Nitrogen cycle of tropical perennial crops under shade trees, 2: cacao [Venezuela]

A cacao (Theobroma cacao) plantation under shade trees was studied in northern Venezuela in order to estimate nitrogen stores in the soil and plantation trees and nitrogen fluxes associated with litterfall, decomposition and harvest. The cacao plants contained 302 kg N/ha, of which woody above-ground parts made up 50%. Fine roots and leaves contained ca. 60 kg N/ha. Litter on the soil surface was sampled quarterly and found to contain from 24 kg N/ha in November to 50 kg N/ha in May, with an annual average of 37 kg N/ha. Shade tree leaves made up 61% of the total nitrogen in the litter on the soil. Mineral soil stores of total nitrogen were 35 x 10('3)kg N/ha, 40% of which was found in the first 20 cm depth. Litterfall rates were studied monthly for a year; 20.9 x 10('3) kg dry litter/ha fell during the year, the major contribution (50%) from shade-tree leaves. The rate of nitrogen transfer with litterfall was 321 kg N/ha/yr. Decomposition rates (k) for shade-tree leaves, cacao leaves, twigs, and flowers 7.5 and fruits were 7.7, 9.5, and 19.7 respectively. In litterbag decomposition experiments, a small increase in per cent N was observed as decomposition progressed for all fractions. Rainfall collected during October and November contained nitrogen predominantly as NH(,4), a fraction of which was retained by the canopy. Nitrogen output by harvest was ca. 45 kg N/ha/yr, with some 20 kg N/ha/yr returned to the field with pod shells after processing. The net harvest output can be amply compensated for by inputs of nitrogen in shade-tree leaf litter alone. Much of this shade-tree nitrogen may be from deeper soil horizons than those exploited by cacao and possibly from biological N(,2)-fixation by the shade trees.