Stabilization of labile carbon in soil microbial biomass and necromass &#8211; A question of nitrogen deficiency?
Authors: Meyer Nele , Sietiö Outi-Maaria , Adamczyk Sylwia , Biasi Christina , Ambus Per , Mganga Kevin , Shrestha Rashmi , Kalu Subin , Martin Angela , Glaser Bruno , Karhu Kristiina 
Published: March 2020

&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;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 (&amp;#8220;microbial N mining&amp;#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.&lt;/p&gt;&lt;p&gt;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 &lt;sup&gt;13&lt;/sup&gt;C labeled glucose to the soil (4 replicates per treatment). As a control to account for background variations in &lt;sup&gt;13&lt;/sup&gt;C, we added &lt;sup&gt;12&lt;/sup&gt;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 &lt;sup&gt;13&lt;/sup&gt;C recovery in soil, microbial biomass, water extractable C, PLFA, amino sugars, and DNA are in progress.&lt;/p&gt;&lt;p&gt;First results indicate that the largest loss of &lt;sup&gt;13&lt;/sup&gt;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 &lt;sup&gt;13&lt;/sup&gt;C glucose remained after 3 month, whereas 40% remained in the fertilized tree treatment. Only small proportions of the recovered &lt;sup&gt;13&lt;/sup&gt;C were present in the pool of water extractable C (&lt;1%) and in living microbial biomass (8&amp;#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 &lt;sup&gt;13&lt;/sup&gt;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.&lt;/p&gt;