Nitrogen Cycling and Assimilative Capacity of Nitrogen and Phosphorus by Riverine Wetland Forests by Mark M. Brinson H. David Bradshaw Emilie S. Kane Department of Bi 01 ogy East Carolina University Greenvi 1 le, North Carol ina 27834 The work upon which this publication is based was supported in part by funds provided by the Office of Water Research and Technology, U.S. Department of the Interior, Washington, D.C., through the Water Resources Research Institute of the University of North Carolina as authorized by the Water Research and Development Act of 1978. Project No. B-114-NC Agreement No. 14-34-0001 -81 07 May 1981 This work was made possible with the assistance and support from a number of people. Martha Jones maintained structure and function in the laboratory environment and performed many of the nutrient analyses. Jerry Freeman contributed greatly to equipment supply and repair. Richard Volk of North Carolina State University kindly made available his mass spectro- meter for our use. Assistance in field work was provided by Randy Creech, Debbie Noltemeier, and Steve Nelson. Most of the figures were drafted by Nancy Edwards of the Regional Development Institute of East Carol ina University. We appreciate the sharing of ideas and have benefited greatly from discussions with Edward J. Kuenzler, Laura A. Yarbro, Patrick J. Mulholland, and Robert P. Sniffen, fellow enthusiasts of North Carolina swamps. Graham J. Davis read an earlier draft of the report and offered helpful comments. We appreciate the efforts of Arlene Hagar who typed the final draft. DISCLAIMER STATEMENT Contents of this publication do not necessarily reflect the views and policies of the Office of Water Research and Technology, U.S. Depart- ment of the Interior, nor does mention of trade names or commercial products constitute their endorsement or recommendation for use by the U.S. Government. ABSTRACT In riverine swamps, opportunities for nutrient exchange between surface water and the sediments of the swamp forest floor occur when streams flood and water overflows into the swamps and when runoff from uplands passes through floodplains. Studies conducted in floodplain swamps of two repre- sentative types in eastern North Carol ina provided insight into these processes. Nitrogen cycling experiments were conducted in both ecosystems with one system being subjected to sustained nutrient loading to assess its assimilative capacity. Labeled (15~)nitrate and ammonium were added to swamp surface water and their diffusion to the forest floor was followed. Of the original nitrate added, 46% remained in the surface water of Tar Swamp and 62% in Creeping Swamp after 2 days. Two days after ammonium treatments, correqeonding levels were 79% and 81%. As indicated by the absence of recoverable N in sediments following nitrate treatments, diffusion of labeled nitrate to the forest floor resulted in its transformation to N2O or N2 by denitrification. Although labeled ammonium also diffused to sediments and accumulated in a sediment-exchangeable form, there was simultaneous efflux of unlabeled ammonium from sediments to the water column. Also, ammonium was readily immobilized from the water by decomposing leaf litter and probably fila- mentous algae, both of which represent short term storages. However, exchangeable ammonium in the sediments was far more important in net accumulation. During the drydown phase, which is an annual summer-fall event in tupelo- cypress swamps, surficial sediment became aerated. Analysis of interstitial water indicated that this stimulated production of ammonium from organic nitrogen (ammonification) and subsequent nitrate production from ammoni um (nitrification). After short term pu1 ses of accumulation of these forms, nitrate was denitrified. Thus, available nitrogen reserves in the sediments were depleted during annual drydown episodes, and the capacity for additional nitrogen assimilation by the sediments was renewed. An experiment was then conducted to determine the capacity of sediments for sustained nutrient assimilation by adding nitrate, ammonium, phosphate, and secondarily treated sewage effluent to surface water in separate chambers at weekly intervals for 46 weeks. Nitrate disappeared rapidly from the surface water between weekly additions and did not accumulate in subsurface water; denitrification was estimated to proceed at a minimal rate of 24.5 g NQ~-N.~-~over the 10-month loading period. Substantial quantities of ammonium accumulated in surface water, and after a lag period, in the exchangeable ammonium fraction of sediment. However, summer drydown depleted these accumulations, presumably by the ni rification-deni trification pathway, for an overall ammonium loss of 13.5 g-m-B-10mo-1 in ammonium treatments. Phosphate added to surface water accumulated as an acid-extractable form in sediments to a level of nearly one-half of total sediment phosphorus by the end of the experiment. Although rates of phosphate addition in these treatments were severalfold higher than the treatment receiving sewage effluent, the inherently phosphate-rich sediments and the lack of an atmos- pheric escape pathway for phosphorus may limit the capacity of the swamp for further phosphate assimilation and 1ong-term sewage appl ication. ABSTRACT (Continued) Studies on the distribution of biomass and nutrients in lateral roots were conducted in the two riverine swamps. Lateral root biomass (2345-2702 g dry wtom-2) and nutrient stocks (for N, P, Ky Ca, Mgy Na) fell within the range for other forested wetlands and uplands. However, Fe concentrations in roots and stocks of Fe per unit area of swamp floor may be severalfold higher than in upland forests, presumably because of greater Fe mobility in wetland sediments and Fe precipitation on root surfaces. In the tupelo swamp, a trend of increasing root biomass with increasing depth is a pattern hitherto unreported for forested ecosystems. TABLE OF CONTENTS Page ACKNOWLEDGMENTS ......................; .. i i ABSTRACT ............................. iii LIST OF FIGURES ......................... vii LISTOFTABLES .......................... ix CONCLUSIONS AND RECOMMENDATIONS ................. xi 1 . INTRODUCTION Contents and Purpose ................... 1 Southeastern River Swamps: Distribution. Structure andFunction ...................... 1 Nutrient Cycling in Swamp Forests ............. 4 Description of Study Area ................. 9 Tar Swamp ....................... 9 Creeping Swamp ..................... 10 2 . WATER-SEDIMENT NITROGEN TRANSFORMATIONS Introduction ....................... 13 Methods .......................... 13 15~Enrichment Experiments ............... 13 Field Work and Sample Collection .......... 13 Methods of Sample Analysis ............. 14 Moisture ..................... 14 Total Kjeldahl Nitrogen ............. 14 15~Analysis ................... 15 Exchangeable NH4 and NO3 ............. 16 Leaves and Woody Matter ............. 16 Surface Water .................. 16 Ammonia Volatilization ................. 16 Ammonium and Nitrate in Interstitial Water ....... 17 Results .......................... 17 15~Experiments .................... 17 Temperature and Dissolved Oxygen .......... 17 Distribution of 15~................. 17 Seasonal Nitrogen Transformations in Interstitial Water ..................... .. 20 Ammonia Volatilization ................. 24 Discussion ........................ 25 3 . SUSTAINED LOADING OF NITROGEN AND PHOSPHORUS TO THE SEDIMENT-WATER SYSTEM Introduction ....................... 27 LIST FIGURES Page 1. Idealized profile of species associations in southeastern bottomland hardwood forests. ................. 3 2. Seasonal changes in the physical and chemical environment of Tar Swamp ......................... 11 3. Losses of inorganic nitrogen and changes in atom % 15~of the surface water of (a) Tar Swamp and (b) Creeping Swamp .... 20 4. Accumulation of 15~in decomposing leaf detritus of Tar Swamp and Creeping Swamp expressed as (a) mg 15~akg leaf-] and (b) ratio of concentration in leaves at the end of the experiment and water at the beginning of theexperiment ........................21 5. Amount of 15~recovered from chambers in Tar Swam after 5 days and Creeping Swamp after 7 days since 18NH4 or 15~0~addition to surface water ............... 22 6. Changes in inorganic nitrogen pools in the interstitial water of surficial sediment of Tar Swamp as related to percentage cover of water in sampling area ............... 23 7. Design of nutrient loading experiment .............. 30 8. Precipitation (a) and water level (b) at Tar Swamp from February 1979 through February 1980 ............. 36 9. Ammonium concentrations of surface water (a) in the NH4 and PNN treatments, (b) in the sewage treatment, and (c) in controls not receiving ammonium loading ........ 40 10. Ammonium concentrations of subsurface water in (a) NHq, PNN, and sewage treatments, (b) PO4 and NO3 treatments, and (c) controls .......................41 11. Exchangeable ammonium concentrations of the surface sediment for (a) NHq, PNN, and sewage treatments, and (b) NO3 and PO4 treatments and controls ............... 42 12. Nitrate concentrations of surface water in the (a) NO3 and PNN treatments and (b) sewage treatment ........... 43 13. Fi l terable reactive phosphorus (FRP) concentrati ons of surface water in (a) PO4 and PNN treatments, (b) sewage treatment, and (c).controls not receiving phosphate loading ....... 44 LIST OF FIGURES (Continued) Page 14. Fi l terabl e reactive phosphorus (FRP) concentrations