Impacts of shrub encroachment on the fraction and stability of soil organic carbon of grassland on the Loess Plateau， and the underlying microbial mechanisms

doi:10.11686/cyxb2025034

Abstract

Abstract:

Shrub encroachment is significantly affecting carbon cycling in grassland ecosystems worldwide， yet its impact on the components and stability of grassland soil organic carbon， and particularly the underlying microbial regulatory mechanisms， remains insufficiently studied. In this study， we focused on the semi-arid grassland of the central Loess Plateau， and analyzed the effects of shrub encroachment on various soil physicochemical and biological properties， organic carbon components and their proportions， microbial community structure， functions， and trophic types， as well as the underlying microbial regulatory mechanisms. The results show that shrub encroachment significantly increased the total organic carbon， dissolved organic carbon， mineral-associated organic carbon （MAOC）， and particulate organic carbon （POC） contents in soil， and the relative proportion of MAOC， but significantly decreased the relative proportion of POC. Furthermore， shrub encroachment significantly altered the bacterial and fungal community composition by affecting soil physicochemical properties， with different impacts on the relative abundance of amplicon sequence variants of different phyla. Shrub encroachment enhanced soil bacterial nitrification， sulfur oxidation， and iron respiration functions， as well as the abundance of fungal saprotrophic and woody saprotrophic type functions， but reduced the abundance of bacterial methylotrophy and aromatic compound degradation functions. The soil microbial residue carbon and bacterial functions were significantly correlated with the contents and proportions of POC and MAOC. A partial least squares path analysis showed that extracellular enzyme activity and microbial residue carbon， related to carbon cycling， were the microbial factors directly regulating the contents and relative proportions of soil organic carbon components， whereas microbial community composition influenced these processes indirectly. These results indicate that shrub encroachment not only increases the total amount of soil carbon and the contents of soil organic carbon components in grassland soils， but also significantly enhances the stability of the soil carbon pool， with extracellular enzymes secreted by soil microbes and microbial residue carbon playing crucial regulatory roles. The findings of this study provide new information about the mechanisms by which microbes regulate organic carbon components in grassland soils under shrub encroachment. These results provide theoretical support for predicting future changes in the soil carbon pool of shrub-encroached grassland on the Loess Plateau.

Key words: semi-arid grasslands on the Loess Plateau, shrub encroachment, organic carbon fractions, soil microorganisms

Yu-du JING, Xiao-wei LIU, Ke LIANG, Jun-hao FENG, Qiang YU, Liang GUO. Impacts of shrub encroachment on the fraction and stability of soil organic carbon of grassland on the Loess Plateau， and the underlying microbial mechanisms[J]. Acta Prataculturae Sinica, 2025, 34(12): 1-15.

Figures/Tables 7

Table 1 Impacts of shrub encroachment on soil physicochemical and biological properties

土壤指标 Soil property	无灌木扩张 NSE	半灌木扩张 SSE	灌木扩张 SE
SWC （%）	14.80±1.01b	20.41±0.80a	21.26±1.18a
pH	8.18±0.01a	8.05±0.01b	8.06±0.02b
SOC （g·kg^-1） DOC （mg·kg^-1）	16.74±0.87c 51.99±2.06b	25.77±0.87b 72.12±4.16a	28.41±0.69a 74.88±2.70a
TN （g·kg^-1）	1.60±0.04b	2.94±0.17a	3.14±0.17a
TP （g·kg^-1）	0.58±0.01c	0.67±0.01b	0.77±0.02a
CA （g·kg^-1）	8.07±0.03b	8.16±0.10b	8.56±0.05a
BG （nmol·g^-1·h^-1）	15.22±0.91c	36.32±3.35a	29.43±1.28b
CBH （nmol·g^-1·h^-1）	9.69±0.60a	9.48±1.51a	10.38±0.17a

Fig.1 Impacts of shrub encroachment on the content and relative proportion of soil organic carbon fractions and microbial necromass carbon

Fig.2 Impacts of shrub encroachment on phylum-level community structures of soil bacteria and fungi

Fig.3 The phylogenetic trees of soil bacterial and fungal ASVs and the correlation between the ASVs and soil basic physicochemical properties

Fig.4 Impacts of shrub encroachment on soil bacterial functions and fungal trophic types

Fig.5 Analysis of the potential drivers of soil organic carbon fraction and the correlation between soil organic carbon fraction and soil microbial properties

Fig.6 The partial-least square path modeling for the regulatory mechanisms of shrub encroachment on soil organic carbon fractions and their ratio to soil total organic carbon

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