Root-microbial interaction accelerates soil nitrogen depletion but not soil carbon after increasing litter inputs to a coniferous forest

Aims Net primary productivity is expected to increase in many forests as Earth warms, which can increase litter inputs to soils and affect carbon (C) and nitrogen (N) dynamics. Understanding how increasing litter inputs affect soil C and N cycling in tropical and subtropical forests is important because they represent some of the most productive ecosystems on Earth, suggesting that small changes in these cycles can have large effects. Methods To test the effects of increased litter inputs and the interactive effect between microbes and roots on soil C and N stocks and dynamics, we manipulated litter inputs and used trenching to exclude roots in a 40-year-old Cunninghamia lanceolata Lamb. (Chinese fir) plantation. At the site, we measured soil C and N pools, soil C-13 and N-15 natural abundance, and potential activities for C-, N-, and phosphorus-acquiring enzymes. Results After four years of experimental treatment, we found that increasing litter inputs reduced total soil N content by 26% relative to background litter inputs, but that increasing litter inputs did not affect soil C content in the plots with roots. In the plots without roots, both soil N and C did not change in response to litter inputs. In the plots with roots, soil delta N-15 increased with increasing litter inputs, but there was no effect in the plots without roots. We found a strong interactive effect between root and litter treatment on soil N pools and delta N-15. The decline in soil N pools and increase in soil delta N-15 were associated with elevated potential enzyme activity for N-acquisition (N-acetyl glucosaminidase). Conclusions Adding litter did not have a significant effect on soil C pools, likely because potential soil C losses were offset by increasing litter-derived C inputs. In contrast to C, adding litter decreased N availability, likely through multiple pathways including gaseous N losses, NO3- leaching, root N uptake, and interactions between saprotrophic microbes and roots during the four-year litter addition experiment. Global changes that increase litter production may lower N pools and imbalance C and N cycling in subtropical coniferous forest ecosystems.