Aspects of nutrient cycling in some indigenous and exotic forests in Westland New Zealand

Abstract

Aspects of nutrient cycling in indigenous and pine forests of the Grey Valley, Westland, were followed over a 2-year period. Four sites representing stages in exotic conversion from mature podocarp-hardwood to 10 year-old radiata pine were selected on similar sites. Nutrient flux in litterfall, rainfall, throughfall and soil drainage beneath the organic horizon and rooting zone were determined. A comparison of throughfall and litterfall was made between mature hard beech and closed-canopy 17 year-old radiata pine. Performance of tension and tray lysimeters was compared on a 1 year-old radiata pine site, at two soil depths. The standing crop of nutrients in components of podocarp-hardwood above-ground biomass was determined.

Throughfall was the principal pathway for transfer of Na, Cl and K from canopy to forest floor. Ca, Mg and P were returned predominantly by litterfall. Nutrients may be recycled considerably faster by canopy leaching than by litterfall.

Hard beech had substantially greater litterfall than podocarp-hardwood (PH 17) or 17 year-old radiata pine due, predominantly, to higher wood-fall. Litterfall in beech and pine was greatest in spring and autumn while in PH 17 a maximum occurred in summer. Litterfall in 10 year-old radiata pine doubled in the second year over the first year after thinning.

Nutrient inputs to the forest floor were considerably greater in PH 17 than 10 year-old or younger pine. Of the three closed-canopy forests (hard beech, podocarp-hardwood and 17 year-old radiata pine) hard beech had largest inputs to the forest floor of Ca and P and lowest inputs of Na and Cl; inputs of K were intermediate. The forest floor of radiata pine had considerably lower inputs of Ca and Mg than for indigenous forest floors.

In soil drainage, there was a net transfer of K, Ca, Mg and P from organic to mineral soil in 1 and 10 year-old radiata pine, but a net uptake from PH 17 organic soil drainage. Net uptake from mineral soil drainage occurred on all sites. It was greatest on the 10 year-old radiata pine site.

Net loss of K from the ecosystem decreased rapidly with increasing biomass. Substantial losses occurred from a 1 year-old radiata pine site. Losses of Ca and Mg were greatest from 10 year-old pine which had recently been thinned. Net losses of P were nil or very low on all sites.

Application of superphosphate fertiliser at the rate of 10 c.p.a. to a 10 year-old radiata pine stand caused negligible increases in leaching of Ca and P below the organic soil and rooting zone.

For a 3-year study it was considered that 5 raingauges, 15-25 throughfall collectors, 20-30 lysimeters and 12-24 littertraps, depending on the site, were adequate to determine nutrient flux in the forested ecosystems studied.