Distribution of organic carbon in physical fractions of soils as affected by agricultural management |
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| Authors: | Sindhu Jagadamma Rattan Lal |
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| Institution: | (1) Environmental Sciences Division, Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN 37830, USA;(2) Carbon Management and Sequestration Center, The Ohio State University, 2021 Coffey Rd, Columbus, OH 43210, USA |
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| Abstract: | Soil organic carbon (SOC) is distributed heterogeneously among different-sized primary particles and aggregates. Further,
the SOC associated with different physical fractions respond differently to managements. Therefore, this study was conducted
with the objective to quantify the SOC associated with all the three structural levels of SOC (particulate organic matter,
soil separates and aggregate-size fractions) as influenced by long-term change in management. The study also aims at reevaluating
the concept that the SOC sink capacity of individual size-fractions is limited. Long-term tillage and crop rotation effects
on distribution of SOC among fractions were compared with soil from adjacent undisturbed area under native vegetation for
the mixed, mesic, Typic Fragiudalf of Wooster, OH. Forty five years of no-till (NT) management resulted in more SOC accumulation
in soil surface (0–7.5 cm) than in chisel tillage and plow tillage (PT) treatments. However, PT at this site resulted in a
redistribution of SOC from surface to deeper soil layers. The soils under continuous corn accumulated significantly more SOC
than those under corn–soybean rotation at 7.5–45 cm depth. Although soil texture was dominated by the silt-sized particles,
most of the SOC pool was associated with the clay fraction. Compared to PT, the NT treatment resulted in (i) significantly
higher proportion of large macroaggregates (>2,000 μm) and (ii) 1.5–2.8 times higher SOC concentrations in all aggregate-size
classes. A comparative evaluation using radar graphs indicated that among the physical fractions, the SOC associated with
sand and silt fractions quickly changed with a land use conversion from native vegetation to agricultural crops. A key finding
of this study is the assessment of SOC sink capacity of individual fractions, which revealed that the clay fraction of agricultural
soils continues to accumulate more SOC, albeit at a slower rate, with progressive increase in total SOC concentration. However,
the clay fraction of soil under native woodlot showed an indication for SOC saturation. The data presented in this study from
all the three structural levels of SOC would be helpful for refining the conceptual pool definitions of the current soil organic
matter prediction models. |
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