Mineralization of two promising green manure species in Red Ferralitic soil (Ferralsol) by the aerobic incubation method

Abstract

An experiment was performed to determine the amount of nitrogen released or immobilized during the decomposition of leaves, stems and the complete air system of two species of green manures: Canavalia ensiformis and Crotalaria juncea as well as to determine the influence of the C:N ratio of these plant residues on this process in a Red Ferralitic soil, by means of the aerobic incubation method. Treatments consisted of doses of 5 t dry weight.ha-1 of the organs or the complete air system of the two species and two check treatments: the control (soil) and a treatment receiving 75 ppm urea N. The experiment was developed under constant temperature and moisture (30oC and 65 % field capactity). The leaves always released N to the soil in equivalent amounts to 40.8 and 34.7 % of the applied N; the stems, with a higher C:N ratio always immobilized soil mineral N, presenting a maximum immobilization near to 60 days, reaching up to 12.4 ppm with crotalaria. Furthermore, such immobilization started to decrease in both species, tending to disappear in time, although it still continued in small extension after 120 days. When the complete air system release was detected 120 days after incubation, in percentages of 12.5 and 18.0 of the N applied to both species respectively. The mineralization of several residues was inversely related to C:N ratio, recording a critical index of 37.6 above which decomposition needs soil mineral N immobilization and below it, decomposition increases mineral N. Nitrification happened quickly independently of the nitrogen source used.