Groundwater Nitrogen and Phosphorus data

This data set contains NO3-N, NH4-N, PO4-P, and Total Nitrogen (TN) concentration (mg/L) measurements from shallow groundwater (GW) samples collected continuously over a three year period. Additional GW data collected included Ec, pH, depth to water table and GW resistance (Ohms). The GW analyzed in this dataset was collected in Beltsville, MD at the Central Maryland Research and Education Center (CMREC) (39°00'46.0"N 76°49'35.7"W). Maryland has a temperate climate with humid dry summers and wet cold winters. The groundwater collection was part of a greater experiment utilizing two different cover crop treatments (enhanced and standard) in two different cash crop residues (Corn and Soybean). The enhanced cover crop treatment utilized interseeding rye, radish and clover into standing cash crops in early fall and then late termination of the cover crop mixture after the cash crops had already been planted. Conversely, the standard cover crop treatment planted rye monoculture in late fall after cash crop harvest and terminated the cover crops in early spring prior to cash crop planting. The data in this data set was collected biweekly from July 2021 until May 2024 using an array of 14 hand dug groundwater wells. Four wells were installed in each cover crop treatment type, two were installed in a permanent grass control strip, and four wells were installed in the riparian forest patch adjacent to the study field. During each sampling event, depth to water table and GW resistivity were measured using a handheld Ohms meter. The wells were purged, allowed time to recharge, and then a 70mL sub-sample was taken back to the lab at the University of Maryland College Park. The Ec (uS/cm) and pH of each sub-sample were measured and then the sub-sample was filtered through a 0.45um filter paper. After filtration the inorganic N concentrations, NO3-N and NH4-N, were determined colorimetrically using the Salicylate Method and the Endol Blue Method respectively. The inorganic P concentration, PO4-P, was determined using the Molybdate colorimetric test. The filtrate was also analyzed for the TN concentration by mixing the filtrate with a strong alkaline persulfate solution and heating the mixture for 1hr at 120 deg C under 16kpa in an autoclave. Following this, the mixture was analyzed for NO3-N and using the respective NO3-N colorimetric methods, giving the total dissolved N concentration. All colorimetric analysis was performed using a DU 7000 Series Beckman Coulter Spectrophotometer and absorbance readings were converted into concentration (mg/L) using equations derived from the standard curves. 

NO3-N (mg/L) = 135.5*absorbance

NH4-N (mg/L) = 55.7*absorbance

TN (mg/L) = 146.7*absorbance - 1.447

PO4-P (mg/L) = 1.764*absorbance + 0.007