Depth patterns of gross nitrogen cycling and soil exoenzyme activities for three northern hardwood forests

Despite the enormous size of the organic nitrogen (N) pool contained
in mineral subsoils, rates of N cycling and soil exoenzyme activities
are rarely measured in soils below 10 or 20 cm depth. Furthermore,
assumed relationships between N mineralization rates and the
activities of various decomposition exoenzymes are poorly
characterized. We measured rates of gross and net N mineralization and
nitrification as well as the potential activities of hydrolytic and
oxidative enzymes at five soil depths (forest floor to 50 cm) in
Spodosols at three hardwood forests of varying age (45 and 100 years
post-harvest and old growth) at and near the Hubbard Brook
Experimental Forest in New Hampshire, USA. As expected, rates of N
cycling and potential enzyme activities per unit soil mass correlated
strongly with soil carbon (C) concentration, and these parameters
declined exponentially with increasing soil depth. After normalization
per unit soil organic matter, N cycling rates and specific enzyme
activities generally decreased little with depth within the mineral
soil. Gross N mineralization rates correlated with specific activities
of those enzymes that hydrolyze cellulose (β-glucosidase,
cellobiohydrolase) and N-rich glucosamine polymers
(N-acetylglucosaminidase), but not those that degrade protein or more
complex C compounds. Hence, gross N cycling appear associated with the
N released during microbial N recycling, rather than from
decomposition of soil organic matter. Across the three stands, the
youngest had a larger ratio of N- to-phosphorus-acquiring enzyme
activities, indicating a greater N demand in younger than older
forests. For all three stands, mineral soil below 10 cm contributed
30–53% of total gross and net N cycling per unit area to 50 cm depth.
Overall, even though microbial N cycling and enzyme activities per
unit soil mass decreased with depth, microbial processes in subsoils
contributed substantially to ecosystem-scale gross N fluxes because of
the sustained microbial activity per unit soil organic matter at depth
and the large size of the organic matter pool in the mineral soil.
These results support the inclusion of often-ignored mineral subsoils
and microbial N recycling in both ecosystem N budgets and in model
simulations, due to their contribution to soil N fluxes and the
importance of microbial N dynamics in forest stands.

An analysis if these data can be found in: Darby, BA, CL Goodale, NA
Chin, CB Fuss, AK Lang, SV Ollinger, GM Lovett. (2020) Depth patterns
and connections between gross nitrogen cycling and soil exoenzyme
activities for three northern hardwood forests.
https://doi.org/10.1016/j.soilbio.2020.107836

These data were gathered as part of the Hubbard Brook Ecosystem Study
(HBES). The HBES is a collaborative effort at the Hubbard Brook
Experimental Forest, which is operated and maintained by the USDA
Forest Service, Northern Research Station.