The addition, decomposition and accumulation of organic matter in some native Nothofagus spp. forests and Pinus radiata plantations in the South Island of New Zealand

Abstract

The addition, decomposition and accumulation of organic matter in beech (Nothofagus spp.) forests and radiate pine (Pinus radiata) plantations were compared in a study conducted over a two-year period from November 1976 to December 1978. A pair of adjacent beech and radiata pine stand was selected in each forest at Granville, Hanmer and Golden Downs (Nelson), located in the South Island of New Zealand. In Golden Downs, a regenerated radiata pine stand was also used.

A quantification of the organic matter content of the forest floor and top-soil (0-20cm) was undertaken in six pairs of adjacent native forest and radiata pine stands, including those at the three forests mentioned above. In Granville forest, carbon dioxide evolution rates from forest floor and mineral N levels in humus and top-soil (0-20cm) were measured directly in the field over two years in both the beech and radiata pine stands used.

Litter-fall data obtained included biomass of litterfall, three-monthly and annual litter-fall budgets of nutrients (N, P, K, Mg, Ca), carbon, water-soluble components (simple carbohydrates, polyphenols and total watersoluble materials), and annual litter-fall budgets of the organic constituents (ether-extractable components, aqueous ethanol-extractable components, holocellulose and residual lignin).

These data showed that in the forests at Granville, Hanmer and Nelson, total annual litter-fall was between 3915 to 7471 kg/ha in beech stands and between 1445 to 5522 kg/ha in radiata pine stands. In all these stands, peak litter-fall occurred in spring (October - November). Beech stand was returning larger amounts of these macro-nutrient elements, and water-soluble and organic constituents than the adjacent radiata pine stand in all forests studied. Annual total macro-nutrient (N + P + K + Mg + Ca) budgets in radiata pine stand accounted between 35 and 65 percent of those in adjacent beech stand.

In the forests at Granville, Hanmer and Nelson, decomposition of the litter in each of the stands used was measured using litter-bag technique from October 1976 to December 1978. Beech (leaf + twig) litter and radiata pine (needle) litter were used in the respective stands. Weights of litter remaining in litter-bags at the end of the 26-month period were between 46 and 54 percent of initial litter weight used for beech, and between 37 and 54 percent for radiata pine. Average first-year weight loss rates (k values) obtained for beech litter ranged from 0.40 to 0.47 and for radiata pine litter from 0.33 to 0.48.

Macro-nutrient concentrations and distribution of the water-soluble and organic constituents in leaf, twig and needle litter determined at various stages of litter decomposition showed that concentrations generally declined with time except those of N and residual lignin. First-year weight loss rates for the macro-nutrient elements and other constituents in all forest stands ranged from low values for residual lignin (0.05 to 0.17) and N (0.10 to 0.27) to high values for water-soluble polyphenols (0.91 to 1.55) and K (0.43 to 1.47). Loss rates of P, K, ether-extractable components and holocellulose for radiata pine litter were greater than those for beech litter, although the reverse order was found for Mg, carbon and residual lignin.

In the quantification of organic matter accumulation in forest stands, the forest floor depths and top mineral soil (0-20cm) in six pairs of adjacent native forest and radiate pine stands at Granville, Hanmer (2 pairs), Nelson, Hochstetter and Peel forest were sampled. Average weight of the forest floor in native forest stands ranged from 25 to 464 tonnes/ha and those in radiata stands ranged from 9 to 79 tonnes/ha. Native forest stands accumulated apparently larger amounts of nutrients in forest floor than their adjacent radiata pine stands. These differences were smaller in the top-soil and in some cases radiata pine stands apparently accumulated larger amounts of top-soil nutrients. In addition, higher levels of exchangeable cations and available phosphorus (Bray-P) were also evident in the top-soil of radiata pine stands, compared to those of beech.

The carbon mineralization field data obtained at Granville forest showed a close relationship existed between CO₂ evolution rates and air temperatures (r = 0.846*** for beech and r = 0.897*** for radiata pine). However, no significant relationship was found between CO₂ evolution rates and soil and/or humus moisture contents. Total CO₂ evolution was about three times greater than expected from the amount of carbon released from annual litter-fall, and this was attributed to contributions from root respiration. Both clearcutting a forest stand and burning of the forest floor were found to result in apparent increases in CO₂ evolution rates.

Mineralization of N in the humus and top-soil (0-20cm) layers in both the beech and radiata pine stands at Granville was found to result mainly in the form of ammonium. Nitrate was frequently not detected (< 0.1 µg/g) or was present in negligibly low concentrations. In the beech stand, ammonium- N levels ranged from 11.0 to 63.1 µg/g for the humus layer and 1.5 to 13.3 µg/g for the top-soil in the period between March 1977 to December 1978. Corresponding values for the radiata pine stand were 22.6 to 85.4 µg/g for the humus layer and 1.4 to 12.2 µg/g for the top-soil. No significant correlations were found between ammonium-N levels and either temperatures, moisture contents or total-N contents for both humus and top-soil samples.

A laboratory incubation experiment was undertaken to evaluate the effects on soil CO₂ evolution and N mineralization due to water extracts of beech and radiata pine litter, particularly those of carbohydrates and polyphenols. It was found that carbohydrates stimulated CO₂ production but had no apparent effect on mineral N levels. Polyphenolic compounds in water extracts of litter and also catechin solution depressed the rate of CO₂ evolution and N mineralization.

A method for the extraction of lipid components of plant materials, suitable for a large number of samples was developed. This method produced consistent results (C.V. ranged from 1 to 7 percent). In the method, plant materials were initially extracted with petroleum ether at 49º C and then followed by 75%-aqueous ethanol at 62º C. The duration was 22 hours for both extractions.