The effect of pH, liming, moisture and temperature on the activity of nitrifiers in a soil under pasture

Abstract

A short-term nitrification assay (SNA) was used to measure the activity of soil nitrifiers in the field in relation to soil pH and seasonal changes in soil temperature and moisture content. At roughly two-monthly intervals over two successive years, samples of the Tokomaru Silt Loam which had been limed in 1982 and/or 1987 were analysed in addition to an unlimed control. The SNA analysis was carried out for a range of pH values between 4.5 and 7.5, obtained by amending the incubation medium with small amounts of HCl or KOH. A quadratic curve was fitted to a plot of SNA value v, incubation pH. The fitted equations were used to calculate the pH optimum for nitrification (pHopt), the SNA value at pHopt (SNAopt) and the SNA value at the soil pH at sampling (SNApH). Values of pHopt and the mean soil pH over the year were higher in soil limed in 1982 than control soil; neither variable showed marked change in either soil over the first 320 days of observation. The addition of lime in 1987 raised the mean soil pH and pHopt in unlimed soil, but had negligible effect on either variable in soil limed five years previously. A covariance-type analysis demonstrated that the same quadratic equation could be fitted to each plot of SNA v. incubation pH for each soil treatment by changing the intercept parameter (C), i.e. the relationship between nitrifier activity and pH in the four soils remained constant over the year. No significant relationships could be found for the four soils between soil pH, pHopt, SNA, soil moisture content, soil temperature and C. However, the slight fluctuation in pHopt tended to follow the variation in soil pH. Values of SNAopt and SNApH showed a more obvious seasonal trend and showed a 1 : 1 relationship over a range of values from 0.015 to 0.110 µmol g-1 h-l; i.e. the nitrifier activity in the soil, irrespective of variations that were random (unknown influences) or associated with seasonal variables (temperature and moisture), was near the optimum with respect to pH. Nitrification activity is dynamic and changes quickly in response to changes in the soil environment. The implications of this with respect to nitrate leaching models include the likelihood that it may not be possible to produce a definitive model which works for all soil types.