MELNHE Overview:
Although temperate forests are generally thought of as N-limited,
resource optimization theory predicts that ecosystem productivity
should be co-limited by multiple nutrients. These ideas are
represented in the Multi-Element Limitation (MEL) model (Rastetter et
al. 2012). To test the patterns of resource limitation predicted by
MEL, we are conducting nutrient manipulations in three study sites in
New Hampshire: Bartlett Experimental Forest, Hubbard Brook
Experimental Forest, and Jeffers Brook in the White Mountain National
Forest. Additional project information can be found at
http://www.esf.edu/melnhe/default.htm
Site Description:
At Bartlett, we have three replicate stands of three ages, young
(clearcut 1982 - 1990), mid-age (clearcut 1970-1979), and mature (cut
1883 - 1915). We have identified stands at Hubbard Brook and Jeffers
Brook that corresponds to the mid-aged and mature stands at Bartlett,
for 4 more stands. Each of the 13 stands has four 1/4 ha (50 m x 50 m)
treatment plots, treated annually beginning in spring 2011, with N (30
kg N/ha/yr as NH4NO3), P (10 kg P/ha/yr as NaH2PO4), N+P, or nothing
(an untreated control). Five stands also have a Ca treatment plot
(1150 kg Ca/ha in the form of CaSiO3).
We are monitoring stem diameter, leaf area, sap flow, foliar
chemistry, leaf litter production and chemistry, foliar nutrient
resorption, root biomass and production, mycorrhizal associations,
soil respiration, heterotrophic respiration, N and P availability, N
mineralization, soil phosphatase activity, soil carbon and nitrogen,
nutrient uptake capacity of roots, and mineral weathering. This data
set includes phosphate, nitrate and ammonium availability measured
using resin exchange strips.
Sampling Design:
In 2011 we began quantifying resin-available N and P in the 4 soil
sampling subplots per plot established in the MELNHE study (see
diagram). Confining sampling in this way ensures that repeated
sampling is done in the same general area. In 2011 two strips for each
inorganic nutrient (1 strip each for PO4, NH4, NO3) were deployed in 2
of the subplots and 1 strip for each inorganic nutrient was deployed
in the other 2 subplots per plot. After 2011 two strips per inorganic
nutrient were deployed per plot in all subplots. To deploy a resin
strip it was inserted under the blade of a knife that had been used to
slice the top ~5 cm of soil (usually O horizon) at a 45 degree angle.
The knife was withdrawn and soil was pressed down to ensure contact
with the strip. Strips were incubated for approximately 2 weeks before
retrieval.
Laboratory procedures:
Anion strips (Ionics AR204-SZRA) were used for NO3 and PO4 retrieval
and cation strips (Ionics CR67-HMR) were used for NH4 retrieval.
Strips for NH4 and NO3 were prepared as follows: 1) Rinse twice in
deionized water (DI) and then shake for 10 minutes in 5% HCl. 2)
Repeat the rinse/shake procedure. 3) Shake in DI for 10 minutes. 4)
Charge (soak) in 1M NaCl for at least 24 hours. 5) Repeat steps 1-3.
6) Store in 1 M NaCl for at least 24 hours prior to use. 7) Rinse in
DI immediately prior to deploying in field.
Strips for PO4 are prepared as follows: 1) Rinse twice in DI and then
shake for 10 minutes in 5% HCl. 2) rinse in DI 3 more times. 3)
Convert to bicarbonate form by shaking for 10 minutes in 0.5 M NaHCO3
andyoung (clearcut 1982 - 1990), mid-age (clearcut 1970-1979), and
mature (cut 1883 - 1915) rinsing with DI. 4) Repeat shaking in
NaHCO3/rinsing 3 more times, for a total of 4 shakes/rinses. 5) Store
in refrigerator for short-term storage and shipment (no more than 1
week).
Strips were collected from the field and were kept damp and
refrigerated until processing. Strips were extracted with the
following procedure: 1) Rinse with DI and gently scrape strips
thoroughly to remove excess soil. 2) Shake strips for 1 hour in
centrifuge tubes with 30 mL each of the following extractants: 1M KCl
for NO3 and NH4 and 0.5M HCl for PO4 strips. 3) Analyze extract
solution using the Lachat autoanalyzer.
Calculations: mg/mL of each inorganic nutrient*30mL extractant= mg
nutrient/strip/incubation days.
Data Location: Data are stored on the Fisk lab drive at Miami
University and on the MELNHE website.
