Long-term effect of tillage, nitrogen fertilization and cover crops on soil organic carbon and total nitrogen content

No-tillage, N fertilization and cover crops are known to play an important role in conserving or increasing SOC and STN but the effects of their interactions are less known. In order to evaluate the single and combined effects of these techniques on SOC and STN content under Mediterranean climate, a long term experiment started in 1993 on a loam soil (Typic Xerofluvent) in Central Italy. The experimental variants are: conventional tillage (CT) and no-tillage (NT), four N fertilization rates (N0, N1, N2 and N3) and four soil cover crop (CC) types (C - no cover crop; NL - non-legume CC; LNL - low nitrogen supply legume CC, and HNL - high nitrogen supply legume CC).The nitrogen fertilization rates (N0, N1, N2 and N3) were: 0, 100, 200, 300kgNha-1 for maize (Zea mays, L.); 0, 60, 120,180kgNa-1 for durum wheat (Triticum durum Desf.); 0, 50, 100, 150kgNha-1 for sunflower (Helianthus annuus L.).From 1993 to 2008, under the NT system the SOC and STN content in the top 30cm soil depth increased by 0.61 and 0.04Mgha-1year-1 respectively. In the same period, the SOC and STN content under the CT system decreased by a rate of 0.06 and 0.04Mgha-1year-1 respectively. During the experimental period, N1, N2 and N3 increased the SOC content in the 0-30cm soil layer at a rate of 0.14, 0.45 and 0.49Mgha-1year-1. Only the higher N fertilization levels (N2 and N3) increased STN content, at a rate of 0.03 and 0.05Mgha-1year-1.NL, LNL and HNL cover crops increased SOC content by 0.17, 0.41 and 0.43MgCha-1year-1 and -0.01, +0.01 and +0.02MgNha-1year-1.Significant interactions among treatments were evident only in the case of the N fertilization by tillage system interaction on SOC and STN concentration in the 0-10. cm soil depth in 2008.The observed SOC and STN variations were correlated to C returned to the soil as crop residues, aboveground cover crop biomass and weeds (C input).We conclude that, under our Mediterranean climate, it is easier to conserve or increase SOC and STN by adopting NT than CT. To reach this objective, the CT system requires higher N fertilization rates and introduction of highly productive cover crops.