Six treatments were compared for their effects on wheat yields, nitrogen (N)
uptake, protein content, and fertiliser N requirements in a long-term rotation
study on a black earth and a red clay in northern New South Wales. Three of
the treatments were lucerne, subterranean clover, and snail medic, all grown
simultaneously from 1988 to 1990 and all followed by 3 years of wheat. The
other 3 treatments were biennial rotations of chickpea–wheat and
long-fallow–wheat as well as a continuous wheat monoculture, all lasting
6 years.
With the exception of the first wheat crop, which experienced very low
growing-season rainfall, lucerne was more beneficial than other legumes to
following wheat crops in terms of yield, protein content, and fertiliser N
requirement. Clover closely followed lucerne in the magnitude of its positive
effects, whereas medic and chickpea produced much smaller effects. Because of
the amount of N removed in the chickpea grain, it appeared that the small
positive effects of chickpea were due to soil N sparing or rapid
mineralisation from crop residues rather than any net contribution of N
fixation to soil N accretion.
Average yields of the 3 wheat crops following lucerne and clover were much
higher than average yields 20 years previously following lucerne, even though
average yields of continuously grown wheat have declined over the past 20
years. However, lucerne eliminated the need for N fertiliser for no more than
2 following wheat crops, and clover for only the first wheat crop. It appears
that the longer duration of lucerne benefits reported in earlier studies was
due to the higher background soil N levels as well as the lower yield
potential in the earlier years. Nevertheless, lucerne lowered the fertiliser
requirement of the third wheat crop by more than 50%. In contrast to
lucerne, annual legumes are probably most beneficial if grown in alternate
years with wheat.
The large benefits of long fallowing particularly on the black earth were
apparently caused by its enhancement of soil moisture and mineral N
accumulation. However, these N effects were surprisingly large considering the
degree of depletion of organic matter in long-fallowed soils.
Nutrients Use Efficiency and Uptake Characteristics of Panicum virgatum for Fodder Production
