TY - JOUR
T1 - Long-term movement of 15N tracers into fine woody debris under chronically elevated N inputs
AU - Currie, William S.
AU - Nadelhoffer, Knute J.
AU - Colman, Benjamin
N1 - Funding Information:
The Harvard Forest is a site in the Long-Term Ecological Research (LTER) network sponsored by the US National Science Foundation. Elevations of experimental plots in the Prospect Hill tract studied here are 370–390 m; monthly mean temperatures are −7◦C in January and 19◦C in July. Precipitation averages 110 cm/yr, distributed fairly evenly throughout the year (Van Cleve and Martin, 1991). We worked in the two forest stands that are part of the ongoing Chronic N study (Aber et al., 1993, Magill et al., 1997). The first is an even-aged red pine (Pinus resinosa Ait.) stand planted in 1926 on land that was previously pastured and cultivated (Motzkin et al., 1999). The second is a predominantly oak (Quercus velutina Lam., Q rubra L., Betula lenta L., Acer rubrum L.) stand. The mixed oak forest is on land that was pastured in the 19th century, reverted to forest by 1860, has regenerated naturally from a mix of disturbances since, and is regionally representative (Aber et al., 1993; Foster 1992; Foster et al., 1992; Motzkin et al., 1999). Soils in both forest stands are coarse-loamy, mixed, frigid Typic Dystrochrepts. Soils are well drained and contain well-defined O horizons (mor type).
Funding Information:
This work was supported by the National Science Foundation, Ecosystems Studies Program (DEB 9815990). Toby Ahrens and Heidi Lux provided field assistance. Jodie Ackerman and Katie Kline provided technical assistance in the laboratory. We thank two anonymous reviewers for thoughtful comments and help to improve the manuscript.
PY - 2002
Y1 - 2002
N2 - Two key questions in the study of large-scale C (carbon) and N (nitrogen) cycling in temperate forests are how N cycling in soil detritus controls ecosystem-level retention of elevated N deposition, and whether elevated N deposition is likely to cause increases in C pools. The large C:N ratios in woody detritus make it a potentially important contributor to N retention, if N immobilization increases, and a potentially important contributor to C sequestration, if pool sizes increase. We studied N concentrations, C:N ratios, and pool sizes of N and biomass in fine woody debris (FWD < 5 cm diam.) 12 years into a long-term N-amendment study in two contrasting forests, a naturally-regenerated forest dominated by Quercus spp., and a 63-yr old plantation of Pinus resinosa. We also quantitatively recovered 15N tracers (originally applied as 15NH4 and 15NO3) in FWD, eight years following their application in the same study, in both ambient and N-amended plots. We used these data to test predictions of tracer redistributions made by a biogeochemical process model that included 15N. Results from the N pool-size analysis and the 15N tracer-recovery analysis indicated that under elevated N inputs of 5 g N m-2 yr-1 (as NH4NO3) over the decadal time period, only 0.15%-0.76% of the elevated N inputs were recovered in FWD of N-amended plots relative to ambient. Any increase in N immobilization in wood appeared to be minimal, in agreement with model predictions. Under N amendments, pool sizes of C in FWD were not significantly different from ambient, whereas pool sizes of N were marginally higher. Patterns of 15NH4 vs. 15NO3 recovery, treatment differences, and forest-type differences suggested that plant uptake, rather than detrital immobilization, was the dominant mechanism of 15N tracer movement into FWD. This result indicates that plant-soil cycling operating over a decadal time scale or longer controls C:N ratios and N pool sizes in woody debris.
AB - Two key questions in the study of large-scale C (carbon) and N (nitrogen) cycling in temperate forests are how N cycling in soil detritus controls ecosystem-level retention of elevated N deposition, and whether elevated N deposition is likely to cause increases in C pools. The large C:N ratios in woody detritus make it a potentially important contributor to N retention, if N immobilization increases, and a potentially important contributor to C sequestration, if pool sizes increase. We studied N concentrations, C:N ratios, and pool sizes of N and biomass in fine woody debris (FWD < 5 cm diam.) 12 years into a long-term N-amendment study in two contrasting forests, a naturally-regenerated forest dominated by Quercus spp., and a 63-yr old plantation of Pinus resinosa. We also quantitatively recovered 15N tracers (originally applied as 15NH4 and 15NO3) in FWD, eight years following their application in the same study, in both ambient and N-amended plots. We used these data to test predictions of tracer redistributions made by a biogeochemical process model that included 15N. Results from the N pool-size analysis and the 15N tracer-recovery analysis indicated that under elevated N inputs of 5 g N m-2 yr-1 (as NH4NO3) over the decadal time period, only 0.15%-0.76% of the elevated N inputs were recovered in FWD of N-amended plots relative to ambient. Any increase in N immobilization in wood appeared to be minimal, in agreement with model predictions. Under N amendments, pool sizes of C in FWD were not significantly different from ambient, whereas pool sizes of N were marginally higher. Patterns of 15NH4 vs. 15NO3 recovery, treatment differences, and forest-type differences suggested that plant uptake, rather than detrital immobilization, was the dominant mechanism of 15N tracer movement into FWD. This result indicates that plant-soil cycling operating over a decadal time scale or longer controls C:N ratios and N pool sizes in woody debris.
KW - Immobilization
KW - Nitrogen retention
KW - Nutrient cycling
KW - Tracer
KW - Uptake
KW - Woody debris
UR - http://www.scopus.com/inward/record.url?scp=0036126474&partnerID=8YFLogxK
U2 - 10.1023/A:1014431304760
DO - 10.1023/A:1014431304760
M3 - Article
AN - SCOPUS:0036126474
SN - 0032-079X
VL - 238
SP - 313
EP - 323
JO - Plant and Soil
JF - Plant and Soil
IS - 2
ER -