Nitrogen deposition and carbon sequestration

Abstract

Nadelhoffer et al. reply — Jenkinson et al. and Sievering are justifiably concerned that our ¹⁵N additions to forest floors do not account for the potential uptake of nitrogen input by forest canopies. We agree that canopies can remove nitrogen from the atmosphere, resulting in inputs to forest floors that are less than the total nitrogen deposition. A North American study¹ has suggested that canopies remove, on average, 16% of total (organic+inorganic) atmospheric nitrogen input to forests, and concluded that nitrogen uptake by the canopy is probably small relative to the nitrogen requirements of trees. Spraying ¹⁵N-labelled ammonium and nitrate on the crowns of five-year-old Norway spruce indicated that foliar uptake in mature forests probably constitutes only a small percentage of annual nitrogen uptake².

Main

Although canopies can remove nitrogen from bulk deposition, the extent to which this nitrogen is biologically available and stimulates wood growth is poorly understood. Throughfall nitrogen fluxes at our sites either exceed the measured bulk nitrogen deposition or closely agree with modelled total deposition³,⁵, so their use as a proxy for nitrogen deposition is reasonable and does not compromise our conclusions.

We think it is unlikely that “pool substitution” of unlabelled inorganic nitrogen for ¹⁵N tracers in soils biased our estimates of throughfall uptake by trees, as claimed by Jenkinson et al., who found this mechanism operating in theoretical models⁶ and in potted, but not in field-grown, plants⁷. Comparison of the amounts of our tracer additions to the amount of unlabelled inorganic nitrogen already present in our soils with those amounts in their theoretical analysis (see Fig. 1 of ref. 6) indicates that pool substitution is unimportant at the low ¹⁵N enrichment used in our throughfall manipulations.

We agree that forest soils can serve as long-term carbon sinks and that carbon turnover times in soil pools are longer than in trees. Nevertheless, as nearly all carbon enters forests through trees, it is here that the influence of nitrogen inputs on carbon uptake is important. We point out, however, that our simple stoichiometric budget (Table 2 of ref. 8) actually factors in the role of soils, with soil storage calculated as the product of the C:N ratio and nitrogen immobilization in soils.

Finally, we recognize that there is a wide range of estimates for nitrogen deposition on temperate forests. This is because of uncertainties and likely but unknown biases in the atmospheric mixing models, emissions data and site-specific wet+dry nitrogen deposition data used to generate spatial predictions of nitrogen deposition on forests. Our temperate-forest deposition value of 5.1×10¹² g nitrogen was derived from a comparison of modelled NO_yand NH_xdeposition estimates⁹. If this value were to be in serious disagreement with estimates based on greatly improved monitoring and modelling efforts, then our assessment of the effects of nitrogen deposition on temperate-forest carbon uptake would need to be revised.

Future estimates of this effect must also consider spatially explicit patterns of nitrogen loss from forests in relation to nitrogen deposition and land use¹⁰, the likelihood of narrowing C:N ratios under increased nitrogen input, and possible nutritional imbalance of tree tissue subjected to high nitrogen input. Meanwhile, our tracer experiments indicate that, although nitrogen deposition in forests accounts for some of the northern-temperate CO₂sink, other factors must account for most of this sink.

See also Jenkinson et al. and Sievering