Root carbon input in organic and inorganic fertilizer-based systems

In agroecosystems, carbon (C) inputs come from applied manures, plant roots and retained shoot residues. Whereas the manure and shoot derived C inputs can be relatively easily determined, high costs associated with root measurements have caused current knowledge of in situ root C input to remain scant, hence it may be considered a "black box". This study aimed at determining root biomass C and nitrogen (N) in response to nutrient management and soil fertility building measures (green manures and catch crops). We made use of one inorganic fertilizer-based and four organic arable cropping systems in an 11-year-old field experiment on a sandy loam soil. Shoots and roots of catch crop mixtures and cereals sampled in 2008 and 2010 were characterized for dry matter (DM) biomass, C and N content. Furthermore, concentrations of other nutrient elements in cereal roots were determined. We also estimated annual C losses through respiration of winter wheat roots. Results show that in the organic systems, catch crop roots accounted for more than 40% C in total plant biomass. For the cereals, root DM biomass was higher in the organic than in the inorganic fertilizerbased system. The shoot-to-root (S/R) ratio was 31-131% higher in inorganic than in similar organic fertilized cereals crops. Annual cumulative C loss through respiration of winter wheat roots was similar across cropping systems. Our findings, which show higher root DM biomass in the organic compared to inorganic fertilizer-based systems, could infer enhanced C sequestration from roots in the former. The results show that use of shoot biomass and fixed S/R ratios to estimate root biomass, in arable systems with different nutrient management strategies, leads to erroneous estimates of root C input