Contrasting contributions of microbial and plant-derived C to soil carbon in desertified grassland restoration

Abstract

Grassland restoration can greatly increase soil organic carbon (SOC). However, the SOC dynamics and its compositional changes during grassland restoration are not fully understood. We investigated the relative contributions of microbial- and plant-derived C to SOC in both topsoil (0–10 cm) and subsoil (10–20 cm) over a 27-year restoration chronosequence in desertified grasslands, using resampling at the same site. We used amino sugars as biomarkers for microbial-derived C, and light fraction organic carbon (LFOC) and particulate organic carbon (POC) as indicators for plant-derived C. Grassland restoration increased SOC from 3.8 to 8.1 g kg⁻¹ in topsoil and from 2.1 to 4.1 g kg⁻¹ in subsoil. Concurrently, microbial necromass carbon (MNC), LFOC, and POC also increased in both soil layers. The proportion of SOC derived from MNC decreased as restoration progressed, while the contribution of LFOC increased, likely due to the continuous input of root biomass and litter after grazing exclusion. These findings support the emerging view that plant-derived C plays a key role in SOC accumulation during the later stages of grassland restoration. Thus, SOC appears less stable and more susceptible to disturbances as restoration progressed, indicating potential risks for C loss. Fungal necromass C was 2.4–2.9 times bacterial necromass C and contributed more to SOC. Random Forest models and correlation analyses revealed that Fe and Al oxides, LFOC, and the SOC/total phosphorus (TP) ratio were key regulators of microbial-derived C in topsoil. Plant-derived contribution to SOC depended primarily on total nitrogen, restoration duration, and TP. We conclude that both microbial- and plant-derived C increased during grassland restoration, but their relative contributions to SOC followed contrasting patterns. These varying C sources during restoration have important implications for SOC stability and vulnerability, and can guide future management strategies for optimizing C sequestration in dryland ecosystems.