Nitrogen Budget and Soil N Dynamics after Multiple Applications of Unlabeled or 15Nitrogen-Enriched Dairy Manure

Repeated N applications to field crops, either as inorganic fertilizers or animal manures, can lead to N buildup in soils with potential long-term environmental hazards. The objective of this 3-yr field study was to monitor total- and mineral-N levels in soil after repeated fertilizer or single or repeated dairy manure applications, and to compute an N balance for the soil-crop system. Unlabeled and 15N-enriched dairy manure were used. The experiment was conducted on a Plano corn silt loam continuously cropped to corn (Zea mays L.) Manure increased total- and NO3–N levels in soil, especially in the 0- to 30-cm depth and in plots receiving frequent and recent manure applications. Manure increased NO3–N in the 0- to 30-cm soil layer more than fertilizer N, whereas the opposite was true in the 30- to 60- and 60- to 90-cm layers. There was a clear NO3–N buildup with repeated manure treatments. Unlabeled N measurements were not accurate enough to track trends in soil total N levels, hampering the calculation of an N balance. 15Nitrogen-labeled manure allowed for direct measurement and provided more accurate estimates of N recovery in soils and crops. During the 3-yr study period, an average of 18% of applied manure 15N was recovered in corn silage and 46% remained in the soil. Unaccounted-for 15N (36%) was assumed to be lost mainly by NH3 volatilization and denitrification. Most (82%) of the 15N remaining in soil was present in the top 30 cm, irrespective of frequency of manure application. Although costly and time-consuming, the use of 15N-labeled manure provided a much better approach to study the fate of manure N within the soil-crop system, compared with unlabeled manure.