The assessment and potential for mitigation of phosphorus losses from Organic soils under intensive dairying

Abstract

The development of marginal land under Organic soils for intensive agricultural use is commonplace around the globe, being driven by population growth and the limited availability of alternative, highly productive soils. The development of Organic soils typically involves drainage and the application of lime and phosphorus (P) fertiliser amendments, to rectify acidic soil conditions and poor fertility for pastoral production. However, due to the poor P retention capabilities of Organic soil (a result of low concentrations of P-sorbing metal oxides) there is an elevated risk of P losses in runoff which can have dire agronomic and environmental consequences. The main aims of this thesis were to identify which properties of Organic soils best indicate the potential scale of P losses, the timeframe over which the risk of P losses is greatest, and how soil and land management variables including soil moisture, liming and fertiliser rates, fertiliser solubility, and mineral content can influence the quantities, forms and fractions of P exported from the landscape. A trial was conducted to quantify the importance of a number of soil physical and chemical properties, and land management activities on the potential for P losses. The results of this trial indicated that the effective management of P losses from Organic soils is critical for at least the first 10 years of development, and that P losses were driven by soil P and mineral concentrations. Management within the first 10 years includes drainage, applications of P fertiliser and liming amendments. To this end, a recently-developed acid Organic soil was used in a runoff trial and three lysimeter studies to determine the influence of these variables: The runoff trial compared the role of soil moisture on the quantities, forms and fractions of P lost in overland and subsurface flow from an Organic soil and a Brown soil. This study demonstrated that soil moisture influenced P loss pathways, but the interactions with soil type (e.g. anion storage capacity) determined the form and quantity of P lost, which may have implications for the drainage or irrigation of Organic soils. The first lysimeter trial determined the influence of liming Organic soils at different fertiliser rates to the forms and fractions of P lost in leachate, retained in the soil and herbage response. The trial found that liming Organic soils had the potential to reduce P concentrations in leachate, and had a positive influence on herbage. The second lysimeter trial determined the importance of P fertiliser solubility for P losses from acid Organic soils at different rates and pH levels. Results showed that the cost of P losses from superphosphate (SSP) treated soils were two to three times greater than reactive phosphate rock (RPR), suggesting that the additional cost of RPR and liming to pH > 5.5 has the potential to offset P losses from SSP fertilisers, making it a more environmentally responsible practice with minimal economic drawbacks. The third lysimeter trial investigated the potential for greater lotus (Lotus pedunculatus var. Grasslands Sunrise) to be used in place of white clover (Trifolium repens) or ryegrass (Lolium perenne) on an acid Organic soil. The trial found no differences in soil P fractions under the three species, and that P uptake into lotus was similar or less than ryegrass and clover, but losses in leachate were commonly greater. Finally, an incubation study was devised to quantify the relative importance of metal oxide concentrations and soil carbon content for P losses, and to determine the potential for the use of mineral amendments to reduce P losses. P losses (as water extractable P; WEP) were reduced by the addition of aluminium sulphate, iron sulphate and calcium sulphate in all cases, and results indicated that P-sorption to aluminium sulphate was the most important factor controlling P release as WEP from these soils. In conclusion, the findings of this study demonstrated that although Organic soils have the potential to lose far greater quantities of P than other soil types, and this is lost predominantly as subsurface flow, there are a number of management practices and approaches that can alleviate environmental risk and require minimal sacrifices to agronomic productivity. This includes identifying the potential for P losses from Organic and Podzol soils using simple soil tests (Olsen P and P retention or anion storage capacity), accounting for the likelihood of P losses in subsurface and overland flow with regard to soil moisture, irrigation and drainage activities, and mitigating the potential for P losses by increasing lime inputs in combination with sparingly-soluble fertilisers (e.g. RPR).