The effect of fertilisation on soil nitrifier activity in experimental grassland plots

Soil nitrification was compared in soils from 89-year-old grassland experimental plots with diverse chemical characteristics. Measurements of NaClO₃-inhibited short-term nitrifier activity (SNA) and deamination of 1,2-diamino-4-nitrobenzene were used to study nitrification and deamination activities, respectively, in soil from each of 12 plots. Using multiple regression analysis, an expression for the relationship between SNA, soil pH and fertiliser N additions was derived which indicated that both the frequency and the quantity of farmyard manure additions were important in determining the rate of nitrification. SNA was greatest where there were large and frequent additions of farmyard manure. In soil with pH below 5.2 SNA was very low or insignificant. The effect of (NH₄)₂SO₄ additions could not be assessed because they acidified the soil. We suggest that additions of farmyard manure increase the potential for NO₃^- leaching or for denitrification. Deaminase assays indicated that soils with a higher pH showed greater N mineralisation than soils with a lower pH, except at the low extreme. There was no obvious relationship between SNA and deaminase activity at higher levels of pH.