## LSU Historical Dissertations and Theses

1988

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Oceanography and Coastal Sciences

Charles W. Lindau

#### Abstract

Laboratory investigations were carried out to study the magnitudes and rates of heterotrophic N$\sb2$ fixation and mineralization of organic N as influenced by controlled redox potentials and pH values. Preincubated Crowley silt loam soil suspensions was amended with 0.50% labelled rice straw (41% $\sp{15}$N excess) and incubated for six weeks in the dark. Effects of all combinations of four redox potentials ($-$100, +100, +300, and +500 mV) and two pH values (5 and 7) were studied. At weekly intervals, soil suspension samples were withdrawn and analysed for N$\sb2$ fixation by using the C$\sb2$H$\sb2$ reduction assay, total organic N, extractable NH$\sb4\sp+$-N, NO$\sb3\sp-$-N, labelled organic N, and labelled extractable NH$\sb4\sp+$-N. The greatest production of C$\sb2$H$\sb4$ was recorded in the $-$100 mV treatment at all weekly samplings, and the smallest C$\sb2$H$\sb4$ production rate was found at +500 mV. The magnitude of C$\sb2$H$\sb4$ production was about twice as great under anaerobic conditions (6.281 $\mu$moles C$\sb2$H$\sb4$/kg/week) as under aerobic conditions (3.630 $\mu$moles C$\sb2$H$\sb4$/kg/week). Compared to production of C$\sb2$H$\sb4$ at pH 7, that at pH 5 showed a net decrease (7.9 $\mu$moles of C$\sb2$H$\sb4$/kg/week ($-$100 mV and pH 7) and 5.6 $\mu$moles C$\sb2$H$\sb4$/kg/week ($-$100 mV and pH 5). At the end of the experiment, the production of C$\sb2$H$\sb4$ at $-$100 mV was significantly greater than those at all other redox potentials. The average C$\sb2$H$\sb4$ production rate corresponded to a N$\sb2$ fixation rate of 10 kg/ha/year if a 3:1 ratio is assumed. The amount of extractable NH$\sb4\sp+$-N released at $-$100 mV over time was significantly greater than the NH$\sb4\sp+$-N released at the other redox potential and pH levels. Significantly greater amounts of NH$\sb4\sp+$-N were released at pH 5 compared to pH 7. The NH$\sb4\sp+$-N released at the end of the incubation period was 55 mg N/kg soil at $-$100 mV and pH 7 compared to 70 mg N/kg soil at the same redox level and pH 5. The nitrification rate was slightly greater at +500 mV than +300 mV and was significantly greater at pH 7 than pH 5, resulting in a significant redox potential by pH interaction. No nitrification was recorded at the +100 and $-$100 mV treatments. The amount of labelled organic N mineralized at pH 5 was about twice that mineralized at pH 7 (0.79 mg $\sp{15}$N/kg soil). A significant redox potential by pH interaction affected the amounts of labelled NH$\sb4\sp+$-N released. The labelled N unaccounted for ranged between 2 to 24% of the total $\sp{15}$N added to the soil.

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