Does terracing enhance soil organic carbon sequestration? A national-scale data analysis in China.

Terracing practice is expected to reserve soil organic carbon (SOC) pools, which are the key components for maintaining soil fertility and land productivity. In China such practice is widespread from center to south since ancient time. In this work, to reveal the spatial-temporal variation characteristics of SOC under different terraced sites in China, we conducted a meta-analysis on 78 studies regarding terracing effects on SOC sequestration. The between-group heterogeneity analysis indicated that terracing land use, age, climatic background, and slope gradient were critical factors for SOC sequestration, while terracing structure and soil depth were not. Overall, for China's landscapes, terracing increased SOC sequestration by 32.4% on average. Relative to other terracing structures, level ditches and half-moon terraces receive more rainwater and fertile topsoil from upper slopes due to their specific concave structures. Terracing in those areas with lower temperatures and less precipitation showed higher SOC sequestration. The extent of SOC sequestration due to terracing was primarily determined by land use type. Reforestation terraces could increase SOC sequestration markedly by eliminating water erosion and related soil carbon loss. The terracing aged 1-2 years leads to a decrease of 6.4% averagely on SOC sequestration, likely because the breakdown of soil aggregates through soil excavation and redistribution improves the decomposition of SOC. Furthermore, terracing aged over five years was more effective on SOC sequestration. Since this study offered a useful synthesis on multiple terracing factors affecting soil carbon in China, it can help to provide a wiser utilization and management of terracing to maximize SOC sequestration, and to make better terracing practices in the context of global change.