Applying C:N ratio to assess the rationality of estimates of carbon sequestration in terrestrial ecosystems and nitrogen budgets

Carbon (C) sequestration in terrestrial ecosystems needs to ensure the reactive nitrogen (Nr) supply. However, the organic C:N ratio is rarely considered in both estimates of C sequestration in terrestrial ecosystems and N budget at regional and global scale. Here we propose an approach for extracting Nr sequestrated in terrestrial ecosystems with C (termed as N rc ) from N budget and then assessing the rationality of estimates of C sequestration in terrestrial ecosystems and N budgets by using the sequestrated organic C:N ratio as a criterion. We extracted N rc from the N budget of IPCC AR5 at global scale and Chinese N budget (Proc Natl Acad Sci (USA) 112:8792, 2015) at regional scale based on the assumptions that there is no net Nr accumulation in agricultural products and no net Nr (except N 2 O) accumulation in the atmosphere at annual temporal scale, and N 2 O is not involved in biological processes. By taking the C sequestration in terrestrial ecosystems from C budget of IPCC AR5 (2.1 Pg C/yr during 2000–2009) to assess the N budget from the same report, the organic C:N ratios calculated by the N rc extracted from both the upper ranges and averages of Nr input and output in global N budgets of IPCC AR5 fell outside the rational range of organic C:N ratio, but the organic C:N ratio (=31) calculated by the N rc (68.1 Tg N/yr) extracted from the lower ranges of the N budget fell very well into the range of organic C:N ratio in global terrestrial ecosystems. N rc extracted from Chinese N budget was 11.6 Tg N/yr in 2010, which could be combined with C into Chinese terrestrial ecosystems in a range between 0.14–0.66 Pg C/yr with a robust estimate of 0.36 Pg C/yr by applying the global terrestrial ecosystem sequestrated organic C:N ratio to calculation. The results indicate that the proposed approach for extracting N rc from N budget works well. Checking the estimated organic C:N ratio in terrestrial ecosystems will help to assess whether the C and/or N budgets are rational or not. Graphical abstract Highlights • An approach is proposed to extract Nr sequestrated with C in terrestrial ecosystems. • The lower ranges of global N budget in IPCC AR5 are rational. • C sequestration in Chinese terrestrial ecosystems is estimated to be 0.36 Pg C/yr in 2010.