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  Soil organic carbon sequestration and modeling under conservation tillage and cropping systems in a rainfed agriculture

  Conservation agriculture is a well-established method for promoting carbon sequestration and reducing greenhouse gas emissions, but little is known about how it affects subtropical dryland farming systems. The goal of this study was to evaluate the potential of conservation agriculture in Pakistan's subtropical dryland to reduce atmospheric CO2 enrichment and alter soil organic carbon fractions. In a field experiment, fallow-wheat (farmers' practice) and the conservation tillage methods minimum tillage (MT), reduced tillage (RT), and zero tillage (ZT) were compared to conventional tillage (CT) in the main plots and the cropping systems sorghum-wheat (S-W) and mungbean-wheat (M-W) to fallow-wheat (F-W) in the sub-plots. Multiple assessments taken over a two-year period revealed that CT plots lacked greater soil organic carbon and its fractions than ZT and RT plots. In comparison to CT, ZT, and RT exhibited higher average total organic carbon (TOC), microbial biomass carbon (MBC), particulate organic carbon (POC), and mineral-associated organic carbon (MOC) concentrations, respectively, of 1.43% and 1.31%, 4.61% and 2.83%, 2.42% and 1.97%, and 1.66% and 1.76%. In comparison to the S-W cropping system, the F-W and M-W cropping systems showed increased MBC and MOC, but POC and TOC were little impacted. The maximum TOC (0.589% and 0.589%), MBC (0.021% and 0.021%), POC (0.195% and 0.192%), and MOC (0.489% and 0.485%) were found in the combinations of ZT with F-W and M-W. Regardless of the cropping systems, cumulative CO2 flow was lowest in ZT plots compared to the other tillage techniques. The CENTURY model confirmed that the use of continuous tillage is a major threat to both soil fertility and production. The study, therefore, concludes that ZT and RT systems in particular are potential possibilities for carbon sequestration in subtropical dryland soils for CO2 reduction.
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  Drivers of the amount of organic carbon protected inside soil aggregates estimated by crushing: A meta-analysis

  Given the importance of soil organic carbon (SOC) stocks and their dynamics in the regulation of climate change, understanding the mechanisms of SOC protection from decomposition is crucial. It is recognized that soil aggregates can provide effective protection of organic carbon from microbial decomposition. Currently, there is no systematic method for estimating the amount of protected carbon within aggregates. However, differences between CO2 emissions from incubation of intact versus crushed aggregates have been widely used as a proxy for SOC physical protection within aggregates. There is no global analysis on this type of experiment yet, nor on the drivers of the amount of SOC physically protected in soils. Using a meta-analysis including 165 pairs of observations from 22 studies encompassing a variety of ecosystems, climate and soil types, we investigated the crushing effects on cumulative carbon mineralization from laboratory incubation experiments. The aggregates were initially separated by either wet sieving or dry sieving before dry crushing. Our results indicated that aggregate crushing led on average to +31 % stimulation of carbon mineralization compared with intact aggregates, which represented 0.65 to 1.01 % of total SOC. This result suggests the mineralization of a previously protected pool of labile organic carbon. The linear regression analysis showed that the crushing effect on carbon mineralization depended on soil characteristics (carbon content, clay content and pH) as well as on aggregate size. Crushing aggregates stimulated carbon mineralization relative to control, up to +63 % in large aggregates (>10 mm), +38 % in large macro-aggregates (2–8 mm), +14 % in small macro-aggregates (0.25–2 mm) and +54 % in micro-aggregates (<0.25 mm). Within each aggregate size-class, the crushing effect depended on the crushing intensity. The destruction of aggregates to <0.05 mm size had a greater effect on carbon mineralization (+130–133 %) than the destruction of aggregates to >2 mm (+3 to 40 %), < 2 mm (+58 to 62 %) and < 0.25 mm (+32 to 62 %) sizes regardless of the initial aggregate size. These results suggest that macroaggregates (>0.25 mm) are less protective than microaggregates (<0.25 mm). Our dataset also show that soil physicochemical characteristics and experimental conditions influenced more the amount of protected SOC than land use and management. Contrary to our expectations the crushing effect was not affected by tillage practices nor land use. Standardizing the experimental conditions of aggregate crushing and subsequent incubation is needed to assess and compare the amount of physically protected SOC in diverse soils, and then to better understand the processes and drivers of SOC protection inside aggregates.