Changes in Soil Organic Matter and Nitrogen Fertility during the Slash-and-Burn and Cropping Phases in Experimental Shifting Cultivation

Abstract

At the experimental field for the study of shifting cultivation set up in the forest of Iriomote Island in Okinawa, changes in pH, organic matter, and nitrogen fertility of soils after forest clearing by burning and during crop cultivation were investigated. The soils were sampled at six periods, I (before forest clearing), II (immediately before burning), III (immediately after burning), IV, V, and VI (10, 20, and 30 months after burning). Tilled and nontilled and other treatment plots were laid out, and cassava was grown twice. Soil pHs rose from periods I to III or IV, and then decreased. Carbon and nitrogen contents, the amounts of mineralizable nitrogen and nitrogen mineralization percentages decreased during cultivation. The amounts of humic and fulvic acids increased from periods I to II and decreased afterwards. The increase in NH₄₋N due to the soil-burning effect was observed. The changes in the above-mentioned parameters during the cultivation period were greater for the first layer (0–5 cm) than for the second layer (5–10 cm), and for the tilled plots than for the non-tilled plots. The nitrification percentages which were originally very low, tended to increase with the rise of soil pH, and became very high after period III. Out of 11 soil samples taken at period III, two samples at the reburnt spots showed remarkable differences in carbon and nitrogen contents, nitrogen fertility, and humus composition compared with the others. Rp type humic acid was transformed into B or A type by reburning.