Cover Crop Effects on Soil N Retention and Supply in Fertilizer-Intensive Cropping Systems (A Review)

Excessive use of nitrogen fertilizers is a common problem for intensively cultivated cropping systems. Introducing cover crops during fallow periods is a good practice for encouraging the sustainable use of N resources in such cropping systems. Although individual previous studies assessing the interception of soil residual N by cover crops and its mineralization for subsequent N supply abound, a comprehensive synthesis of cover crop effects on N retention and supply from such studies is still lacking, especially in fertilizer-intensive cropping systems. This paper aims to assess the influences of cover crop practices on soil N retention and supply on the basis of published literatures. Cover crop biomass accumulation, N absorption capacity, root-induced rhizosphere deposition, N-stratification and biological nitrification inhibition all play a part in temporal N storage and leaching reduction. Excessive pre-absorption, residue-induced immobilization or the negative priming effect can result in a negative after-effect, increasing the risk of yield reduction and reducing the likelihood of cover crop introduction. Comparisons between different cover crops, especially the mostly reported leguminous and non-leguminous species, are carried out in this review and the influences of cover crop management practices including species selection, planting and termination time are also discussed. There is a need to develop adaptive cover crop management practice for specific cropping systems based on its effects on N retention and supply.