Lucero Mariani

SOM management in the tropics: Why feeding the soil macrofauna?

This paper synthesises information on the food requirements of soil macroinvertebrates and some of their effects on soil organic matter dynamics. Some clues to techniques that would optimise their activities through organic matter management are suggested. Soil macroinvertebrates can consume almost any kind of organic residues in mutualistic association with soil microflora. Significant amounts estimated at several T per ha of predominantly easily assimilable C are used yearly in natural ecosystems as energy to sustain these activities. Sources of C used are highly variable depending on the feeding regime. The largest part of the energy assimilated (e.g., 50% by the tropical earthworm Millsonia anomala) is actually spent in burrowing and soil transport and mixing. Bioturbation often affects several thousand tons of soil per hectare per year and several tenth of m3 of voids are created in soil. A great diversity of biogenic structures accumulate and their nature and persistance over time largely controls hydraulic soil properties. The OM integrated into the compact biogenic structures (termite mounds, earthworm globular casts) is often protected from further decomposition. Most management practices have negative effects on the diversity and abundance of macroinvertebrate communities. Structures inherited from faunal activities may persist for some weeks to years and the relationship between their disappearance and soil degradation is rarely acknowledged. When SOM supply is maintained but diversity is not, the accumulation in excess of structures of one single category may have destructive effects on soil. It is therefore essential to design practices that provide the adequate organic sources to sustain the activity and diversity of invertebrates. Special attention should also be paid to the spatial array of plots and rotations in time.

Seed dispersion by surface casting activities of earthworms in Colombian grasslands

The effects of Martiodrilus sp. (Oligochaeta, Glossoscolecidae) on the soil seed banks was investigated in a Colombian savanna and two intensive pastures. Germination and washing–sieving methods were used to compare seed density, diversity, species composition and germination rates in earthworm casts and the surrounding soil. Although large amounts of seeds were present in casts (163.65, 156.84 and 60.36 seeds per 100 g of dry casts in the savanna, the old and young pastures, respectively), germination rates were 3–40 times lower than in the surrounding soil, likely due to degradation during the gut transit. The number of viable seeds present in casts was 0.40, 7.46 and 1.99 seeds per 100 g of dry casts in the savanna, the old and young pastures, respectively. Species composition of viable seeds was quite different in casts compared to soil, probably because of selective seed ingestion by earthworms. Viable seeds deposited in surface casts each year represented 0.65%, 16.17% and 8.24% of the total viable seed bank of the soil, in the savanna, the old and young pastures, respectively. In the savanna and the old pasture, species composition in casts was more similar to the vegetation than species composition in the soil was. This may indicate that ingested seeds that survive gut transit have a greater chance to germinate than those of the soil seed bank, providing vegetation cover is sufficiently opened to enable germination processes. Thus, casts may be considered as a regeneration niche for plant species, and earthworm activity a factor that enhances, in some cases, the expression of the soil seed bank in the standing vegetation.