Journal Paper Digests

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Journal Paper Digests 2022 #3

  • Estimating cation exchange capacity from agronomic soil tests: Comparing Mehlich-3 and ammonium acetate sum of cations
  • Evaluation of Mehlich-3 for determination of cation exchange capacity in Kansas soils
  • The meaning of net zero and how to get it right

The meaning of net zero and how to get it right

The concept of net-zero carbon emissions has emerged from physical climate science. However, it is operationalized through social, political and economic systems. We identify seven attributes of net zero, which are important to make it a successful framework for climate action. The seven attributes highlight the urgency of emission reductions, which need to be front-loaded, and of coverage of all emission sources, including currently difficult ones. The attributes emphasize the need for social and environmental integrity. This means carbon dioxide removals should be used cautiously and the use of carbon offsets should be regulated effectively. Net zero must be aligned with broader sustainable development objectives, which implies an equitable net-zero transition, socio-ecological sustainability and the pursuit of broad economic opportunities.

Net-zero emissions is more than a concept of physical climate science. It is implemented in social, political and economic contexts. This Perspective proposes seven attributes that are critical for the practical and effective implementation of net zero.

Estimating cation exchange capacity from agronomic soil tests: Comparing Mehlich-3 and ammonium acetate sum of cations

Soil cation exchange capacity (CEC) measures the capability of soils to hold and release nutrients. It is an important measure for classifying soils and planning nutrient additions, but it is not included in routine soil tests offered to farmers. In this study, we compared CEC measured with standard laboratory practices (neutral ammonium acetate [AAc]) with CEC estimated from agronomic soil test data (Mehlich-3 and AAc). We applied previously published estimation methods to a dataset of 48 soil samples. Based on the results, traditional methods underestimate measured cation exchange capacity by 36% for Mehlich-3 and 24% for AAc. The estimation can be improved by including aluminium (12% overestimate) or by fitting a localized pedotransfer function (3% underestimate), which offers a possibility for informing land managers about their soil nutrient holding capacity without adding costs to the analysis.

Evaluation of Mehlich-3 for determination of cation exchange capacity in Kansas soils

Soil testing laboratories across the United States have implemented the Mehlich-3 (M3) extraction method for phosphorus (P) and other nutrients. Though M3 is known to work well as a multinutrient extractant, it raises concerns for measuring exchangeable cations, particularly in calcareous soils. The objectives of this study were (a) to evaluate M3 as an extractant for base cations for Kansas soils, (b) to identify a range of soil pH where M3 may be a suitable replacement for ammonium acetate (AA), and (c) to examine the relationship between cation exchange capacity (CEC) summation and CEC displacement. A study was conducted evaluating 308 soils collected from across Kansas and extracted using both the M3 and AA soil extraction methods. Cation exchange capacity was estimated using the summation approach from measurements of both the M3 and AA, and the displacement method. Results indicate a strong correlation among M3 and AA methods for K, Mg, and Na (R-2 = .98, .96, .97, respectively). However, these relationships were considerably weaker for extractable Ca (R-2 = .78), where extractable Ca was higher in the M3 extracts for high pH soils. Estimated CEC was also affected by soil pH for the M3 method, with an estimated critical pH value of 7.3. Extractable Ca and CEC for soils with a pH below 7.3 showed a strong correlation between M3 and AA methods (R-2 = .9). Using M3 as a multinutrient extractant can improve turnaround time for sample analysis in soil testing laboratories. However, the use of the M3 procedure in soils with a pH of 7.3 or above will likely result in overestimation of exchangeable Ca and CEC.

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