Root‐Derived Carbon and the Formation and Stabilization of Aggregates

Abstract

It is hypothesized that particulate organic matter (POM) contributes to aggregate stability. However, little is known about the dynamics of the POM fraction or its role in aggregate formation. A simulated no‐till study was conducted to examine changes in free and aggregate‐associated POM during the decomposition of in situ 14C‐labeled roots during a 1‐yr incubation in a loess‐derived silt loam. Two water pretreatments (capillary‐wetted and slaked) were applied to soil samples collected during the incubation, and the samples were then wet sieved to obtain five aggregate size fractions. Densiometric separations were used to isolate free and released POM (frPOM) and intraaggregate POM (iPOM). Small macroaggregates (250–2000 μm) were enriched in iPOM‐14C on Day 0 which suggested that many of these aggregates formed around cores of new, root‐derived POM during the growth and senescence of the oat plants. Slaking resulted in the disruption of many of the small macroaggregates (250–2000 μm) and a large increase in frPOM‐14C on Day 0. The amount of 14C released into the frPOM pool with slaking declined with time. In contrast, there was a significant linear increase in the amount of new, root‐derived iPOM‐14C in large microaggregates (53–250 μm) that were released when unstable macroaggregates (>250 μm) slaked. These data support the hypothesis that new microaggregates are formed within existing macroaggregates and provide strong evidence that, in no‐till, aggregate formation and stabilization processes are directly related to the decomposition of root‐residue and the dynamics of POM C in the soil.

Soil microaggregate and macroaggregate decay over time and soil carbon change as influenced by different tillage systems

“…These changes in the short term can be insignificant, yet SOC content for NT soil aggregates increased over time, consistent with the findings of Sainju et al (2008) and West and Post (2002). Stable macroaggregates are enriched in new SOC compared with unstable macroaggregates (Gale et al 2000), especially in relatively undisturbed systems like NT, where new root-derived intraaggregate particulate organic matter is important in stabilizing small macroaggregates. Although the differences in SOC and STN contents for soil with different tillage treatments were not significantly different (p < 0.05), all tillage systems' SOC and STN contents at the top 0 to 15 cm (0 to 6 in) depth were different from that for the baseline.…”

Soil organic matter, biota and aggregation in temperate and tropical soils - Effects of no-tillage

“…This results in localized drying of the soil and cohesion of soil particles around the roots. The crucial role played by roots in the formation of waterstable aggregates has been recognized for some time [244], but only recently published results [116,208,257] have reported that roots are more important than shoots in the formation of aggregates and stabilization of aggregate-associated SOM (Tab. III).…”

“…This fine POM becomes increasingly encapsulated with minerals and microbial products, forming new microaggregates (53-250 µm) within the macroaggregates [10,116,150,207,230]. With active root growth stabilizing macroaggregates, intense biological activity (induced by root exudation) may also cause further encrustation of microbial products and mineral particles, forming microaggregates around root-derived POM [116].…”

“…This fine POM becomes increasingly encapsulated with minerals and microbial products, forming new microaggregates (53-250 µm) within the macroaggregates [10,116,150,207,230]. With active root growth stabilizing macroaggregates, intense biological activity (induced by root exudation) may also cause further encrustation of microbial products and mineral particles, forming microaggregates around root-derived POM [116]. It has been found that this microaggregate formation within macroaggregates is crucial for the long-term sequestration of C [231,232] because microaggregates have a greater capacity to protect C against decomposition compared with macroaggregates [24,40,234].…”

Soil physico-chemical changes following application of municipal solid waste leachates to grasslands

“…Root growth and physiology also produced localised soil physical properties around the root, such as compaction (Misra et al, 1986). The drying of soil by roots together with the organic binding material produced in the rhizosphere is expected to enhance soil structural stability (Bowen and Rovira, 1991;Gale et al, 2000).…”