While I was reading a recent BBC article on so-called "carbon farming", I started to wonder, is crop production a net source or a net sink of carbon emissions? Even with tillage and ploughing it seems implausible that it emits more than it sequesters (it's plants after all, consuming carbon is what they do). Are there any studies on the topic, how much it sequesters, how much it emits? What are the numerically represented advantages of no-till, no-plough systems, are they tiny, are they huge?
Food is responsible for a third of global greenhouse gas emissions, and crops are no exception. For crop production, there are two major categories of emissions sources: Land use, and agricultural production. For crops, the relevant sources within these categories are:
- Land use
- Land use change (e.g. deforestation)
- Cultivated soils
- Drainage and burning of soils, including peatlands
- Agricultural production
- Emissions from fertilizer application
- Methane from rice
- Fuel use from on-farm machinery
- Energy for fertilizer production
- Burning of agricultural waste
The specific impact of each varies by crop and farming method. The 2018 paper "Reducing food's environmental impacts through producers and consumers" (pdf) includes this chart showing the range of impact for several agricultural sources at wheat farms, from a meta-analysis of studies covering nearly 40,000 farms around the world:
You can see that the largest impact categories are related to fertilizer, pesticides, and energy use on the farm (electricity and fuel). These are things that can't be reduced by the no-till, no-plough methods mentioned in the question.
While that figure is for wheat, there are also differences depending on which type of crop is being farmed. Here's another graphic from Our World in Data:
You can see that some crops (such as tree nuts) actually sequester carbon -- however the trade-off here is the high water footprint.