Effects of micronutrient fertilization on soil carbon pools and microbial community functioning

Fertilization with macronutrients, e.g., nitrogen (N) and phosphorus (P), has wide-ranging and well-characterized effects on terrestrial ecosystem functions. In contrast, influences of other elements, e.g., sodium (Na), calcium (Ca), and potassium (K), hereafter, ‘micronutrients’, are poorly understood. To determine effects of macronutrients, micronutrients, and their interactions on soil carbon (C) cycling, we conducted a full factorial experiment where treatments consisted of fertilizing experimental plots with Na, Ca, and K (each manipulated individually) and macronutrients (N + P added together). Each year of the experiment we measured aboveground plant biomass and sampled soils to quantify mineralizable C pools and active microbial biomass. To further explore micronutrient effects on microbial-mediated soil C cycling, in the final year of the experiment we assessed microbial C cycle functions using a catabolic profiling technique and quantified multiple additional soil C pools: particulate (POM-C), mineral-associated (MAOM-C), and total (TC). Macronutrients (N + P) affected nearly all measured variables – when N + P was added, active microbial biomass increased, catabolic profiles were altered, and mineralizable C, POM-C, and TC pools all increased. Micronutrients also had effects on soil C cycling that were generally smaller than effects of macronutrients – Ca addition reduced mineralizable C pools relative to the other treatments and K addition increased POM-C and TC pools. Further, Ca + K addition altered microbial catabolic profiles, but only when N + P was also added. Our study reveals several influences of micronutrients on soil C cycling – specifically, that Ca and K may influence the size of soil C pools by influencing the C cycle functions of soil microbial communities.