Eucalyptus globulus harvest residue management effects on soil carbon and microbial biomass at 1 and 5 years after plantation establishment

Abstract

Residues from harvest of eucalypt plantations represent a significant proportion of site organic matter and nutrients, and their management may influence longer term plantation productivity through changes in soil organic matter and nutrient supply. Experiments at two sites in south-western Australia were established to investigate the effects of harvest residue management on soil fertility and plantation productivity. The sites were a relatively fertile red earth, and a poorer-fertility grey sand. Harvest residue treatments were burning, complete removal of the above-ground material, retention and uniform distribution on site, and double the normal quantity of residues retained and distributed on site. Soil microbial biomass, an indicator of longer term (potentially several rotations) impacts of management on soil organic matter, was measured 1 and 5 years after planting. Retention of residues had minimal influence on total soil C and N, or C oxidized by 333 mM permanganate, but it significantly increased surface soil (0–5 cm) microbial biomass carbon (MBC) and nitrogen (MBN), as well as increasing the MBC and MBN quotients (ie. MBC and MBN as a proportion of the total soil C and N). Effects of residue management on MBC and MBN were generally highly significant 1 year after establishment, but differences between treatments had decreased after 5 years. Residue retention significantly increased MBC at both sites down to 20 cm depth, and significantly increased MBN at the red earth site at 1 year after establishment, but effects at 5 years were only evident in the surface (0–5 cm) soil. At 1 year, MBN in the surface (0–5 cm) soil was more responsive to residue retention than MBC, as the MBN quotient increased more than 2-fold at the red earth site, compared with only a 30% increase in MBC quotient. Higher MBC and MBN levels are due to greater availability of substrate derived from residues early in the rotation. The effect of residue management on microbial biomass was reduced at 5 years, and confined to the surface soil (0–5 cm), suggesting that availability of the organic material from the residues had declined. The decline was mainly due to utilization by the microbial biomass (respiration) or conversion to less labile forms.