mg C ha–1 year–1 along the south coast of Western Australia. Lower but still significant sequestration rates were found for kikuyu in South Australia (0.26 ± 0.13 Mg C ha–1 year–1). No changes in SOC were found for panic–Rhodes grass pasture systems in the northern district of Western Australia. Additionally, we found no changes in SOC when kikuyu-based pastures were established on formerly cropped paddocks in the Namoi Catchment of New South Wales. Stable isotope results corroborated these findings and suggested that, where SOC has accumulated, the gains have been dominated by SOC derived from the perennial vegetation and have been concentrated in the upper 10 cm of soil.