Groundwater Nitrogen Source Identification and Remediation

Project Work Plan

In order to implement the recommendations described above, this project will (1) evaluate relative inputs of natural and fertilizer N in the Texas High Plains and Rolling Plains, (2) quantify nitrogen inventories under different agricultural land use management schemes, (3) identify and demonstrate strategies for reducing groundwater nitrogen levels (e.g. N crediting), and (4) transfer the results to farmers and others throughout the region through field days, workshops, and other venues.

Quantify Relative Inputs of Natural and Fertilizer Nitrogen in the Texas High Plains

Preliminary work in the Texas High Plains indicated that 74% (median) of total nitrate inventories in soil profiles is from mineralization of natural SOM in the soil profile that developed during initial cultivation. Support for the attribution of nitrate to mineralization of SOM during initial cultivation was provided by bulges of nitrate levels in the deeper part of the profiles that extended into old soil water (pre-cultivation) and were dated using chloride concentration analyses. These results were preliminary and need more support and justification. UT-BEG was not able to compare native rangeland sites with nearby cultivated sites to quantify changes in organic carbon and nitrogen to support these results because suitable native rangeland sites were not available in the southern High Plains.

The proposed work is designed to quantify the changes between native rangeland sites and cultivated sites that are in close proximity to each other. UT-BEG will use data from the USDA Bushland site near Amarillo where an area of the research station has been maintained under rangeland management and is adjacent to cropland. Previous drilling has shown that there is very low nitrate under native rangeland at this site and there are high nitrate bulges that extend into old soil water under cropland, similar to results from the southern High Plains.

UT-BEG will measure organic carbon and total nitrogen in the native and cropland profiles to quantify the reduction in organic carbon from the native to the cropland site. These data will be used to determine if changes in organic carbon and nitrogen can account for the increased nitrate found in profiles under cropland. This is an ideal system to conduct this comparison because of the close proximity of the native and cropland systems and the similarity in soil types between the two settings. UT-BEG will also examine carbon-13 isotopes on the organic carbon which may provide insights on the impact of the shift from native vegetation to cropland on the relative proportions of soil organic carbon derived from the native system versus the cultivated system. Soil and plant N will also be determined from long-term Conservation Reserve Program (CRP) fields to provide proof of concept that SOM may be a potential source of nitrate. Mass balance studies will also be conducted to evaluate the relative inputs of nitrogen from different sources.

These results should be applicable to the entire Texas High Plains and much of the Rolling Plains Seymour aquifer regions where up to 70% of wells exceed the 10 mg/L NO3-N maximum contaminant level. Unfortunately there are no appropriate rangeland sites in the Seymour aquifer region to apply this approach, but perhaps long-term CRP fields could serve as a substitute. Nitrate derived from mineralization of native SOM does not constitute a continuous input to the system and should move through the groundwater system as a pulse. UT-BEG will evaluate this process through mass balance analyses.

Quantify Nitrogen Inventories under Different Agricultural Land Use Management Schemes

Previous drilling and analyses have shown different nitrate inventories under various agricultural management scenarios. This drilling has been conducted at many sites in the Texas High Plains and Rolling Plains, including research farms associated with Texas Tech, such as La Mesa Research Farm in Dawson county, AgCares Farm, and Halfway Farm. Historical data of water and nutrient applications have been recorded at these sites. The drilling and sampling at these sites will be used to connect agricultural management practices and accumulation and/or leaching of nitrates in the subsurface. Historical records of agricultural practices will be examined for dryland and irrigated sites. The above farms offer ideal locations for conducting this work because of the detailed records on water and nutrient applications. The quality of the irrigation water has also been analyzed. Highest nitrate-N inventories were found under irrigated agriculture, moderate levels of nitrate were found under dryland agriculture, and very low levels of nitrate were found under rangeland settings.

UT-BEG will examine these data to determine relationships between nitrogen application rates and subsurface inventories. Based on these data and findings from UT-BEG, Texas AgriLife Research will develop recommendations on nitrogen application rates for farmers. In addition to the amount of nitrogen applied, the timing of the application process is also critical. Previous studies indicate that ~50% of the nitrogen is applied pre-plant (Bronson et al., 2006) and this nitrate may be much more vulnerable to leaching because the crop has not developed to utilize the applied N. These data are very important to minimize future groundwater nitrate contamination. Texas AgriLife Research will work with UT-BEG to determine best management practices (BMPs) related to nitrogen fertilizer applications for producers. UT-BEG will also evaluate nitrate input from mineralization of SOM. This mechanism may contribute substantial amounts of nitrate to the system. UT-BEG will supplement existing data by drilling additional boreholes in Lynn county where extremely high groundwater nitrate contamination is found, provided site access can be obtained.

One promising BMP is nitrogen crediting. This practice will be demonstrated and verified by Texas AgriLife Research located at the Vernon Center. Side-by-side plots of N crediting and conventional fertilization practices will be demonstrated to farmers in the region and benefits of N crediting documented, both economically and environmentally. This is greatly needed to provide assurance to farmers in the region that the nitrogen in the irrigation water (which in some soils leaches quickly through the root zone) is truly available for crop uptake and that reductions in fertilizer applications will not adversely impact crop yields and their livelihoods.

Due to the quick leaching alone, it is likely that a one to one accounting of the nitrogen in the irrigation water is not possible. This will also vary depending on the irrigation method used. These impacts must be quantified and demonstrated to area farmers to obtain widespread adoption. Primary crops in the region include cotton, bermudagrass hay, wheat, and peanuts. Demonstrations at the Chillicothe Experiment Station will target three different irrigation systems cropped to cotton: subsurface drip, furrow, and overhead irrigation. Nutrient management strategies will be based on the crop's agronomic: 1) N requirements; 2) N and phosphorous (P) requirements; 3) N requirements minus irrigation N credit; 4) N and P requirements minus irrigation N credit and 5) the control (N supplied through irrigation water only). Soil samples will be taken to a depth of 36 inches following each growing season. Samples will be segmented (0-6, 6-12, 12-18, 18-24, and 24-36 inches) and analyzed for nitrate, ammonium, total N, and phosphorus. Irrigation water samples will also be taken weekly throughout the irrigation season and analyzed for nitrate. One irrigation treatment based on a pre-selected evapotranspiration replacement level will be used among the three irrigation systems.

To transfer the results and recommendations of the project to farmers directly and via project partners, AgriLife Research will work with TWRI and UT-BEG to develop and distribute a fact sheet, technical report, handouts, presentations, and refereed journal publications and posters (as appropriate). This information will be presented at a minimum of 1 field day at the demonstration site, 2 national meetings, 2 regional meetings, and 3 workshops/stakeholder meetings held to discuss nitrate and irrigation strategies. These educational materials will primarily target the Texas High Plains and Rolling Plains regions but will have applicability to similar systems both regionally and nationally. To ensure the long-term delivery of these materials, findings, and recommendations, the educational materials will subsequently be made available to AgriLife Extension and other project partners for use at other venues in the region.

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