Evaluation of carbon mineralization and its temperature sensitivity in different soil aggregates and moisture regimes: A 21-year tillage experiment

The interplay between external residue addition, and soil organic carbon dynamics and mineralization kinetics: Experiences from a 12-year old conservation agriculture

Maintaining soil carbon is vital under changing climate. Conservation agriculture (CA) is reported to have potential to store soil organic carbon (SOC). The impact of carbon inputs on SOC dynamics and mineralization kinetics, and the priming effect of residue addition under long-term CA in subtropical regions, however, are not clear or adequately evaluated. Therefore, we studied these under a 12-year-old CA-based pigeon pea-wheat cropping system with permanent broad bed with residue (CA-PBB), permanent flatbed with residue (CA-PFB), permanent narrow bed with residue (CA-PNB), and conventional till (CT) treatments. Also, an incubation study was undertaken to understand better the processes involved. Results showed that CA treatments significantly enhanced the total SOC compared to CT practice, and, among them, the CA-PFB exhibited highest total SOC with 36.6% and 35.8% higher values at 0-5 and 5-15 cm depths, respectively. The CA-PFB followed by CA-PBB and CA-PNB had significantly higher carbon management index and carbon retention efficiency than CT. The CA-PFB also showed higher carbon sequestration rates of 68.4 and 188.8 kg ha-1 year-1, surpassing values of 8.4 and 52.9 kg ha-1 year-1 under CT at 0-5 and 5-15 cm depth, respectively. Furthermore, soil incubation study revealed that the CA systems had higher cumulative mineralization values at 0-5 cm soil layer but lower at 5-15 cm soil compared to CT, indicating a considerable improvement in SOC at 5-15 cm soil depth. On the contrary, the SOC decay rate was higher under CA than CT, and at 35 °C than at 15 °C. A positive priming effect was also observed, depending on the substrate type, pigeon pea residue exhibiting higher priming effect than wheat residue. Thus, these studies show that residue input increases cumulative mineralization and SOC decay rate vis-à-vis helps to sequester carbon in the recalcitrant fraction, leading to higher stable carbon in soil.

Understanding how conservation tillage promotes soil carbon accumulation: Insights into extracellular enzyme activities and carbon flows between aggregate fractions

Conservation tillage has been shown to mitigate climate change by promoting the sequestration of soil carbon (C) in agroecosystems. However, knowledge on how conservation tillage accumulates soil organic C (SOC), especially at the aggregate scale, remains limited. This study aimed to clarify the effects of conservation tillage on SOC accumulation by measuring hydrolytic and oxidative enzyme activities and C mineralization in aggregates and developing an extended scheme of C flows between aggregate fractions using the 13C natural abundance (δ13C) method. Topsoils (0-10 cm) were sampled from a 21-year tillage experiment located in the Loess Plateau of China. Compared with conventional (CT) and reduced tillage with straw removal (RT), no-till (NT) and subsoiling with straw mulching (SS) enhanced the proportions of macro-aggregates (> 0.25 mm) (by 12-26%) and SOC contents in bulk soils and all aggregate fractions (by 12-53%). In bulk soils and all aggregate fractions, SOC mineralization and the activities of hydrolases (β-1,4-glucosidase, β-acetylglucosaminidase, β-xylosidase, and cellobiohydrolase) and oxidases (peroxidase and phenol oxidase) were 9-35% and 8-56% lower, respectively, under NT and SS than under CT and RT. Partial least squares path model revealed that reductions in the activities of hydrolases and oxidases and increases in macro-aggregation decreased SOC mineralization in bulk soils and macro-aggregates. Furthermore, Δ13C values (aggregate-associated δ13C - bulk-soil δ13C) increased with decreasing size of soil aggregates, suggesting that C is younger in larger aggregates than in smaller aggregates. The probability of C flows from large to small soil aggregates was lower under NT and SS than under CT and RT, indicating that young SOC with low rates of decomposition in macro-aggregates was better protected under NT and SS. Overall, NT and SS enhanced SOC accumulation in macro-aggregates by decreasing the activities of hydrolases and oxidases and C flows from macro- to micro-aggregates, which promoted C sequestration in soils. The present study provides improved insights into the mechanism and prediction of soil C accumulation under conservation tillage.

Influencing factors and spatiotemporal heterogeneity of net carbon sink of conservation tillage: evidence from China

Conservation tillage is an important reform of traditional tillage, which has significant carbon sequestration and emission reduction effects. It is important to investigate the influencing factors and spatiotemporal heterogeneity of net carbon sink of conservation tillage for realizing the "dual carbon" target, and facilitating agricultural sustainable development. This study used the coefficient accounting method to calculate the carbon sink and carbon emission of conservation tillage in China from 2000 to 2019, respectively. Based on this, the net carbon sink of conservation tillage was measured. Then, the spatiotemporal heterogeneity of influencing factors on net carbon sink of conservation tillage was analyzed by using the geographically and temporally weighted regression model. The results showed that (1) the net carbon sink of conservation tillage in China was significant and had potential to have a constant rise; (2) spatially, the net carbon sink of conservation tillage changed more variably in longitudinal direction. Specifically, the promotion effect of conservation tillage machinery gradually decreased from west to east. The planting structure and conservation tillage promotion intensity played key roles in improving net carbon sink of conservation tillage. (3) Temporally, the effect of conservation tillage machinery showed positive effect of decreasing yearly, while the positive effect of promotion intensity increased year by year. Planting structure and economic development negatively affected improvement on the net carbon sink of conservation tillage and the negative effect increased year by year. Additionally, the effect of education on the net carbon sink shifted from positive to negative over time. The study aims to provide a reference for the government to promote conservation tillage according to local conditions and to achieve the "dual carbon" target.

The carbon-quality temperature hypothesis: Fact or artefact?

Climate warming can reduce global soil carbon stocks by enhancing microbial decomposition. However, the magnitude of this loss remains uncertain because the temperature sensitivity of the decomposition of the major fraction of soil carbon, namely resistant carbon, is not fully known. It is now believed that the resistance of soil carbon mostly depends on microbial accessibility of soil carbon with physical protection being the primary control of the decomposition of protected carbon, which is insensitive to temperature changes. However, it is still unclear whether the temperature sensitivity of the decomposition of unprotected carbon, for example, carbon that is not protected by the soil mineral matrix, may depend on the chemical recalcitrance of carbon compounds. In particular, the carbon-quality temperature (CQT) hypothesis asserts that recalcitrant low-quality carbon is more temperature-sensitive to decomposition than labile high-quality carbon. If the hypothesis is correct, climate warming could amplify the loss of unprotected, but chemically recalcitrant, carbon and the resultant CO₂ release from soils to the atmosphere. Previous research has supported this hypothesis based on reported negative relationships between temperature sensitivity and carbon quality, defined as the decomposition rate at a reference temperature. Here we show that negative relationships can arise simply from the arbitrary choice of reference temperature, inherently invalidating those tests. To avoid this artefact, we defined the carbon quality of different compounds as their uncatalysed reaction rates in the absence of enzymes. Taking the uncatalysed rate as the carbon quality index, we found that the CQT hypothesis is not supported for enzyme-catalysed reactions, which showed no relationship between carbon quality and temperature sensitivity. The lack of correlation in enzyme-catalysed reactions implies similar temperature sensitivity for microbial decomposition of soil carbon, regardless of its quality, thereby allaying concerns of acceleration of warming-induced decomposition of recalcitrant carbon.