Journal Paper Digests

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Journal Paper Digests 2018 #6

  • Temperature sensitivity of soil organic carbon decomposition increased with mean carbon residence time: Field incubation and data assimilation
  • Networking our science to characterize the state, vulnerabilities, and management opportunities of soil organic matter.
  • Soil pH as the chief modifier for regional nitrous oxide emissions: New evidence and implications for global estimates and mitigation.
  • Assessment of ecosystem resilience to hydroclimatic disturbances in India
  • Assessing uncertainties in crop and pasture ensemble model simulations of productivity and N2O emissions
  • Is Rock-Eval 6 thermal analysis a good indicator of soil organic carbon lability? - A method-comparison study in forest soils. Use of the USDA National Cooperative Soil Survey Soil Characterization Data to Detect Soil Change: A Cautionary Tale.

Temperature sensitivity of soil organic carbon decomposition increased with mean carbon residence time: Field incubation and data assimilation

Authors: Zhou, XH; Xu, X; Zhou, GY; Luo, YQ

Source: GLOBAL CHANGE BIOLOGY, 24 (2):810-822; FEB 2018

Abstract: Temperature sensitivity of soil organic carbon (SOC) decomposition is one of the major uncertainties in predicting climate-carbon (C) cycle feedback. Results from previous studies are highly contradictory with old soil C decomposition being more, similarly, or less sensitive to temperature than decomposition of young fractions. The contradictory results are partly from difficulties in distinguishing old from young SOC and their changes over time in the experiments with or without isotopic techniques. In this study, we have conducted a long-term field incubation experiment with deep soil collars (0-70 cm in depth, 10 cm in diameter of PVC tubes) for excluding root C input to examine apparent temperature sensitivity of SOC decomposition under ambient and warming treatments from 2002 to 2008. The data from the experiment were infused into a multi-pool soil C model to estimate intrinsic temperature sensitivity of SOC decomposition and C residence times of three SOC fractions (i.e., active, slow, and passive) using a data assimilation (DA) technique. As active SOC with the short C residence time was progressively depleted in the deep soil collars under both ambient and warming treatments, the residences times of the whole SOC became longer over time. Concomitantly, the estimated apparent and intrinsic temperature sensitivity of SOC decomposition also became gradually higher over time as more than 50% of active SOC was depleted. Thus, the temperature sensitivity of soil C decomposition in deep soil collars was positively correlated with the mean C residence times. However, the regression slope of the temperature sensitivity against the residence time was lower under the warming treatment than under ambient temperature, indicating that other processes also regulated temperature sensitivity of SOC decomposition. These results indicate that old SOC decomposition is more sensitive to temperature than young components, making the old C more vulnerable to future warmer climate.

Networking our science to characterize the state, vulnerabilities, and management opportunities of soil organic matter

Authors: Harden, JW; Hugelius, G; Ahlstrom, A; Blankinship, JC; Bond-Lamberty, B; Lawrence, CR; Loisel, J; Malhotra, A; Jackson, RB; Ogle, S; Phillips, C; Ryals, R; Todd-Brown, K; Vargas, R; Vergara, SE; Cotrufo, MF; Keiluweit, M; Heckman, KA; Crow, SE; Silver, WL; DeLonge, M; Nave, LE

Source: GLOBAL CHANGE BIOLOGY, 24 (2):e705-e718; FEB 2018

Abstract: Soil organic matter (SOM) supports the Earth’s ability to sustain terrestrial ecosystems, provide food and fiber, and retains the largest pool of actively cycling carbon. Over 75% of the soil organic carbon (SOC) in the top meter of soil is directly affected by human land use. Large land areas have lost SOC as a result of land use practices, yet there are compensatory opportunities to enhance productivity and SOC storage in degraded lands through improved management practices. Large areas with and without intentional management are also being subjected to rapid changes in climate, making many SOC stocks vulnerable to losses by decomposition or disturbance. In order to quantify potential SOC losses or sequestration at field, regional, and global scales, measurements for detecting changes in SOC are needed. Such measurements and soil-management best practices should be based on well established and emerging scientific understanding of processes of C stabilization and destabilization over various timescales, soil types, and spatial scales. As newly engaged members of the International Soil Carbon Network, we have identified gaps in data, modeling, and communication that underscore the need for an open, shared network to frame and guide the study of SOM and SOC and their management for sustained production and climate regulation.

Soil pH as the chief modifier for regional nitrous oxide emissions: New evidence and implications for global estimates and mitigation

Authors: Wang, YJ; Guo, JH; Vogt, RD; Mulder, J; Wang, JG; Zhang, XS

Source: GLOBAL CHANGE BIOLOGY, 24 (2):E617-E626; FEB 2018

Abstract: Nitrous oxide (N2O) is a greenhouse gas that also plays the primary role in stratospheric ozone depletion. The use of nitrogen fertilizers is known as the major reason for atmospheric N2O increase. Empirical bottom-up models therefore estimate agricultural N2O inventories using N loading as the sole predictor, disregarding the regional heterogeneities in soil inherent response to external N loading. Several environmental factors have been found to influence the response in soil N2O emission to N fertilization, but their interdependence and relative importance have not been addressed properly. Here, we show that soil pH is the chief factor explaining regional disparities in N2O emission, using a global meta-analysis of 1,104 field measurements. The emission factor (EF) of N2O increases significantly (p < .001) with soil pH decrease. The default EF value of 1.0%, according to IPCC (Intergovernmental Panelon Climate Change) for agricultural soils, occurs at soil pH 6.76. Moreover, changes in EF with N fertilization (i.e. EF) is also negatively correlated (p < .001) with soil pH. This indicates that N2O emission in acidic soils is more sensitive to changing N fertilization than that in alkaline soils. Incorporating our findings into bottom-up models has significant consequences for regional and global N2O emission inventories and reconciling them with those from top-down models. Moreover, our results allow region-specific development of tailor-made N2O mitigation measures in agriculture.

Assessment of ecosystem resilience to hydroclimatic disturbances in India

Authors: Sharma, A; Goyal, MK

Source: GLOBAL CHANGE BIOLOGY, 24 (2):E432-E441; FEB 2018

Abstract: Recent studies have shown an increasing trend in hydroclimatic disturbances like droughts, which are anticipated to become more frequent and intense under global warming and climate change. Droughts adversely affect the vegetation growth and crop yield, which enhances the risks to food security for a country like India with over 1.2 billion people to feed. Here, we compared the response of terrestrial net primary productivity (NPP) to hydroclimatic disturbances in India at different scales (i.e., at river basins, land covers, and climate types) to examine the ecosystems’ resilience to such adverse conditions. The ecosystem water use efficiency (WUEe: NPP/Evapotranspiration) is an effective indicator of ecosystem productivity, linking carbon (C) and water cycles. We found a significant difference (p<.05) in WUEe across India at different scales. The ecosystem resilience analysis indicated that most of the river basins were not resilient enough to hydroclimatic disturbances. Drastic reduction in WUEe under dry conditions was observed for some basins, which highlighted the cross-biome incapability to withstand such conditions. The ecosystem resilience at land cover and climate type scale did not completely relate to the basin-scale ecosystem resilience, which indicated that ecosystem resilience at basin scale is controlled by some other ecohydrological processes. Our results facilitate the identification of the most sensitive regions in the country for ecosystem management and climate policy making, and highlight the need for taking sufficient adaptation measures to ensure sustainability of ecosystems.

Assessing uncertainties in crop and pasture ensemble model simulations of productivity and N2O emissions

Authors: Ehrhardt, F; Soussana, JF; Bellocchi, G; Grace, P; McAuliffe, R; Recous, S; Sandor, R; Smith, P; Snow, V; Migliorati, MD; Basso, B; Bhatia, A; Brilli, L; Doltra, J; Dorich, CD; Doro, L; Fitton, N; Giacomini, SJ; Grant, B; Harrison, MT; Jones, SK; Kirschbaum, MUF; Klumpp, K; Laville, P; Leonard, J; Liebig, M; Lieffering, M; Martin, R; Massad, RS; Meier, E; Merbold, L; Moore, AD; Myrgiotis, V; Newton, P; Pattey, E; Rolinski, S; Sharp, J; Smith, WN; Wu, LH; Zhang, Q

Source: GLOBAL CHANGE BIOLOGY, 24 (2):E603-E616; FEB 2018

Abstract: Simulation models are extensively used to predict agricultural productivity and greenhouse gas emissions. However, the uncertainties of (reduced) model ensemble simulations have not been assessed systematically for variables affecting food security and climate change mitigation, within multi-species agricultural contexts. We report an international model comparison and benchmarking exercise, showing the potential of multi-model ensembles to predict productivity and nitrous oxide (N2O) emissions for wheat, maize, rice and temperate grasslands. Using a multi-stage modelling protocol, from blind simulations (stage 1) to partial (stages 2-4) and full calibration (stage 5), 24 process-based biogeochemical models were assessed individually or as an ensemble against long-term experimental data from four temperate grassland and five arable crop rotation sites spanning four continents. Comparisons were performed by reference to the experimental uncertainties of observed yields and N2O emissions. Results showed that across sites and crop/grassland types, 23%-40% of the uncalibrated individual models were within two standard deviations (SD) of observed yields, while 42 (rice) to 96% (grasslands) of the models were within 1 SD of observed N2O emissions. At stage 1, ensembles formed by the three lowest prediction model errors predicted both yields and N2O emissions within experimental uncertainties for 44% and 33% of the crop and grassland growth cycles, respectively. Partial model calibration (stages 2-4) markedly reduced prediction errors of the full model ensemble E-median for crop grain yields (from 36% at stage 1 down to 4% on average) and grassland productivity (from 44% to 27%) and to a lesser and more variable extent for N2O emissions. Yield-scaled N2O emissions (N2O emissions divided by crop yields) were ranked accurately by three-model ensembles across crop species and field sites. The potential of using process-based model ensembles to predict jointly productivity and N2O emissions at field scale is discussed.

Is Rock-Eval 6 thermal analysis a good indicator of soil organic carbon lability? - A method-comparison study in forest soils

Authors: Soucemarianadin, L; Cecillon, L; Chenu, C; Baudin, F; Nicolas, M; Girardin, C; Barre, P

Source: SOIL BIOLOGY & BIOCHEMISTRY, 117 108-116; FEB 2018

Abstract: Soil respiration tests and abundance of particulate organic matter (POM) are considered as classical indicators of the labile soil organic carbon (SOC) pool. However, there is still no widely accepted standard method to assess SOC lability and the pertinence of these two time-consuming methods to characterize SOC turnover can be questioned. Alternate ways of determining the labile SOC fraction are thus much needed. Thermal analyses, in particular Rock-Eval 6 (RE6) analysis has shown promising results in the determination of SOC biogeochemical stability.Using a large set of samples (n = 99) of French forest soils representing contrasted pedoclimatic conditions, including deep samples (up to 0.8 m depth), we compared three different methods used for SOC lability assessment. We explored whether respired-C isolated by a 10-week laboratory soil respiration test, POM-C isolated by a physical SOC fractionation scheme (particle-size > 50 pm and d < 1.6 g cm(-3)) and several RE6 parameters were comparable and how they correlated.As expected, respired-C (mg CO2-C.g(-1) SOC) and POM-C (% of total SOC) fractions strongly decreased with depth. RE6 parameters showed that SOC from deeper soil layers was also thermally less labile, more oxidized and H-depleted. Indeed, SOC from deeper soil layers had lower proportion of thermally labile SOC, higher T50-Hc-pyR (temperature at which 50% of the pyrolysable hydrocarbons were effectively pyrolyzed) and T50-CO2-OX (temperature at which 50% of the CO2 gas had evolved during the oxidation phase), larger oxygen index, and smaller hydrogen index. Surprisingly, the two classical indicators of the labile SOC pool (respired-C and POM-C) were only marginally correlated (p = 0.051) and showed layer-specific correlations. Similarly, respired-C was poorly correlated to RE6 parameters. Conversely, the POM-C fraction showed a strong negative correlation with T50-HC-PYR (rho = -0.73) and good correlations with other RE6 parameters.Our study showed that RE6 parameters were good estimates of the POM-C fraction, which represents a labile SOC pool with a residence time of ca. a couple decades that is meaningful regarding SOC stock changes upon modifications in land management. RE6 thermal analysis could therefore be a fast and cost-effective alternative to more time-consuming methods used in SOC pool determination, and may be integrated into soil monitoring networks to provide high-throughput information on SOC dynamics.

Use of the USDA National Cooperative Soil Survey Soil Characterization Data to Detect Soil Change: A Cautionary Tale

Authors: Tomer, MD; James, DE; Schipper, LA; Wills, SA

Source: SOIL SCIENCE SOCIETY OF AMERICA JOURNAL, 81 (6):1463-1474; NOV-DEC 2017

Abstract: Recently, the USDA-NRCS National Cooperative Soil Survey Soil Characterization Database (NSCD) was reported to provide evidence that total nitrogen (TN) stocks of agricultural soils have increased across the Mississippi basin since 1985. Unfortunately, historical changes in methods used to measure TN were not fully accounted for in that report. We used NSCD archives to calibrate between wet (pre-1995) and dry (post-1995) digestion methods used in measuring TN and soil organic carbon (SOC), then evaluated temporal trends in SOC and TN stocks with data from 423 Alfisol and 900 Mollisol profiles representing the US Corn Belt. Data were grouped by moisture regime, farming history (presence of Ap horizon), and depth (0-20, 20-60, and 60-100 cm). Regressions showed geographic and textural influences on SOC, and that SOC increased with time among farmed soils, particularly at 20 to 60 cm. Soil TN was dependent on SOC, especially at the surface (R-2 > 0.71), and decreasing TN trends with time were found among farmed soils above 60-cm depth. Increases in C to N ratios further suggested TN has been slowly stabilizing within soils of the US Corn Belt. However, C to N ratios <9.0 were prevalent at 60-100 cm depth (>60% frequency), indicating large TN stores remain in these soils. An increasing trend in SOC, TN, and C to N ratios among non-farmed, aquic Mollisols suggested SOC and TN accumulations in wet soils typically located below croplands. Results suggest slow improvement in agricultural soils. However, resampling has not been broadly undertaken for NSCD soil profiles, hence use of this database to detect soil change should be approached cautiously.

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