Comparison of understorey biological nitrogen fixation and biomass production in grassed‐down conventional and organic apple orchards in Canterbury, New Zealand

Abstract

In a previous study, the understorey biomass production and biological nitrogen (N) fixation of a grassed‐down organic apple orchard were presented. The aim of this paper is to report the results of a similar study of two conventional orchards in a nearby location and to compare the present results with those of the organic orchard. Biological N₂ fixation was determined in the field using the ¹⁵N isotopic dilution technique and the experiments were conducted over a two‐year period. Present results showed that substantial amounts of N (112 to 143 kg N ha^‐1.2 years^‐1) were fixed in the understorey of the conventional orchard and these were not significantly different from those of the organic orchard. However, the N₂ fixation was sustained in the conventional, but not in the organic orchards in the second year, probably due to regular additions of fertilizers in the conventional orchards. In both orchards, N₂ fixation was better correlated with clover than total dry matter yield. Seasonal effects found were highest N₂ fixation and biomass production occurring during late spring and early summer and lowest during winter. Climatic factors were investigated in one of the conventional orchards and it was found that seasonal effects were related to a combination of temperature and moisture deficit effects.