**2.6. N balance**

fact that the soil was quite stony, the apparent density required a correction which was executed considering the percentage of gross elements (those that did not pass through a 2 mm

two 30 cm long and 2.65 cm diameter wide cylindrical samples were taken at the plots of the studied treatments. These two samples were transferred to a hermetic 2 L pot with a rubber septum (**Figure 4**). Then, the pots of the different plots were inserted in a hole next to the studied plot and were covered with the soil of the same hole and kept to incubate for 24 hours. In such a manner, the actual soil temperature and its changes could be mimicked. After 24 hours, 10 mL samples of the atmosphere of each pot were stocked in Vacutainer® tubes for their

O determination by gas chromatography (Unicam 8925). The same process was fol-

in the denitrification process and the ammonium oxidation in the nitrification process [33].

O produced in the incubation with an atmosphere of 5% C<sup>2</sup>

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

H2

due to denitrification (Ndeni) [8] (Knowles, [25]).

O production rate (N<sup>2</sup>

Oprod) was measured

Oem. To achieve this,

O reduction to

comprises the

area in each plot

Oprod,

) (Air Liquide, SA) were injected mak-

blocks N<sup>2</sup>

H2

Oem, N<sup>2</sup>

H2

sieve) (**Table 4**) and their density, which is acknowledged as 2.65 g cm−3 [37].

*O production and total denitrification at the arable layer*

in the arable layer at the same treatments and times described for N<sup>2</sup>

From January 23 to September 18, 2003, the N<sup>2</sup>

lowed for other pots to which 100 mL of acetylene (C<sup>2</sup>

ing the atmosphere of the pot rich in acetylene by 5% (**Figure 4**). C<sup>2</sup>

The cumulated N loss was calculated by the integration of the diary rates of N<sup>2</sup>

To determine the N extraction of wheat in the aerial part at harvest, a 0.25 m<sup>2</sup>

O + N<sup>2</sup>

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

*2.4.2. N2*

86 Global Wheat Production

ulterior N<sup>2</sup>

Therefore, the N<sup>2</sup>

joint production rate of N<sup>2</sup>

and Ndeni over time.

**2.5. Plant N extraction**

N2

The N balances are based on the application of the mass conservation principle to the cultures. Thus, the variation of N stocked in a system equals the difference among the inputs and outputs to the system [31]. Often, the N balance is inferred considering the inputs and outputs in relation to the main source, the soil Nmin. This is considered available to plants in spite of the fact that it can also be consumed by microorganisms, dissipate as gas, or leach through the soil [34]. The balance was assessed as described in Eq. (6) [29]:

$$\text{NminS} + \text{Min} + \text{F} = \text{Nbb} + \text{Nabr} + \text{Nix} + \text{N}\_2\\\text{Oem} + \text{NminAH} + \text{Nc} \tag{6}$$

where, NminS: Nmin quantity in soil before seeding (kg N ha−1), Min: soil N fraction in the soil due to the conversion of organic nitrogen to ammonium (kg N ha−1) (It is calculated from the adjustment of the balance for the control treatment, i.e., 0 (kg N ha−1)), F: fertilizer N applied (kg N ha−1), Nab: N absorbed by the aerial part of the plant (kg N ha−1), Nabr: N absorbed by the roots of the plant (kg N ha−1) (It was estimated that this was 25% of Nab [38]), Nl: leached N (kg N ha−1), N<sup>2</sup> Oem: N emitted as N<sup>2</sup> O (kg N ha−1), NminAH: Nmin in soil after harvest (kg N ha−1), Nc: Not accounted for.

Thus, a positive Nc means that: (i) the inputs have been overestimated due to an error in its calculation or/and experimental error, (ii) the outputs have been underestimated for the same reasons as in (i), (iii) there are other N outputs that have not been considered, and (iv) a combination of all or some of the situations described here occurs.

N2 Oem was considered in the balance instead of N<sup>2</sup> Oprod or Ndeni since these last two refer to gases produced in the arable layer and thus may not exit the system.
