Positive and negative priming effects in an Ultisol in relation to aggregate size class and biochar level

Abstract

The priming effect in soils is known to depend in part on the size distribution and decomposability of aggregate-associated soil organic carbon. However, the biochar-induced carbon mineralization priming effects within aggregates of different size classes are poorly understood. In this study, the bulk soils were firstly separated into three fractions (>1 mm macroaggregate, 0.25−1 mm macroaggregate, and <0.25 mm microaggregate). Then, the bulk soils and different aggregate-size fractions were incubated for 180 days following the incorporation of biochar (C4 source) with two levels (2.5 % and 5.0 %). Sources of C decomposition (biochar derived C, primed C, and basal soil derived C) were partitioned. The percent of biochar-derived CO2 emission to the total mineralized C increased as the aggregate size decreased. The short-term positive priming effects appeared in the early stage for all three aggregate-size fractions. Predominantly negative priming effects were observed after 56 days of incubation in the bulk soils and>0.25 mm macroaggregates. However, a weak negative priming effect was only detected after 180 days at a high biochar level in the <0.25 mm microaggregates. Furthermore, much stronger repression of soil C mineralization was induced by a higher biochar level, with 10.1–53.4 % higher negative priming effects under 5.0 % biochar treatments than 2.5 % biochar treatments over 180 days. The results suggest that a greater portion of total readily decomposable soil organic C in >0.25 mm macroaggregates has probably been protected by biochar amended and made unavailable. Our study indicates that the priming effects of biochar on the soil organic carbon in Ultisols depend on the aggregate size.