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• Temperature was most important regulator of plant litter decay and retention of C in soil. • Precipitation plays an interactive role with temperature. • More than ½ of litter C was lost <1 yr and only about 5–12% remained after 8–10 yr. • Soil properties did not play a discernible role in these processes. • No-till slowed plant litter decay, but only initially in arid environments. Identifying the primary regulators of plant litter decay in agricultural soils is important for understanding ecosystem function now and for managing post-harvest litter in a warmer world. We conducted a litter decay study at 11 sites in Canada and New Zealand with diverse soils and climates. We incorporated 13C-labelled barley litter into the surface 10 cm of soil and monitored amount remaining over 8–10 years; at five sites litter was also applied to the soil surface to simulate no-tillage. Our objectives were to evaluate litter decay as influenced by soil type, tillage practice, and environmental conditions, and quantify the most important factors controlling C retention in soil. Loss of C via decomposition occurred quickly – more than half was lost within 1 year and only about 5–12% remained at the end of the experiment. A double exponential decay model, based on temperature and developed from the first 5 years of the study, accurately described litter decay, but only after re-tuning using measurements from the full experimental period. Including precipitation in the model further improved its fit. Soil properties exerted minimal discernible influence on the amount of litter C remaining, implying that properties such as the amount and surface area of clay minerals, were less important than climate in limiting litter decay or enhancing retention of C in soil. Comparison of litter application treatments showed that no-till slowed decay of plant litter, but only for a short time (∼1 yr) and only in environments with a mean annual precipitation of <1000 mm. These findings have implications for the role of soils in climate mitigation. If only 10% of added plant litter C remains in soil beyond a few years, regardless of climate, residue placement, or soil type, then rates of soil gain are limited without substantive increases in residue inputs. [ABSTRACT FROM AUTHOR]

• Soil organic carbon (SOC) stabilities were studied in two agroforestry systems. • We studied hedgerow and shelterbelt systems and their component land-uses (cropland vs forest). • The biological stability of SOC was higher in hedgerow than in shelterbelt system. • The biological stability of SOC was higher in cropland than in forest land-use. • Agroforestry system by land-use interactively affected the thermal stability of SOC. Agroforestry systems play important roles in carbon (C) sequestration in western Canada. However, the effect of agroforestry system type and their component land-uses (forested area and cropland) on soil organic C (SOC) stability is poorly understood. We studied SOC stability in surface soils from two agroforestry systems (shelterbelt system and hedgerow system) and their component land-uses (forested area and cropland) in western Canada. A 61-day laboratory incubation was used to assess SOC biological stability, and compared to thermal stability measured during ramped combustion. The shelterbelt system had a lower biological stability than the hedgerow system as indicated by the higher cumulative soil respiration and SOC loss during incubation. Cumulative respiration and SOC loss during incubation in the forested area was 1.63 and 1.57 times that in the cropland, indicating a lower SOC biological stability in the former. Thermal stability indicators, CO 2 -T 50 (the temperature at which half of the CO 2 is produced), DSC-T 50 (the temperature at which half of the exothermic energy of the SOC is released) and ROI (return on energy investment) were influenced by the interactive effects of the agroforestry system by land-use, while TG-T 50 (the temperature at which half of the exothermic mass is lost) and Ed (energy density) were influenced by land-use and the interactive effects of agroforestry system by land-use. Hedgerow cropland had a higher SOC thermal stability (higher TG-T 50) than the hedgerow forested area, while there was no difference among shelterbelt cropland, and the forested areas of shelterbelt and hedgerow system. We conclude that planting trees on agricultural land to form agroforestry systems alters SOC biological and thermal stabilities, and the hedgerow system may have a more stable SOC than the shelterbelt system, thus, maintaining the hedgerow system may enhance C stability, promote C sequestration and climate change mitigation. [ABSTRACT FROM AUTHOR]