Nutritional relationships between pampas grass (Cortaderia spp.) and Pinus radiata

Gadgil, R. L.; Sandberg, A. M.; Allen, P. J.

The results are reported of greenhouse trials investigating (1) the response of pampas grass (Cortaderia selloana and C. jubata, introduced species) to (a) N fertilizer (ammonium nitrate at rates equivalent to 0, 50, 100, 150 or 200 kg N/ha) added to a coastal sand, and (b) P fertilizer (sodium dihydrogen phosphate at rates equivalent to 0, 50, 100, 150 or 200 kg N/ha) and/or ammonium nitrate added to a silty clay loam soil, and (2) the effect of decomposing pampas plant residues (resulting from herbicide (glyphosate) treatment) on growth of pot-grown Pinus radiata seedlings. Addition of fertilizers increased growth of pampas. The response of pampas to addition of P was greatest when N was also added. Pampas residues resulting from herbicide treatment reduced growth of P. radiata seedlings. The results of 2 field trials in New Zealand are also reported. In the first field trial, volunteer pampas seedlings were measured in a 1-yr-old P. radiata plantation on a clay soil in the Maramarua Forest. Superphosphate (170 g) was applied by hand to the base of each seedling after planting and three months later the whole area was broadcast sown with the legume Lotus uliginosus (2 kg seed plus 10 kg serpentine superphosphate/ha). Increased pampas growth was associated with the addition of superphosphate. The second field trial was established in 1983 in a 12-yr-old second rotation stand growing on coastal sand in the Waiuku Forest. Factorial combinations of 2 levels of N treatment (0 or 200 kg N/ha as broadcast calcium ammonium nitrate) and 2 levels of pampas control (no control or application of glyphosate in June and November in 1983 and 1984) were applied to 0.09 ha plots. Basal area of P. radiata was measured in 1983, 1985, 1986 and 1987. The basal area of plots treated with glyphosate and N fertilizer together was greater than in the other plots. The results suggested that fertilizer N immobilised by decomposing pampas residues was released gradually and utilized by P. radiata for up to 4 years after treatment. Where pampas growth is uncontrolled, a large proportion of the fertilizer intended to promote P. radiata growth may never reach the trees.