Fate Of Pre-Plant And Split-Applied 15Nitrogen Enriched Urea In Corn

Abstract

Nitrogen (N) is essential to produce high yielding corn (Zea mays L.), but excess fertilization before rapid corn N uptake may result in N loss from the soil-corn system that reduces fertilizer N use efficiency, causes an economic loss for producers, and negatively impacts the environment. Urea fertilizer applications at planting are common, but recent years of wet springs in the U.S. upper Midwest has fostered a greater interest in split-applications that may avoid early spring N loss and improve fertilizer N supply to the crop. Fertilizer N rate, application timing, soil physical and chemical properties, and weather patterns can modify fertilizer-derived N distribution (FDN) in the soil profile, uptake by the crop, and potential for N loss in the year of application and subsequent years. Thus, the objectives of this study were to investigate the effects of fertilizer N rate and application timing on 1) FDN distribution and form in the soil, 2) FDN and soil-derived N (SDN) uptake and partitioning by the corn crop, and 3) fertilizer recovery and reuse in the soil-corn system over two consecutive growing seasons. Three studies were initiated in both 2014 (Becker14, Clara City14, Waseca14) and 2015 (Becker15, Lamberton15, Waseca15) in Minnesota, at sites that represented agronomically important soils. Urea fertilizer (46% N) was applied at planting at 45 kg N ha-1 increments from 0 to 270 or 315 kg N ha-1. An additional treatment was split-applied as 45 kg N ha-1 at planting and 90 kg N ha-1 when the corn had four fully developed leaves (V4). Labeled 15N urea (5 atom %) fertilizer was applied to microplots in the 45, 135, and 225 kg N ha-1 treatments, as well as the 45/90 kg N ha-1 split-application treatment. Soil samples were collected in the first growing season within eight days of 15N urea fertilizer application (PA), when the corn had eight fully developed leaves (V8), at tasseling (R1), and post-harvest (PHY1). In the second growing season, soil samples were collected at pre-plant (PPY2) and post-harvest (PHY2). Aboveground plant samples were collected at V8, R1, and physiological maturity (R6) in the first growing season, and R6 in the second growing season. At PA, 63 to 112% of FDN was recovered from the top 60 cm of the soil profile across all sites, except Becker14 and Clara City14 where 55 and 43% of the applied FDN were recovered by corn averaged across all treatments, respectively. Low recovery of FDN at Becker14 was likely due to N leaching through the loamy sand soil profile following greater-than-normal April through June precipitation. At Clara City14, low recovery of FDN was due to NH3 volatilization from inadequate incorporation of urea into the soil profile. Of the total FDN in the soil at PA, 72 to 90% was in the soil organic N fraction and temporarily protected from loss. The majority of soil FDN was in the top 15 cm of the soil profile, but FDN was observed in all soil sampling depths indicating that leaching of NO3- and soluble organic FDN is rapid irrespective of the soil texture. The soil inorganic FDN concentration was greatest immediately after fertilization but decreased to background levels (<10% of the applied N rate) by V8 or R1. Likewise, the concentration of FDN in corn biomass was greatest early in the season but decreased as soil FDN concentration decreased and as the corn increasingly assimilated inorganic SDN. At the end of the first growing season, approximately 20 to 47% of FDN was recovered in the soil-crop system. Aboveground corn FDN recovery ranged from low values of 2.8 to 4.4% at Becker15 (loamy sand) to higher values of 34.0 to 41.9% at Lamberton15 (loam) that reflected the N loss potential of each soil. Very low FDN recovery at Becker15 indicates that urea fertilizer applied at planting should not be done at sites with coarse-textured soils. Averaged across sites and treatments, 28% of the aboveground FDN was in the stover, 69% was in the grain, and 4% was in the cobs. Because only a small portion of FDN was returned to the soil-crop system from the first year corn residue and because soil organic FDN was fairly stable, ≤ 7.4 kg FDN ha-1 was assimilated in the aboveground biomass across all sites at the end of the second year, indicating residual FDN does not supply an agronomically important amount of N to the succeeding crop. The 45/90 kg N ha-1 split-application improved corn grain yield over the 135 kg N ha-1 single application at planting on coarse-textured soils by 3.3 Mg ha-1 on average but there was no yield improvement on fine-textured soils. Likewise, the split-application had greater FDN uptake values than the 135 kg N ha-1 treatment during the early vegetative development stages but did not differ from the 135 kg N ha-1 treatment at R6 in either aboveground FDN recovery or the soil FDN content. Partitioning of FDN to plant parts were similar between the split- and the single application. These results indicate that split-applications will likely have the greatest improvement in fertilizer use efficiency and grain production on coarse-textured soils that are prone to significant N loss. Overall, this study illustrated that a single or split-application of urea may be an acceptable management strategy when the soil and environmental conditions do not favor N loss. However, if spring conditions continue to be wetter-than-normal, urea applications done at planting or as early split-applications are poorly retained in the soil resulting in poor recovery efficiency by the corn crop and recycling for future crops. Other N sources may need to be considered, such as polymer-coated urea or anhydrous ammonia, that delay nitrification until later in the growing season for decreased risk of leaching and denitrification. Delaying the split-application a few weeks longer may be another way to avoid spring leaching events for improved FDN recovery. While fertilizer N significantly increased yield at 11 of 12 site-years in this study, ultimately, SDN accounted for most of the total N uptake and yield potential of the crop. This result re-emphasizes the importance of maintaining soil health and its N supplying capacity because its depletion will result in increasingly expensive input costs to achieve similar production levels.