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The implications of lag times between nitrate leaching losses and riverine loads for water quality policy.

ABSTRACT: Understanding the lag time between land management and impacts on riverine nitrate-nitrogen (N) loads is critical to understand when action to mitigate nitrate-N leaching losses from the soil profile may start improving water quality. These lags occur due to leaching of nitrate-N through the subsurface (soil and groundwater). Actions to mitigate nitrate-N losses have been mandated in New Zealand policy to start showing improvements in water quality within five years. We estimated annual rates of nitrate-N leaching and annual nitrate-N loads for 77 river catchments from 1990 to 2018. Lag times between these losses and riverine loads were determined for 34 catchments but could not be determined in other catchments because they exhibited little change in nitrate-N leaching losses or loads. Lag times varied from 1 to 12 years according to factors like catchment size (Strahler stream order and altitude) and slope. For eight catchments where additional isotope and modelling data were available, the mean transit time for surface water at baseflow to pass through the catchment was on average 2.1 years less than, and never greater than, the mean lag time for nitrate-N, inferring our lag time estimates were robust. The median lag time for nitrate-N across the 34 catchments was 4.5 years, meaning that nearly half of these catchments wouldn't exhibit decreases in nitrate-N because of practice change within the five years outlined in policy.

SUBMITTER: McDowell RW

PROVIDER: S-EPMC8360963 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:To evaluate the process of nitrate accumulation and leaching in surface and ground water, we conducted simulated rainfall experiments. The experiments were performed in areas of 5.3 m2 with bare slopes of 3° that were treated with two nitrogen fertilizer inputs, high (22.5 g/m2 NH4NO3) and control (no fertilizer), and subjected to 2 hours of rainfall, with. From the 1st to the 7th experiments, the same content of fertilizer mixed with soil was uniformly applied to the soil surface at 10 minutes before rainfall, and no fertilizer was applied for the 8th through 12th experiments. Initially, the time-series nitrate concentration in the surface flow quickly increased, and then it rapidly decreased and gradually stabilized at a low level during the fertilizer experiments. The nitrogen loss in the surface flow primarily occurred during the first 18.6 minutes of rainfall. For the continuous fertilizer experiments, the mean nitrate concentrations in the groundwater flow remained at less than 10 mg/L before the 5th experiment, and after the 7th experiment, these nitrate concentrations were greater than 10 mg/L throughout the process. The time-series process of the changing concentration in the groundwater flow exhibited the same parabolic trend for each fertilizer experiment. However, the time at which the nitrate concentration began to change lagged behind the start time of groundwater flow by approximately 0.94 hours on average. The experiments were also performed with no fertilizer. In these experiments, the mean nitrate concentration of groundwater initially increased continuously, and then, the process exhibited the same parabolic trend as the results of the fertilization experiments. The nitrate concentration decreased in the subsequent experiments. Eight days after the 12 rainfall experiments, 50.53% of the total nitrate applied remained in the experimental soil. Nitrate residues mainly existed at the surface and in the bottom soil layers, which represents a potentially more dangerous pollution scenario for surface and ground water. The surface and subsurface flow would enter into and contaminate water bodies, thus threatening the water environment.

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The implications of lag times between nitrate leaching losses and riverine loads for water quality policy.

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