Isotopic evidence for changes in residue decomposition and N-cycling in winter flooded rice fields by foraging waterfowl

Winter flooded rice fields can serve as substitute habitat for migratory waterfowl. Not much is known about the effects of the foraging waterfowl on nutrient cycling in rice production systems. This study quantifies the effect of foraging waterfowl on decomposition of rice residue and N-cycling in a winter flooded rice field in the Sacramento Valley, California. Along two transects in a field, pairs of control plots and exclosure plots, which excluded waterfowl, were set up. The original straw residue within the inner 2 m2 of the 3 m×3 m plots was replaced by 15N labeled straw residue. The labeled residue was subsequently followed into the light fraction and mineral fraction of the soil through the winter flooding period. Foraging waterfowl increased the loss of 15N from the residue from 83 to 89%, but did not affect the mass decomposition and loss of total N of the residue. No significant effect of the waterfowl was seen in the recovery of the residue N in the total soil N pool and in the light fraction, although there was a low recovery of residue N within these pools in the control treatments. The recovery of residue N in the NH4+-N pool after winter flooding was significantly lower in the presence of waterfowl. The increased loss of residue N and lower recovery of residue N in the presence of waterfowl may indicate that the rate of N-cycling was increased. A better understanding of the N-cycle in winter flooded rice fields in the presence of waterfowl is needed to assess the potential benefits of winter flooding for the rice farmers.