NITROGEN AND WATER FLOWS UNDER PASTURE-WHEAT AND LUPIN-WHEAT ROTATIONS IN DEEP SANDS IN WESTERN-AUSTRALIA - 1 - NITROGEN-FIXATION IN LEGUMES, NET N MINERALIZATION, AND UTILIZATION OF SOIL-DERIVED NITROGEN
Gc. Anderson et al., NITROGEN AND WATER FLOWS UNDER PASTURE-WHEAT AND LUPIN-WHEAT ROTATIONS IN DEEP SANDS IN WESTERN-AUSTRALIA - 1 - NITROGEN-FIXATION IN LEGUMES, NET N MINERALIZATION, AND UTILIZATION OF SOIL-DERIVED NITROGEN, Australian Journal of Agricultural Research, 49(3), 1998, pp. 329-343
Detailed studies on the efficiency with which pastures and crops use s
oil-derived nitrogen (N) in southern Australia are limited. Inefficien
cies in the N cycle are indicated by widespread soil acidification and
low N status in wheat grain. The aims of this study were to document
rates of Nz fixation by subterranean clover-based pastures and narrow-
leaf lupin, plant uptake of soil-derived N, mineralisation of organic
N during legume and cereal phases, and export of N from pastures, lupi
n, and wheat in relation to climate and soil water. These measurements
were undertaken in a rotation experiment conducted on a deep sand loc
ated in the northern wheatbelt of Western Australia at a site with a l
ong-term average rainfall of 460 mm. The rotations examined over 3 yea
rs were 2 years pasture-wheat and lupin-wheat. The N-15 natural abunda
nce technique was used to differentiate soil-derived N from atmospheri
c N in legumes. Biomass production, grain yields, and N contents were
standard plant measurements in all treatments. Net N mineralisation be
tween growing seasons was ascertained by measuring changes in soil ino
rganic N to 1.5 m. Growing season net N mineralisation was determined
using an in situ method in which soil cores were isolated from plant r
oots. Anion exchange resin was used to trap leached below the depth of
the soil cores. Nitrogen fixation by subterranean clover in a mixed p
asture ranged from 29 to 162 kg N/ha, whereas Np fixation by lupins wa
s less variable, ranging from 90 to 151 kg N/ha. Pastures were large c
onsumers of soil-derived N (range 58-154 kg N/ha), with capeweed being
the most important sink (range 38-120 kg N/ha). In comparison, wheat
and lupins were inefficient users of soil N, removing 29-51 kg N/ha wi
thin a season. Another 31-67 kg N/ha of inorganic N in soil was not ut
ilised by wheat or lupin. Annual net N mineralisation ranged from 80 t
o 130 kg N, confirming the high rate of decomposition of organic matte
r in the sandy soil. Mineralisation over summer and autumn, when crop
and pastures were not grown, supplied similar to 25% of the inorganic
N produced in soil profiles in 1995 and 20-40% in 1996. The study indi
cated that legumes used in rotations with cereals on deep sands were a
ble to add adequate organic N to soil to insure rates of net N mineral
isation sufficient to support cereal yields in excess of current shire
averages. However, in practice, the asynchrony in supply and demand f
or N resulted in the inefficient use of soil-derived N by wheat.