<p>It has been assumed for a long time that stable soil organic carbon (SOC) results from selective preservation of plant residues. Yet, a new paradigm points to a more active role of microorganisms in building SOC storage. In this context, even labile C, such as sugars, may persist in soil for a long time due to their incorporation into microbial biomass and ultimately necromass. The latter is considered as a relatively stable pool. However, little is known about the cycling of labile C through the microbial biomass and the turnover time of its residues. Unraveling the mechanisms and regulating factors would be critical for understanding SOC stabilization in soil.</p><p>We assume that the fate of labile C is mainly driven by microbial nitrogen (N) demand and supply. Specifically, we hypothesize that (1) high N demand forces microbes to decompose N-rich substances (&#8220;microbial N mining&#8221;), such as amino sugars, leading to a rapid turnover of microbial necromass, and that (2) labile C is stabilized in microbial necromass when N demand is met.</p><p>To investigate these hypotheses, we set up a greenhouse pot experiment including four treatments: (1) bare soil, (2) bare soil+N, (3) tree, and (4) tree+N. The soil is a sandy and nutrient poor forest soil from southern Finland. Trees are 1 m high pines (Pinus Sylvestris), which are supposed to induce microbial N deficiency by exuding easily degradable C compounds and by competing with microbes for mineral N. In order to follow to fate of labile C, we added trace amounts of <sup>13</sup>C labeled glucose to the soil (4 replicates per treatment). As a control to account for background variations in <sup>13</sup>C, we added <sup>12</sup>C glucose to another set of pots (4 replicates per treatment). Up to now, we sampled the soil 1 day, 3 days, 8 days, 1 month, 3 months, 6 months, 9 months, and 1 year after glucose addition. Measurements of the <sup>13</sup>C recovery in soil, microbial biomass, water extractable C, PLFA, amino sugars, and DNA are in progress.</p><p>First results indicate that the largest loss of <sup>13</sup>C tracer occurred in the unfertilized tree treatment, i.e., where N demand was high but N supply was low. Here, only 22% of the <sup>13</sup>C glucose remained after 3 month, whereas 40% remained in the fertilized tree treatment. Only small proportions of the recovered <sup>13</sup>C were present in the pool of water extractable C (<1%) and in living microbial biomass (8&#177;3%, 3 days after glucose addition). As protection by clay minerals and aggregates is likely not a relevant process in this sandy soil, we suspect the remaining <sup>13</sup>C to be stabilized in microbial residues, but depending on N demand. We assume that microbial necromass accounts for a considerable proportion to total SOC storage, especially under conditions of adequate nitrogen supply.</p>
Dynamics of Soil Microbial N-Cycling Strategies in Response to Cadmium Stress