Carbon losses from pyrolysed and original wood in a forest soil under natural and increased N deposition

Abstract. Pyrogenic organic matter (PyOM) plays an important role as a stable carbon (C) sink in the terrestrial ecosystems. However, uncertainties remain about in situ turnover rates of PyOM in soil, the main processes leading to PyOM C and nitrogen (N) losses from the soil, and the role of N availability in PyOM cycling in soils. We measured PyOM and native soil organic carbon losses from the soil as carbon dioxide and dissolved organic carbon (DOC) using additions of highly 13C-labelled PyOM (2.03 atom %) and its precursor pinewood during one year in a temperate forest soil. The field experiment was carried out under ambient and increased mineral N deposition (+60 kg N ha−1 yr−1). The results showed that after one year: (1) 0.5% of PyOM-C and 22% of wood-C were mineralized as CO2, leading to an estimate of minimum turnover time of 191 and 4 yr respectively, (2) the quantity of PyOM and wood lost as dissolved organic carbon was negligible (0.0004 ± 0.0003% and 0.022 ± 0.007 respectively); and (3) N additions decreased cumulative PyOM mineralization by 43%, but did not affect cumulative wood mineralization and did not affect the loss of DOC from PyOM or wood. We conclude that mineralization to CO2 was the main process leading to PyOM losses during the first year of decomposition in a forest soil, and that N addition can decrease PyOM C cycling while leaving unaltered wood C cycling.