Afforestation is a good strategy for climate change mitigation through increasing carbon stocks. This study determined changes in soil respiration (SR) brought about by the afforestation of high quality agricultural land in a temperate-humid region (Galicia, NW Spain), identified the variables that explain the observed changes and determined the main factors regulating SR temporal variation. Paired plots of fertile soils (cropped vs. afforested plots) were established in two similar areas (Pontevea and Laraño) where afforestation with Populus x euroamericana (Dode) Guinier was carried out in the same year. Different management practices and crop rotations were used (maize–pasture, Laraño and maize–fallow, Pontevea). The SR was measured in situ with a CO2 static chamber every 15 days (every month in winter) for 16 months; soil temperature (Ts) and soil moisture content (W) were also measured. In both areas, significant differences (p < 0.05) in SR between paired plots were related to soil organic C content and SR was mainly influenced by Ts, except during the summer period where SR fluctuations were accompanied by W fluctuations. These findings show that growing pasture crops on high quality land can prevent the loss of soil N and C and probably improve the greenhouse gas balance in the system.