## 1 Introduction  
Soil organic carbon (SOC) and total nitrogen (STN) play an important role in the global biogeochemical cycling of C and N within and between terrestrial ecosystems and between atmospheric and terrestrial pools [1; 2; 3]. Whilst measurements of SOC and STN in soils are relatively straightforward, their quantification, and therefore their significance, is often hampered by marked spatial variability [4]. This includes variability in both the vertical (soil depth) and horizontal directions [5]. An understanding of this variability is clearly important for an improved understanding of the cycling of C and N and for increasing the precision of estimates of C and N stocks. In soils, labile carbon and nitrogen originate from dissolved organic matter (DOM), which is defined as the entire pool of water-soluble organic matter that is either absorbed on soil or sediment particles or dissolved in interstitial pore water [6; 7]. Practically, it is often considered to be any organic matter fraction in solution that passes through a 0.45-\(\upmu\)m filter [8; 9]. Water-extractable organic matter (WEOM), extracted from soil under various laboratory conditions, is widely considered to be a substitute for in situ DOM [10; 11]. Dissolved organic matter comprises only a small part of the total soil organic matter, nevertheless, the dissolved fraction is generally a reflection of the total soil organic matter, because the soluble phase tends to be in equilibrium with the solid phase [11]. As soil DOM is significantly correlated with CO\({}_{2}\) and N\({}_{2}\)O emissions it can be a sensitive indicator of carbon and nitrogen mineralisation, and reflects environmental perturbations [12; 13], particularly through its impact on the eutrophication of water bodies [14]. Various factors can control the dynamics of DOM, among which soil temperature and soil moisture are considered to be the two most important natural environmental drivers, often explaining trends in soil surface DOM concentrations [15]. Increasing soil temperatures can also stimulate the decomposition of soil organic matter (SOM) through enhanced microbial activity [16]. In general, the relationships between DOM concentrations and soil moisture vary considerably among different studies [15]. Soil pH has often been shown to have a positive impact on DOM release from surface soils in laboratory studies, while these effects seem to be small in the field [15]. All of the factors controlling DOM can vary with topographic position (e.g., a higher content of organic matter and water content in the lower slope positions compared to the upper slopes due to water-related movement) [17]. Due to the difficulty of soil sampling and associated costs, most studies evaluating dissolved organic matter rely on information obtained from relatively few representative sampling points or profiles [18; 19; 20]. Given the known spatial variability in soil properties [21; 22], there is often considerable uncertainty about the representativeness of this sampling approach and how the values relate to actual stocks or concentrations. Clearly, more intensive assessments are required in order to obtain a better understanding of the variability in dissolved organic matter, together with an examination of the reason(s) for any spatial variation. Quantification of ecosystem C and N stocks and their changes over time is a major challenge and requires the identification of an appropriate sampling strategy that accurately accounts for the often-large variability in soil properties. Assessment of the impact of sampling scale on the spatial variation of soil carbon and nitrogen and their relationship with environmental variables can also contribute to an improved understanding of causally related variations. The development of more robust sampling strategies will also improve the inputs to biogeochemical process-based models that are used for simulating soil carbon and nitrogen dynamics.