The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 245

ammonification, nitrification and assimilation into the microbial biomass.
The location of such a reserve of organic N is not clear, but it could occupy
an adsorbed phase on surfaces in the soil matrix, which can be released
partly by salt solutions (Reemtsma et al ., 1999). This would be consistent
with the greater amounts of DON released from soil by 0.5 M K 2 SO 4 com-
pared with water.
In the soil leachate experiment, concentrations of DON were signifi-
cantly larger in leachates from cores with vegetation. The total amount of
DON leached from the cores with vegetation was 0.23 g N m −2 , which
was four times greater than that leached from cores without vegetation
and 20% less than the DON extracted with water from the soil cores in the
incubation experiment. Thus, removing the vegetation cover and root mat
eliminated an important source of DON. In the cores without vegetation,
some root material remained, the decay of which would account for the
larger concentration of DON leached over the first 14 days compared with
subsequent days. The greater leaching from cores with vegetation suggests
that much of the DON was leached from foliage and plant residues in the
root mat and/or exuded from roots. In forest soils, it has been suggested
that the major source of DON in soil water was water-soluble organic
material leached from freshly shed leaf litter (e.g. Qualls and Haines, 1991).
Plant roots are also known to release a complex mixture of organic
compounds that include sugars, amino acids and organic acids into the
rhizosphere as part of normal growth. If the DON in the leachates
was derived predominantly from the plant roots and litter, then the
water-extractable DON pool in the soil contributed very little to the DON
in leachates.
Role of DON in N mineralization
One of the hypotheses regarding the DON pool is that it is an intermediate
in the mineralization process and a source of readily mineralized organic
N (Appel and Mengel, 1990). In sandy soils, DON extractable with 0.01 M
CaCl 2 correlated positively with plant uptake and soil inorganic N
(Appel and Mengel, 1993), possibly because the sandy soils contained
<50mgCkg −1 dry soil. The podzol studied here contained 278 mg C kg −1
dry soil and the relationship between the readily mineralized N pool and
the mineral N pool possibly was obscured by the quantity of DON. Thus
the DON pool may contain a smaller active fraction that is turning over
rapidly. For example, microbial metabolites, such as amino acids derived
from the large microbial N pool, have a rapid turnover rate in some
soils (Jones, 1999). Alternatively, the organic N mineralized may have
been replaced by DON desorbed from a larger pool of adsorbed organic
matter.