Fine root production and turnover in Brazilian Eucalyptus plantations under contrasting nitrogen fertilization regimes
Résumé
Nitrogen fertilization increased largely over the last decade in tropical eucalypt plantations but the behaviour of belowground tree components has received little attention. Sequential soil coring and ingrowth core methods were used in a randomized block experiment, from 18 to 32 months after planting Eucalyptus grandis, in Brazil, in order to estimate annual fine root production and turnover under contrasting N fertilization regimes (120 kg N ha_1 vs. 0 kg N ha_1). The response of growth in tree height and basal area to N fertilizer application decreased with stand age and was no longer significant at 36 months of age. The ingrowth core method provided only qualitative information about the seasonal course of fine root production and turnover. Mean fine root biomasses (diameter <2 mm) in the 0–30 cm layermeasured bymonthly coring amounted to 0.91 and 0.84 t ha_1 in the 0 N and the 120 N treatments, respectively. Fine root production was significantly higher in the 0 N treatment (1.66 t ha_1 year_1) than in the 120 N treatment (1.12 t ha_1 year_1), probably as a result of the greater tree growth in the control treatment throughout the sampling period. Fine root turnover was 1.8 and 1.3 year_1 in the 0 N and the 120 N treatments, respectively. However, large fine root biomass (diameter <1 mm) was found down to a depth of 3 m one year after planting: 1.67 and 1.61 t ha_1 in the 0 N and the 120 N treatments, respectively. Fine root turnover might not be insubstantial in deep soil layers where large changes in soil water content were observed.