Based on in situ soil incubations, we found that warming significantly increased annual rates of net N mineralization (P < 0.01) (Fig. a). The average increase of net N mineralization in the heated area relative to the control area was about 45% over 7 years of warming; a mean annual increase in the net N mineralization of 27.4 kg N ha−1 in the organic and mineral soil (0–10 cm) horizons. We observed a progressive increase in NO3− production over the 7-year study period (Fig. b). By the 5th year of warming, the production of NO3− had increased by a factor of 3 in the heated area, with 25% of the NH4+ produced during the mineralization process transformed to NO3−.
Fig. 1 a Net N mineralization in the control (open bars) and heated (filled bars) areas. Bars represent mean net N mineralization rates of subplots (n = 10) in kg N ha−1year−1 + 1SE. b Net nitrification (more ...)
Net N mineralization and nitrification varied both annually and seasonally in the heated and control areas. We defined spring as April–May, summer as June–August, autumn as September–October and winter as November–March. Peaks in net N mineralization occurred in the summer months, averaging 45 and 37% of the annual totals in the control and heated areas, respectively (Fig. ). In the spring, net mineralization averaged 21% of the annual total in the control area and 23% in the heated area. In the fall, mineralization averaged 30% in the control area and 31% in the heated area. Over the winter, mineralization averaged 4% in the control area and 9% in the heated area. These seasonal trends were also observed for net nitrification, with peaks occurring in the summer months, averaging 55 and 49% of the annual totals in the control and heated areas, respectively (Fig. ). In the spring, net nitrification averaged 25% of the annual total in the control area and 32% in the heated area. In the fall, nitrification averaged 18% in the control area and 11% in the heated area. In the winter, nitrification averaged 3% in the control area and 8% in the heated area. While the overall trends were the same for both areas, the proportion of N mineralized in the summer decreased in response to warming, while the proportion of N mineralized in the winter increased with warming. The proportion of net nitrification occurring in the spring and winter increased in response to warming and decreased in the fall and summer.
Fig. 2 a Monthly net nitrification in the control (white circles) and heated (filled circles) areas. Points represent mean net nitrification rates of subplots (n = 10) in kg N ha−1year−1 + 1SE. (more ...)
Over 7 years of warming, we saw no significant solution or gaseous losses of N. While we saw occasional spikes in NH4+ and NO3− solution losses in the heated and control areas, there was no significant effect of warming on solution losses through time. Gaseous N2O fluxes from both heated and control areas were near detection limits, and no discernable pattern emerged over years of warming (Online Resource 1).
Foliar percent N has been significantly higher in the heated area since the 2nd year of warming (P < 0.05 for each year) (Fig. a). While the dominant species (oak, red maple and white ash) all showed higher percent N with warming in most years, red maple had the largest and most consistent response to warming. When foliar N was expressed as a percent increase over the controls, red maple shows a 25% increase in leaf N by year 6 (Fig. b).
Fig. 3 a Foliar %N in control (open circle) and heated (filled circle) leaves ±1SE. Percent N is not available for year 4. A pre-treatment correction factor was applied to the %N values. b Percent change in leaf N content in the heated area relative (more ...)
Overall, we saw no significant stand-level responses of leaf N resorption efficiency with warming. However, we did see significant species-level differences (Fig. a–c). Red maple had the highest leaf N resorption compared to red oak and white ash in the warmed area (P < 0.01 for each year), and its leaf N resorption efficiency increased relative to the control by year 3 (P < 0.05 for each year). By year 6, oaks had a significant decrease in N resorption in response to warming (P = 0.04). White ash had a variable response in leaf resorption, with a positive response to warming in year 2 and a negative response to warming in year 3 (P = 0.05 and 0.01, respectively).
Fig. 4 Average percent N resorption for a red maple b red oak and c white ash in the control (open circles) and heated (filled circles) areas. Lines represent %N values retained in the trees during senescence by species ±1SE. A pre-treatment (more ...)
We sampled NRA during an expected peak in seasonal NO3− availability in 2009. Overall, oaks had the highest NRA in the heated and control areas, followed by white ash and red maple (Table ).
Table 1 Nitrate reductase activity corrected for canopy mass in 2008 (μmoles NO2− m−2 litter mass h−1) by species in the control and heated areas ±1SE; P = 0.77, 0.042, and 0.23 for red maple, white (more ...)
Warming had significant effects on plant C cycling. Overall, there was an increase in C storage with warming (Melillo et al. 2011
). In addition, after correcting for pre-treatment differences, we saw a significant increase in the relative growth rate in trees on the heated area through time (P
= 0.007) (Fig. a). We saw varied species-specific responses to warming. Red maple showed the largest and most consistent response to warming, doubling its relative growth rate in the warmed areas (P
= 0.01) (Fig. b). While red oak had the greatest increase in biomass and trends toward increased relative growth rate in the heated area, the difference was not significant (P
= 0.20) (Fig. c). White ash showed no consistent pattern of increased relative growth rates with warming (P
= 0.47) (Fig. d).
Fig. 5 a Average relative growth rate for heated (filled circle) (n = 74) and control (n = 83) (open circles) areas ±1SE. b Relative growth rate of oak species (red and black oaks) in control (open circles) (n (more ...)
In addition to increases in woody biomass, we saw overall increases in litterfall with warming (60 kg C ha−1). Litterfall was used here as a proxy for canopy mass. Oak litterfall showed the greatest increase in response to warming by year 5 (P = 0. 006), with an average annual increase in litterfall of 35% relative to the control over the past 7 years. The other dominant species showed more variation through time, with no clear pattern emerging.