Soil aggregates indirectly influence litter carbon storage and release through soil pH in the highly alkaline soils of north China

BACKGROUND: Soil aggregate-size classes, structural units of soil, are the important factors regulating soil organic carbon (SOC) turnover. However, the processes of litter C mineralization and storage in different aggregates-size classes are poorly understood, especially in the highly alkaline soils of north China. Here, we ask how four different aggregate sizes influence rates of C release (C(r)) and SOC storage (C(s)) in response to three types of plant litter added to an un-grazed natural grassland. METHODS: Highly alkaline soil samples were separated into four dry aggregate classes of different sizes (2–4, 1–2, 0.25–1, and <0.25 mm). Three types of dry dead plant litter (leaf, stem, and all standing dead aboveground litter) of Leymus chinensis were added to each of the four aggregate class samples. Litter mass loss rate, C(r), and C(s) were measured periodically during the 56-day incubation. RESULTS: The results showed that the mass loss in 1–2 mm aggregates was significantly greater than that in other size classes of soil aggregates on both day 28 and day 56. Macro-aggregates (1–2 mm) had the highest C(r) of all treatments, whereas 0.25–1 mm aggregates had the lowest. In addition, a significant negative relationship was found between C(s)/C(r) and soil pH. After incubation for 28 and 56 days, the C(s) was also highest in the 1–2 mm aggregates, which implied that the macro-aggregates had not only a higher CO(2) release capacity, but also a greater litter C storage capacity than the micro-aggregates in the highly alkaline soils of north China.