Nutrient use efficiency and utilisation in east coast Australian Eucalyptus forests

Abstract

Nutrient use efficiency (NUE) has been used as a method to evaluate the utilisation and processes of cycling of nutrient in forests. In publications, different methods have been used to calculate NUE, but as efficiency they are all expressed as ratios and these cover the basic areas of: (1) absorption from the soil and uptake, (2) efficiency in their requirement or utilisation of nutrients including physiological efficiency and (3) efficiency in the retention and internal re-utilisation of nutrients. Few studies address NUE based on complete nutrient cycling information but use indices based on litterfall or foliage nutrient concentrations. In this study different expressions of NUE were defined and evaluated for N and P using data on nutrient cycles on species in the genus Eucalyptus in regrowth and mature native eastern Australian forests. It has been hypothesised that NUE increases with decreasing nutrient availability however increasing such efficiency has a cost reflected in reduced productivity. The hypothesis was proven for all expressions of NUE correlating NUE against estimates of soil N or P availability but there were differences between coastal and tableland Eucalyptus forests. The level of significance varied for different types of NUE and in these ecosystems P was of greater significance than N. This reflected the importance of P in relation to productivity on many of the weathered soils or the limited value of the soil available N indices. It is suggested that the difference expressions of NUE differ in their significance with species so some will be relatively more efficient in terms of uptake, others in utilisation and others in redistribution. Only in extreme situations of nutrient availability do species have relatively high efficiency for all methods NUE calculation. It is proposed that these differences are of importance in site/species distribution and more importantly in mixed stands where they provide advantages either at different stages of stand development or after significant disturbances such as fire.