Abstract: It remains unclear how soil microbial metabolism responds to organic input and the driving factors during soil carbon fixation and fertilization in newly reclaimed farmlands. We conducted a field experiment to explore the effects of different organic inputs on soil nutrients, organic carbon fractions, extracellular enzyme activities, microbial metabolism, and microbial carbon utilization efficiency in a newly reclaimed farmland with a rice-wheat rotation in Jiande of Zhejiang in 2022. Five treatments were implemented with equivalent C return in addition to conventional chemical fertilizers (NPK): 1) NPK alone (control), 2) NPK + manure + maize straw (MS), 3) NPK + manure (M), 4) NPK + straw biochar-based manure (MBF), and 5) NPK + straw biochar (MB). The results showed that organic inputs significantly enhanced soil nutrients, soil organic carbon fractions, and microbial activity compared to NPK. In terms of soil nutrient improvements, the order was M > MBF > MS > MB, while crop yield followed the order of MS > M > MBF > MB. The active organic carbon contents followed the order of M > MS > MBF > MB, with a 91.7% increase in the M treatment compared with NPK. For recalcitrant organic carbon contents, the sequence was MB > MBF > M > MS, with a 160.7% enhancement in the MB treatment than NPK. The microbial biomass showed the order of M > MS > MBF > MB. Microbial biomass carbon, nitrogen, and phosphorus in the M treatment was increased by 81.1%, 140.9%, and 261.1%, respectively compared with NPK. Extracellular enzyme activities followed the order of MS > M > MB > MBF. The MS treatment increased C cycle-related enzyme activities (β-glucosidase, β-xylosidase, and β-cellobiohydrolase) by 176.3%, 180.4%, and 439.2%, respectively, and N cycle-related enzyme activity (N-acetyl-β-glucosaminidase) by 331.4% compared with NPK. Results of Mantel correlation analysis and redundancy analysis showed that dissolved organic carbon and microbial biomass carbon were the primary drivers of extracellular enzyme activities during the wheat and rice growing seasons. Enzyme vector models and partial least squares path modeling revealed that soil microbial metabolism in the newly reclaimed farmland was constrained by both carbon and phosphorus contents. Organic inputs alleviated phosphorus limitation by improving soil nutrient availability and decreased microbial carbon use efficiency by increasing active organic carbon content. In summary, organic inputs played a positive role in soil carbon fixation and fertilization in the newly reclaimed farmland. Among the treatments, MBF showed the best comprehensive effect on soil carbon fixation and fertilization.

Key words: newly reclaimed farmland, straw returning, stoichiometry, extracellular enzyme activity, microbial nutrient limitation, microbial carbon use efficiency