Soil Teeming with Life: New Frontiers for Soil Science - Sustainable Management of Soil Organic Matter - page 410

minimizing losses of N through leaching by careful soil management that
takes account of soil microbial processes.
Measuring the process of mineralization has for many years proved
difficult, but is essential if we wish to be able to quantify and/or manage
nutrient transformations within a given system. Using the recently devel-
oped pool dilution techniques, we now realize that flows of nitrogen in soils
that result from gross mineralization processes are very much larger than
previously thought. Goulding et al . have shown that gross transformations
of N may exceed 18 mg kg day −1 , indicating that such fluxes may be many
times the uptake of N by crop plants, although in this example the high
mineralization rates were accompanied by high immobilization. Method-
ological problems remain in the use of such techniques, though they do pro-
vide a real indication of microbial activity and allow improved insight into
the competition between plants and microbial populations for nitrogen.
The concept that mineralization of organic nitrogen is a necessary pre-
requisite for plant nitrogen acquisition has been questioned in a number of
recent studies. Goulding et al . have found that extractable soil N in a range
of arable soils in the south of England contains 55-65% of total soluble N in
organic form, which clearly indicates the importance of possible N loss in
this form and may indicate a possible source of N for plants. Nasholm et al .
(1998) found rates of 15 N-labelled glycine uptake by coniferous trees, dwarf
shrubs and grasses that were comparable with that of NH 4 + . Uptake of
organic N is known to be of importance in upland and boreal vegetation,
and has often been assumed to be mediated by ectomycorrhizas (Turnbull
et al ., 1996). It has been suggested by Jonasson and Shaver (1999) that this
may explain the reason why some plants adapted to these habitats show
relatively little response to additions of inorganic N.
An understanding of the factors contributing to the turnover and
decomposition of dissolved organic matter (DOM) fractions is important in
allowing us to predict losses and potential mineralization from this source.
Several papers in this topic have identified the importance of DOM in
contributing to losses of organic carbon with associated nutrients from soil
profiles by leaching (Kaiser et al ., McTeirnan et al ., and Marschner and
Bredow). It is likely, however, that site properties, such as hydrology, play
an important role in mediating such losses (McTiernan et al .). Chapman
et al . found that concentrations of dissolved organic nitrogen (DON)
remained constant despite changes in net mineralization of N and sug-
gested that this could be explained by the equilibrium between the DON
produced and a larger reserve pool. The central role of organic matter as an
intermediate in the process of mineralization (Appel and Mengel, 1990)
underlines the importance of developing a better understanding of

dissolved organic fractions in nutrient cycling processes.
The storage of nutrients in soils is closely linked to the availability
and throughput of organic matter derived from plants. Current increases