Tree species and stand density: Effects on soil organic matter content, decomposability and susceptibility to microbial priming
Authors: Menyailo Oleg , Sobachkin Roman , Makarov Mikhail , Cheng Chih-Hsin 
Published: March 2022

&lt;p&gt;Forest stand density has been shown to have different albeit small effects on soil carbon. We hypothesized that the absence of density effect on soil carbon (C) storage can be explained by a loss of old soil C. This replacement of old by fresh C results in zero net C sequestration by soils but could alter the quality of soil organic matter.&amp;#160;&amp;#160; We used one afforestation experiment in Siberia, in which three tree species (spruce, larch and Scots pine) were grown for the last 30 years at 18 levels of stand density, ranging originally from 500 to 125,000 stems per ha. We selected five density levels and studied C and nitrogen (N) contents in mineral soils at 0-5 cm depth. The age of soil C was measured under larch and spruce for three levels of density by radiocarbon (&lt;sup&gt;14&lt;/sup&gt;C) dating. In all soil samples we determined stability of soil organic matter (SOM) to mineralization (decomposability) and to elevated input of readily decomposable C &amp;#8211; glucose (primability).&amp;#160; Stand density affected soil C and N contents differently depending on the tree species. Only under spruce both C and N contents were increasing with density, under larch and pine the covariation was insignificant, while N tended even to decline with density increase.&amp;#160; With the &lt;sup&gt;14&lt;/sup&gt;C data we were able to show strong dilution of old SOM by fresh C derived from the trees, the effect was stronger with higher density. This provides first evidence that density increase increases the fractions of new C versus old C and this can happen without altering the total C contents like under larch. While stand density altered soil C and N contents only under spruce, it altered C decomposability under all tree species: with density increase the C decomposability (per unit of C) declined under spruce but increased under larch and pine. This is relevant to predicting C losses from forest soils with different tree species and densities.&amp;#160; Higher losses would occur under larch and pine with higher densities, but increase of density under spruce would reduce the C losses from soil. Furthermore, while no significant covariation of stand density with C primability was detected, we first observed strong tree species effects on C primability. Twice as much C is lost from soil under larch than under spruce and pine by equal addition of C-glucose.&amp;#160; This indicates that elevated C deposition from roots and exudates to soil as predicted due to elevated CO&lt;sub&gt;2&lt;/sub&gt; concentration would most strongly accelerate soil C turnover and C losses under larch than under spruce and pine. Overall, tree species altered the susceptibility of soil C to elevated C input and stand density had strong effect on the decomposability of SOM, which is important parameter of C stability. The effect of stand density is thus important to consider even if stand density does not affect total soil C content.&amp;#160;&lt;/p&gt;