The effect of cropping practice on soil carbon in Victoria

The effect of cropping practice on soil carbon in Victoria

The PICCC soil carbon project, led by DEPI’s Fiona Robertson, has recently published a paper on a series of field trials assessing the impact of farm management practices on soil carbon.

Soil organic stocks (SOC) were compared under treatments in three long-term (12, 28 and 94 year old) field experiments in two contrasting environments (Mallee and Wimmera regions). Inclusion of bare fallow in crop rotations reduced SOC (by 1.4–2.4 Mg C/ha, or 8–12%) compared with continuous cropping. In some circumstances, zero tillage and stubble retention increased SOC (by up to 1.5 Mg C/ha, or 8%), but not in others.

Similarly, including a pulse crop (field pea, where the grain was harvested) or a leguminous pasture (medic or lucerne) phase in rotations increased SOC in some circumstances (by ~6-8 Mg C/ha, or 29–35%, and by 3.5 Mg C/ha, or 21%, respectively) but not in others. However inclusion of a vetch green manure or unfertilised oat pasture in the rotation did not significantly increase SOC compared with continuous wheat.

Researchers expect that the variable SOC responses observed were likely to be partly due to differences in nitrogen and water availability (and their effects on carbon inputs and decomposition) and partly to other, unidentified, interactions. They concluded that the management practices examined so far may not reliably increase SOC on their own, but that significant increases in SOC are possible in some circumstances through long-term use of multiple practices such as stubble retention + zero tillage + legume N input + elimination of fallow. Further trials are aiming to identify other interactions and influencing factors.