Volk Matthias, Bassin Seraina, Lehmann Moritz F, Johnson Mark G, Andersen Christian P
Agroscope Zürich, Air Pollution/Climate Group, Reckenholzstrasse 191, 8046 Zürich, Switzerland.
Department Environmental Science, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland.
Soil Biol Biochem. 2018;125:178-184. doi: 10.1016/j.soilbio.2018.07.014.
We followed soil C fluxes in a subalpine grassland system exposed to experimentally increased atmospheric N deposition for 7 years. Earlier we found that, different from the plant productivity response, the bulk soil C stock increase was highest at the medium, not the high N input as hypothesized. This implies that a smaller N-deposition rate has a greater potential to favor the biological greenhouse gas-sink. To help elucidate the mechanisms controlling those changes in SOC in response to N deposition, we produced four soil density fractions and analyzed soil organic C concentration [SOC], as well as δC signatures (δC) of SOC components. Soil respired CO (δC) was analyzed to better distinguish seasonal short term dynamics from N-deposition effects and to identify the predominant substrate of soil respiration. Both at the start of the experiment and after 7 years we found a strong, negative correlation between [SOC] and δC of the soil density fractions in the control treatment, consistent with an advanced stage of microbial processing of SOC in fractions of higher density. During the experiment the [SOC] increased in the two lighter density fractions, but decreased in the two heavier fractions, suggesting a possible priming effect that accelerated decomposition of formerly recalcitrant (heavy) organic matter pools. The seasonal pattern of soil δC was affected by weather and canopy development, and δC values for the different N treatment levels indicated that soil respiration originated primarily from the lightest density fractions. Surprisingly, [SOC] increases were significantly higher under medium N deposition in the <1.8 fraction and in bulk soil, compared to the high N treatment. Analogously, the depletion of δC was significantly higher in the medium compared to the high N treatment in the three lighter fractions. Thus, medium N deposition favored the highest C sequestration potential, compared to the low N control and the high N treatment. Clearly, our results show that it is inappropriate to use plant productivity N response as an indicator for shifts in SOC content in grassland ecosystems. Here, isotopic techniques illustrated why atmospheric N deposition of 14 kg N ha yr is below, and 54 kg N ha yr is above a threshold that tips the balance between new, assimilative gains and respiratory losses towards a net loss of [SOC] for certain soil fractions in the subalpine grassland.
我们在一个亚高山草原系统中追踪土壤碳通量，该系统已进行了7年的实验性大气氮沉降增加处理。此前我们发现，与植物生产力响应不同，土壤总碳储量的增加在中等氮输入水平时最高，而非如假设的那样在高氮输入水平时最高。这意味着较低的氮沉降速率更有可能促进生物性温室气体汇。为了帮助阐明控制土壤有机碳（SOC）响应氮沉降变化的机制，我们制备了四个土壤密度级分，并分析了土壤有机碳浓度[SOC]以及SOC组分的δC特征（δC）。对土壤呼吸释放的二氧化碳（δC）进行分析，以更好地区分季节性短期动态与氮沉降效应，并确定土壤呼吸的主要底物。在实验开始时和7年后，我们都发现对照处理中土壤密度级分的[SOC]与δC之间存在强烈的负相关，这与高密度级分中SOC的微生物处理处于高级阶段一致。在实验期间，两个较轻密度级分中的[SOC]增加，而两个较重密度级分中的[SOC]减少，这表明可能存在激发效应，加速了先前难分解（重）有机物质库的分解。土壤δC的季节性模式受天气和冠层发育的影响，不同氮处理水平的δC值表明土壤呼吸主要源自最轻的密度级分。令人惊讶的是，与高氮处理相比，中等氮沉降下<1.8级分和土壤总体中的[SOC]增加显著更高。类似地，在三个较轻密度级分中，中等氮处理下δC的消耗比高氮处理显著更高。因此，与低氮对照和高氮处理相比，中等氮沉降有利于最高的碳固存潜力。显然，我们的结果表明，将植物生产力对氮的响应用作草地生态系统中SOC含量变化的指标是不合适的。在此，同位素技术说明了为什么每年每公顷14千克氮的大气氮沉降低于阈值，而每年每公顷54千克氮的大气氮沉降高于该阈值，该阈值会使新的同化增益与呼吸损失之间的平衡向亚高山草地某些土壤级分的[SOC]净损失倾斜。
