Are pasture soils models of soil biological fertility?
Summary
Given the relative health of pastoral soils in comparison to cropped soils, Stirling and Lodge suggest that pastoral areas might be chosen as ecological reference sites for agricultural management. In general, Stirling and Lodge’s study illuminates important differences between biological processes under a range of pastures in two different climatic zones. The study also provides land managers with an array of indicators that could be used as measures of soil health in pastoral regions. It is vital that robust indicators of soil health are continually developed and refined by scientists as well as assessed and monitored by land managers to ensure that we maintain and improve the productivity and health of an important economic asset into the future.
Research findings
Australia’s most productive pastures occur in areas of relatively high rainfall. In fact, it has been estimated that 50% of Australian cattle sales and 40% of sheep sales occur in pastures that receive over 600mm of rain per year (Stirling and Lodge 2005). Given the importance of these high rainfall pasturelands to the Australian economy, it is critical that we ensure their productivity into the future by protecting soil health.
It is surprising therefore that very little broad-based research has occurred to illuminate the biological processes underpinning soil health in high rainfall pastoral regions. A study in the Australian Journal of Soil Research seeks to rectify this situation by investigating wide-ranging factors that play a role in maintaining soil health in two important pastoral regions of Australia (Stirling and Lodge 2005).
Stirling and Lodge chose to investigate 40 sites, all of which receive a relatively high per annum rainfall of between 675mm and 692mm. The sites chosen, however, occur in two quite different climatic regions. One of these regions, near Tamworth and Armidale in New South Wales, has a pattern of predominantly summer rainfall, while sites near Hamilton in Western Victoria and South Australia receive primarily winter rainfall.
Stirling and Lodge investigated a wide array of indicators in order to gain a broad-brush understanding of soil health in the two climatic regions, including the influence of pasture type and climate on soil biological processes. The authors chose to investigate soil processes under four common pastures: subterranean clover, lucerne, phalaris and, in the winter zone only, perennial ryegrass. Samples were taken from several sites in each zone under each pasture type.
In the laboratory, a range of properties of each sample were measured each sample as indicators of soil health. Total organic nitrogen and carbon were assessed as a measure of the resources available to maintain the soil ecology, while labile carbon was measured to provide an indication of carbon readily available as a food source for the soil biota (in the form of amino acids, sugars or organic acids). Microbial biomass provided the authors with an indication of the total size of the soil microbial community, while the activity of this community was also investigated using a measure of enzyme activity. Finally, the number and variety of soil nematodes in each sample was measured to provide an indication of soil nutrient availability, soil structure, and soil decomposition channel.
Plant parasitic nematodes
Stirling and Lodge found that several species of plant parasitic nematodes, especially lesion nematodes, were wide-spread across both climatic zones and all pasture types. However, levels of these nematodes were not high enough to pose a problem for pasture growth, as they often do for cereal crops. The authors concluded that pasture species or pasture management practices may be less advantageous to lesion nematodes than some crop species or crop management practices.
Free-living nematodes
Stirling and Lodge also assessed levels of free-living nematodes to provide an indication of soil biological health across three indices:
- Enrichment index (EI) – a measure of soil nutrients available as a resource for the soil biota;
- Structure index (SI) – a measure of the degree of soil structure
- Channel index (CI) – a measure of the soil’s primary channel of decomposition.
The enrichment index was found to be highest in the winter zone, particularly under lucerne and subterranean clover. In particular, the existence of the bacterial feeding nematode family Rhabditidae indicated a large bacterial population at the winter sites as a food source for these nematodes. By way of contrast, Rhabditidae was rarely found in any of the summer rainfall sites. The enrichment index was found to be particularly low in the subterranean clover and lucerne pastures of this climatic zone. The authors concluded that low numbers of Rhabditidae in this zone may have been the result of low soil moisture content at the time of sampling – Rhabditidae feeds on bacteria living in films of moisture and is therefore only supported in soil with high moisture content.
The structure index was used as a measure of soil structure, and was indicated by the presence of an omnivorous nematode that is sensitive to soil disturbance by cultivation, heavy metals, arsenic, acid or nitrogen fertilizer. Unlike agricultural soils, pastoral soils are not often subject to disturbance and could therefore be expected to have good soil structure. Contrary to this prediction, only two pastoral types in the winter zone (perennial ryegrass and phalaris) were demonstrated to have a high structure index. In fact, the summer zone had a significant proportion of sites with poor soil structure: these sites were often acidic, planted with lucerne or subterranean clover, and had low soil moisture content of only 5-11%.
Climatic zone appeared to be a strong determining factor for the decomposition channel index of each site. In the summer zone, fungal decomposition predominated, while bacterial composition prevailed in the winter zone (as indicated by differences in the ratio of bacterial to fungal feeding nematodes). In consuming bacteria, ‘microbivorous’ nematodes take in more nitrogen than necessary for their body structure. This excess nitrogen is excreted as ammonia, available as nitrogen in the soil for uptake by plants and bacteria (Ferris 1998). Stirling and Lodge surmise that the two climatic zones therefore experience very different patterns of nutrient cycling.
Soil organic matter and microbial communities
Climatic zone played a strong influence on the level of nitrogen as well as the level of total organic carbon at each site. In the winter zone, levels of carbon and nitrogen, as well as levels of labile carbon, were found to be significantly higher than in the summer zone. As a consequence of this larger available food source, sites in the winter zone were also shown to support significantly higher bacterial populations. The authors conclude that predictable rainfall in the winter zone facilitates higher levels of pasture growth, which in turn generates a higher degree of soil organic matter in this zone. Bacterial populations and microbial activity were found to be particularly low in lucerne pastures in the summer zone (as also reflected by the low enrichment index at these sites).
Cropping or pasture
In general, the investigation demonstrated that pastures in both climatic zones were relatively healthy – especially when compared to land used for cropping. In a previous study, total carbon levels in the summer rainfall zone were found to be 44% lower in cropped land than in adjacent pastures (Lefroy et al. 1993). Supporting the results of this previous study, levels of microbial activity in the pastures investigated by Stirling and Lodge were found to be much higher than levels observed in cropped soil in the northern grain-growing region (Bell et al. unpublished). By way of explanation, the authors concluded that pastoral land benefits from relatively little disturbance as well as the enhancement of grass or legume growth.
References
Ferris H (1998) ‘The Role of Nematodes in Soil Fertility’.
Stirling GR and Lodge GM (2005) A survey of Australian temperate pastures in summer and winter rainfall zones: Soil nematodes, chemical and biochemical properties’. Australian Journal of Soil Research 43: 887-904. Read Abstract.
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