Shifts in litterfall and dominant nitrogen sources after expansion of shrub thickets.
Woody encroachment into herbaceous ecosystems is emerging as an important ecological response to global change. A primary concern is alterations in C and N cycling and associated variations across a variety of ecosystems. We quantified seasonal variation in litterfall and litter N concentration in Morella cerifera shrub thickets to assess changes in litterfall and associated N input after shrub expansion on an Atlantic coast barrier island. We also used the natural abundance of (15)N to estimate the proportion of litterfall N originating from symbiotic N fixation. Litterfall for shrub thickets ranged from 8,991 +/- 247 to 3,810 +/- 399 kg ha(-1) year(-1) and generally declined with increasing thicket age. Litterfall in three of the four thickets exceeded previous estimates of aboveground annual net primary production in adjacent grasslands by 300-400%. Leaf N concentration was also higher after shrub expansion and, coupled with low N resorption efficiency and high litterfall, resulted in a return of as much as 169 kg N ha(-1) year(-1) to the soil. We estimated that approximately 70% of N returned to the soil was from symbiotic N fixation resulting in an ecosystem input of between 37 and 118 kg ha(-1) year(-1) of atmospheric N depending on site. Considering the extensive cover of shrub thickets on Virginia barrier islands, N fixation by shrubs is likely the largest single source of N to the system. The shift from grassland to shrub thicket on barrier islands results in a substantial increase in litterfall and foliar N concentration that will likely have a major impact on the size and cycling of ecosystem C and N pools. Increasing C and N availability in these nutrient-poor soils is likely to permanently reduce cover of native grasses and alter community structure by favoring species with greater N requirements.
Brantley
Steven T
ST
Department of Biology, Virginia Commonwealth University, Richmond, VA 23284, USA.
Young
Donald R
DR
D039903:Actinobacteria / Q000378:metabolism; D002244:Carbon / Q000032:analysis*; D017753:Ecosystem; D029590:Myricaceae / Q000737:chemistry*; D009584:Nitrogen / Q000032:analysis*; D009586:Nitrogen Fixation / Q000502:physiology; D009587:Nitrogen Isotopes / Q000032:analysis; D018515:Plant Leaves / Q000737:chemistry; D006109:Poaceae / Q000737:chemistry; D012621:Seasons; D012987:Soil / Q000032:analysis*; D014768:Virginia
D009587:Nitrogen Isotopes; D012987:Soil; D002244:Carbon; D009584:Nitrogen