Variability of phosphorus content in dry-land pasture surface soils of the Moe River catchment

Leading organisers and collaborators:

Future Farming Systems Research Division, Department of Primary Industries.

Contact:

David Reez ,   Olga Vigiak,  David Shambrook

Context:

Topsoil phosphorus is an important driver of pasture productivity, but can also lead to nutrient losses from runoff events. Phosphorus content of topsoils varies according land management and soil type. Whilst the impact of land management practices (such as fertiliser management, tillage operations, and grazing intensity) on soil phosphorus content is well known, the interactions between land management and soil characteristics are less well described. The aim of this research was to explore soil and land management interactions and phosphorus content for dryland pasture topsoils in the Moe River catchment (Gippsland, Victoria). Soils in this region were grouped according to soil hydrology (profile permeability and structure) and topsoil organic matter content. Four land management systems (beef, extensive dairy, medium-input dairy, and intensive dairy) were defined to reflect different fertiliser regimes and grazing intensities. Topsoil samples were taken on at least 12 representative paddocks for each soilland management combination, leading to 138 paddocks being sampled. Samples were taken at two depths (0-2 cm and 0-10 cm) to assess any stratification in phosphorus content. Samples were analysed to measure Colwell dissolved phosphorus (PCol, mg kg-1), Phosphorus Buffering Index (PBI), total phosphorus (TP, mg kg -1 ), and phosphorus soluble in CaCl2 (PCaCl2). Statistical variability of each phosphorus measurement in relation to the three factors (soil group, land management, and soil depth) is presented and discussed in the light of soil productivity and risk of nutrient runoff losses.

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Colwell P

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REML estimates Least Significant Intervals (LSI, at P = 0.05) of PCol(mg kg -1 , back transformed from glog scale) across HRUs (first letter indicates management system, B=beef, D1=intensive dairy, D2=pasture dominant dairy, D34= feed dominant and extensive dairy; the second letter indicates the soil group – refer to Table 1).

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Table 2. Estimated PCol mean and 95% confidence interval (mg kg -1 ) for HRUs

Colwell P measurements indicate that land management is the dominant influence on distribution and retention, although there is some influence according to soil group.

Sampling Sites and Soil Types

Table 1. Soil types within the Moe River catchment, with representative topsoil texture and organic carbon content and percentage of area extent in the catchment. AD class indicates the soil type defined within Accountable Dairying; P-group indicates soil grouping for P sampling purposes.

 

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Soil groups comprise several soil types, taking into account surface texture and profile differences. Note differences in texture.

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Summary of Results

Statistical analysis of P chemical species distribution across sites confirms the literature on P parameters, with PCol being different across industry types, and TP and PBI being mostly different across soil types. However, interactions of management x soil were detected (PCol) and this may require further work for confirmation. Distribution of PCA was strongly and linearly correlated to PCol /PBI ratio, confirming literature (Moody, 2011). This study highlights that high environmental risk is associated with combinations of intensive management choices (PCol) with low P retention soil properties (PBI, TP).

Stratification

Stratification indicates the level of difference of phosphorus between 0-2 cm and 0-10 cm depth. There are differences between the soil groups. Soil group A has marked stratification compared with the other soil groups. This is interpreted as being due primarily to lighter surface textures and/or organic matter compared with soil groups B and D.

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CaCl2 P (PCA)

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Figure 5. REML estimates Least Significant Intervals (LSI, at P = 0.05) of PCA (mg kg -1 , back transformed from glog scale) across HRUs (first letter indicates management system, B=beef, D1=intensive dairy, D2=pasture dominant dairy, D34= feed dominant and extensive dairy; the second letter indicates the soil group – refer to Table 1).

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PCA approximates P solution content in runoff; high values of PCA equate to a higher risk of P loss (Moody, 2011). PCA was strongly related to the ratio of Col P to Phosphorus Buffering Index (PBI). Dairy II showed the highest environmental risk potential.