关键词: 氮沉降, 养分限制, 生态化学计量, 落叶松次生林

Abstract: Exploring the effects of nitrogen (N) addition on the stoichiometric characteristics of rhizosphere and bulk soil, microorganisms, and extracellular enzymes can provide theoretical insights into the impacts of N deposition on soil carbon dynamics and nutrient limitation status. We conducted a field experiment in a secondary Larix gmelinii forest in the Greater Khingan Mountains to examined the effects of N addition on the stoichiometric characteristics of soil, microorganisms and extracellular enzymes, as well as extracellular enzyme activities. There were four treatments: control (CK), low N (LN), medium N (MN), and high N (HN), with the rate of 0, 25, 50, and 75 kg N·hm^-2·a^-1, respectively. The results showed that N addition significantly increased total soil organic carbon (SOC), total nitrogen (TN), SOC∶TN and SOC∶TP in rhizosphere soil, as well as TN, SOC∶TP and TN∶TP in bulk soil, but significantly decreased total phosphorus (TP) in bulk soil. SOC and SOC∶TN in bulk soil were significantly increased under LN and MN treatments. Nitrogen addition significantly increased microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), MBC∶MBN, MBC∶MBP, and MBN∶MBP in rhizosphere soil. In addition, microbial biomass phosphorus (MBP) in rhizosphere soil, as well as MBC, MBN, MBC∶MBN, MBC∶MBP, and MBN∶MBP in bulk soil significantly increased under LN and MN treatments. Enzyme activities related to carbon, nitrogen, and phosphorus acquisition, and extracellular enzyme nitrogen-to-phosphorus ratio (E_N∶P) in both rhizosphere and bulk soils were enhanced by N addition. Nitrogen addition significantly decreased the extracellular enzyme carbon-to-nitrogen ratio (E_C∶N) and carbon-to-phosphorus ratio (E_C∶P) in rhizosphere soil by 10.9%-17.6% and 7.0%-9.0%, respectively. E_C∶P in bulk soil increased significantly by 7.2%-7.4% under LN and MN treatments. Results of enzyme vector analysis showed that N addition alleviated carbon limitation in rhizosphere microorganisms and phosphorus limitation in bulk soil microorganisms. In rhizosphere soil, E_C∶N showed a linear negative correlation with MBC∶MBN and SOC∶TN, while E_C∶P showed a linear negative correlation with MBC∶MBP and SOC∶TP. In bulk soil, E_C∶P was positively correlated with SOC∶TP and MBC∶MBP. In summary, the stoichiometric characteristics of soil, microorganisms, and extracellular enzymes in rhizosphere and bulk soils responded differently to N addition. Our results would provide a theoretical basis for understanding the rhizosphere effects on forest soil nutrient cycling under N deposition.

Key words: nitrogen deposition, nutrient limitation, ecological stoichiometry, Larix gmelinii secondary forest