Three representative UK moorland soils show differences in decadal release of dissolved organic carbon in response to environmental change

Moorland carbon reserves in organo-mineral soils may be crucial to predicting landscape-scale variability in soil carbon losses, an important component of which is dissolved organic carbon (DOC). Surface water DOC trends are subject to a range of scaling, transport and biotic processes that disconnect them from signals in the catchment's soils. Long-term soil datasets are vital to identify changes in DOC release at source and soil C depletion. Here we show, that moorland soil solution DOC concentrations at three key UK Environmental Change Network sites increased between 1993-2007 in both surface-and sub-soil of a freely-draining Podzol (48% and 215% increases in O and Bs horizons, respectively), declined in a gleyed Podzol and showed no change in a Peat. Our principal findings were that: (1) considerable heterogeneity in DOC response appears to exist between different soils that is not apparent from the more consistent observed trends for streamwaters, and (2) freely-draining organo-mineral Podzol showed increasing DOC concentrations, countering the current scientific focus on soil C destabilization in peats. We discuss how the key solubility controls on DOC associated with coupled physico-chemical factors of ionic strength, acid deposition recovery, soil hydrology and temperature cannot readily be separated. Yet, despite evidence that all sites are recovering from acidification the soil-specific responses to environmental change have caused divergence in soil DOC concentration trends. The study shows that the properties of soils govern their specific response to an approximately common set of broad environmental drivers. Key soil properties are indicated to be drainage, sulphate and DOC sorption capacity. Soil properties need representation in process-models to understand and predict the role of soils in catchment to global C budgets. Catchment hydrological (i.e. transport) controls may, at present, be governing the more ubiquitous rises in river DOC concentration trends, but soil (i.e. source) controls provide the key to prediction of future C loss to waters and the atmosphere

Spatial variability in properties affecting organic horizon carbon storage in upland soils

Quantifying the amount and distribution of soil organic carbon (SOC) within natural soils is important for sample design, C budgeting/pool estimation, and understanding SOC turnover at a process level. We examined the distribution of SOC across a typical UK upland, moorland catchment to establish the amount and spatial structure of variability in key soil properties affecting SOC stocks, namely O horizon C content, bulk density (DB) and horizon depth. Organic horizons of Histosols and Gleysols had greater SOC contents but smaller bulk densities than Podzols and Leptosols. Consequently, SOC density differences between soils were minimized and horizon depth variation became crucial to the measurement of SOC stocks. However, individual Podzol profiles stored appreciable amounts of SOC in O horizons (up to 50 kg m–2). Geostatistical analyses showed spatially structured variance in many properties relating to SOC storage at both plot (variograms reaching sills at ranges 3–8 m) and catchment scales (ranges 437–529 m). The increase in variance from plot to catchment scales was large for O horizon depth. However, DB showed complex scale and soil type inter-relationships, with similar variance at different scales. We show that detailed soil investigations spanning multiple spatial scales are necessary to quantify soil C storage properties for purposes of hydro-ecological modeling and C budgeting at small catchment scales. This has implications for upscaling to regional or national soil C databases

Root development impacts on the distribution of phosphatase activity:Improvements in quantification using soil zymography

Abstract Zymographic methods for the 2D distribution of phosphatase activity in soils have markedly advanced our understanding of root-soil-microbiota interactions. Robust quantitative approaches for 2D assays, which use 4-methylubelliferyl phosphate (4-MUP), are needed to advance a mechanistic understanding of enzyme behaviour and distribution in soils. We present improvements to the method for phosphatase zymography in rhizobox studies, involving (1) a systematic evaluation of 4-methylumbelliferone (4-MU)-based calibration functions in relation to image exposure time and (2) the development of advanced image analysis tools for lateral and longitudinal distributions of phosphatase activity along barley roots (Hordeum vulgare L., cv Optic). Exposure time (<1–32 s) affected the slope and intercept of 4-MU calibration equations by 4.4- and 5.8-fold, respectively. In lateral root profiles, a linear relationship was found between phosphatase activity and root hair length at 0 cm (7 nKat mm-2), 0.2 cm (48 nKat mm−2), and 2 cm (234 nKat mm−2) distance from the root tips (r = 0.9795, p < 0.0001); an algorithm designed to optimise estimates of phosphatase activity longitudinally confirmed this relationship (r = 0.9462, p < 0.0001). To improve the precision and accuracy of fluorescence-based soil zymography, careful control of calibration and imaging conditions and further development of advanced image analysis techniques are recommended