Frances C. Hoyle

Soil microbial biomass-Interpretation and consideration for soil monitoring

Since 1970, measurement of the soil microbial biomass (SMB) has been widely adopted as a relatively simple means of assessing the impact of environmental and anthropogenic change on soil microorganisms. The SMB is living and dynamic, and its activity is responsible for the regulation of organic matter transformations and associated energy and nutrient cycling in soil. At a gross level, an increase in SMB is considered beneficial, while a decline in SMB may be considered detrimental if this leads to a decline in biological function. However, absolute SMB values are more difficult to interpret. Target or reference values of SMB are needed for soil quality assessments and to allow ameliorative action to be taken at an appropriate time. However, critical values have not yet been successfully identified for SMB. This paper provides a conceptual framework which outlines how SMB values could be interpreted and measured, with examples provided within an Australian context.

Seasonal changes in microbial function and diversity associated with stubble retention versus burning

The long-term (16-year) effect of stubble management (i.e. retained or burnt) on the size of the microbial community (microbial biomass-C and -N), microbial community structure (PLFA), and function (CO2-C evolution, gross N transformation rates, enzymatic activity, and community level physiological profiles) was investigated on 4 occasions during a single wheat-growing season using soil collected from the low-rainfall (<250 mm) region of Western Australia. Significant differences (P < 0.001) in microbial community structure and function were determined for different sampling times by phospholipid fatty acid (PLFA) analyses and community level physiological profiles (CLPP). However, neither PLFA nor CLPP analyses identified differences between stubble treatments. In contrast to total soil organic matter-C, for which no treatment differences were evident, microbial biomass-C was 34% and CO2-C evolution 61% greater in stubble-retained treatments than in burnt-stubble treatments in the 0-0.05 m soil layer. Seasonal increases in microbial biomass-C (P < 0.001) were on average twice as large and CO2-C evolution (P < 0.001) nearly 4 times greater in September during crop flowering compared with other sampling times. In contrast, microbial biomass-N remained constant throughout the entire sampling period. Stubble- retained treatments also demonstrated significantly greater (P < 0.05) levels of arginine ammonification, acid phosphatase and β-glucosidase enzyme activity on average compared with burnt-stubble treatments. However, the effect (P = 0.05) of stubble treatment on gross N mineralisation, nitrification, or immobilisation rates was seasonally dependent with burnt-stubble treatments demonstrating lower gross N mineralisation rates than retained- stubble treatments in November. Gross N mineralisation was lower (37-83% on average) than potential gross nitrification rates (estimated in the presence of excess NH4 + ) measured from May to September. The rate of potential gross nitrification was observed to decline significantly (P = 0.06) in November and as a result, more closely matched gross N mineralisation rates. Potential gross nitrification rates were also up to 6 times greater than microbial immobilisation of NH4 + , indicating that this would be the primary consumptive process in the presence of NH4 + . Whilst potential nitrification rates in the presence of excess NH4 + were high, low soil NO3 − concentrations indicate that plant/microbial demand for NO3 − and NH4 + exceeded the supply capacity. For example, actual gross nitrification rates (determined in the presence of 15 N-labelled NO3 − ) were only greater than gross N mineralisation in May, indicating N supply constrained nitrification at other sampling times. Findings illustrate that increased wheat yields of 31% in this study were associated with the retention of stubble. Further they demonstrate that changes in stubble management significantly influenced the mass and activity of microorganisms (and in some cases N cycling), whilst having little influence on community diversity. Additional keywords: 15 N isotopic pool dilution, FLUAZ, nitrogen, carbon.

Capacity for increasing soil organic carbon stocks in dryland agricultural systems

Assessment of the potential for soil carbon sequestration based on soil type, land use, and climate scenarios is crucial for determining which agricultural regions can be used to help mitigate increasing atmospheric CO2 concentrations. In semi-arid and Mediterranean-type environments, soil organic carbon (SOC) storage capacity is rarely achieved under dryland agricultural systems. We aimed to assess both actual (measured) and attainable (modelled) SOC stock values for the dryland agricultural production zone of Western Australia. We measured actual SOC storage (0–0.3 m) and known constraints to plant growth for a range of soils types (3–27% clay) and land uses (continuous cropping, mixed cropping, annual and perennial pastures) on the Albany sand plain in Western Australia (n = 261 sites), spanning a rainfall gradient of 421–747 mm. Average actual SOC stocks for land use–soil type combinations ranged from 33 to 128 t C/ha (0–0.3 m). Up to 89% of the variability in actual SOC stock was explained by soil depth, rainfall, land use, and soil type. The scenarios modelled with Roth-C predicted that attainable SOC values of 59–140 t C/ha (0–0.3 m) could be achieved within 100 years. This indicated an additional storage capacity of 5–45% (7–27 t C/ha) depending on the specific land use–soil type combination. However, actual SOC in the surface 0–0.1 m was 95 to >100% of modelled attainable SOC values, suggesting this soil depth was ‘saturated’. Our findings highlight that additional SOC storage capacity in this region is limited to the subsoil below 0.1 m. This has implications for management strategies to increase SOC sequestration in dryland agricultural systems, as current practices tend to concentrate organic matter near the soil surface.

Potentially mineralisable nitrogen: relationship to crop production and spatial mapping using infrared reflectance spectroscopy

Accurate and rapid prediction of the spatial structure of soil nitrogen (N) supply would have both economic and environmental benefits with respect to improved inorganic N fertiliser management. Yet traditional biochemical indices of soil N supply have not been widely incorporated into fertiliser decision support systems or environmental risk monitoring programs. Here we illustrate that in a low-input, semi-arid environment, potentially mineralisable N (PMN, as determined by anaerobic incubation) explained 21% of wheat grain yield (P = 0.003), whereas there was no significant relationship between wheat grain yield and inorganic N fertiliser application. We also assessed the spatial pattern of PMN using a structured grid soil sampling strategy over a 10-ha area (180 separate samples, 0–0.1 m). PMN in each soil sample was determined by standard biochemical analysis and also predicted using a fourier transform infrared spectrometer (FTIR). Findings illustrate that FTIR was able to significantly predict (P < 0.001) PMN values in soil and has the advantage of enabling high sample throughput and rapid (within minutes) soil analysis. Given the relatively low cost of FTIR machines and ease of use, such an approach has practical application in situations where analysis cost or access to equipped laboratories has hindered the measurement and monitoring of soil N supply within paddocks and across regions.

Microbial response to the addition of soluble organic substrates

Soil microbial activity is often limited by the absence of readily available carbon (C) based substrates. Addition of a range of soluble organic substrates to soil has been shown to either accelerate or constrain the rate of CO2-C evolution. The aim of this study was to investigate the capacity of the microbial population to become activated in response to small additions of glucose-C (10–50 µg C/g soil) and 19 other soluble organic substrates (30 µg C/g soil) in soil either amended or not with cellulose. Rapid utilisation (equivalent to 25–35%) of added glucose was demonstrated in an initial flush of respiratory activity measured as CO2-C. However, the cumulative amount of respired C in 23 days indicated no additional release of CO2-C from the native soil organic matter (SOM) following application of glucose to soils, and a highly variable secondary phase of C mineralisation distinct from the initial glucose mineralisation phase. Although several C substrates resulted in the evolution of ‘extra’ CO2-C, no obvious association was observed between the response and the chemical structure of each substrate.

Microbial respiration, but not biomass, responded linearly to increasing light fraction organic matter input: Consequences for carbon sequestration

Rebuilding ‘lost’ soil carbon (C) is a priority in mitigating climate change and underpinning key soil functions that support ecosystem services. Microorganisms determine if fresh C input is converted into stable soil organic matter (SOM) or lost as CO2. Here we quantified if microbial biomass and respiration responded positively to addition of light fraction organic matter (LFOM, representing recent inputs of plant residue) in an infertile semi-arid agricultural soil. Field trial soil with different historical plant residue inputs [soil C content: control (tilled) = 9.6 t C ha−1 versus tilled + plant residue treatment (tilled + OM) = 18.0 t C ha−1] were incubated in the laboratory with a gradient of LFOM equivalent to 0 to 3.8 t C ha−1 (0 to 500% LFOM). Microbial biomass C significantly declined under increased rates of LFOM addition while microbial respiration increased linearly, leading to a decrease in the microbial C use efficiency. We hypothesise this was due to insufficient nutrients to form new microbial biomass as LFOM input increased the ratio of C to nitrogen, phosphorus and sulphur of soil. Increased CO2 efflux but constrained microbial growth in response to LFOM input demonstrated the difficulty for C storage in this environment.

Spatially governed climate factors dominate management in determining the quantity and distribution of soil organic carbon in dryland agricultural systems.

Few studies describe the primary drivers influencing soil organic carbon (SOC) stocks and the distribution of carbon (C) fractions in agricultural systems from semi-arid regions; yet these soils comprise one fifth of the global land area. Here we identified the primary drivers for changes in total SOC and associated particulate (POC), humus (HOC) and resistant (ROC) organic C fractions for 1347 sample points in the semi-arid agricultural region of Western Australia. Total SOC stock (0–0.3 m) varied from 4 to 209 t C ha−1 with 79% of variation explained by measured variables. The proportion of C in POC, HOC and ROC fractions averaged 28%, 45% and 27% respectively. Climate (43%) and land management practices (32%) had the largest relative influence on variation in total SOC. Carbon accumulation was constrained where average daily temperature was above 17.2 °C and annual rainfall below 450 mm, representing approximately 42% of the 197,300 km2 agricultural region. As such large proportions of this region are not suited to C sequestration strategies. For the remainder of the region a strong influence of management practices on SOC indicate opportunities for C sequestration strategies associated with incorporation of longer pasture phases and adequate fertilisation.

Estimating the economic value of soil organic carbon for grains cropping systems in Western Australia

Soil organic carbon (SOC) has the potential to benefit soil function and fertility, and in agricultural production systems, it is considered integral to sustainable farming. We analyse the value of SOC in cropping systems of the south-west of Western Australia in terms of agronomic benefits from increasing productivity (through increased plant-available water-holding capacity) and reducing fertiliser use (due to increased mineralisation of nitrogen). We also present the potential value of SOC in terms of sequestration benefit if landholders were able to participate in a carbon-sequestration program. We estimate the marginal value of SOC (the value of a soil with more SOC, by 1 t C/ha, than a standard soil) to be AU

Response of microbial biomass and CO2-C loss to wetting patterns are temperature dependent in a semi-arid soil

7.1–8.7/t C.ha.year, depending on rainfall zone and crop type. Approximately 75% of this value is the estimated sequestration value, 20% is the nitrogen-replacement value, and 5% is the estimated productivity improvement value. Over 50 years, this equates

Temperature and stubble management influence microbial CO2–C evolution and gross N transformation rates

130–160/t C.ha depending on the rainfall zone. These values are sensitive to variations in fertiliser and carbon prices. Our results imply this it is unlikely that the SOC benefits will drive practice change in the south-west of Western Australia.