Cellulose, Peat, and Mixed Vegetation in situ Decomposition in a Fen Exposed to Increasing Nitrogen Treatments, 2012-2015

Development of the oil sands has led to increasing atmospheric N
deposition, with values as high as 17 kg N ha-1 yr-1; regional
background levels <2 kg N ha-1 yr-1. To examine responses to N
deposition, over five years, we experimentally applied N (as NH4NO3)
to a fen near Mariana Lake, Alberta, at rates of 0, 5, 10, 15, 20, and
25 kg N ha-1 yr-1, plus controls (no water or N addition). We examined
the effects of N addition on cellulose, peat, and mixed vegetation
placed in the fen from 2012-2015 and collected after 5 and 17 months.
Decomposition of cellulose filter paper increased with increasing N
addition; the slopes of the responses did not differ between years,
although decomposition was faster for filter paper placed in the field
in 2014 than in either 2012 or 2013. In contrast, decomposition of
Sphagnum moss was unaffected by N addition. Vascular plant litter
decomposition decreased with N addition in 2015, but not 2014, Water
addition alone had no significant effect on cellulose decomposition k
values in any of the three years (p > 0.26) or on vascular plant
litter mass in either year (p = 0.81). However, Sphagnum mass loss was
significantly higher in the control treatment than in 0 kg N ha-1 yr-1
treatment (p = 0.0046) averaged over the two years of decomposition.
Assessment of decomposition and its controls may be especially
important in peatlands, as the development and persistence of peat
depends on an excess of NPP over decomposition throughout the peat
profile. There is evidence that increasing N deposition/availability
stimulates cellulose decomposition in surface fen peat, as we found
previously at Mariana Lake Bog, however, bog material decomposition
appears to be more complicated.

油砂开发活动导致大气氮沉降持续升高，沉降通量最高可达17 kg N ha⁻¹ yr⁻¹，而区域背景值不足2 kg N ha⁻¹ yr⁻¹。为探究生态系统对氮沉降的响应，本研究在五年周期内，于加拿大阿尔伯塔省马里亚纳湖附近的一处富营养泥炭沼泽（fen）中开展野外施氮实验：施氮剂为硝酸铵（NH₄NO₃），施氮梯度设置为0、5、10、15、20、25 kg N ha⁻¹ yr⁻¹，另设对照组（不施加水与氮）。本研究针对2012至2015年间布设、并分别在布设后5个月和17个月回收的纤维素、泥炭及混合植被样品，检测了氮添加对其分解过程的影响。纤维素滤纸的分解速率随施氮水平升高而加快；尽管2014年布设的滤纸分解速率快于2012年与2013年布设的样品，但不同年份间的响应斜率无显著差异。与之相反，泥炭藓（Sphagnum moss）的分解过程不受氮添加的影响。2015年维管植物枯落物的分解速率随施氮量升高而降低，但2014年未出现该趋势。仅施加水分对三年间的纤维素分解速率常数（k值）均无显著影响（p > 0.26），对两年间的维管植物枯落物生物量也无显著影响（p = 0.81）。不过，综合两年分解周期的观测数据来看，对照组的泥炭藓质量损失率显著高于0 kg N ha⁻¹ yr⁻¹施氮组（p = 0.0046）。在泥炭湿地中，分解过程及其调控因子的研究尤为重要，因为泥炭的形成与持续存在依赖于整个泥炭剖面内净初级生产力（NPP）高于分解消耗的盈余状态。已有研究表明，氮沉降/氮有效性升高会促进表层富营养泥炭沼泽泥炭中的纤维素分解——这与我们此前在马里亚纳湖酸沼（Mariana Lake Bog）中的发现一致，但酸沼泥炭的分解过程似乎更为复杂。