Afforestation of severely desertified land in semi-arid areas promotes soil carbon and nitrogen accumulation through microbial necromass

Afforestation is an important measure used to expand carbon (C) sequestration and mitigate climate change. However, the impact of soil microbial necromass on soil C and nitrogen (N) sequestration still remain unclear following afforestation of severely desertified land. We investigated 10-, 20-, and 40-year-old Caragana microphylla and Pinus sylvestris var. mongolica plantations (±2 years) established on mobile dunes in northern China, taking mobile dunes and natural grassland and forest as references. We then collected topsoil (0 − 20 cm) samples and measured amino sugar. Afforestation would ideally increase soil organic carbon (SOC) and total nitrogen (TN) storage by 26.3 and 2.5 Tg, respectively, after 40 years based on the baseline of the year 2000. It would take more than 110 years to realize SOC and TN restoration, taking natural grassland as a reference. Microbial necromass were highest in natural ecosystems, followed by plantations and mobile dunes. Afforestation promoted the accumulation of microbial necromass, leading to an increase in SOC and TN. Microbial necromass contributed more to TN (24.4 to 49.5%) than to SOC (22.3 to 41.0%). Fungal necromass were approximately four times greater than bacterial necromass. Improving soil physical properties and mitigating N limitation are potential effective methods for increasing microbial necromass. The composite structural equation model reveals the mechanism by which microbial necromass promote soil carbon sequestration after afforestation in severely desertified land in semi-arid areas.