EVALUATION OF PERCOLATION AND NITROGEN LEACHING FROM A SWEET PEPPER CROP GROWN ON AN OXISOL SOIL IN NORTHWEST PUERTO RICO Eric Harmsen, Joel Colon, Carmen Lis Arcelay and Dionel Cadiz Agricultural and Biosystems Engineering Department University of Puerto Rico – Mayagüez Campus

This Material Is Based on Research Supported by USDA/CSREES Grant No. CBAG-82 in Tropical/ Subtropical Agriculture Research

BACKGROUND N itrate leaching through the soil profile can result in a significant loss of fertilizer N, and may be a source groundwater contamination. N itrate leaching through the soil profile can result in a significant loss of fertilizer N, and may be a source groundwater contamination. Highly weathered soils may exhibit substantial anion exchange capacity (AEC), which retards the movement of anions such as NO 3 - through the profile. Highly weathered soils may exhibit substantial anion exchange capacity (AEC), which retards the movement of anions such as NO 3 - through the profile. However, AEC decreases as soil pH increases by agronomic practices such as liming. However, AEC decreases as soil pH increases by agronomic practices such as liming.

OBJECTIVE To evaluate the effects of liming and fertigation on the leaching of nitrogen through a highly weathered tropical soil To evaluate the effects of liming and fertigation on the leaching of nitrogen through a highly weathered tropical soil

Study Area – NW Puerto Rico

Field Study Field Study 2002 and 2003 at the UPR Experiment Station at Isabela, PR, using sweet peppers (Capsicum annuum, “Biscayne”) as the test crop and 2003 at the UPR Experiment Station at Isabela, PR, using sweet peppers (Capsicum annuum, “Biscayne”) as the test crop. 2 lime levels (Lime or No lime); 2 fertigation frequencies (Weekly or Bi-weekly). 2 lime levels (Lime or No lime); 2 fertigation frequencies (Weekly or Bi-weekly). Soil samples collected every two weeks at 20 cm increments, down to 80 cm. Soil samples collected every two weeks at 20 cm increments, down to 80 cm. Analyzed for NH 4 + and NO 3 - concentrations by steam distillation (Mulvaney, 1996). Analyzed for NH 4 + and NO 3 - concentrations by steam distillation (Mulvaney, 1996).

Experimental Layout

Estimating Nitrogen Leaching L NO3 = 0.01 ρ b NO 3 PERC / θ vol L NH4 = 0.01 ρ b NH 4 PERCθ vol L NH4 = 0.01 ρ b NH 4 PERC / θ vol

Water Balance Perc = (R-RO)+Irr-ET c +  S Perc = Deep percolation (R –RO) = Rainfall – Runoff Irr = Irrigation based on ET pan ET c = Evapotranspiration based on Penman-Monteith method.  S = Change in stored water

IRR = ET pan = K c K p E pan IRR = ET pan = Evapotranspiration based on pan K c = Crop coefficient K p = Pan coefficient E pan = Pan evaporation

ET c = K c ET o ET c = Evapotranspiration based on Penman- Monteith method. K c = Calibrated crop coefficient ET o = Reference evapotranspiration

Calibration of the Water Balance Equation Calibration variables: (RO/R) Calibration variables: (RO/R) Calibration target: average soil moisture content Calibration target: average soil moisture content

RESULTS

Depth% Sand 1 % Silt 1 % Clay 1 Soil Classification Bulk DensityPorosity 0-20 cm silty clay cm clay cm clay cm clay Depth Hydraulic Conductivity (cm/day) In-Situ Field Capacity Year 1 Site In-Situ Field Capacity Year 2 Site Moisture Content at 0.33 bar Pressure Moisture Content at 15 bar Pressure Available Water Holding Capacity (AWHC) 0-20 cm cm cm cm Properties of the Coto Clay

Evapotranspiration – 2003 Season

Calibrated Soil Moisture Content RO/R =

Estimated Percolation

Soil Nitrate Concentration at cm Depth 2002

Nitrate, ammonium and nitrate plus ammonium (Total) leached during Year 1 and 2 for the four experimental treatments UnitsLF1LF2NLF1NLF2LF1LF2NLF1NLF2 NO3kg/ha NH4kg/ha Totalkg/ha Total%

Percent of Nitrogen Leached Relative to Applied

How much N was leached during the flooding events? The estimated amount of nitrogen lost (average of all treatments) on April 6, 2002 and April 10, 2003 was 19.6 kg/ha and 20.1 kg/ha, respectively. The estimated amount of nitrogen lost (average of all treatments) on April 6, 2002 and April 10, 2003 was 19.6 kg/ha and 20.1 kg/ha, respectively. For years 1 and 2 this represented 34% and 60% of the total N lost by leaching during the two seasons, respectively. For years 1 and 2 this represented 34% and 60% of the total N lost by leaching during the two seasons, respectively.

The method used in this study may underestimate nitrogen losses, because the contribution of unsaturated flow to the percolation was not considered 2003

Conclusions No clear difference in nitrogen leaching was observed for the lime and no-lime treatments. No clear difference in nitrogen leaching was observed for the lime and no-lime treatments. No clear difference in nitrogen leaching was observed between the fertigation treatments. No clear difference in nitrogen leaching was observed between the fertigation treatments. The average percent of nitrogen (nitrate plus ammonium) leached during the 1st and 2nd season, relative to the amounts applied, were 26% and 15%, respectively. The average percent of nitrogen (nitrate plus ammonium) leached during the 1st and 2nd season, relative to the amounts applied, were 26% and 15%, respectively.

Conclusions cont. Leaching events were associated with large rainstorms, suggesting that leaching of N would have occurred regardless of the irrigation scheduling method used. Leaching events were associated with large rainstorms, suggesting that leaching of N would have occurred regardless of the irrigation scheduling method used. During the first and second seasons, respectively, 34% and 60% of the total N lost by leaching occurred during a single day (April 6 in 2002 and April 10 in 2003) when flooding was observed in the study areas. During the first and second seasons, respectively, 34% and 60% of the total N lost by leaching occurred during a single day (April 6 in 2002 and April 10 in 2003) when flooding was observed in the study areas. The N leaching results from this study may be underestimated because the contribution of percolation from unsaturated flow was not considered. The N leaching results from this study may be underestimated because the contribution of percolation from unsaturated flow was not considered.

Future Work Estimate N leaching rates using the Darcy velocity results, estimated from the tensiometers. Estimate N leaching rates using the Darcy velocity results, estimated from the tensiometers. Calibrate and validate a numerical model (HYDRUS-2D) to simulate water and nitrogen movement. Calibrate and validate a numerical model (HYDRUS-2D) to simulate water and nitrogen movement. Use the numerical model to develop best management practices in terms of maximizing N uptake and minimizing N leaching. Use the numerical model to develop best management practices in terms of maximizing N uptake and minimizing N leaching.