Dynamics of soil fungal and bacterial biomass in a
temperate climate alley cropping system
Stefan Seiter
a,*, Elaine R. Ingham
b, Ray D. William
a aDepartment of Horticulture, Oregon State University, ALS 4017, Corvallis, OR 97331, USA bDepartment of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331, USAReceived 13 April 1998; accepted 16 November 1998
Abstract
Soil bacterial and fungal dynamics were measured in an alley cropping system using direct microscopy techniques. The alley cropping system involved hedgerows of alder trees (Alnus rubra) and sweet corn (Zea mays) grown in the alleys. Trees were periodically coppiced and prunings were incorporated into the soil as green manure. Active fungal and bacterial biomass were greatest in tree rows and declined with distance from the trees. Active fungal biomass was greatest at the ®rst year July sampling, ranging from 44mg g dry soilÿ1in the tree row to 22mg g dry soilÿ1in the middle of the alley. Bacterial biomass in
all sampling locations peaked during May before coppicing of trees and cultivation of the alley. Bacterial and fungal biomass in the middle of the alley were similar to sweet corn monocropping plots throughout the growing season. The results suggested that the relatively low pruning biomass that can be produced and incorporated into the soil of temperate climate alley cropping systems compared to tropical alley cropping systems has little effect on microbial biomass. However, additional green manure in the form of leaf fall, additional below ground substrate from tree roots, and favorable conditions in untilled tree rows contribute to higher soil fungal and bacterial counts in and near the tree rows.#1999 Elsevier Science B.V. All rights reserved.
Keywords:Soil fungi; Soil bacteria; Green manure; Tree prunings;Alnus
1. Introduction
Alley cropping systems represent hybrids of agri-cultural and natural ecosystems. In these cropping systems, annual crops are grown in alleys between hedgerows of trees. Trees are coppiced periodically and prunings are incorporated into the soil as green manure. Alley cropping systems are designed to inte-grate the soil enriching process of forest ecosystems into agricultural production systems (Nair, 1984;
Kang et al., 1985). A primary objective is to create ef®cient nutrient cycles in which nutrients from prun-ings and other plant litter are effectively recycled into new plant biomass. The microbial biomass plays a central role in this recycling process (Paul and Clark, 1989). Microbes are responsible for the biochemical degradation of the organic litter and convert nutrients from organic to plant available, mineralized forms (Coleman and Crossley, 1996). More than 90% of all nutrients pass through the microbial biomass to higher trophic levels (Kennedy, 1995).
The composition of the microbial community in¯u-ences the transformation of plant residues into soil
*Corresponding author. Tel.: 5442; fax: +541-737-3479; e-mail: seiters@bcc.orst.edu
organic matter and plant available nutrients (Beare et al., 1993; De Ruiter et al., 1993). Therefore, separate assessment of the decomposer groups is essential to understand soil microbial dynamics. Also necessary are methods that determine whether fungi and bacteria (the two dominant decomposer groups) are functional. Failing to distinguish between active and inactive microbial biomass may lead to erroneous conclusions regarding nutrient cycling processes (Hunt et al., 1987). An assessment of active biomass is also impor-tant when considering long-term development and sustainability of an ecosystem (Klein et al., 1995).
Various methods are available to study the active portion of the soil microbial biomass. Substrate-induced respiration (SRI) measures temporal biomass changes and activity as affected by management (Horwath and Paul, 1994). However, SIR measures only the activity of microbes that respond to additions of glucose and fails to account for the whole functional soil microbial community. Enzyme activity also has been correlated with active microbial biomass. How-ever, Kuperman and Carreiro (1997) point out that enzymes, due to their potential for persisting in the soil, re¯ect both the activity of microbes that grew in the past and those when sampled. Both enzyme activity and SRI fail to distinguish between the main decomposer groups. In contrast, microscopic methods offer opportunities to determine the composition of the microbial biomass and an assessment of its activity.
The composition of the microbial biomass is in¯u-enced by quantity and quality of organic matter input (Killham, 1994). Both vary considerably between tree rows and alleys of an alley cropping system. The major components of organic matter input to the soil in alleys are crop residues and tree prunings. In tree rows, leaf litter and prunings are added to the soil. Substantial organic matter input also is derived from below-ground sources such as root exudates, sloughed nodules (when nodulated N-®xing trees are grown), and decomposing roots (Brussaard et al., 1993). Dif-ferent management of organic matter inputs also in¯uences the microbial composition (Neher, 1995). Leaving plant residue on the surface (i.e., prunings and leaf litter in tree rows) selects for fungi while incor-porating prunings and crop residue into the soil of the alleys will shift the balance towards bacteria (Hendrix et al., 1986; Holland and Coleman, 1987).
The objective of our study was to determine the in¯uence of alley cropping on the dynamics of soil bacteria and fungi. Because of the green manure additions, we expected to ®nd greater microbial bio-mass in alley cropping systems compared to monocrop systems. We also expected to ®nd variations in micro-bial biomass and composition in different locations of the alley cropping system due to differences in organic matter input and management. We hypothesized that if trees provide microbial substrate in the form of addi-tional green manure and labile, nitrogen rich com-pounds from root litter, then microbial activity will be higher in soil close to the trees compared to soil away from the trees. In addition, we hypothesized that differences in organic matter input and management will shift the microbial composition toward bacteria in the alleys and fungi in the tree row.
2. Materials and methods
2.1. Study site and field operations
The study was conducted at the Oregon State Uni-versity Horticultural Research Farm in Corvallis on a Chehalis clay loam soil. Low precipitation during the growing season requires regular irrigation of row crops, which in our study was supplied by overhead sprinklers. In April 1991, one-year-old red alder (Alnus rubra) seedlings were planted in plots measur-ing 9 by 4.5 m (Fig. 1). From 1991 to 1995, sweet corn (Zea maysvar. ``Jubilee'') was planted in alley crop-ping plots and monocropcrop-ping plots. The experimental design involved a randomized block design, replicated in three blocks.
Corn seedbed preparation in the alleys involved chisel plowing and rotary hoeing. Each year, all plots were fertilized with 170 kg N, 130 kg P, 100 kg K haÿ1. Nitrogen was split into two applications.
The ®rst application was broadcast at planting in ammonium form; the second was side dressed as urea, 6 weeks after planting. No pesticides were applied. Weeds were controlled by hand and mechanical cul-tivation. After corn harvest, corn stalks were mowed with a ¯ail mower and left on the soil surface until next year's seedbed preparation.
June (i.e., ®rst coppicing). Shredded prunings were incorporated into the top 10 cm of the soil with a rotary hoe during seedbed preparation. A second coppicing was performed six weeks after planting when the corn reached a height of 30 cm. These prunings were not shredded and left in the tree row on the soil surface.
2.2. Soil sampling and analysis
Soil microbial biomass in the alley cropping system were measured six times during the experiment: before seedbed preparation in May 1994 and 1995, in the middle of the growing season in July 1994 and 1995, and after the growing season in October 1993 and 1995. Three replicate 2 cm diameter, 5 cm deep soil cores were collected at three distances from the trees: in the tree row; 50 cm from the trees (inside alley); and 150 cm from the trees (middle alley). In 1995, replicate samples also were collected in corn monocropping plots (Fig. 1). Soil samples were put in plastic bags, immediately placed in a cooler, trans-ported to the laboratory, and stored in a refrigerator to ensure accurate measurement of the active portion of total fungi and bacteria (Morris et al., 1997).
Active fungal and active bacterial biomass were determined by measuring the length and width of ¯uorescein diacetat (FDA) stained hyphae and count-ing the number of FDA-stained bacteria, respectively, using epi-¯uorescent microscopy. Total fungal bio-mass was determined using DIC microscopy by mea-suring the length and width of all hyphae in a known volume of soil (Ingham and Klein, 1984). Total bac-terial biomass was determined by counting the num-bers and measuring diameters of all bacteria in soil suspension stained with FITC and ®ltered on Nucle-pore black-stained ®lters (Babiuk and Paul, 1970). Microbial biomass was determined by converting biovolume to biomass using standard factors recom-mended by Van Veen and Paul (1979).
Soil samples were collected before tree planting in May 1991 to establish baseline values of soil chemical properties. Four years later, in May 1995, soil in the alley cropping and monocropping plots was sampled from three soil depths: 0±15, 15±30, and 30±45 cm. In alley croppingsystems,soil was collected 50 cm(insidealley) and 150 cm from the trees (middle alley). Each sample represented composites of 15 thoroughly mixed sub-samples. Samples were air dried for 72 h at 808C. Soil samples were analyzed for soil organic matter (SOM) (Nelson and Sommers, 1982) and total Kjeldahl nitrogen (Isaac and Johnson, 1976).
3. Results
3.1. Fungal biomass
On several sampling dates, signi®cant differences in active fungal biomass between the three sampling locations in the alley cropping system were observed (Fig. 2(a)). Highest active fungal biomass occurred in the tree row, intermediate biomass on the inside alley and lowest biomass in the middle alley. The exception to this trend was the sampling in May 1994 when fungal biomass was similar in all three locations. In July of both sampling periods, we observed no sig-ni®cant differences between the two alley locations. However, fungal biomass in both alley locations was signi®cantly lower than that in the tree row.
Active fungal biomass also changed seasonally. From May to July 1994, activity increased in all locations. Active fungal biomass increased
mately 12-fold in the tree row and fourfold in the alley. In 1995, seasonal trends differed between sampling locations. Fungal biomass increased in the tree row between May and July and decreased in October. In the alley, we observed a decrease from May through July and October. Seasonal differences in the two alley locations were insigni®cant, except for the decrease from July to October in the middle of the alley.
Trends in total fungal biomass were less consistent compared to active fungal biomass. We observed signi®cantly higher biomass in the tree rows only on two sampling dates (Fig. 2(b)). On other dates, we found similar biomass in the three sampling locations or differences that ®t no particular pattern. Seasonal changes of total fungal biomass mirrored those in active fungal biomass, both peaking in July. In 1995, total fungal biomass in the tree row remained high from July through October but decreased signi®cantly in the alley. Comparisons between the same dates of the two sampling periods show that fungal biomass was similar in July 1994 and 1995, but was higher in May and October of the second sampling period.
3.2. Bacterial biomass
Mean active bacterial biomass was highest in the tree rows, except in May 1995 (Fig. 3(a)). Differences between tree rows and alleys were statistically sig-ni®cant (p0.05) only in the last two sampling dates.
Seasonal trends of active bacterial biomass were inconsistent across sampling locations and years. For example, in 1994, we found no signi®cant changes from May to July. In 1995, however, we observed seasonal variations that differed between sampling locations. Active fungal biomass in the tree row increased from May to July and decreased in October. In the alley, on the other hand, the biomass decreased from May through July and October.
In 1995, we observed similar total bacterial biomass in all sampling locations (Fig. 3(b)). The variations on earlier sampling dates did not follow a particular pattern. Seasonal trends of total bacterial biomass were different from those of fungal biomass. Bacterial biomass was greatest in May and decreased through July and October during both sampling periods while total fungal biomass was usually highest in July. We also observed a year to year difference in bacterial biomass. Biomass in 1994 was signi®cantly higher than that in 1995.
3.3. Ratio of fungi to bacteria
We observed signi®cant differences in the ratio of total fungi to bacteria between the sampling locations on two sampling dates (Fig. 4). In July 1994 and October 1995, this ratio was more than three times larger in the tree row than in the alley. Seasonal trends in the ratio of fungal to bacterial biomass generally
were characterized by an increase from May to July followed by a decrease in October. However, it remained high in the tree row during October 1995 while it dropped signi®cantly in the alley.
The ratio of total fungi to bacteria in the tree row and on the inside of the alley was higher in 1995 compared to the same months in 1993 and 1994. However, in the middle of the alley, the values in October 1993 and 1995 were similar and had decreased to below one.
3.4. Comparison between alley cropping and monocropping systems
We found higher active and total fungal, and active bacterial biomass in the tree row of the alley cropping system compared to the monocropping system. How-ever, when we compared the soil in the monocropping system with the soil in the alley cropping area that received the shredded prunings but was treated the same otherwise (i.e., sampling location ``middle alley''), bacterial and fungal biomass was similar throughout the growing season.
We also compared soil chemical properties in the cropping area of both cropping systems. Differences were observed in the top 15 cm of the soil but not in the soil layers below (Table 1). We observed higher organic matter content and Kjedahl-N in the top soil of the alley cropping locations. However, organic matter content in the monocropping and all locations of the alley cropping system were below the baseline values of samples taken in 1991, before the ®rst season of alley cropping (Table 2).
4. Discussion
We attribute higher active fungal biomass in and near the tree row to increased substrate availability. In the middle of the alley, prunings of the ®rst coppicing constituted the only green manure input. In contrast, in
Fig. 3. Bacterial biomass of (a) active bacteria and (b) total bacteria in monocropping and three sampling locations of an alley cropping system. Soil samples for ``tree row'' were collected between trees of the double tree row, ``inside alley'' samples were collected at a distance of 0.5 m from trees, and ``middle alley'' samples were collected at a distance of 1.5 m from trees. Values shown are mean values of three replicate samples. Error bars represent standard errors of the means.
the tree row and the inside of the alley, green manure originated from prunings of two coppicings and leaf fall at the end of the growing season. The additional green manure was not incorporated but stayed on the soil surface near the trees. More than other decom-posers, fungi are able to use plant residues that are deposited on the soil surface because of their ability to grow across air ®lled space (Killham, 1994; Neher, 1995). The increased fungal biomass in the tree row and the inside alley during the July and October samplings re¯ects the use of the additional organic material as a fungal substrate. Surface tree litter near the trees may have contributed indirectly to the increased microbial activity because it provided sub-strate for the soil fauna. We observed that tree leaves appeared to be a favorite food source for earthworms (Lumbricus terrestris) on our study site. Several weeks
after the trees had shed their leaves, most tree litter had been pulled into earthworm channels (data not shown). Similar to Hauser (1992), we observed sig-ni®cantly more earthworm casts in the vicinity of the hedgerows where earthworms provide physical habitat and nutrient-rich substrate for soil bacteria and fungi. No signi®cant differences in microbial biomass between locations were observed at the May sampling dates. Various management factors contributed to this observation. For example, each year, 2 t haÿ1of corn
residue was left shredded on the soil surface of the alley during the winter fallow. This residue provided ample substrate for microbes in the alley, effectively eliminating any differences in substrate availability between locations in the ®eld at the May sampling date. Cultivation in the alley (but not the tree row) after the May sampling most likely ampli®ed differences in
Table 1
Soil organic matter (SOM) and soil nitrogen (Kjedahl-N) in alley cropping and monocropping sampling locations
Year Soil depth (cm) Sampling location % SOM % Soil nitrogen
1995 0±15 Inside alley 2.41a 0.150a
Middle alley 2.37a 0.142ab Monocrop 2.25b 0.137b 15±30 Inside alley 2.15b 0.132a Middle alley 2.13b 0.132a Monocrop 2.05b 0.129a 30±45 Inside alley 2.22a 0.141a Middle alley 2.07a 0.132a Monocrop 2.04a 0.132a
1991 0±15 All 2.57 0.135
15±30 2.52 0.134
30±45 2.54 0.136
1991 measurements represent baseline values before alley cropping was practiced. 1995 measurement were taken in May, before the fourth year of sweet corn alley cropping. ``Inside alley'' and ``middle alley'' indicate sampling locations 50 cm and 150 m from trees, respectively. Values are means of three measurements each composed of 15 sub-samples. Different letters denote statistically significant differences between sampling locations within the indicated soil depth at ap-value of 0.05.
Table 2
Above-ground organic dry matter (DM), carbon (C) and nitrogen (N) added to the soil in the alley cropping system (values are means of three samples)
Year Above-ground organic matter input
Prunings 1st coppice Prunings 2nd coppice Leaf fall
microbial biomass at later dates. Numerous studies showed that cultivation is detrimental especially to the growth of fungi because fungi strands are easily broken during break-up of soil particles and recover only slowly from tillage disturbance (Hendrix et al., 1986; Holland and Coleman, 1987; AndreÂn et al., 1990; Kennedy, 1995). A third management factor that most likely ampli®ed differences at the later sampling dates was coppicing (carried out after the May sampling dates). Coppicing of alley cropping trees increases the speed of ®ne root turnover (Fer-nandes, 1991) and the exudation of nitrogen rich root compounds (Sanginga et al., 1990). In a separate alley cropping study in adjacent ®eld plots, we found that Alnus sinuta trees labeled with 15N released a sub-stantial amount of nitrogen from roots into the soil. Soil microbe mediated transfer of15N to corn plants growing at a distance of 0.63 cm occurred within six weeks after coppicing (Seiter et al., 1995). Therefore, additional substrate was provided to the fungi and bacteria in the tree row and inside alley location.
Lack of moisture caused low fungal and bacterial biomass in October of the ®rst year. Because of the warm growing season, the corn was harvested early (September 8) and less than 12 mm of rainfall fell between the time when irrigation was turned off and microbial biomass sampling on October 15 (climate data not shown). In 1995, the harvest occurred later (September 19) extending the irrigation into mid-September. In addition, more rain fell prior to the soil sampling, creating favorable soil conditions and a less dramatic drop of bacterial biomass. Under the present climate conditions, a peak of microbial growth usually occurs in spring, explaining the high values of bacter-ial biomass in the May samplings. During this time, rapid plant growth of the native alley vegetation (i.e. weeds during winter fallow) and trees is accompanied by high root activity, which provides the necessary substrate for the microbial development (Lynch and Painting, 1980; Patra et al., 1995). Bacterial biomass remained high from May to July. The high July values in the alleys may actually present a second peak during which bacteria, after being reduced during seedbed preparation, responded to favorable conditions pro-vided by rapid corn root growth coupled with ample soil moisture from irrigation.
Soil bacterial and fungal biomass in the middle of the alley were similar to soil in monocropping systems
suggesting that a yearly organic matter input of less than 1 t haÿ1t to the soil of the alley was too small to
signi®cantly affect fungal and bacterial biomass. Sig-ni®cant changes of microbial biomass were found in alley cropping systems in tropical regions where pruning yield was much greater than that in our study. For example, Van der Mersch et al. (1993) found consistently higher microbial biomass in alley crop-ping systems than in a monocropcrop-ping control after adding 21.7 t haÿ1 of Leuceana leucocephala and
15.23 t haÿ1of Senna siamea prunings in one year.
Similarly, Kachaka et al. (1993) found signi®cantly higher microbial biomass C in soil that was amended with alley cropping prunings compared to controls. In other studies with high pruning inputs, however, results were less conclusive. For example, Haggar et al. (1993) added 7.9 t haÿ1 of Erythrina and
11.4 t haÿ1yrÿ1 of Gliricidia prunings in an alley
cropping system in Costa Rica. They observed that microbial N was 30% higher in alley crop treatments than the monocrop only at 105 days after the applica-tion of prunings but not at earlier or later sampling dates. Mazzarino et al. (1993) found no effect of pruning additions on microbial C and N in one season unless carbon inputs in the form of prunings and crop residues were more than 400% higher than in a control.
5. Conclusion
Differences in the response of fungi and bacteria to the alley cropping treatment demonstrated the need to distinguish between these two decomposer groups. Neher (1995) suggested that inclusion of perennial crops in agricultural systems shifts the soil foodweb more toward a natural ecosystem characterized by a higher fungi to bacterial ratio. In the uncropped area of the alley cropping system, we were able to con®rm this hypothesis. However, since trends in the cropped area were inconsistent, we concluded from our study that the amount of incorporated prunings is critical. Alley cropping systems are not shifted toward a fungal dominated ecosystem when pruning biomass is small. The effects of larger green manure quantities on multiple trophic food web levels need to be examined to draw further reaching conclusions.
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