Conference presentation
The role of nitrogen and pyrogenic carbon in defining soil organic matter fraction stability
American Geophysical Union Fall Meeting, Vol.2023
American Geophysical Union
AGU23 (11/12/2023–15/12/2023)
13/12/2023
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Abstract
The balance of soil organic matter accumulation v degradation underpins both global carbon budgets and terrestrial ecosystem fertility. There is mounting evidence for matrix-defined limits on the accumulation of the fine, microbially-mediated, materials (MAOM) that comprise the majority of soil organic matter. This suggests an outsize role for the relatively small pool of coarse, plant-derived materials (POM) in dictating terrestrial organic matter stocks. But the mechanisms driving POM stabilisation remain unclear. Here we work to redress this knowledge gap by building an isotope-based framework to quantify the movement of carbon and nitrogen between the two fractions. We developed this framework using 312 agricultural soils, spanning 11 soil types, from across the Australian continent. Soils were physically separated into coarse (>53 μm) and fine (<53 μm) fractions, and then analysed for, 1) the proportion of pyrolised organic matter (PyOM), 2) isotopic composition of organic nitrogen δ15N and carbon (δ13C), and, 3) key soil matrix properties like clay, iron, and aluminium. The continent's long and intense fire history complicates conventional interpretation of POM v MAOM fractions: chemically recalcitrant PyOM accounted for <1% to >70% of material in both size fractions. After accounting for PyOM uncertainty, the more weathered subsoils (10-30 cm depth) fit a trophic isotope model where microbially processing leads to MAOM enriched in heavy isotopes of both N and C relative to POM. However, this did not hold in the surface soils (0-10 cm), where δ13CMAOM – δ13CPOM ranged from -13‰ to +11‰ (0.4 ± 3 ‰) and δ15NMAOM – δ15NPOM ranged from -10‰ – +9‰ (1 ± 2 ‰). This apparent shift from POM to MAOM degradation with depth supports hypotheses around mineral saturation controlling POM availability. However, the picture is complicated by the lack of relationship in the relative enrichment of the δ15NPOM and δ13CPOM pool, which suggests that nutrient supply can be decoupled from the overarching organic matter stabilisation trend. Our findings highlight the need to consider nitrogen dynamics within soil organic matter pools.
Details
- Title
- The role of nitrogen and pyrogenic carbon in defining soil organic matter fraction stability
- Creators
- Naomi S. Wells - Lincoln UniversityJeff Baldock - CSIRO Land and WaterMark Farrell - CSIRO Agriculture and Food
- Publication Details
- American Geophysical Union Fall Meeting, Vol.2023
- Conference
- AGU23 (11/12/2023–15/12/2023)
- Publisher
- American Geophysical Union
- Identifiers
- 991013337983702368
- Academic Unit
- Faculty of Science and Engineering; Science
- Language
- English
- Resource Type
- Conference presentation