Effect of cover crops on leaching of dissolved organic nitrogen and carbon in a maize-cover crop rotation in Mediterranean Central Chile

Protection and management of water quality across agricultural landscapes requires a sound understanding of runoff and/or leaching of nutrients and other agrichemicals from agricultural production systems to receiving waters. We, in a large leaching columns experiment, studied the losses of dissolved organic N (DON), dissolved organic carbon (DOC), dissolved inorganic N (DIN) and total dissolved N (TDN) from maize cultivation on a coarse-textured soil in in Mediterranean Central Chile. The combined effects of cover crops and inorganic N fertilisation rates were evaluated on nitrogen and carbon leaching loads (DIN, DON and DOC) and ratios of soluble components (DON:DIN, DON:TDN and DOC:DON). A total of 52 soil columns for 13 treatments (4 replicates) were established to evaluate leaching of dissolved N and C forms from: 1) continuous bare soil (fallow) compared with a continuous cover crop (Lolium multiflorum or Trifolium repens), with 0 or 150 kg N ha-1 applied; and 2) maize-fallow and maize-cover crop rotations with two different N doses (250 or 400 kg N ha-1) for the maize and cover crops (L. multiflorum and/or T. repens). We found that inclusion of a grass cover crop (L. multiflorum) and optimal N fertilisation (250 kg N ha-1) treatment resulted into lower DIN losses from the study columns. However, in trial 1, the DON load from the treatments with continuous grass cover crop L. multiflorum was on average twice the DIN load. In trial 2, the crop rotation of maize cultivation with 400 kg N ha-1 applied and inclusion of a legume cover crop T. repens resulted into the highest DIN loads, while a crop rotation of maize with 250 kg N ha-1 applied and inclusion of a grass cover crop L. multiflorum had the lowest DIN loads. However, the latter rotation gave significantly higher DON loads than the maize-fallow treatments. In trials 1 and 2, inclusion of L. multiflorum enhanced soil organic pools and microbial activity, and thus increased the amount of DON and DOC susceptible to leaching. Overall, the rotation with maize with 250 kg N ha-1 applied and L. multiflorum as cover crop generated the lowest amount of TDN leaching from the soil columns. We recommend this to be further studied in field conditions as a best management practice for reducing the risk of diffuse pollution of surface water bodies and groundwater from maize cultivation in Mediterranean Central Chile.