PARENT SESSION
Poster Session 24: Agro-Ecology
Wednesday, August 10, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal
Spatially-variable ammonium fixation in an irrigated agricultural system in northwestern Mexico.
Ahrens, Toby1, Matson, Pamela1, Fendorf, Scott1, 1 Stanford University, Stanford, CA
ABSTRACT- Recently-fixed ammonium (NH4+ ) in clay lattices in soils can impact nitrogen (N) uptake by crops, nitrate (NO3-) leaching from the rooting zone, and nitrogenous gas emissions from soils in agricultural systems. The amount and availability of N retained in 2:1 clay minerals depends on several factors, including the type of clay minerals, availability of N in the soil matrix, and wetting and drying events of a soil. We have carried out a set of experiments to determine how the spatial variability of these factors influences landscape-level fate and transport of nitrogen in an irrigated semi-tropical agricultural system in the Yaqui Valley, Sonora, Mexico. A range of vertisols and aridisols in the Valley were subjected to a set of laboratory experiments to determine the quantity of non-exchangeable inorganic N in actively managed farmer fields. We also analyzed soils from experimental fertilization and irrigation response trials at the International Maize and Wheat Improvement Center. Typical CEC values for surface soils in this region are 33 meq 100 g-1 soil and 88 meq 100 g-1 clay. Potassium (K) fixation isotherms suggest that the upper 15cm of soil are able to fix between 930 and 1400 kg K ha-1, equivalent to 330-500 kg NH4+ -N ha-1. Fixation potentials in deeper soil horizons (60-90 cm) are considerably greater, but it is unclear how deep NH4+ ions are leached in this system. Our results suggest that the kinetics of fixation and remineralization of this reservoir of non-exchangeable N could have important and spatially-variable impacts on gaseous and aqueous losses of N from the soil, and management practices such as fertilizer and irrigation regimes likely influence the size of this reservoir.
Key words: ammonium fixation, vertisols