Age-dependent predictors of soil multifunctionality in larch plantations: roles of nutrients and microbial networks

Understanding how stand development is linked to soil multifunctionality (SMF) is essential for sustaining ecosystem functions in plantation forests.

In this study, five stand age gradients (21, 26, 32, 37, and 47 a) of larch (Larix principis-rupprechtii) plantations were analyzed in northern China. SMF was quantified using a multithreshold approach, and a structural equation model was employed to resolve pathways through which stand age influences SMF via soil physicochemical properties, microbial diversity, and co-occurrence network.

Soil organic carbon (SOC) and total nitrogen (TN) accumulated during early (21 and 26 a) to mid-aged stages (32 and 37 a). Specifically, SOC increased from 31.37 to 41.06 g·kg^-1, while TN increased from 2.29 to 3.20 g·kg^-1, before declining slightly in mature stands. Soil pH decreased from 6.67 to 6.31 along the stand age gradient, indicating intensified soil acidification. Microbial α-diversity increased from early to mid-aged stands and declined in mature (47 a) stands, with bacterial Chao1 index peaking at 37 a (P < 0.01) and fungal Chao1 index peaking at 32 a (P < 0.01). In contrast, microbial co-occurrence networks exhibited a continuous increase in structural complexity. SMF peaked in mature stands and was positively associated with SOC, TN, microbial α-diversity, and network complexity, but negatively associated with soil pH. Stand age influenced SMF primarily through indirect pathways. Specifically, SOC improved SMF mainly by promoting microbial α-diversity, whereas TN contributed primarily by strengthening bacterial network complexity. In contrast, soil pH exerted a persistent negative constraint on SMF by limiting microbial turnover. Notably, microbial network complexity explained variation in SMF more effectively than microbial diversity alone, particularly at higher multifunctionality thresholds.

Overall, SMF in larch plantations was linked to a hierarchical, multi-pathway cascade in which soil nutrient dynamics structured microbial interaction networks, thereby amplifying functional integration, and informing stand age-specific management strategies.