Abstract
AbstractContrasting tillage strategies not only affect the stability and formation of soil aggregates but also modify the concentration and thermostability of soil organic matter associated with soil aggregates. Understanding the thermostability and carbon retention ability of aggregates under different tillage systems is essential to ascertain potential terrestrial carbon storage. We characterized the concentration and thermostability of soil organic carbon (SOC) within various aggregate size classes under both zero and conventional tillage using novel Rock‐Eval pyrolysis. The nature of the pore systems was visualized and quantified by X‐ray computed tomography to link soil structure to organic carbon preservation and thermostability. Soil samples were collected from experimental fields in Botucatu, Brazil, which had been under zero‐tillage for 2, 15 and 31 years, and from adjacent fields under conventional tillage. Soils under zero‐tillage significantly increased pore connectivity whilst simultaneously decreasing interaggregate porosity, providing a potential physical mechanism for protection of SOC in the 0–20‐cm soil layer. Changes in the soil physical characteristics associated with the adoption of zero‐tillage resulted in improved aggregate formation compared to conventionally tilled soils, especially when implemented for at least 15 years. In addition, we identified a chemical change in composition of organic carbon to a more recalcitrant fraction following conversion to zero‐tillage, suggesting aggregates were accumulating rather than mineralizing SOC. These data reveal profound effects of different tillage systems upon soil structural modification, with important implications for the potential of zero‐tillage to increase carbon sequestration compared to conventional tillage.Highlights Different tillage systems may affect SOC thermostability and C retention potentials of soil aggregates. SOC thermostability was characterised by Rock‐Eval pyrolysis and pore systems were quantified by X‐ray CT within aggregate size classes. Profound effects of zero versus conventional tillage upon soil structural modification were observed Important implications for zero‐tillage to increase C sequestration versus conventional tillage.
Highlights
The quantity of CO2 released to the atmosphere from agricultural soils is mainly dependent on the rate of soil organic carbon (SOC) formation versus decomposition (Trumbore, 1997)
We hypothesize that (a) the minimized mechanical disturbance associated with zero-tillage will allow regeneration of the soil porous architecture over time and increase soil porosity through the development of continuous pores, (b) this will increase stable macroaggregates under long-term zero-tilled soils, (c) there will be a greater fraction of microaggregates formed within macroaggregates as the rate of macroaggregate formation and degradation is reduced under zero-tillage, and (d) zerotilled aggregates will contain more organic carbon than conventionally tilled aggregates, with an increase in the “labile” proportion, as characterized by its thermostability, due to a reduction in aggregate porosity limiting microbial access
The increased amount of soil organic carbon in the microaggregates formed within macroaggregates (mM) fraction in long-term zero-tilled soils, in addition to the increase in mean weight diameter, indicates stronger potential for soil aggregation and accumulation of SOC under zero-tillage compared with conventional tillage
Summary
The quantity of CO2 released to the atmosphere from agricultural soils is mainly dependent on the rate of soil organic carbon (SOC) formation versus decomposition (Trumbore, 1997). Turnover of labile organic matter occurs over intervals ranging from hours to years and is highly influenced by soil management practices, whereas stable organic matter turnover occurs on timescales ranging from decades to centuries (Feng, Plante, Aufdenkampe, & Six, 2014) Both fractions can be found in aggregates of all sizes and contribute to the regulation of organic carbon storage duration (Bongiorno et al, 2019). We hypothesize that (a) the minimized mechanical disturbance associated with zero-tillage will allow regeneration of the soil porous architecture over time and increase soil porosity through the development of continuous pores, (b) this will increase stable macroaggregates under long-term zero-tilled soils, (c) there will be a greater fraction of microaggregates formed within macroaggregates as the rate of macroaggregate formation and degradation is reduced under zero-tillage, and (d) zerotilled aggregates will contain more organic carbon than conventionally tilled aggregates, with an increase in the “labile” proportion, as characterized by its thermostability, due to a reduction in aggregate porosity limiting microbial access
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