Comparative effects of subterranean clover, medic, lucerne, and chickpea in wheat rotations, on nitrogen, organic carbon, and moisture in two contrasting soils

Abstract

The effects of subterranean clover, medic, and lucerne, grown simultaneously from 1988 to 1990, followed by wheat from 1991 to 1993, and of chickpea and long fallow in alternate years with wheat, on soil total nitrogen (N), nitrate-N (NO^-₃-N), organic carbon (C), and moisture were measured over 6 years in 2 long-term experiments on a black earth (pellic vertisol) and red clay (chromic vertisol) in northern New South Wales.

The accretion of soil total N in the black earth and NO^-₃-N in both soils was higher after lucerne than after other legumes, and NO^-₃ in the black earth remained high after lucerne to the full depth (120 cm) of measurement during the following 3 years of wheat growing. Clover had the next largest effect on total N and NO^-₃ accretion, and chickpea had the smallest effect except in the red clay where chickpea increased NO^-₃ more than medic in 1990. However, none of the annual legumes had much effect on NO^-₃ after the first year of cropping and their small residual effects, if any, were confined to the top 30 cm of soil. Levels of total N accretion after lucerne were higher than previously measured, because of the greater depth of measurement, but were similar on a per unit depth basis.

High levels of NO^-₃ -N after long fallow, especially in the black earth, which tended to be higher than after medic or chickpea, were probably caused by accelerated mineralisation of organic N which has declined more in this rotation than in any other.

There was no accumulation of organic C during the legume growing period in any rotation, and C tended to be lower after chickpea than after other legumes. Organic C was almost always lowest in the long fallow treatment. Summer-growing grasses, which occurred in all treatments to varying degrees, may have caused the organic C accumulation during the 3 years of cropping.

In the first year of wheat growing, soil water was lower after lucerne than after other treatments and highest after long fallow, continuous wheat, and chickpea. It was replenished in the red clay to field capacity in all treatments by high rainfall during the fallow before the first wheat crop but not in the black earth, which failed to reach field capacity in any treatment even 2.5 years after the pasture legume phase.