Indices of soil nitrogen availability in five Tasmanian Eucalyptus nitens plantations

Abstract

Several soil analyses were used to estimate available N in surface soils (0–10 cm) over a 2-year period at 5 sites that supported 1- to 4-year-old Eucalyptus nitens plantations, and once in subsoils (10–120 cm) at 3 of these sites. Soils were derived from basalt (1 site previously pasture, 1 Pinus radiate, and 2 native forest) or siltstone (previously native forest). Soil analyses examined were total N, total P, total C, anaerobically mineralisable N (AMN), hot KCl-extractable N (hot KCl-N), and NH4+ and NO3– in soil solution and KCl extracts. AMN, KCl-extractable NH4+ and NO3–, and soil solution NH4+ and NO3– varied considerably with time, whereas hot KCl-N, total N, total P, and total C were temporally stable except for a gradual decline in total C with time at one site. Only total P was correlated with net N mineralisation (NNM) across all sites (r2 = 0.91, P < 0.05, n = 5). At 2–3 years after planting, soil solution and KCl-extractable NO3– dropped below 0.1 mm N and 1 μg N/g soil, respectively, at sites with NNM ≤24 kg N/ha.year (n = 3). Sites with NNM ≤24 kg N/ha.year also had ≤0.8 Mg P/ha. Although concentrations of indices of soil N availability decreased with depth, the contribution of subsoil (10–120 cm depth) to total profile N availability was estimated to be at least twice that of the top 10 cm. At an ex-pasture site, high concentrations of mineral N were found at 75–105 cm depths (KCl-extractable N, 289.3 μg N/g soil; 2.8 mm mineral N in soil solution), which may have become available to plantations as their root systems developed.