Abstract
Two of the biggest problems facing humankind are feeding an exponentially growing human population and preventing the accumulation of atmospheric greenhouse gases and its climate change consequences. Refined agricultural practices could address both of these problems. The research addressed here is an exploration of the efficacy of alternative agricultural practices in sequestering carbon (C). The study was conducted in Zimbabwe with the intent to (a) demonstrate the utility of micrometeorological methods for measuring carbon dioxide (CO2) exchange between the surface and the atmosphere in the short-term, and (b) to quantify differences in such exchange rates for a variety of agricultural practices. Four Bowen ratio energy balance (BREB) systems were established on the following agricultural management practices: (1) no-till (NT) followed by planting of winter wheat (Triticum aestivum), (2) NT followed by planting of blue lupin (Lupinus angustifolios L.), (3) maize crop residue (Zea mays L.) left on the surface, and (4) maize crop residue incorporated with tillage. Over a period of 139 days (from 15 June to 31 October 2013) the winter wheat cover crop produced a net accumulation of 257 g CO2-C m-2, while the tilled plot with no cover crop produced a net emission of 197 g CO2-C m-2 and the untilled plot with no cover emitted 235 g CO2-C m-2. The blue lupin cover crop emitted 58 g CO2-C m-2, indicating that winter cover crops can sequester carbon and reduce emissions over land left fallow through the non-growing season. The micrometeorological methods described in this work can detect significant differences between treatments over a period of a few months, an outcome important to determine which smallholder soil management practices can contribute towards mitigating climate change.
Highlights
Though most greenhouse gas (GHG) emissions are from fossil fuel combustion, agriculture has a unique and important role in mitigating GHG emissions and their relationship to climate change
Four Bowen ratio energy balance (BREB) systems were established on the following agricultural management practices: (1) no-till (NT) followed by planting of winter wheat (Triticum aestivum), (2) NT followed by planting of blue lupin (Lupinus angustifolios L.), (3) maize crop residue (Zea mays L.) left on the surface, and (4) maize crop residue incorporated with tillage
The relationships of other variables such as moisture and temperature can be distinguished, and annual and interannual totals of flux can be measured and compared for various combinations of management practices. These results underscore the problem of bare-fallow for both tilled and land left untilled, and highlights the value of cover crops during the non-growing season. These results indicate that BREB micrometeorological systems can be used to distinguish short-term differences in days, weeks, and months between agricultural practices in a temperate and moderately dry climatic regime in Zimbabwe, despite instrument challenges such as remote power and environmental influences such as sporadic turbulence, rainfall, and irrigation
Summary
Though most greenhouse gas (GHG) emissions are from fossil fuel combustion, agriculture has a unique and important role in mitigating GHG emissions and their relationship to climate change. Agriculture is a source of approximately 10% of total GHG emissions (FAO, 2013), yet it has the potential to mitigate GHG emissions by sequestering carbon (C). Agriculture’s mitigation potential is explained in part by the size of the top three active reservoirs of C in the C cycle. The oceans play an important role in C sequestration and are the largest active reservoir with 38,000 Pg C, though only 700-1,000 Pg C of this total are at shallow depths where interactions with the atmosphere take place. Fossil fuels make up the second largest active reservoir with 5,000-10,000 Pg C. With about 12% of the global land surface under cultivation, agricultural C sequestration is an important low-cost mitigation strategy with substantial co-benefits (Smith et al, 2007)
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