N-15-NITROGEN MINERALIZATION FROM BACTERIA BY PROTOZOAN GRAZING AT DIFFERENT SOIL-MOISTURE REGIMES

The predation by protozoa of bacteria was studied in relation to soil moisture regimes. In soil microcosms, maintained at three soil moisture levels, it was shown that the activity of protozoa was inhibited as soon as pores with pore necks > 3-mu-m were devoid of water. The activity of protozoa reduced the number of bacteria and increased the mineral N content in the soil under more favourable moisture conditions. In a series of experiments in which planted soil was subjected to soil moisture regimes with either continuous moist or fluctuating moist conditions, it was shown that protozoa generally stimulated the mineralization of N and the uptake of N by plants 5-10% compared to soils without protozoa. In addition the turnover of introduced N-15-labelled bacterial N was higher in soils with protozoa than in soils with only bacteria. Even though protozoa were forced to encyst upon drying of the soil, they reacted very rapidly to remoistening. Under conditions with modest fluctuating soil moisture conditions, protozoan activity resulted in an even higher plant N-15-N uptake than that in soils kept continuously moist. When fluctuations in the soil moisture content were more intense and applied more frequently, protozoa still stimulated the mineralization of N. Thus their more restricted activity tended to yield less N available for plant uptake than in continuously moist soils. Protozoa did not reduce bacterial numbers in all incubations and protozoan numbers were not affected by the soil moisture regimes. An important regulation by soil moisture fluctuations on the effect of food web interactions between bacteria and protozoa to mineralization of N is hypothesized.