Author:
Armstrong R. D.,McCosker K.,Millar G.,Probert M. E.
Abstract
The feasibility of using legume leys to redress declining levels of soil
nitrogen (N) fertility on the heavy clay Vertisols of the northern Australian
grain belt depends partly on the ability of plant residues to supply N
directly to subsequent cereal crops. An alternative is the use of fertiliser N
in continuous cereal cropping. Two experiments were conducted (one in the
field, the other under polyhouse conditions) to compare the uptake of N from
either plant residues or ammonium sulfate fertiliser that had been labelled
with 15N.
In a field trial, 15N-labelled shoots of grain sorghum
and Desmanthus virgatus and ammonium sulfate were
applied to micro-plots and the flux of the added N between different soil
pools and a wheat crop was followed over 219 days. Only small amounts of
residue-derived N (<5%) were recovered in the mineral N of the soil
at a depth of 0-10 cm, whereas over 88% of the fertiliser N was present
as mineral N soon after adding the fertiliser. Soil microbial biomass-N was
increased following addition of residues. Recovery of added
15N in the wheat crop was much higher from the
fertiliser (35%) than from the 2 residue sources (<5%).
The pot trial compared a wider range of 15N-labelled
residues (shoot and root residues of
Desmanthus virgatus, Lablab
purpureus, and sorghum) with several rates of ammonium sulfate, applied in the
presence and absence of non-labelled grain sorghum residues, over 4 cropping
cycles. Dry matter production and N uptake were increased by application of
fertiliser N, although the response was reduced in the presence of
non-labelled sorghum residues; responses to residue N were much smaller than
those to fertiliser N. In the first crop following residue application
<7% of residue N was recovered, increasing to 12-23% over the
4 crops. Recovery of fertiliser N by the crops increased with the rate of
application, and also depended on whether it was applied together with
residues.
A feature of the results, in both the field and pot experiments, was the large
proportion of applied 15N that could not be accounted
for in either the soil or the crops, and these losses have been attributed to
denitrification.
Subject
General Agricultural and Biological Sciences
Cited by
8 articles.
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