Icelandic grasslands as long-term C sinks under elevated N inputs

Abstract. About 10 % of the anthropogenic CO₂ emissions have been absorbed by northern terrestrial ecosystems during the past decades. It has been hypothesized that part of this increasing carbon (C) sink is caused by the alleviation of nitrogen (N) limitation by increasing anthropogenic N inputs. However, little is known about this N-dependent C sink. Here, we studied the effect of chronic seabird-derived N inputs (47–67 kg N ha⁻¹ yr⁻¹) on the net soil organic C (SOC) storage rate of unmanaged Icelandic grasslands on the volcanic Vestmannaeyjar archipelago by using a stock change approach in combination with soil dating. We studied both early developmental soils (50 years) and mature soils (1,600 years), and for the latter we separated between decadal (topsoil) and millennial (total soil profile) responses, where the SOC stocks in the topsoil accorded to 40–50 years of net SOC storage and those in the total soil to 1,600 years of net SOC storage. We found that enhanced N availability – either from accumulation over time, or seabird derived – increased the net SOC storage rate. Under low N inputs, the early developmental soils were weak decadal C sinks (0.018 ton SOC ha⁻¹ yr⁻¹), but this increased quickly under elevated N inputs to 0.29 ton SOC ha⁻¹ yr⁻¹, thereby equaling the decadal SOC storage rate of the unfertilized mature site. Furthermore, at the mature site, chronic N inputs not only stimulated the decadal SOC storage rate, but also the millennial SOC storage was consistently higher at the high N input site. Hence, our study suggests that Icelandic grasslands, if not disturbed, can remain C sinks for many centuries under current climatic conditions and that chronically elevated N inputs can induce a permanent strengthening of this sink.