The effect of incorporation of low nitrogen organic matter into soil on legume growth, yield, nodulation and nitrogen fixation: A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science with Honours

Abstract

Narrow-leafed lupin (Lupinus angustifolius) and field pea (Pisum sativum L.) were grown in four levels of soil incorporated organic matter (0, 5, 10 and 20 t ha⁻¹ ). Wheat (Triticum aestivum L.) straw was the organic matter used and was incorporated 3 days prior to sowing, with sowing undertaken 10 November 2008. Two quadrats of 0.1 m² in size from each plot of above and below ground biomass were collected every 7 days to determine biomass accumulation and nodulation score. At final harvest, two 0.5 m² quadrat cuts from each plot of above ground biomass were threshed to determine total, seed and straw dry weights. Prior to threshing, 10 random plants from each sample were taken aside for yield component analysis. Carbon and nitrogen analysis of final harvest above ground biomass was undertaken for each plot. A significant interaction (P<0.05) between lupin biomass accumulation and 0 t ha⁻¹ straw incorporation occurred 9 WAS in which biomass was greater at this straw incorporation rate. Legume species affected biomass accumulation with lupin having a greater accumulation (P<0.05) than pea in below and above ground biomass except during the middle of the trial period at which time pea accumulation was greater. At 6, 9 and 19 WAS significant differences (P<0.05) occurred between biomass accumulation and straw incorporation rates in which 0 t ha⁻¹ provided greater accumulation at 6 and 9 WAS, with 10 t ha⁻¹ having the greater accumulation at 19 WAS. Mean seed weight was significantly effected by straw incorporation rate, with the 0 t ha⁻¹ incorporation rate having a lower mean seed weight compared to all other straw treatments. Species differences (P<0.05) occurred between C and N concentrations of the final harvest above ground biomass. Peas had a significantly greater No/o, with lupin having a greater Co/o and wider C:N ratio. Nitrogen content of above ground biomass equated to 188 and 170 kg N ha⁻¹ for lupin and pea respectively but was not significantly different. Total and straw harvest dry weights were significantly (P<0.05) higher for lupin than pea, with pea having a significantly (P<0.05) greater harvest seed weight and HI. Legume species effected nodulation score (P<0.05), with pea having a greater nodulation score during the early to mid period of the trial. Lupin had a significantly greater (P<0.05) number of pods m⁻², pod plant⁻¹ and seeds m⁻² compared to pea. Regression analysis (P<0.01) showed that total harvest dry weight resulted in an increase in C%, with a decline in C% and C:N ratio with increasing seed dry weight. An increase in seed dry weight resulted in increased N%, with a decrease in N% as straw dry weight increased. C% and C:N ratio both increased with increasing straw dry weight. C% increased with increasing pods m⁻², pods plant⁻¹ and seeds m⁻²,with the C:N ratio increasing with increasing number of seeds m⁻². Mean seed weight was also affected by legume species (P<0.05), with pea having a greater mean seed weight. Regression analysis showed that as N% increased so did mean seed weight, with C% and C:N ratio declining with increasing seed weight (P<0.01). A positive relationship (P<0.01) was found between pea total biomass accumulation and nodulation score. Incorporation of organic matter in the form of wheat straw as a source of carbon for N₂ fixation by Rhizobium is recommended as it significantly improves mean seed weight and causes no detrimental effect in biomass accumulation, nodulation, and yield in either lupin or field pea.