The efficiency of organic C sequestration in deep soils is enhanced by drier climates

Abstract

Accurate assessment of organic C sequestration in deep soils is crucial to C management and understand the role of deep-rooted vegetation in the C cycle. Trees in drylands usually develop roots to access deep water resources. Deep soils typically contain large stores of sequestrated C because the microbial activities that decompose C are limited and C turnover time is long. However, we know little about whether root water uptake can benefit organic C sequestration in deep soils and the effect of precipitation on organic C sequestration. To address this, we selected five sites along a precipitation gradient from 422 mm to 606 mm on China’s Loess Plateau, and collected soil samples down to 1000 cm to measure soil organic C (SOC) content and soil water content (SWC) in both apple orchards and arable lands. We found that SOC storage (SOCS) and soil water storage (SWS) of two vegetation types in 0–800 cm soil layers increased significantly with increasing mean annual precipitation (MAP). Apple orchards showed greater SOC sequestration, particularly in deep soils (200–1000 cm), across each precipitation gradient relative to the corresponding arable lands. The ΔSOCS (difference in SOCS between apple orchards and the corresponding arable lands) in deep soils gradually decreased as MAP increased, and ΔSOCS for MAP = 422 mm was almost twice as great as that for MAP = 606 mm. Moreover, the ratio of ΔSOCS/ΔSWS in deep soils significantly increased as MAP decreased in the interval 400–610 mm. This indicates that the efficiency of SOC sequestration in deep soils is enhanced in a drier climate. The findings here indicate that deep soils may contribute greatly to organic C sequestration, and may provide insights into the water-C relationships in deep soils.