Microbial C and N in revegetated wheatbelt 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 degradation. Microbial processes will play an important role in the future recovery of these soils by re-establishing organic nutrient cycles in the absence of external inputs of fertilizers. The microbial biomass of mineral 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 microbial C and N contents of humus and leaf-litter materials, in addition to mineral soils, was examined using samples from sites in the Western Australian wheatbelt revegetated with eucalypts, under pasture or native Banksia woodland. Estimates of microbial C and N obtained using the NPC method were compared with those estimated by the substrate-induced respiration method, and oxidizable C and soluble N extracted after fumigation. Agreement between the methods to estimate microbial C and N was linear and highly significant, with the similar regression relationships obtained for mineral soil and organic samples. We concluded that the ninhydrin method can give a reliable estimate of biomass in organic materials as well as in mineral soil. Microbial C in our samples was 194–354 kg ha −1 with the ratio of microbial C-to-total C being within the range of values reported for other soil and organic materials. However, 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 ecosystem. 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 substrates. The microbial fraction forms a substantial pool (8.4–14.7%) of N in the revegetated and native woodland ecosystems with the potential to make a major contribution to nutrient cycling.