Samples were collected using the split-PVC corer method one time for
the pre-treatment in fall (September) 2015, and three times each year
in 2016 and 2017, in the spring (May-June), summer (August), and fall
(October-November).
In the split-PVC corer method, a 5 cm diameter split PVC corer is used
to take all samples. A split PVC corer consists of a piece of 2 inch
(5 cm) PVC pipe, about 15-20 cm long, split lengthwise on both sides.
The corer is actually in two pieces. We put the corer together along
the cuts, and duct-tape one side -- the "hinge" side.
Holding the corer firmly together, we hammer it 10-15cm into the
ground. The corer is removed and then opened with the intact soil core
inside. The soil is split into 2 layers (Oi/Oe/Oa and A mineral
horizons). Each horizon is measured and placed into a sample bag. We
typically collect 2-8 cores per site, compositing all cores by
horizon.
Sample collection for determination of in situ net N mineralization
and nitrification rates followed the buried-bag method. Paired soil
cores were removed from plots. One core in each pair was returned
intact to the laboratory, and the other core in each pair was sealed
in polyethylene bags and immediately re-inserted into the ground for
4-8 weeks for in situ incubation, after which it was removed and
returned intact to the laboratory.
Samples were stored at 4°C between sampling and analysis (from less
than 1 week to up to 3 weeks). Soils were manually homogenized: all
large rocks, roots, and other non-decomposed organic material were
removed, and samples were removed, and samples were thoroughly mixed.
All samples were held at field moisture before analysis. Soil water
content was determined gravimetrically.
Microbial biomass C and N content was measured using the chloroform
fumigation-incubation method (Jenkinson and Powlson 1976). Soils were
fumigated to kill and lyse microbial cells in the sample. The
fumigated sample was inoculated with fresh soil and sealed in a jar,
and microorganisms from the fresh soil grew vigorously using the
killed cells as substrate. The flushes of carbon dioxide (CO2) and 2 M
KCl extractable inorganic N (NH4+ and NO3-) released by the actively
growing cells during a 10-day incubation at field moisture content
were assumed to be directly proportional to the amount of C and N in
the microbial biomass of the original sample. A proportionality
constant (0.41) was used to calculate biomass C from the CO2 flush in
the fumigated samples. Biomass N is the total inorganic N flush in the
fumigated samples.
Inorganic N and CO2 production were also measured in
"control" samples. Control samples were prepared in the same
fashion as those listed above, but were not fumigated. These
laboratory incubations provided estimates of microbial respiration and
potential net N mineralization and nitrification. Microbial
respiration was quantified from the amount of CO2 evolved over the
10-day laboratory incubation. Potential net N mineralization and
nitrification were quantified from the accumulation of NH4+ plus NO3-
and NO3- alone during the 10-day laboratory incubation. We measured 2
M KCl extractable inorganic N in the fresh soil samples to determine
the initial soil NO3- and NH4+ concentrations. Carbon dioxide was
measured by thermal conductivity gas chromatography. Inorganic N was
measured colorometerically using an autoanalyzer.
Denitrification enzyme activity was measured using the short-term
anaerobic assay described by Smith and Tiedje (1979). Homogenized
soils were amended with NO3- (100 mg N kg-1), dextrose or glucose (40
mg kg-1), chloramphenicol (10 mg kg-1), and acetylene (10 kPa), and
were incubated under anaerobic conditions for 90 minutes. Gas samples
were taken at 30 and 90 minutes, stored in evacuated glass tubes and
analyzed for N2O by electron capture gas chromatography. For more
information on any of the methods described above, refer to Standard
Soil Methods for Long-Term Ecological Research (1999).
Organic matter was measured on composite samples for each soil horizon
by loss on ignition (LOI) over 4 h at 450°C for the 2015 pre-treatment
soils only.
In situ net N mineralization and net nitrification rates were
quantified from the accumulation of NH4+ plus NO3- and NO3- during the
4-8 week in situ (field) incubation. We measured 2 M KCl extractable
inorganic N in the fresh and in situ incubated soil samples to
determine soil NO3- and NH4+ concentrations. Inorganic N was measured
colorometerically using an autoanalyzer.
All results are expressed on a per gram of dry soil basis. Values can
be converted to a “per area” basis using data on the mass of different
soil horizons found elsewhere on the data page of this website.
The sample dates are associated with the following project timeline:
2015-09-23: Fall 2015 (Pre-treatment)
2016–1-27: 1st Icing
2016-06-12: Spring 2016
2016-08-24; Summer 2016
2016-10-18: Fall 2016
2017-01-14: 2nd Icing
2017-05-10: 2017 Spring
2017-08-28: 2017 Summer
2017-11-01: 2017 Fall
