Abstract:
Effects of three mulches of tree foliage and twigs (red alder, Douglas-fir, and
salal) and homogenized, partially decomposed material (forest floor) on soil macronutrients
(Ca, K, Mg, P, NO3, and NH4) were evaluated in the summer of 1997. These
effects were measured with buried, coated ion-exchange resin bags at two different times
(2 and 12 weeks) and at two different depths (2 and 15 cm) in homogenized soil cores.
Over the time of this study various mulches had no significantly different impact
on nutrient ions sorbed on the buried resins. The results indicated that in the early stages
of decomposition, chemical differences in the plant material evaluated do not
differentially influence soluble nutrients sorbed on buried resins. However, some of this
lack of difference may have been due to the preparation of the mulches. The preparation
included chopping and shredding, which may have eliminated differences that exist in a
more natural setting. The effect of forest floor material was significant on three of six
response variables. The interaction of forest floor and mulch was significant for all
variables except K. Whether the interaction decreased or increased the mean amount of a
given response variable was dependent on the type of mulch material. The exception was
NO3, which in all cases increased in the presence of forest floor in combination with
mulch. The results indicate that microbial populations that reside in the forest floor
interact differently with the types of plant mulches placed over them.
The effect of time was significant for all response variables with K, Ca, and Mg
responding in a proportional manner. Nitrate and ammonium concentrations increased
exponentially over time, which indicated a rather vigorous biological population involved
in decomposition of nitrogen-containing organic compounds.
The effect of depth was significant for all variables except for P. More nutrients
accumulated at the 15 cm depth than at the 2 cm depth, which was not expected. This
possibly indicates high immobilization due to microbes at the surface. The results of P
may have been due to the type of resin used and the pH of the soil cores.
The results suggest that forest floor material has more influence with above
ground additions of soil macro-nutrients than fresh plant material. However, the detected
interactions of these two types of materials suggest that ion cycling can be influenced
very early in the litter decomposition process.
A second study tested the effect of saturated soil coated resin bags versus non-coated
bags. The results strongly suggest that the effect of coating resin bags with a thin
slurry of saturated soil paste significantly decreases variation in tests of Ca, Mg, NO3, and
NH4. The minimal amount of work with this procedure suggests that it is advantageous
for evaluating most macro-nutrients with resin bags, particularly NO3 and NH4.
