Abstract

Drying/wetting (D/W) and freezing/thawing (F/T) cycles can strongly influence the turnover of soil organic carbon (SOC). Both events can accelerate the particulate organic matter (POM) decomposition because of macroaggregate (>250 μm) disruption and consequent release of formerly protected POM fraction. We test this hypothesis by adding 14 C-glucose and 13 C-lignocellulose derived to loamy soil from a humid temperate ancient forest ( Araucaria araucana ). We applied four F/T (-18/20 oC) and four D/W cycles (-5.0 bars/field capacity), including a no cycle (No cycles) and a control soil (without substrate addition and cycles). The CO 2 efflux and priming effect (PE), the acceleration or retardation of soil organic carbon (SOC) mineralization shortly after fresh organic matter addition were determined weekly during 28 days of incubation. The density fractions (free, fPOM; occluded, oPOM; and heavy, Hf) from macro- (>250-μm), micro- (250-53 μm), and Silt+clay aggregates size (<53-μm) was evaluated. Both F/T and D/W decreased fPOM in the macroaggregate and the Silt+clay aggregate size, while in microaggregates, POM fractions remained constant. The oPOM differences were only recorded in the bulk soil. Freezing and thaw, however, released Hf decomposable organic C from Silt+clay aggregate size. After the first cycle, D/W lignocellulose derived CO 2 was always higher (50 mg kg -1 ) than F/T, and No cycles, having no differences amongst the last two. Glucose derived CO 2 increased sharply until day three, and thereafter, CO 2 steadily accumulate till day 28 with or without cycles. Both D/W and F/T induced a negative PE in both lignocellulose and glucose. However, F/T was the most depleted (-125 mg CO 2 kg -1 ) after four cycles. The negative PE was interpreted as microorganisms having a preferential use C substrate that could offset the native SOC lost by positive SOC balance (63- 134 mg kg -1 ). Here we propose a conceptual model (Fig. 1) where negative PE and the soil C balance are shown. Our results supported the idea that F/T and D/W are not equivalent disrupting events and D/W caused a large release of fPOM from disturbed macroaggregates.