Nitrification in coniferous forest soils

Abstract

Net nitrification rates tend to be low or negligible in the forest floor of many coniferous forests of North-East Scotland. The most likely process controls are substrate availability, pH, allelopathy, water potential, nutrient status and temperature. These are discussed in relation to field and laboratory studies of net and potential rates of nitrification.Fungi make up by far the largest part of the nitrifier community in the coniferous forest floor. Very little is known about the distribution and activity of autotrophs in these systems, although it is certain that in vitro evidence suggesting autotrophs cannot nitrify at pH levels characteristic of coniferous forest soils is unrealistic.Because of the metabolic diversity of nitrifying fungi, a variety of organic and inorganic nitrification pathways may exist in coniferous forests. The possible involvement of free radicles in fungal nitrification in coniferous forest soils is also suggested.A complete understanding of nitrification in coniferous forest soils can only result from field characterisation of N flux such as through the use of 15N. This must be combined with ecophysiological characterisation of the organisms involved in order that the complexity of nitrification in coniferous forest soils can be resolved.

Inorganic nitrogen availability after severe stand-replacing fire in the Greater Yellowstone ecosystem

“…Nitrification may cease if the NH 4 ϩ supply declines below some threshold or if microbial assimilation of NH 4 ϩ preempts uptake by nitrifiers (49), as our data suggest. Factors other than NH 4 ϩ supply may also limit nitrification in coniferous forest soils (50,51). Low pH has been associated with low rates of nitrification (52,53), although Booth et al (54) did not find a significant effect of pH.…”

Untitled

“…Emmer and Tietema (1990) found in a forest soil an optimum for nitrification at 25 • C. However, mineralization and nitrification in summer are very small due to very small soil water contents (Figures 3 and 4). Myers et al (1982) found a linear relationship between net nitrogen mineralization rate and soil water content and some nitrifying soil microbes are very sensitive to water stress (Killham, 1990). …”

“…No nitrogen of the fertilizer is lost through the soil at the grazed innerdune plot and only 13% at the exclosure of the innerdune site. The larger flux at the foredune site might be due to a raised nitrification rate in the calcareous soil with a higher pH at the foredune site (Dancer et al, 1973;Killham, 1990;Nyborg and Hoyt, 1978). The larger flux at the fertilized exclosure at the innerdune site compared to the grazed and non-fertilized exclosure was possibly due to fertilizer taken up by the vegetation and subsequent mineralisation and nitrification of senescing leaves and dead standing biomass.…”

“…Consequently, the ammonium flux is the smallest, while the nitrate flux is the largest at the foredune site (MS1). Relatively high soil temperature in summer (about 18 • C) should cause relatively large mineralization rates (Freijer, 1994;Stanford et al, 1973) and nitrification rates (Emmer and Tietema, 1990;Killham, 1990). Emmer and Tietema (1990) found in a forest soil an optimum for nitrification at 25 • C. However, mineralization and nitrification in summer are very small due to very small soil water contents (Figures 3 and 4).…”

“…These differences are very small in the summer months and increased in the autumn (Figure 7). During winter these differences exist because of the raised nitrification rates at the calcareous foredune site (Dancer et al, 1973;Killham, 1990;Nyborg and Hoyt, 1978). Consequently, the ammonium flux is the smallest, while the nitrate flux is the largest at the foredune site (MS1).…”