School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEffects of mulching logging residue on soil water relations, soil temperature and early growth of Pinus radiata on sandy soils in south-west Victoria( 1990)The timber industry in Australia is becoming increasingly dependent on plantations of exotic softwoods, with Pinus radiata D. Don (Monterey pine) the most important species planted in Victoria and South Australia. In general, these plantations have been highly productive in the first rotation. However, in the second rotation there is evidence of a decline in productivity on relatively infertile sandy soils, particularly where the litter and logging residue remaining after clearfelling is burnt. Research has indicated that this decline can be arrested by careful control of cultural inputs, such as chemical weed control, fertiliser and planting of legumes, or by mulching the litter and logging residue. This thesis reports on a study that examined the interacting effects of mulch, weed control and fertiliser on soil and plant water relations, weed biomass, soil temperature, root development, tree growth, and biomass and nutrient accumulation during the first five years following re-establishment of a P. radiata plantation. Mulch, weed control and fertiliser were applied (present, absent) in a 2 x 2 x 2 split-plot factorial design, resulting in eight treatment combinations. Mulch was applied prior to planting; weed control prior to planting and at regular intervals thereafter to age two years; fertiliser at 1.5, 10 and 14 months after planting, a total of 58.1 kg ha-1 of nitrogen and 19.1 kg ha-1 of phosphorus (as part of a complete mineral mix, including zinc). During the first two years after planting, soil and plant moisture levels, soil temperature and tree growth were monitored at four-weekly intervals, and weed biomass three-monthly. Annual growth was also monitored to age five years. Root development, tree biomass and nutrient accumulation were examined in detail at age two years. In the absence of mulch and weed control, soil moisture was significantly reduced during the first two summers after planting due to evapotranspiration losses, with a consequent reduction in tree growth. Both mulch alone and weed control alone were similarly effective in conserving soil moisture in the surface 10 cm of soil, through a reduction in evaporation and transpiration losses respectively. There was evidence that transpiration losses from weeds in mulch had a greater impact than evaporation losses from bare soils at depths from 10 to 40 cm. Maximum soil moisture levels during the summer months occurred where the mulch and weed control treatments were applied together. Mulch reduced weed biomass on average by half for the first two years after planting compared to that on bare soils with no weed control, though weed biomass by mid- to late- summer was similar for both treatments. Weeds which establish in the mulch treatment benefit from improved soil moisture availability as do the P. radiata trees. Mulch significantly reduced soil temperatures compared to those under bare soils during the spring and summer months, particularly in the surface 10 cm of soil. This is likely to have led to less favourable conditions for root growth and mineralisation of soil nitrogen under mulch during spring, when soil moisture availability is not limited by any of the treatments. During summer however, conditions were more favourable under mulch due to higher soil moisture availability. Where mulch and weed control were both applied, root length at age two years was almost twice compared to that for weed control alone. This difference occurred in the surface 10 cm of soil, and is attributed to the combined effect of higher soil moisture availability and stable temperatures within the optimum range for root growth under mulch during the summer months. Roots were also found in the mulch, which indicates that P. radiata roots can either extract nutrients, water or both from the mulch itself. All growth parameters show that in the absence of fertiliser, the responses to mulch alone and weed control alone were similar up to age five years, and that growth was significantly increased when both treatments were applied. These responses were however confounded, as severe zinc deficiency was induced on the weed control alone treatment. In the presence of fertiliser, the responses to mulch alone, weed control alone and both treatments combined were similar by age four years. The largest response to fertiliser was for the weed control alone treatment, probably due to the correction of zinc deficiency. A fertiliser response was initially found where both the mulch and weed control treatments were applied, but by age four years this response was no longer detectable. Growth at age five years was a maximum on, and not significantly different between, the mulch and weed control, mulch and fertiliser, weed control and fertiliser, and mulch, weed control and fertiliser treatment combinations. The nitrogen concentration in needles less than one year-old was at or below the critical level of 14 g kg-1 for all treatment combinations at age two years, except for the weed control alone treatment which was affected by severe zinc deficiency. Despite this, foliage on the treatments associated with good growth did not show obvious nitrogen deficiency symptoms. It is likely that there was net immobilisation of nitrogen under the mulch treatment during the first two years, but subsequent to this it is considered that the mulch would have been a source of nitrogen to the P. radiata trees. Foliar concentrations of both phosphorus and potassium were adequate for all treatment combinations. Foliar concentrations of zinc were at or below the critical level of 10-11 mg kg-1 for those treatments showing maximum growth, which indicates that demands were just being met. They were well below the critical level for the weed control alone treatment. There is clear evidence that the mulched litter and logging residue is a source of zinc to the replanted P. radiata. Relationships between concentration of nutrients in biomass components and tree diameter were generally weak and not significant, and thus nutrient accumulation closely mirrored the pattern of biomass production. The results show that mulching of the litter and logging residue in situ has the beneficial effects of conserving soil moisture, which in combination with more favourable soil temperature conditions during summer, resulted in satisfactory early growth. The results also indicate that mulch is a source of nutrients to the re-established crop. Although care must be taken in extrapolating these results, forest managers should consider mulching as an alternative site preparation technique to burning, particularly on infertile sandy soils.