Changes in soil microbial and nematode communities during ecosystem decline across a long-term chronosequence

Following the creation of new land surfaces, there is an initial build-up phase, but in the prolonged absence of catastrophic disturbance an ecosystem decline phase has often been observed. While a number of studies have investigated the changes in soil biota that occur during the build-up phase, few studies have investigated how the soil food web changes during the ecosystem decline phase, even though such studies may assist our understanding of biotic factors that contribute to long-term ecosystem changes. We investigated the response of soil microbial and nematode communities to ecosystem decline by studying each of four stages of a long-term (280,000 year) forested chronosequence caused by uplift of marine terraces in the Waitutu region of Fiordland National Park, New Zealand. With increasing chronosequence age there were large increases in ratios of C to N, C to P, and N to P in both the organic layer and mineral soil layer, indicative of greater nutrient (notably P) limitation over time. Variables related to soil microbial biomass and activity were lower on the older terraces when expressed on a per unit soil C basis, reflecting that the quality of the soil organic matter, which is the resource that supports microbial metabolism, declined over time. This in turn had important consequences for population densities of soil nematodes and enchytraeids, including both microbe-feeding and predatory groups. There were significant increases in the fungal:bacterial biomass ratio and in the fungal-feeding:bacterial-feeding nematode densities. Taken collectively, our results suggest a decline in soil microbial activity and soil fauna, and an increase in the relative importance of the fungal-based (vs. bacterial-based) energy channel during long-term ecosystem development on terraces of marine origin. This corroborates the hypothesis that the studied sites represent a retrogressive shift in soil organic matter quality over a long-term chronosequence.