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There is a need to quantify agriculture's potential to sequester carbon (C) to inform global approaches aimed at mitigating climate change effects. Many factors including climate, crop, soil management practices, and soil type can influence the contribution of agriculture to the global carbon cycle. The objective of this study was to investigate the C sequestration potential of conservation agriculture (CA) (defined by minimal soil disturbance, maintaining permanent soil cover, and crop rotations). This study used micrometeorological methods to measure carbon dioxide (CO2) flux from several alternative CA practices in Harare, central Zimbabwe. Micrometeorological methods can detect differences in total CO2 emissions of agricultural management practices; our results show that CA practices produce less CO2 emissions. Over three years of measurement, the mean and standard error (SE) of CO2 emissions for the plot with the most consistent CA practices was 0.564 ± 0.0122 g CO2 m−2 h−1, significantly less than 0.928 ± 0.00859 g CO2 m−2 h−1 for the conventional tillage practice. Overall CA practices of no-till with the use of cover crops produced fewer CO2 emissions than conventional tillage and fallow. [ABSTRACT FROM AUTHOR]

Quantification of the emission rates of various gases from soils at night remains a challenge, confronting climate science (in the case of CO2 and CH4) and agriculture science (for NH3 and N2O, among others). In the stable conditions prevailing at night, concentrations of such emitted gases build up at the surface during the night, with intermittent interruptions commonly attributed to the passage of packets of turbulence. The utility of conventional micrometeorological experimental methods in such circumstances is questionable, and chamber methods have been developed to meet the challenge. Here, a statistical approach is proposed, in which micrometeorological field data are used to replicate the likely characteristics of a chamber experiment, yielding estimates of surface fluxes at the surface itself and not at some height above it. The methodology proposed is developmental at this time, with details intended to correspond to the use of both closed and vented chambers. Its application to three recent field studies is explored: (1) a study of nocturnal CO2 emission from two test areas (one previously tilled and the other not) in Ohio in 2015; (2) a similar experiment conducted in Zimbabwe in 2013 (one area previously tilled and a second left fallow), and (3) an investigation of NH3 effluxes from a crop previously treated with urea ammonium nitrate (UAN), in Illinois in 2014. There are few measurements with which to compare the results presented here, however the values obtained are within the range of available field data. [ABSTRACT FROM AUTHOR]

• The representativeness of measurements to the BREB procedure is quantified. • Considerations of fetch, footprints and undesirable wind directions are obviated. • Heat budget contributions not considered in the basic BREB approach are revealed. • The singularities usually confronted in BREB analysis are avoided. The addition of measurements of downward-looking infrared surface temperature to the standard suite of Bowen Ratio Energy Balance observations permits a direct examination of the adequacy of fetch. Furthermore, when the observed rates of change of surface temperature are examined (using multiple regression) in the light of the total turbulent heat exchange (the sum of sensible and latent heats) and the total heat energy available (due to net radiation, corrected for the ground heat exchange) a quantification of the rate of heat storage otherwise unaccounted for (e.g. by the canopy) is revealed. The statistical methodology minimizes the difficulties often encountered near dawn and dusk when the ratio of sensible to latent heat fluxes approaches -1. Analysis making use of all available data is then possible, without data exclusions being applied. Four months of data obtained at a site in the Zimbabwe highland region in 2014 are used to illustrate this augmented Bowen Ratio analysis methodology. The analysis reveals that the heat storage terms tend to dominate the heat energy budget for periods following dawn and dusk, and that most of the heat storage is associated with the uppermost layers of soil rather than the vegetative canopy. [ABSTRACT FROM AUTHOR]

• Results of studies in Ohio indicate heat storage rates typically maximizing at about 40 W m-2 in mid-morning over maize. • During the period of most rapid growth, the storage term peaks at above 60 W m-2. • Ohio heat storage rates are similar to small-plot results from earlier studies in Zimbabwe. • Photosynthesis accounts for about as much of the apparent heat storage as does biomass heating. • Comparisons between BREB and ABRA procedures indicate differences, mainly in the periods near dawn and dusk. • For CO 2 fluxes, eddy covariance is an optimum measurement technique provided the site is large enough. An experiment conducted over a field of maize in Ohio during the growing season of 2015 provided measurements sufficient to compare among three methods for quantifying net ecosystem exchange of sensible heat, latent heat and carbon dioxide: eddy covariance (EC), the familiar Bowen ratio energy balance (BREB) method and a statistical procedure making use of surface temperature measurements by downward-looking infrared thermometry (the augmented Bowen ratio analysis – ABRA). EC and BREB yield run-by-run estimates of fluxes. The third method (ABRA) gives average values over prescribed ensembles of data. The Ohio observations support the conventional understanding that EC is the preferred methodology provided the site is large enough. Both BREB and ABRA can make use of measurements closer to the surface than EC, and hence are more suited for studies of small test plots. The ABRA method provides results agreeing with conventional BREB and EC in daytime except for periods near dawn and dusk when heat storage can be a substantial contributor in the surface heat energy budget. The unattributed heat storage in the morning found in analysis of observations made elsewhere is substantiated. It is concluded that the apparent heat storage term in the surface heat budget is most influenced by heating of crop biomass and the energy demands of photosynthesis. The comparisons provide clear evidence of the difficulties associated with application of all gradient-based methodologies at night (affecting both BREB and ABRA) when atmospheric stability often prohibits reliable association of fluxes with gradients. CO 2 measurements reveal substantial differences among the three different methodologies (ABRA, BREB and EC), with EC appearing to be more robust and therefore preferred if the test area is large enough. [ABSTRACT FROM AUTHOR]