Cooper, Hannah
(2020)
The role of zero-tillage in climate change mitigation.
PhD thesis, University of Nottingham.
Abstract
An estimated 11% of the world’s land surface is under cultivation for crop production, and an additional 6.6% of the land will need to come into cultivation to feed a population of 9.1 billion people in 2050. Agricultural soils are a significant source of greenhouse gas emissions, contributing one-fifth of CO2 emissions globally, one-third of CH4 emissions and two-thirds of N2O emissions. Changes in the management of agricultural soils can affect their role as a source or sink in the global carbon cycle, and the scale and composition of their greenhouse gas emissions. Therefore, understanding the role of different crop cultivation regimes in regulating these processes is a key global challenge
Conventional tillage is a traditional technique for crop production which has been practiced for centuries. This approach is beneficial in destroying large soil clods, burying weeds, preparing the seedbed and incorporating crop residues into the soil profile. However, through the process of loosening the upper 15-20 cm of soil, this management can result in large scale topsoil erosion. The annual soil loss from a plough-based farming averages 1.5 mm of erosion, which is almost 90 times faster than which new soil is produced. Zero-tillage, whereby the soil surface is not mechanically loosened, and the crop residue remains on the surface, is an effective management strategy to reduce soil erosion and limit water runoff. As a result, there is extensive evidence that zero-tillage is beneficial for the functioning and quality of the soil in many agricultural settings. There has, however, been contrasting evidence surrounding the climate change mitigation capabilities of zero-tillage in comparison to conventional tillage.
The current literature suggests that zero-tillage practices may reduce greenhouse gas emissions in terms of decreased CO2 emissions, but an increase in the more damaging N2O emissions may offset any potential benefits. Uncertainty also remains regarding the extent to which zero-tillage can sequester more carbon throughout the soil profile, with some studies highlighting incorrect sampling and calculation methodologies which have overestimated the potential of zero- tillage.
This project assesses the extent to which zero and conventional tillage can play a role in climate change mitigation across commercial and experimental farms, as well as evaluating the magnitude of change over time, and across contrasting climates. The results indicate an increase in soil organic carbon stocks in zero- tilled compared to conventionally tilled soils which was both microbially and physically mediated. X-ray Computed Tomography revealed that zero-tillage significantly increased soil porosity (with a mean increase of 13%) and pore connectivity (by a mean increase of 4%) in the long-term but decreased inter aggregate porosity (by 4%), providing a potential physical mechanism for the protection of soil organic carbon. Carbon analysis indicated that zero-tilled soils feature a large labile pool comprising the increased presence of lignin and polysaccharides. Moreover, soils under zero-tillage for 31 years in Brazil also showed a shift in the chemical composition of organic carbon to a more recalcitrant fraction following long-term zero-tillage, suggesting soils were accumulating rather than mineralising soil organic carbon. Furthermore, across both the U.K and Brazilian soil samples, global warming potential was at least 30% lower from zero-tilled soils compared to conventionally tilled soils, primarily driven by lower CO2 emissions under zero-tillage. Moreover, the net global warming potential of zero-tillage systems decreased further with time in management. However, further factors, including the effect of management on crop yield and nutrient efficiency need to be explored to understand the broader implications of widespread adoption. This study demonstrates that soil management practices strongly influence many biophysical and chemical properties, mitigating CO2 emissions, alongside delivering further soil health benefits. These improvements, which will be leading drivers in the adoption of zero-tillage systems for farmers, include improved soil quality, increased soil pore connectivity, and reduced soil erosion. These effects have the potential to benefit soils globally, provided economic mechanisms are available to promote the implementation of zero-tillage as a mitigation strategy of global warming.
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