Converting grassland to arable cropping leads to reduced soil organic carbon (SOC) stocks and thereby affects soil and climate protection goals. However, in forage production systems, silage maize produces considerable amounts of herbage and metabolizable energy yields for ruminants. To answer the question whether using direct drilling, as a minimal soil invasive technique, is able to prevent SOC stock degradation without loss of herbage yield in ley-arable systems, a two-year field experiment was set up on a sandy loam soil in northern Germany. In pre-management, a 10-year-old grassland sward was eradicated with a glyphosate and then sown with silage maize with (i) direct drilling (no-till, NT); and (ii) by conventional mouldboard ploughing (conventional tillage, CT). Each treatment included a non-N-fertilized (N0) and a 90 kg N ha−1 year−1 fertilized (N1) treatment. All silage maize treatments were analyzed in terms of above- (ANPP) and belowground net primary production (BNPP) and potential C input in comparison with the grassland control (GC) during the two years after conversion to arable silage maize. Quantification of SOC stocks and origin of C derived from residual plant material was estimated by the determination of natural 13C isotope abundance. Long-term impacts on SOC of a proposed ley-arable system (three years of grassland followed by three years of silage maize) using NT and CT practices were predicted using a soil C model. Results showed that ANPP and BNPP did not differ (p > 0.05) between tillage treatments and the two experimental years. Fertilization increased ANPP only in the second year with a lower fraction of roots belowground in comparison with N0 (p < 0.05). According to the 13C signature the total of silage maize-derived C after two years was up to 2.5–3.2 t C ha−1 in both tillage treatments, of which ∼76% can be explained by the measured C inputs from plant residues. The NT treatment showed lower amounts of decay of previously sequestered C in the upper 10 cm during the two-year period after land use change to silage maize. However, long-term predictions revealed ongoing breakdown of soil C regardless of tillage management used for continuous silage maize after conversion from grassland. The insertion of a three-years grass phase followed by three years of silage maize established with NT techniques was able to maintain SOC stocks in the long-term.