Particle Size and Biochemical Fractionation of Soil Organic Carbon Associated with the Roots of some Rangeland Plants

Soil organic carbon (SOC) is released from decomposition of plant residues, while root secretion products in rhizosphere are also a substantial source of SOC input to soil. Binding SOC to clay minerals leads to increase aggregate stability and protect organic carbon against microorganisms. Organo-mineral complexes have important role in decreasing organic carbon decomposition. Assessment of organic carbon particle size and biochemical fractionation is an appropriate approach to investigate organic carbon dynamics and durability against microorganisms in rhizosphere as a hot spot of activity.  

The study area was a semi-arid rangeland with the main plants species including five perennial rangeland species: crested wheat grass (Agropyron cristatum), astragalus (Astragalus verus), sheep fescue (Festuca ovina), phlomis (Phlomis oliveri), feverfew (Tanacetum parthenium). Whole soil surrounding plant roots with all roots was taken for each plant. Three sample with different distances from root surface were taken by applying this procedure: sample A: The soil which is adhered to the root surface and separates quickly from roots after drying, sample B: The soil in root zone, which is not stuck and almost is so close to roots, sample C: The soil which is wholly far from root area and apparently not affected by roots. Intact samples removed from ground and transferred quickly to laboratory to separate roots and soils with different distances from root surface by drying the root system before shaking. Particle size fractionation was done by wet sieving of aggregates and SOC in different aggregate sizes was measured by wet combustion method. Biochemical fractionation of SOC was done by acid hydrolysis method to study organic carbon stability at different distances from root surface.

ANOVA results showed a significant effects of plants and distance from root surface on aggregate size classes. The results showed the increasing amounts of microaggregates at root vicinities compare to macroaggregates. By increasing distance from root surface, the >2 mm aggregates increased, but, the amount of <0.15 mm aggregates decreased significantly. Toward root surface from C to A locations, the mean weight diameter (MWD) of soil aggregates decreased due to decreasing macro-aggregates at root vicinity. Maenwhile, SOC increased approaching to root surface due to root exudates and rhizodeposits. The highest and lowest of SOC content were found in the A location of Feverfew and the C location of Astragalus (4.16 and 0.82%), respectively. The OC contents in root vicinity were higher than other locations due to high root exudates and rhizodeposits which had C-containing molecules. Soil OC contents had significant correlation with measured soil parameters. The highest SOC content was found in micro-aggregate and in vicinity of roots. Low-decomposed OC, which has crucial role in linking microaggregates to make macroaggregates, led to high OC contents in macroaggregates. Soil OC biochemical fractionation demonstrated higher OC contents in recalcitrant pool at further distances from root surface, while by going toward root vicinity the amounts of OC in water soluble and labile pool increased. In average for A locations, 66% of total OC was measured as water soluble fraction, while for C location, the average fraction of labile and recalcitrant pools from total OC were found 62.5% and 50%, respectively. As the root exudates had fresh OC such as carbohydrates and sugars, the concentration of OC in water soluble and labile pools were so high at root vicinity. Moreover, OC in labile and water soluble pools had high correlation coefficient and, contributed to high fractions of total OC in root vicinity. Whilst C in recalcitrant pool were found higher in further distances from root surface, because activities of microorganisms and the fresh OC were decreased toward bulk soil.

This study investigated the effect of root activities of five perennial rangeland plants on the particle size and biochemical fractionation of soil OC at different distances from root surface. In root vicinity due to addition of fresh OC from roots to soil and higher microorganisms’ activities, mineral particles were aggregated to micro-aggregates which contained a large fraction of soluble and labile Soil OC. But, recalcitrant OC were dominated in macro-aggregates far from root surface. Rangeland plants with various root systems and characteristics had strong impact on particle size and biochemical fractionation of soil OC which needs more investigation. Durability of biochemical C pools has important role in carbon dynamic and stability in soil.