Long-term effects of soil organic matter burial on carbon sequestration

Abstract

Subsoils play an important role within the global C cycle, since they have high soil organic carbon (SOC) storage capacity due to generally low SOC contents. However, measures for enhancing SOC storage, such as no-tillage, agroforestry and application of animal manure, commonly focus on topsoils. In this dissertation, the option of actively burying SOC-rich soil material through tillage practices was studied for the first time assessing SOC sequestration in deep ploughed arable and forest soils as well as in fossil ridge and furrow cropland under forest with buried topsoils. Assessed deep-ploughed cropland soils contained on average 42±18% more SOC than the reference plots down to 100 cm. In contrast, total SOC stocks in deep ploughed forest soils were not significantly higher than in their respective reference soils. The ‘newly formed’ topsoil of the deep ploughed soils still contained less SOC than the reference topsoil, 14% and 37% under cropland and under forest, respectively. On average, the fraction of mineralised SOC assessed in one-year laboratory incubations, was 32% lower in incubated buried than in reference topsoils. Sandy cropland buried topsoil stripes and reference topsoils had the lowest mineralisable SOC fraction. Buried SOC was partly to fully preserved, most effectively in sandy cropland soils, but selective preservation of certain, most stabile SOC fractions and preferential decomposition of labile fractions, such as the commonly most easily decomposable free light fraction, could not explain SOC stability. Land use history of many of the studied sandy sites as heath was identified as an important aspect concerning SOC stabilisation. Medieval ridge-and-furrow cultivation led to the burial of former topsoil under ridges. SOC content was 0.4-0.9 g kg-1 higher at ridges than at reference positions, indicating long-term preservation of former topsoil SOC by burial under ridges, although enhanced SOC stocks at ridges due to carbon burial could not be significantly confirmed for all sites. In conclusion, deep ploughing led to enhanced subsoil SOC contents and stocks compared to a reference plot in forests and cropland. Deep ploughing increased the storage space for SOC by mechanically translocating topsoil material into the subsoil. SOC accumulation in the new topsoil horizons additionally determine the effect of deep ploughing on full profile SOC stocks.