MICROBIAL C AND N IN REVEGETATED WHEAT-BELT SOILS IN WESTERN-AUSTRALIA - ESTIMATION IN SOIL, HUMUS AND LEAF-LITTER USING THE NINHYDRIN METHOD

Areas of wheatbelt soils in Western Australia are being replanted with non-local eucalypt species to mitigate the effects of land degradatio n. Microbial processes will play an important role in the future recov ery of these soils by re-establishing organic nutrient cycles in the a bsence of external inputs of fertilizers. The microbial biomass of min eral soils can be conveniently estimated from the release of ninhydrin -positive compounds (NPC) following fumigation with chloroform (Amato and Ladd, 1988) The reliability of the NPC method to estimate microbia l C and N contents of humus and leaf-litter materials, in addition to mineral soils, was examined using samples from sites in the Western Au stralian wheatbelt revegetated with eucalypts, under pasture or native Banksia woodland. Estimates of microbial C and N obtained using the N PC method were compared with those estimated by the substrate-induced respiration method, and oxidizable C and soluble N extracted after fum igation. Agreement between the methods to estimate microbial C and N w as linear and highly significant, with the similar regression relation ships obtained for mineral soil and organic samples. We concluded that the ninhydrin method can give a reliable estimate of biomass in organ ic materials as well as in mineral soil. Microbial C in our samples wa s 194-354 kg ha-1 with the ratio of microbial C-to-total C being withi n the range of values reported for other soil and organic materials. H owever, the amounts of microbial N in the soil, FH humus and litter of the revegetated ecosystems were high, particularly in relation to the total amount of N in the soil horizon. Microbial N ranged from 31 to 73 kg ha-1 and comprised 5.9-14.7% of the N in the soil-humus-litter e cosystem. Microbial C-to-N ratios were low and remarkably consistent ( 5.13 +/- 1.11) despite the wide C-to-N ratio of the soil and litter su bstrates. The microbial fraction forms a substantial pool (8.4-14.7%) of N in the revegetated and native woodland ecosystems with the potent ial to make a major contribution to nutrient cycling.