Canopy and understory nitrogen additions differently affect soil microbial residual carbon in a temperate forest

Atmospheric nitrogen (N) deposition in forests can affect soil microbial
growth and turnover directly through increasing N availability and
indirectly through altering plant-derived carbon (C) availability for
microbes. This impacts microbial residues (i.e., amino sugars), a major
component of soil organic C (SOC). Previous studies in forests have so far
focused on the impact of understory N addition on microbes and microbial
residues, but the effect of N deposition through the plant canopy, the
major pathway of N deposition in nature, has not been explicitly explored.
In this study, we investigated whether and how the quantities (25 and 50
kg N ha-1 yr-1) and modes (canopy and understory) of N addition affect
soil microbial residues in a temperate broadleaf forest under 10-yr N
additions. Our results showed that N addition enhanced the concentrations
of soil amino sugars and microbial residual C (MRC) but not their relative
contributions to SOC, and this effect on amino sugars and MRC was closely
related to the quantities and modes of N addition. In the topsoil, high-N
addition significantly increased the concentrations of amino sugars and
MRC, regardless of the N addition mode. In the subsoil, only canopy N
addition had a positive effect on amino sugars and MRC, implying that the
indirect pathway via plants plays a more important role. Neither canopy
nor understory N addition significantly affected soil microbial biomass
(as represented by phospholipid fatty acids), community composition, and
activity, suggesting that enhanced microbial residues under N deposition
likely stem from increased microbial turnover. These findings indicate
that understory N addition may underestimate the impact of N deposition on
microbial residues and SOC, highlighting that the processes of canopy N
uptake and plant-derived C availability to microbes should be taken into
consideration when predicting the impact of N deposition on the C
sequestration in temperate forests.