Role and function of green manure crops in mobilisation and utilisation of soil legacy phosphorus in cropping systems : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

In many agroecosystems, significant quantities of phosphorus (P) applied in mineral and organic forms have accumulated in the soil as “legacy P” which represents a resource that may be used to maintain crop production with reduced P inputs. The objective of this study was to investigate and quantify the impact of various green manure plant species on legacy P mobilisation and utilisation in selected volcanic soils. To achieve this objective, a combination of root study container experiments; short- and long-term glasshouse pot trials; and an analysis of soil from an established field experiment were carried out in this PhD study. A variety of potential green manure species were assessed in volcanic soils derived from pumice and ash. Analyses included plant yield, P uptake, and root organic anion exudation and soil P fractionation, microbial P, and phosphatase enzyme activities. Results showed that plant available inorganic P in excess of agronomic optima prevented the mobilisation of legacy soil P. Lupin (Lupinus angustifolius) and pea (Pisum sativum) were the best performing green manure species with respect to legacy soil P mobilisation with a rhizosphere P depletion of 8-11%. Lupin was better than pea, which was at least partly attributed to a combination of fine root hairs and organic anion exudation. Inclusion of lupin and pea green manures in the absence of P inputs significantly increased cereal crop yield (27 - 35%) and P uptake (15 - 29%) over two rotations compared with fallow, which was mainly attributed to mobilisation of legacy soil P. However, results from a field trial with continuous P inputs showed that, after 5 years, legume green manure did not improve maize yield and P uptake. This was likely due to a combination of factors including high soil P status via continued P inputs and the fact that green manure was left on the surface rather than incorporated into the soil. The findings of this study clearly demonstrated that legume green manures, when managed properly, have the potential to enhance soil legacy P mobilisation and thus increase P use efficiency in temperate crop systems, which in turn may reduce P inputs required to maintain production.