Towards a better understanding of the origin and fate of fine colloidal and dissolved black carbon in agro-ecosystems

黑碳（BC）包括各种有机燃烧残留物，它们通过大气沉积进入土壤，例如来自植被火灾和/或以生物炭形式直接施用黑碳。黑C具有很强的抗逆性，因此在几十年内在土壤中积累。然而，人们对土壤中BC的降解和流失却知之甚少。近年来的一些研究表明，BC以溶解BC （DBC）的形式存在于水中，其中包括< 450 nm的颗粒。然而，从土壤中以水可提取BC （WEBC）的形式损失到这一溶解部分的直接证据是缺失的。我假设I)耕地土壤中的大部分BC以可水转运形式（WEBC）存在，ii) WEBC主要由胶体到纳米级颗粒组成，可以通过分析和制备场流分馏（FFFprep）来表征。此外，我假设这个WEBC是由微生物BC降解产生的。为了验证这些假设，我将首先调查田间试验中不同BC输入的耕地土壤的WEBC含量和组成（0,4.5和9吨生物炭/公顷，与堆肥一起施用于灌木林）。在BC分析（氧化为苯多羧酸（BPCA法））之前，将从土壤中提取WEBC并过滤到< 450 nm。其次，为了测定胶体至纳米级BC的丰度，将建立FFFprep方法，并对其进行优化，以制备分离BC颗粒。这些可以用于BPCAs的后续分析，作为芳香缩合程度的标记。为了阐明微生物BC降解对WEBC形成的贡献，通过FFFprep和随后的BPCA分析对5年孵育实验样本（包括无菌和非无菌变异）进行了表征。这些试验将首次研究土壤中WEBC的大小分布及其在降解过程中的变化程度。本研究将进一步揭示生物炭施用量是否与土壤中WEBC相关。然后，我将能够提供第一个想法，如果降解成细胶体和纳米颗粒可能与土壤中的碳流失和全球碳循环有关，至少在十年的时间尺度上。

Black carbon (BC) comprises a variety of organic combustion residues, which enter the soil by atmospheric deposition, e.g. from vegetation fires and/or by direct application of BC in form of biochar. Black C is highly recalcitrant and hence accumulates in soil within several decades. However, the degradation and loss of BC in soil is hardly understood. In the last years several studies showed that BC occurs in water in the form of dissolved BC (DBC), which includes particles < 450 nm. Though, a direct evidence for loss of BC from soil in form of water extractable BC (WEBC) to this dissolved fraction is missing. I hypothesize that i) a large part of BC in arable soils exists in water-transportable form (WEBC), and that ii) WEBC consist mainly of colloidal to nano-sized particles, which can be characterized via analytical and preparative field flow fractionation (FFFprep). Besides, I assume that this WEBC is produced by microbial BC degradation. To test these hypotheses, I will first investigate arable soils from field experiments with different BC inputs on their WEBC content and composition (0, 4.5 and 9 t biochar/ha applied with compost to coppice plantations). WEBC will be extracted from soil and filtrated to < 450 nm prior to BC analyses (oxidation to benzene polycarboxylic acids (BPCA method). Secondly, for the determination of BC abundance in colloidal- to nano-size classes an FFFprep method will be established and later on optimized for preparative isolation of BC particles. These can then be used for subsequent analysis of BPCAs, which serve as marker for the degree of aromatic condensation. To elucidate the contribution of microbial BC degradation to WEBC formation, samples of a 5-year incubation experiment (including sterile and non-sterile variants) are characterized by FFFprep and subsequent BPCA analyses. These experiments will first time investigate the size distribution of WEBC in soil and to which degree it will change during degradation. This study will further reveal if biochar application rates correlate with WEBC in soil. I will then be able to provide first ideas if degradation of BC to fine-colloidal and nanoparticles might be relevant for BC losses from soil and the global carbon cycle at least in decadal time-scales.