Standard methods for assessment of soil biodiversity and land use practice.

 

Characterization of the soil biota and methodological approaches 

Key Functional Groups 

The taxonomic diversity of the soil biota is so high that inevitably some selection must be made. The taxonomic groups described below were selected on the basis of their diverse functional significance to soil fertility (hence the term "target taxa"); and their relative ease of sampling.

1) Earthworms, which influence both soil porosity and nutrient relations through channeling, and ingestion of mineral and/or organic matter.

 

2) Termites and ants, which influence a) soil porosity and texture through tunnelling, soil ingestion and transport, and gallery construction; b) nutrient cycles through transport, shredding and digestion or organic matter.

 

3) Other macrofauna such as woodlice, millipedes and some types of insect larvae which act as litter transformers, with an important shredding action on dead plant tissue, and their predators (centipedes, larger arachnids, some other types of insect)

 4) Nematodes, which a) influence 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 have very high generic and species richness.

5) Mycorrhizas, which associate with plant roots, improving nutrient availability and reducing attacks by plant pathogens. 

6) Rhizobia and, when relevant, other N-fixing microsymbionts which transform N2 into forms available for plant growth.

 

7) Microbial biomass, which is an indirect measure of the total decomposition and nutrient recycling community of a soil. Microbial biomass is contributed by three very diverse taxa: fungi, protists and bacteria (including archaea and actinomycetes), but it is not usually practical to separate these during measurements. Microbial biomass estimation usually depends on relatively crude chemical methods (lysis of cells, followed by determinations of total N (and P), conversion of these values to a C equivalent, and comparisons with unlysed control samples). It may thus have relatively low resolution, but assesses the decomposer community as a whole.

Sampling design: overall strategy

Macrofauna, microbiota and soil (for physical and chemical analyses) are sampled in transects, for which the optimum size is 40 x 4m. However, for the quantitative sampling of termites and for a number of above-ground studies (particularly plant functional attributes and C sequestration) quadrats of 40 x 5m have been deployed, and it seems advisable to standardize both above-ground and below-ground work at 40 x 5m (Figure 2). In further amendments to the procedures, pitfall trapping of surface-active invertebrates and a 100m qualitative transect for termites have been added to the sampling. These can take place along one flank of the transect (pitfalls) or in parallel at about 5-10m distance (termite transect). These modifications are intended, in part, to contribute elements of true biodiversity to the dataset by achieving resolution at the species level, but also to mitigate the variability of data from short transects on groups with typically patchy distributions. Replication of transects in each site is also desirable, as it facilitates statistical analysis of the data obtained, though this may not always be practical where time and funding are limited.

NB/ In small plots, highly dissected cropping systems or on difficult terrain, it is not necessary for the transect to be both linear and contiguous. For example, where the greatest linear dimension of a particular land-use is <40m, two parallel transects of 20m sample with the same theoretical efficiency as one of 40m. Similarly, a transect can be bent through angles up to 90o to sample plots of irregular shape or to avoid significant natural features such as streams, steep slopes or rock outcrops. Tree falls should, however, be included in the transect if this is appropriate to its existing line and length, and not bypassed. 

Land Use Selection and Characterization.

Soil biota are expected to vary with land-use and their diversity to broadly diminish along the chronosequence represented by undisturbed forest, logged-over forest, recently cleared and burned forest, cropping systems, derived pastures and recently established fallow. In any locality, therefore, baseline sampling must be carried out in whichever land use can be identified as the most natural (undisturbed) control site available, preferably closed-canopy forest. However, full site characteristics and classification (and therefore accurate site description) cannot be obtained from apparent land use alone. Concurrent or prior sampling must therefore be carried out for a suite of basic physical and chemical soil properties, including bulk density, texture (S/S/C ratios), pF, pH, total C, total N, exchangeable cations, available P, CEC, Al3+ and H+ . It is suggested that soil cores taken for these analyses should be from completely undisturbed ground but immediately adjacent to each monolith trench (the outer trench wall is probably the best place), thus providing the opportunity for correlating soil properties with the presence/absence of particular taxa and functional groups. A precise site history is also desirable (though not always obtainable), together with GPS coordinates, altitude, slope, aspect, annual rainfall, mean temperature and humidity, rainy days, length of dry season, and cumulative seasonal rainfall up to the sampling date. Description of sites can be completed by the above-ground vegetation character. Features such as mean canopy height, crown cover percent, basal area, domin cover/abundance scores for ground flora, litter accumulation and abundance, plant species and generic richness may assist in arranging sites along botanical diversity gradients which have some relationship to their actual positions in the chronosequences and disturbance intensifications.

CIFOR/ICRAF report.

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