Carbon Dynamics of Surface Residue– and Root‐derived Organic Matter under Simulated No‐till

Abstract

No‐till practices have the potential to increase soil organic C, but little is known about the relative contribution of surface residue and roots to soil organic C accumulation. In a simulated no‐till experiment, we studied the fate of 14C‐labeled surface residue and in situ roots during a 1‐yr incubation. Soil samples collected during the incubation were chemically dispersed and separated into five particle size and density fractions. The organic C, 14C, and total N content of each fraction was determined. Alkali traps were used to measure 14C losses due to respiration. After 360 d, 66% of the 14C contained in the surface residue on Day 0 had been respired as 14CO2, 11% remained in residue on the soil surface, and 16% was in the soil. In comparison, 56% of the root‐derived 14C in the soil was evolved as 14CO2 and 42% remained in the soil. The large (500–2000 μm) and small (53–500 μm) particulate organic matter (POM) fractions together contained 11 to 16% of the initial root‐derived 14C in the soil. In contrast, POM contained only 1 to 3% of the inital surface residue–derived 14C. These data show clear differences in the partitioning of surface residue– and root‐derived C during decomposition and imply that the beneficial effects of no‐till on soil organic C accrual are primarily due to the increased retention of root‐derived C in the soil.

Crop Residues: The Rest of the Story

“…The data show a slow decrease in C:N ratio (105, 67, 47, and 27 when measured 0, 12, 24, and 36 months after harvest, respectively). In contrast to the oat residue used in the laboratory study (8), corn residue can have C:N ratios exceeding 200, while corn grain has a C:N ratio of ∼40. It is therefore not scientifically defensible to use short-term laboratory data to predict longterm residue carbon mineralization rates in the field!…”

Soil aggregation dynamics and carbon sequestration

“…As fine iPOM is formed it gradually becomes encrusted with clay particles and microbial products (t2 to t3) to form microaggregates within macroaggregates (Six et al, 1998(Six et al, , 1999. In isotope tracer studies, labeled C was redistributed from macroaggregates to microaggregates with time (Gale and Cambardella, 2000) suggesting that microaggregates are formed within macroaggregates (Oades, 1984). Eventually, the binding agents in macroaggregates degrade, resulting in loss of macroaggregate stability (t4) and the release of stable microaggregates, which become the building blocks for the next cycle of macroaggregate formation (Tisdall and Oades, 1982).…”

“…Classical methods: Traditional techniques of dry and wet sieving of aggregates are severely constrained in the quantification of C in different size fractions. Sieving and pre-treatment procedures performed prior to the determination of aggregate size distribution affect the SOC distribution in aggregates (Gale and Cambardella, 2000). Dry sieving is a preferable technique over wet sieving for maintaining a relatively intact habitat and activity of soil microorganisms (Schutter and Dick, 2002), which, however, reflects dry field conditions with no rain or irrigation.…”

“…Literature is replete with information on the effects of land management and living biomass on the C cycle and sequestration (Lal et al, 1998). There are also numerous studies with regard to the importance of SOM to soil fertility, soil erosion, and plant growth (Gale and Cambardella, 2000). These studies have improved our understanding of the interrelationships between soil biological properties and SOC, and the implications of crop and soil management to C sequestration (Six et al, 1999), but information on the fundamental physical and chemical processes influencing formation and stabilization of aggregates in relation to SOC sequestration is limited.…”

Fungal response from oat (Avena sativa)plants and surface residue in relation to soil aggregation and organic carbon

“…In a pot study under growth chamber conditions, Gale and Cambardella (2000) found that a greater amount of 14 C was retained in the soil from oat roots (42%) compared to oat leaves left to decompose on the surface (16%). Those authors suggested that a portion of root OC is rapidly released into the soil after plant senescence, followed by increases in stabilized, protected, slow decomposition forms (Gale and Cambardella 2000). The authors observed rapid initial decomposition with 80% of coarse roots decomposed within 90 days.…”