Root carbon input in organic and inorganic fertilizer-based systems

Background and aims: In agroecosystems, carbon (C) inputs come from plant roots, retained shoot residues and in some cases from applied manures. Manure and shoot derived C inputs are relatively easy to determine. Conversely, high costs associated with root measurements have caused knowledge on root C input to remain scant. This study aimed at determining macro-root C input and topsoil root related respiration in response to nutrient management and soil fertility building measures. Methods: We sampled roots and shoots of cereals and catch crops in inorganic and organic fertilizer-based arable cropping systems in a long-term experiment in 2 years, 2008 and 2010. Sampled shoots and macro-roots of catch crop mixtures and cereals were characterized for dry matter (DM) biomass (C was estimated as 45 % of DM biomass). We also measured topsoil root-related soil respiration throughout the growing season of winter wheat by subtracting soil respiration from soil with and without exclusion of roots. Results: Catch crop roots accounted for more than 40 % of total plant C. For spring barley in 2008 and spring wheat in 2010, root C was higher in the organic than in the inorganic fertilizer-based systems. However, for winter wheat in 2008 and spring barley in 2010, there were similar amounts of root C across systems. The measurements of topsoil root-derived respiration also showed no difference across systems, despite large differences in harvested cereal yields. Cereal biomass shoot-to-root (S/R) ratio was higher (31–131 %) in inorganic than in organic fertilizer-based systems. Conclusions: Our findings show that macro-roots of both cereal crops and catch crops play a relatively larger role in organically managed systems than in mineral fertilizer based systems; and that the use of fixed biomass S/R ratios to estimate root biomass leads to erroneous estimates of root C input