Soluble Organic Nitrogen Pools in Forest soils of Subtropical Australia

Soil soluble organic N (SON) plays an important role in N biogeochemical cycling. In this study, 22 surface forest soils (0–10 cm) were collected from southeast Queensland, Australia, to investigate the size of SON pools extracted by water and salt solutions. Approximately 5–45 mg SON kg⁻¹, 2–42 mg SON kg⁻¹ and 1–24 SON mg kg⁻¹ were extracted by 2 M KCl, 0.5 M K₂SO₄ and water, on average, corresponding to about 21.1, 13.5 and 7.0 kg SON ha⁻¹ at the 0–10 cm forest soils, respectively. These SON pools, on average, accounted for 39% (KCl extracts), 42% (K₂SO₄ extracts) and 43% (water extracts) of total soluble N (TSN), and 2.3% (KCl extracts), 1.3% (K₂SO₄ extracts) and 0.7% (water extracts) of soil total N, respectively. Large variation in SON pools observed across the sites in the present study may be attributed to a combination of factors including soil types, tree species, management practices and environmental conditions. Significant relationships were observed among the SON pools extracted by water, KCl and K₂SO₄ and microbial biomass N (MBN). In general, KCl and K₂SO₄ extracted more SON than water from the forest soils, while KCl extracted more SON than K₂SO₄. The SON and soluble organic C (SOC) in KCl, K₂SO₄ and water extracts were all positively related to soil organic C, total N and clay contents. This indicates that clay and soil organic matter play a key role in the retention of SON in soil.