Litter Decomposition in Response to Nitrogen Addition and Soil Warming at Harvard Forest 2010-2012

The purpose of this study is to examine whether two environmental change
stressors (warming and nitrogen deposition) differentially impact litter
decomposition. We investigated this using a two year litterbag decomposition
experiment at the chronic N amendment experiment and the Barre Woods Soil
warming experiment, and measured litter decay dynamics, enzyme activities and
litter chemistry.
In both years mass loss of the mixed litter was suppressed under N addition,
with most of the mass loss observed in the first year compared to the second
year (70% and 30% of total mass loss, respectively). Both years showed either
increased activity for some hydrolytic enzymes (e.g. cellobiohydrolase) or no
difference (e.g. ß-N-acetylglucosaminidase) with increased N. The lignolytic
enzymes (e.g. peroxidases) showed no difference in activity in the first year,
but had a highly reduced activity in year 2 under elevated N conditions. Soil
warming did not significantly affect litter mass loss, and only had an effect on
the activity of a few enzymes.
In the oak reciprocal litterbag study, decay of oak litter originating from
the highest N addition plot was negatively affected by simulated N deposition in
the first year of decomposition, while after two years, simulated N deposition
negatively affected all litter, and litter originating from the highest N
addition plot decayed more slowly than control litter even without added N (i.e.
in the control plot). In addition, in the first year of decomposition lignolytic
enzyme activities were suppressed in litter originating from the N addition
treatments, but due to simulated N deposition in year two.

本研究旨在探究两种环境变化胁迫因子——增温（warming）与氮沉降（nitrogen deposition），是否会对枯落物分解（litter decomposition）产生差异化影响。我们依托长期氮添加试验与Barre Woods土壤增温试验，开展了为期两年的枯落物袋分解实验（litterbag decomposition experiment），测定了枯落物分解动态、酶活性及枯落物化学组成。

两年实验周期内，氮添加处理均抑制了混合枯落物的质量损失；相较于第二年，第一年的质量损失占总质量损失的比例更高，分别为70%与30%。随着氮添加水平提升，部分水解酶（如纤维二糖水解酶cellobiohydrolase）的活性有所上升，而部分酶（如β-N-乙酰氨基葡萄糖苷酶ß-N-acetylglucosaminidase）的活性无显著变化。木质素分解酶（lignolytic enzymes，如过氧化物酶peroxidases）在第一年的活性无明显差异，但在第二年高氮处理下其活性大幅降低。土壤增温对枯落物质量损失无显著影响，仅对少数酶的活性产生调控作用。

在栎树互置枯落物袋实验中，分解第一年，源自最高氮添加样地的栎树枯落物的分解过程会被模拟氮沉降抑制；而在两年分解周期结束后，模拟氮沉降会对所有栎树枯落物的分解产生负面影响，且即便在未添加氮的对照样地中，源自最高氮添加样地的枯落物分解速率仍慢于对照枯落物。此外，分解第一年，源自氮添加处理样地的枯落物其木质素分解酶活性受到抑制；而在第二年，该抑制效应则由模拟氮沉降引发。