54 Baars. Mycorrhizae

Presently much controversy exists over increasing populations
and food production maintenance. For many countries the envis-
aged prosperity of the Green Revolution became a cold shower
and landed them not only with huge foreign debts but also with
degraded ecosystems, and loss of bio-diversity. Degradation
rendered much of the world's agricultural land useless and
available land could no longer produce enough food to sustain
increasing populations and generate foreign currency to pay
debts. Since the start of this Revolution many effort have
been made to develop alternative sustainable methods to halt
degradation and provide people with methods to maintain at
least sustenance levels. Such methods emerged after it was
realised that many imported agricultural systems were totally
incompatible with third world natural ecosystems. Tropical
rainforests could not be transformed into high yielding wes-
tern style monocropping agro-systems without paying a severe
penalty. Many countries have become blatantly aware of this.
In some cases this resulted in irreversible loss of productiv-
ity. Since most future expansion of agricultural land will be
at the expense of tropical rainforests, the focal point of
this study is directed at the consequences of disturbance on
mycorrhizae population, and its relation to crop decline which
gives cause for shifting cultivation.

Most tropical rainforest soils are marginal with respect to
crop production. They are low in organic matter, CEC, fertili-
ty and are P deficient. Without disturbance, these factors are
maintained by the finely tuned nutrient cycling mechanisms
characteristic of this environment. The ecosystem is in equil-
ibrium. Micro-organisms play a major role in maintaining this
equilibrium. Notably mycorrhizae forming fungi contribute
heavily to nutrient availability and water management. Once
forests have been disturbed by burning and subsequent cultiva-
tion practices, the role of mycorrhizae forming fungal popula-
tions is (temporarily) curtailed. A major link in future
sustainability has been heavily impeded.

Shifting cultivators abandon their plots after three years of
cropping in deforested rainforest soils. Cropping potential
has declined. Generally this is attributed to depleted nut-
rient stocks. Richards in his classical book The Tropical
Rainforest suggests that nutrient stocks in rainforests are
low and that when these are cut and burned, nutrient losses
result in rapid crop productivity decline. Among others, Jor-
dan showed that this is not the case and nutrient stocks are
in fact higher then before the slash and burn. However, these
nutrients do not seem to be available to crop plants. Fallow
vegetation, on the contrary, is able to use these locked-up
resources. Jordan postulated several possible causes for crop
decline of which weeds, insect consumption, erosion, nematodes
and heavy metal toxicity were major suspects. Despite extensi-
ve research he does not supply a definite answer to what caus-
es decline while stocks are sufficiently high for adequate
crop production. In this paper, by highlighting research
evidence from literature, it is attempted to link this pheno-
mena to mycorrhizae forming fungi. Regarding these fungi,soil
chemical-, physiological-, biological-, and ecological aspects
are studied to underwrite the hypothesis that mycorrhizae
presence or absence is partially or fully responsible for crop
decline. The term Parasymbiont defines mycorrhizas' atavistic
nature, neither being a true parasite or symbiont, and able to
express either association as exogenous circumstances may
demand.

Decreased nutrient availability is the reason for fallow and
shifting cultivation in tropical rainforest. If these systems
are not properly managed then the only alternative is to
resort to external inputs to maintain cropping potential on
the same plot of land. Decreased cropping potential is gener-
ally attributed to decreased fertility, weed infestation,
lowered pH or organic matter content. On some soils either one
or a combination of these aspects may be the reason for crop
decline. On tropical rainforest soils however non of these
likely agronomical causes rang true and the question of crop
decline following their cutting has remained open. The aim of
this study is to establish a definite connection between crop
decline in cultivated tropical rainforest soils after the
initial three years of cropping on one hand and mycorrhizae
forming fungi on the other.


CONTENTS:

Summary 1
Aim 2
Hypothesis 3
Introduction 4
History 4

Chapter 1
Mycorrhizae forming fungi in tropical ecosystems 5
Definition 5
Main types 5
General characteristics 5

Chapter 2
Mycorrhizal types 7
Ecto-Mycorrhizae (ECM) 7
Vesicular arvuscular mycorrhizae (VAM) 7
Arbuscules 10
Vesicules 10
Hyphae 10
Vam status vs Plant Species 11
Rhizosperic extension 11
Carbohydrate household 12
Phosphate nutrition 12
VAM and heavy metal toxicity 13
Climatic stress 13
Pathogens and VAM 14
Interactive symbiosis 14
Other effects 14
VAM diversity 15
Effectiveness 15

Chapter 3
Effects of disturbance on tropical ecosystems
and Mycorrhizae 17
Introduction 17
Crop cycle 18
Soil chemical aspects 19
Organic matter 19
Mycorrhizae 21
Soil biological aspects 22
Spore density 23
Mycorrhizal decline 24
Physiological aspects 29
Organic acid toxicity 29
Heavy metal toxicity 29
Recognition 29
Survival mechanism 30
Hormones 31
Ecological aspects 32
Selection and specifity 32
Mycorrhizal Succession 33

Chapter 4
Mycorrhizae, beneficial or parasymbiont 35
Atavistic nature 35
Substantiation 36
Orchids 38

Conclusion & discussion 39
Hypothetical scenario 40
Recommendation 41
Compost and mulch 41
Inoculation 42

Notes 45
Bibliography 49
Acknowledgements 50