EFFECT OF SUPERPHOSPHATE ADDITION ON N-MINERALIZATION IN SOME AUSTRALIAN FOREST SOILS

Pinus radiata stands are reported to accumulate increased N on addition of phosphatic fertilizers. Field and laboratory studies were initiated to determine if addition of superphoshate increases soil N mineralization in forest soils. In a field experiment, application of 200 kg P ha-1 as superphosphate to trenched plots in a Pinus radiata stand increased accumulated soil mineral-N contents by 122% and 82% above the control on two occasions. Application of 500 kg P ha-1 as superphosphate either alone or in combination with lime (10 Mg ha-1) increased in situ soil net N mineralization in a dry sclerophyll eucalypt forest from 20.7 (control) to 28.3 (+P) and 30.2 (+P+lime) kg N ha-1 yr-1 20 cm-1. Uptake by the vegetation accounted for all of the mineralized nitrogen. In a 180 day laboratory incubation using undisturbed soil columns (0-20 cm) from nine forest sites, seven soils showed a positive response to superphosphate (100 kg P ha-1) addition, with increases in N mineralization ranging from 14% to 117%. The response of N mineralization to superphosphate addition was not related to initial soil organic C, total N or P content in the 0-20 cm layer. However, soil pH and organic C combined to account for 76% of the variation in N mineralization response (P < 0.01). In the 0-5 cm layer of untreated soils, soil organic P content could explain 71% of the variation in net N mineralization. Addition of superphosphate appears to increase N mineralization in several Australian forest soils and the improved availability of N will enhance tree growth rates. The mechanisms underlying the response are still poorly understood and thus also our ability to predict its significance for tree nutrition on specific forest sites.