Soil Respiration During Three Cropping Cycles of Durum Wheat under Different Tillage Conditions in a Mediterranean Environment

This study analysed respiration from soil cultivated with wheat under a Mediterranean climate and submitted to two tillage treatments: no tillage (NT) and reduced tillage (RT). Carbon dioxide (CO2) emissions were measured by static chambers in three growing seasons (2015-18) in the framework of a long-term experiment of different tillage systems established in 2002. Tillage management remained the same throughout the trial, while chemical weed control was carried out in response to the actual weed content. Environmental variables were monitored, including the soil total organic carbon (TOC) content, water-extractable organic carbon (WEOC), soil temperature and water content. Total CO2 emission, as mean of the studied seasons, did not significantly differ between NT and RT (2.35 and 2.24 t CO2-C ha(-1) for NT and RT, respectively), although differences were found at the growth cycle scale. Total annual CO2 emission from NT was stable, being the same along the three cycles, while total RT CO2 emission decreased from the first to the third cycle. The TOC content was almost stable, while WEOC showed large variability that could explain the variability of CO2 emissions for RT over short time scales and was probably because of weed control in the middle of the second growing cycle. CO2 emissions were driven by soil water content via the impact on water filled pore space, and soil temperature range, that is daily maximum minus minimum temperature. In conclusion, for this winter wheat NT did not improve the balance of soil CO(2 )emissions with respect to RT.