Conventional agriculture creates a special ecosystem by mixing the topsoil (and compacting it) through tillage, removing plant canopies that protect the soil, adding fertilizers and biocides, and removing harvests. A more sustainable agriculture minimizes topsoil disturbance, reduces inputs, and substitutes organic for mineral fertilizers (Doran and Werner, 1990).
Plowing in conventional agriculture incorporates crop residues into the soil profile to produce homogeneous soils that favor the bacteria, protozoa, and bac-tivorous nematode portions of the underground food web; in contrast, minimal tillage leaves organic residues on the surface and a rich organic layer near the surface, enhancing the fungal, Collembola, and earthworm portions of the underground food web (Hendrix et al., 1986; Lee and Pankhurst, 1992). The protozoan communities differ between the two systems in the greater prominence of r-selected colpodid ciliates (reflecting less species diversity) in conventional fields (Foissner, 1992; Bamforth, unpublished data). The biomass of amoebae and flagellates, however, is greater in the surface layer of ecofarmed systems (DeRuiter et al., 1993) and is associated with increased nitrogen mineralization (DeRuiter et al., 1993). Using testacea as bioindicators, Wodarz et al. (1992) found organically farmed field and vineyard soils showed improved soil conditions over conventionally farmed counterparts.
Organic fertilizers, especially straw and animal manures, are more similar to natural organic substrates than chemical fertilizers. Microbial and protozoan activity is highest in organically enriched soils (Schnurer et al., 1985; Aescht and Foissner, 1991; 1992), and is usually accompanied by increases of most soil fauna, especially earthworms (Doran and Werner, 1990), which enhance protozoan biodiversity.
The higher protozoan activity in soils under nontillage and organic fertilizer management is enhanced by other fauna, especially earthworms, which disperse bacteria and their protozoan predators to new locations, through burrowing movements and passing ingested cysts through guts, providing new hot spots and releasing greater quantities of nutrients, which have led to increased plant yields in a few cases (Brown, 1995). Ingested active protozoa furnish a highly assimilable food source, sustaining the fauna that enhance microbial and protozoan activities (Brown, 1995). Thus, high protozoan biodiversity usually reflects earthworm abundance.
The application of biocides often influences other organisms besides those targeted. Herbicides have little effect on protozoa, although they may influence them indirectly by altering bacterial nitrogen activities and by modifying the environment in eliminating the vegetation over the soil. Insecticides and fungicides are more toxic, as shown in the study of Petz and Foissner (1990) on the effects of lindane, an insecticide, and mancozeb, a fungicide, on the soil ciliate and testacean communities of a spruce forest. The insecticide decreased both numbers and species, and altered the community structure of ciliates by increasing the abundance of several colpodids. This result shows the value of multispecies-monitoring studies, and also the value of biodiversity to an ecosystem, allowing response to changing conditions (Bamforth, 1995b). The insecticide exerted less effect on testacea, and the fungicide exerted little influence on both groups. The investigation used a randomized block design and extended the study period to the 90 days needed to ascertain if the biocide caused acute toxicity (Domsch et al., 1983). This type of study shows the precision that protozoan bioindicators can provide to assess agroecosystem conditions.
The heavy machinery used in modern farming causes soil compaction, destroying not only the worm and root channels that reduce soil porosity and the larger fauna, but also reducing pore spaces in which bacteria and their protozoan predators live. Compaction reduces testacean species diversity and eliminates large forms (Berger et al., 1985), and a number of studies relating pore space to protozoan activity (e.g., Rutherford and Juma, 1992; England et al., 1993 ) show less activity in smaller spaces. Griffiths and Young (1994) found the same trend and concluded that compaction influences protozoa indirectly by producing anaerobic conditions that inhibit protozoa and reduce the metabolism and reproduction of their bacterial prey.
A vital part of agricultural management is soil conservation and restoration, which can be monitored by analyzing the protozoan community to assess the degree of the comprehensive biological activity to productive farming (Yeates et al., 1991; Wodarz et al., 1992).
Was this article helpful?