Apparent microbial carbon use efficiency in the fertosphere: short-term responses to nitrogen fertilizer granule dissolution in tropical soil

Understanding short-term microbial responses to N fertilizer granule dissolution is essential for elucidating how C substrates are transformed within localized fertilizer–soil microsites, hereafter the fertosphere. We conducted a 20-d laboratory incubation to evaluate the mineralization and apparent microbial C use efficiency (aCUE) of four 14 C-labeled substrates — glucosamine, glucose, glycine, and malic acid — added to a tropical clayey soil treated with six granular N fertilizers: ammonium nitrate, ammonium sulfate, diammonium phosphate (DAP), potassium nitrate, struvite, and urea. Apparent microbial C use efficiency was operationally defined as the fraction of substrate label allocated to the slower-turnover pool of a double first-order exponential model, representing a kinetic proxy rather than a direct measure of biosynthetic efficiency. Nitrogen fertilizers stimulated 14 C mineralization of labile substrates, particularly urea, which induced the highest 14 CO 2 emission and the lowest aCUE, concurrent with a pronounced alkalinization (pH up to 8.9) driven by urea hydrolysis. In contrast, DAP and struvite increased glycine half-life and aCUE relative to the unfertilized control, consistent with relief of P limitation in this low-P Ferralsol. Glucose and malic acid were predominantly allocated to the slower (anabolic) pool, whereas glucosamine and glycine were primarily respired. The aCUE reduction was greater for N-containing than for C-only substrates. Overall, granular N fertilizers did not promote short-term retention of low-molecular-weight C substrates in the fertosphere and may instead accelerate their mineralization. These findings underscore that microsite-scale variations in aCUE can transiently modulate soil C turnover in fertilized tropical systems.