Source: Food Climate Research Network
Alternative cropping systems such as organic or conservation agriculture are often expected to lead to enhanced soil carbon storage as compared with conventiaonl systems, and therefore to hold potential to contribute to climate change mitigation via carbon sequestration.
However existing reviews comparing them to conventional agriculture report contradictory results. To resolve this uncertainty, this study presents the results of a 16 year experiment in northern France which carefully compares the development of soil organic carbon levels for four cropping systems: conventional, low-input, organic and conservation agriculture.
The results find that only conservation agriculture and to a lesser extent organic cropping systems significantly increased soil carbon storage. For temperate climates, the authors conclude, this suggests that alternative cropping systems can make a contribution to climate change mitigation.
For each of the four different cropping systems considered (conventional, low-input, conservation and organic), this study aimed to calculate the inputs and outputs of carbon within the system as a whole (both above and below ground) and importantly to measure and explain the causes of soil carbon storage. The different cropping systems studied were characterised as follows.
Conventional (CON): regionally representative cropping system, with inputs used at levels designed to maximise yields.
Low input (LI): Limitation of external inputs, with reduced soil tillage, better targeted fertilisation and reduced pesticide use as compared to conventional agriculture.
Conservation (CA): Suppression of soil tillage, more diversified crop successions and permanent plant cover (direct seeding with permanent plant cover called cover crop)
Organic (ORG) minimising impacts on soil, water and air quality. Systemic prevention of weeds, pests and diseases combined with nutrient self sufficiency.
To see the crops used in the rotations, click here.
In each of the experimental plots for the cropping systems, after harvesting, crop residues were left on the soil surface. To ensure that the results captured only carbon inputs from plant growth on the experimental plots and not from carbon imported from outside the system, no organic fertilizer (i.e. manure) was used. Data on soil properties, including organic carbon, was collected on each plot, down to a depth of 30cm (the base of the plow layer). These data points were then used to parameterise a soil carbon model, which then provided a means to quantitatively evaluate the flows of carbon within each cropping system.
Between the cropping systems, considerable differences in yields and carbon storage were observed. Conventional wheat yields were similar to the regional averages. In comparison, wheat yields for low-input, conservation, and organic system reached, 91%, 66%, and 55% of this respectively. For soil organic carbon, after the 16 years, levels did not change significantly in either the conventional (+3%) or low-input systems (+1%). However, significant increases in soil organic carbon were observed for the conservation agriculture (+24%) and organic (+12%) systems. This equates to a rate of 0.63 and 0.28 tons per hectare per year.
Based on the modelling, the authors concluded that between the cropping systems, there was no evidence to suggest a difference in the rate of soil organic carbon decomposition (i.e. soil mineralisation). This is despite the fact that for the conservation agriculture system, no ploughing took place. Therefore, the differences in soil carbon storage were predominantly (although not entirely), the result of different levels of carbon input from crops grown as part of each system.
The level of carbon input to the system from cash crops grown was considerably lower for conservation agriculture and organic systems, due to their comparatively lower yields. However, this was compensated for by carbon input from alfalfa and in the case of conservation agriculture, catch and cover crops. Total carbon input from all crops (above and below ground) was greatest for conservation agriculture (5.41 t ha-1 yr-1), followed by conventional (4.09 t ha-1 yr-1), low-input (3.81 t ha-1 yr-1), and organic (2.87 t ha-1 yr-1). With conservation agriculture and organic at either end of the carbon input ranking, what this shows is that total carbon inputs were only weakly correlated with soil carbon storage. Rather, what distinguishes them both, is the larger below ground carbon inputs generated over the study period, which for conservation and organic systems were 1.86 and 1.07 tons of carbon per hectare annually, whereas for both conventional and low-input systems, below ground inputs totalled 0.82 tons per hectare
Ultimately, the researchers conclude that alternative arable systems do have the potential to sequester organic carbon in temperate climate conditions, but that this potential is driven primarily by the carbon inputs to the system, rather than by the effect of reduced soil decomposition through lower tillage.