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Concern over the impacts of anthropogenic greenhouse gas emissions has led to proposals to offset CO2 emissions by mechanisms that could increase carbon storage in soils. Estimates of the potential efficacy of various strategies vary, but an issue common to all of them is how to verify the magnitude and stability of the results of interventions. Soil sinks are “out of sight” which contrasts with strategies like reforestation. Below-ground carbon sinks, whether they be organic or inorganic, pose substantially more challenging issues for quantification and monitoring. ERW seeks to apply Ca, Mg-rich silicate rocks to enhance the consumption of CO2 by weathering reactions. Measurement of base cation losses from soils is used for quantifying CO2 uptake. Assessing CO2 uptake this way requires assessment of small differences between heterogeneous end members. Geochemical tracers can be used to estimate basalt input assuming that the endmembers are distinct. To compensate for open system behavior normalization to an “immobile” element is necessary. The limitation is typically the highly heterogenous nature of soils. Data from these settings often have high covariances. We reanalyzed published data on amended soils using Monte Carlo uncertainty analysis. We find that in a number of cases the Ca and Mg differences in pre- and post-amendment soils are not significantly different from zero (1 s.e.). High soil chemistry variances makes quantification of small differences difficult. Techniques for estimating relevant sample size and power for noisy data sets and modest effect sizes are well developed in other fields and can be appropriately applied to ERW problems. A simplified example using Lehr’s approximate rule for a two-sided test with s.d. for the pre- and post- data sets = 0.1 and the effect size (net change) = 0.03 yields a sample size n = 178 to obtain a sample power of 0.8 at the 95% CI, an optimistic estimate. Appropriate experimental design for ERW will require substantial sampling effort and rigorous a priori statistical assessment. Current studies so far cannot achieve this. The ocean ∆CO2/∆ALK ratio used to estimate CO2 uptake is important and unlikely to be as high as most studies have assumed. 

Concern over the impacts of anthropogenic greenhouse gas emissions has led to proposals to offset CO2 emissions by mechanisms that could increase carbon storage in soils. Estimates of the potential efficacy of various strategies vary, but an issue common to all of them is how to verify the magnitude and stability of the results of interventions. Soil sinks are “out of sight” which contrasts with strategies like reforestation. Below-ground carbon sinks, whether they be organic or inorganic, pose substantially more challenging issues for quantification and monitoring. ERW seeks to apply Ca, Mg-rich silicate rocks to enhance the consumption of CO2 by weathering reactions. Measurement of base cation losses from soils is used for quantifying CO2 uptake. Assessing CO2 uptake this way requires assessment of small differences between heterogeneous end members. Geochemical tracers can be used to estimate basalt input assuming that the endmembers are distinct. To compensate for open system behavior normalization to an “immobile” element is necessary. The limitation is typically the highly heterogenous nature of soils. Data from these settings often have high covariances. We reanalyzed published data on amended soils using Monte Carlo uncertainty analysis. We find that in a number of cases the Ca and Mg differences in pre- and post-amendment soils are not significantly different from zero (1 s.e.). High soil chemistry variances makes quantification of small differences difficult. Techniques for estimating relevant sample size and power for noisy data sets and modest effect sizes are well developed in other fields and can be appropriately applied to ERW problems. A simplified example using Lehr’s approximate rule for a two-sided test with s.d. for the pre- and post- data sets = 0.1 and the effect size (net change) = 0.03 yields a sample size n = 178 to obtain a sample power of 0.8 at the 95% CI, an optimistic estimate. Appropriate experimental design for ERW will require substantial sampling effort and rigorous a priori statistical assessment. Current studies so far cannot achieve this. The ocean ∆CO2/∆ALK ratio used to estimate CO2 uptake is important and unlikely to be as high as most studies have assumed. 

Abstract Lowland tropical forest soils are relatively N rich and are the largest global source of N₂O (a powerful greenhouse gas) to the atmosphere. Despite the importance of tropical N cycling, there have been few direct measurements of N₂(an inert gas that can serve as an alternate fate for N₂O) in tropical soils, limiting our ability to characterize N budgets, manage soils to reduce N₂O production, or predict the future role that N limitation to primary productivity will play in buffering against climate change. We collected soils from across macro‐ and micro‐topographic gradients that have previously been shown to differ in O₂availability and trace gas emissions. We then incubated these soils under oxic and anoxic headspaces to explore the relative effect of soil location versus transient redox conditions. No matter where the soils came from, or what headspace O₂was used in the incubation, N₂emissions dominated the flux of N gas losses. In the macrotopography plots, production of N₂and N₂O were higher in low O₂valleys than on more aerated ridges and slopes. In the microtopography plots, N₂emissions from plots with lower mean soil O₂(5%–10%) were greater than in plots with higher mean soil O₂(10%–20%). We estimate an N gas flux of ∼37 kg N/ha/yr from this forest, 99% as N₂. These results suggest that N₂fluxes may have been systematically underestimated in these landscapes, and that the measurements we present call for a reevaluation of the N budgets in lowland tropical forest ecosystems. 

Abstract We use the Multiple Element Limitation (MEL) model to examine responses of 12 ecosystems to elevated carbon dioxide (CO₂), warming, and 20% decreases or increases in precipitation. Ecosystems respond synergistically to elevated CO₂, warming, and decreased precipitation combined because higher water‐use efficiency with elevated CO₂and higher fertility with warming compensate for responses to drought. Response to elevated CO₂, warming, and increased precipitation combined is additive. We analyze changes in ecosystem carbon (C) based on four nitrogen (N) and four phosphorus (P) attribution factors: (1) changes in total ecosystem N and P, (2) changes in N and P distribution between vegetation and soil, (3) changes in vegetation C:N and C:P ratios, and (4) changes in soil C:N and C:P ratios. In the combined CO₂and climate change simulations, all ecosystems gain C. The contributions of these four attribution factors to changes in ecosystem C storage varies among ecosystems because of differences in the initial distributions of N and P between vegetation and soil and the openness of the ecosystem N and P cycles. The net transfer of N and P from soil to vegetation dominates the C response of forests. For tundra and grasslands, the C gain is also associated with increased soil C:N and C:P. In ecosystems with symbiotic N fixation, C gains resulted from N accumulation. Because of differences in N versus P cycle openness and the distribution of organic matter between vegetation and soil, changes in the N and P attribution factors do not always parallel one another. Differences among ecosystems in C‐nutrient interactions and the amount of woody biomass interact to shape ecosystem C sequestration under simulated global change. We suggest that future studies quantify the openness of the N and P cycles and changes in the distribution of C, N, and P among ecosystem components, which currently limit understanding of nutrient effects on C sequestration and responses to elevated CO₂and climate change.