Land use affects temperature sensitivity of soil organic carbon decomposition in macroaggregates but not in bulk soils in subtropical Oxisols of Queensland, Australia

Abstract

The turnover of soil organic carbon (SOC) under different land uses plays an important role in terrestrial carbon (C) cycling. However, the effects of warming on C decomposition dynamics have not been resolved due to seemingly inconsistent results among different experiments. Hence, the objective of this work was to assess soil aggregate-associated C and temperature sensitivity of SOC decomposition within bulk soils and aggregates under two contrasting land management systems in subtropical southern Queensland.We examined the temperature sensitivity of SOC decomposition (Q10) and activation energy required for soil C mineralization (Ea) from bulk soils (<8 mm), large macroaggregates (2−8 mm) and small macroaggregates (0.25−2 mm), as well as key C and N cycling enzyme activities in the surface (0−10 cm) soils of an Oxisol. The two land uses studied were: undisturbed rainforest and 115-year-old kikuyu grass (Pennisetum clandestinum) pasture. Bulk soils as well as their large and small macroaggregates were separately incubated at two temperatures (17 °C and 27 °C) for 124 d. We observed that the pasture soils had more small macroaggregates but less microaggregates than the forest soils. However, all fractions in the forest contained significantly higher SOC than those in the pasture. Potential soil enzyme activities per unit SOC within bulk soils and macroaggregates for N-acetyl β-glucosaminidase, α-glucosidase, and phenol oxidase were higher in the pasture than in the forest. The cumulative C mineralization values of bulk soils under the forest vegetation were 16 % higher than for pasture at 17 °C and 97 % higher than pasture at 27 °C. Cumulative C mineralization from large macroaggregates of both land uses were more than that from small macroaggregates. However, the Ea and Q10 values of bulk soils were similar for both the forest and pasture soils. For the large macroaggregates, the Ea value was ∼78 % higher for the pasture soil than for the forest soil, with the Q10 value also being 73 % higher in the pasture soil. However, the opposite was observed for the small macroaggregates, with Ea and Q10 values being higher in the forest soil. The contrasting nature of Q10 from large and small macroaggregates in Oxisols suggests that the feedback from SOC decomposition in response to changing temperature, that is, global warming is closely associated with soil aggregation.