Below-ground biodiversity
“Best bet” Land-use Systems
Country reports
Alternatives to Slash-and-Burn in Brazil
Global Environmental Concerns
Unique id: IDAB3JXB
Source file: D:\Projects\ASB\ASB Country and Thematic reports\Brazil country report\ASB Brazil Summary Report.xml
Authors: S. Vosti, C. L. Carpentier, J. Witcover, . Carvalho dos Santos, E. Muńoz Braz, J. Ferreira Valentim, S. J. de Magalhăes de Oliveira, C. Palm, F. de Souza Moreira, A. Cattaneo, A. Gillison, A. Mansur Mendes, V. Rodrigues, T. C. de Araújo Gomes, M. V. Neves d’Oliveira, E. do Amaral, S. Fujisaka, C. Castilla, T. Tomich, D. Bignell, D. Gonçalves Cordeiro, A. Hermes Vieira, R.S. Correira da Costa, M. Faminow, M. Locatelli, M. Swift, S. Weise, M. van Noordwijk, N. Sampaio, I. L. Franke, H. J. Borges de Araujo, L. M. Rossi, E. Barros, B. Feigl, S.P. Huang, J. Cares, C. Pinho de Sá, . Carneiro, P. Woomer
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Background
ASB’s research on the impact of land use change on the below-ground biotic community arose because of the critical role of this community in shaping an ecosystem. Soil biological processes are essential for maintaining ecosystem functions such as the decomposition of organic matter and the cycling of nutrients.
There is limited knowledge, however, of the extent to which the biota below ground, and the functions its species perform, is dependent on the biota above ground, and vice-versa. This knowledge gap makes it difficult not only to predict the effects of land-use change on ecosystem processes but also to evaluate other scenarios, such as the effects of climate change or agricultural intensification on ecosystems. There is also limited knowledge of the taxonomy of many below-ground groups, which has led most scientists to use functional groups, in which fauna are categorized by their role in soil functions or processes. The biodiversity working group selected the following target functional groups for study on the basis of their diverse functional significance to soil fertility and overall ease of sampling across a range of LUS:
• Earthworms, which influence both soil porosity and nutrient relations through the channelling and ingestion of mineral and/or organic matter.
• Termites and ants, which influence (a) soil porosity and texture, through tunneling, soil ingestion and transport, and gallery construction; and (b) nutrient cycles, through transport, shredding and digestion of organic matter.
• Other macrofauna, such as woodlice, millipedes and certain insect
larvae, and their predators (centipedes, larger arachnids and some other types
of insects), which act as litter transformers and shredders of dead plant
tissue.
• Nematodes, which (a) influence soil turnover in their roles as root grazers, fungivores, bacterivores, omnivores and predators; (b) occupy existing small pore spaces in which they are dependent on water films; and (c) usually havevery high generic and species richness. Small size, high abundance and multifunctionality should make nematodes highly sensitive to disturbance.
• Mycorrhizae, which associate with plant roots, improving nutrient availability and reducing attacks by plant pathogens.
• Rhizobia, which transform N2 into forms available for plant growth.
• Overall microbial biomass, which is an indirect measure of the total decomposition and nutrient recycling function of a soil. It is constituted by fungi, protists and bacteria (including archaea and actinomycetes).
The working research questions for the group are listed in the first column of Table 6. A series of common protocols specific to the faunal groups being measured were employed at each site. These protocols ranged from the most desirable (detailed and intensive sampling) to the more practical (less detailed and intensive, given time and resource limitations and the desire not to disturb farmers’ fields).
Table 6. Key ASB questions regarding the functional implications of below-ground biodiversity (BGBD)
|
ASB question |
Affirmative evidence |
Qualifying comments |
Functional implications |
|
|
|
|
|
|
1.
Does LUS change affect BGBD? |
Macrofauna, termites,
nematodes, mycorrhizae, rhizobia |
Not all countries
or sites |
Sustainability or
renewal of soil fertility may be compromised |
|
2.
Does agricultural intensification reduce BGBD or affect community
composition? |
Macrofauna, termites (reduction and
community change); nematodes (community change); cf. mycorrhizae
(increase and community change) |
Not all countries
or sites. Trends different within macrofauna (termites vs earthworms) and
between macrofauna and smaller biota1 |
Management systems
and site histories may be influential |
|
3.
Does agricultural diversification promote or sustain BGBD? |
Macrofauna, termites |
Agroforestry retains macrofaunal diversity in 3
countries, but trend is opposite for smaller biota |
Canopy cover
promotes the large biota, but agroforestry is variable in its nature and
effects |
|
4.
Is extreme disturbance highly damaging to BGBD? |
Macrofauna, termites |
Loss of canopy
reduces some macrofauna, but others are unaffected. No consistent
evidence for smaller biota |
Soil ecosystem
engineers may be more vulnerable |
|
5.
Is BGBD linked to AGBD or production? |
Termites Rhizobia |
Link to woody basal
areas and plant functional modi Link to shoot dry
weight |
Termites are good
indicators of niche diversity High soil abundance
may promote plant production |
|
6.
Is BGBD influenced by proximity to forest? |
Macrofauna, termites |
New cropfields and
small cropfields are more forest-like. Intermediate disturbance favours ants
and earthworms |
Short-fallow
rotations are damaging to soil biotas |
|
7.
Are there effects on abundance and biomass independent of BGBD? |
Macrofauna Microbial biomass |
Earthworms promoted
at intermediate disturbance without great diversity Diminishes with
agricultural intensification |
Soil biota are
robust, except at extremes of disturbance Indicative
of lowered biological activity. |
|
|
|
|
|
1 ‘Smaller biota’ means nematodes, mycorrhizae and rhizobia.
In
• Macrofauna: total richness, total biomass, earthworm biomass, termite density and ant density.
• Microbial biomass: relative to carbon and nitrogen.
• Mycorrhizae: spore numbers, species numbers.
• Nematodes: genera and family density, population density, nematode diversity (three indices), trophic function, disturbance level and decomposition pathway.
• Rhizobia: numbers and population efficiencies.
Samples were taken from the same
sites used to sample carbon storage. A 25 m x 4 m transect was drawn for each
research plot. Macrofauna were extracted from monoliths; microfauna from soil
cores.
Full analysis of the below-ground data has been complicated
by problems with taxonomic identification and the highly heterogeneous
distribution patterns of soil organisms, which are rarely amenable to
statistical analysis based on the normal distribution. Given this
heterogeneity, low confidence intervals for estimates of abundance are rarely
obtained (M. Swift and D. Bignell, personal communications). A global synthesis
of the evidence in answer to each research question is given in Table 6. In the case of
• Microbial biomass (N and C) and total soil carbon. These are variables that are indicative of general soil health or quality rather than biodiversity. As shown in Figure 9, forest (DFOR) had higher values for all these variables than for other land uses. It can thus be concluded that these soils are able to support lower biological activity than the forest.
• Rhizobia. As Figure 10 shows, there are differences in diversity at the genus (strain) level. The prevalance of Bradyrhizobium spp, which are slow to very slow growers, compared with other, faster growing strains, such as Rhizobium, Sinorhizobium, Mesorhizobium and Allorhizobium, is affected by land use change. This indicates a simultaneous change in ecosystem function.
• Arbuscular mycorrhizae. Spore numbers were highest in crops and pasture and lowest in agroforests. However, more species were found in forest and fallows and fewer in pastures and agroforests.
• Nematodes. Data
on nematodes, unique to the Brazilian benchmarks, are presented in Table
7. Nematode abundance is lowest in
agroforests and food-crop fields and highest in pasture. All three diversity
indices are consistent in showing that the lowest diversity is associated with
pasture and food-crop fields and the highest with fallows and agroforests.
However, the reduction in generic richness (and associated diversity indices)
in pasture and food-crop fields is not reflected to the same extent by the
indices of trophic diversity, trophic dominance and the abundance (percentage
of total) of plant-feeding and bacterial-feeding groups. This supports the
conclusion that these fauna remain
functionally robust over the broad range of land uses, land covers and degrees
of disturbance surveyed. The fallow population is noticeably different in
functional composition, with more bacterial feeders. The maturity index,
however, clearly distinguishes food-crop fields as the most disturbed form of
land use with respect to effects on soil biota. This index broadly assesses the
balance between colonizers (species with high rates of reproduction and which
tolerate disturbance) and persisters (species that typically have long
life-cycles and low rates of reproduction). On this basis, the three tree-based
systems (secondary forest, agroforest and fallow) can be seen as more stable
habitats than the two non-tree systems (pasture and food-crop fields).
• Soil macrofauna(earthworms, ants and termites). Table 8 presents the data on these. In terms of response to land use change, the general trend is similar to that for nematodes. The diversity in agroforests is nearly as rich as in forests, so it is quite possible that the ecosystem functions remain intact. The same is substantially true for fallows, although the decline in earthworm biomass warrants further investigation. The decline of earthworms and termitesin pastures indicates significant disruption of the biological processes that regulate soil fertility. This is doubtless a factor at work in the long-term decline in the productivity of pastures and may also prevent the conversion of pasture to cropped fields.
Overall, the data obtained by the working group indicate
that below-ground fauna are sensitive to changes in land use. The trends for
macrofauna and nematodes are clearest, but more sampling is need for all
classes of fauna.
Figure 9. (a) Microbial biomass (µg N/soil and µg C/g soil) and (b) soil carbon (dag/kg) and organic matter (dag/kg) in different LUS
AGF CROP
DFOR FAL PAST
AGF CROP
DFOR FAL
PAST Notes: 1. Microbial biomass and soil carbon were measured in
the first 20 cm of the soil. 2. Upper-case and lower-case letters denote separate
groups within each figure; means with different letters differ by 5% (Tukey
test). 3. LUS labels refer to AGF = agroforestry; CROP = annual
crop; DFOR = forest; FAL = secondary fallow; PAST = pasture