Recently Deposited Organic Matter in Soil Water‐Stable Aggregates

Abstract

Some suggest that young labile soil organic matter accumulates preferentially in water‐stable macroaggregates (>250 µm) where it acts as a transient binding agent. We determined the proportions of recently deposited C (<15 yr) in water‐stable aggregate size fractions using 13C natural abundance. The experiment was performed on a silty loam (fine‐loamy, mixed, frigid Aeric Haplaquept) on plots that were under either continuous corn (C4) or a mixed C4‐C3 rotation for 15 yr and that had previously been under a permanent meadow (C3). Field‐sieved (6 mm) air‐dry soil samples (0‐10 cm) were either dry sieved or wet sieved with slaking to obtain a range of aggregate sizes. Total organic C contents and natural 13C abundances were determined in each aggregate size fraction, which allowed calculation of the proportions of aggregate C derived from either C3 or C4 (recently deposited C). The proportion of water‐stable macroaggregates in the meadow soil and their C3‐derived C contents decreased with corn cropping. The net loss of C3‐derived C was greater for larger macroaggregates (>2 mm) than for smaller ones. Assuming first‐order kinetics, the estimated half‐life of the C3‐derived C of stable aggregates >2 mm was 13 yr, which corresponds to that reported for macro organic matter in similar systems. Analysis of water‐stable macroaggregates under corn showed that they were enriched in recently deposited C relative to microaggregates and to the whole soil, which partly compensated for their loss in C3‐C. On average, 20% of the C in water‐stable aggregates >1 mm was derived from corn whereas this value was down to 9% in the whole soil and 1% in the microaggregates. The results of this study provide further quantitative evidence that slaking‐resistant macroaggregates are enriched in, and probably stabilized by, recently deposited organic matter.

Protected organic matter in water-stable aggregates as affected by mineral fertilizer and manure applications

“…This suggests that OM in microaggregates was less mineralizable than that in macroaggregates. This supports other findings on Gleysol from Eastern Canada, which showed that OM associated with microaggregates had a slower turnover rate and is more processed than that associated with macroaggregates (Angers and Giroux 1996;Monreal et al 1997).…”

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

“…10), based on the concepts presented in Figure 2 and additional data collected, specific to NT and CT systems. According to the model, fresh plant material provides the POM that acts as a nucleation site for the growth of fungi and other soil microbes [5,150,209]. Microbial growth and the resulting production of extracellular polysaccharides bind residue and soil particles into macroaggregates.…”

Soil quality attributes, soil resilience, and legacy effects following topsoil removal and one-time amendments

“…Water-stable aggregation (WSA) is often used as an index of soil quality and can serve as an indicator of a soil's resistance to water and wind erosion (Angers et al 2008). Nonetheless, although much research has been conducted on the dynamics of SOM fractions (e.g., Six et al 1999;Lal 2004) and their relationships with WSA (e.g., Angers and Giroux 1996;Chenu et al 2000), little is known about SOM and WSA recovery with time on eroded soils and their relationships with soil resilience and resistance. Similarly, the legacy effect of amendments (manure and fertilizer) applied to eroded surfaces has not been quantified.…”