Amounts, forms and availability of nitrogen and phosphorus in soil under conventional and organic cropping: A dissertation submitted in partial fulfilment of the requirement for the degree of Bachelor of Science with Honours at Lincoln University

Abstract

Organic farming is increasingly being promoted as a sustainable alternative to conventional farming. There is, however, a need for a greater understanding of how the organic system can function sustainably within a New Zealand agricultural setting. This study investigated the amounts, forms and availability of nitrogen (N) and phosphorus (P) in soil under conventional and organic cropping. Soil samples were taken to a depth of 70 cm from the conventionally managed Lincoln Mixed Cropping Farm (LCF) and the organically managed Biological Husbandry Unit (BHU) in Canterbury. These sites were chosen for comparison because they had the same soil type (Wakanui-fine sandy loam) and provided an opportunity to examine the effect of 25 years of contrasting management on nutrient amounts and availability. Analyses included organic carbon (C), total N and total P measurements. A 7-day anaerobic incubation was used to determine potentially mineralisable nitrogen (PMN), while isotopic exchange kinetics (IEK) was used to determine soil inorganic P availability. It was found that past management systems had not substantially affected organic C, total N and P levels of the topsoil (0-15 cm) but had significantly affected amounts of C and nutrients in the subsoil (15- 70 cm). Amounts of N, organic C and organic P were gre~ter at the BHU, which was attributed to the utilisation of deep rooting species. Inorganic P was greater in the LCF subsoil which may have been partially due to preferential leaching given the comparative lack of deep rooting species and the apparent high P fixing capacity (determined by IEK) of the subsoil. Greater pools of solution and readily available inorganic P in the BHU 0-7 .5 cm of soil indicated that the different cropping management systems had modified soil properties. The greater amounts detected at the BHU may reflect tillage regimes, intensity of production and the absence of animal grazing in the system. This study demonstrates the important role the subsoil plays regarding the nutrient fertility of cropping systems.