Zacheis Amy, Ruess Roger W, Hupp Jerry W
Department of Biology and Wildlife, University of Alaska, 211 Irving, Fairbanks, AK, 99775, USA.
Institute of Arctic Biology, University of Alaska, Fairbanks, AK, 99775, USA.
Oecologia. 2002 Feb;130(4):600-608. doi: 10.1007/s00442-001-0837-9. Epub 2002 Feb 1.
Lesser snow geese (Anser caerulescens caerulescens) and Canada geese (Branta canadensis) use several salt marshes in Cook Inlet, Alaska, as stopover areas for brief periods during spring migration. We investigated the effects of geese on nitrogen cycling processes in Susitna Flats, one of the marshes. We compared net nitrogen mineralization, organic nitrogen pools and production in buried bags, nitrogen fixation by cyanobacteria, and soil and litter characteristics on grazed plots versus paired plots that had been exclosed from grazing for 3 years. Grazed areas had higher rates of net nitrogen mineralization in the spring and there was no effect of grazing on organic nitrogen availability. The increased mineralization rates in grazed plots could not be accounted for by alteration of litter quality, litter quantity, microclimate, or root biomass, which were not different between grazed and exclosed plots. In addition, fecal input was very slight in the year that we studied nitrogen cycling. We propose that trampling had two effects that could account for greater nitrogen availability in grazed areas: litter incorporation into soil, resulting in increased rates of decomposition and mineralization of litter material, and greater rates of nitrogen fixation by cyanobacteria on bare, trampled soils. A path analysis indicated that litter incorporation by trampling played a primary role in the nitrogen dynamics of the system, with nitrogen fixation secondary, and that fecal input was of little importance.
小雪雁（Anser caerulescens caerulescens）和加拿大雁（Branta canadensis）在春季迁徙期间会短暂利用阿拉斯加库克湾的几个盐沼作为中途停留地。我们研究了雁群对其中一个盐沼——苏西特纳滩地氮循环过程的影响。我们比较了放牧地块与已禁牧3年的配对地块的净氮矿化、埋藏袋中的有机氮库和产量、蓝藻的固氮作用以及土壤和凋落物特征。春季时，放牧区域的净氮矿化速率较高，且放牧对有机氮的有效性没有影响。放牧地块矿化速率的增加无法用凋落物质量、凋落物数量、小气候或根系生物量的改变来解释，这些在放牧地块和禁牧地块之间并无差异。此外，在我们研究氮循环的这一年，粪便输入非常少。我们认为践踏有两个作用，可以解释放牧区域氮有效性更高的原因：一是将凋落物混入土壤，导致凋落物材料的分解和矿化速率增加；二是在裸露、被践踏的土壤上，蓝藻的固氮速率更高。路径分析表明，践踏导致的凋落物混入在该系统的氮动态中起主要作用，固氮作用其次，而粪便输入的重要性不大。
