1. The legacy of cropping history reduces the recovery of soil carbon and nitrogen after conversion from continuous cropping to permanent pasture

• Authors:
  • Orton, T. G.
  • Jones, A. R.
  • Dalal, R. C.
• Source: Science Article
• Volume: 216
• Year: 2016
• Summary: Enhancing soil organic carbon (SOC) and total nitrogen (N) is considered an important step in mitigating greenhouse gas emissions and improving soil fertility. The loss of SOC and N generally observed during cropping may be reversed by converting such land use to permanent pasture. However, large uncertainties remain around the processes that govern how much C and N may be sequestered from this conversion in soils worldwide. Here, we sampled soils across 10 paddocks on 20-year old grass pasture sites with a chronosequence of cropping history in order to quantify changes in SOC and N after the conversion of long-term cropping to pasture land use in a semi-arid region of southwest Queensland, Australia. The average rate of change in SOC stocks under pasture in the top 0-0.1m soil layer was approximately 0.1tCha-1yr-1, consisting of an increase in SOCC4 (pasture) of 0.2tCha-1yr-1 and a decrease in SOCC3 (pre-pasture) of 0.1tCha-1yr-1. The decrease in SOCC3 was enhanced at sites with greater years under cropping, indicating a reduced potential for SOC sequestration at sites with longer duration under cropping. The loss of total nitrogen (N) under cropping was also partially recovered with the introduction of permanent perennial pastures. A significant, positive correlation between soil aggregation and mineralisable N under cropping suggested that soil structure has a strong influence over N stability in the soil. However, soil aggregation and mineralisable N did not improve under pastures, indicating that the loss of soil fertility and structure under cropping remained a residual effect that was not recovered within 20 years of permanent pastures in this semi-arid subtropical environment. We suggest here that the resilience of ecosystems to recover soil fertility under pastures declines with greater years under cropping. © 2015 Elsevier B.V.

2. Strategic tillage in no-till farming systems in Australia's northern grains-growing regions: II. Implications for agronomy, soil and environment

• Authors:
  • Freebairn, D. M.
  • Dalal, R. C.
  • Seymour, N. P.
  • Bell, M. J.
  • Moody, P. W.
  • Dang, Y. P.
  • Walker, S. R.
• Source: Soil and Tillage Research
• Year: 2015
• Summary: In semi-arid sub-tropical areas, a number of studies concerning no-till (NT) farming systems have demonstrated advantages in economic, environmental and soil quality aspects over conventional tillage (CT). However, adoption of continuous NT has contributed to the build-up of herbicide resistant weed populations, increased incidence of soil- and stubble-borne diseases, and stratification of nutrients and organic carbon near the soil surface. Some farmers often resort to an occasional strategic tillage (ST) to manage these problems of NT systems. However, farmers who practice strict NT systems are concerned that even one-time tillage may undo positive soil condition benefits of NT farming systems. We reviewed the pros and cons of the use of occasional ST in NT farming systems. Impacts of occasional ST on agronomy, soil and environment are site-specific and depend on many interacting soil, climatic and management conditions. Most studies conducted in North America and Europe suggest that introducing occasional ST in continuous NT farming systems could improve productivity and profitability in the short term; however in the long-term, the impact is negligible or may be negative. The short term impacts immediately following occasional ST on soil and environment include reduced protective cover, soil loss by erosion, increased runoff, loss of C and water, and reduced microbial activity with little or no detrimental impact in the long-term. A potential negative effect immediately following ST would be reduced plant available water which may result in unreliability of crop sowing in variable seasons. The occurrence of rainfall between the ST and sowing or immediately after the sowing is necessary to replenish soil water lost from the seed zone. Timing of ST is likely to be critical and must be balanced with optimising soil water prior to seeding. The impact of occasional ST varies with the tillage implement used; for example, inversion tillage using mouldboard tillage results in greater impacts as compared to chisel or disc. Opportunities for future research on occasional ST with the most commonly used implements such as tine and/or disc in Australia's northern grains-growing region are presented in the context of agronomy, soil and the environment.

3. Aggregate hierarchy and carbon mineralization in two Oxisols of New South Wales, Australia

• Authors:
  • Daniel, H.
  • Lockwood, P. V.
  • Wilson, B. R.
  • Rabbi, S.
  • Young, I. M.
• Source: SOIL & TILLAGE RESEARCH
• Volume: 146
• Issue: Pt. B
• Year: 2015
• Summary: The conventional model of aggregate formation suggests a hierarchy where micro-aggregates with lower porosity and therefore reduced soil organic carbon (SOC) mineralization form inside macro-aggregates. This model has however been questioned for highly weathered Oxisols where inconclusive results regarding the presence of aggregate hierarchy have been obtained to date. We hypothesized that in Oxisols (i) an aggregate hierarchy would be present (ii) the porosity of micro-aggregates would be lower than that of macro-aggregates and (iii) pore geometry of aggregates would influence SOC mineralization. We collected topsoils from Oxisols in northern New South Wales, Australia from which macro-aggregates (>250µm), micro-aggregates (53-250µm) and <53µm fractions were isolated from bulk soil by wet sieving. 3D images of macro- and micro-aggregates were produced using X-ray computed tomography (µCT) showing the presence of micro-aggregates inside macro-aggregates, which confirmed the presence of an aggregate hierarchy in the Oxisols studied. Macro-aggregates were more common and SOC in higher concentrations in forest systems compared with agricultural (the cultivation or pasture) land-uses, but aggregate geometry differed little between the land-uses studied. The porosity of macro-aggregates (4%) was significantly lower than micro-aggregates (5.5%). Despite the differences in pore geometry between macro- and micro-aggregates, SOC mineralized (SOCmin) during a 2-month incubation (at 25°C) was similar in macro- (3% of SOC concentration) and micro-aggregates (2.8% of SOC concentration). We conclude that although aggregate hierarchy exists in these soils and that aggregate geometry did differ between aggregate size classes, there was no evidence to support the porosity exclusion principle and the assumption that SOC is preferentially stabilized within micro-aggregates in these soils.

4. An analysis of the socio-economic factors influencing the adoption of conservation agriculture as a climate change mitigation activity in Australian dryland grain production

• Authors:
  • Dargusch, P.
  • Rochecouste, J. -F
  • Cameron, D.
  • Smith, C.
• Source: Agricultural Systems
• Volume: 135
• Year: 2015
• Summary: The cropping sector in Australia contributes 2.5% of national greenhouse gas emissions, not accounting for the historical loss of soil carbon. The Australian Government is developing policy initiatives targeted at farmers to encourage changes in management practices that aim to reduce emissions from the agricultural sector. The main policy proposal being developed is a market-based mechanism to pay farmers from an Emissions Reduction Fund using methodologies specified under the Australian Carbon Farming Initiative. The adoption of conservation agriculture practices in the dryland grain sector in Australia shows the potential to achieve emissions reductions in the order of three million tCO2e annually. This paper presents a series of systems models that describe the process of how Australian dryland grain farmers decide to change and adopt conservation agriculture practices. Results indicate that a number of economic and social factors drive the rate of practice change, and change seems to be motivated mostly by the pursuit of productivity benefits rather than environmental benefits. We postulate that it may be more effective for climate policy to directly target the adoption of conservation agriculture practices amongst Australian dryland grain farmers by promoting the crop productivity benefits likely to be achieved by such practices, rather than attempting to develop a market-based mechanism for carbon payments. Under this approach, emissions reduction outcomes and carbon payments would not be the primary driver for changing farming practices, but rather a concurrent benefit.

5. Assessment of precipitation and temperature data from CMIP3 global climate models for hydrologic simulation

• Authors:
  • Peel, D. J.
  • McMahon, M. C.
  • McMahon, T. A.
  • Karoly, D. J.
• Source: HYDROLOGY AND EARTH SYSTEM SCIENCES
• Volume: 19
• Issue: 1
• Year: 2015
• Summary: The objective of this paper is to identify better performing Coupled Model Intercomparison Project phase 3 (CMIP3) global climate models (GCMs) that reproduce grid-scale climatological statistics of observed precipitation and temperature for input to hydrologic simulation over global land regions. Current assessments are aimed mainly at examining the performance of GCMs from a climatology perspective and not from a hydrology standpoint. The performance of each GCM in reproducing the precipitation and temperature statistics was ranked and better performing GCMs identified for later analyses. Observed global land surface precipitation and temperature data were drawn from the Climatic Research Unit (CRU) 3.10 gridded data set and re-sampled to the resolution of each GCM for comparison. Observed and GCM-based estimates of mean and standard deviation of annual precipitation, mean annual temperature, mean monthly precipitation and temperature and Koppen-Geiger climate type were compared. The main metrics for assessing GCM performance were the Nash-Sutcliffe efficiency (NSE) index and root mean square error (RMSE) between modelled and observed long-term statistics. This information combined with a literature review of the performance of the CMIP3 models identified the following better performing GCMs from a hydrologic perspective: HadCM3 (Hadley Centre for Climate Prediction and Research), MIROCm (Model for Interdisciplinary Research on Climate) (Center for Climate System Research (The University of Tokyo), National Institute for Environmental Studies, and Frontier Research Center for Global Change), MIUB (Meteorological Institute of the University of Bonn, Meteorological Research Institute of KMA, and Model and Data group), MPI (Max Planck Institute for Meteorology) and MRI (Japan Meteorological Research Institute). The future response of these GCMs was found to be representative of the 44 GCM ensemble members which confirms that the selected GCMs are reasonably representative of the range of future GCM projections.

6. Nitrogen management is the key for low-emission wheat production in Australia: A life cycle perspective

• Authors:
  • Dalal, R. C.
  • Wang, W.
• Source: European Journal of Agronomy
• Volume: 66
• Year: 2015
• Summary: Farm management affects the global greenhouse gas (GHG) budget by changing not only soil organic carbon (SOC) stocks and nitrous oxide (N2O) emissions but also other pre-farm, on-farm and off-site emissions. The life cycle assessment (LCA) approach has been widely adopted to assess the "carbon footprint" of agricultural products, but rarely used as a tool to identify effective mitigation strategies. In this study, the global warming impacts of no-till (NT) vs. conventional till (CT), stubble retention (SR) vs. stubble burning (SB), and N fertilization (NF) vs. no N fertilization (N0) in an Australian wheat cropping system were assessed using in situ measurements of N2O fluxes over three years, SOC changes over forty years and including other supply chain GHG sources and sinks. The results demonstrated the importance of full GHG accounting compared to considering SOC changes or N2O emissions alone for assessing the global warming impacts of different management practices, and highlighted the significance of accurately accounting for SOC changes and N2O emissions in LCAs. The GHG footprints of wheat production were on averaged 475kg carbon dioxide equivalent (CO2-e) ha-1 (or 186kg CO2-e t-1 grain) higher under NF than N0. Where fertilizer N was applied (70kgNha-1), the life cycle emissions were 200kg CO2-e ha-1 (or 87t-1 grain) lower under NT than CT and 364kg CO2-e ha-1 (or 155t-1 grain) lower under SR than SB. Classification of the emission sources/sinks and re-calculation of published data indicated that under the common practices of SR combined with NT, N-related GHG emissions contributed 60-95% of the life cycle emissions in the predominantly rain-fed wheat production systems in Australia. Therefore, future mitigation efforts should aim to improve N use efficiency, explore non-synthetic N sources, and most importantly avoid excessive N fertilizer use whilst practising NT and SR.

7. Nutrient density as a metric for comparing greenhouse gas emissions from food production

• Authors:
  • Kingwell, R. S.
  • Behrendt, R.
  • Eckard, R. J.
  • Doran-Browne, N. A.
• Source: CLIMATIC CHANGE
• Volume: 129
• Issue: 1-2
• Year: 2015
• Summary: Dietary Guidelines for many countries recommend that people should eat 'nutrient dense' foods, which are foods with a high nutrient to energy ratio; and that people should limit their intake of saturated fat, added salt or added sugar. In addition, consumers and environmentalists increasingly want their food to be produced with a low impact on the environment, including reduced greenhouse gas emissions (GHGE), yet agriculture is a major source of CH4 and N2O emissions, as well as producing CO2 emissions. Current research on GHGE from agriculture does not incorporate the nutritional value of the foods studied. However, the nutritional content of food is important, given the prevalence of malnutrition, including obesity (due to over-consumption of foods high in energy yet low nutritional density), and the negative health impacts they produce. This paper introduces the metric, emissions/unit nutrient density, and compares the results with three other metrics: emissions intensity (t CO(2)e/t product), emissions/t protein and emissions/GJ. The food products examined are wheat flour, milk, canola oil, lean lamb, lean beef, untrimmed lamb and untrimmed beef. The metric t CO(2)e/unit nutrient density was the preferred metric to use when examining GHGE from food production because it compares different types of products based on their nutritional value, rather than according to singular nutrients such as protein, or specific attributes such as product weight or energy content. Emissions/unit nutrient density has the potential to inform consumer choices regarding foods that have a higher nutritional content relative to the GHGE generated. Further analysis would be useful to develop and expand the use of this metric further.

8. Characterisation of the soil microbial community of cultivated and uncultivated vertisol in Australia under several management regimes.

• Authors:
  • Adetutu, E. M.
  • Shrestha, K.
  • Stevens, S.
  • Shrestha, P.
  • Walsh, K. B.
  • Ball, A. S.
  • Midmore, D. J.
• Source: AGRICULTURE ECOSYSTEMS & ENVIRONMENT
• Volume: 199
• Year: 2015
• Summary: Soil management is known to affect microbial populations relevant to nutrient cycling and plant health. We investigated the effects of several cropping management practices on a Central Queensland vertisol, including the application of liquid biological inoculums, green manuring and conventional chemical fertiliser. Soil microbial load and diversity was indexed using soil respiration, Biolog Ecoplate and FF microplates and PCR-DGGE. Compared to cultivated soil, uncultivated vertisol, represented by virgin brigalow soil, possessed 87% higher soil nitrate than cultivated soils, and significantly higher microbial catabolic potential, as observed in Biolog substrate utilisation patterns. In cultivated soil, there was little difference between treatments in these substrate utilisation patterns, but large changes associated with season. However, the results of 16S rDNA and Internal Transcribed Spacer region based DGGE profiles were consistent with an increase in bacterial diversity and a decrease in fungal diversity in amended cultivated soils relative to the unfertilised cultivated treatment.

9. Quantifying the response of cotton production in eastern Australia to climate change

• Authors:
  • Kouadio, L.
  • Power, B.
  • Cockfield, G.
  • Mushtaq, S.
  • White, N.
  • Williams, A.
• Source: CLIMATIC CHANGE
• Volume: 129
• Issue: 1-2
• Year: 2015
• Summary: The paper evaluates the effect of future climate change (as per the CSIRO Mk3.5 A1FI future climate projection) on cotton yield in Southern Queensland and Northern NSW, eastern Australia by using of the biophysical simulation model APSIM (Agricultural Production Systems sIMulator). The simulations of cotton production show that changes in the influential meteorological parameters caused by climate change would lead to decreased future cotton yields without the effect of CO2 fertilisation. By 2050 the yields would decrease by 17 %. Including the effects of CO2 fertilisation ameliorates the effect of decreased water availability and yields increase by 5.9 % by 2030, but then decrease by 3.6 % in 2050. Importantly, it was necessary to increase irrigation amounts by almost 50 % to maintain adequate soil moisture levels. The effect of CO2 was found to have an important positive impact of the yield in spite of deleterious climate change. This implies that the physiological response of plants to climate change needs to be thoroughly understood to avoid making erroneous projections of yield and potentially stifling investment or increasing risk.

10. Estimate of the global warming potential of the Tasmanian pyrethrum industry in comparison to other crops, potato and onion.

• Authors:
  • Pethybridge, S. J.
  • Hay, F.
• Source: Web Of Knowledge
• Issue: 1073
• Year: 2015
• Summary: A desktop study was undertaken to assess the contribution of the pyrethrum industry in Tasmania, Australia to on-farm greenhouse gas (GHG) emissions in comparison to potato and onion. Pyrethrum used significantly less diesel in transport than potato or onion. This was mainly due to the lower weight of dried flowers harvested from pyrethrum crops in comparison to the weight of crop from potato and onion fields (60 t/ha). First harvest pyrethrum required a similar amount of diesel to potato and onion for tractor and harvester operations. However, older pyrethrum crops required only 41% of the diesel used in potato and onion crops due mainly to the perennial nature of pyrethrum and the absence of cultivation in years subsequent to planting. Pyrethrum required greater inputs (kg a.i./ha) of herbicides and lower inputs of fungicides than potato and onion. All three crops required little input of insecticides. Pyrethrum had substantially lower inputs of nitrogen (N) and phosphorus (P) than potato or onion. First-harvest pyrethrum required only 38 and 59% of the N required by potato and onion, respectively. Moreover, pyrethrum required lower potassium (K) than potato, but higher K than onion. The electricity consumption associated with irrigation of pyrethrum and onion were similar, and amounted to 39% that of potato, due to the higher irrigation requirement of potato (5.1 ML/ha) in comparison to pyrethrum and onion (2.0 ML/ha) in Tasmania. The global warming potential (GWP) per hectare associated with planting to harvest of first year pyrethrum (18 months) and subsequently from one harvest to another (12 months) was estimated at 4,128.8 and 2,184.7 kg CO 2-e/ha, respectively. By comparison the GWP resulting from planting to harvest of potato and onion in Tasmania (4 months) was estimated at 5,284.0 and 4,875.3 kg CO 2-e/ha. The GWP of first year pyrethrum was therefore 78% that of potato and 85% that of onion, while the annual GWP per hectare of older pyrethrum fields was 41% that of potato and 45% that of onion. This study was based on strict assumptions and on generic emission factors for greenhouse gases. Therefore caution is required with the absolute figure for GWP. However, this study indicated that the on-farm production of pyrethrum is a relatively low contributor to GHG production on a per hectare basis, in comparison to other annual crops often grown in rotation.