Mark Conyers

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.

Influence of dryland agricultural management practices on the acidification of a soil profile

Abstract The high cost of transporting limestone for distances up to 300 km to acidic cropping soils in Australia has caused interest in evaluating the effect of management practices on soil acidification. Practices which minimise acidification will minimise the use and hence cost of liming. A long term rotation trial which started in 1979 at Wagga Wagga in southeastern Australia provided an ideal opportunity to evaluate the effects of some rotations, stubble handling and tillage practices on the relative rates of soil profile acidification over a period of 12 years. The soil had an initial pH (CaCl 2 ) of 4.93 and 0.13% total N. All 13 treatments acidified the soil over the 12 years of the trial, at rates equivalent to a loss of 46 to 95 kg CaCO 3 ha −1 year −1 . Although there were differences between treatments in their relative rates of acidification only the management of N, as fertiliser or via legumes in the rotation, appears to offer any scope for minimising acidification rates. Most treatments influenced the stratification of the soil profile more than they influenced the net rate of acidification. All 13 treatments had minimum pH and minimum variance in pH at 5–10 cm depth, the depth range into which crop seed is generally placed. Direct drilling resulted in a higher pH (CaCl 2 ) at 0–5 cm depth than conventional (2 or 3 passes) cultivation but the reverse was true at 5–10 cm. Direct drilling resulted in a stratification of soil pH which was exaggerated by burning stubble, as opposed to retaining it, and minimised by an increasing intensity of cultivation. There was no difference in soil profile acidification between lupin crops ( Lupinus augustifolius ), L, and subterranean clover pasture ( Trifolium subterraneum ), C, in 1:1 rotations with wheat ( Triticum aestivum ), W. The addition of a second wheat phase, LWW, made no difference to the soil pH compared to the LW rotation. The effects of mowing and mulching of subterranean clover on soil pH were not significantly different to the effects of grazing by sheep. The addition of 100 kg N ha −1 year −1 as urea in three split applications to continuous crops of wheat caused the surface 10 cm of soil to be acidified by 0.4 pH units compared with where no N fertiliser was applied. The greater acidification extended to at least 15 cm depth. In our system, which uses only annual species and relies on N inputs from legumes or fertiliser, there has so far been little evidence that realistic management practices will minimise acidification and the requirement for lime.

Perennial pastures for recharge control in temperate drought-prone environments. Part 1: productivity, persistence and herbage quality of key species

Abstract Perennial-based pasture swards potentially offer land managers the capacity for recharge control in temperate cropping zone environments to satisfy the dual role of fostering increased agricultural productivity and reduced deep drainage. This study evaluated the productivity, persistence and herbage quality of lucerne (Medicago sativa L.), phalaris (Phalaris aquatica L.), chicory (Cichorium intybus L.), perennial veldt grass (Ehrhata calcycina Sm.), grazing brome (Bromus stamineus E. Desv.), plantain (Plantago lanceolata L.), Rhodes grass (Chloris gayana Kunth), tall fescue (Festuca arundinacea syn. Lolium arundinaceum Schreb. syn. Schedonorus phoenix (Scop.) Holub.) and cocksfoot (Dactylis glomerata L.) in two contrasting environments in the cropping zone of southern New South Wales (NSW), Australia. The performance of two cultivars with contrasting levels of summer activity of each of the latter two species was also assessed. Lucerne was the most productive species evaluated, producing 54–85% more herbage than phalaris, the next most productive species. Lucerne was also the most persistent species with a higher basal frequency than all other species during the experimental period and, averaged across samplings, had the highest crude protein (22.3%) in the leaf and stem of any species. Chicory herbage had the highest dry matter digestibility (76.7%) and ash content (15.1%) and lowest neutral (35.4%) and acid detergent fibre contents (21.8%) compared with the other species. The more summer-dormant cultivars of cocksfoot (cv. Kasbah) and tall fescue (cv. Fraydo) were both found to be more persistent than their semi-summer-active counterparts (cvv. Currie and Demeter, respectively), demonstrating the importance of summer dormancy for the persistence of both species in these environments. Tall fescue cv. Fraydo was equally persistent yet produced only 42–51% of the cumulative biomass of phalaris over 5 years, indicating that tall fescue is not a viable species in these drought-prone environments, nor were plantain and grazing brome due to their inferior productivity and persistence. The study highlighted the lack of viable perennial pasture options currently available in cropping zone environments of southern NSW other than lucerne, phalaris and the summer-dormant cultivar of cocksfoot, Kasbah. Chicory and perennial veldt grass, with further breeding and selection under Australian environmental conditions, could have the potential to be viable perennial pasture options for the cropping zone of southern NSW.

Effect of plant residue return on the development of surface soil pH gradients

Abstract In the field, surface soil pH gradients were observed under senescing plants over late spring and summer. A soil incubation experiment was conducted (119 days, 20  °C) to provide direct evidence of the influence of plant residue incorporation on soil pH. This was investigated in terms of plant residue type (wheat and subterranean clover) and dry matter addition rate (0, 6.25, 12.5 and 25.0 g kg–1), as well as the soil layer of incorporation (0–2.5 and 7.5–10 cm) and moisture regime (continuously moist and moist-dry cycles). During incubation, moist unamended soils slowly acidified. In contrast, the addition of plant residue resulted in a rapid (day 0–7) increase of soil pH due to the association, and particularly oxidation, of added organic anions. This was followed by a gradual (day 7–119) pH decline attributed to the mineralization and subsequent nitrification of added organic N. The addition of 12.5–25.0 g kg–1 of cereal crop residues, and 6.25–25.0 g kg–1 of legume-based pasture residues, resulted in a net alkalization of the surface 2.5 cm of soil. It was therefore concluded that surface soil pH gradients observed in the field were largely attributable to an increase of pH at the surface 2.5 cm in response to plant residue return. The magnitude of such gradients will be particularly large with the return of large quantities of plant residues of high ash alkalinity in soils of relatively low initial pH and biological activity, and when the surface of the soil is exposed to moist-dry cycles.

A conceptual framework for improving the P efficiency of organic farming without inputs of soluble P fertiliser

The issues for P availability to agricultural plants are 3-fold: the solution concentration of P during early growth (intensity factor); the quantity of P in the soil ‘bank’ to meet plant and animal needs (capacity or quantity factor); and the rate at which P becomes available from mineral and organic sources (kinetic factor). These three needs can be met by: (1) applying traditional (manure, compost) and novel (biosuper) sources; (2) modifying or selecting plants for their root architecture, phytase activity, carboxylate excretion, and P translocation inefficiency; (3) encouraging rhizosphere conditions which favour phytase and carboxylase activity; (4) developing symbioses with mycorrhiza and Penicillium species; (5) developing exoenzyme products which release inositol P; (6) adjusting soil pH to 6–7 for maximum availability of native mineral P sources or lowering pH for maximum availability of reactive rock P; (7) developing rotations which maximise organic P cycling. These 7 broad groupings of strategies to improve P nutrition each operate by 1 or more of the 3 mechanisms of quantity, intensity, and kinetic factors. The possible application of these strategies to ‘organic’ farming is outlined in this review. However, a successful application of these strategies might also improve the P efficiency of conventional agriculture.

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.

Lime responses by barley as related to available soil aluminium and manganese

Twenty-three surface soils (0-10 cm) sampled from major New South Wales lime trials were incubated at six lime rates, from 0 to 10 t/ha, and used in pot trials with barley (Hordeurn vulgare cv. Schooner) which was grown for five weeks. Each replicate pot was soil tested for exchangeable cations (Ca, Mg, K, Na, Mn, Al), pH in 0.01 M CaCl2, and Al in the 0.01 M CaCl2 extract by pyrocatechol violet (total Al) and by reaction for 15 s in 8-hydroxyquinoline (monomeric Al). The latter was also converted to the activity of Al3 in the 0.01 M CaCl2 extract. The soil tests were compared for their prediction of the dry matter yield of whole tops of barley. The four tests for aluminium (exchangeable Al as Al/ECEC; total 0.01 M CaCl2 extractable Al; monomeric 0.01 M CaCl2 extractable Al; and Al3+ activity in 0.01 M CaCl2) were each better indicators of the infertility of the acid soils than soil pH. The prediction of the dry matter yield of barley by the four aluminium tests was improved by the inclusion of exchangeable soil manganese in the statistical analysis. The use of 0.01 M CaCl2 is recommended as a routine extraction procedure for diagnosing Al toxicity. Soil Mn should also be measured and included in correlations with barley growth. Pyrocatechol violet is the recommended analytical procedure for Al owing to its comparative simplicity.

Crop responses to lime in long-term pasture-crop rotations in a high rainfall area in south-eastern Australia

A long-term trial, known as ‘managing acid soils through efficient rotations’ (MASTER), commenced in 1992 to develop and demonstrate a cropping system that is economically viable on the highly acid soils of the traditional permanent pasture region in south-eastern Australia, so that their fertility is sustained or improved. There were 2 permanent pasture systems and 2 pasture–crop rotations, each with and without lime. This paper reports the effect of lime on crop production over the first cycle (6 years). On annual pasture–crop rotations, lime significantly increased the dry matter production at anthesis and grain yields of wheat (cv. Dollarbird) compared with the unlimed treatments. Averaged across years from 1992 to 1997 (excluding the severe drought year 1994), wheat crops produced 1.6 t/ha more grain on the limed treatments than on the unlimed treatments (3.6 v. 2.0 t/ha). On perennial pasture–crop rotations, the lime effects varied with crops grown at each phase and year. For example, despite being tolerant of acidity, oats (cv. Yarran) responded to lime in 1996. Likewise, triticale (cv. Abacus) responded to lime in 1997. Wheat (cv. Dollarbird) that is moderately tolerant to acidity responded to lime in phase 6 from 1992 to 1997 excluding 1994 (3.5 v. 1.7 t/ha). Acid-tolerant wheat varieties, triticale, and narrow-leaf lupins are considered the most viable crops for the soil and climatic conditions encountered in this high rainfall (5000—800 mm per annum) area of south-eastern Australia.

Pasture and sheep responses to lime application in a grazing experiment in a high-rainfall area, south-eastern Australia. I. Pasture production

‘Managing Acid Soils Through Efficient Rotations (MASTER)’ is a long-term pasture–crop rotation experiment commenced in 1992. One of the objectives was to demonstrate the extent of crop, pasture, and animal responses to lime on a typical acidic soil in the 500–800 mm rainfall zone in south-eastern Australia. Two types of pastures (perennial v. annual pastures) with or without lime application were established in 1992. This paper presents the results of the pasture dry matter (DM) responses to lime application over 6 years from 1992 to 1997. Results showed that both perennial and annual pastures responded positively to lime on a highly acidic soil on the south-west slopes of New South Wales. Averaged across pasture types and 5 growing seasons, the limed pastures produced 18% more pasture DM (520 kg/ha, P < 0.05) than the unlimed pastures. Significant responses to lime were detected on perennial pastures (610 kg DM/ha, P < 0.05), but not on annual pastures, although the limed annual pastures produced more DM (420 kg/ha, P = 0.20) than the unlimed annual pastures. There was a large seasonal variation in pasture growth rate with the significant lime responses in winter and spring on both perennial pastures (P < 0.05) and annual pastures (P < 0.10 in winter and P < 0.05 in spring), but no responses in autumn and summer on either perennial or annual pastures. The extra growth in winter is of importance as winter is the period when feed is normally inadequate and limits stocking rates. It is recommended that perennial-based pastures should be promoted for the purposes of productivity, in terms of increasing pasture production and improving feed quality, and for the environmental benefits in terms of alleviating the soil acidity problem and reducing the risk of dryland salinity in the high-rainfall zone in south-eastern Australia.