Differential Utilization and Allocation of Nitrogen Sources in Larix olgensis A. Henry Seedlings

Despite a plethora of studies in recent years focusing on the impact of nitrogen source addition on plant responses, there remains a lack of clarity regarding the differential utilization and distribution patterns of various nitrogen sources by Larix olgensis A. Henry seedlings. Specifically, the mechanisms by which ammonium nitrogen, nitrate nitrogen, and urea are differentially absorbed and distributed among different organs within the plant, as well as how these processes couple with rhizosphere soil microbial processes, still await elucidation. This study, conducted under field experimental conditions, employed a combination of 15N isotopic tracing, soil physicochemical property measurements, enzyme activity analysis, and microbial community functional analysis to investigate the effects of three nitrogen sources (NH4+, NO3−, and urea) and their varying addition levels on nitrogen absorption and distribution in Larix olgensis A. Henry seedlings. The results indicate that nitrogen source type significantly influences the nitrogen absorption rate and internal distribution patterns of plants. Within 24 h, seedlings preferentially absorb ammonium nitrogen and retain a higher proportion of newly absorbed nitrogen in their roots. The high ammonium chloride (GN) treatment group exhibited the highest 15N abundance in the root region, suggesting rapid root assimilation and short-term underground retention. By 48 h, the 15N abundance and AT% values in most organs across different treatment groups were significantly higher than those at 24 h, facilitating the transport of nitrate nitrogen and urea to stems and leaves, indicating a gradual shift in nitrogen distribution towards the aboveground parts. Moderate nitrogen addition improved soil nutrient conditions, altered pH and conductivity, enhanced nitrogen transformation processes related to urease and nitrate reductase, and increased microbial diversity and metabolic functions related to carbon metabolism, nitrogen metabolism, and energy metabolism. Soil pH, total nitrogen (TN), ammonium nitrogen (NH4+-N), and organic carbon (OC) are core environmental factors driving the differentiation of soil microbial community structure, with distinct specificity in the response of microbial groups across different taxonomic levels to soil physicochemical properties.