Effects of phosphorus and nitrogen on growth of pasture plants and VAM fungi in SE Australian soils with contrasting fertiliser histories (conventional and biodynamic)
Megan Ryan, Julian Ash,
Introduction
Biodynamic agriculture is practiced as an alternative to conventional agriculture in many areas around the world (Kirchmann, 1994; Lytton-Hitchins et al., 1994; Reganold, 1995; Murata and Goh, 1997; Vereijken et al., 1997; van Mansvelt et al., 1998). Biodynamic farm management practices reflect a desire to improve the healthiness of produce, which is believed to occur through harnessing biological processes and eliminating use of pesticides, herbicides, synthetic veterinary medicines and readily soluble fertilisers (Kirchmann, 1994).
As biodynamic farmers do not apply soluble fertilisers, it is possible that the soil biological community may adapt to play a greater role in plant nutrition (Ritz et al., 1997). Hence, the relationships between soil nutrient concentrations and plant growth may differ between biodynamic farms and conventional farms and this could be reflected in their response to additions of soluble nutrients. Australian soils are particularly suitable to test such ideas, as both total and extractable P concentrations are generally low. Extractable P concentrations are generally 10–400 μg g−1, and sometimes as low as 1 μg g−1 (Lindsay, 1985). While conventional farmers overcome this problem through applying fertilisers containing readily soluble P, plant growth on organic or biodynamic farms, where these fertilisers are not applied, is likely to be limited by P (Dann et al., 1996).
To examine whether long-term biodynamic management had changed the ability of the soil biological community to influence plant nutrient uptake, a glasshouse experiment was conducted which involved additions of fertilisers containing soluble P and N to soil sampled from three paired adjacent conventional and biodynamic farms. Growth of two pasture species was used as a bioassay of the ability of the soil to provide nutrients to plants. The level of colonisation by vesicular-arbuscular mycorrhizal (VAM) fungi indigenous to each soil was also examined as they are a prominent part of the soil community, are involved in plant uptake of nutrients particularly P (Bolan and Robson, 1983), and have been reported to adapt in abundance and function in response to nutrient inputs (Johnson, 1993).
Farm management
At three locations in the Goulburn River Valley near Shepparton, Victoria, Australia (145°10′E, 36°20′S), a biodynamic dairy farm was matched with a neighbour with similar soil type, herd breed, farm area and a history of consistent conventional management. These are referred to as Farm Pairs A, B and C.
The farms consisted primarily of permanent summer-irrigated perennial pastures used to support free-ranging dairy cattle. These pastures had not been cultivated for, on average, 35 years and
Comparisons between soils
Table 1 presents the soil extractable P and total N concentrations in the six soils used in the experiment, after they had been bulked and sieved. Extractable P concentrations were 2–3 times higher and total N concentrations 1.3–2 times higher in the conventional soils.
Fig. 1 presents the VAM colonisation levels for plants grown in each soil without added nutrients. For clover, the VAM colonisation level was significantly higher in the biodynamic soils and did not differ significantly within
VAM fungi
Addition of P markedly reduced VAM colonisation in clover (Fig. 3(a)) and shoot P concentration had a strong negative linear correlation with VAM colonisation (Fig. 7). Shoot and root P concentrations have often been reported to exhibit this relationship with VAM colonisation (Lu et al., 1994; Ryan, 1998). This is consistent with the impact of P being mediated through the effects of phospholipid content of root cells on root cell membrane permeability and exudation of the carbohydrates on which
Conclusions
The response of plants and VAM fungi growing in soils from conventional and biodynamic farms to additions of soluble P and N did not differ, in spite of the biodynamic soils not having received soluble P and N fertilisers for 17 years. While the biomass and community structure of the soil ecosystem may vary in response to farming system, it cannot be assumed that fundamentally different processes will be governing important pathways, such as plant nutrient uptake, or that the soil ecosystem