Extensive areas of land in central Queensland, cleared and sown to buffel grass (Cenchrus ciliaris) pasture, are now showing symptoms of decline. Nitrogen deficiency has reduced pasture productivity. The restoration of productivity of these 'rundown' pastures needs to be addressed and practices aimed at increasing the turnover of nitrogen or adding nitrogen to the soil-plant system were investigated.
The effect of a single cultivation on the mineralization of soil nitrogen and productivity of buffel pastures was assessed. Treatments were four levels of soil disturbance - nil, 45% (chisel plough), 64% (disc plough) and 100% (off-set disc plough) combined with four rates -0, 60, 120, 240 kg N/ha of nitrogen fertilizer and supplementary irrigation. Results showed an initial depression in buffel yield with disturbance, due to reduced basal cover, but a recovery by the end of summer. There was a trend towards higher dry matter yields (30% increase) and higher nitrogen yield (50% increase) in grass tops from moderate (64% disturbance) cultivation. The nitrogen concentration (% N) in the grass was improved for three months following cultivation. Data on the nitrogen balance show four times more nitrogen in the buffel roots (140-170 kg N) than in the shoots (40 kg N) and only 6 kg N/ha in the litter. Potential netmineralization of nitrogen from the soil was 72 kg N/ha, or only 2% of the total N reserves in the upper 40 cm of soil. Data suggest that nitrogen in buffel grass shoots was retranslocated to belowground parts during winter.
A 20-year-old Gayndah buffel pasture was cultivated with off-set disc, chisel, blade or disc ploughs in winter (August). Though nitrogen concentration (% N) in grass tops was increased during the four months following cultivation, cultivation treatments failed to improve nitrogen availability as measured by nitrogen uptake in grass tops. None of the cultivation treatments improved grass growth.
The effectiveness of five pasture renovation techniques (ploughing, fallow-resow, oversowing a legume, nitrogen fertilizer and burning) was assessed over three years in a 14-year-old Biloela buffel pasture. The pasture was moderately nitrogen deficient and applying nitrogen fertilizer significantly increased dry matter yields to a maximum of 11 000 kg/ha. Some renovation practices (one pass with a disc or chisel plough, burning or oversowing with Fitzroy Stylo) failed to improve pasture yield in any year. However, pasture production was increased 57% and nitrogen uptake in grass tops increased 50% when buffel grass was resown after a 3-month cultivated fallow. In old buffel grass pastures, large reserves of nitrogen occur in the grass roots and microbial biomass. Cultivated fallow released nitrogen from these sources and enhanced the availability to resown grass.
A survey of 18 plantings of the legume Stylosanthes scabra cv. Fitzroy ranging in age from 2.6 to 10.7 years found no increase in total nitrogen, organic carbon or potentially mineralizable nitrogen level in the soil growing legume. However, monitoring of small plots of Fitzroy over four years identified an increase in total soil nitrogen level of 35 kg/ha/year. There was no improvement in grass yield when grown with Fitzroy.
To identify the sites for nitrogen relocation in old buffel grass pasture, K15 NO3 was applied in the field. The 15Npulse was taken up quickly by the grass and over 54 weeks, 33% was recovered from the plant fractions (tops, litter, roots) and the balance was located in the soil. Within the plant, 15N was initially highest in plant tops but this declined over time and was off-set by increases in the roots. In the soil, reductions in the proportion of 15N in mineral nitrogen and microbial biomass were reflected in increases in the non-biomass organic nitrogen pool. The grass roots and non-biomass organic nitrogen fraction in the soil appear to be the main sites for nitrogen relocation in both new and old buffel grass pastures.
The importance of internal translocation of nitrogen in meeting the deficiency between soil nitrogen supply and plant nitrogen requirement for growth are discussed. There was a 22 to 50% decline in nitrogen in buffel grass tops at the end of the growth season and a growth requirement for nitrogen that was 40 to 50% greater than the estimated soil mineralization potential. Nitrogen mineralization potential, estimated from mineral nitrogen accumulation in bare-fallow plots, amounted to 1 to 2% of the total soil nitrogen reserves. The additional nitrogen requirement to restore pasture production to the pre-rundown condition would require an increase in net mineralization to about 4% of total soil nitrogen.
It is concluded from this study that successful renovation of rundown buffel grass pasture by cultivation requires a period of bare fallow which would be best achieved in practice through crop-pasture rotation systems.