Management Options for Carbon Sequestration in Croplands Evaluated in Long-Term Field Experiments in Northern Europe.

Improved management of croplands is a win-win strategy resulting in both increased soil fertility and carbon sequestration. We quantified the effect of N fertilization, organic amendments and ley-arable rotations versus continuous annual cropping systems on soil carbon stocks by analyzing data from long-term field experiments in Nordic countries. Increasing net primary production was found to be the main driver for higher  soil carbon storage. Mineral N fertilization increased soil carbon stocks by about 1-2 kg C ha^-1 for each kg of N applied to cropland. Ley-arable rotations, being a combination of annual and perennial crops, are expected to have C stocks intermediate between those of continuous grass- and croplands. A summary of data from 15 long-term sites showed that on average 0.5 Mg ha^-1 yr^-1 (range 0.3 to 1.1; median 0.4 Mg ha^-1 yr^-1) more carbon was retained in soils in ley-arable compared to exclusively annual systems, depending on species composition, management, soil depth and the duration of the studies. Retention factors calculated for straw, manure, sawdust, peat, sewage sludge and composted household waste varied widely from about 15% for above-ground crop residues to about 90% for composted household waste in a decadal time scale. Retention of roots and rhizodeposits was higher than for above-ground residues. We also emphasize that increased soil carbon stocks not always lead to carbon sequestration of atmospheric CO₂ and that C sequestration not always leads to mitigation of greenhouse gas emissions. Consequences of different land use and management are discussed, taking into account two critical boundaries – the limited area of agricultural land on Earth and requirements to produce sufficient food, fibres and energy for a growing population.