Enhancing soil organic carbon (SOC) and total nitrogen (N) is considered an important step in mitigating greenhouse gas emissions and improving soil fertility. The loss of SOC and N generally observed during cropping may be reversed by converting such land use to permanent pasture. However, large uncertainties remain around the processes that govern how much C and N may be sequestered from this conversion in soils worldwide. Here, we sampled soils across 10 paddocks on 20-year old grass pasture sites with a chronosequence of cropping history in order to quantify changes in SOC and N after the conversion of long-term cropping to pasture land use in a semi-arid region of southwest Queensland, Australia. The average rate of change in SOC stocks under pasture in the top 0-0.1m soil layer was approximately 0.1tCha-1yr-1, consisting of an increase in SOCC4 (pasture) of 0.2tCha-1yr-1 and a decrease in SOCC3 (pre-pasture) of 0.1tCha-1yr-1. The decrease in SOCC3 was enhanced at sites with greater years under cropping, indicating a reduced potential for SOC sequestration at sites with longer duration under cropping. The loss of total nitrogen (N) under cropping was also partially recovered with the introduction of permanent perennial pastures. A significant, positive correlation between soil aggregation and mineralisable N under cropping suggested that soil structure has a strong influence over N stability in the soil. However, soil aggregation and mineralisable N did not improve under pastures, indicating that the loss of soil fertility and structure under cropping remained a residual effect that was not recovered within 20 years of permanent pastures in this semi-arid subtropical environment. We suggest here that the resilience of ecosystems to recover soil fertility under pastures declines with greater years under cropping. © 2015 Elsevier B.V.
