CARBON, WATERED ENERGY FLUXES IN AGRICULTURAL SYSTEMS OF AUSTRALIA AND NEW ZEALAND
A comprehensive understanding of the effects of agricultural management on climate–crop interactions has yet to emerge. Using a novel wavelet–statistics conjunction approach, we analysed the synchronisation amongst fluxes (net ecosystem exchange NEE, evapotranspiration and sensible heat flux) and seven environmental factors (e.g., air temperature, soil water content) on 19 farm sites across Australia and New Zealand. Irrigation and fertilisation practices improved positive coupling between net ecosystem productivity (NEP = −NEE) and evapotranspiration, as hypothesised. Highly intense management tended to protect against heat stress, especially for irrigated crops in dry climates. By contrast, stress avoidance in the vegetation of tropical and hot desert climates was identified by reverse coupling between NEP and sensible heat flux (i.e., increases in NEP were synchronised with decreases in sensible heat flux). Some environmental factors were found to be under management control, whereas others were fixed as constraints at a given location. Irrigated crops in dry climates (e.g., maize, almonds) showed high predictability of fluxes given only knowledge of fluctuations in climate (R2 > 0.78), and fluxes were nearly as predictable across strongly energy- or water-limited environments (0.60 < R2 < 0.89). However, wavelet regression of environmental conditions on fluxes showed much smaller predictability in response to precipitation pulses (0.15 < R2 < 0.55), where mowing or grazing affected crop phenology (0.28 < R2 < 0.59), and where water and energy limitations were balanced (0.7 < net radiation ∕ precipitation < 1.3; 0.27 < R2 < 0.36). By incorporating a temporal component to regression, wavelet–statistics conjunction provides an important step forward for understanding direct ecosystem responses to environmental change, for modelling that understanding, and for quantifying nonstationary, nonlinear processes such as precipitation pulses, which have previously defied quantitative analysis.
Cleverly, J., Vote, C., Isaac, P., Ewenz, C., Harahap, M., Beringer, J., Campbell, D., Daly, E., Eamus, D., He, L., Hunt, J., Grace, P., Hutley, L., Laubach, J., McCaskill, M., Rowlings, D., Rutledge Jonker, S., Schipper, L., Schroder, I., Teodosio, B., Yu, Q., Ward, P., Walker, J., Webb, J. and Grover, S., 2020. Carbon, water and energy fluxes in agricultural systems of Australia and New Zealand. Agricultural and Forest Meteorology, 287, p.107934.
AN AGRICULTURAL PRACTISE WITH CLIMATE AND FOOD SECURITY BENEFITS: “CLAYING” WITH KAOLINITIC CLAY SUBSOIL DECREASED SOIL CARBON PRIMING AND MINERALISATION IN SANDY CROPPING SOILS
As the agricultural sector seeks to feed a growing global population, climate-smart agriculture offers opportunities to concurrently mitigate climate change by reducing greenhouse gas emissions and/or increasing carbon storage in soils. This study examined the potential for clay addition to reduce CO2 emissions from plant residues and soil organic matter in a sandy soil. Soils were sourced from a 15-year-old field trial where claying (200 t ha-1) had already demonstrated improvements in water infiltration, grain yield and profits. Isotopically labelled plant residues (wheat, canola, or pea) were used to separate residue-derived and soil-derived CO2 sources from a nil-clay control, a historically clayed, and two freshly created soils with either high (10%) or low (3%) subsoil clay additions. Laboratory incubations demonstrated that historically clayed soils released less CO2 from plant residues and soil organic matter. Clay addition also decreased the priming effect of adding fresh residue to soils. The results from clay experimentally added in the laboratory varied. Differences in chemical and biological indicators (pH, microbial biomass C and N, extractable organic C and N, NO3-, NH4+, , abundance of bacterial, archaeal, fungal , LMCO, GH48 and CbhI genes) did not correlate with patterns of CO2emissions across treatments.. While claying practices have previously demonstrated benefits to crop productivity, this research demonstrates long-term changes in carbon-cycling that could promote greater carbon sequestration.
Grover, S., Butterly, C., Wang, X., Gleeson, D., Macdonald, L., Hall, D. and Tang, C. (2019). An agricultural practise with climate and food security benefits: “Claying” with kaolinitic clay subsoil decreased soil carbon priming and mineralisation in sandy cropping soils. Science of The Total Environment, p.134488
SOIL GREENHOUSE GAS EMISSIONS FROM AUSTRALIAN SPORTS FIELDS
Managed turf is a potential net source of greenhouse gas (GHG) emissions. While most studies to date have focused on non-sports turf, sports turf may pose an even greater risk of high GHG emissions due to the generally more intensive fertiliser, irrigation and mowing regimes. This study used manual and automated chambers to measure nitrous oxide (N2O) and methane (CH4) emissions from three sports fields and an area of non-sports turf in southern Australia. Over 213 days (autumn to late spring), the average daily N2O emission was 37.6 g N ha−1 day−1 at a sports field monitored at least weekly and cumulative N2O emission was 2.5 times higher than the adjacent non-sports turf. Less frequent seasonal sampling at two other sports fields showed average N2O daily emission ranging from 26 to 90 g N ha−1 day−1. Management practices associated with periods of relatively high N2O emissions were surface renovation and herbicide application. CH4 emissions at all of the sports fields were generally negligible with the exception of brief periods when soil was waterlogged following heavy rainfall where emissions of up to 1.3 kg C ha−1 day−1 were recorded. Controlled release and nitrification inhibitor containing fertilisers didn't reduce N2O, CH4 or CO2 emissions relative to urea in a short term experiment. The N2O emissions from the sports fields, and even the lower emissions from the non-sports turf, were relatively high compared to other land uses in Australia highlighting the importance of accounting for these emissions at a national level and investigating mitigation practices.
Riches, D., Porter, I., Dingle, G., Gendall, A. and Grover, S. (2019). Soil greenhouse gas emissions from Australian sports fields. Science of The Total Environment, p.134420
EFFECTS OF DISTANCE FROM CANAL AND DEGRADATION HISTORY ON PEAT BULK DENSITY IN A DEGRADED TROPICAL PEATLAND
Over recent decades, the combination of deforestation, peat drainage and fires have resulted in widespread degradation of Southeast Asia's tropical peatlands. These disturbances are generally thought to increase peat soil bulk density through peat drying and shrinkage, compaction, and consolidation. Biological oxidation and fires burning across these landscapes also consume surface peat, exposing older peat strata. The prevalence and severity of deforestation, peat drainage and fire are typically greater closer to canals, built to drain peatlands and provide access routes for people. We compared bulk densities of 240 cm peat profiles from intact forests and degraded peatlands broadly, and also assessed differences between degraded peatlands near-to-canals (50–200 m from the nearest canal) and far-from-canals (300+ m from the nearest canal). The effects of vegetation type and fire frequency on bulk density, irrespective of the distance from canal, were also investigated. Mean bulk density values ranged between 0.08 and 0.16 g cm−3 throughout the 240 cm peat profiles. Drainage of peat near-to-canals increased bulk density of peat above the minimum water table depth. Degradation by deforestation and fire also increased bulk densities of upper peat strata, albeit with greater variability. Peat sampled further from canals experienced less intense water table drawdowns, buffering them from drainage effects. These areas were also more commonly forested and burnt less frequently. Differences in bulk densities below minimum water table levels are less clear, but may reflect lowering of the current peat surface in degraded peatlands broadly. These results clearly show that important differences in bulk density exist across degraded peatlands that are spatially dependent on distance from canals and disturbance history. These landscape features should be taken into account when designing future bulk density sampling efforts and peatland restoration programs, or when extrapolating from existing sources in order to make accurate inferences from them.
Sinclair, A., Graham, L., Putra, E., Saharjo, B., Applegate, G., Grover, S. and Cochrane, M. (2019). Effects of distance from canal and degradation history on peat bulk density in a degraded tropical peatland. Science of The Total Environment, 699, p.134199
SOIL SECURITY FOR AUSTRALIA
Soil Security is an emerging sustainability science concept with global application for guiding integrated approaches to land management, while balancing ecosystem services, environmental, social, cultural, and economic imperatives. This discussion paper sets the scene for an Australian Soil Security framework as an example of how it might be developed for any country, defining the key issues and justification for Soil Security, as well as detailing implementation requirements and benefits; two examples of beneficial outcomes are provided in terms of facilitating decommoditization of agricultural products and the impact of urban encroachment on productive land. We highlight research gaps, where new knowledge will contribute to well-rounded approaches that reflect differing stakeholder perspectives. We also provide key nomenclature associated with a potential Soil Security framework so that future discussions may use a common language. Through this work we invite scientific and policy discourse with the aim of developing more informed responses to the myriad of competing demands placed on our soil systems.
Bennett, J.M.; McBratney, A.; Field, D.; Kidd, D.; Stockmann, U.; Liddicoat, C.; Grover, S. (2019). Soil Security for Australia In: Sustainability , 11, 1-15
THE SHORT-TERM EFFECTS OF LIMING ON ORGANIC CARBON MINERALISATION IN TWO ACIDIC SOILS AS AFFECTED BY DIFFERENT RATES AND APPLICATION DEPTHS OF LIME
Two acidic soils (initial pH, 4.6) with contrasting soil organic C (SOC) contents (11.5 and 40 g C kg−1) were incubated with 13C-labelled lime (Ca13CO3) at four different rates (nil, target pH 5, 5.8 and 6.5) and three application depths (0–10, 20–30 and 0–30 cm). We hypothesised that liming would stimulate SOC mineralisation by removing pH constraints on soil microbes and that the increase in mineralisation in limed soil would be greatest in the high-C soil and lowest when the lime was applied in the subsoil. While greater SOC mineralisation was observed during the first 3 days, likely due to lime-induced increases in SOC solubility, this effect was transient. In contrast, SOC mineralisation was lower in limed than in non-limed soils over the 87-day study, although only significant in the Tenosol (70 μg C g−1 soil, 9.15%). We propose that the decrease in SOC mineralisation following liming in the low-C soil was due to increased microbial C-use efficiency, as soil microbial communities used less energy maintaining intracellular pH or community composition changed. A greater reduction in SOC mineralisation in the Tenosol for low rates of lime (0.3 and 0.5 g column−1) or when the high lime rate (0.8 g column−1) was mixed through the entire soil column without changes in microbial biomass C (MBC) could indicate a more pronounced stabilising effect of Ca2+ in the Tenosol than the Chromosol with higher clay content and pH buffer capacity. Our study suggests that liming to ameliorate soil acidity constraints on crop productivity may also help to reduce soil C mineralisation in some soils.
Grover, S., Butterly, C., Wang, X. and Tang, C., 2017. The short-term effects of liming on organic carbon mineralisation in two acidic soils as affected by different rates and application depths of lime. Biology and Fertility of Soils, 53(4), pp.431-443
WATER BALANCE COMPLEXITIES IN EPHEMERAL CATCHMENTS WITH DIFFERENT LAND USES: INSIGHTS FROM MONITORING AND DISTRIBUTED HYDROLOGIC MODELING
Although ephemeral catchments are widespread in arid and semiarid climates, the relationship of their water balance with climate, geology, topography, and land cover is poorly known. Here we use 4 years (2011–2014) of rainfall, streamflow, and groundwater level measurements to estimate the water balance components in two adjacent ephemeral catchments in south‐eastern Australia, with one catchment planted with young eucalypts and the other dedicated to grazing pasture. To corroborate the interpretation of the observations, the physically based hydrological model CATHY was calibrated and validated against the data in the two catchments. The estimated water balances showed that despite a significant decline in groundwater level and greater evapotranspiration in the eucalypt catchment (104–119% of rainfall) compared with the pasture catchment (95–104% of rainfall), streamflow consistently accounted for 1–4% of rainfall in both catchments for the entire study period. Streamflow in the two catchments was mostly driven by the rainfall regime, particularly rainfall frequency (i.e., the number of rain days per year), while the downslope orientation of the plantation furrows also promoted runoff. With minimum calibration, the model was able to adequately reproduce the periods of flow in both catchments in all years. Although streamflow and groundwater levels were better reproduced in the pasture than in the plantation, model‐computed water balance terms confirmed the estimates from the observations in both catchments. Overall, the interplay of climate, topography, and geology seems to overshadow the effect of land use in the study catchments, indicating that the management of ephemeral catchments remains highly challenging.
Dean, J.,Camporese, M.,Webb, J.,Grover, S.,Dresel, P.,Daly, E. (2016). Water balance complexities in ephemeral catchments with different land uses: Insights from monitoring and distributed hydrologic modeling In: Water Resources Research, 52, 4713 - 4729
FIRE IN AUSTRALIAN SAVANNAS: FROM LEAF TO LANDSCAPE
Savanna ecosystems comprise 22% of the global terrestrial surface and 25% of Australia (almost 1.9 million km2) and provide significant ecosystem services through carbon and water cycles and the maintenance of biodiversity. The current structure, composition and distribution of Australian savannas have coevolved with fire, yet remain driven by the dynamic constraints of their bioclimatic niche. Fire in Australian savannas influences both the biophysical and biogeochemical processes at multiple scales from leaf to landscape. Here, we present the latest emission estimates from Australian savanna biomass burning and their contribution to global greenhouse gas budgets. We then review our understanding of the impacts of fire on ecosystem function and local surface water and heat balances, which in turn influence regional climate. We show how savanna fires are coupled to the global climate through the carbon cycle and fire regimes. We present new research that climate change is likely to alter the structure and function of savannas through shifts in moisture availability and increases in atmospheric carbon dioxide, in turn altering fire regimes with further feedbacks to climate. We explore opportunities to reduce net greenhouse gas emissions from savanna ecosystems through changes in savanna fire management.
Beringer, J, Hutley, L, Abramson, D, et al, and Grover, S 2015, 'Fire in Australian savannas: From leaf to landscape', Global Change Biology, vol. 21, no. 1, pp. 62-81
A MICROMETEOROLOGICAL TECHNIQUE FOR DETECTING SMALL DIFFERENCES IN METHANE EMISSIONS FROM TWO GROUPS OF CATTLE
Potential approaches for reducing enteric methane (CH4) emissions from cattle will require verification of their efficacy at the paddock scale. We designed a micrometeorological approach to compare emissions from two groups of grazing cattle. The approach consists of measuring line-averaged CH4 mole fractions upwind and downwind of each group and using a backward-Lagrangian stochastic model to compute CH4 emission rates from the observed mole fractions, in combination with turbulence statistics measured by a sonic anemometer. With careful screening for suitable wind conditions, a difference of 10% in group emission rates could be detected. This result was corroborated by simultaneous measurements of daily CH4 emissions from each animal with the sulfur hexafluoride (SF6) tracer-ratio technique.
Laubach, J, Grover, S, Pinares-Patino, C and Molano, G 2014, 'A micrometeorological technique for detecting small differences in methane emissions from two groups of cattle', Atmospheric Environment, vol. 98, pp. 599-606.
OCCASIONAL LARGE EMISSIONS OF NITROUS OXIDE AND METHANE OBSERVED IN STORMWATER BIOFILTRATION SYSTEMS
Designed, green infrastructures are becoming a customary feature of the urban landscape. Sustainable technologies for stormwater management, and biofilters in particular, are increasingly used to reduce stormwater runoff volumes and peaks as well as improve the water quality of runoff discharged into urban water bodies. Although a lot of research has been devoted to these technologies, their effect in terms of greenhouse gas fluxes in urban areas has not been yet investigated. We present the first study aimed at quantifying greenhouse gas fluxes between the soil of stormwater biofilters and the atmosphere. N2O, CH4, and CO2 were measured periodically over a year in two operational vegetated biofiltration cells at Monash University in Melbourne, Australia. One cell had a saturated zone at the bottom, and compost and hardwood mulch added to the sandy loam filter media. The other cell had no saturated zone and was composed of sandy loam. Similar sedges were planted in both cells. The biofilter soil was a small N2O source and a sink for CH4for most measurement events, with occasional large emissions of both N2O and CH4 under very wet conditions. Average N2O fluxes from the cell with the saturated zone were almost five-fold greater (65.6 μg N2O–N m− 2 h− 1) than from the other cell (13.7 μg N2O–N m− 2 h− 1), with peaks up to 1100 μg N2O–N m− 2 h− 1. These N2O fluxes are of similar magnitude to those measured in other urban soils, but with larger peak emissions. The CH4 sink strength of the cell with the saturated zone (− 3.8 μg CH4–C m− 2 h− 1) was lower than the other cell (− 18.3 μg CH4–C m− 2 h− 1). Both cells of the biofilter appeared to take up CH4 at similar rates to other urban lawn systems; however, the biofilter cells displayed occasional large CH4 emissions following inflow events, which were not seen in other urban systems. CO2 fluxes increased with soil temperature in both cells, and in the cell without the saturated zone CO2 fluxes decreased as soil moisture increased. Other studies of CO2 fluxes from urban soils have found both similar and larger CO2 emissions than those measured in the biofilter. The results of this study suggest that the greenhouse gas footprint of stormwater treatment warrant consideration in the planning and implementation of engineered green infrastructures.
Grover, S, Cohan, A, Chan, H, Livesley, S, Beringer, J and Daly, E 2013, 'Occasional large emissions of nitrous oxide and methane observed in stormwater biofiltration systems', Science of the Total Environment, vol. 465, pp. 64-71
AMMONIA EMISSIONS FROM CATTLE URINE AND DUNG EXCRETED ON PASTURE
Twelve cattle were kept for three days in a circular area of 16 m radius on short pasture and fed with freshly-cut pasture. Ammonia (NH3) emissions from the urine and dung excreted by the cattle were measured with a micrometeorological mass-balance method, during the cattle presence and for 10 subsequent days. Daily-integrated emission rates peaked on Day 3 of the experiment (last day of cattle presence) and declined steadily for five days thereafter. Urine patches were the dominant sources for these emissions. On Day 9, a secondary emissions peak occurred, with dung pats likely to be the main sources. This interpretation is based on simultaneous observations of the pH evolution in urine patches and dung pats created next to the circular plot. Feed and dung samples were analysed to estimate the amounts of nitrogen (N) ingested and excreted. Total N volatilised as NH3 was 19.8 (± 0.9)% of N intake and 22.4 (± 1.3)% of N excreted. The bimodal shape of the emissions time series allowed to infer separate estimates for volatilisation from urine and dung, respectively, with the result that urine accounted for 88.6 (± 2.6)% of the total NH3 emissions. The emissions from urine represented 25.5 (± 2.0)% of the excreted urine-N, while the emissions from dung amounted to 11.6 (± 2.7)% of the deposited dung-N. Emissions from dung may have continued after Day 13 but were not resolved by the measurement technique. A simple resistance model shows that the magnitude of the emissions from dung is controlled by the resistance of the dung crust.
Laubach, J, Taghizadeh-Toosi, A, Gibbs, S, Sherlock, R, Kelliher, F and Grover, S 2013, 'Ammonia emissions from cattle urine and dung excreted on pasture', Biogeosciences, vol. 10, no. 1, pp. 327-338
THE LINK BETWEEN PEAT HYDROLOGY AND DECOMPOSITION: BEYOND VON POST
Hydrology is central to the formation, growth and utilisation of peat soils. However, peat presents a difficult medium in which to measure hydrologic properties, due to its soft structure and high water content. The von Post scale is a widely used measure of the extent of decomposition of peat, which can be applied in the field or the laboratory without specialised equipment. von Post decomposition has been shown to correlate with hydraulic conductivity. However, it is a categorical and subjective measure of decomposition. Technological advances now enable peat chemistry to be described quantitatively and objectively. 13C nuclear magnetic resonance (NMR) is now routinely used to describe peat chemistry, and the capability of this technique has been extended by the development of the molecular mixing model (MMM). The MMM predicts the molecular composition of organic material from the spectral information provided by 13C NMR. We found significant relationships between the hydrologic properties hydraulic conductivity and water yield and peat chemistry as described by 13C NMR and the MMM. These relationships have potential application in all fields where the hydrologic properties of peat soils are of interest (i.e. peat mining, peatland restoration, catchment management) and also in modelling of peatland development and responses to climate change.
Grover, S and Baldock, J 2013, 'The link between peat hydrology and decomposition: Beyond von Post', Journal of Hydrology, vol. 479, pp. 130-138
LAND USE CHANGE AND THE IMPACT ON GREENHOUSE GAS EXCHANGE IN NORTH AUSTRALIAN SAVANNA SOILS
Savanna ecosystems are subjected to accelerating land use change as human demand for food and forest products increases. Land use change has been shown to both increase and decrease greenhouse gas fluxes from savannas and considerable uncertainty exists about the non-CO2 fluxes from the soil. We measured methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) over a complete wet-dry seasonal cycle at three replicate sites of each of three land uses: savanna, young pasture and old pasture (converted from savanna 5–7 and 25–30 yr ago, respectively) in the Douglas Daly region of Northern Australia. The effect of break of season rains at the end of the dry season was investigated with two irrigation experiments. Land use change from savanna to pasture increased net greenhouse gas fluxes from the soil. Pasture sites were a weaker sink for CH4 than savanna sites and, under wet conditions, old pastures turned from being sinks to a significant source of CH4. Nitrous oxide emissions were generally very low, in the range of 0 to 5 μg N2O-N m−2 h−1, and under dry conditions soil uptake of N2O was apparent. Break of season rains produced a small, short lived pulse of N2O up to 20 μg N2O-N m−2 h−1, most evident in pasture soil. Annual cumulative soil CO2 fluxes increased after clearing, with savanna (14.6 t CO2-C ha−1 yr−1) having the lowest fluxes compared to old pasture (18.5 t CO2-C ha−1 yr−1) and young pasture (20.0 t CO2-C ha−1 yr−1). Clearing savanna increased soil-based greenhouse gas emissions from 53 to ∼ 70 t CO2-equivalents, a 30% increase dominated by an increase in soil CO2 emissions and shift from soil CH4 sink to source. Seasonal variation was clearly driven by soil water content, supporting the emerging view that soil water content is a more important driver of soil gas fluxes than soil temperature in tropical ecosystems where temperature varies little among seasons.
Grover, S, Livesley, S, Hutley, L, Jamali, H, Fest, B, Beringer, J, Butterbach-Bahl, K and Arndt, S 2012, 'Land use change and the impact on greenhouse gas exchange in north Australian savanna soils', Biogeosciences, vol. 9, no. 1, pp. 423-437
CARBON CHEMISTRY AND MINERALIZATION OF PEAT SOILS FROM THE AUSTRALIAN ALPS
The carbon chemistry of 10 profiles of peat soil has been described in detail using 13C nuclear magnetic resonance (NMR) spectroscopy. The changes with depth in the allocation of signal to different carbon functional groups were consistent with an increase in the extent of decomposition (EOD) of the organic material with depth. This increase in EOD with depth is typical of peat soils. Incubation experiments were carried out on peats spanning the range of EODs encountered, to investigate the effect upon mineralization of substrate quality (as defined by 13C NMR spectroscopy), water content and particle size. The confounding factors of depth, water content, bulk density, aeration and carbon content were eliminated by incubating ground peat material in a sand matrix. The size of the mineralizable carbon pool and the rate of carbon mineralization were both significantly affected by substrate quality, water content and particle size. Substrate quality had the greatest effect upon the size of the mineralizable carbon pool: as substrate quality decreased, so too did the size of the mineralizable carbon pool. Water content had the greatest effect upon the rate of carbon mineralization, which increased and then decreased as water content increased, with a maximum rate constant at a volumetric water content of 0.37 cm3 cm−3.
Grover, S and Baldock, J 2012, 'Carbon chemistry and mineralization of peat soils from the Australian Alps', European Journal of Soil Science, vol. 63, no. 2, pp. 129-140
ACCUMULATION AND ATTRITION OF PEAT SOILS IN THE AUSTRALIAN ALPS: ISOTOPIC DATING EVIDENCE
Bog peat soils have been accumulating at Wellington Plain peatland, Victoria, Australia for the last 3300 years. Now, dried peat soils are common adjacent to bog peats. The 14C basal age of dried peat is not different from the 14C basal age of bog peat, which supports the theory that dried peat formed from bog peat. A novel application of 210Pb dating links the timing of this change with the introduction of livestock to Wellington Plain in the mid‐1800s. Physical loss of material appears to have been the dominant process removing material as bog peats drained to form dried peats, as indicated by the mass balances of carbon and lead. This research has implications for the post‐fire and post‐grazing restoration of bogs in Victoria's Alpine National Park, and the contribution of peat soils to Australia's carbon emissions.
Grover, S, Baldock, J and Jacobsen, G 2012, 'Accumulation and attrition of peat soils in the Australian Alps: Isotopic dating evidence', Austral Ecology, vol. 37, no. 4, pp. 510-517
SEASONAL VARIATION AND FIRE EFFECTS ON CH4, N2O AND CO2 EXCHANGE IN SAVANNA SOILS OF NORTHERN AUSTRALIA
Tropical savanna ecosystems are a major contributor to global CO2, CH4 and N2O greenhouse gas exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact upon savanna soil microbial processes involved in N2O and CH4 exchange. We measured soil CO2, CH4 and N2O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniatatrees above sorghum grass) at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is a frequent natural and management action in these savanna (every 1–2 years). There was no seasonal change and no fire effect upon soil N2O exchange. Soil N2O fluxes were very low, generally between −1.0 and 1.0 μg N m−2 h−1, and often below the minimum detection limit. There was an increase in soil NH4+ in the months after the 2008 fire event, but no change in soil NO3−. There was considerable nitrification in the early wet season but minimal nitrification at all other times.
Savanna soil was generally a net CH4 sink that equated to between −2.0 and −1.6 kg CH4 ha−1 y−1 with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry season significantly reduced soil gas diffusion and as a consequence soil CH4 uptake. There were short periods of soil CH4 emission, up to 20 μg C m−2 h−1, likely to have been caused by termite activity in, or beneath, automated chambers. Soil CO2 fluxes showed a strong bimodal seasonal pattern, increasing fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH4 fluxes, but a much stronger relationship with soil CO2fluxes, explaining up to 70% of the variation in unburnt treatments. Australian savanna soils are a small N2O source, and possibly even a sink. Annual soil CH4 flux measurements suggest that the 1.9 million km2 of Australian savanna soils may provide a C sink of between −7.7 and −9.4 Tg CO2-e per year. This sink estimate would offset potentially 10% of Australian transport related CO2-e emissions. This CH4 sink estimate does not include concurrent CH4 emissions from termite mounds or ephemeral wetlands in Australian savannas.
Livesley, S, Grover, S, Hutley, L, Jamali, H, Butterbach-Bahl, K, Fest, B, Beringer, J and Arndt, S2011, 'Seasonal variation and fire effects on CH4, N2O and CO2 exchange in savanna soils of northern Australia', Agricultural and Forest Meteorology, vol. 151, no. 11, pp. 1440-1452
SOILS, CROP NUTRIENT STATUS AND NUTRIENT DYNAMICS ON SMALL-HOLDER FARMS IN CENTRAL TIBET, CHINA
Little is known about the soils that support agriculture in Tibet. The aim of this paper is to investigate the physical and chemical properties of Tibet’s agricultural soils, the nutritional status of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) crops, and the sustainability of current soil management practices. Physical descriptions of Tibet’s agricultural soils were based on soil pits dug at three locations across Tibet’s agricultural zone. Chemical analyses were conducted on soils from seven sites across the zone. Nutritional constraints to agriculture were identified through leaf tissue tests on wheat and barley crops from 23 fields. These results, combined with published information on farm inputs and yields, provided insight into the sustainability of current nutrient practice. Soils were found to be silty or sandy clay loams with alkaline reaction, low organic content and low K and Zn status. Leaf analysis revealed one third to one half of cereal crops were marginal or deficient for K, Zn and Mg. Most farmers export grain and import only nitrogenous and phosphatic fertilizers leading to a nutrient imbalance. A balanced fertilizer program is required to halt nutrient depletion and increase grain production. Reduced tillage and crop residue retention are needed to improve soil health.
Paltridge, N, Grover, S, Gouyi, L, Tao, J, Unkovich, M, Tashi, N and Coventry, D 2011, 'Soils, crop nutrient status and nutrient dynamics on small-holder farms in central Tibet, China', Plant and Soil, vol. 348, no. 12, pp. 219-229
SPECIAL—SAVANNA PATTERNS OF ENERGY AND CARBON INTEGRATED ACROSS THE LANDSCAPE
Savannas are highly significant global ecosystems that consist of a mix of trees and grasses and that are highly spatially varied in their physical structure, species composition, and physiological function (i.e., leaf area and function, stem density, albedo, and roughness). Variability in ecosystem characteristics alters biophysical and biogeochemical processes that can affect regional to global circulation patterns, which are not well characterized by land surface models. We initiated a multidisciplinary field campaign called Savanna Patterns of Energy and Carbon Integrated across the Landscape (SPECIAL) during the dry season in Australian savannas to understand the spatial patterns and processes of land surface–atmosphere exchanges (radiation, heat, moisture, CO2, and other trace gasses). We utilized a combination of multiscale measurements including fixed flux towers, aircraft-based flux transects, aircraft boundary layer budgets, and satellite remote sensing to quantify the spatial variability across a continental-scale rainfall gradient (transect). We found that the structure of vegetation changed along the transect in response to declining average rainfall. Tree basal area decreased from 9.6 m2 ha−1 in the coastal woodland savanna (annual rainfall 1,714 mm yr−1) to 0 m2 ha−1 at the grassland site (annual rainfall 535 mm yr−1), with dry-season green leaf area index (LAI) ranging from 1.04 to 0, respectively. Leaf-level measurements showed that photosynthetic properties were similar along the transect. Flux tower measurements showed that latent heat fluxes (LEs) decreased from north to south with resultant changes in the Bowen ratios (H/LE) from a minimum of 1.7 to a maximum of 15.8, respectively. Gross primary productivity, net carbon dioxide exchange, and LE showed similar declines along the transect and were well correlated with canopy LAI, and fluxes were more closely coupled to structure than floristic change.
Beringer, J, Hacker, J, Hutley, L, Leuning, R, Arndt, S, Amiri, R, Bannehr, L, Grover, S and et al, 2011, 'Special - Savanna patterns of energy and carbon integrated across the landscape', Bulletin of the American Meteorological Society, vol. 92, no. 11, pp. 1467-1485
THE IMPORTANCE OF TERMITES TO THE CH4 BALANCE OF A TROPICAL SAVANNA WOODLAND OF NORTHERN AUSTRALIA
Termites produce methane (CH4) as a by-product of microbial metabolism of food in their hindguts, and are one of the most uncertain components of the regional and global CH4exchange estimates. This study was conducted at Howard Springs near Darwin, and presents the first estimate of CH4 emissions from termites based on replicated in situ seasonal flux measurements in Australian savannas. Using measured fluxes of CH4 between termite mounds and the atmosphere, and between soil and the atmosphere across seasons we determined net CH4 flux within a tropical savanna woodland of northern Australia. By accounting for both mound-building and subterranean termite colony types, and estimating the contribution from tree-dwelling colonies it was calculated that termites were a CH4 source of +0.24 kg CH4-C ha−1 y−1 and soils were a CH4 sink of −1.14 kg CH4-C ha−1 y−1. Termites offset 21% of CH4consumed by soil resulting in net sink strength of −0.90 kg CH4-C ha−1 y−1 for these savannas. For Microcerotermes nervosus (Hill), the most abundant mound-building termite species at this site, mound basal area explained 48% of the variation in mound CH4 flux. CH4 emissions from termites offset 0.1% of the net biome productivity (NBP) and CH4 consumption by soil adds 0.5% to the NBP of these tropical savannas at Howard Springs.
Jamali, H, Livesley, S, Grover, S, Z, D, Hutley, L, Cook, G and Arndt, S 2011, 'The importance of termites to the CH4 balance of a tropical savanna woodland of northern Australia', Ecosystems, vol. 14, no. 5, pp. 698-709