Below ground carbon inputs (BGC) to soil are the sum of root biomass and rhizodeposition carbon. They play a major role in the carbon cycle in soils, since this carbon may be more stable than the above ground inputs. As this part of the plant is inherently difficult to measure in the field, it is usually estimated from yield in order to supply soil models with input data. While fertilization intensity considerably affects above ground biomass, its influence on BGC inputs is largely unclear, especially with respect to the subsoil.
Therefore, we determined net root biomass and rhizodeposition C of field-grown maize and wheat at harvest in different farming systems (bio-organic, conventional) and fertilization treatments (zero, manure, mineral) along an intensity gradient in two Swiss long-term field trials, using isotope labelling techniques.
As expected, increasing fertilization intensity resulted in a strong increase of above ground biomass. However, BGC inputs were similar among treatments on both sites irrespective of soil depth. The proportions of rhizodeposition C of BGC inputs averaged 54 to 63% and were, therefore, much larger than the widely adopted 40% for field-grown cereals. They increased with soil depth and were highest under sole organic fertilization.
Our findings imply that yield-independent values provide closer estimates for BGC inputs to soil of cereals in different farming systems than yield-based functions. Also, fertilization has only little potential to alter absolute amounts of BGC inputs to deep soil in order to sequester carbon in the long term.
Reference:
Hirte, J., Leifeld, J., Abiven, S., Oberholzer, H.-R., Mayer, J., 2018. Below ground carbon inputs to soil via root biomass and rhizodeposition of field-grown maize and wheat at harvest are independent of net primary productivity. Agriculture, Ecosystems & Environment 265, 556–566. doi:10.1016/j.agee.2018.07.010