Changes in soil CO2 and O2 concentrations when radiata pine is grown in competition with pasture or weeds and possible feedbacks with radiata pine root growth and respiration

Abstract

This study examined the reciprocal effects of growing ryegrass, lotus and other weed species in competition with radiata pine on soil CO2 and O2 concentrations and on the growth and root respiration of the radiata pine. Soil O2 concentrations decreased and soil CO2 concentrations increased with increasing soil depth. Radiata pine plus competing species slightly reduced soil O2 concentrations and markedly increased soil CO2 concentrations (up to 40 mmol mol−1) compared with radiata pine alone. The dry weights of shoots and roots, and the root respiration rates of radiata pine grown with competing vegetation were much less than those for radiata pine alone. This probably was not solely caused by competition for nutrients water or light since adequate water and nutrients were supplied to all treatments and the radiata pine overtopped the competing vegetation. When radiata pine roots were raised in NaHCO3 solutions equivalent to a range of CO2 concentrations, succinate dehydrogenase activity (a metabolic indicator of mitochondrial respiration) and elongation rates of roots decreased as CO2 concentrations increased from 0 to 40 mmol mol−1. This suggests that the elevated CO2 concentrations found in the experiments in soil was the cause, at least in part, of the reduced growth of radiata pine in competition with other species.