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• Field experiment, incubation, and meta-analysis were combined. • Limitation of SOC mineralization in NT was observed with the experimental duration. • Stable SOC fractions led to limitation of SOC mineralization in NT. • Macro-aggregate protection from mineralization accelerated SOC accumulation in NT. • SOC sequestration in NT are underestimated when ignoring the experimental duration. Conservation tillage incorporating no-till (NT) is often recommended to reduce soil organic carbon (SOC) mineralization and sequester SOC, thus mitigating carbon dioxide production. The rates of SOC accumulation and mineralization are significantly affected by the time since the introduction of NT, however, there is currently poor understanding of exactly how rates change over time under long-term NT. Thus, an 11-year field experiment, a 60-day laboratory incubation, and a meta-analysis were used to better understand the effects of NT, experiment duration, and their interaction on SOC accumulation and mineralization. Our field experiment showed that, compared with plow tillage (PT), NT had a higher proportion of more stable SOC fractions and significantly increased SOC concentration by 12.6 % in the 5th year of the field experiment, and 52.2 % in the 11th year. A significant interaction between tillage and experiment duration was observed for SOC mineralization using laboratory incubation. NT decreased bulk SOC mineralization by 20.1 % in the 5th year of the field experiment and by 44.8 % in the 11th year when compared with PT. This was associated with an increase in SOC concentration and a decrease in SOC mineralization from macro-aggregates (>2 mm) over time and thus the accumulation of SOC was attributed to the physical protection of SOC within macro-aggregates. Meta-analysis confirmed these results and found that, compared with PT, NT significantly decreased SOC mineralization by ∼11 % when experiment duration was ≤8 years and by ∼27 % when it was 8–16 years (P < 0.05). This indicated that SOC mineralization was reduced in NT to an increasing degree as experiment duration increased and additional SOC could be sequestered with an experiment duration of up to 16 years. [ABSTRACT FROM AUTHOR]

Sequestering more carbon in soil is an effective approach to mitigate climate change. The temporal variability of soil organic carbon (SOC) that reflects its loss and accumulation potential is important to model SOC sequestration under climate change, but it is still poorly understood under biochar addition. To identify the temporal variability of SOC, induced by climate conditions (i.e. precipitation and temperature) under biochar addition, a field experiment with the biochar addition rates of 0 (CK), 1.8 (BL), 3.6 (BM), and 7.2 (BH) mega gram (Mg) ha−1 yr−1 and a laboratory incubation under controlled conditions at 20, 40, and 70% moisture under 10 and 20 °C in CK and BH soils was conducted. The results showed that biochar addition increased average annual SOC sequestration rates by 31.8–47.8% (369.8–556.6 kg ha−1 yr−1), and led to higher stock compared with CK. However, the annual sequestration rates of biochar addition showed a decreasing trend from 2014 (0.8–4.0 Mg ha−1 yr−1) to 2018 (−7.1 to −3.5 Mg ha−1 yr−1), and BH had the lowest sequestration rate from 2017 to 2018 compared with CK. Because the coefficients of variation of SOC concentration under biochar addition (1.2–11.0%) were significantly higher than those of CK (0.8–2.9%), indicating that biochar enhanced the temporal variability of SOC. The incubation study also confirmed that higher temporal variability under biochar addition resulted from increased sensitivity of SOC mineralization to temperature at 0–10 cm and to moisture at 10–20 cm layer. The high temporal variability was not conducive to the rates of carbon inputs retention in soil, and BH decreased by 20.4% compared with CK. Therefore, higher temporal variability induced by biochar addition constrains the potential for SOC sequestration, and middle and low rates (1.8–3.6 Mg ha−1 yr−1) of biochar addition are promising and cost-effective options in the North China Plain. Image 1 • Soil organic carbon was affected by temperature and moisture under biochar addition. • Biochar enhanced the temperature sensitivity of carbon at 0–10 cm layer. • Biochar enhanced the moisture sensitivity of carbon at 10–20 cm layer. • Middle and low rates of biochar addition were promising options. [ABSTRACT FROM AUTHOR]

• Higher specific SOC mineralization but lower Q 10 was observed under maize season. • Lower specific SOC mineralization but higher Q 10 was observed in macro-aggregates. • NT with SR promoted macro-aggregation and decreased specific SOC mineralization. • NT with SR improved Q 10 at low-middle moisture. • Increased moisture could decrease Q 10. Soil organic carbon (SOC) mineralization is regulated by temperature and moisture. No-till (NT) and straw returning (SR) have been widely adopted to sequester SOC, but information about the effects of temperature and moisture on SOC mineralization in NT and SR is limited, particularly SOC mineralization in different aggregate size classes. To identify the responses of SOC mineralization to temperature and moisture in NT and SR, undisturbed soils were sampled from a factorial experiment of tillage (NT and plow tillage [PT]) and straw (SR and straw removal [S0]) in a wheat-maize cropping system and incubated at moisture levels of 40%, 70%, and 100% at 15 °C and 25 °C. The results showed that maize season increased SOC mineralization but decreased its temperature sensitivity (Q 10) compared with the wheat season under different moisture levels. Because NT and SR promoted macro-aggregation and low mineralization was observed in macro-aggregates, NT significantly decreased the absolute (per unit soil) and specific (per unit SOC) mineralization compared with PT, regardless of the soil temperature, moisture, depth, and sample date (P < 0.05). The SR significantly increased absolute SOC mineralization; Whereas, there was no significant difference in specific mineralization and was even 24.4% lower at 5–10 cm depth in the maize season than that of S0 (P < 0.05). However, macro-aggregates had 13.6%-37.3% and 11.9%-19.9% higher Q 10 values than the micro-aggregates in the wheat and maize seasons, respectively. This led to a higher Q 10 under NT and SR at 40% moisture level, indicating that NT and SR constrained SOC accumulation under global warming. Increases in moisture ranging from 40% to 100% could decrease Q 10 , thus no significant difference between NT and PT, and SR and S0 at 100% level. Conclusively, NT with SR decreases SOC mineralization but increases its Q 10 , indicating that the loss of SOC is currently low but could accelerate under future warming. Our study also provided an effective approach that increased moisture could constrain the improvement of Q 10 induced by NT with SR. [ABSTRACT FROM AUTHOR]

• C sequestration and mineralization showed negative correlations at 0–10 cm layer. • Macro-aggregates (>2 mm) were the main carriers of SOC sequestration. • No-till improved C stock in macro-aggregates due to less mineralization. • Straw returning increased C concentration due to low mineralization quotient. Understanding the process of soil organic carbon (SOC) sequestration and mineralization in aggregates is pertinent to mitigate climate change and minimize risks of soil degradation. Thus, soil samples were obtained after a 10-year field experiment to identify the influences of tillage on aggregate-associated SOC sequestration and mineralization in the North China Plain (NCP). Four tillage practices investigated were as follows: no-till with straw retention (NTS, conservation tillage), rotary tillage with straw incorporation (RTS), conventional tillage with straw incorporation (CTS), and conventional tillage with straw removal (CT). Significantly negative correlations were observed between SOC concentration and potentially mineralized carbon in aggregates under different treatments for the 0–10 cm soil layer. The large macro-aggregates (>2 mm) with the highest proportion of size distribution represented the major pool of SOC stock (47.3–51.2%) and mineralization amount (38.2–43.6%) in the 0–30 cm layer, followed by that in the small macro-aggregates (0.25–2 mm), regardless of tillage practices. However, the mineralization quotient (mineralization per unit SOC concentration) of macro-aggregates (>0.25 mm) was lower than that for the other size classes. The NTS enhanced the macro-aggregate formation in the 0–20 cm layer and associated SOC concentration in the 0–10 cm layer. Furthermore, NTS decreased total potential mineralization in the 0–30 cm layer compared with the other tillage practices, attributed to decrease in the large macro-aggregates (30.0–51.4%) with low particulate organic carbon (POC) concentration. The NTS with low straw inputs had higher incremental efficiency with straw incorporation than that in the RTS and CTS by 45.0% and 13.5%, respectively (P < 0.05). Overall, the higher proportion of macro-aggregates recorded under NTS decreased carbon mineralization, and consequently, increased incremental efficiency with straw incorporation, and improved SOC sequestration in the surface soil layer in the NCP. [ABSTRACT FROM AUTHOR]

• No-till with straw retention (NTS) increased particulate organic C (POC) over time. • NTS increased mean weight diameter (MWD) over time. • High POC and MWD contributed to low C mineralizability. • Negative correlations were observed between C mineralizability and concentration. • NTS sequestered more C over time by reducing mineralizability. Conservation tillage has been widely practiced to increase soil organic carbon (SOC), and reducing SOC mineralization has the potential to improve accumulation. To assess the effects of conservation tillage on characteristics of SOC mineralization and accumulation, as well as their relationship, a site experiment was established in 2008 using data from 2012 to 2019 under wheat-maize cropping system. Four treatments were investigated: no-till with straw retention (NTS, conservation tillage), no-till with straw removal (NT), conventional tillage with straw incorporation (CTS), conventional tillage with straw removal (CT). The results showed that NTS significantly improved SOC concentration than the other treatments at 0–10 cm soil depth (P < 0.05) by 5–18% in 2012 and 31–93% in 2019 (increasing increments over time). There was a negative correlation (R2 = -0.60) between SOC concentration and mineralizability. On average, the SOC mineralizability under NTS was 39.0 mg C kg−1 SOC at 0–10 cm depth, significantly lower than CTS and CT (P < 0.05). Furthermore, relative SOC mineralizability of NT/NTS, CTS/NTS, and CT/NTS was>1 and showed a linear relationship with time, indicating that NTS can increase SOC accumulation by reducing SOC mineralizability over time. A negative correlation was observed between SOC mineralizability and mean weight diameter (MWD) (R2 = -0.34), and the highest MWD under NTS, suggesting reduced mineralizability due to enhanced physical protection of soil macro-aggregates. Furthermore, the particulate organic carbon (POC) concentration was highly correlated with MWD (R2 = 0.76). Conclusively, results elucidated that higher POC under conservation tillage contributed to the macro-aggregate formation and aggregate stability at 0–10 cm depth, and thus protected SOC from mineralization, and additional SOC sequestration could be accelerated by improving the duration of conservation tillage. [ABSTRACT FROM AUTHOR]