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Philip A. Throop and Eric J. Hanson

Absorption of “N-enriched fertilizer by young `Bluecrop' bushes was compared following applications on six dates between April and Sept. Ammonium sulfate solutions containing 2.1 g N (10.2 atom % 15N) were dripped directly into the root zone of single bushes. Soil covers and irrigation were used to maintain similar soil moisture conditions during treatment periods. Bushes were excavated after two weeks of exposure, and separated into roots, stems, and current-season growth (shoots, leaves, fruit). Tissues were dried, weighed, and analyzed for 15N and 14N by mass spectrometry. Bushes treated in May, June and July absorbed a greater percentage of applied N (6-8%) than bushes treated in Apr, Aug or Sept (1-3%). Results indicate that fertilization between late May and late July may result in the greatest efficiency of fertilizer use.

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Philip A. Throop and Eric J. Hanson

Rates of absorption of 15N-enriched ammonium sulfate by young `Bluecrop' highbush blueberries (Vaccinium corymbosum L.) were compared following applications on six dates between late April and September. Ammonium sulfate solutions containing 2.1 g N (10.2 atom % 15N) were dripped directly into the root zone of single bushes. Soil covers and irrigation were used to maintain similar soil moisture conditions during treatment periods. Treated bushes from each application date were excavated after 2 weeks of exposure and separated into roots, stems, and current season's growth (new shoots, leaves, fruit). Tissues were dried, weighed, and analyzed for 15N and 14N by mass spectrometry. Soils were also analyzed for labeled and nonlabeled N. Bushes treated in late May, June, and July absorbed a greater percentage of applied N (6% to 9%) than bushes treated in April, August, or September (1% to 3%). Absorption of N appeared to be affected more by the demand of the plants than soil N availability. Plants absorbed N most efficiently during active growth between late bloom and fruit maturity.

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Eric J. Hanson, Philip A. Throop, Sedat Serce, John Ravenscroft and Eldor A. Paul

Highbush blueberries (Vaccinium corymbosum L.) are long lived perennial plants that are grown on acidic soils. The goal of this study was to determine how blueberry cultivation might influence the nitrification capacity of acidic soils by comparing the nitrification potential of blueberry soils to adjacent noncultivated forest soils. The net nitrification potential of blueberry and forest soils was compared by treating soils with 15N enriched (NH4)2SO4, and monitoring nitrate (NO3 --N) production during a 34-day incubation period in plastic bags at 18 °C. Net nitrification was also compared by an aerobic slurry method. Autotrophic nitrifiers were quantified by the most probable number method. Nitrate production from labeled ammonium (15NH4 +) indicated that nitrification was more rapid in blueberry soils than in forest soils from six of the seven study sites. Slurry nitrification assays provided similar results. Blueberry soils also contained higher numbers of nitrifying bacteria compared to forest soils. Nitrification in forest soils did not appear to be limited by availability of NH4 + substrate. Results suggest that blueberry production practices lead to greater numbers of autotrophic nitrifying bacteria and increased nitrification capacity, possibly resulting from annual application of ammonium containing fertilizers.