Soil organic carbon monitoring to assess agricultural climate change adaptation practices in Navarre, Spain

Climate change adaptation strategies are needed for agriculture, one of the most vulnerable human activities. In Navarre, North of Spain, ongoing adaptive management practices were identified and promoted in the framework of a regional adaptation strategy. Most include practices aiming to increase topsoil organic carbon (SOC) in agricultural land. In this work, the effectiveness of these practices (conservation agriculture, crop rotations, additions of organic matter, irrigation, and controlled grazing management) was assessed by means of monitoring SOC in a network of 159 agricultural fields across the region. These fields were selected across bioclimatic zones, where soil vulnerabilities and land-uses were previously assessed, to represent the most widespread conditions in the region. A sampling protocol designed to compare SOC stocks in plots with equal soil conditions within each zone, and with or without adaptive practices, allowed the determination of their effect size (measured as response ratios, RR). Exogenous organic matter addition was the most effective practice for SOC storage (RR 95% confidence interval (CI) [1.25-1.37]) across the region. Controlled grazing also resulted in net SOC gains (RR CI [1.13-1.42]) in temperate and semiarid grasslands. Conservation agriculture seemed to be more effective in the driest zone (RR CI [1.30-1.53]) than in the more humid ones (RR CI [0.98-1.21]). Irrigation also displayed a net positive effect (RR CI [1.17-1.34]), modulated by irrigated crop management, whereas crop rotations had an overall negative impact vs. monoculture (RR CI [0.84-0.96]), likely by their interaction with irrigation. These results confirm the variability in SOC responses to changes in management, and SOC as an indicator for assessing regional adaptation practices, although other biophysical, agronomic, and socio-economic factors also need to be accounted for.