Earthworms and phosphorus in Amazonian upland soils / Christienne Nicole Pereira.

Tese (mestre) - Cornell University, 2003

The Amazon Rainforest has been called 'the lungs of the Earth,' however deforestation has been occurring at an unprecedented rate. Part of the deforestation cycle has been due to overuse of pastures leading to soil degradation and land-use choices by, oftentimes, absentee landowners. Phosphorus is known to be the most limiting soil nutrient of these land-use systems and across the tropics. It is important to better understand how best to manage P within Amazonian land-use systems through better understanding how land-use and soil biological components influence P to better understand options for sustainable land-use in an extremely important ecosystem. Agroforestry systems have been suggested as an alternative to slash-and-burn agriculture and as a strategy to improve soil chemical and physical characteristics of degraded lands, especially degraded pastures, in the Amazon basin (Fernandes et al., 1995). The use of perennial tree crop systems may also lower economic risk and increase diversity of the agricultural system, improve standard of living, and decrease deforestation pressures (Fernandes and Matos, 1995; Lal, 1991; McGrath et al., 2000b; Sanchez et al., 1985). Some believe earthworms to be among the most important macrofauna transforming the above-ground litter entering the soil, and earthworms are known to increase soil P availability. They are abundant and produce up to 400 Mg soil (dry wt.) ha-¹ yr-¹ (Barois and Lavelle, 1986) in the humid tropics, which is continuously recycling P and adding to soil available and moderately available P and sustaining soil resistant P over time. We investigated the effect of large earthworms (Family Glossoscolecidae), land-use, i.e., agroforestry systems (AGR), pasture (PAS), and secondary forest (SEC), and specific agroforestry tree species, i.e., áraça-boi (Eugenia stipitata), Brazil nut (Bertholletia excelsa), cupuaçu (Theobroma grandiflorum), and pupunha (Bactris gasipaes), on soil P availability of upland Oxisols of the central Amazon basin. The land-use systems were established in 1991 and underwent different management regimes, with low-input fertilization in AGR and PAS, and no fertilization in SEC. A modified sequential P extraction (Tiessen and Moir, 1993; Hedley, 1982) was used to measure P pools, and total N and other available nutrients were measured. We found that the earthworms studied increased organic hydroxide P, where fertilization increased inorganic hydroxide P. Inorganic P was increased by fertilization, and organic P was increased by earthworm gut passage and/or selection of ingested materials. Earthworm gut passage and/or selective ingestions additionally increased available P (sum of resin and bicarbonate fractions) and moderately available P (sum of hydroxide and dilute acid fractions), and P fertilizer application and land-use increased available P The use of a modified sequential fractionation produced fewer differences between earthworm casts and soils than were expected. We suggest the use of a condensed extraction with three extractions (Available P, Moderately Available P, and Resistant P) that provide an ecologically based understanding of the P availability in soil. Earthworm casts were estimated to constitute 41.0, 38.2, and 26.0 kg Available P ha-1 (sum of resin and bicarbonate fractions) in the agroforestry system, pasture, and secondary forest, respectively. We found that pupunha increased resin P and Brazil nut increased bicarbonate organic P. Fertilization increased the hydroxide organic P. Additionally, áraça-boi increased hydroxide organic P. Pupunha and Brazil nut increased soil available P (sum of available Hedley fractions - AP) and fertilization increased moderately available P (sum of moderately available Hedley fractions - MAP). This suggests earthworms and the use of pupunha and Brazil nut in agroforestry systems with moderate fertilization will better maintain AP and MAP in soils of the central Brazilian Amazon than other tree species and land-use systems studied with no earthworms.