Emissions of Greenhouse Gases from Cropland Irrigated with Untreated Sewage Effluent in the Mezquital Valley, Mexico.

Irrigation with waste water adds labile carbon and nitrogen compounds to the soil, and

when applied by flooding it rapidly changes the soil's atmosphere and redox potential.

In the Mezquital Valley more than 90 000 ha is irrigated with wastewater from Mexico City, and enhanced emissions of CO2 and N2O follow each flooding.

We measured the emissions of these two gases from a field irrigated periodically with

wastewater and under three crops, namely alfalfa, rye-grass and maize, using static chambers for 21 months. We also measured emissions from a field growing rain-fed maize before and shortly after two rain events. The data from repeated measurements from the same chambers are correlated in time, and so we modeled the ante-dependence and we fitted the models by the residual maximum likelihood (reml) method.

Carbon dioxide was the main gas emitted with peaks up to 448 mg C as CO2 m^-2 hour^-1.

Emissions of N2O were largest under maize (up to 2.98 mg N as N2O m^-2 hour^-1) and peaked in particular irrigation events either as the infiltrating water replaced the gases from air-filled pores or several days after irrigation as excess nitrogen and fresh sources of carbon were mineralized. Processes operating during the few days during and immediately after irrigation seem to determine the dynamics of gaseous production in this agricultural ecosystem.

Despite the large additions of nitrogen, the emissions of N2O are 30 to 70% less than the default values of the Intergovernmental Panel on Climate Change (IPCC). It seems that nitrogen is used efficiently.