Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2007
Publication Date: 12/21/2007
Citation: Smith, M.W., Wood, B.W., Raum, W.R. 2007. Recovery and partitioning of nitrogen from early spring and midsummer applications to pecan trees. Journal of the American Society for Horticultural Science. 132(6):758-763. Interpretive Summary: Nitrogen (N) management is a major cost in pecan orchard enterprises. Because of decreasing energy resources required to produce N and to the increasing expense of N-fertilizer, it is important to devise orchard management practices that reduce usage while still meeting tree and orchard needs. Bearing pecan trees were studied regarding their uptake of N from soils, when the N was needed, and where it was found within trees. It was found that pecan trees maintained with adequate N nutrition derived the majority of N used for annual parts from stored N pools, although applied N is also rapidly absorbed and transported to N sinks. These results emphasize the importance of maintaining an annual N fertility program for current and future production and provides guidelines for improvements in N management of pecan orchards.
Technical Abstract: Effective nitrogen (N) management promotes consistent and abundant pecan [Carya illinoinensis (Wangenh.) C. Koch] production while minimizing waste. Recovery and partitioning characteristics of N potentially affects N management decisions; for this reason, we report certain N characteristics exhibited by trees in a bearing ‘Pawnee’ orchard. Nitrogen was applied pre-budbreak (PBB) as a single 10 Mar. application at 1.689 g•cm-2 cross-sectional trunk area or a split application in March (70%) followed by a mid summer application during rapid fruit development (RFD) on 28 July (30%) (i.e., PBB + RFD) using 15N-enriched fertilizer. Recovery of N by trees the first year was 7.2% from the PBB application and 11% from the RFD portion of the split application. Nitrogen application was 210% larger at PBB (March) than at RFD (July) resulting in 118% more N absorbed. At harvest in November, fruit contained 41% and 36% of total N recovered during the first year from the PBB and RFD treatments, respectively. About 3% of the total fruit N was derived from fertilizer (NDF) absorption during the current year. Recovery was 12% for the PBB treatment and 19% for the RFD treatment by the end of the second growing season, with 93% more N absorbed from the PBB application. Nitrogen recovered from the PBB application increased about 50% while trees were dormant, but there was little change in N recovery when applied during RFD. During the year of application, NDF was similar in shuck, shell and kernel tissue when 15N enriched fertilizer was applied PBB. When applied at RFD more NDF was in the kernel than the shuck and shell, indicating rapid absorption and transport to the fruit, especially to the developing kernel. In both treatments, most fruit N was derived from tree storage reserves. In the second year, NDF was highest in shucks and lowest in kernels for the PBB application; thus, N enrichment from the previous year was being depleted. In contrast, NDF was higher in kernels than shucks and shells when 15N-enriched fertilizer was applied during RFD the previous year, indicating that N applied during RFD the previous July was being absorbed in the latter part of the subsequent growing season. This study demonstrates that pecan trees maintained with adequate N nutrition derived the majority of N used for annual parts from stored N pools, although applied N was also rapidly absorbed and transported to N sinks. Dependence on endogenous N pools explains why pecans usually require at least two years to respond when N is withheld from well-managed trees. These results emphasize the importance of maintaining an annual N fertility program for current and future production.