Irrigation with wastewater is an increasing practice around the world triggered by the large needs of water for agriculture. Many times this source of water is added by flooding to the fields, creating short temporal oxygen-depleted environments, where nitrous oxide (N2O) emissions are promoted. Agriculture is responsible for up to 60% of the global annual emissions of this gas, and its emission factors (EF) must be reported. During 21 months, we installed closed chambers to measure the N2O emissions from a wastewater irrigated plot in the Mezquital Valley. Here, alfalfa, rye grass, and maize were grown in succession, receiving mainly organic N contained in the untreated wastewater and the residues of the previous crop; only maize received extra mineral fertilization of 100 kg of N ha-1. We obtained a time series where the gaps were filled by linear interpolation. EF and N2O direct emissions were estimated by the Tier 1 approach of the IPCC. We found that the alfalfa and rye grass, which only receive organic nitrogen, produced minor N2O emissions, that reflected in lower EF (0.004 and 0.001, respectively), compared with the default value of the IPCC (0.01). In contrast, maize, which receives organic and inorganic N, lost 1% of this N input as N2O, the same as the default value (0.01). It seems that the form of N, the crop age and type influence greatly the N dynamics in this wastewater irrigated land-use system. Therefore, the Tier I approach of the IPCC seems to underestimate the N efficiency of this agroecosystem and overestimates the N2O direct emissions from alfalfa and rye grass.
Keywords: Alfalfa; Linear interpolation; Maize; Mezquital Valley; Organic amendments; Rye grass.
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