Atmospheric nitrogen deposition promotes carbon loss from peat bogs

Abstract

Peat bogs have historically represented exceptional carbon (C) sinks because of their extremely low decomposition rates and consequent accumulation of plant remnants as peat. Among the factors favoring that peat accumulation, a major role is played by the chemical quality of plant litter itself, which is poor in nutrients and characterized by polyphenols with a strong inhibitory effect on microbial breakdown. Because bogs receive their nutrient supply solely from atmospheric deposition, the global increase of atmospheric nitrogen (N) inputs as a consequence of human activities could potentially alter the litter chemistry with important, but still unknown, effects on their C balance. Here we present data showing the decomposition rates of recently formed litter peat samples collected in nine European countries under a natural gradient of atmospheric N deposition from approximately 0.2 to 2 g.m(-2).yr(-1). We found that enhanced decomposition rates for material accumulated under higher atmospheric N supplies resulted in higher carbon dioxide (CO2) emissions and dissolved organic carbon release. The increased N availability favored microbial decomposition (i) by removing N constraints on microbial metabolism and (ii) through a chemical amelioration of litter peat quality with a positive feedback on microbial enzymatic activity. Although some uncertainty remains about whether decay-resistant Sphagnum will continue to dominate litter peat, our data indicate that, even without such changes, increased N deposition poses a serious risk to our valuable peatland C sinks.

Fig. 1.

Hourly CO₂ emission from litter peat samples after 4 and 10 days of incubation in relation to atmospheric N deposition in study bogs. Relationships were explained by a logarithmic regression for both incubation periods [y = 0.98 + 0.21ln(x), R² = 0.75, P < 0.01 and y = 0.49 + 0.11ln(x), R² = 0.73, P < 0.01, respectively]. Each value is the mean (± 1 SE) of three to six litter peat samples.

Fig. 2.

Relationships between atmospheric N deposition and activity of phosphatase, β-glucosidase, and chitinase at the end of incubation. A positive relationship was found for phosphatase (y = 4.1 + 7.5x, R² = 0.91, P < 0.01; n = 12) and β-glucosidase (y = 2.0 + 1.3x, R² = 0.81, P < 0.01; n = 12), whereas a negative relationship was found for chitinase [y = 1.7 − 0.1ln(x), R² = 0.50, P = 0.01; n = 12]. Values are means of three to six litter peat samples for each bog (SE was <15% of the corresponding mean value).

Fig. 3.

Increasing N deposition was accompanied by an increasing activity of phenol oxidase (y = 0.25 + 0.28x, R² = 0.71, P < 0.01; n = 12) and by a decreasing trend of the ratio between mean concentration of polyphenols released at the end and at the beginning of incubation (y = 0.58 − 0.06x, R² = 0.43, P = 0.02; n = 12). Each value is the mean of three to six litter peat samples (± 1 SE). A negative correlation was found between the activity of phenol oxidase and the final to initial ratio of polyphenol concentration (r = 0.59, P = 0.04; n = 12). dicq, 2,3-Dihydroindole-5,6-quinone-2-carboxylate.

Fig. 4.

N deposition and release of DOC. At the end of the incubation period the amount of DOC released was generally higher than the correspondent amount released at the beginning of incubation. Litter peat samples collected under higher atmospheric N deposition released a significantly greater amount of DOC compared with samples receiving a lower N input [y = 4.3 + 2.4ln(x), R² = 0.61, P < 0.01; n = 12]. Each value is the mean (± 1 SE) of three to six litter peat samples.