Breaking the Ice – Consequences of northwards extending agriculture for soil organic matter cycling under a changing climate

土壤储存着最大的陆地碳(C)库，是大气碳库的数倍。土壤有机碳(SOC)储量最高的是在北纬高纬度地区，那里的土壤大多是未开垦的，而且不连续或连续冻结。与此同时，预计北纬地区的气温上升也最强，使该地区成为气候碳循环反馈的全球热点。多年冻土的融化将明显导致有机碳的损失，从而增加大气中的二氧化碳。然而，全球变暖还将对北极圈永久冻土的碳循环产生另一种间接但强烈的影响：人类干预。随着北方可耕种土地面积的增加和生活条件的改善，将发生并开始发生广泛的土地利用变化，这被认为将改变一系列生物和非生物土壤属性。土地利用的变化可能因此放大土壤变暖效应。如果不量化这些变化和影响SOC循环的过程的相关后果，就无法预测人类干预对气候-碳循环反馈的影响。为了校准能够可靠地预测气候变化下与土地利用变化相关的有机碳储量变化的模型，该项目将涉及以下方面：与非永冻土相比，永久冻土对耕作的直接生物和非生物响应，农业土壤(北方森林转变为农田和草地)的长期有机碳储量和质量发展，以及由于土地利用变化而导致的微生物碳利用效率及其温度敏感性的潜在变化。后者对于理解耕作土壤在碳循环方面与未耕作土壤相比将如何应对气候变化至关重要。为了量化碳利用效率，将应用一种新的18O标记方法。加拿大最西北部的育空省被确定为完美的研究区域。由于黄金潮，它已经有一个多世纪的农业历史，即使在自然植被下的土壤仍然受到浅层冻土影响的地区也是如此。因此，可以用时序方法研究土地利用变化的影响。在相对较小的距离内，冻土深度变化很大，这使得研究土地利用变化的影响也可以沿着温度序列进行。农业主要发生在河流沉积物上，这一事实限制了混杂因素。大多数情况下，农民进行小规模混合耕作，因此可以同时研究退耕还林还草。接洽的农民非常开放和合作，导致可以抽样的大量和平衡的地点，所有这些都很容易接近。该项目的结果将使人们对富碳北部地区的土壤碳循环有一个新的看法：永久冻土融化和森林砍伐。

Soils store the largest terrestrial carbon (C) pool and a multiple of the atmospheric C pool. The highest soil organic C (SOC) stocks are found in high northern latitudes, where soils are mostly uncultivated and discontinuously or continuously frozen. At the same time, the strongest temperature increase is also expected in northern latitudes, making this region a global hot spot for climate carbon-cycle feedbacks. Thawing of permafrost will evidently lead to losses in SOC and thereby increase atmospheric CO2. However, global warming will also have another indirect, but strong influence on the carbon cycle of northern circumpolar permafrost soils: human interventions. With increasing area of land that can be cultivated as well as improved living conditions in the North, extensive land use changes will occur and started to occur, which are acknowledged to change a wide range of biotic and abiotic soil properties. The land use changes may thereby amplify soil warming effects. Without quantifying these changes and associated consequences for processes affecting SOC cycling, the impact of human interventions on climate-carbon cycle feedbacks cannot be predicted. To calibrate models that can reliably predict SOC stock changes under a changing climate with associated land use changes, the following aspects will be addressed in the project: Direct biotic and abiotic responses of permafrost soils to cultivation as compared to non-permafrost soils, long-term SOC stock and quality development of agricultural soils (boreal forests converted to cropland and grassland) and potential changes in microbial carbon use efficiency and its temperature sensitivity due to land use change. The latter is critical to understand how cultivated soils will respond to climate change as compared to uncultivated soils with regard to carbon cycling. To quantify carbon use efficiencies, a novel 18O-labeling approach will be applied. The Yukon, the north-westernmost province of Canada, was identified as the perfect research area. Due to the ‘goldrush’, it has an agricultural history of more than a century, even in areas where soils under natural vegetation are still influenced by shallow permafrost. Land use change effects can thus be studied in a chronosequential approach. In relative small distance, permafrost depth varies greatly, enabling to study land-use change effects also along a thermosequence. The fact that agriculture occurs mainly on river sediments constrains confounding factors. Most often, farmers conduct small-scale mixed farming, thus conversion from forest to cropland and grassland can be studied at the same time. The approached farmers have been very open and cooperative, resulting in a large and balanced number of sites that can be sampled, all of which being easily accessible. Results of the project will enable a new view on soil C cycling in the C-rich Northern regions with double pressure: permafrost thawing and deforestation.