Increased greenhouse gas fluxes from no-till soils explained with an integrative surface and soil air gas flux estimation approach

Background/Question/Methods

A variety of mitigation strategies are being explored to combat the rising concentration of atmospheric carbon dioxide.  Among them is increasing storage of carbon in soils by reducing the frequency of agricultural tillage – a practice that can also conserve soil, water, and nutrients.  This approach has immense promise because of the amount of land used for agriculture and the speed at which tillage practices can be changed.  However, there have been contradictory findings about the impact tillage has on global warming because it can sometimes lead to disproportionate increases in the very strong greenhouse gases of nitrous oxide and methane, even though more carbon is stored in the soils.  These strong greenhouse gases are produced when part of the soil lacks oxygen, but it has been difficult in the past to measure when and why that occurs.  We measured the soil structures responsible for strong greenhouse gas production with a novel set of field and laboratory techniques to determine how agricultural practices and natural environmental variation affect the mechanisms responsible for strong greenhouse gas production. 

Results/Conclusions

Simultaneous field gas flux studies and laboratory soil incubation experiments allowed us to compare the response of the same soils to natural and manipulated changes to tillage, soil moisture, and organic matter.  A new method to calculate in situ soil gas flux that integrates soil air concentration gradients and chamber flux measurements provided improved surface flux estimates and partitioned estimates of gross soil methane production and consumption.  We found that gross methane consumption was much greater in no-till soils than conventionally tilled soils.  Gross methane production was also greater in no-till soils – these estimates demonstrated that production may occur frequently in soils even when the related surface flux measurements overwhelmingly show net methane consumption.  Surface flux estimates calculated with the integrative technique were typically 30-80% different from the traditionally calculated chamber method flux estimates, sometimes up to 300% different.  Soil moisture was less important than tillage in explaining field measured surface flux or gross methane production but was a primary predictor in the laboratory experiments.  With the help of this integrative gas flux estimator we determined that no-till soils host hidden methane production which causes their surface fluxes to be impacted by environmental changes differently than tilled soils.