Indices for characterising spatial variability of soil nitrogen semi-arid grasslands of Northwestern Australia

Within-site heterogeneity of soil N in arid and semi-arid grasslands is often less pronounced than in shrublands and, depending on the index used, can be difficult to detect. We compared biological and chemical indices of soil N from two perennial tussock grasslands of similar soil type in sub-tropical, northwestern Australia. We aimed to add to the minimal information on spatial heterogeneity of soil N in Australian arid environments and to identify indices suited to routine analysis in monitoring and rehabilitation programs in Australiaʹs rangelands. Soils were collected from two sites per grassland type at the end of spring (dry season) from immediately adjacent to tussocks and open areas between tussocks and from 0–2 and 2–5 cm depth. Amounts of N mineralized in leached aerobic incubations (81 d, 40°C, 0–2 cm only) differed significantly between grasslands and between sampling positions (greater adjacent to tussocks) and ranged from 88 to 158 μg g−1. Only NO3−-N was produced during aerobic incubations and was about 20% of total N, irrespective of grassland and sampling position. In contrast, N produced during anaerobic incubations (7 d, 40°C) and by extraction in hot KCl was less than 1% of total N. Both of these indices were poorly correlated with others and were either highly variable or did not detect a pronounced depth effect. Oxidisable C and absorbance260 of 10 mM NaHCO3 extracts were better correlated with our aerobic index of N availability but relationships differed between grasslands. We recommend either absorbance260 of mild-salt extracts (soils leached and then aerobically incubated) or total N for relative estimates of N mineralization potential in hot, arid environments where soils are routinely `field-dryʹ. Strong correlations between variables in this study indicate that heterogeneity in relatively undisturbed, semi-arid soils of similar mineralogy and history is of sufficient magnitude for identifying appropriate indices of N availability.