Effect of cropping practices on soil organic carbon: evidence from long-term field experiments in Victoria, Australia

Despite considerable research, predicting how soil organic carbon (SOC) in grain production systems will respond to conservation management practices, such as reduced tillage, residue retention and alternative rotations, remains difficult because of the slowness of change and apparent site specificity of the effects. We compared SOC stocks (equivalent soil mass to ~0–0.3m depth) under various tillage, residue management and rotation treatments in three long-term (12-, 28- and 94-year-old) field experiments in two contrasting environments (Mallee and Wimmera regions). Our hypotheses were that SOC stocks are increased by: (1) minimum tillage rather than traditional tillage; (2) continuous cropping, rather than crop–fallow rotations; and (3) phases of crop or pasture legumes in rotations, relative to continuous cropping with cereals. We found that zero tillage and stubble retention increased SOC in some circumstances (by up to 1.5MgCha-1, or 8%) but not in others. Inclusion of bare fallow in rotations reduced SOC (by 1.4–2.4MgCha-1, or 8–12%) compared with continuous cropping. Including a pulse crop (field pea, where the grain was harvested) in rotations also increased SOC in some instances (by ~6–8MgCha-1, or 29–35%) but not in others. Similarly, leguminous pasture (medic or lucerne) phases in rotations either increased SOC (by 3.5MgCha-1, or 21%) or had no significant effect compared with continuous wheat. Inclusion of a vetch green manure or unfertilised oat pasture in the rotation did not significantly increase SOC compared with continuous wheat. The responses in SOC to these management treatments were likely to be due, in part, to differences in nitrogen and water availability (and their effects on carbon inputs and decomposition) and, in part, to other, unidentified, interactions. We conclude that the management practices examined in the present study may not reliably increase SOC on their own, but that significant increases in SOC are possible under some circumstances through the long-term use of multiple practices, such as stubble retention+zero tillage+legume N input+elimination of fallow. The circumstances under which increases in SOC can be achieved require further investigation.