Greenhouse gas emissions and soil organic matter dynamics in woody crop orchards with different irrigation regimes.

Water scarcity in arid, semiarid and dry regions is a limiting factor for the development of sustainable agriculture. As a consequence, the adoption of new strategies such as regulated deficit irrigation (RDI) to reduce water and energy consumption will be essential. Decreases in irrigation water content may also have positive effects on soil C cycle. Thus, an experiment was setup in three woody crop orchards during two years, with the objective of assessing if RDI can reduce soil CO and NO emissions, modify soil inorganic C and organic C quality and stability and affect soil aggregation. Soil CO and NO emissions were measured every two weeks while soil samplings were carried out every three months. Results indicated that decreases in soil moisture by RDI implementation were related to significant decreases in CO emissions in all crops. RDI contributed to an average decrease, compared with full irrigation, of 1088-1664 g CO m in the experimental period. Furthermore, CO emission was negatively correlated with inorganic C, suggesting the protective effect of soil carbonates towards organic matter. RDI also contributed to significantly decrease soil NO emissions. However, NO emission patterns did not directly follow soil moisture patterns and were constant in the experimental period. RDI contributed to an average decrease, compared with full irrigation, of 90-409 mg NO m. No physicochemical property was significantly affected by irrigation regime. Although microbial biomass was not significantly affected by RDI, β-glucosidase activity was significantly higher under full irrigation during the warm seasons, with significant positive correlation with CO emissions. This seems to suggest that a significant fraction of CO emitted from soil derives from organic matter degradation, which is limited with low water content. So, RDI could contribute to promote soil C sequestration by reduced greenhouse gas emissions, with no negative effects on soil structure at short-term.