## 1 Introduction  
The drive to decrease anthropogenic CO\({}_{2}\) emissions has led to increased harvesting of woody biomass for energy generation [1, 2, 3]. This practice poses a risk to soil fertility as more nutrients are removed in biomass that would have previously been left on the forest floor to decompose and return nutrients to the soil. The risk is greatest for nutrient poor, acidic forest soils [4, 5, 6]. North American studies have found biomass harvesting to result in a 17% to 50% net loss of calcium (Ca), 20% to 57% loss of potassium (K), and 16% to 45% loss of magnesium (Mg) [7]. Similar reductions have been identified in studies in Sweden [8], Finland [9], Belgium [10], Germany [11] and across all Nordic countries and the UK [12]. Anthropogenic nitrogen and sulphur deposition have also driven nutrient losses, especially of base cations [13, 14, 15]; as can increased forest productivity, e.g., due to rising levels of N deposition and atmospheric CO\({}_{2}\)[16; 17; 18]. These trends have important implications for soil acidification and sustainable forest management.  
Forest soils rely on soil mineral weathering to replace losses of base cations (BC), buffer acidification processes and sustain forest growth [19]. Many temperate and boreal forests are located on naturally acid soils with low mineral weathering rates [20; 21; 22; 23], making them more vulnerable to acid deposition and biomass harvesting. It is very important to both policymakers and the forest industry to be able to quantify a sustainable level of biomass extraction that allows forest productivity to be maintained without progressively impoverishing forest soils [6; 24].  
Nutrient mass balance calculations or 'budgets' are typically used to assess soil impacts by comparing nutrient inputs via atmospheric deposition, mineral weathering and organic matter decomposition with losses/exports via leaching and biomass removal [25]. Results are compared with the stock or reservoir of available or total nutrients in the soil to determine future sustainability. For base cations, the soil reservoir of exchangeable Mg, Ca and K held on the cation-exchange complex is thought to be most important and readily measured by soil extractions using concentrated salts (salt-extractable exchangeable pools; [8; 10; 11]). Such an approach has been applied in a number of studies to estimate the ability of forest soils to sustain a range of forest management practices under different climate scenarios [7; 8; 11]. Results are often presented in terms of the number of harvesting cycles/forest rotations that can be sustained before the stock of soil base cations finally runs out. In reality, impacts on forest nutrition and growth rates are likely to occur well before the soil stock is emptied. Accurate estimates of soil base cation stocks and budgets are important to guide forest policy on sustainable levels of biomass extraction [26].  
The objectives of this study are: (1) to quantify the forest base cation (Ca, Mg, and K) balances and their temporal change for ten long term monitoring, UK ICP Level II sites, covering different soil types and tree species--oak, Scots pine and Sitka spruce; and (2) to assess the impact of different harvesting scenarios (Conventional, Whole-tree harvesting with green brash (brush removed before needle drop) and Whole-tree harvesting with brown brash (brush removed after needle drop)) on long-term nutrient sustainability of the UK's most productive tree species--Sitka spruce, across the main forest soil types. A simple decision matrix is produced to guide the sustainable harvesting practice of Sitka spruce on different soil types.