Distribution of organic carbon and nutrients in soil aggregates under different stand types of Cunninghamia lanceolata in southern Guangxi of China

Revealing the dynamics of soil organic carbon (C-org) and nutrients at aggregate scales is essential for improving our understanding of soil C-org mitigation and nutrient restitution in the Cunninghamia lanceolata plantations. In this study, soil C-org, total nitrogen (N-tot), available phosphorus (P-ava), and exchangeable cations (including calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+)) received the analysis in aggregate fractions acquired from 0 to 10 cm and 10-20 cm depths in the three different stand types, namely mixed stands of Cunninghamia lanceolata and Michelia macclurei (stand A), Cunninghamia lanceolata and Mytilaria laosensis (stand B), and pure stand of Cunninghamia lanceolata (stand C). Soil aggregates were classified into macro-aggregates (>2 mm), meso-aggregates (2-0.25 mm), and micro-aggregates (<0.25 mm) by the dry-sieving process. The two mixed stands displayed higher level of soil aggregate stability than the pure stand. Moreover, micro-aggregates acted as the main fractions that carried soil C-org, N-tot, and P-ava, and both micro-aggregates and macro-aggregates referred to the main fractions that carried exchangeable cations. As for the soil nutrient stocks, only the exchangeable K+ stock of the pure stand dominated among the C-org and nutrient stocks, in addition, the rest of the other nutrient stocks of the mixed forests (stands A and B) took an advantage over the pure stand. Moreover, the C-org and nutrient stocks in stands A and B were reflected in the macro-aggregates, differently, those of stand C were mainly reflected in the micro-aggregates. Thus, selecting suitable broadleaf tree species mixed with Cunninghamia lanceolata can alleviate the reduction of soil aggregate stability and the loss of soil nutrients, thus promoting soil resources to be sustainably utilized and protecting soil quality and health in southern Guangxi of China.