There is one important concern for sustainable nitrogen management at a wastewater land application site, which is the potential nitrogen contamination to groundwater caused by nitrogen leaching. Nitrogen mass balance has been proposed as an effective approach to control the nitrogen leaching and to remove the risk of nitrogen contamination to groundwater at wastewater land application sites. However, the quantification of nitrogen in mass balance needs tremendous investigation. This paper was to report the field research regarding nitrogen fate and transformation at wastewater land application sites, to supply the data for nitrogen mass balance, and to provide the information for sustainable nitrogen management. Field study of nitrogen was conducted at a wastewater land application site located in the City of Lubbock, Texas, USA. Total nitrogen, nitrate nitrogen, total Kjeldahl nitrogen, and ammonia nitrogen were analyzed in applied wastewater, leached wastewater, and soils. There was no nitrogen contamination to groundwater found during the investigation at the site designed and managed by nitrogen mass balance. The results can supply information for effective sustainable nitrogen management and be used to improve the effectiveness and efficiency at a wastewater land application site. This paper also calls on further and more detailed field investigation and quantification of nitrogen with an aim to obtain more data and knowledge for advanced nitrogen mass balance and sustainable nitrogen management at a wastewater land application site.