Fertilizer and previous land use effects on C and N mineralization in soils from Eucalyptus globulus plantations

Abstract

Rates of microbial respiration and net N mineralization were measured in soils from Eucalyptus globulus Labill. plantations established on ex-pasture and ex-native forest sites using a long-term laboratory aerobic incubation (226 d at 20°C) in leaching microlysimeters. The plantations had been fertilized with either N or P, or both N and P 15 mo prior to soil sampling. Carbon and net N mineralization rates in soils from plantation sites were compared with rates from adjacent pasture and forest sites. Potentially mineralizable N (N 0) and net mineralization rate constant ( k) were determined using a first-order kinetic model. Application of nutrients to eucalypt plantations significantly influenced net N mineralization processes and the effects differed with previous land use. For both ex-native forest and ex-pasture plantation sites, addition of N fertilizer markedly increased (about 2-fold) k, and reduced N 0. Addition of P in the absence of N to the ex-native forest plantation soil reduced cumulative net N mineralization. Both the amount and rate of net N mineralization were associated with previous land use. The proportion of total soil N mineralized during incubation was similar in the ex-native forest plantation, ex-pasture plantation and pasture soils and higher than in the native forest soil. The relative amounts of CO 2-C respired per unit of net N mineralized differed between the land use units and was greatest on the native forest site, reflecting the high C-to-N ratio of soil organic matter and potential for N immobilization in soil from the native forest relative to the managed ecosystems. Amounts of net N mineralized (kg ha −1 in 0–200 mm soil) during the initial 28 d aerobic incubation declined in the order: pasture (29)>ex-pasture (18)>ex-native forest (14)>native forest (3), reflecting the low net N mineralization in native forest soil compared to managed ecosystems. The reason for the large difference (about 40%) between the amounts of net N mineralized from soils of the pasture and the ex-pasture plantation sites is uncertain and requires further investigation.