Gross nitrogen transformations in adjacent native and plantation forests of subtropical Australia

Abstract

The impact of land-use change on soil nitrogen (N) transformations was investigated in adjacent native forest (NF), 53 y-old first rotation (1R) and 5 y-old second rotation (2R) hoop pine (Araucaia cunninghamii) plantations. 15N isotope dilution method was used to quantify gross rates of N transformations in aerobic and anaerobic laboratory incubations. Results showed that the land-use change had a significant impact on the soil N transformations. Gross ammonification rates in the aerobic incubation ranged between 0.62 and 1.78 mg N kg-1 d-1 and were significantly lower in NF and 1R soils than in 2R soils. Gross nitrification rates ranged between 2.1 and 6.6 mg N kg-1 d-1 and were significantly higher in NF soils than in plantation soils. Similarly, rates of gross mineralisation measured in the anaerobic incubation were significantly higher in NF soils compared to plantation soils. Gross nitrification and mineralisation rates were highly correlated with soil C:N ratio (r = -0.66, P<0.01 for both gross nitrification and mineralisation) which in turn was highly correlated with root (r = 0.8315, P<0.01), litter (L) (r = 0.76, P<0.01), and fermentation (F) (r = 0.66, P<0.01) layer C:N ratios. Nitrification was the dominant soil N transformation process in the contrasting forest ecosystems. There is evidence to suggest that heterotrophic nitrifiers may undertake a significant portion of nitrification.