This study provides a more comprehensive characterization of nitrogen cycling than has previously been documented for cultivated cranberry beds. Highlights of this study include data from five conventionally cultivated cranberry beds. Our data demonstrate that the amount of nitrogen held in the plants and the soil (average = 82 kg N/ha) exceeds the annual inorganic nitrogen inputs (average = 50 kg N/ha).We considered other potential sources of nitrogen, and for most of the growing season, early May through August, dissolved organic nitrogen (DON) dominates extractable nitrogen pools in the bed (5-25 kg/ha), while amounts of extractable inorganic nitrogen are quite low (0-2 kg/ha). The large pool of DON may represent an additional nitrogen input that helps to settle the discrepancy in the mass balance equation.
Cranberry (Vaccinium macrocarpon), is a low-growing, woody, evergreen vine that is native to Wisconsin. The cranberry industry in Wisconsin has the highest amount of acreage and production in North America (~17,000 acres). Ammonium-based fertilizers are applied at rates of 40 – 50 kg N/ha, much lower than the nitrogen additions for most agronomic crops. Cranberries are also different in that they are very sensitive to large inputs of nitrogen; they cause cranberries to have excess vegetative growth at the expense of yield (Figure 1). Therefore applications of nitrogen fertilizer are carefully regulated by growers.
Although inorganic nitrogen additions from fertilizers are carefully managed, dissolved organic nitrogen is currently not considered a nitrogen source for the plants. Therefore it is not included in growers’ management plans. However, there is evidence that because cranberry roots form an association with ericoid mycorrhizal fungi, they may be able to access dissolved organic nitrogen (Bending and Read 1996). Unlike fertilizer additions, the dissolved organic nitrogen pools are naturally replenished by leaf litter additions and fine root turnover throughout the season. Further research on the cranberry-ericoid mycorrhizal fungi association will provide growers with important information that may help them to take advantage of a naturally existing relationship.
My short-term objectives are to measure extractable soil nitrogen pools. I looked at three different forms of nitrogen: ammonium, nitrate, and dissolved organic nitrogen (DON). I also explore the impacts of a relationship between cranberry roots and mycorrhizal fungi in the field on nutrient management. The intermediate-term objectives are to share the results with the growers. Over the long-term, I hope that the growers may find my research useful and incorporate some of the findings into their nutrient management plans. By 2008, cranberry growers will be required to provide annual nutrient management plans to the Wisconsin Department of Natural Resources, in an attempt to more carefully monitor and regulate surface and groundwater inputs, including additions made by fertilizers. Furthermore, a better understanding of the patterns of nitrogen cycling and use by cranberries may increase production profitability and sustainability.
Data were collected from five conventionally cultivated beds in Central Wisconsin (Wood and Juneau Country). All beds are planted with the variety Stevens (Vaccinium macrocarpon Ait. Stevens). A grid of 35 points, 5 columns and 7 rows, was laid out in each bed (approximately 366 m x 46 m). Soil N pools were measured by taking three 15 cm deep cores from 6 randomly chosen points in the grid at three times throughout the growing season. One additional core was placed in a sealed polyethylene bag, and inserted back into the hole, to measure net mineralization and DON accumulation in the field by the buried bag technique. The two other cores were brought back to the laboratory for determination of TN, nitrate and ammonium. The extracts from the initial samples were frozen (-20 ° C) and then later analyzed pairwise with the incubated samples. The incubated samples were left in the field for 5 -7 weeks.
Samples were processed within 48 hours of collection. A 1:10 w/v soil to 1M KCl was used for extraction for both soil cores and buried bag samples, samples were shaken for 1 minute, and then allowed to extract for 24 hrs at room temperature. Samples were then filtered with a 0.2 micron glass fiber filter. Total dissolved nitrogen (TDN), nitrate and ammonium were determined colorimetrically on a Labsystems Multiskan EX microplate reader with absorbance at 650 nm using the colorimetric Berthelot assay. TDN measurements were made using this assay, following a persulfate digestion and a reduction with De Varda’s metal. Nitrate samples were reduced to ammonium with DeVarda’s metal and then read colorimetrically with the Berthelot assay. Samples were analyzed for ammonium directly. Dissolved organic nitrogen was determined by subtracting ammonium and nitrate from the total nitrogen values.
Sub-samples of roots were taken from the top layer of each core and stored in 50% ethanol. Samples were cleared and stained using the methods described in Scagel (2003) and Grace and Stribley (1991). Using a line intercept method (Giovannetti and Mosse, 1980), root length colonization (percent root length containing ERM structures), was determined with a dissecting microscope (40x to 90x magnification).
Flooding events were measured with grab samples in three different beds during major water influxes during the growing season, including trash flood (spring), and the harvest flood (late fall). Precipitation was estimated with the National Atmospheric Deposition data. Irrigation inputs were measured with ion exchange columns. Irrigation rates were provided by the growers. Groundwater was measured from mini-piezometers placed in the field.
It is estimated that cranberries have about 70 kg N/ha in current season shoots and fruit, yet growers don’t apply more than 45 kg N/ha annually. What factors contribute to the difference between what is added in nitrogen fertilizer and what accumulates in the current season’s growth? I have applied a mass balance approach to help answer this question. Other potential inputs include nitrogen from precipitation, irrigation, floodwater, and mineralization.Notrogen values are based on composite data from our unpublished work and previous extension publications (Hart 2000).
Measuring extractable soil nitrogen pools was one of the most important components of the mass balance equation. I have found that there are relatively large pools of dissolved organic nitrogen (5 – 25 kg/ha), in comparison to inorganic nitrogen pools, which are much smaller (0 – 2 kg/ha).
These extractable soil nitrogen pools have been incorporated into the mass balance equation. Even after accounting for inorganic nitrogen inputs, there is still a discrepancy (average = 48 kg N/ha) between calculated residual = average 34 kg N/ha and the actual residual = 82 kg N/ha. I looked for another possible nutrient source for these plants. Accounting for the accumulation of DON from buried bags (about 19 kg N/ha) does not completely balance the equation, but it does help resolve some of the discrepancy found between nitrogen inputs and nitrogen held in the plants. This is seen in the recalculated residual.
Bending GD, Read DJ. 1996. Nitrogen mobilization from protein-polyphenol complex by ericoid and ecomycorrhizal fungi. Soil Biology and Biochemistry 28(12):1603 – 1612.
Gionvanetti, M., B. Mosse. 1980. An evaluation of techniques for measuring vesicular-arbuscular infection in roots. New Phytologist 84:489-500.
Grace, C., D.P. Stribley. 1991. A safer procedure for routine staining of vesicular-abuscular mycorrhzial fungi. Mycological Research 95:1160-1162.
Kosola KR, Workmaster BAA. 2007. Mycorrhizal colonization of cranberry: effects of cultivar, soil type, and leaf litter composition. Journal of the American Society for Horticultural Science. 132:1-8.
Nitrogen for bearing cranberries in North America. Editor J. Hart. 2000. Oregon State University Extension Service. EM8741.
Scagel C. 2005. Mycorrhizal status of sand-based cranberry (Vaccinium macrocarpon) bogs in southern Oregon. Small Fruits Review 2(1):31-41.
Educational & Outreach Activities
- 1) Thesis: Characterizing Nitrogen Cycling in Cultivated Cranberry Beds, December 2007
2) Annual Grower Meeting Cranberry School, January 2005, Wisconsin Dells Wisconsin
Title: Mycorrhizae and Water Management
3) National Association of Cranberry Researchers and Extension Workers Meeting, November 2005, British Columbia
Title: Characterizing Nitrogen Cycling from Cultivated Cranberry Beds
The short-term impact of the results from my project will include writing them up as a part of my dissertation and preparing a peer-reviewed journal article. The information will also be disseminated to growers through a newsletter article and through grower meetings. This information may also provide useful background information for research on further aspects of nitrogen cycling in cranberry beds, and overall nutrient management. A project investigating nitrate contamination of irrigation water is already being conducted, and also there are plans to look at nitrification in cranberry beds. Over the long-term, these projects will provide a more comprehensive look at nitrogen cycling in cranberry beds, and provide growers and managers with important information to help improve profitability and sustainability of this crop.
For the past three years I have worked at four different marshes and I have interacted with the cranberry farmers on each of them. The work that I have done is a characterization study, and therefore the information that I have gathered is laying the groundwork for future research. I have documented the relatively large pools of extractable DON in the soil and that there is ericoid mycorrhizal colonization of the cranberry roots. I have placed that information within the context of inorganic nitrogen management, which growers currently practice, through the mass balance approach. Future work may be able to demonstrate that the plants are actively using the DON in field. Additionally, the data will provide useful background information to other related research in our lab which is investigating the impact of nitrate in groundwater on yield.