Plantation understorey legume functional groups enhance soil organic carbon sequestration by promoting species richness

Abstract

Despite evidence showing that plant diversity or tree mixtures increase carbon storage in experimental grasslands or mixed forests, the effects of understorey plant communities on changes in soil organic carbon (SOC) storage in planted forests remain unclear, especially for monoculture plantations. Therefore, based on field observations in monoculture plantations on the Loess Plateau in China, we investigated the direct effects of understorey species richness (USR) and plantation age on SOC storage, and identified the indirect effects of understorey legume species richness (ULSR) and soil moisture (SM) mediated by USR. Results indicated that plantation age and USR significantly increased SOC sequestration with almost equal effects. The changes in the relationship between USR and SOC storage varied along the water availability gradient, from significantly positive (in plots with SM ≤11.14%) to insignificant (in plots with SM >11.14%). We observed that ULSR could increase SOC sequestration by promoting USR, while SM had negative and positive effects on ULSR and USR, respectively. Our findings can provide insights into plantation management in semiarid regions: Thinning mature plantations and introducing understorey legumes can increase SOC sequestration and maintain water resource sustainability.

Effect of plantation age on plant and soil C:N:P stoichiometry in Kentucky bluegrass pastures

“…The C:N ratio in leaves in the increasing process of plantation ages is dependent on the changes in leaf C and N concentration from 2 to 9 years of plantation age. The change in C concentrations in leaves may be ascribed to the photosynthetic capacity of Kentucky bluegrass among different plantation ages (Wang C. et al, 2023). The higher plant density in the early stages of plantation often limits the spatial distribution of light (Aerts, 1996;Xu et al, 2023), wherein lower-height plants may not receive sufficient light for photosynthesis and carbon sequestration (Poorter et al, 2012), resulting in the lower leaf C in 2-3 years of plantation age.…”

“…In addition, Kentucky bluegrass uptakes soil nitrogen to supply plant growth, and the soil nitrogen can be gradually consumed with the increase in plantation age (Han et al, 2019). The decrease in soil nitrogen concentration is insufficient to meet plant requirements, leading to a decrease in leaf nitrogen concentration (Shi et al, 2015;Wang C. et al, 2023) when plantation age is below 6 years. However, when the plantation age is over 6 years, the accumulation of plant residues gradually increases soil nitrogen, and this added nitrogen can be released into the soil again (Hu et al, 2019;Zhang et al, 2021), which can provide relatively sufficient soil nitrogen for plant uptake, leading to an increase in leaf N when plantation age ranges from 6 to 9 years.…”

“…Therefore, these results indicated that leaves contribute to more N accumulation for Kentucky bluegrass plants during 2 to 3 years of plantation age, while roots contribute to more N accumulation for Kentucky bluegrass from 4 to 9 years of plantation age. A lower C:N ratio after 4 years of plantation age indicates that litter decomposition and nutrient return become fast (Ning et al, 2021;Wang C. et al, 2023), and this would be conducive to greater nutrient transfer to the soil from 4 to 9 years of plantation age.…”

“…Kentucky bluegrass (Poa pratensis), a perennial rhizomatous grass with high tolerance to environments, has become increasingly important (Pi et al, 2015;Wu et al, 2023) because of its ability to be productive in cool temperatures and high soil moisture conditions (DeKeyser et al, 2015;Zhu et al, 2023). This grass has been widely grown in Eurasia (Cui et al, 2020), Asia (Wang C. et al, 2023), and North America (Folck et al, 2023;Toledo et al, 2023) due to its unique ecological advantage: the relatively longer time intervals between tilling, which reduce the risk of soil erosion compared to annual grass pastures (Lindsey et al, 2020).…”