Effect of Controlled Redox Potential and pH on Heterotrophic Nitrogen Fixation and Mineralization in Crowley Silt Loam Soil.

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 At weekly intervals, soil suspension samples were withdrawn and analysed for N$\sb2$ fixation by using the C$\sb2$H$\sb2$ reduction assay, total organic N, extractable NH$\sb4\sp+$-N, NO$\sb3\sp-$-N, labelled organic N,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