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We quantify the difference between the human-caused sediment yield and the natural rates of soil production and bedrock erosion in a now largely deforested tropical highland. The present-day rate of soil loss in the Upper Mahaweli catchment, Sri Lanka, is calculated by using suspended river-load fluxes. These data provide spatially averaged sediment yields of 130-2100 t*[km.sup.-2]*[yr.sup.-1]. Local rates of soil loss from agricultural plots on hillslopes are as high as 7000 t*[km.sup.-2]*[yr.sup.-1]. By comparison, natural rates of sediment generation, as determined by measuring cosmogenic [sup.10]Be in quartz from sediments and soils, are only 13-30 t*[km.sup.-2]*[yr.sup.-1]. The natural rates presented here provide a benchmark against which recent erosion rates, determined by various sediment gauging techniques, can be referenced. In the Sri Lankan highlands, these results suggest that soil is now being lost 10-100 times faster from agriculturally utilized areas than it is being produced. Keywords: cosmogenic nuclides, erosion, sediment yield, tropical highlands, Sri Lanka.

The tectonically active Taiwan orogen features numerous rivers that yield a high amount of sediment with fluxes exceeding 104 t/km2/yr. Amongst these, the landslide‐dominated Liwu River is well studied regarding its dynamic surface processes. However, the quantification of denudation in the Liwu Basin is still an ongoing task as rates obtained to date are subject to substantial differences depending on methods that differ in their spatio‐temporal scales. We constrain an upper limit of global denudation using the cosmogenic nuclide 10Be(meteoric) and its ratio to stable 9Be. Meteoric cosmogenic 10Be is delivered to Earth's surface by precipitation, whereas stable 9Be is released from rock weathering. In contrast to in situ cosmogenic 10Be measured in quartz, the 10Be(meteoric)/9Be ratio can be analyzed in quartz‐poor settings. 10Be(meteoric)/9Be‐derived denudation rates (Dmet) vary from 8.1 to >30 mm/yr in the Liwu mainstem, and from 3.4 to 21.5 mm/yr in the tributaries. These new Dmet are among the highest cosmogenic nuclide‐derived rates ever measured. Most of these rates agree with rates from sediment gauging or channel incision. We propose that stochastic landsliding plays a major role in denudation processes here. Using a soil‐bedrock mixing model and published riverine organic 14C data as a soil tracer, we estimate the fractional contribution of bedrock landslide material to mainstem sediments to be 55%–97%, which explains the magnitude and large variability (4‐fold) in Dmet. We demonstrate the complexity associated with denudation rates determination in landslide‐dominated routing systems; but also the potential of 10Be(meteoric)/9Be for tracing stochastic landsliding processes. Plain Language Summary: Many of Earth's rivers that transfer sediments with an extremely high rate are found in the Taiwan mountain belt, where earthquakes and typhoons are very common. Determination of the sediment removal rate in these rivers is crucial for understanding of the evolution of mountain landscapes through time and their impact on ocean chemistry and atmospheric trace gases. We applied a denudation rate meter to river sediments from one of the fastest‐eroding catchments in Taiwan, the Liwu River: the meteoric cosmogenic isotope beryllium‐10 (10Be) delivered by rainfall normalized to stable 9Be. This geochemical tool is particularly suited for fast‐eroding settings given the high delivery rate of meteoric 10Be to Earth's surface. The measured 10Be(meteoric)/9Be‐derived denudation rates in the Liwu Basin are highly variable and range from ∼3 to >30 mm/yr. We find that stochastic bedrock landsliding is likely the cause for such magnitude and large variability in denudation. Liwu river sediments constitute a mixture between steadily eroding surface soil and bedrock materials released from depths that lack meteoric 10Be. Our findings demonstrate the substantial potential of 10Be(meteoric)/9Be to trace sources of rainfall‐triggered landslides. Key Points: We apply the cosmogenic nuclide 10Be(meteoric) and its ratio to 9Be in one of the world's fastest‐eroding basin10Be(meteoric)/9Be ratios record extremely high denudation rates (>10 mm/yr)A soil‐bedrock mixing model reveals that bedrock landslides are the cause of the high denudation rates [ABSTRACT FROM AUTHOR]

That silicon is an important element in global biogeochemical cycles is widely recognised. Recently, its relevance for global crop production has gained increasing attention in light of possible deficits in plant-available Si in soil. Silicon is beneficial for plant growth and is taken up in considerable amounts by crops like rice or wheat. However, plants differ in the way they take up silicic acid from soil solution, with some species rejecting silicic acid while others actively incorporate it. Yet because the processes governing Si uptake and regulation are not fully understood, these classifications are subject to intense debate. To gain a new perspective on the processes involved, we investigated the dependence of silicon stable isotope fractionation on silicon uptake strategy, transpiration, water use, and Si transfer efficiency. Crop plants with rejective (tomato, Solanum lycopersicum , and mustard, Sinapis alba) and active (spring wheat, Triticum aestivum) Si uptake were hydroponically grown for 6 weeks. Using inductively coupled plasma mass spectrometry, the silicon concentration and isotopic composition of the nutrient solution, the roots, and the shoots were determined. We found that measured Si uptake does not correlate with the amount of transpired water and is thus distinct from Si incorporation expected for unspecific passive uptake. We interpret this lack of correlation to indicate a highly selective Si uptake mechanism. All three species preferentially incorporated light 28 Si, with a fractionation factor 1000×ln⁡(α) of -0.33 ‰ (tomato), -0.55 ‰ (mustard), and -0.43 ‰ (wheat) between growth medium and bulk plant. Thus, even though the rates of active and passive Si root uptake differ, the physico-chemical processes governing Si uptake and stable isotope fractionation do not. We suggest that isotope fractionation during root uptake is governed by a diffusion process. In contrast, the transport of silicic acid from the roots to the shoots depends on the amount of silicon previously precipitated in the roots and the presence of active transporters in the root endodermis, facilitating Si transport into the shoots. Plants with significant biogenic silica precipitation in roots (mustard and wheat) preferentially transport silicon depleted in 28 Si into their shoots. If biogenic silica is not precipitated in the roots, Si transport is dominated by a diffusion process, and hence light silicon 28 Si is preferentially transported into the tomato shoots. This stable Si isotope fingerprinting of the processes that transfer biogenic silica between the roots and shoots has the potential to track Si availability and recycling in soils and to provide a monitor for efficient use of plant-available Si in agricultural production. [ABSTRACT FROM AUTHOR]

Meteoric 10Be (10Bemet) concentrations in soil profiles have great potential as a geochronometer and a tracer of Earth surface processes, particularly in fine-grained soils lacking quartz that would preclude the use of in situ produced 10Be (10Bein situ). One prerequisite for using this technique for accurately calculating rates and dates is constraining the delivery, or flux, of 10Bemet to a site. However, few studies to date have quantified long-term (i.e., millennial) delivery rates, and none have determined a delivery rate for an eroding soil. In this study, we compared existing concentrations of 10Bein situ with new measurements of 10Bemet in eroding soils sampled from the same depth profiles to calibrate a long-term 10Bemet delivery rate. We did so on the Pinedale (~21-25 kyr) and Bull Lake (~140 kyr) glacial moraines at Fremont Lake, Wyoming (USA), where age, grain sizes, weathering indices, and soil properties are known, as are erosion and denudation rates calculated from 10Bein situ. After ensuring sufficient beryllium retention in each profile, solving for the delivery rate of 10Bemet, and normalizing for paleomagnetic and solar intensity variations over the Holocene, we calculate 10Bemet fluxes of 1.46 (±0.20) x 106 atoms cm-2 yr-1 and 1.30 (±0.48) x 106 atoms cm-2 yr-1 to the Pinedale and Bull Lake moraines, respectively, and compare these values to two widely used 10Bemet delivery rate estimation methods that substantially differ for this site. Accurately estimating the 10Bemet flux using these methods requires a consideration of spatial scale and temporally varying parameters (i.e., paleomagnetic field intensity, solar modulation) to ensure the most realistic estimates of 10Bemet-derived erosion rates in future studies. [ABSTRACT FROM AUTHOR]