Thierry Desjardins

Pasture damage by an Amazonian earthworm

Almost all cultivated soils undergo some reduction in the porosity of the surface layers, and nowhere is this more evident than in tropical rainforests that have been converted to pastures. Following deforestation in an area of Costa Rica, soil bulk density has been shown to increase rapidly after conversion to pasture, leading to poor drainage and a reduced rate of gaseous diffusion. These factors limit methane consumption and promote the anaerobic production of methane. A similar effect on methane flux has been found in upland soils in the Brazilian Amazonian basin after conversion from forest to pasture,. Increases in atmospheric methane are therefore not limited to emissions from flooded soils, as forest-to-pasture conversion promotes the anaerobic mineralization of organic matter by changing the physical properties of soil.

Changes of the forest-savanna boundary in Brazilian Amazonia during the Holocene revealed by stable isotope ratios of soil organic carbon

The possibility of ecosystem boundary changes in northern Brazilian Amazonia during the Holocene period was investigated using soil organic carbon isotope ratios. Determination of past and present fluctuations of the forest-savanna boundary involved the measurement of natural 13C isotope abundance, expressed as δ13C, in soil organic matter (SOM). SOM 13C analyses and radiocarbon dating of charcoal fragments were carried out on samples derived from soil profiles taken along transects perpendicular to the ecotonal boundary. SOM δ13C values in the upper soil horizons appeared to be in equilibrium with the overlying vegetation types and did not point to a movement of the boundary during the last decades. However, δ13C values obtained from deeper savanna and forest soil layers indicated that the vegetation type has changed in the past. In current savanna soil profiles, we observed the presence of mid-Holocene charcoals derived from forest species: fire frequency at that time was probably greater, and more extensive savanna may have resulted. Isotope data and the presence of these charcoals thus suggest that the forest-savanna boundary has shifted significantly in the recent Holocene period, forest being more extensive during the early Holocene than today. During the middle Holocene, the forest could have strongly regressed, and fires appeared, with a maximum development of the savanna vegetation. At the beginning of the late Holocene, the forest may have invaded a part of this savanna, and fires occurred again.

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.

Effects of earthworm inoculation on soil organic matter dynamics of a cultivated ultisol

In Peruvian Amazonia, cropping techniques manipulating the biological processes of soil fertility have been tested to increase productivity and sustainability of crops. Special attention was paid to earthworms since theircommunities are dominant in natural ecosystems and severely depleted in cultivated soils, and also because their populations can be manipulated. The objective of this work was to evaluate the impact of the endogeic earthworm Pontoscolex corethrurus on soil organic matter dynamics, by comparing treatments with and without earthworms. Carbon dynamics is described using particle-size fractionation and in situ natural isotopic labelling (carbon 13) obtained by shift of C3 to C4 vegetation. After 6 years of maize cultivation, the organic carbon stock of the 0-10 cm layer decreased respectively by 4 and 27 % in the control and the earthworm inoculated treatments. For the inoculated treatment this decrease mainly occurred in the large particle size (plant residues) Seventy % of the carbon derived from forest was lost during 6 years in the 2000-200 pm fractions in the inoculated treatment and 19 % in the control. However, the incorporation of carbon derived from maize in soil, especially in the large particle fractions (> 50 μm), was lower in the earthworm inoculated treatment. Accordingly, the proportions of carbon derived from forest and from maize were the same in the two treatments. Thus, the main effect of earthworm inoculation was a more important mineralisation of the carbon derived from forest and maize, especially in the large particle size fractions (> 50 μm).

Carbon content in Amazonian Oxisols after forest conversion to pasture.

Soil plays an important role in the C cycle, and substitution of tropical forest by cultivated land affects C dynamic and stock. This study was developed in an area of expansion of human settlement in the Eastern Amazon, in Itupiranga, State of Para, to evaluate the effects of native forest conversion to Brachiaria brizantha pasture on C contents of a dystrophic Oxisol. Soil samples were collected in areas of native forest (NF), of 8 to 10 year old secondary forest (SF), 1 to 2 year old SF (P1-2), 5 to 7 year old SF (P5-7), and of 10 to 12 year old SF (P10-12), and from under pastures, in the layers 0-2, 2-5 and 5-10 cm, to evaluate C levels and stocks and carry out separation of OM based on particle size. After deforestation, soil density increased to a depth of 5 cm, with greater increase in older pastures. Variation in C levels was greatest in the top soil layer; C contents increased with increasing pasture age. In the layers 2-5 and 5-10 cm, C content proved to be stable for the types of plant cover evaluated. Highest C concentrations were found in the silt fraction; however, C contents were highest in the clay fraction, independent of the plant cover. An increase in C associated with the sand fraction in the form of little decomposed organic residues was observed in pastures, confirming greater sensitivity of this fraction to change in soil use.