Soil nitrogen dynamics and crop residues. A review

Nitrogen (N) is a major fertiliser for agriculture and food production. About 67.84 million tons of N are annually applied to agricultural fields, without which nearly half of the world's population would not be alive today. Returning plant residues to the soil is an alternative and sustainable way of N fertilisation. Although impacts of returning plant residues on plant available N in soil have been widely studied, there is still no systematic review of their mechanisms and models. In this review we highlight the following advances: (1) When plant residues are returned to the soil, N undergoes biotic immobilisation-remineralisation, abiotic immobilisation, soil organic N mineralisation and plant residue organic N mineralisation. (2) Plant residues modify inorganic N fate using three mechanism mineralisation, immobilisation-mineralisation and immobilisation, depending on plant residue nature and soil properties. (3) The use of plant residue C/N ratio is not always effective to predict the effect of plant residues. Instead, soil properties and the forms of carbon and nitrogen should be considered. (4) Mineralisation always promotes N uptake by crops and increases the risk of N loss. In addition, although net immobilisation is involved in immobilisation-mineralisation and immobilisation, it does not necessarily induce lower crop nitrogen uptake. Results also depend on the synchronism between the changing soil inorganic N and the crop N uptake. (5) N loss during mineralisation can be reduced by an immobiliser. Net N immobilisation during immobilisation-mineralisation and immobilisation can be reduced by changing the timing of ploughing and fertilising or by changing the plant residues placement.