Carbon and Nutrient Losses in Rice Field Drainage Water.

In flooded irrigated rice systems, much of the water applied to a field may run off field into surface waters. The amount of carbon and nutrient lost from rice systems in drainage water is not known, nor is it usually accounted for in annual nutrient budgets. Research was conducted on eight rice fields over a two year period to determine seasonal net losses of dissolved organic carbon (DOC) and nutrients (N, P and K). In California, rice fields are typically flooded during the growing season, harvested and subsequently reflooded over winter to facilitate straw decomposition. Net loss was calculated as the amount of nutrients leaving the field as drainage minus the incoming nutrients in the irrigation water. A negative net loss value indicates that the field was a sink for that nutrient. In all cases net losses were greater during the winter than during the growing season. Net DOC losses ranged from -14 to 20 and 0 to 190 kg ha^-1 during the growing season and winter flooding period, respectively. Ammonium-N losses ranged from -0.6 to 1 and 0 to 3.3 kg ha^-1 during the growing season and winter flooding period, respectively. Nitrate-N losses were negligible in both seasons. Dissolved inorganic P (DIP) losses ranged from -1 to 1 and 0 to 10 kg ha^-1 during the growing season and winter flooding period, respectively. During the growing season there was no net K losses from rice fields (on average -5 kg ha^-1), however, during the winter K losses ranged from 0 to 99 kg ha^-1.  For nutrients where net losses were detected, losses increased with increased water flow. Thus, reducing the amount of water flowing through fields should reduce losses. During the growing season under low water input fields were sinks for DOC and all measured nutrients.