Applied Soil Ecology 15 2000 49–59
The effect of slashmulch and alleycropping bean production systems on soil microbiota in the tropics
M. Rosemeyer
a,∗
, N. Viaene
b
, H. Swartz
c
, J. Kettler
d,1
a
Department of Agronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
b
Departement Gewasbescherming Department of Crop Protection, Centrum voor Landbouwkundig Onderzoek Agricultural Research Center, Burg. Van Gansberghelaan 96, 9820 Merelbeke, Belgium
c
Environmental Affairs, Idaho Power Co., P.O. Box 70, Boise, ID 83707, USA
d
International Honors Program, Global Ecology Faculty, 19 Braddock Park, Boston, MA 02116, USA
Abstract
Mulch applied to tropical soil may increase soil health both through stimulation of microbiota beneficial to plant nutri- ent uptake and the suppression of plant disease. To test this hypothesis, we compared beans in three cropping systems: 1
mulched with secondary vegetation slashmulch, 2 mulched with foliage from alleycropped nitrogen-fixing trees, Cal- liandra calothyrsus, Gliricidia sepium and Inga edulis alleycropmulch, and 3 unmulched, for their effect on positive
microbial interactions nodulation with Rhizobium and arbuscular mycorrhiza AM, and pathogenic associations foliar and root diseases in long-term field experiments in Costa Rica. In some dry years slashmulched beans nodulated significantly
more than the unmulched, though in wet years the nodule biomass was not significantly different between treatments. Beans grown in alleycropping systems mulched with foliage from Calliandra and Inga had lower nodule biomass than beans in
slashmulched and Gliricidia mulched plots at 3 and 5 weeks, probably due to high nitrogen levels from the decomposing vegetation of the tree mulch. Roots of bean plants were 95–98 colonized by AM fungi and there were no differences
between the slashmulched beans and the unmulched treatments. Slashmulch had no effect on the foliar diseases angular leaf spot and web blight, due to their naturally low incidence in that year; slashmulched decreased the severity of anthracnose
caused by Colletotrichum lindemuthianum. Slashmulch decreased severity of a Fusarium-type root rot, but increased a Rhizoctonia-type. In the laboratory, microbial activity as determined by measurements of CO
2
respiration was greater in the slashmulched than the unmulched system due to the high respiration of the mulch material. Our hypothesis that the
use of mulch would favor mutualistic symbionts was not supported, which may be due to weather and nutrient interference. However, the hypothesis that mulch would reduce disease incidence was supported for certain diseases. A comparison of the
microbial community with those of nearby natural systems can help provide a standard for the assessment of microbial health in agroecosystems and allow us to avoid the circularity of defining microbial communities as ‘healthy’ because they are found
in cropping systems whose plants appear without disease. © 2000 Published by Elsevier Science B.V.
Keywords: Soil health; Plant disease; Alleycrop; Mulch; Mycorrhiza; Nodulation; Costa Rica
∗
Corresponding author. Tel.: +1-608-265-9367; fax: +1-608-262-5217.
E-mail addresses: merosemeyerfacstaff.wisc.edu M. Rose- meyer, n.viaeneclo.fgov.b N. Viaene, h swartzhotmail.com
H. Swartz, jkettlerexcel.net J. Kettler
1
Tel.: +1-617-267-0026.
1. Introduction
Soil health has been broadly defined as “the con- tinued capacity of the soil to function as a vital living
system, within ecosystem and land use boundaries, to sustain biological productivity, promote the quality of
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50 M. Rosemeyer et al. Applied Soil Ecology 15 2000 49–59
air and water environments and maintain plant, ani- mal and human health” Doran and Safley, 1997. A
healthy soil is one that can provide the service of or- ganic matter breakdown and nutrient release such that
plant and animal biomass are supported. The micro- biota are critical to the wide array of functions in the
soil contributing to soil health including: 1 decompo- sition of organic matter and with microfauna provide
nutrients to sustain biological productivity of plants and animals, 2 participation in symbioses with plants
facilitating nutrient uptake and 3 suppression of pathogens through competition or antagonism, thereby
sustaining plant and animal health. Microbiota are also the foundation of the detritus food web, which because
of efficient transfer between trophic levels, supports lengthy food webs of organisms which are the basis
for integrated soil function Coleman and Crossley, 1996.
Microbes constitute about one quarter of all living biomass on earth Jong, 1989 and during the early
stages of the earth’s history, the major nutrient cy- cles have existed with microorganisms and one-celled
organisms alone. Our present-day nutrient cycles, re- gardless of the inclusion of animals and plants, are
not profoundly different from their pre-historic an- tecedents Knoll and Awrawik, 1983. In other words,
microbes are key facilitators in the major nutrient cycles. Energy from the decomposition of carbon
compounds such as polysaccharides, cellulose, and lignin fuels heterotrophic microbes that are responsi-
ble for nutrient transformations, such as asymbiotic nitrogen fixation, protein and amino acid decompo-
sition, mineralization and immobilization of nitrogen N and phosphorus and mineral transformations
Roper and Orphel-Keller, 1997. Microorganisms are responsible for significant nutrient transformations
involving both macro and micronutrients: N, P, K, S, Fe, K, Ca, Mg, Mn, Al, Zn, Se Alexander, 1997.
These microbially-driven processes influence nutrient availability and ultimately soil health and quality.
Since they are such a critical component of soil and ecosystem function, these microbial groups, if sensi-
tive to disturbance, may serve as important indicators of ecosystem function Turco et al., 1994.
The use of microflora as potential indicators of soil health is not new. Waksman, in his 1927 textbook
Principles of Soil Microbiology, considers several mi- crobial criteria as indicators of soil fertility, including
nitrogen fixing capacity, nitrifying capacity, numbers of microbes, cellulose decomposition rate and carbon
dioxide CO
2
evolution. Nevertheless, as an indicator of soil health microbes have been little used because of
their small size and the difficulty in culturing them, and most importantly the lack of knowledge of the uncul-
tured species. At present, uncultured species are nine times greater than those able to be reproduced on me-
dia Roper and Orphel-Keller, 1997. Recently various techniques have been used to characterize microbial
community structure or function, e.g. fatty acid methyl ester FAME and rRNA fingerprinting to determine
types of organisms present and BIOLOG plates for substrate utilization Heuer and Smalla, 1997. How-
ever, currently these techniques present difficulties in addressing the question of soil health because they are
able to resolve the species level and because of the vast gap in knowledge concerning these organisms and
their ecological roles.
There is increasing evidence of changes in certain microbial populations with cultural management and
agrochemical application Roper and Orphel-Keller, 1997. Studies assessing the effect of agrochemicals
on microbes have roughly identified three sensitivity groups: low, medium and high Domsch et al., 1983.
Along with other microbial indicators these sensitiv- ity groups may contribute to identifying a healthy
soil or at least identify which groups may have lower than optimum activity when converting from
high agrochemical use to low input agroecosystems. High sensitivity groups include nitrifying bacteria,
nitrogen-fixing bacteria and actinomycetes responsi- ble for some steps in organic matter decomposition
Domsch et al., 1983. Additionally if certain critical groups, e.g. ammonia oxidizers and basidiomycete
fungi that decompose lignin, have a low physiological tolerance to certain environmental disturbances, these
might also serve as indicators of soil health under certain conditions Andrén et al., 1999. Molecular
techniques are being developed to assess their diver- sity Gollwitzer, 1999.
Disease suppression is also an important function of the soil microbial component in contributing to
soil health. From a soil manager’s perspective the ab- sence of disease in a particular crop plant may define
a healthy soil Hornby and Bateman, 1997. Although this clearly involves a set of defined value judgements,
the absence of disease, whether it be in a crop plant
M. Rosemeyer et al. Applied Soil Ecology 15 2000 49–59 51
or generally throughout an ecosystem, is an important aspect of ‘maintaining biological productivity’ in gen-
eral or ‘plant, animal and human health’ specifically. Likewise plant symbioses with microbes especially
under low external input situations low agrochemical use are critical for plant health and productivity. Ar-
buscular mycorrhiza AM fungi, with their broad host range have been instrumental not only in plant nutri-
ent uptake but also soil aggregation. Water stable ag- gregates increase with mycorrhizally-secreted protein,
glomalin Wright and Upadhyaya, 1998. Therefore, high colonization of roots with AM fungi might indi-
cate optimum levels of water stable soil aggregation important in providing space for soil atmosphere and
water infiltration. Nitrogen-fixing bacteria are essen- tial for legumes where no nitrogenous fertilizers are
applied. In addition, the lack of this part of the micro- bial complement may indicate other missing elements
in the system.
Since cropping systems can affect soil biota and thereby affect soil health, we studied three systems
to see how Costa Rican farmers could best manage their soil. The slashmulch system is pre-Hispanic in
origin but, unlike many traditional systems, still rel- evant to Costa Rican bean production. After about 2
years of fallow, bean seed is broadcast into the second growth vegetation, which is cut down to form a mulch
through which the bean seeds emerge. Bean acreage in the slashmulch system has not changed much over
the last 20 years and still accounts for some 30–40 of Costa Rican bean production, 60 of which is sold
off the farm Rosemeyer, 1995. Demand for higher production has caused shortened fallows and beans
are produced every year. Consequently the composi- tion of the secondary vegetation changes as dicotyle-
donous plants are replaced with monocots that are less productive for slashmulch beans. Alleycropped plant-
ing of leguminous trees that are coppiced yearly for mulch might circumvent this problem by their contin-
uous production of high-quality foliage.
In a series of experiments spanning more than a decade, we have explored agronomic and nutrient cy-
cling aspects of the slashmulch, alleycropmulch and unmulched systems Rosemeyer and Gliessman, 1992;
Rosemeyer, 1994; Kettler, 1997a,b. Soil health is one component of our assessment and here we test the
hypotheses that slashmulch and alleycropmulch sys- tems provide greater soil health via three contributions
of the microbiota: facilitation of symbioses nodula- tion and AM of beans, disease suppression and in-
creased microbial respiration.
2. Materials and methods
