The effect of delaying autumn incorporation of green manure crop on N mineralization and spring wheat (Triticum aestivum L.) performance

The use of legumes as green manuring crops does involve a potential risk of N leaching losses over the winter period. The susceptibility of crop residue-derived N to losses and the pre-crop value of a green manuring crop can be manipulated by proper timing of incorporation into soil. In this study, mineralization of C and N was investigated in a range of low temperatures, including thawing and freezing, that are characteristic to autumn green manure incorporation and its decomposition. The pre-crop effect of green manuring was further tested with spring wheat under field conditions. We hypothesized that delaying green manure incorporation in the autumn would reduce the risk of N losses from the field and maximize the N transfer to a successive spring wheat crop. To test the hypothesis, N mineralization was followed in a laboratory experiment where red clover (Trifolium pratense L.) shoots were incubated at 4-8degreesC for 40-80 days to simulate early autumn, delayed autumn and late autumn incorporation of a green manuring crop, followed by an incubation at -2degreesC or at -2degreesC to 4degreesC for 25 days to simulate winter conditions. In a simultaneous field experiment, we measured the effect of delayed autumn incorporation of common vetch (Vicia sativa L.) green fallow on spring wheat performance. In the laboratory experiment, significant N mineralization during incubation was detected when simulating both early autumn and delayed autumn incorporation. In contrast, no net N mineralization was detected when simulating late incorporation. In the field experiment, the N supply from soil to spring wheat was higher in the late and delayed incorporation treatments than in early or spring incorporation of green manure. Late incorporation also produced most wheat grain. We conclude that different amounts of N become available to wheat, depending on the time of incorporation of green manure residues in soil. This difference is due to temperature. Late or delayed incorporation of green manure residues has the potential to reduce the susceptibility of mineral N to leaching and yields more N available to a subsequent crop.