Management of tree pruning residues to improve soil organic carbon in olive groves

Abstract

Olive groves are dominant in the eastern part of Andalusia (Spain) and therefore any change in their management can have a significant environmental impact. The management of olive tree pruning (burning in field versus crushing and amendment in the soil) can affect nitrogen and carbon dynamics in soils. In this study CO2 production, magnitude of priming effect and the dynamics of available nitrogen during the decomposition of: i) unburned pruning, and ii) remains resulting from the burning of pruning (ash and biochar) were evaluated in two soils of contrasting soil organic carbon levels. The residence time of biochar refractory carbon exceeded 13 years whereas for unburned pruning it was longer than 2.7 years. There was no priming effect when ash or biochar were applied to soils. However, the priming effect was positive when the soils were amended with unburned pruning and it was significantly correlated with water-soluble carbon. Ash or biochar did not change the amount of available nitrogen in the soil. Nevertheless, the amendment of unburned pruning reduced the levels of available nitrogen (from 91.9 to 21.8 μg NO3−–N g−1 to 55.6 and 8.3 in the soils of high and low soil organic carbon respectively) and increased soil nitrogen retention, at least temporarily. As a strategy of soil organic carbon accumulation, the amendment of unburned pruning was significantly more effective (≈819 kg C ha−1 year−1) than that of biochar and ash resulting from burned pruning (≈19 kg C ha−1 year−1).