Response of Water and Nitrogen Losses to Water Management Practices and Green Manure Application in Lowland Paddy Fields

Abstract

The complexity of agricultural water management practices has challenged researchers attempting to determine how hydrological pathways affect water and nitrogen (N) losses. Field experiments in a typical rice agricultural catchment in the Zhanghe Irrigation District in China were conducted in 2014 and 2015 to evaluate and quantify the connections between hydrological pathways and water and N losses. Four water management periods—namely, the irrigation period (IP), the rainfall period (RP), the normal-drainage period (DP), and the nonirrigation-drainage period (NP)—were defined based on climate conditions and artificial irrigation and drainage periods. The experimental results indicated that most water and N losses occurred in DP and RP and that lateral seepage and tailwater were the main sources of the observed drainage. Low average concentrations of ammonia N (NH4+─ N, 0.373 mg L−1) and nitrate N (NO3−─ N, 0.285 mg L−1) and low proportions of dissolved N (42.2–54.0%) were detected in association with green manure application, whereas the average concentration of total N (TN, 1.376 mg L−1) was relatively high, which resulted in slight N pollution in the studied agricultural area. The concentrations and proportions of dissolved N were greastest in DP and RP. The average daily mass fluxes of NH4+─ N, NO3−─ N, and TN were 0.027, 0.020, and 0.095 kg ha−1 day−1, respectively, and the magnitudes in DP and RP were significantly greater than those in IP and NP. The average concentrations of NH4+─ N, NO3−─ N, and TN were not correlated to drainage discharge according to a Pearson correlation analysis, whereas a power function can quantify the relationship between N mass fluxes and discharge during various water management periods. The average thresholds of drainage discharge, at which N export from paddy fields equalled to zero, were 45.2 m3 day−1 ha−1 for NH4+─ N, 51.9 m3 day−1 ha−1 for NO3−─ N, and 55.5 m3 day−1 ha−1 for TN. These findings indicate that controlling drainage discharge is important for the reduction of water loss and N loss from paddy fields.