Changes in soil organic carbon over 20 years after afforestation in semiarid SE Spain

- No differences in SOC among the 10 soil-preparation treatments after 20 years.
- An increasing mean SOC in the afforestation subplots was found.
- No differences in SOC among afforestation, abandoned croplands and cereal crops.
- Potential for SOC sequestration in the following years of semiarid afforestation.

This paper analyses the changes on SOC in semiarid abandoned farmland located in SE Spain partially afforested with Pinus halepensis Mill. in winter 1995-1996. The study tests the effects that 10 soil-preparation treatments before planting exerted on SOC, evaluates the effects of land-use change on SOC, and simulates SOC dynamics using the RothC model. The land-use changes studied were from cereal crops to abandoned croplands and from cereal crops to afforestation, in comparison to a native forest in equilibrium (maximum SOC reached). The results show that there were no differences in SOC among the soil-preparation treatments applied 20 years ago. An increasing mean SOC was recorded in the afforestation subplots. These increases were not always significant from one sampling period to another and were related to silvicultural practices and climate. SOC in the afforested land was influenced by the soil carbonate content (+), pH (−), and pine density (+). After 20 years, the mean SOC values in the afforested land were higher than in abandoned croplands and cereal crops (12.23 Mg C ha−1, 9.52 Mg C ha−1 and 8.02 Mg C ha−1, respectively) though no statistical differences were registered among these land uses. In all cases SOC differed with respect to the native pine forest, which showed 40.59 Mg C ha−1. Our results indicated that not enough time had elapsed to detect significant cumulative effects in SOC due to slow dynamics of carbon in the soil in the semiarid climate, the low inputs of organic matter to the soil and, probably, the low soil clay content in the study area. At the end of this study, the SOC in the afforested area was 69.9% less than in the native pine forest, indicating a clear potential for SOC sequestration in the following years. The RothC model was able to simulate the measured SOC increases due to the abandonment of agriculture and the afforestation of the cereal crops. In a 20-year model, we found a fit for the time course of SOC in abandoned croplands of RMSE < 10%, and a RMSE = 33% for the afforested area. Although the model in all cases overestimated the SOC measured, our results could be used as general indicators for predictions of the time course of SOC and potential carbon sequestration in semiarid afforested areas.