Journal Paper Digests

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Journal Paper Digests 2019 #12

  • Reducing Sampling Uncertainty in Aeolian Research to Improve Change Detection
  • Deep soil flipping increases carbon stocks of New Zealand grasslands
  • How ecologists define drought, and why we should do better

Reducing Sampling Uncertainty in Aeolian Research to Improve Change Detection

Authors: Webb, NP; Chappell, A; Edwards, BL; McCord, SE; Van Zee, JW; Cooper, BF; Courtright, EM; Duniway, MC; Sharratt, B; Tedela, N; Toledo, D

Source: JOURNAL OF GEOPHYSICAL RESEARCH-EARTH SURFACE, 124 (6):1366-1377; JUN 2019

Abstract: Measurements of aeolian sediment transport support our understanding of mineral dust impacts on Earth and human systems and assessments of aeolian process sensitivities to global environmental change. However, sample design principles are often overlooked in aeolian research. Here we use high-density field measurements of sediment mass flux across land use and land cover types to examine sample size and power effects on detecting change in aeolian transport. Temporal variances were 1.6 to 10.1 times the magnitude of spatial variances in aeolian transport for six study sites. Differences in transport were detectable for >67% of comparisons among sites using similar to 27 samples. Failure to detect change with smaller sample sizes suggests that aeolian transport measurements and monitoring are much more uncertain than recognized. We show how small and selective sampling, common in aeolian research, gives the false impression that differences in aeolian transport can be detected , potentially undermining inferences about process and impacting reproducibility of aeolian research.Plain Language Summary Aeolian sediment transport, including wind erosion and dust emission, impacts agricultural production and food security, nutrient cycling, water resources, and climate. Measuring aeolian sediment transport is therefore important for developing an understanding of its impacts on Earth systems and society. However, little consideration has been given to how many samples are needed to measure aeolian transport and detect its change across space and through time. We investigate how sample size, design, and decisions about the precision of change detection affect aeolian transport monitoring. Using field measurements, we show that traditional approaches in aeolian research with small sample sizes and selective placement of equipment are often unable to detect change and support robust inferences about aeolian processes. Unless large numbers of samples are us ed, uncertainty in field measurements can be so large that i! t undermines our understanding of how and why aeolian sediment transport rates change across space and through time.

Deep soil flipping increases carbon stocks of New Zealand grasslands

Authors: Schiedung, M; Tregurtha, CS; Beare, MH; Thomas, SM; Don, A

Source: GLOBAL CHANGE BIOLOGY, 25 (7):2296-2309; JUL 2019

Abstract: Sequestration of soil organic carbon (SOC) has been recognized as an opportunity to off-set global carbon dioxide (CO2) emissions. Flipping (full inversion to 1-3 m) is a practice used on New Zealand’s South Island West Coast to eliminate water-logging in highly podzolized sandy soils. Flipping results in burial of SOC formed in surface soil horizons into the subsoil and the transfer of subsoil material low in SOC to the “new” topsoil. The aims of this study were to quantify changes in the storage and stability of SOC over a 20-year period following flipping of high-productive pasture grassland. Topsoils (0-30 cm) from sites representing a chronosequence of flipping (3-20 years old) were sampled (2005/07) and re-sampled (2017) to assess changes in topsoil carbon stocks. Deeper samples (30-150 cm) were also collected (2017) to evaluate the changes in stocks of SOC previously buried by flipping. Density fractionation was used to determine SOC stability in recent and buried tops oils. Total SOC stocks (0-150 cm) increased significantly by 69 +/- 15% (179 +/- 40 Mg SOC ha(-1)) over 20 years following flipping. Topsoil burial caused a one-time sequestration of 160 +/- 14 Mg SOC ha(-1) (30-150 cm). The top 0-30 cm accumulated 3.6 Mg SOC ha(-1) year(-1). The chronosequence and re-sampling revealed SOC accumulation rates of 1.2-1.8 Mg SOC ha(-1) year(-1) in the new surface soil (0-15 cm) and a SOC deficit of 36 +/- 5% after 20 years. Flipped subsoils contained up to 32% labile SOC (compared to < 1% in un-flipped subsoils) thus buried SOC was preserved. This study confirms that burial of SOC and the exposure of SOC depleted subsoil results in an overall increase of SOC stocks of the whole soil profile and long-term SOC preservation.

How ecologists define drought, and why we should do better

Authors: Slette, IJ; Post, AK; Awad, M; Even, T; Punzalan, A; Williams, S; Smith, MD; Knapp, AK

Source: GLOBAL CHANGE BIOLOGY, NIL_1-NIL_8; JUL 19 2019

Abstract: Drought, widely studied as an important driver of ecosystem dynamics, is predicted to increase in frequency and severity globally. To study drought, ecologists must define or at least operationalize what constitutes a drought. How this is accomplished in practice is unclear, particularly given that climatologists have long struggled to agree on definitions of drought, beyond general variants of “an abnormal deficiency of water.” We conducted a literature review of ecological drought studies (564 papers) to assess how ecologists describe and study drought. We found that ecologists characterize drought in a wide variety of ways (reduced precipitation, low soil moisture, reduced streamflow, etc.), but relatively few publications (similar to 32%) explicitly define what are, and are not, drought conditions. More troubling, a surprising number of papers (similar to 30%) simply equated “dry conditions” with “drought” and provided little characterization of the drought conditions stu died. For a subset of these, we calculated Standardized Precipitation Evapotranspiration Index values for the reported drought periods. We found that while almost 90% of the studies were conducted under conditions quantifiable as slightly to extremely drier than average, similar to 50% were within the range of normal climatic variability. We conclude that the current state of the ecological drought literature hinders synthesis and our ability to draw broad ecological inferences because drought is often declared but is not explicitly defined or well characterized. We suggest that future drought publications provide at least one of the following: (a) the climatic context of the drought period based on long-term records; (b) standardized climatic index values; (c) published metrics from drought-monitoring organizations; (d) a quantitative definition of what the authors consider to be drought conditions for their system. With more detailed and consistent quantification of drough t conditions, comparisons among studies can be more rigorous! , increasing our understanding of the ecological effects of drought.

Customising public digital soil mapping products with mass preserving splines

*There are now plenty of soil maps which digitally represent the soil variation across landscapes in both the lateral and vertical dimens...… Continue reading

Journal Paper Digests

Published on January 28, 2020

Journal Paper Digests

Published on January 27, 2020