Environmental Engineering Reference
In-Depth Information
the technique integrates the on-site soil water dynamics only if the soil water content
at the beginning of the incubation period is representative of the entire incubation
period. The following specifics to the assay were drawn from Hart et al. (
1994
).
Soil cores are extracted with thin-walled PVC or metal cylinders, sharpened at
1 end to provide a cutting surface. Recommended cylinder dimensions are 5 cm
i.d. and 12 cm length. Longer cylinders allow for deeper soil samples but are more
difficult to remove intact cores from the cylinders and require larger incubation bags.
Cylinders are traditionally handled in groups of three. Cylinders are hand driven into
the soil until 2 cm of the cylinder is above the soil surface. Three cylinders should be
used for each 50 m
2
of area; these serve as replicates. Cylinders are removed and the
soils are removed from the cylinders resulting in a 0-10 cm sampling depth. Soils
from three cylinders are composited for pre-incubation analysis. These will serve to
provide baseline (time 0) data for initial concentrations of NH
4
+
and NO
3
.Addi-
tional soil samples are taken (pre- and post-incubation) to determine gravimetric soil
water content. As the laboratory analysis is conducted on field moist soil, soil
moisture content is needed to express the data on a dry weight basis. Each of the
remaining soil samples are enclosed in polyethylene bags (15-30
m thick), the bags
are sealed with plastic ties, and returned to their respective bore holes. The bags are
covered with leaf litter, if present. If not, the bags should be covered with a small
amount of similar soil to prevent temperature extremes. Bags are removed after a 1 or
2 month incubation period. Samples with perforated bags should be discarded.
Consecutive buried bag incubations will provide seasonal patterns of net N minerali-
zation and nitrification. Microbial activity will continue after the bags are retrieved.
Therefore, the samples should be kept cool (2-5
C) during short-term storage (
μ
2d)
and frozen for long-term storage. Inorganic N is extracted from soil subsamples with
2
M
KCl in the lab. See Hart et al. (
1994
) for the sample preparation and extraction
procedures. Filtered extracts are analyzed for NH
4
+
-N and NO
3
-N (see Mulvaney
1996
). Net N mineralization is calculated as the change in inorganic N (NH
4
+
-N and
NO
3
-N) content over the incubation period; net nitrification is calculated as the
change in NO
3
-N content over the incubation period. Net P mineralization can also
be determined with this method.
7.5.4.3 Resin Core Method
The
in situ
intact-core resin-bag method was developed to measure N mineraliza-
tion and nitrification under the existing field soil conditions with respect to temper-
ature and water content (Distefano and Gholz
1986
). This system consists of an
intact soil core within a PVC or metal cylinder with an ion exchange resin bag at
each end. This allows water but not ions to flow through the soil core and eliminates
the static moisture regime inherent to the buried bag method. No resin bag is needed
at the top of the cylinder if the soil core does not receive leachate from overlying
soil horizons or is exempt from inundation.
Wienhold et al. (
2009
) presented a modified version of the ion resin exchange
technique in which they use a resin bag at the bottom only. A metal cylinder
