Nitrate and phosphorus loads
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Daily nitrate-N, soluble reactive phosphorus and total phosphorus loads in subsurface drainage were quantified in an agricultural farm field in eastern Indiana (Davis Purdue Agricultural Center).
Site description: The data was collected from the field W at Davis Purdue Agricultural Center (DPAC) located in eastern Indiana. Field W is relatively flat (slope < 1%), with 0.16 km2 total area, divided into four plots, northwest (NW), southwest (SW), northeast (NE), and southeast (SE) with areas ranging from 3.5 ha to 3.7 ha. The four soil series at the site range from very poorly to somewhat poorly drained, with a small portion of moderately well drained series. The subsurface drainage system was installed in 2004, with 10-cm laterals having an approximate depth of 1 m and spacing of 14 m, resulting in a drainage intensity of 1.1 cm day-1 and drainage coefficient of 1 cm day-1. Drainage in the SE and NW plots was controlled at two different levels during some periods depending on the season, while the SW and NE were allowed to drain freely. This field has been in a corn-soybean rotation since 2011 and in continuous corn before that, and was managed using chisel-plow tillage in the fall and field cultivator tillage in the spring during the study period. Nitrogen (N) and phosphorus (P) fertilizers were applied at different rates prior to and after planting corn. Phosphorus was also applied prior to soybean planting in two of the three soybean years. The rate and timing of fertilizer applications were uniform for all plots and were based on Purdue Extension recommendations. Further details of the site management and data are available in Abendroth et al. (2017). More information about this site and fertilizer application can be found in Saadat et al., 2018.
Sampling strategy and load calculation: Automated water samplers (ISCO) were used to draw samples from the drainage outlet flow of each plot. Samples were collected every hour when flow was present except during winter, and combined into weekly composite samples varying in length from twice a week to biweekly. During the winter, water samples were collected manually to avoid freezing problems, approximately every week whenever flow was present. Samples were kept frozen until analysis and then analyzed on a SEAL Analytical AQ2 auto-analyzer to be tested for nitrate+nitrite-N (referred to nitrate-N), soluble reactive phosphorus (SRP) and total phosphorus (TP) according to US EPA methods.
Daily nitrate-N, SRP and TP concentration values needed for the load calculations were estimated using linear interpolation. After estimating daily concentrations, daily loads were calculated by multiplying the daily drain flow by estimated daily concentrations.
References: Saadat, S., Bowling, L., Frankenberger, J. and Kladivko, E., 2018. Nitrate and phosphorus transport through subsurface drains under free and controlled drainage. Water research, 142: 196-207.