1. Soil fauna dynamics and soil chemical and physical properties in abandoned pastures and agroforestry systems in the Central Amazon
ND04_FEER_02
Ilse L. Ackerman1, Juliete M. T. Queiroz2, Christienne Kuczak1, Erick C. M. Fernandes1, Marco A. Rondon1, Eleusa Barros3, Elisa Wandelli2, Johannes Lehmann1
*1 Department of Crop and Soil Sciences, Cornell University, Ithaca, NY 14853; *2Centro de Pesquisa Agroflorestal da Amazônia da Embrapa (EMBRAPA-CPAA), AM-10, km 29, Manaus, AM, ; *3Instituto Nacional de Pesquisas da Amazônia, Rua Nelson Batista Sales 114, Cj Petro-Aleixo, Manaus, AM. Correspondence:
1. INTRODUCTION
Earthworms, termites and leaf cutter ants of the macrofauna group that are the more readily accessible and potentially manageable 'ecosystem engineers'. Earthworms and termites not only ingest large amounts of litter and soil, but also actively move around in soil and thereby play a major regulatory role in the dynamics of litter, SOM and soil nutrients. Leaf cutter ants have been shown to decrease deep soil resistance to penetration and in increase deep soil fertility (Moutinho, 1998; Brener & Silva, 1995). Several authors have stressed the need for research on the role of soil macrofauna in restoring land productivity (Urbanek, 1989; Abbott, 1989; Lavelle, 1994).
We are measuring the impact of previous land use, mulching, and biomass removals on the dynamics of earthworms, termites and leaf cutter ants and their impacts on C and nutrients in abandoned pastures and 9-year-old agroforestry systems established on abandoned pasture land. Where forest is converted to pasture, the initial increase in dead standing wood appears to be responsible for population explosions of mound-building termites. Paradoxically, while termites have an important role in cycling the dead woody material, their mounds in pastures appear to resist vegetation establishment thereby limiting aboveground biomass accumulation.
Figure 2. Aerial view of the experiment site showing Primary forest in the background, 4-year-old agroforestry systems in the center and regenerating forests on abandoned pastures in the lower left hand portion. The entire scene covers approximately 15 hectares.
Primary forest
Agroforestry I
Agroforestry II
Agroforestry III
Secondary forest on abandoned pasture
Figure 3: Contents of C, N and P in termite mounds versus control soil
Figure 4: Inorganic phosphorous fractions in earthworm casts and adjacent soils in agroforestry systems and secondary forest on abandoned pastures
Figure 1: Photo sequence showing large amounts of woody biomass on soil surface in a recently established pasture that appears to result in the proliferation of termite mounds within 5-8 years after pasture establishment. The mound structures are hard and many remain devoid of vegetative cover for as long as years after mounds start forming.
4. RESULTS
Macrofauna Groups: Our preliminary data show distinct changes in the soil macrofauna family profiles for secondary forest on abandoned pastures versus four agroforestry systems. The 4-year-old palm based and high input silvopastoral systems favored the proliferation and/or invasion of earthworms. This is most likely a response to the higher and better quality litter inputs in these systems as compared to abandoned pastures (Figure 1 below). We are now sampling the same systems at 9 years and will also characterize the distribution of soil fauna with depth in an attempt to investigate relationships with rooting depth of the vegetation (Figure 2 below).
C and nutrients: Our data show that termite mounds have significantly higher C contents as compared to adjacent soil. This is likely to be a function of the higher clay contents in the mounds. Termite mounds may play a role in carbon sequestration, perhaps in part offsetting their negative effect on aboveground C accumulation. N and K contents of mounds are significantly higher than in adjacent soil. While Ca and Mg contents are lower in mounds, the available P content is similar. We are currently sampling more sites to obtain a more robust data set.
Phosphorus fractions : There is a significant difference in inorganic P levels for earthworm casts between system type, i.e. ASII and secondary forest and Pi levels in all fractions are significantly greater for earthworm casts than control soil. Most of the Pi, however, appears in the least available NaOH extractable (Fe oxide-bound) fraction. We are currently completing assays for organic P.
Plant families and Biomass: Plant biomass is significantly lower on mounds relative to adjacent soil. Plants from the Asteraceae, Fabaceae, Poaceae and Melastomataceae tend to dominate the mound structures (Figure 5). We are continuing to evaluate the extent of mounds, plant distribution and biomass.
Figure 5: Plant Families and Biomass on Mounds versus Adjacent Soil
2. OBJECTIVES
(1) To quantify their extent and impact on soil C and nutrients in pasture, secondary forest regenerating on abandoned pasture and agroforestry systems established on abandoned pastures.
(2) To identify the factors responsible for limiting vegetation growth on termite mounds over time and to identify plant species capable of colonizing termite mound structures
(3) To quantify the dynamics of soil macrofauna groups in pastures, abandoned pastures and agroforestry systems on abandoned pastures
(4) To assess the impact of macrofauna groups on C and P fractions in the soil.
3. METHODS & MEASUREMENTS
Site: The study area consists of sites at the EMBRAPA field station located along highway BR-174 north of Manaus, Amazonas. These include actively grazed pastures, a 7-year secondary forest, and 8-year-old agroforestry systems (Fernandes et al., 2001; McCaffery et al., this session). The following measurements are in progress:
1. Termite mounds and carbon dynamics: Extent of land coverage by termite mounds;
Compare the following parameters on and off termite mounds: Above- and belowground plant biomass; Soil carbon content and fractions (POM, ELF); Soil respiration; Microbial biomass and respiration; Termite respiration;
2.1 Biomass limiting physical factors on and off termite mounds : Soil resistance to penetration; Soil bulk density, texture, and moisture; Soil hydrophobicity and water infiltration; Air permeability;
2.2 Biomass limiting chemical factors on and off termite mounds : Soil pH; Macro- and micronutrients, Al3+
3. Abandoned pasture versus agroforestry system management impacts on the dynamics of soil macrofauna groups
4. Soil macrofauna activity impacts on soil P fractions (modified Hedley extraction).
5. DISCUSSION
Data on soil fauna impacts on C, nutrients, plant distribution and productivity is still being collected. We estimate that all sampling will be completed in the next 3 months. The data and observations presented here must be considered as preliminary.
Preliminary results show that despite trends of increased contents C, N, and K in termite mound structures both vegetation regeneration and primary productivity appears to be lower on mounds relative to adjacent soils. Methane emission from termite mounds is higher than from surrounding soil (See Rondon et al., this session).
Although inorganic P fractions are higher in earthworm casts, most of the Pi is in the least available Fe-oxide-bound fraction. It will be interesting to evaluate the P contributions of the organic fractions since the readily available Pi is ~0.1 mg/kg. We expect the organic P data by March 2001.
The Asteraceae and Fabaceae plant families have species with apparently significant potential to colonize mound structures and contribute to increasing primary productivity.
Figure 1: Macrofauna Groups in Abandoned Pasture
and Agroforestry Systems
Figure 2: Macrofauna Groups by Soil Depth in
an Abandoned Pasture
6. REFERENCES CITED see Handout