Birch hardwood fibers with kraft lignin as peat substitutes: Effects on plant growth and rhizosphere microbiome

• Birch hardwood fibers replaced up to 50% of peat in growing media. • Kraft lignin enhanced the efficacy of growing media at minimal concentrations. • Birch hardwood fibers with kraft lignin enriched microbial decomposers. • The chemistry of growing media and the microbiome jointly impacted plant growth. Research on peat alternatives for horticultural growing media used in containerized plant production has gained interest due to greenhouse gas emissions linked to peat extraction and the resulting EU and German national goals. We evaluated birch hardwood fibers as partial peat substitutes and, for the first time, investigated the effects of incorporating kraft lignin into birch hardwood fibers on nutrient dynamic, plant performance, and rhizosphere microbiome. Two independent greenhouse experiments were conducted using basil ( Ocimum basilicum ) and marigold ( Tagetes patula ) in growing media containing 0–100% birch hardwood fibers, with or without the addition of kraft lignin, compared with white peat. Growing media containing 30–50% birch hardwood fibers supported plant development comparable to or exceeding that observed in peat, particularly for basil. The 100% birch hardwood fibers significantly inhibited development in both plant species. The addition of 2% kraft lignin enhanced growth of basil and marigold, while higher concentrations inhibited germination. Nutrient analysis revealed pronounced nitrogen immobilization in birch hardwood fiber-rich growing media, irrespective of kraft lignin concentration. Rhizosphere bacterial, archaeal, and fungal community composition differed significantly between peat and birch hardwood fiber growing media. Birch hardwood fibers with kraft lignin increased the relative abundance of several lignocellulolytic taxa. Nevertheless, these shifts were not consistently associated with improved nutrient availability or plant performance. Overall, birch hardwood fibers, particularly when supplemented with low concentrations of kraft lignin, can replace up to 50% of peat in horticultural growing media. Nonetheless, nitrogen immobilization and the dose-dependent effects of kraft lignin remain important limitations.