Microbial Flora

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The different chemical and physical condition of the soil on different plots has led to substantial changes in the population and activity of micro-oragnisms that transform carbon, nitrogen, sulphur and phosphorus compounds. These microbial changes have themselves led to differences in the soils' chemical properties and to the growth and chemical composition of the herbage on the plots. Some of these changes are outlined below.

The differences in soil carbon content and distribution could be due to differences in carbon addition associated with dry matter production and grazing or to differences in the rate of carbon breakdown and redistribution by micro-organisms or mesofauna, or to some combination of these. Glucose induced respiration (V_max) was much smaller on the fertiliser-treated plots, and particularly small on the acid plot 7 (Table 1). The glucose concentration at half V_max (K_m) was also smaller on these same plots and this is reflected in the variation in biomass C.

The ratio of microbial to organic carbon was however only substantially depressed on plot 7. When expressed on a gravimetric basis there is no clear pattern in the production of carbon dioxide per gram of soil, though that from plot 7 is very large. When the variation in carbon content is taken into account the specific respiration (qCO₂) is much higher on this plot. It appears the micro-organisms on plot 7 are particularly stressed by their environment, for both K_m and V_max are much smaller than for the other plots. The biomass C and the ratio of biomass to soil carbon are both much smaller on plot 7 suggesting that the microbial population is being stressed by the environment on this plot; this reduced biological activity is responsible for the accumulation of C on plot 7.

Table 1. Measures of microbial activity on a selection of the plots.

Table 2. Log₁₀ of the most probable number g^-1 and short term nitrifier activity (SNA, µg NO₂-N g^-1 h^-1)

One might also expect that there are comparable differences in activity of micro-organisms involved in soil phosphorus and sulphur transformation, comparable to the changes described above for carbon and nitrogen. The modification to all of these could also have implications for loss of nutrients to the atmsophere (some in the form of greenhouse gases) or to ground water, where they may lead to eutrophication.