Some insights into the health of Dermosols around Tasmania, Australia.
Although research on the effects of agricultural management on Tasmanian soils has been undertaken at specific sites (Cotching et al. 2002; Sparrow et al. 1999) the ability to report on soil condition, and specifically trends in soil condition, around the State has typically been limited to qualitative comments in State of the Environment reporting (Tasmanian Planning Commission 2009). In 2004, the Department of Primary Industries, Parks, Water and Environment (DPIPWE) initiated a project to measure and monitor the condition of a variety of soil types under a range of land uses over time and to gather data on the condition of the State's valuable soil resource. Ultimately, the aim of the project is to provide information on how soil condition may be changing over time. Initially funded by the Australian government and supported by the three Tasmanian Natural Resource Management (NRM) regions, this project collected baseline data to enable future quantifiable assessments of trend against a range of soil condition indicators. Each site is to be revisited and resampled every 5 years to provide further data for comparison against the baseline dataset. An overview of the project, together with some findings from the initial sampling round, are provided in Cotching and Kidd (2010). The present paper identifies some of the soil changes that have occurred based on the results of two separate sampling rounds, 5 years apart, for one Tasmanian soil type. It is not the intention of this paper to identify reasons why changes have occurred, nor to relate changes to management practices.
Soils provide a wide range of ecosystem services, many of which are often forgotten in our busy daily routines. They provide the medium in which most of our food and other organic resources are grown; they support a range of environmental and biodiversity systems (including carbon sequestration), filter water that flows into streams and rivers, provide homes, habitats and resources for a range of flora and fauna and contribute to the overall diversity of our planet. Soil health can be defined as the ability of soils to sustainably provide these ecosystem services. Soil condition measures the current state of the soil against a variety of soil properties (indicators) considered relevant to soil health. Although different soils may have varying capacity to undertake ecosystem services, changes in soil condition may improve or reduce this capacity.
Dermosols, according to the Australian soil classification (Isbell 2002), represent those soils that have a moderate or better structure within the B2 horizon and lack texture contrast between the A and B horizons. In Tasmania, Dermosols typically comprise clay loam topsoils grading to light clay or clay subsoils. Cotching (2009) reports that, based on an interpretation of historical land systems information, Dermosols are the dominant soil order that occurs in Tasmania (24% vs the next most common, Organosols, at almost 15%).
Previous work by Cotching et al. (2002) assessed 15 Dermosol sites for changes associated with three forms of agricultural management: (1) long-term pasture; (2) cropping with shallow tillage; and (3) cropping with more rigorous tillage. That work identified significant changes in a variety of soil parameters that could be attributed to land management and to the duration of cropping history. Although considered fairly robust, Cotching et al. (2002) consider that the Dermosols are generally less well drained and less robust soils than the Ferrosols of northern Tasmania, yet they are often cultivated as intensively. Chilvers (1996) provided management guidelines to help maintain or improve soil condition for both these soil types. The effects of intensive cropping on Ferrosols have been reported by Sparrow et al. (1999), who identified correlations between declines in organic carbon content and length of cropping history.
The Soil Condition Evaluation and Monitoring (SCEAM) project was initiated to establish baseline data on soil condition for a variety of soil-land use combinations around the State. Repeat sampling at each site every 5 years is planned to develop a dataset that enables identification of trends in soil condition over time. Several key soil properties were identified as being important indicators of soil condition, including soil pH, organic carbon content, bulk density and aggregate stability, and this paper reports specifically on changes that have occurred in these soil properties between the two sampling rounds completed to date.
Materials and methods
The SCEAM project sampled 285 sites around Tasmania between May 2004 and October 2008, encompassing a wide range of soil type and land use combinations. Of these sites, 59 are Dermosols under an agricultural land use, whereas a further 25 are under plantation or native forest.
Soil sites suitable for investigation under SCEAM were identified using available soil maps of Tasmania plus local knowledge and contacts in agricultural industries. The relatively large number of sites on Dermosols recognises the contribution made by this soil type to agriculture in Tasmania. Of the 59 agricultural sites sampled, 10 are under long-term pasture (five organic operations, two irrigated pasture operations, one dryland pasture operation and two dryland pasture operations on north-facing slopes), 32 are under intensive cropping and 17 under perennial horticulture. The sites were sampled over a 3-year period and at different times of the year. The distribution of the sites used in the present study is shown in Fig. 1.
Once a paddock with the required soil type and land use was identified, a random point in the paddock was located precisely using differential global positioning system (DGPS). A 50-m transect was then run out across the slope and the bearing from the origin recorded. Two or three test holes were augered close to the transect to confirm uniformity of soil type along the length of the transect. Soil samples used to measure indicators of soil health were taken along this transect (although this is a different strategy for sample collection to the grid approach described in McKenzie et al. (2002), it is still considered a valid statistical approach (N. McKenzie, pers. comm.). Three types of soil sample were collected from each selected site, as described below.
1. Close to the start of the transect, a soil pit was excavated to 100 cm and described using standard terminology (National Committee on Soil and Terrain 2009). Samples were collected by horizon unless the soil layer exceeded 30 cm thickness, in which case the layer was subdivided into layers of equal thickness and sampled accordingly. The purpose of the soil pit, and the samples collected from it, was to confirm the soil classification and provide a full profile description and supporting analysis for the site (samples from the soil pit were not used to measure indicators of soil health).
2. Along the transect, samples were collected from two depths every 2 m. The depths of the samples taken were 0-75 mm and a subsoil depth 75 mm thick at some depths from 100 to 300 mm such that sampling across soil layers could be avoided. Subsoil sampling depth was consistent at each site but differed between sites depending on the thickness of topsoil layers. Samples from each depth were bulked, air dried, subsampled using a splitter and sent for analysis. A standard suite of nutrient analyses was performed by the CSBP Laboratory (Perth, WA, Australia) on each subsample, including tests for the key indicators of soil condition pH (method 4A1; Rayment and Lyons 2011) and organic carbon (air dry, method 6A1; Rayment and Lyons 2011).
3. Finally, at three points along the transect (-12.5, 25 and 37.5 m), samples were taken from each of the two depth layers for bulk density and aggregate stability assessment. Bulk density samples were collected using a cylinder 75 mm deep with a 75 mm diameter, which was carefully hammered into the soil using a suitable dolly and driver. Aggregate stability samples were collected using a spade and carefully packed to avoid damaging aggregates. These samples were processed in the DPIPWE laboratories in Launceston (Tas., Australia). Aggregate stability of water-stable aggregates of up to 0.25 mm was subsequently determined using a modified wet sieving technique after Laffan et al. (1996).
Sampling was repeated at each site approximately 5 years after initial sampling by returning to the same DGPS coordinates and running the 50-m transect out on the same bearing as for the original sampling. Where possible, resampling was undertaken at a similar time of year to the original sampling. However, some sites were sampled at different times of the year and, particularly for cropping sites, at a different phase of the crop rotation to the original sampling, perhaps providing an explanation for different results between the two sampling rounds.
A paired-sample t-test was used to determine whether changes that occurred between the two sampling rounds were significant for each of the four indicators of soil condition, namely pH, organic carbon, bulk density and aggregate stability.
The results for each soil condition indicator and land use type are presented in Tables 1-3.
For the Dermosols under cropping, the paired-sample t-test indicated that the decrease in organic carbon, increase in bulk density, increase in pH and decrease in aggregate stability identified were all significant (P<0.05) within topsoil samples. Similar changes occurred within the subsoil and again all changes were identified as significant (P<0.05).
For Dermosols under pasture, a decrease in organic carbon and an increase in bulk density were both considered significantly different (P<0.05) within the topsoil. Within the subsoil, only a significant increase in bulk density was measured.
Dermosols under perennial horticulture showed the least change, with a decrease in aggregate stability in both topsoil and subsoil samples being the only indicator identified as significantly different (P<0.05) by the paired-sample t-test.
In addition, the number of sites within each land use identified as showing an adverse change is given in Table 4. In the context of this paper, an adverse change is considered to occur in one direction only for each soil property and is identified in Table 4. In defining these directions of change as adverse it is acknowledged that, depending on the starting value for a particular soil property, a change in the 'adverse' direction could be considered beneficial, or that a change in the opposing direction could also be considered detrimental. For example, generally speaking, declining pH is considered detrimental for the soil. However, if the starting pH is above 7, then a reduction to around pH 6-6.5 might be considered an improvement. Similarly, sites where bulk density is already low may be considered to be further declining in condition if bulk density values continue to fall.
Changes against targets
At the commencement of the project target values (or, where appropriate, ranges) were established for the various indicators of soil condition for the Dermosols, together with other soil and land use types included in the project (but not reported in this paper). Some targets for a particular indicator were the same across most soil types and land uses, whereas others differed or were related to the local rainfall regimen (a rainfall of greater or less than 800 mm was used). For example, topsoil pH (1 : 5 water) range for all soil types is 5.5-7.0 except under forestry, where the target range is 4.0-7.0; however, topsoil organic carbon targets vary depending on soil type, land use and rainfall. These targets have been developed by drawing on published information (e.g. Cotching et al. 2002), trial results and local knowledge. The information was used to create a draft set of targets that was circulated for review before being discussed at a workshop of relevant experts to achieve an overall consensus. The targets are presented in Table 5 as values (ranges) that arc considered reasonable for the soil types and land uses they represent within Tasmania. The targets have been developed to provide an indicator for further investigation should soil property values be identified that fall below or outside the respective targets. The target values are not intended to be used to label soils as healthy or unhealthy because many other factors can play a part in determining soil health.
Table 6 lists the sites for each land use, sample depth and soil property that are recorded as falling outside the target value (or range) for each of the two sampling rounds completed so far. It is evident from the data that there is an increase in the number of sites not meeting the target values for one or more soils indicators for each of the three land use types assessed. For example, under cropping, the number of sites not meeting organic carbon targets has increased from eight to 13 in the topsoil; under Dermosols, subsoil bulk density and aggregate stability both have increased numbers of sites that do not meet the respective target values. Under pasture, there has been an increase in the number of sites not meeting topsoil bulk density and organic carbon targets. Only with pH has the number of sites meeting target values remained constant or declined for each land use type.
The results suggest that, for Dermsols sampled as part of the SCEAM project, soil health appears to be declining for some sites under each of the three land use types investigated, as indicated by significant adverse changes in one or more of the soil condition indicators of pH, organic carbon content, bulk density and aggregate stability.
Increases in topsoil and subsoil pH under cropping supports the general observation that many Tasmanian cropping soils are not suffering from acidification. This can probably be attributed to the long history of liming that is undertaken by many farmers around the State.
Organic carbon levels are recorded as declining significantly under both cropping and pasture management, although only within the topsoil of pasture sites. The increase in the number of sites where topsoil carbon levels now fall below the identified target is also indicative that soil carbon levels are continuing to decline in many instances. The decline under pasture comes as something of a surprise because perennial systems are expected to be in carbon equilibrium and these sites require further investigation and analysis to determine the reasons for this unusual trend.
Bulk density has increased in both subsoil and topsoil for pasture and cropping land uses but only in the subsoil under perennial horticulture. Increased bulk density is not necessarily an issue if initial bulk densities were recorded as low. Mean topsoil bulk density under cropping varied from 0.99 g [cm.sup.-3] in the first round of sampling to 1.2 g [cm.sup.-3] in the second round, with corresponding values of 0.87 and 1.01 g [cm.sup.-3] under pasture. Although current topsoil bulk density averages under both cropping and pasture are not excessive, the concern is that the general trend appears to be increasing bulk density.
Declining aggregate stability under cropping suggests that continued use and cultivation of Dermosols is weakening aggregate bonding. This may be related, in part, to lowering soil carbon levels, but the reasons for change in the subsoil under perennial horticulture are less clear and need further investigation.
It should be noted that results for organic carbon have not been corrected for changes in bulk density between sampling rounds. Consideration was given to making this correction but discussion with others (e.g. J. Sanderman, pers. comm.) suggest the non-continuous sampling of soil carbon with depth makes the adjustment for bulk density somewhat simplistic and unreliable and that for the purpose of this project, measurements of carbon concentration rather than carbon stock are probably more appropriate.
It is acknowledged that accurate measurement of some soil properties, aggregate stability for example, is complicated by natural variability in the soil, consistency of sample collection and storage, analytical procedure and even by variation of the operative collecting the sample or performing the analysis. Other sources of 'error' may be due to the resampling occurring at a different time of the year to baseline data collection (although this was minimised as much as possible), or that the paddock was in a different phase of crop rotation or soil management or just that soil conditions were different because of natural seasonal variation in climate or soil moisture content. For these reasons, it is also not reliable to make judgements on trends in soil condition based on the present two sampling time points on a graph and so the interpretation of the above results needs to be treated as preliminary. It is hoped that monitoring over a longer time period will enable reliable conclusions on trends in soil condition to be determined.
Changes under cropping are more common than under either pasture or perennial horticulture, most likely due to the more intensive management of soils under cropping. Investigations into the reasons for the changes require farmer surveys on their land management practices. The increase in the number of cropping sites with values for at least one soil condition indicator now falling outside the suggested targets compared with the original time of sampling suggests an unfavourable change in soil health may be occurring, probably driven by declining soil organic matter due to oxidation processes associated with tillage. More regular soil amendments and land management practices like minimum tillage and ley phases may be part of the solution.
Despite the limitations to measuring soil properties, some of which have been discussed earlier, many of the initial results from the SCEAM project support findings of other authors (e.g. Cotching et al. 2002; Sparrow el al. 1999) that soil condition in Tasmania continues to decline, and the results should be considered as an early warning to possible long-term future trends.
The author acknowledges the support of the following for contributions to the SCEAM project: The Australian Government for the initial funding for this project under Caring for our Country; the Tasmanian NRM regions; the various DPIPWE staff who have worked on the project collecting and processing samples; and the farmers and land managers who have allowed us repeated access to their farms or forestry coups to collect soil samples.
C. J. Grose
Department of Primary Industries, Parks, Water and Environment, PO Box 46, Kings Meadows, Tas. 7249, Australia. Email: firstname.lastname@example.org
Chilvers WJ (1996) 'Managing Tasmania's cropping soil: a practical guide for farmers.' (Department of Primary Industries and Fisheries: Hobart)
Cotching WE (2009) 'Soil health for farming in Tasmania.' (Bill Cotching: Devonport, Tas.)
Cotching WE, Kidd DB (2010) Soil quality evaluation and the interaction with land use and soil order in Tasmania, Australia. Agriculture, Ecosystems & Environment 137, 358-366. doi:10.10l6/j.agee.2010. 03.006
Cotching WE, Cooper J, Sparrow LA, McCorkell BE, Rowley W (2002) Effects of agricultural management on Dermosols in northern Tasmania. Australian Journal of Soil Research 40, 65-79. doi: 10.1071 /SR01006
Isbell RF (2002) 'The Australian soil classification.' Revised edn. (CSIRO Publishing: Melbourne)
Laffan MD, Grant JC, Hill RB (1996) A method for assessing the erodibility of Tasmanian forest soils. Australian Journal of Soil and Water Conservation 9, 16-22.
McKenzie N, Henderson B, McDonald W (2002) 'Monitoring soil change. Principles and practices for Australian conditions.' CSIRO Land and Water Technical Report 18/02. (CSIRO)
National Committee on Soil and Terrain (2009) 'Australian soil and land survey field handbook.' 3rd edn. (CSIRO Publishing: Melbourne)
Rayment GE, Lyons DJ (2011) 'Soil chemical methods: Australasia.' Australian soil and land survey handbook series. (CSIRO Publishing: Melbourne)
Sparrow LA, Cotching WE, Cooper J, Rowley W (1999) Attributes of Tasmanian Ferrosols under different agricultural management. Australian Journal of Soil Research 37, 603-622.
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Table 1. Paired sample t-test results for each soil property for Dermosols under cropping between baseline samples (Sampling Round 1) and the first 5-year interval (Sampling Round 2) Sampling Sampling Round 1 Round 2 No. sites Mean Variance Mean Variance F-value Organic carbon Topsoil 32 3.66 3.15 3.05 1.61 0.0013720 Subsoil 32 2.86 1.92 2.36 1.72 0.0000200 pH Topsoil 32 6.10 0.25 6.30 0.28 0.0039713 Subsoil 32 6.06 0.32 6.22 0.33 0.0080471 Bulk density Topsoil 32 1.00 0.02 1.17 0.02 0.0000002 Subsoil 32 1.15 0.02 1.28 0.03 0.0000001 Aggregate stability Topsoil 32 74.76 120.57 65.94 173.18 0.0034722 Subsoil 32 63.53 240.58 55.97 465.52 0.0164151 Table 2. Paired sample t-test results for each soil property for Dermosols under pasture between baseline samples (Sampling Round 1) and the first 5-ycar interval (Sampling Round 2) Sampling Sampling Round 1 Round 2 No. sites Mean Variance Mean Variance F-value Organic carbon Topsoil 10 5.48 2.29 4.34 1.62 0.00561 Subsoil 8 2.97 0.69 3.02 1.07 0.86607 pH Topsoil 10 5.88 0.18 5.81 0.18 0.47700 Subsoil 8 5.64 0.23 5.69 0.28 0.55265 Bulk density Topsoil 10 0.91 0.02 1.06 0.02 0.00062 Subsoil 8 1.11 0.05 1.28 0.07 0.01645 Aggregate stability Topsoil 10 83.00 224.22 78.80 152.62 0.12283 Subsoil 8 72.88 574.70 68.25 717.07 0.19039 Table 3. Paired sample t-test results for each soil property for Dermosols under perennial horticulture between baseline samples (Sampling Round 1) and the first 5-year interval (Sampling Round 2) Sampling Sampling Round 1 Round 2 No. sites Mean Variance Mean Variance P-value Organic carbon Topsoil 17 3.69 1.65 3.33 1.36 0.07236 Subsoil 17 2.41 1.04 2.41 0.68 0.99850 pH Topsoil 17 6.37 0.27 6.48 0.15 0.36555 Subsoil 17 6.42 0.22 6.52 0.17 0.40388 Bulk density Topsoil 17 1.05 0.04 1.10 0.04 0.25429 Subsoil 17 1.25 0.04 1.31 0.05 0.26309 Aggregate stability Topsoil 17 74.76 120.57 65.94 173.18 0.01189 Subsoil 17 61.94 315.56 50.94 451.81 0.04347 Table 4. Proportion (%) of Dermosol sites in each land use category and at each sampling depth showing a change in a defined direction for each of four soil properties considered relevant to soil condition between baseline sampling results and the subsequent 5-ycar interval results Decrease Increase Decrease in in organic Decrease in bulk aggregate carbon in pH density stability Dermosols under pasture Topsoil 90 63 90 70 Subsoil 63 25 87 88 Dermosols under cropping Topsoil 84 31 88 59 Subsoil 84 31 84 59 Dermosols under perennial horticulture Topsoil 71 35 71 76 Subsoil 41 24 71 71 Table 5. Target values (or ranges) of soil properties for soil health for productive agriculture in Tasmania ESP, Exchangeable sodium percent Land use Soil property Soil orders categories Soil pH (A) (in water) All Pastures, cropping and horticulture Forestry Organic C (% w/w) Chromosols, Calcarosols, All Kurosols, Podosols, Rudosols, Sodosols, Tenosols Dermosols, Ferrosols, Cropping and Hydrosols horticulture Pastures and forestry Vertosols All Extractable All Pastures phosphorus (Olsen P mg [kg.sup.-1]) Bulk density (Mg All All [m.sup.-3]) Aggregate stability Ferrosols, Vertosols All (% >0.25 mm) Dermosols, Calcarosols, All Hydrosols Chromosols, Kurosols, All Podosols, Sodosols, Tenosols Rudosols All ESP (%) All All Annual Target rainfall value Depth (B) (mm) or range Surface 5.5-7.0 Subsurface 5.2-7.0 Surface + subsurface 4.0-7.0 Surface >2 Subsurface >1 Surface >800 >3 <800 >2 Subsurface >800 >3 <800 >1.5 Surface + subsurface >800 >4 <800 >2 Surface >4 Subsurface <600 >3 Surface 23-28 Surface + subsurface <1.2 Surface + subsurface >70 Surface + subsurface >60 Surface + subsurface >40 Surface + subsurface >30 Surface + subsurface <6.0 (A) Soil water ratio 1:5. (B) Surface = 0-75 mm; subsurface = 75 mm thick sampled between 75 mm and 300 mm. Table 6. Number of sites in each sampling round that fall outside the target value (or range) for each soil property, land use type and layer sampled (shaded cells indicate where the number of sites has increased from the first to second round) R1, baseline; R2, subsequent 5-year interval Organic PH Bulk Aggregate carbon density stability No. sites R1 R2 R1 R2 R1 R2 R1 R2 Dermosols under pasture Topsoil 10 0 2 2 1 0 2 1 1 Subsoil 8 6 6 2 2 3 5 2 2 Dermosols under cropping Topsoil 32 8 13 3 3 2 9 6 14 Subsoil 32 16 22 7 5 12 23 15 17 Dermosols under perennial horticulture Topsoil 17 4 5 1 0 4 4 1 5 Subsoil 17 13 12 0 0 8 12 8 12
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|Date:||Sep 1, 2015|
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