The productivity and nitrogen economy of artificial ecosystems comprising various comninations of perennial legumes and coniferous tree species

Abstract

The effect of the perennial legumes Lotononis bainesii, Desmodium uncinatum, and Phaseolus atropurpureus on the growth of the indigenous conifers Araucaria cunninghamii and Agathis robusta and the exotic pines Pinus elliottii var. elliottii, P. taeda, and P. caribaea var. hondurensis was tested over a 5 year period on a lateritic podzolic soil. The legumes grew vigorously but under the dense canopy of the exotic pines began to decline in the third season. They depressed the yield of the exotic pines but stimulated that of the native conifers. They greatly increased the nitrogen content of the standing crop (vegetation plus litter) of native conifers, but had little or no effect on the nitrogen content of exotic pines.

No change in soil nitrogen status was detected below a depth of 4 in., but the amount of total nitrogen in the 0-4 in. horizon was increased by Phaseolus and Desmodium, as was the nitrogen content of the whole soil-plant system. These increases occurred mainly in association with the native conifers, since the exotic pines shaded the legumes and reduced their effectiveness.

The marked stimulation of the native conifers by the leguminous cover crops confirms previous findings that nitrogen is the primary limiting nutrient for these trees on lateritic podzolic soils in the coastal lowlands of southern Queensland. Exotic pines did not benefit from the presence of legumes, yet produced dry matter yields of 5-6 tons/acre/year. There is evidence of substantial accretion of nitrogen, amounting to 20-45 lb/acre/year, to ecosystems dominated by Pinus spp., even in the absence of legumes; most of this extra nitrogen was contained in the trees.