Agroforestry Systems 48: 61–77, 2000.
 2000 Kluwer Academic Publishers. Printed in the Netherlands.
Defining competition vectors in a temperate alley cropping
system in the midwestern USA
3. Competition for nitrogen and litter decomposition dynamics
S. JOSE1, A. R. GILLESPIE1, *, J. R. SEIFERT2, D. B. MENGEL2 and
P. E. POPE1
1
Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 479071159, USA; 2 Department of Agronomy, Purdue University, West Lafayette, IN 47907-1159,
USA (*Author for correspondence: E-mail: [email protected])
Key words: 15N, decomposition, fertilizer use efficiency, nutrient competition, nutrient release
Abstract. An experiment was conducted in an 11-year-old black walnut (Juglans nigra L.), red
oak (Quercus rubra L.), maize (Zea mays L.) alley cropping system in the midwestern USA to
examine the extent of tree-crop competition for nitrogen and decomposition dynamics of tree
leaves and fine roots. A below-ground polyethylene root barrier (1.2 m deep) isolated black
walnut roots from maize alleys in half the number of plots providing two treatments viz. ‘barrier’
and ‘no barrier’. The percentage of N derived from fertilizer (%NDF) and fertilizer use efficiency (%UFN) were determined using 15N enriched fertilizer. Further, maize grain and stover
biomass, tree leaf biomass, tissue N concentration, and N content were quantified in both
treatments. The ‘barrier’ treatment resulted in a significantly greater grain (67.3% more) and
stover (37.2% more) biomass than the ‘no barrier’ treatment. The %NDF in both grain and stover
was higher in the ‘no barrier’ treatment as a result of competition from tree roots for water and
mineralized N in soil. Maize plants growing in the ‘no barrier’ treatment had a lower %UFN
than those in the ‘barrier’ treatment due to their smaller size and inability to take up fertilizer.
Analysis of tree leaf and fine root decomposition patterns revealed faster release of N (39% over
15 days for black walnut and 17.7% for red oak) and P (30% over 15 days for both species)
from roots compared to the leaves of both species. Following an early release of P (11.3% over
45 days), red oak leaves exhibited significant immobilization for the rest of the incubation period.
The data indicate that competition for N from fertilizer is minimal since nutrient acquisition is
not simultaneous among black walnut and maize. However, competition for mineralized N in
soil can exist between black walnut and maize depending on water availability and competition. Tree leaves and fine roots can enhance soil nutrient pools through the addition of soil carbon
and nutrients. Tree fine roots seem to play a more significant role in nutrient cycling within the
alley cropping system because of their faster release of both N and P as compared to leaves.
Selection of tree species and their phenology will impact the magnitude and rate of nutrient
cycling.
Introduction
Concerns over the long-term sustainability of intensive monoculture systems
have resulted in an interest in agroforestry systems in the temperate region.
Alley cropping, an intercropping system that addresses both environmental
and economic concerns, is becoming a popular agroforestry practice, especially in the midwestern US (Garrett and Buck, 1997). The trees often planted
in these systems include hardwood species such as black walnut (Juglans nigra
62
L.), oaks (Quercus spp.), and pecan (Carya pecan (Marsh.) Engl. Graebn)
combined with crops such as maize (Zea mays L.), soybean (Glycine max L.
(Merr.)), wheat (Triticum spp.) and oats (Avena spp.). One of the most studied
alley cropping systems in the US is the black walnut-based system. However,
most of the published works relate to system design, silvicultural aspects,
and economics (Garrett and Kurtz, 1983; Kurtz et al., 1984; Garrett et al.,
1991).
Results of tropical alley cropping trials, where most alleycropping research
has been conducted, have confirmed both above and below-ground interaction between trees and crops. Although below-ground competition for water
was found to be the major factor causing yield reductions in alley cropping
systems of the semi-arid tropics (Singh et al., 1989; Ong et al., 1991), aboveground competition for light has been reported as the limiting factor in
temperate alley cropping systems (e.g., Chirko et al., 1996).
In previous work, we examined competition for light and water among trees
and maize as determinants of crop yield and found that trees were successful
in competing for water, reducing maize yields in a temperate US alley
cropping system (Gillespie et al., this issue; Jose et al., this issue). Growing
season water uptake by maize plants was reduced by 31.4% when tree roots
were present in the cropping alley. Since plants take up most of their nutrients in the transpirational stream of water, it is reasonable to expect a change
in normal nutrient uptake due to decreased water uptake. Furthermore, like
water competition, trees may compete for nutrients, also altering maize growth
and yield. Since the magnitude of nutrient competition within temperate alley
cropping systems is unknown, the first objective of our study was to quantify
the competition for N between black walnut and maize using 15N-labelled
fertilizer.
The role of woody perennials in restoring or maintaining soil fertility has
long been recognized. Evidence exists from tropical agroforestry experiments,
where leguminous trees are interplanted with crop plants, that tree mulch
additions can meet crop nutrient requirements (Palm, 1995). Although nutrient
dynamics have been the subject of several agroforestry experiments in the
tropics (Palm and Sanchez, 1991; Schroth et al., 1992; Lehmann et al., 1995)
such studies are only beginning in temperate agroforestry systems (e.g.
Thevathasan and Gordon, 1997).
The two major mechanisms by which trees enhance soil fertility in agroforestry systems are the addition of organic matter through leaf and root decay,
and biological nitrogen fixation by leguminous trees. For temperate systems,
however, the trees often planted are non-nitrogen fixers selected for their
product value rather than nutrient input. However, these trees can substantially increase the organic matter input into the intercropped system through
the turnover of leaves and roots. Knowledge about the release of nutrients
through the decomposition of leaves and roots will help in estimating the
inorganic fertilizer requirements of both crops and trees in such systems.
Despite their importance, such studies have not been reported in temperate
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alley cropping systems involving fine hardwood species. Hence, the second
objective of this study was to investigate the decomposition dynamics of
leaves and fine roots of two of the most popular tree species used in alley
cropping systems in the midwestern US, black walnut and red oak (Quercus
rubra L.). Specifically, we tried to determine the rates of decomposition and
patterns of N and P release of leaves and fine roots from these two species.
Materials and methods
Study area
The present study forms part of a larger trial which includes a randomized
complete block design with 12 plots for black walnut-maize alley cropping
and 12 plots for red oak-maize alley cropping, established in 1985 in Indiana,
USA (see Gillespie et al., this issue for a description of the entire trial). Only
six plots from the black walnut-maize alley cropping trial were used for
assessing interspecific competition using 15N fertilizer. Of the six plots, three
plots were subjected to a root pruning treatment in the spring of 1995 where
polyethylene barriers, 1.2 m deep, were installed on both sides of the tree rows
1.2 m from the trunks of the trees. These barriers were used to prevent tree
roots from growing into the alley. Thus, the ‘barrier’ plots served as the nocompetition treatment whereas the ‘no barrier’ plots provided the tree-crop
competition treatment. Of the 24 plots in the larger trial, only eight plots
representing the intact system without any root pruning (four in black walnut
and four in red oak) were used in the decomposition study. Each plot is
100 m long and 11 m wide, having a row of trees on both sides of the cropping
alley. The spacing between tree rows is 8.5 m (current intra- row tree spacing
of 2.5 m) and six rows of no-till maize are planted within the alleys which
have a north-south orientation. Maize was fertilized each growing season at
the rate of 168 kg N ha–1, which is typical for the midwestern US. Table 1
gives the soil chemical characteristics for the study area.
Microplots and 15N application
Maize (Pioneer 3293) was planted on May 22, 1996 at a population of 65,
000 plants ha–1 in rows 0.76 m apart. Insecticide and herbicide were applied
as recommended. In each plot, two 2.5 m × 0.76 m microplots were established (10-12 plants per plot), one each in the first row (edge) and third row
(middle) of maize for the application of labeled fertilizer. Microplots were
laid out in such a way that one of the black walnut trees in the tree row was
in the center and could be selected as the target tree for 15N sampling (Figure
1). On May 30, each microplot was supplied with 15N enriched (NH4)2SO4
(approximately 5% atom excess) at a rate of 168 kg of N per hectare. The
rest of the plot area (except the second and fourth row where regular
64
Table 1. Surface soil chemical properties of an Ultic Hapludalf silt loam within an alley cropping
system in the midwestern USA.
Soil property
1992
1995
pH
Nitrogen (kg ha–1)
Phosphorus (kg ha–1)
Potassium (kg ha–1)
Calcium (kg ha–1)
Magnesium (kg ha–1)
CEC (cmol kg–1)
Organic matter (%)
0006.36
iN/A
0061.6
0254
1954
0390
0006.7
0001.9
0006.15
2464a
0061.6
0242
2380
0324
0007.2
0002.2a
a
Analysis was done on samples collected in 1997.
Figure 1. Microplot layout showing fertilizer application scheme and location of the target
tree in an alley cropping system in the midwestern USA. Microplots were located at the center
of each larger plot (100 m × 11 m).
65
(NH4)2SO4 was applied at the same rate; see Figure 1) was supplied with
NH4NO3 at an equal rate (168 kg N ha–1).
Six plants (above-ground portions only) from the center of each microplot
row were harvested at physiological maturity (early October), dried at 65 °C,
and separated into grain and stover (including cobs) components. Tree leaves
were sampled on July 3. For this purpose, the tree canopy was divided into
an upper and lower half. Then, ten leaves were collected from all four cardinal
directions for both halves. These leaves were combined to give one composite
sample per tree. Total canopy leaf biomass was quantified using equations
developed by Jose (1997). All samples (maize grain, stover, and tree leaves)
were first coarse-ground and then fine-ground using a coffee grinder, and
subsequently analyzed. To prevent any cross-contamination of the 15N plant
material, the grinders were thoroughly cleaned between samples. Subsamples
of the ground plant material were analyzed for total N and 15N (Michigan State
University, East Lansing, MI, USA) following the protocol of Harris and
Paul (1989). Percentage of plant N derived from fertilizer (%NDF), a measure
of the relative amounts of N a crop obtains from the soil and from applied
fertilizer, was calculated according to Wienhold et al. (1995) as:
%NDF =
% 15N excess samples %
× 100
15
N excess fertilizer
Percent utilization of fertilizer N (%UFN), a measure of fertilizer use efficiency, was calculated for maize plants as follows (Wienhold et al., 1995;
Barber et al., 1996):
%UFN =
%NDF × S
R
where S = kg N ha–1 in grain or stover and R = kg N ha–1 applied.
Decomposition study
Field procedures
Fine roots (< 2 mm in diameter) were collected in June of 1996 from trees
growing in the alley cropping systems by digging pits measuring approximately 1 m × 1 m to a depth of 30 cm near the base of trees. Following collection, roots were washed in distilled water to remove adhering soil particles
and freeze-dried. Freeze drying was chosen over oven drying as freeze drying
preserves carbohydrate fractions which have been linked to the early stages
of fine root decomposition (McClaugherty et al., 1982). Following freezedrying, subsamples were oven-dried at a temperature of 65 °C. This was done
to calibrate for differences in initial moisture content of samples which were
66
freeze-dried and subsequently retrieved samples which were oven-dried. Root
decomposition bags were made by placing 500 mg of fine root tissue in
0.5 mm nylon mesh bags measuring 8 cm × 8 cm.
Freshly fallen black walnut leaves were collected during late August using
litter traps. Since oak leaves remained on the trees until the end of October,
they were collected on November 1. The leaves were air-dried and subsamples were oven-dried to account for differences in initial moisture content of
air-dried samples and retrieved samples which were oven-dried. Litter decomposition bags were made by placing 10 g of air-dried leaves (for compound
black walnut leaves a rachis:lamina weight ratio of 30:70 was used) in 2 mm
nylon mesh bags measuring 20 cm × 20 cm.
On 31 August 1996, litter bags containing leaves and roots (except red
oak leaf bags which were placed on 9 November 1996) were installed in four
black walnut and four red oak alley cropping plots. All root bags were placed
in the mineral soil at a depth of 10 cm. Leaf litter bags were placed directly
on the mineral soil. All bags were installed in the center of each plot at a
distance of 4.25 m from the tree rows.
On each sampling date, soil samples used to calculate gravimetric soil
moisture content were collected from each plot in sealed bags. Soil temperature was also recorded in each plot using a piercing probe digital thermometer. Figure 2 shows the temporal variation in average soil moisture and
temperature in black walnut and red oak alley cropping systems during the
study period.
Laboratory procedures
Two bags of leaf litter and roots were collected for each species after 15, 45,
105, 195, and 235 days from each plot (except for red oak leaf samples which
were collected after 15, 45, 105, and 180 days). Adhering soil was rinsed
carefully from the residues with a minimum of distilled water (using a
0.106 mm sieve) to reduce artificial leaching (Lehmann et al., 1995). The
samples were then dried at 65 °C for 48 hours and weighed. Mass loss was
calculated as the difference between initial weight and the weight remaining
at each sampling date.
The samples were then ground and redried for 24 hours at 65 °C. Carbon
and nitrogen were determined on a CNS analyzer (Leco Scientific Instruments,
MI, USA). Subsamples were digested using a perchloric acid-hydrogen
peroxide technique (Adler and Wilcox, 1985) for P determination. The
phospho-molybdate blue method was used to determine the P concentration
in the digest solution using a spectrophotometer (Olsen and Sommers, 1982).
Data analysis
Data were analyzed using Student’s t test. Treatment effects were considered
significant if P < 0.05. Tests of normality and equal variance were run with
67
Figure 2. Temporal variation in gravimetric soil moisture and soil temperature (measured to a
depth of 10 cm) in midwestern USA black walnut and red oak alley cropping systems during
the study period 1996–1997.
each analysis. All tests were carried out using SigmaStat (Jandel Scientific
Inc.).
Results and discussion
Above-ground biomass and nitrogen content
Maize grain and stover biomass varied significantly between ‘barrier’ and ‘no
barrier’ treatments (Table 2). The inter-row difference in grain and stover
biomass was not significant within each treatment, except for stover in the
‘no barrier’ treatment (P = 0.0451). When maize plants were isolated from
the below-ground competition by a root barrier, grain yield increased to 6.76
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Table 2. Corn grain and stover biomass, N concentration and content, %NDF and %UFN in ‘barrier’ and ‘no barrier’ treatments in a midwestern USA
alleycropping trial.
Treatment
Row
Biomass (Mg ha–1)
N content (kg ha–1)
NDF (%)
Grain
Stover
Grain
Stover
Grain
Stover
(Grain+Stover)
%UFN
Barrier
1
3
Meana
7.0 (0.8)
6.6 (0.4)
6.8 (0.4)
5.8 (0.5)
6.0 (0.3)
6.0 (0.3)
107 (15)
097 (7)
102 (8)
76 (7)
77 (5)
76 (4)
42.9 (3.8)
42.6 (3.1)
42.7 (2.2)
46.9 (3.6)
45.9 (2.8)
46.4 (2.0)
48.3 (5.4)
46.0 (5.7)
47.2 (5.6)
No barrier
1
3
Mean
3.6 (0.7)
4.5 (0.4)
4.0 (0.4)
3.9 (0.6)
4.7 (0.3)
4.3 (0.3)
068 (11)
077 (5)
073 (6)
52 (6)
56 (4)
54 (4)
54.7 (1.9)
47.3 (2.1)
51.0 (2.1)
57.4 (1.5)
49.0 (1.8)
53.2 (2.1)
40.0 (6.2)
37.8 (1.6)
38.9 (3.9)
< 0.001
0.004
000.013
00.004
00.021
00.043
00.073
P valueb
Values in parentheses denote standard errors of means.
a
Mean indicates the treatment mean, i.e. average of the first and third row.
b
P value indicates the significance between treatment means.
69
mg ha–1 from 4.04 mg ha–1 in the ‘no barrier’ treatment. Similar results have
been reported from the tropics where below-ground root barriers increased
grain yields to 1.63 mg ha–1 from 0.42 mg ha–1 in a sorghum (Sorghum bicolor
Moench)-leucaena (Leucaena leucocephala Lam. de Wit) alley cropping
system (Singh et al., 1989). These authors concluded that the yield reduction
in sorghum was mainly due to competition for water.
Our research has shown that competition for water is likely the major factor
causing yield reductions in red oak and black walnut-maize temperate alley
cropping systems (Jose et al., in press). However, it is possible that tree roots
may also successfully compete for the available nutrients in the system thereby
reducing maize nutrient uptake. For example, root zone treatments had an
impact on tree foliar N concentrations. Leaf N was 18.4% higher in ‘no
barrier’ trees than the ‘barrier’ trees (Table 3). It is reasonable to suspect
that ‘no barrier’ tree roots were acquiring N from a greater soil volume
including the fertilized alley, resulting in a higher N concentration in their
leaves.
Uptake of fertilizer nitrogen and fertilizer use efficiency
The N derived from fertilizer (NDF) in both grain and stover showed significant differences between treatments (Table 2). And the percentages of NDF
in grain and stover were similar in each treatment. Variation between the first
and third row was significant only for stover fertilizer uptake within the ‘no
barrier’ treatment. Plants growing in the ‘barrier’ treatment had a lower%
NDF in both grain and stover as compared to the grain and stover in the ‘no
barrier’ treatment. In other words, maize plants in the ‘barrier’ treatment were
taking up a higher percentage of their N from the nitrogen already present in
the soil (hereafter referred to as soil N) than from the applied fertilizer.
Tree fine roots (which are active in nutrient uptake) are normally concentrated in the same surface soil layer where most crop roots are present (Kang,
1993). In our previous work, similar patterns of root distribution were
observed in both black walnut and maize (Jose et al., this issue). Both species
had most of their roots concentrated in the top 30 cm soil layer. This indicates that black walnut and maize explore the same soil layer for nutrients.
Considering the fact that black walnut begins growth early in the growing
season (by April), a high demand for nutrients can be expected during that
period. Maize is normally planted in May (e.g. on May 22 in our study). It
is possible that by the time maize is planted, black walnut has already taken
up most of the soil N that mineralizes each spring, forcing maize plants to
depend more on fertilizer N for growth and development when interplanted
in an alley cropping system. Thus, the higher %NDF observed in the ‘no
barrier’ plants may be the result of soil N depletion due to competing tree
roots. Alternately, one could argue that N mineralization may be lower in the
‘no barrier’ alley as a result of reduced water availability. As mentioned
earlier, maize plants in the ‘barrier’ treatment exhibited no difference between
70
the first and third rows in grain/stover NDF. However, plants in the ‘no barrier’
treatment always had higher grain/stover NDF for the first row, the row closest
to the trees, indicating less available soil N close to the tree row or less ability
to take up N because of lower water uptake.
The percentage of N derived from fertilizer in tree leaves was only 1.9%,
indicating that 98% of the canopy N was derived from sources other than the
fertilizer (Table 3). These other sources can be mineralized soil N as well as
N retranslocated from the previous year’s storage. Since senescing black
walnut leaves retranslocate 30–45% of their N before leaf fall (Jose and
Gillespie, unpublished data), a portion of the retranslocated N can be expected
to be from previous years’ fertilizer uptake. Long-term studies monitoring the
15
N distribution in different tree parts are needed to better understand annual
and cumulative N cycling dynamics and fertilizer use.
Walnut leaves become fully developed in seven weeks, and peak leaf N
concentration is reached in eight to nine weeks (Drossopoulos et al., 1996).
In general, maize plants start their growth when black walnut leaves have
reached maturity. Thus, competition for fertilizer N is minimal between black
walnut and maize plants due to the temporal difference in N acquisition.
However, since soil N and water are depleted at a faster rate in the ‘no barrier’
treatment, maize plants are likely to experience N deficiency without supplemental fertilizer N. Further, in the event of an early maize planting,
competition for fertilizer N may be much higher than that observed in this
study. Thus, management practices such as tree root pruning, time of planting,
and rate and placement of fertilizer can be used to control tree-crop competition for N.
Utilization of fertilizer N was greater in the ‘barrier’ treatment than in the
‘no barrier’ treatment (Table 2). Maize plants utilized 47.2% of the applied
fertilizer in the ‘barrier’ treatment as compared to 38.9% in the ‘no barrier’
treatment. This reduction in fertilizer use efficiency where roots intermingled may be associated with reduced maize growth due to tree competition
Table 3. Percentage of nitrogen derived from fertilizer (%NDF), percentage utilization of
fertilizer nitrogen (%UFN), and foliage parameters for trees in the ‘barrier’ and ‘no barrier’
treatments in an alley cropping system in the midwestern USA.
Parameters
Average dry leaf biomass (kg tree–1)
Leaf nitrogen concentration (%)
Canopy nitrogen content (kg tree–1)
NDF (%)
UFN (%)
P valuea
Treatment
Barrier
No barrier
5.62 0(0.32)
2.67 0(0.11)
0.15 0(0.00)
0.001 (0.16)
0.001 (0.07)
4.72
3.16
0.15
1.96
0.72
Values in parentheses denote standard errors of means.
a
P value indicates the significance between treatment means.
(0.86)
(0.04)
(0.03)
(0.89)
(0.38)
0.38
0.02
0.99
0.10
0.03
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for water as well as fertilizer. Maize growth was much lower in the ‘no barrier’
treatment than in the ‘barrier’ treatment as reflected by the lower stover and
grain biomass. As suggested by Wienhold et al. (1995), under reduced growth,
maize plants are unable to utilize available fertilizer. Alternately, bigger plants
with larger root systems in the ‘barrier’ treatment were able to explore more
soil for water, soil N, and fertilizer N, leading to greater efficiency in
fertilizer use.
Decomposition patterns
Leaves and roots exhibited different rates of mass loss during the incubation
period, mass loss being greater for roots than for leaves in both species
(Figure 3). Significant mass losses occurred within a short period in both
substrates. For example, initial decomposition during the first 15-day period
resulted in a 22.7% mass loss in black walnut roots whereas it was only 10.1%
in red oak roots. Leaf litter exhibited a slower decomposition rate during this
Figure 3. Changes in residual mass and carbon of the leaves and roots of black walnut and red
oak as a percentage of original mass and carbon in alley cropping systems in the midwestern
USA. Error bars represent one standard error of the mean.
72
period as compared to roots with only 7 and 4.4% mass losses in black walnut
and red oak, respectively. Leaf litter exhibited an additional mass loss of 7.1%
for black walnut and 6.7% for red oak over the next 30 days. During this
period, red oak roots lost only 1.4% of their mass whereas black walnut roots
had an additional mass loss of 12.8%. Subsequent mass loss during the rest
of the incubation period was much lower in leaves and roots for both species
(Figure 3).
The rapid mass loss observed during the early phase of decomposition in
our study follows that of Melillo et al. (1989) who described a two phase
decay sequence for red pine needles. The first phase is characterized by a
constant fractional mass loss whereas mass loss becomes imperceptible during
the second phase. The rapid mass loss during the early phase may be related
to microbial utilization of highly labile components of the substrate such as
nonstructural carbohydrates. When these are depleted, the rate of mass loss
is also decreased (McClaugherty et al., 1984). In our study, carbon loss from
the decaying substrates followed similar patterns as those of mass loss. In
general, carbon loss was much greater during the first 45-day period, suggesting a rapid depletion of the labile carbon sources by microorganisms
(Figure 3).
The observed difference in decay rates between substrates (leaves and roots)
is largely a function of substrate quality. We calculated the carbon:nitrogen
ratio (C:N ratio) which has widely been recognized as a measure of substrate
quality (McClaugherty et al., 1985). In substrates with high C:N ratios, it has
been found that lignin is the major factor controlling decomposition (Berg and
McClaugherty, 1989; Melillo et al., 1989). We observed an initial C:N ratio
of 26 and 58 for black walnut and red oak roots, respectively. The lower C:N
ratio of the black walnut roots indicates a better substrate quality with a faster
decay rate. Among leaves, black walnut had a lower C:N ratio (37) than red
oak (50). Nevertheless, the observed differences in decay rates (and nutrient
release patterns) may not be solely due to differences in substrate quality.
For example, red oak leaves started the decay process in early November when
black walnut leaves had already incubated for 70 days. Thus, the initial phase
of decomposition in both black walnut and red oak leaves took place under
different environmental conditions. Further, the role of juglone (an allelochemical produced by black walnut) in controlling the decay process of black
walnut leaves still remains unclear.
The variations in leaf and root decomposition rates may also be partially
explained by the differences in microenvironmental conditions. In our study,
roots were buried in the soil where soil moisture would be higher and temperatures more stable, allowing a larger microbial population to colonize and
decompose roots as compared to the surface-applied leaves. Since the soil
moisture and temperature patterns were similar in both the black walnut and
red oak alley cropping systems (Figure 2), it is reasonable to assume that the
interspecific differences in decay for roots were not a result of the environment. Rather, as discussed earlier, substrate quality is likely responsible for
73
this phenomenon. However, differences in leaf decomposition between black
walnut and red oak must be attributed to different substrate quality as well as
variations in environmental conditions.
Nutrient release patterns
N and P release patterns exhibited clear differences between substrates as well
as species (Figure 4). The leaf litter of both black walnut and red oak showed
very little nutrient release during the incubation period. Walnut leaves released
about 4.9% of their N in 45 days and exhibited no further release thereafter.
In comparison, red oak leaves released as much as 9% of their N within the
first 45 days. However, N release decreased thereafter. A similar trend was
observed during red oak root decomposition (Figure 4). Following a rapid
release of N (17.7%) within the first 15 days, nutrient release decreased rapidly
leading to a net increase in N content at 130 days. Black walnut roots released
Figure 4. Patterns of N and P release in leaves and roots of black walnut and red oak in alley
cropping systems in the midwestern USA. An increase from 100% indicates net immobilization
and a decrease from 100% indicates net mineralization. Error bars represent one standard error
of the mean.
74
a greater proportion of their N (39%) in 15 days with no apparent increase in
N content until the end of the incubation period.
The rate of P release was higher for black walnut leaves than for red oak
leaves (Figure 4). As discussed earlier, black walnut and red oak leaves decomposed under different environmental conditions (Figure 2), making a direct
comparison problematic. For example, black walnut leaves released as much
as 33.7% of their P in 45 days, compared to only 11.3% from red oak leaves.
Further, following the early release of P, red oak leaves exhibited significant
increase in P content whereas no net increase was observed for black walnut
leaves during the 235 day period. The beginning of our leaf incubation
experiment in both species coincided with their respective natural leaf fall.
This allowed us to mimic what happens naturally though making interspecific
comparisons difficult.
Although black walnut leaves were releasing a greater proportion of their
P than red oak leaves, an opposite trend was observed for root tissue. Both
black walnut and red oak roots released approximately 30% their P during
the first 15 days of incubation. Red oak roots released an additional 23.8% P
within the next 30 days whereas black walnut roots exhibited no further
release.
Conclusions
We observed reductions in grain and stover biomass as a result of belowground competition from the black walnut trees. Although competition for N
was suspected as one of the reasons for the observed yield reductions, our
results indicate that competition for applied fertilizer N was not as severe as
expected. Tree roots were taking up soil N and water before maize was
planted, leaving a relatively N-poor soil or a soil with low N availability. This
is suggested as one of the reasons for the higher %NDF in plants growing in
the ‘no barrier’ treatment. That is, little mineralized N remained after tree
uptake, and thus maize had to utilize the N available in fertilizer.
It has been suggested that crop N uptake is the prime determinant of grain
yield under low fertility conditions (Muchow and Sinclair, 1995). Corn was
supplied with 168 kg N ha–1 which is typical for corn production in the midwestern US. However, there was lower N use efficiency for corn in the ‘no
barrier’ treatment than in the ‘barrier’ treatment. This indicates that plants in
the ‘no barrier’ treatment were unable to utilize the fertilizer to the same extent
as plants in the ‘barrier’ treatment. Lower soil moisture and a reduction in
corn water uptake for plants growing in the ‘no barrier’ treatment have been
found in this alley cropping system (Jose et al., this issue). Thus, we suspect
that competition for water from tree roots is responsible for the reduction in
biomass in the ‘no barrier’ corn, causing a lower efficiency of fertilizer use.
To calculate the inorganic fertilizer requirements in an alley cropping
system, the amount of nutrients released by decomposing litter components
75
must be estimated. In general, nutrient release from stover is taken into account
for making fertilizer recommendations. However, because of the scarcity of
information, nutrient release from tree leaves and roots is often ignored in
temperate alley cropping systems. Several studies have examined in situ
decomposition dynamics of leaves (Blair, 1988; Adams and Angradi, 1996)
and fine roots of a number of temperate tree species in forests (McClaugherty
et al., 1984; Fahey et al., 1988). However, only a few studies have been
reported in the temperate agroforestry literature where nutrient dynamics were
studied to determine the role of trees in enhancing soil fertility. For example,
recent research in Canada revealed that poplar (Populus spp.) leaf litter can
provide higher nitrification rates, soil carbon content, and crop N uptake
adjacent to trees in alley cropping systems (Thevathasan and Gordon, 1997).
In Michigan (USA), Bross et al. (1995) observed a high rate of mass loss
and N mineralization from black locust (Robinia pseudoacacia L.) mulch in
nine weeks. In our study, only 5% of the black walnut leaf N was mineralized during the first year. Black walnut roots were able to release as much as
39% of their N in 15 days.
The N and P remaining in the undecomposed litter after the first growing
season will have residual effects on subsequent crops. Research in agronomy
has proven residual nutrient effects for several years on subsequent crops
(Sisworo et al., 1990). More research is needed to quantify the residual effects
of litter in both tropical and temperate alley cropping systems. Although N
and P are added to our alley cropping systems from the decomposing tree litter
(leaves and roots), a considerable amount of fertilizer input is still needed to
maintain productivity. However, as organic matter is added and soil meso
and macrofauna become abundant (farm chemicals may also influence meso
and macrofaunal populations), a more favorable environment for decomposition can be expected. This will not only enhance soil physical properties and
moisture retention, but also accelerate nutrient release, thereby increasing soil
fertility levels. Further, surface soil disturbance may accelerate nutrient release
patterns directly or indirectly. Eventually, all these effects will help to reduce
the need for inorganic fertilizer inputs into the system.
Acknowledgements
This research was funded by the North Central Sustainable Agriculture
Research and Education program of the United States Department of
Agriculture under cooperative agreement no. 92-COOP-1-7266. The authors
express their sincere thanks to S. Bell, K. Rodkey, and S. Munoz for their help
at various stages of this project. Scientific Contribution No. 15714 of the
Purdue University Agricultural Research Program.
76
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