Variation in soil net mineralization rates with dissolved organic carbon additions

Northeastern forest are generally considered to be nitrogen limited, even though soil microbial populations may in fact be limited by the amount of carbon required as an energy source. Field studies have shown that with increasing N availability, net mineralization rates increase initially but then decrease over time. One possible reason for a reduction in net mineralization rates is that labile soil carbon, such as dissolved organic carbon (DOC), becomes depleted. Five-week laboratory incubations of organic and mineral soil samples with different carbon amendments were performed in order to determine the effect of carbon quality on potential net nitrification and net mineralization rates. Net nitrogen immobilization was greater in soils incubated with glucose amendments than those incubated with more complex carbon compounds such as humic acid. Although this resulted in lower net mineralization rates overall, net immobilization is an indicator of immediate microbial response to the carbon substrate. Net nitrification was not detected, likely due to the short duration of the incubations. Measurements of gross mineralization rates may be necessary to see an overall effect of carbon substrate on N cycling in these soils.