Responses of soil carbon, nitrogen, and phosphorus stoichiometry to afforestation of severely desertified land in northern China

Large-scale afforestation is being implemented globally to promote carbon sequestration in ecosystems, address climate change, and restore degraded ecosystems. However, the impacts of trees and shrubs on soil stoichiometry in severely desertified land remain unclear. We investigated the topsoil (0 − 20 cm) carbon, nitrogen, and phosphorus (C:N:P) stoichiometry of Pinus sylvestris var. mongolica and Caragana microphylla plantations established on mobile sand dunes in northern China. Soil organic carbon (SOC, 2.6 to 11.3 g kg⁻¹) and total nitrogen (TN, 0.3 to 1.2 g kg⁻¹) concentrations increased by 3.2 to 6.5 times the values in mobile dunes, on average, after 40 years of afforestation. Total phosphorus (TP, 0.1 to 0.2 g kg⁻¹) in the 40-year-old P. sylvestris and C. microphylla plantations increased by 15.1 and 24.3 %, respectively. Well-constrained C:N ratios (7.7 to 10.1 on average) indicated strong coupling relationship between SOC and TN. The average C:P (7.2 to 43.8) and average N:P (0.8 to 4.4) ratios increased with increasing afforestation age due to disproportionate changes in the three elements, but the rate of increase under C. microphylla decreased. N:P ratios under P. sylvestris increased with increasing afforestation age, indicating that long-term P. sylvestris afforestation may become P-limited. Environmental factors explained 75 % of the total variation of soil C:N:P stoichiometry. The improved plant diversity, productivity, and soil physical properties after afforestation improved the soil C:N:P stoichiometry. Climate and terrain had negative impacts on this stoichiometry. We suggest combining multiple species for afforestation to improve species diversity and providing supplemental P in later stages of afforestation to counteract the future P limitation. Our findings provide new knowledge on the relationships between afforestation and soil stoichiometry in severely desertified areas, and provide a reference for optimizing afforestation.