Ammonification and nitrification in acid forest soils : A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Horticultural Science (Honours) in the University of Canterbury

Abstract

A substrate amendment technique was used to identify the mechanism of N0₃⁻-N production in two Pinus radiata samples. Site 1 had an initial pH of 4.68 and a predominant N-form of NH4⁺. The initial pH of site 2 was 5.95 and N0₃⁻ was the predominant form of N. Nitrogen was added at 150μg/g of oven dry humus (in peptone and ammonium sulphate) and C was added at 550μg/g as starch. The samples amended with peplone (organic N) increased N03- production compared to the control after a 42 day incubation. The addition of ammonium sulphate inhibited the production of N0₃⁻-N for site 1 but significantly increased it in the site 2 sample. The starch amendment (a source of readily-mineralisable C) produced N0₃⁻-N levels equivalent to those of the control for site 1 but significantly suppressed N0₃⁻-N production in the site 2 sample. These results suggested that site 1 produced N0₃⁻ heterotrophically whereas in site 2 N0₃⁻ production occurred via the autotrophic pathway. A further incubation used acetylene to block the autotrophic oxidation (by Nitrosomonas) of NH4⁺ to N0₂⁻ and chlorate to block the oxidation of N0₂⁻ to N0₃⁻ by the autotroph Nitrobacter. These inhibitors were not expected to affect N0₃⁻ production by the heterotrophs. The results showed that both acetylene and chlorate interfered with the heterotrophic production of N0₃⁻ occurring in the site 1 sample. Acetylene effectively blocked the production of N0₃⁻ in the site 2 sample which also showed a significant increase in the levels of NH4⁺ -N compared to the control. Chlorate reduced N0₃⁻ production in the site 2 sample but did not inhibit it. The effect of rates of N at 150 and 300μg/g. added in ammonium sulphate and peptone. and rates of readily-mineralisable C at 550, 1100, 2200, 11000μg/g, added as starch, were used to investigate the mechanism of heterotrophic nitrification in the site 1 sample during a 35 day incubation. The addition of ammonium sulphate inhibited N0₃⁻-N production while peptone promoted it. The starch amendment at 550μg/g increased the production of N0₃⁻-N above the control level. Starch additions above 1100μ gig resulted in the immobilization of inorganic N. These results suggested that the limitation to heterotrophic nitrification in the site sample was a lack of readily -mineralisable C.