Graeme Poile

Soil carbon dynamics under different cropping and pasture management in temperate Australia: Results of three long-term experiments

In addition to its important influence on soil quality and therefore crop productivity, soil organic carbon (SOC) has also been identified as a possible C sink for sequestering atmospheric carbon dioxide. Limited data are available on the impact of management practices on the rate of SOC change in agricultural soils in Australia. In this paper, results of three long-term trials (13–25 years) located near Wagga Wagga in temperate Australia were used to assess C dynamics under different tillage and stubble management practices, and under cropping intensities in pasture/crop rotations. Experimental results confirm the importance of management practices and pasture in determining first the steady-state SOC concentrations that are characteristic of given rotations and crop management systems, and second the rates of change of SOC concentrations as they approach steady-state concentrations in agricultural soils of this agro-ecological zone. A long-term crop/pasture experiment at a site with initial high SOC showed that the rate of SOC change in different treatments ranged from –278 to +257 kg C/ha.year over 0–0.3 m soil depth. Under continuous cropping, even under conservation agriculture practices of no-tillage, stubble retention, and crop rotation, the high initial SOC stock (0–0.3 m) present after a long-term pasture phase was, at best, maintained but tended to decrease with increased tillage or stubble burning practices. The effect of tillage was greater than that of stubble management. Increases in SOC were observed only in rotations incorporating a pasture phase. Our results suggest that improved soil nutrient and grazing management of permanent pasture can lead to an increase of 500–700 kg C/ha.year where the initial SOC concentrations are well below steady-state concentrations that could be expected after long periods of improved management. No difference was found between perennial pasture and annual pasture to the depth measured (0–0.3 m). Our results suggest that pasture holds the key to maintaining, and even increasing, SOC under crop/pasture in this environment.

Soil carbon stocks under different pastures and pasture management in the higher rainfall areas of south-eastern Australia

In Australia, pastures form the basis of the extensive livestock industries and are important components of crop rotation systems. Despite recent interest in the soil carbon sequestration value of pastures in the mitigation of climate change, little information is available on the soil carbon sequestration potential of pastures in New South Wales farming systems. To quantify the soil carbon stocks under different pastures and a range of pasture management practices, a field survey of soil carbon stocks was undertaken in 2007 in central and southern NSW as well as north-eastern Victoria, using a paired-site approach. Five comparisons were included: native v. introduced perennial, perennial v. annual, continuous v. rotational grazing, pasture cropping v. control, and improved v. unimproved pastures. Results indicated a wide range of soil organic carbon (SOC) stocks over 0–0.30 m (22.4–66.3 t C/ha), with little difference when calculated based on either constant soil depth or constant soil mass. Significantly higher SOC stocks were found only as a result of pasture improvement using P application compared with unimproved pastures. In this case, rates of sequestration were estimated to range between 0.26 and 0.72 t C/ha.year, with a mean rate of 0.41 t C/ha.year. Lack of significant differences in SOC stocks for the other pastures and pasture management practice comparisons could be due to inherent problems associated with the paired-site survey approach, i.e. large variability, difficulties in obtaining accurate site history, and the occasional absence of a valid control as well as the likely lower rates of SOC sequestration for these other comparisons. There is a need for scientific long-term trials to quantify the SOC sequestration potential of these other pastures and pasture management practices.

Comparison of three carbon determination methods on naturally occurring substrates and the implication for the quantification of 'soil carbon'

Accounting for carbon (C) in soil will require a degree of precision sufficient to permit an assessment of any trend through time. Soil can contain many chemically and physically diverse forms of organic and inorganic carbon, some of which might not meet certain definitions of ‘soil carbon’. In an attempt to assess how measurements of these diverse forms of C might vary with analytical method, we measured the C concentration of 26 substrates by three methods commonly used for soil C (Walkley–Black, Heanes, and Leco). The Heanes and Leco methods were essentially equivalent in their capture of organic C, but the Leco method captured almost all of the inorganic C (carbonates, graphite). The Heanes and Walkley–Black methods did not measure carbonates but did measure 92% and 9%, respectively, of the C in graphite. All three of the common soil test procedures measured some proportion of the charcoal and of the other burnt materials. The proportion of common organic substrates (not the carbonates, graphite, or soil) that was C by weight ranged from ~10% to 90% based on the Heanes and Leco data. The proportion of the organic fraction of those same substrates, as measured by loss-on-ignition, that was C by weight ranged from 42% to 100%. The relationship between Walkley–Black C and total C (by Heanes and Leco) showed that Walkley–Black C was a variable proportion of total C for the 26 substrates. Finally, the well-known, apparent artefact in the Cr-acid methods was investigated: dichromate digestion should contain at least 7–10 mg C in the sample or over-recovery of C might be reported. Our observation that common soil C procedures readily measure C in plant roots and shoots, and in burnt stubble, means that there will likely be intra-annual variation in soil C, because avoidance of these fresh residues is difficult. Such apparent intra-annual variation in soil C will make the detection of long-term trends problematic.

Three long-term trials end with a quasi-equilibrium between soil C, N, and pH: an implication for C sequestration

The aim of this study was to assess the long-term changes in some key soil chemical properties at the completion of three long-term trials in south-eastern Australia and the relationship between those soil properties. From a soil organic matter perspective, the build-up of carbon (%C) requires an accumulation of nitrogen (%N), and the build-up of %C and %N fertility comes at the cost of soil acidity. Rotation, tillage, and stubble practices combine to alter the quantity, quality (C : N), and the depth distribution of organic matter in a soil, but the three soil chemical properties reported here seem to also be in quasi-equilibrium at the three long-term sites. The consequence is that if the build-up of soil organic matter leads to soil acidification, then the maintenance of agricultural production will require liming. The emission of CO2 when limestone reacts with soil acids, plus the C cost of limestone application, will negate a proportion of the gains from C sequestration as organic matter in soil. Such cautionary information was doubtless unforeseen when these three long-term trials were initiated.

pH buffering: the chemical response of acidic soils to added alkali.

Most studies about lime requirement and the pH buffering capacity of soil have either calibrated buffer solutions to soil-CaCO 3 reactions or have correlated pH buffering with soil properties such as clay and organic matter content. A different approach is to address two fundamental questions: where do OH - ions react when they are added to acid soils, and can we quantify these reactions? The experimental hypothesis was that alkali added to a soil (cmol/kg) could be accounted for quantitatively by summing the increase in effective cation exchange capacity (ECEC) (cmol/kg), the decrease in exchangeable A1 (cmol/kg), and, possibly, the decrease in exchangeable Mn (cmol/kg). It was presumed that this model would begin to fail as pH approached neutrality, where the precipitation of CaCO 3 would start to exert an influence on pH buffering capacity. Hence, this model was intended to account for the reaction of applied OH- in the approximate range 4 < pH(CaCl 2 ) < 7. Twelve soils were titrated with Ca(OH) 2 at near constant ionic strength and soil:solution ratio so as to minimize changes in (H + ) caused by redistribution between solution and adsorbed/exchangeable phases. After 16 h of reaction, the exchangeable cations were measured in each soil at each concentration of Ca(OH) 2 addition. The data indicated that exchangeable Mn was not a sink for OH- under the experimental conditions. Precipitation of Ca 2+ occurred during the reaction period, causing an apparent loss of the applied alkali. However, this reaction could be expected to reverse over a longer period of time and thus was considered an experimental artifact rather than a component of buffering capacity. The experimental artifact could be overcome by expressing pH buffering in terms of the measured slope of pH change versus the sum of measured changes in ECEC plus exchangeable Al. It was concluded that in the pH(CaCl 2 ) range 4 to 7, the soil sink for added alkali can be accounted for quantitatively by the increase in ECEC plus the decrease in exchangeable Al. Therefore, soil pH BC in the alkaline direction can be estimated from changes in ECEC and exchangeable Al.

Novel barley (Hordeum vulgare L.) germplasm resistant to acidic soil

Improving the resistance of barley (Hordeum vulgare L.) to acidic soils is an important goal of several barley breeding programs around the world. The identification and utilisation of novel barley sources resistant to aluminium (Al) may provide a significant and rapid advance towards that goal. Barley standards and screening protocols for selecting barley germplasm resistant to Al in nutrient solution and acidic soil were reevaluated. The assays used were quantitative in nature and were suitable for genotypic- and seedling-based selections. Although there was a broad agreement between the solution culture assays and soil assays in the ranking of genotypes it obscured the fact that misclassification of genotypes is common. Brindabella was shown to be better suited than Dayton (the current barley standard resistant to Al) as the Australian standard for resistance to acidic soils. A seedling-based Al pulse-recovery assay and an acidic soil assay were used to characterise 41 genotypes from the South and East Asian Barley Core Collection (SEA-BCC). In addition, in the acidic soil assays several standard barley and wheat genotypes were included. Three SEA-BCC genotypes were more resistant than Dayton to acidic soil while several others were similar to Dayton. The most resistant SEA-BCC genotypes Honen, Ohichi and Zairai Tanbo were of Japanese origin. Misclassification of barley genotypes and wheat genotypes for resistance to soil acidity between solution culture and acid soil assay provided strong evidence for the unsuitability of solution culture assay. Although in solution culture several barley genotypes were sensitive relative to wheat, in acidic soil they were not different from wheat. While the quest for resistant barley to acidic soils similar or better than resistant wheat still continues, it may be an unnecessary endeavour.

Tillage does not increase nitrous oxide emissions under dryland canola (Brassica napus L.) in a semiarid environment of south-eastern Australia

Dryland cereal production systems of south-eastern Australia require viable options for reducing nitrous oxide (N2O) emissions without compromising productivity and profitability. A 4-year rotational experiment with wheat (Triticum aestivum L.)–canola (Brassica napus L.)–grain legumes–wheat in sequence was established at Wagga Wagga, NSW, Australia, in a semiarid Mediterranean-type environment where long-term average annual rainfall is 541mm and the incidence of summer rainfall is episodic and unreliable. The objectives of the experiment were to investigate whether (i) tillage increases N2O emissions and (ii) nitrogen (N) application can improve productivity without increasing N2O emissions. The base experimental design for each crop phase was a split-plot design with tillage treatment (tilled versus no-till) as the whole plot, and N fertiliser rate (0, 25, 50 and 100kgN/ha) as the subplot, replicated three times. This paper reports high resolution N2O emission data under a canola crop. The daily N2O emission rate averaged 0.55g N2O-N/ha.day, ranging between –0.81 and 6.71g N2O-N/ha.day. The annual cumulative N2O-N emitted was 175.6 and 224.3g N2O-N/ha under 0 and 100kgN/ha treatments respectively. There was no evidence to support the first hypothesis that tillage increases N2O emissions, a result which may give farmers more confidence to use tillage strategically to manage weeds and diseases where necessary. However, increasing N fertiliser rate tended to increase N2O emissions, but did not increase crop production at this site.

Soil potassium relationships, uptake efficiency and availability for six distinctive soils in central and southern New South Wales, Australia

Most soils in eastern Australian contain abundant soil potassium (K) reserves, and it is often assumed that there are no problems with soil K status. However, soil K deficiency has been reported in selected locations, and for viticulture, there are potential problems with high soil K concentrations due to the application of winery wastewater. This study investigated different soil K variables and plant variables for six soils with distinctive properties from across central and southern New South Wales to determine the presence of soil K deficiency and to understand the effect of adding K on the dynamics of soil K availability. A glasshouse experiment compared the selected soils under three fertiliser K rates with forage kale as the test species. Highly significant differences (P < 0.001) were found for soil and fertiliser K rate effects for three measures of soil K (solution K, soln K; exchangeable K, exch. K; tetraphenyl borate K; TBK). Significant soil and fertiliser rate effects were detected (P < 0.001 and P = 0.04 respectively) for the plant shoot (stem and leaf) biomass and nutrient uptake efficiency (UPE) index, but no plant K deficiency was detected; in fact, luxury K consumption was likely. Quantification of K efficiency indices (UPE and utilisation efficiency, UTE) demonstrated significant differences between the soils in the ease with which K was removed. This was illustrated by the negative correlation between both UPE and UTE with final exch. K. From soil properties potentially related to soil K variables, a significant linear regression relationship (P = 0.05) was found for TBK with illite and clay content. By contrast, a linear regression relationship between exch. K and illite content only was weaker (P = 0.09). These relationships show how soil properties (especially mineralogy) can predict soil K variables. A significant positive log–log relationship was found between exch. K or TBK for 37 Queensland soils and the same soil K measures from this study, consistent with this relationship. This relationship indicates that TBK can be effectively predicted from measuring exch. K for a wide range of soils across eastern Australia, but more research is required to understand the value of TBK to predict soil K availability.

Simultaneous measurement of exchangeable Al and other cations in acidic soils

It is both time consuming and costly to undertake two extractions of acidic soils when there is a need to measure exchangeable Al along with the other cations. There is some evidence that, although 1 M KCl is the standard procedure for exchangeable Al, the extraction of soil with 0.1 M BaCl2 + 0.1 M NH4Cl gives similar values. It would then be possible to measure all cations from one extraction. There is also concern that the assumption of trivalence of Al does not hold true in all situations, as commonly held to be true in the literature of the 1950s to the 1970s. Two experiments were conducted: the first a simple comparison of three extraction procedures in common use in New South Wales and the second a repeated comparison of two extractions but with more detailed measurements to enable interpretation of the results. During the second experiment we also measured the charge on the extracted Al by titration. The three methods for extraction of Al gave similar results despite very different procedures with respect to physical mixing, soil–solution contact time and strength of electrolyte, indicating that the pool of exchangeable Al was operationally well defined. The average charge on KCl-extracted Al was within error of 3 moles per mole of Al, supporting the current trivalent model of Alex. The 0.1 M BaCl2 + 0.1 M NH4Cl procedure estimated Alex successfully on acid soils of low effective cation exchange capacity (ECEC) (<10 cmolc/kg) and so can be used for extraction of all cations. However, as ECEC increased the 0.1 M BaCl2 + 0.1 M NH4Cl extraction tended to underestimate Alex compared with KCl on soils with above ~1.5 to 2 cmolc/kg of Alex.

Biomass, feed quality, mineral concentration and grain yield responses to potassium fertiliser of dual-purpose crops

ABSTRACT The effect of increasing rates of potassium (K) fertiliser was studied on a soil with base exchangeable K values ≤  0.1 cmol(+)/ kg by assessing biomass, feed quality, mineral concentration and grain yield for wheat, triticale and canola. The crops showed variable biomass response to K rates at early plant growth stages, however at anthesis there were significant positive biomass responses to increasing rates of K. A positive K rate effect was detected on feed quality with significant increases in metabolisable energy. Irrespective of sampling time, increasing K rate resulted in significant increases in crop K concentration and mineral ratios, such as the Tetany index, but significant decreases in Ca and Mg concentrations. Final grain yield was significantly increased by K rate and there were beneficial effects on grain quality (thousand grain weight). Overall, this study highlights the importance of K to production responses and implications for the health and productivity of grazing livestock.