Investigation of the interactions between soil acidity, phosphorus biochemistry and dynamics and legumes in acid grassland soils : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Soil acidity and associated phosphorus (P) deficiency and aluminium (Al) toxicity are major constraints to agricultural production worldwide. For instance, in New Zealand steep lands, the so-called hill and high country, many commercial legume species fail to establish and persist in these acidic, low fertility environments. This PhD research project investigated (1) the impact of phosphogypsum (PG) on soil fertility and Al speciation in the soil solution and (2) the impact of lime-induced pH elevation on P biochemistry and dynamics in the rhizosphere of lupins and under open field grassland conditions. For the first part of the investigation, two experiments were conducted, each with a specific objective; in the first experiment (Chapter 2) the objective was to compare the effects of PG amendment, soluble fertilizer, and lime on short-term lucerne yield and P and sulphur (S) uptake in two different acidic soils under controlled environment conditions. The objective of the second experiment (Chapter 3) was to examine the impact of PG on Al speciation in the porewaters of both planted and incubated (unplanted) soils using the Visual Minteq Model. These two complementary experiments revealed that (1) PG has increased P and S bioavailability and therefore improved lucerne P and S uptake and yield. However, the application of PG to low pH soils necessitates its combination with lime because it has been found that pH was the most important factor controlling the nutrition and growth of lucerne as evidenced by the large difference in the yield and P and S uptakes between PG alone and PG combined with lime. The second key result (2) was that PG reduces soil exchangeable Al and monomeric Al3+in the soil solution if applied at 1-3 t ha-1. Higher application rates could acidify the soil and displace Al from the soil exchangeable sites into the soil solution. The mechanisms by which PG reduced Al3+ activity included the immobilization process through sulphate (SO42-) and fluoride (F-) binding and via precipitation reactions. For the second part of the investigation, three experiments were conducted (Chapters 4, 5 and 6), each with a specific objective. Experiment 1 (Chapter 4) objective was to examine the effects of increasing soil pH from 5.3 to 6.0 using lime on P-related processes and dynamics in the rhizosphere of two lupins (Lupinus polyphyllus and Lupinus angustifolius) after 11 weeks of plant growth in pots under glasshouse conditions. Experiment 2 (Chapter 5) was conducted to examine the impact of soil pH increase to near-neutral (pH 6.3) using lime on (1) acid phosphatase activity and labile P (DGT-P) distribution patterns in the rhizosphere of Lupinus angustifolius grown in two contrasting acid pasture soils, using innovative imaging techniques, (2) root morphological and physiological root traits. Experiment 3 (Chapter 6) was carried out in the field to study and quantify the effects of liming on P biochemistry and fractionation during 18 months on a long-term (+60 years) permanent fertilized grassland. All three experiments investigated the same soil (Mt Grand soil), collected from Central Otago, NZ—they proved unanimously that liming increases P availability and increases the mineralization of labile and moderately labile organic P (Po) in this soil. For instance, in the field experiment, labile inorganic P (Pi) increased by 42% at pH 7.0 compared to pH 5.4, while labile and moderately labile Po decreased by 33% and 25%, respectively. It was concluded from Chapters 4 and 5 that increasing soil pH above 6.0 negatively affects Lupinus angustifolius growth and P acquisition processes such as organic anions exudation and fine root length, while Lupinus polyphyllus was unresponsive to liming. Another conclusion drawn from the field trial (Chapter 6) is that liming enhances the mobilization of the historically applied P fertilizer and promotes Po mineralization.