R.G. McBride et al. Applied Soil Ecology 15 2000 243–251 247
3. Results and discussion
3.1. Experiment 1: addition of rye 3.1.1. Organic acids in leachate
Only low concentrations 450 mmoll of formic and acetic acids were detected in the leachate from
treatments receiving 23 and 34 g of rye Fig. 1. These acids were primarily detectable during the first 24 h
following rye application, although formic acid was detected after 180 h in one sample of each of the two
rye rates. None of the other organic acids were present in detectable concentrations during the 180 h incu-
bation period. These findings were consistent with a study conducted by Schwartz and Martin 1955, who
observed a roughly 70 increase in the concentrations of acetic and formic acids after 24 h following incor-
poration of fresh alfalfa Medicago sativa L.. Simi-
Fig. 1. Formic acid A and acetic acid B concentration and the pH C of the soil leachate after the incorporation of rye. Mean
separation of pH data by LSD. Means with no common letter are different at the 5 level within each sample data. All data points
are averages of three pots sampled.
larly, Baziramakenga et al. 1995 showed that formic and acetic acids were the dominant acids released fol-
lowing incorporation of dried quackgrass Elytrigia repens L. Nevski into soil. They reported that these
two acids accounted for 75 of the total aliphatic acids present in the soil during their 8-week experiment.
An additional effect of the rye treatment was a sig- nificant increase F-test of rye×time interaction effect,
p0.01 in the pH of the soil solution at each sam- ple date after time zero, compared with the control
Fig. 1. At this higher solution pH 7.0–7.5, acetic acid pK
a
= 4.76 and formic acid pK
a
= 3.75 are al-
most exclusively in the dissociated, anionic form. The dissociated form of these acids does not have a toxic
effect on root-knot nematodes, even at high concen- trations Djian et al., 1991. It is unclear what mech-
anism was responsible for the rise in pH; the addition of Ca and Mg in the rye would not explain such a
large increase.
3.2. Nematode population The failure to detect high concentrations of low
molecular weight organic acids in the rye-treated soils did not correspond to a failure to alter the root-knot
nematode population in the treatments. The total num- ber of nematodes found in the soil following rye ad-
ditions might not be the best measurement of the rye-treatment effect. For example, nematodes in the
soil may be observed to be alive following rye addi- tions, but may no longer be able to infect a host plant.
For this reason, tomato plants were planted into the treated soil to serve as direct bio-indicators of nema-
tode activity as measured by the ability of the nema- todes to infect the plants.
The growth of the tomato shoots and roots was sig- nificantly enhanced with rye treatment F-test of rye
main effect, p0.05 Fig. 2. Seven days prior to planting, the solution pH of the rye-treated soils Fig.
1 was significantly higher than the unamended con- trol, and was much closer to the tomato plant’s op-
timum range of 6.0–6.5 Gould, 1992. Although all the pots received adequate supplemental fertilization
and none of the plants were deficient in water or in plant nutrients, differences in plant nutrient concentra-
tions were observed. The tomato plants grown in the rye-amended soil showed a highly significant F-test
of rye main effect, p0.01 decrease in the concen-
248 R.G. McBride et al. Applied Soil Ecology 15 2000 243–251
Fig. 2. Transformed weight of tomato shoots and roots grown in soil amended with different rates of fresh rye. Weights represent
an average of 15 reps. Means with no common letter within a row are significantly different at the 5 level. LSD 0.05=0.316 for
dry shoot weight, and LSD 0.05=0.621 for fresh root weight.
tration of Fe compared with the unamended control. There were highly significant increases p0.01 in
the concentrations of P, K, and Mn, and a signifi- cant increase in Mg concentration p0.05 over the
plants grown in the soil receiving no rye Table 1. The observed rise in solution pH might contribute to in-
creased P availability, but K and Mg availability would remain relatively unchanged, and Mn solubility would
have been expected to decline. The differences in plant size were possibly due to damage by the nematode
populations.
The root-knot nematode populations on the toma- toes were significantly p0.001 suppressed by
Table 1 Nutrient concentration of tomato leaves from plants grown in a
non-amended soil and a soil amended with fresh rye shoots Tomato leaf
nutrient content Treatment in grams of rye
S.E.
a
23 34
Phosphorus 0.30 a
0.40 b 0.45 c
0.01 Potassium
1.57 a 1.89 b
2.11 c 0.04
Magnesium 0.27 a
0.31 ab 0.31 b 0.01
Manganese µgg 38.6 a
66.0 b 63.2 b
3.10 Iron µgg
45.4 a 36.0 b
37.2 b 2.46
a
S.E. of the mean=square root mean square errorreps; mean square error computed from the model Y
ij
= µ+T
i
+ S
j
+ e
ij
, where T
i
represents rye treatments, and S
j
represents sampling dates replications; means with no common letter within a row are
significantly different at the 5 level LSD.
the addition of the rye, as measured by the visual root-gall rating of the root system Fig. 3. When
nematode-induced root galls were counted, damage was reduced from a mean of 8.3 gallsg of root in the
0 g rye treatment to a mean of 4.8 gallsg in the 23 g rye treatment and 3.0 gallsg in the 34 g rye treatment
with a standard error of the difference of 0.78 Fig. 3. The root damage reduction is an indication that the
addition of rye to the soil adversely affected the abil- ity of root-knot nematodes to parasitize the tomato
plants in the study, with the high rye addition rate being the most effective. Similar results have been
found with cotton Gossypium hirsutum L. following rye additions McBride et al., 1999.
Researchers have suggested that the addition of a labile C source to the soil results in an increase in
bacteria and fungi that utilize the added C for energy Linford et al., 1938. Populations of nematodes that
feed on the bacteria and fungi would in turn increase. With the overall nematode population increasing, it
is likely that organisms such as predatory nematodes, mites, and nematode trapping fungi, which prey on
nematodes, would also increase. The plant-parasitic nematodes, having received no direct benefit from the
C source, would suffer from the increase in predator populations.
3.3. Experiment 2: fate of added organic acids The concentrations of acetic, propionic, butyric,
and valeric acids in the soils declined significantly p0.01 from a maximum of 630 mmoll formic
acid to a minimum of 10 mmoll acetic acid dur- ing the 10 h incubation Fig. 4. This finding clearly
shows the rapid disappearance of these low molecular weight organic acids from the soil solution.
It is possible that the acids were adsorbed, com- plexed, or degraded within the soil. All three of these
mechanisms of removing organic acids from solution would have the result of limiting their interaction
with nematodes. For example, at low soil pH the edge hydroxyls of clay minerals can be protonated and
participate in a ligand exchange with organic acids. In addition, proton bonding could theoretically occur
between the mineral surface and an acid in the dis- sociated form. This mechanism of removal would be
possible in this experiment where the soil solution pH was 4.3. The organic acids used are largely dissociated
R.G. McBride et al. Applied Soil Ecology 15 2000 243–251 249
Fig. 3. Nematode-induced root damage to tomato roots grown in soil amended with different rates of fresh rye. The gall count A is expressed as nematode induced gallsg. The visual index B is expressed as percent of the root mass damaged. The gall counts were
transformed by taking the square root of the data to stabilize variance. Means with no common letter with are significantly different at the 5 level. Data represent an average of 15 plants. LSD 0.05=1.6 for gall counts, and LSD 0.05=3.9 for visual index.
pK
a
3.8–4.9 at a pH4.3 and therefore are likely to participate in this type of reaction. Aliphatic acids
are known to have chelating characteristics, forming complexes with metals in the soil Stevenson and
Ardakani, 1972; Brynhildsen and Rosswall, 1997.
Fig. 4. Decline in five low molecular weight organic acids in soil water over time. The five acids declined significantly over time as measured by analysis of variance at the 0.05 level, with data points representing averages of three pots. Standard errors were 59 for formic,
32 for acetic, 52 for propionic, 61 for butyric, and 106 for valeric acid.
Acids, such as citric and oxalic form stable complexes, while acids like formic do not Fox and Comer-
ford, 1990. Microbial degradation is also a potential mechanism for removal of organic acids from the soil
solution.
250 R.G. McBride et al. Applied Soil Ecology 15 2000 243–251
4. Conclusions
Kategori