Manipulating nitrogen release from nitrogen-rich crop residues using organic wastes under field conditions

Following mineralization of N-rich crop residues, large amounts of mineral N can be released into the soil. Manipulating N mineralization of crop residues may be an option to reduce N[O.sub.3.sup.-] in soil. The potential to manipulate the N release from vegetable crop residues by using organic wastes was tested under field conditions. At the start of the experiment, cauliflower (Brassica oleracea L. van botrytis L.) residues (~73 Mg flesh matter [ha.sup.-1]) together with an immobilizer waste (~5 Mg C [ha.sup.-1] of straw, green waste compost, sawdust, or paper sludge) were incorporated into a silt loam soil. After 154 d, a remineralizing waste (~1 Mg C [ha.sup.-1] of vinasse or dairy sludge) was incorporated. During the field experiment, the mineral N content in the soil was measured at regular time intervals, and net N release, N[O.sub.3.sup.-] leaching, and denitrification were simulated using a N mineralization-immobilization model coupled to a N[O.sub.3.sup.-] leaching model. Straw, green waste compost, and sawdust were able to immobilize between 54 and 68% of the N released by the cauliflower residues and reduced N[O.sub.3.sup.-] leaching by 56 to 68%. Paper sludge released an extra amount of N due to its low C/N ratio. No consistent remineralization of N could be found in any of the treatments, probably due to an unsuitable composition of the remineralizer wastes. Manipulating N release of N-rich crop residues by using organic wastes may be a suitable method to reduce N[O.sub.3.sup.-] leaching; however, stimulating remineralization of immobilized N by the start of the following spring may not be easy to achieve. Abbreviations: CD, coefficient of determination; CRM, coefficient of residual mass.