to remove any clogging by ochre precipitates before the growing
season. Nevertheless, the discharge from one LWC lysimeter ceased
in 2009 and could not be re-established by flushing. As the degree of
soil water saturation, redox potential and pore water composition
in this particular lysimeter deviated from those in other replicates,
and as ochre clogging was confirmed and more shallow growth of
plant roots was detected upon dismantling of the lysimeters at the
end of experiment, this lysimeter was omitted from further analysis
as an outlier.
At the end of experiment, the soil monoliths were dismantled
using a simple piston developed for the purpose (Fig. 4). Undisturbed
cores and composite samples were taken from each soil
horizon.
2.3. Soil redox potential
An automatic data acquisition system was constructed consisting
of a Pt electrode in each soil horizon of a lysimeter, a common
Ag/AgCl reference electrode (Inlab 301), a saturated KCl salt bridge
(Linebarger et al., 1975) and a data logger (Agilent 34980A, multiplexer
module 34923A, input impedance 10 G) scanning the Pt
electrodes at 10-min intervals (Fig. 5). Two salt bridges facilitated
the use of two common reference electrodes for 50 Pt electrodes
and protected the reference electrodes from any contamination by
direct contact with soil. The Pt electrodes were checked before
installation and after the experiment in a standard redox buffer
solution (220 mV for Ag/AgCl, L4619, DIN 38404, Schott). The soil
redox potential was continuously measured in the Ap, Bg2, Bgjc,
BCg and Cg horizons. The functioning of the redox electrodes and
salt bridges was monitored by continuous measurement of the
redox buffer solution (220 mV, Schott, changed weekly) and by
manual measurements (a Pt electrode, a calomel reference electrode
(Orion 90-01) and a high-impedance voltmeter (P5E, Jensen
instruments)). A correction factor due to the bridge connection (fs)
was calculated on the basis of the reference measurements of the
buffer solution, but because this factor was small (<10 mV), it was
ignored in the final calculations. Redox readings (Em, mV) measured
at a given soil temperature Tsoil (K) were converted to the redox
potentials (Eh) relative to the standard hydrogen electrode at 298 K
(Eh, mV) using Eq. (1), in which Eref was 245 mV for the calomel
and 199 mV for the Ag/AgCl reference electrode, and fT was 0.6 and
0.7, respectively (Hills and Ives, 1961; Sawyer and Roberts, 1974):
Eh =
Em +
Eref +
fT (298
−
Tsoil) (1)
2.4. Soil moisture and temperature
The soil moisture content (v, m3m−3), electrical conductivity
(dS m−1, data not shown) and temperature (Tsoil, K) were measured
continuously at 10-min intervals with 5TE probes and Em50 data
loggers (Degacon Devices, Inc., USA) in the Ap, Bg2 and BCg soil horizons
(Fig. 5). Pre-testing with ECH2O-TE probes had revealed that
they were unsuitable for acid conditions, as their plating corroded
in four months. Therefore, 5TE probes made of stainless steel were
chosen in the study. Before installing the probes, their function was
checked in quartz sand, and they were separately calibrated for
each horizon after Cobos and Chambers (2010) using the logged
raw data and the gravimetrically determined soil volumetric moisture
content at the end of experiment. The probes were uninstalled
with the surrounding soil, and the probe readings just before uninstalling
were compared to the volumetric soil moisture content at
the end of experiment. The volumetric soil moisture was calculated
from the gravimetric moisture content (Gardner, 1986) using the
bulk densities (b) determined in triplicate with a core (volume
0.25 dm3) of these given soil horizons. The coefficients of determination
(R2) for the calibration curves were 0.95, 0.93 and 0.76
for the Ap, B2g and BCg horizon, respectively, and calibrated data
were used for calculating the soil moisture contents in different soil
horizons. The degree of water saturation (S) for each horizon was
calculated using Eq. (2):
S
= v
s
(2)
where v is volumetric water content and s is saturated water
content determined for the given horizons.
Air temperature was measured using K-type thermocouples
and continuously logged with a data logger (Agilent
34980A, multiplexer module 34921A) in the open air, under
the heat insulation and within the plant canopy. The comparison
of soil temperature measured by 5TE probes at
10-cm depth and the air temperature in the plant canopy
demonstrated a linear relationship (the coefficient of determination
R2 was 0.92) when the soil temperature exceeded
0.1 ◦C.
2.5. Soil and water pH
As continuous measurement of the soil pH was not feasible and
the temporal pH changes were expected to be gradual, soil pH was
measured manually (IQ 160 ISFET pH meter with a built-in thermistor,
PHW37-SS, Hach Company, USA) through access holes in
the core wall at monthly intervals (Fig. 5). The pH of pore water
การแปล กรุณารอสักครู่..