Different Amounts of Added Litter Do Not Affect Long-Term Carbon Mineralization and Stabilization in Topsoils and Subsoils

Abstract

Background: Over half of global soil organic carbon (SOC) stocks are stored in subsoil. A larger fraction of carbon (C) input may be retained and stabilized in subsoil as compared to topsoil. This retained fraction determines C sequestration efficiency, which may also depend on the amount of C input. Aims: This study aimed to determine how litter decomposition and stabilization differ between topsoil and subsoil and how these processes respond to increasing amounts of added litter. Methods: A field incubation experiment was initiated at three forest sites (Braunschweig, Hohes Holz and Krofdorf). At each site, 13C-enriched pre-decomposed beech litter was added at doses of 2–64 g kg−1 of soil and buried in mesocosms in topsoils and subsoils. After 1, 2 and 4 years, samples were excavated and size-fractionated (<20 µm). Results: Different amounts of substrate addition did not affect the mineralization of this substrate or the formation efficiency of new fine fraction carbon (OCfine). Furthermore, soil depth had little impact, with 75% and 71% of the added substrate mineralized in the topsoils and subsoils, respectively, after 4 years. Substrate addition also did not significantly enhance the mineralization of native SOC. The lowest formation of new OCfine was observed at the Braunschweig site characterized by sandy soils, suggesting a potential link between soil texture and OCfine formation. Conclusions: The SOC formation efficiency is decoupled from the amount of C added; nevertheless, increasing the amount of C input will lead to a continuous linear rise in SOC stocks.