SOIL NITROGEN STORAGE AND ASSOCIATED REGULATION FACTORS IN AN ACACIA HYBRID PLANTATION CHRONOSEQUENCE IN SOUTHERN VIETNAM

The study was designed to quantify the soil total nitrogen (STN) storage (SNS) and its controlling factors in Acacia hybrid (Acacia auriculiformis x A. mangium) plantations in five different ages (2-, 4-, 6-, 8-, and 10-years-old) in Langa -Dongnai Forestry Company, Southern Vietnam. A total of 75 soil samples were taken at five different soil depths from 0-100 cm. The STN concentration, SNS, soil properties, and plant biomass features (i.e., understorey vegetation biomass and litter biomass) were estimated. The STN concentration significantly increased with increasing forest stand age, whereas significantly decreased with increasing soil depth in all stand ages. The SNS in the uppermost 1 m soil layer in 2-, 4-, 6-, 8, and 10-year-old stands was 6.83, 9.50, 12.30, 17.15, and 19.41 Mg. ha-1, respectively. The SNS was focused mainly on the topsoil layers (0-40 cm). The SNS in the upper 0-40 cm soil layer accounted for over 50% of the total soil N stocks and the accumulation revealed decreasing tendencies with an increase of soil depth in all five stands. The soil organic carbon concentration and soil particle size fractions (i.e., soil silt concentration) were detected as the leading environmental parameters modulating the SNS variance. Our findings illustrate that considerations of N sequestration potential in Acacia hybrid plantations must take stand age into account and offer significant information for a better comprehension of the key environmental parameters driving SNS in Southern Vietnam.