Foliar Litter Nitrogen Dynamics as Affected by Forest Gap in the Alpine Forest of Eastern Tibet Plateau

et al. (2014) Foliar Litter Nitrogen Dynamics as Affected by Forest Gap in the Alpine Forest of Eastern Tibet Plateau. PLoS ONE 9(5): e97112. doi:10.1371/journal.pone.0097112 Foliar Litter Nitrogen Dynamics as Affected by Forest Gap in the Alpine Forest of Eastern Tibet Plateau Qiqian Wu 0 Fuzhong Wu 0 Wanqin Yang 0 Yeyi Zhao 0 Wei He 0 Bo Tan 0 Ben Bond-Lamberty, DOE Pacific Northwest National Laboratory, United States of America 0 1 Institute of Ecological Forestry, Sichuan Agricultural University , Chengdu, Sichuan , China , 2 Center for Ecological Research, Northeast Forestry University , Harbin, Heilongjiang , China There is increasing attention on the effects of seasonal snowpack on wintertime litter decomposition, as well as the processes following it, in cold biomes. However, little information is available on how litter nitrogen (N) dynamics vary with snowpack variations created by tree crown canopies in alpine forests. Therefore, to understand the effects of seasonal snowpack on litter N dynamics during different critical stages, litterbags with fir (Abies faxoniana), birch (Betula albosinensis), larch (Larix mastersiana) and cypress (Sabina saltuaria) foliar litter were placed on the forest floor beneath snowpack created by forest gaps in the eastern Tibet Plateau. The litterbags were sampled at the onset of freezing, deep freezing, thawing and growing stages from October 2010 to October 2012. Mass loss and N concentrations in litter were measured. Over two years of decomposition, N release occurred mainly during the first year, especially during the first winter. Litter N release rates (both in the first year and during the entire two-year decomposition study period) were higher in the center of canopy gaps than under closed canopy, regardless of species. Litter N release rates in winter were also highest in the center of canopy gaps and lowest under closed canopy, regardless of species, however the reverse was found during the growing season. Compared with broadleaf litter, needle litter N release comparisons of gap center to closed canopy showed much stronger responses to the changes in snow cover in winter and availability of sunshine during the growing season. As the decomposition proceeded, decomposing litter quality, microbial biomass and environmental temperature were important factors related to litter N release rate. This suggests that if winter warm with climate change, reduced snow cover in winter might slow down litter N release in alpine forest. - Introduction Nitrogen (N) released from plant litter plays a crucial role in maintaining soil fertility and ecosystem productivity in most terrestrial ecosystems [1], especially in N-limited ecosystems [2,3]. The present consensus is that vegetation type (referred to as litter quality) is the primary determining factor in the rate of N release during decomposition at the local scale, and climate (temperature, moisture, vegetation and soil types) controls the global-scale patterns in N release during decomposition [1,4,5]. However, increasingly well-documented evidence has accumulated demonstrating that seasonal snowpack and the related seasonal freezethaw pattern have strong effects on wintertime litter decomposition, microbial activity, and the following decomposition process in cold biomes [610]. According to IPCC [11], ongoing winter warming and extreme weather events are changing the patterns of seasonal snowpack and freeze-thaw cycles in cold biomes, and subsequently affected matter cycling. Therefore recent research has directed much more attention to the effects of simulated changes in seasonal snowpack and freeze-thaw cycles on litter decomposition and soil biological and biochemical processes [8 10,12]. At the local scale, however, the dynamic pattern of seasonal snowpack in the high-altitude frigid forest ecosystem is greatly influenced by forest gaps and crown canopies. However, so far not a lot of information has been available on the affects of seasonal snowpack gradient created by forest gaps and crown in winter canopies on nitrogen dynamics. In high-altitude frigid forest ecosystems, the interception of canopy on snow accumulation and the effects of canopy shading on snow ablation in winter often create a snowpack gradient on the forest floor between the center of a gap and under the forest canopy [13]. However, snowpack gradients vary during different critical stages. In deep winter, snowpack depth decreases from the gap center to under the crown canopy, owing to the crown canopy intercepting snow and low temperatures; while in late winter and early spring, snowpack depth increases from the gap center to under the canopy, owing to the shading sunlight that the crown canopy provides, which leads to a lower rate of snow ablation beneath the canopy. The differences in the dynamic pattern of snow accumulation and ablation from the gap center to under the canopy are influenced by the pattern of seasonal freeze-thaw cycles during different stages of the year [5]. In the growing season the crown canopy intercepts rainfall and creates shade, which and subsequently effects transpiration, and therefore also affects the dynamics of soil moisture and temperature [14]. Some studies have shown that large gaps significantly reduce microbial activity and decomposition rates by changing environmental conditions that should consequently reduce N cycling rates in a subtropical forest ecosystem [15]. Theoretically, the effect of forest gaps on litter decay and N release during the snowy season is different from those in the growing season, owing to both the canopy intercepting snow and reducing its accumulation, and the snowpack providing insulation. In addition, the accumulation and ablation of snow and ice in forest gaps compared with under canopies may affect the process of N dynamics in litter through leaching, mechanical disruption, and the effects of biological activity. As yet, little information has been available on the effect of winter snowpack created by forest gaps and crown canopies on the dynamics of litter N release. Litter quality is the internal determining factor regarding the rate of early litter decomposition, especially as it relates to N release from foliar litter [16,17]. Foliar litter with lower C/N and lignin/N ratios are most favorable for microbial growth and invertebrate digestion. In turn, they have higher rates of decay and N release [17]. Theoretically, the process of foliar litter decay in cold biomes differs from warm biomes that are not influenced by seasonal snowpack snowmelt leaching, mechanical disruption of freeze-thaw cycles, and the influence of microbial degradation of cryophile and cold resistant microorganisms Recently, Zhu et al. [9] have demonstrated that frequent freeze-thaw cycles and litter chemical properties determine winter decomposition, while microbe-related factors play more important roles in determining decomposition during the subsequent gr (...truncated)