Natural 13C abundance and carbon storage in Danish soils under continuous silage maize

Silage maize has replaced fodder beet as energy-rich winter forage for housed cattle. In Denmark silage maize now occupies more than 4% of the arable land, but little is known on the effect of continuous silage maize on soil carbon dynamics under NW European cropping conditions. Soil total-C and natural 13C abundance were determined every 2-3 years in four Danish arable soils (0-20 cm) continuously cropped to maize for 14 years under identical climatic conditions. A set of soils was sampled after 11 years with maize and analysed for total-C and maize-derived C in >2 mm plant residues, in <2 mm soil, and in 250-2000 μm particulate organic matter (POM). Inputs of C in maize roots and stubbles induced an average annual increase in soil total-C of 90-470 kg C ha−1. When soils were amended with an additional annual input of 8 t DM ha−1 in chopped above-ground maize biomass, soil C storage increased 110-940 kg C ha−1 per year. No changes in soil C/N ratio were observed. Carbon from maize roots and stubbles accumulated in the soil at an annual rate of 250-490 kg C ha−1; with an additional annual input of 8 t DM ha−1, the accumulation rate was 710-980 kg maize-C ha−1. After 14 years with continuous silage maize, maize-derived C accounted for 7-18% of the total soil C; with additional maize biomass inputs, maize-C made up 18-31%. The retention in soil of the C added in above-ground maize biomass averaged 11-15%. The >2 mm maize residues accounted for 7-21% of all the maize-C retained in soil after 11 years of maize cropping. Between 17 and 41% of the maize-C found in <2 mm soil was associated with POM. The amount and composition of the POM reflected the intensity of maize residue inputs but POM did also include biologically recalcitrant organic matter derived from pre-maize vegetations. In Danish light-textured soils with moderate C concentrations and with a history of cereal dominated cropping, continuous silage maize was able to increase soil C storage. Changes in 13C natural abundance showed that the accumulation of maize-C was substantial even in soil showing moderate changes in soil total-C.