**2.5. Plant N extraction**

To determine the N extraction of wheat in the aerial part at harvest, a 0.25 m<sup>2</sup> area in each plot was randomly chosen and the plants in that area were cut to the ground. Grain and straw were separated and dried in an oven at 70°C for 48 h at least to determine biomass. Then, the samples

**Figure 4.** Incubation pot before it was buried and bag and syringe with acetylene.

were ground and sieved through 0.5 and 1 mm sieves respectively. The total N was determined both for straw and grain by Kjeldahl procedure [5] with a Kjeltec Auto sampler System 1035 (Tecator). The N absorption by the aerial part of the plant (Nab) was determined as the sum of the products of the N concentration at grain and straw times their respective biomasses.

The yielded grain quantity datum, needed for the previous calculation was determined by harvesting the central 1.5 m wide aisle of each plot. Yield was referred to a humidity of 120 g kg−1.
