Gc. Anderson et al., NITROGEN AND WATER FLOWS UNDER PASTURE-WHEAT AND LUPIN-WHEAT ROTATIONS IN DEEP SANDS IN WESTERN-AUSTRALIA - 2 - DRAINAGE AND NITRATE LEACHING, Australian Journal of Agricultural Research, 49(3), 1998, pp. 345-361
Quantification of nitrate (NO3-) leaching is fundamental to understand
ing the efficiency with which plants use soil-derived nitrogen (N). A
deep sand located in the northern wheatbelt of Western Australia was m
aintained under a lupin (Lupinus angustifolius)-wheat (Triticum aestiv
um) and a subterranean clover (Trifolium subterraneum) based annual pa
sture-wheat rotation from 1994 to 1996. Fluxes of water and NO3- throu
gh, and beyond, the root-zone were examined. Drainage was calculated o
n a daily basis from measurements of rainfall, evapotranspiration, and
the change in soil water content to a depth of 1.5 m. Evapotranspirat
ion was estimated from Bowen ratio measurements, and soil water conten
t was determined by time domain reflectrometry. Soil was sampled in la
yers to 1.5m at the onset of winter rains and analysed for NO3-. Ceram
ic suction cups were installed at 0.25, 0.4, 0.6, 0.8, 1.0, 1.2, and 1
.4 m to sample soil solution from June to mid August. The NO3- leached
from each layer was computed by multiplying the daily drainage throug
h each layer by the estimated concentration of NO3- within the layer.
The estimated concentration of NO3- in a layer was calculated by takin
g into account NO3- either entering that layer through mineralisation
and leaching or leaving the layer through plant uptake. Mineral N was
added to the surface 0.2 m in accordance with measured rates of net N
mineralisation, and daily N uptake was calculated from the measured ab
ove-ground plant N derived from soil N. Root sampling was undertaken t
o determine root length density under pastures, lupin, and wheat. Cumu
lative drainage below 1.5 m was similar under wheat and lupin, and acc
ounted for 214 mm from 11 May to 15 August 1995 and 114 mm from 2 July
to 15 September 1996. The cumulative evapotranspiration (E-a) over th
ese periods was 169 mm from a wheat crop in 1995, and 178 mm from a lu
pin crop in 1996. The amount of NO3- in soil at the start of the growi
ng season was affected by previous crop, with a lower range following
wheat (31-68 kg N/ha) than following legumes (40-106 kg N/ha). These l
arge quantities of NO3- in the soil at the break of the season contrib
uted substantially to NO leaching. Leaching of NO3- below 1.5 m in whe
at crops accounted for 40-59 kg N/ha where these followed either lupin
or pasture. In contrast, less NO3- was found to leach below 1.5m in p
astures (17-28 kg N/ha). Greater N uptake by capeweed (Arctotheca cale
ndula L.) than by either wheat or lupin was the main reason for the lo
wer amount of NO3- leached in pastures.