『Abstract
Sodium polytungstate (SPT) is currently the material of choice
for soil density fractionation (DF). We recently detected high
levels of N in several types of commercially available SPT (0.74-1.4
mg g-1), raising a concern that undesirable chemical
effects on soils may occur during the DF procedure. To address
this concern, we conducted two experiments to examine effects
of SPT on C and N in the resulting soil fractions. First we suspended
A-horizon material from three soil types of greatly differing
mineralogy for 24 h in solutions containing three types of commercially
obtained SPT and commercial SPT that had been passed through cation
exchange resin columns. we compared %C, %N, 15N and
13C values in treated and untreated soils. We also
spiked SPT with tracer-level 15NH4+
to measure potential NH4+ absorption
by the soil fractions. Results suggest that the N-rich commercially
available SPT can have a considerable effect on δ15N
values, likely due to the presence of 15N-enriched
NH4+ in the SPT. In one of our
soils, 15N enrichment of 3‰ was observed associated
with overnight soaking in N-rich SPT (0.74 mg g-1).
By contrast, when using SPT with low levels of N (0.05 mg g-1),
no significant changes in 15N were observed. The remaining
soil (after suspension and rinsing) was similar in %C, %N, 15N
and 13C to the untreated bulk soil, suggesting that
suspension of soil in SPT with low N levels purchased from the
manufacturer or else through treatment with cation exchange resins
does not greatly alter these variables. Low-N SPT is available
commercially although it must be specifically requested from the
manufacturer and is currently more expensive to purchase. Our
results confirm that SPT tested and known to be low in C and N
(<0.06 mg g-1) does not adversely contaminate soils
during the soil density fractionation procedure. If using newly
purchased or recycled SPT with higher N or C levels than this,
we recommend thorough testing for possible contamination effects
prior to use. However we caution against using SPT that contains
N or C levels >0.5 mg g-1.
Keywords: Soil organic matter dynamics; Stable isotope ratios;
Carbon; Nitrogen; Sodium polytungstate; SPT; Density fractionation;
Natural abundance of 15N and 13C』
Introduction
Methods
Results and discussion
Acknowledgments
References