Effects of fire and harvesting on nitrogen transformations and ionic mobility in soils of Eucalyptus regnans forests of south-eastern Australia

Summary Effects of fire and forest harvesting on inorganic-N in the soil, on net N-mineralization, and on the leaching of NO_inf3^sup--N and metallic cations were measured in forests of Eucalyptus regnans following a severe wildfire in 1983. E. regnans regenerates profusely by seed after fire, and this study compared unburnt forest with forests burnt at varying intensities (surface fire and crown fire), and with logged and burnt forest (slash fire). Total inorganic-N in soil (0–5 cm) increased with increasing fire intensity to a maximum of 158 g g^-1 in the slash fire plot (compared with 51 g g^-1 in the unburnt forest) over the first 205 days after fire. Total inorganic-N returned to a concentration equal to that in the unburnt forest after 485 days at the slash fire plot, and after only 205 days at the surface fire plot. Studies of net mineralization in situ and of NO_inf3^sup--N in soil solution support the hypothesis that inorganic-N was immobilized in all of the burnt forests; microbial immobilization after fire is identified as a key process in N-conservation, limiting the substrate available for nitrification and thereby limiting the loss of N from the system by leaching. The concentrations of NO_inf3^sup--N and metallic cations in soil solution increased with increasing fire intensity. For the first 318 days after the fire, [NO_inf3^sup--N] in soil solution at 10 cm averaged 0.6 g ml^-1 in the unburnt forest, 9.7 mg l^-1 in the surface fire plot, 26 mg l^-1 in the crown fire plot, and 70 mg l^-1 in the slash fire plot. The concentration of metallic cations in soil solution was significantly correlated with [NO_inf3^sup--N], the observed order of mobility being Ca²⁺>Mg²⁺>K⁺>Na⁺. Processes which limit the production and persistence of NO_inf3^sup--N in soil solution following disturbance will significantly reduce nutrient losses or redistribution.