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CHANGES IN SOIL PHOSPHORUS DURING 200 YEARS OF SHIFTING CULTIVATION IN INDONESIA
Author(s) -
Lawrence Deborah,
Schlesinger William H.
Publication year - 2001
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(2001)082[2769:cispdy]2.0.co;2
Subject(s) - environmental science , phosphorus , soil water , soil fertility , shifting cultivation , cycling , nutrient , slash and burn , nutrient cycle , ecology , agronomy , agriculture , biology , soil science , chemistry , geography , forestry , organic chemistry
We investigated the long‐term effects of shifting cultivation on soil phosphorus pools to understand the effects of repeated intense disturbance on nutrient cycling in rain forests and to assess the sustainability of a widespread tropical agricultural system. We studied 24 sites that formed a gradient of cultivation history near a village in West Kalimantan, Indonesia, where long‐fallow shifting cultivation has been practiced for at least 200 yr. The sites, including primary forest, had undergone 0–10 prior cycles (each ∼20 yr) of cultivation and forest regrowth. The number of swidden–fallow cycles experienced at a given site was determined through interviews. Using sequential chemical extraction of surface soils, we inferred changes in the distribution and availability of soil phosphorus during 200 yr of shifting cultivation. We examined the relative importance of fire vs. secondary forest biota in mediating the changes observed by heating soils in the laboratory to simulate the conditions of a slash fire and assessing data on root presence by depth. Over the first four cycles (∼80 yr), total phosphorus in the top 30 cm of soil increased by a mean of 73 kg·ha −1 ·cycle −1 (3.7 kg·ha −1 ·yr −1 ), much of it in the organic NaOH‐extractable and the inorganic, concentrated HCl‐extractable, fractions. Accounting for differences in original site fertility, total P did not change significantly over 2–10 cycles (∼200 yr), but the distribution shifted from both available and highly recalcitrant inorganic forms to available organic and occluded inorganic forms. NaOH‐P i decreased from 28% to 13%, and residual P decreased from 30% to 10% of total P. In contrast, NaOH‐P o increased from 13% to 26%, and cHCl‐P i increased from 15% to 40%. The significant increase in soil organic phosphorus in combination with greater concentrations of fine roots deep in the soil profile suggest that deep‐rooting fallow vegetation, rather than fire, is responsible for increasing, and maintaining, total phosphorus. The magnitude of changes induced by the fire are not great enough to account for the increases observed over several cycles, especially considering coincidental losses. Despite losses of nonoccluded P i , we believe the availability of phosphorus may have been enhanced over 200 yr due to an increase in the stable organic phosphorus fraction. Long‐fallow shifting cultivation, as practiced in West Kalimantan, can continue to support rural populations for some time.