Local controls modify the effects of timber harvesting on surface soil carbon and nitrogen dynamics

Managing for structural complexity to enhance forest ecosystem health and
resiliency is increasingly incorporated in silvicultural treatments. High
spatial variability in stands managed for structural complexity could
obscure the effects of forest management on surface soils. Yet few studies
have assessed how within-stand variation in forest structure and other
local controls influence surface soil organic matter dynamics over time
following timber harvests. We used a stratified random sampling design to
capture variation in stand age, legacy structure, soil type, and
topography in a second-growth, oak-hardwood forest in the northeastern
U.S. We compared surface soil carbon and nitrogen content and availability
in 15 harvested stands managed to promote tree regeneration (n = 144
plots) and five unharvested controls (n = 48 plots). We also examined
changes over time since harvest in just the harvested stands using a
22-year chronosequence. Forest management strongly influenced surface soil
carbon and nitrogen dynamics. The timber harvests had lower soil carbon
and nitrogen, microbial biomass, and carbon mineralization but higher
nitrogen mineralization. These differences were more pronounced in the
drier, less fertile soil type than in more moist, fertile soils. Across
the 22-year chronosequence, topography, soil type, and downed woody
material density dictated the direction of changes in surface soil carbon
and nitrogen over time. Soil carbon and nitrogen accrued over time at
drier, higher elevation (~300 m) sites and under higher densities of fine
woody material but declined at lower elevations (~180 m) and under lower
fine woody material. Proximity to legacy trees was associated with higher
soil carbon and nitrogen concentrations and availability. Our findings
underscore the importance of silvicultural practices that retain
structural legacies and downed woody material in shaping surface soil
carbon and nitrogen dynamics over time. Our results also highlight how
accounting for spatial variation in local controls on soil carbon and
nitrogen, such as topography, can improve detection of changes from forest
management practices that increase spatial heterogeneity within stands,
such as irregular shelterwood and seed tree regeneration methods.