STUBBLE RETENTION AND TILLAGE IN A SEMIARID ENVIRONMENT .2. SOIL MINERAL NITROGEN ACCUMULATION DURING FALLOW

The accumulation of soil mineral nitrogen (SMN) was studied over 4 yea rs at two sites in north-west Victoria, Australia in response to fallo w treatments. The four combinations of stubble management (with and wi thout) and tillage (with and without) in Is-month-long fallows of a fa llow-wheat rotation were compared with a tilled summer fallow prior to field pea and wheat crops in rotation. Across both sites, root-zone S MN ranged from 66 to 222 kg N h(-1) during the fallows and at sowing t ime was generally less at Dooen than at Walpeup despite greater total soil N (Dooen 0.11%, Walpeup 0.05%). At Dooen, the long fallow with st ubble retention and tillage accumulated 46 kg N h(-1) more SMN than th e short fallow after field pea. In one year, stubble retention increas ed SMN by 20 kg N ha(-1) but reduced it by an average of 26 kg N h(-1) in two of the four years. At Walpeup, SMN increased in the pea-wheat rotation by an average of 46 kg N h(-1) above the tilled fallow withou t stubble in two of the four years. The inclusion of field pea in the rotation increased SMN at depth by the second cycle of the rotation at Walpeup only. Stubble retention and tillage had little effect on SMN accumulation at Walpeup. The distribution of SMN in the soil profile w as different every year at each site despite some years having similar total SMN. There was a tendency for stubble retention to depress SMN in the surface layers possibly through immobilisation and denitrificat ion.