Use less, lose less: obtaining and maintaining an environmentally and agronomically sustainable farming system with phosphorus

Abstract

Long-term application of phosphorus (P) in excess of crop or pasture requirements has led to accumulation of P within many well developed agricultural soils. Due to the increased risk of P loss to surface waters from such soils and growing concerns over the security of future P supply, more sustainable farming systems are essential. This thesis aimed to investigate the effectiveness of three potential mitigation strategies to reduce P loss from P enriched pastoral soils. The simplest strategy would be to cease P fertiliser application until soil P concentrations decrease to an acceptable level. Standard soil test methods and isotope exchange kinetics were employed using soil samples from long-term fertiliser trials, spanning 7 – 26 years and four soil types, to determine the rate of decline in soil P over time. Both the agronomically and environmentally important pools of soil P showed exponential decline over time following a halt to P fertiliser application. The rate of decline in water extractable P (WEP), a proxy for dissolved reactive P (DRP) in surface runoff, was dependant on the initial Olsen P concentration and the P retention, a measure of anion storage capacity, while the rate of decline in Olsen P was found to be dependent on the initial WEP concentration. Using these relationships an equation was generated to determine the time it would take for the P concentration of a given soil to decrease to a target value, set at 0.02 mg P L-1 for WEP and 25 mg P kg-1 for Olsen P. Applying these equations it was calculated that it would take between 22 – 44 years to reach the environmental WEP target but only 0 – 7 years to reach the agronomic Olsen P target. This, combined with limited data available for one of the trial sites on the dry matter yield response, suggests that decreasing P loss from high P soils through withholding P fertiliser is slow and is likely to result in a reduction in pasture yields. Two further mitigation strategies aimed at decreasing P loss within an acceptable timeframe while maintaining productivity were investigated through two lysimeter trials. The first trial investigated the effect of a re-introduction of conventional tillage during pasture renewal on subsurface P loss across four contrasting soil types. Tillage decreased the DRP load in leachate collected over the first five weeks of the trial across three of the four soil types by 30 – 70 % compared to the treatment simulating direct drill reseeding. However, thereafter no effect was noted, possibly due to the reformation of macropores. Over the 451 days of the trial WEP decreased by 28 – 43 % for the tilled soils but only 12 – 26 % for the direct drill soils. This study suggests that tillage is not an effective method to decrease subsurface P losses in the long-term but may be effective at decreasing P loss to surface runoff. The second trial investigated the implementation of a cut and carry system with the addition of nitrogen (N) fertiliser to boost plant growth and the phytoextraction of P across three soil types. For two of the soils N application increased pasture production and decreased the load of DRP leached by 53 – 76 % and the load of total dissolved P (TDP) by 39 – 53 % compared to when no N was applied. Furthermore, compared to the no P treatment, applying P at half the rate designed to maintain soil P concentration, decreased the load of DRP and TDP in leachate by a 62 – 68 % and 54 – 59 % respectively, due to immobilisation of P within the microbial biomass. A high sorption capacity, leading to slow release of P to the soil solution, was seen as the probable reason for the lack of treatment effect in the third soil. In conclusion decreasing soil P concentrations by cessation of P fertiliser is unlikely to reduce P loss within a reasonable timeframe and may negatively impact on pasture production. However, if implemented in combination with tillage or enhanced phytoextraction of P through increased N application we have the potential to decrease P loss to surface waters and maintain a productive farming system. However consideration of the soil type and P loss pathway is essential when selecting the mitigation technique.