Unraveling the long-term stabilization mechanisms of organic materials in soils by physical fractionation and NMR spectroscopy

Abstract

International audienceThe fundamental mechanisms whereby organic inputs stabilize in soil are poorly resolved, which limits our current capacity to predict the dynamics of soil organic matter (OM) turnover and its influence on soil quality and functioning. Here we fractionated soil OM from long-term experimental field plots either unamended or amended with two organic materials of different quality (i.e., solid cattle manure and crop residues) for 44 years into five measurable and meaningful pools directly related to conceptual preservation mechanisms: dissolved OM, mineral-free particulate OM located outside aggregates (unprotected from decomposition), OM occluded within both macroaggregates and microaggregates (weakly and strongly protected by physical mechanisms, respectively), and OM intimately associated with soil mineral particles (protected by chemical mechanisms). Compared to the unamended soil, the application of cattle manure and crop residues increased total organic C content by 35 and 10%, respectively. Most of these increases (up to 60 and 72% for cattle manure and crop residues, respectively) were explained by the mineral-associated OM pool, followed by the intra-microaggregate OM fraction. In general, the distribution and dynamics of N content paralleled those of C content. As determined by a range of modern nuclear magnetic resonance (NMR) techniques, including 13C cross polarization magic angle spinning (MAS), 1H high resolution (HR)-MAS, and 1Hsingle bond13C heteronuclear single quantum coherence HR-MAS NMR, the mineral-associated OM fraction was found to be predominately of microbial origin, unlike free and intra-aggregate OM pools, which were dominated by plant structures at different stages of decomposition. As a whole, our results indicate that the main mechanism by which organic inputs are stabilized and OM accrues in soils is not the physical and chemical protection of undecayed or partially degraded organic structures, but the adsorption on mineral surfaces of microbial biomass and microbial by-products resulting from microbial growth, transformation, and degradation processes. It is possible that organic amendments increase more than previously thought the microbial populations of the soil, which live, thrive, and die in close association with the mineral surfaces. This mechanism appears to be enhanced with the addition of stable organic materials

Effects of different regimes of fertilization on soil organic matter under conventional tillage

“…Following a high input of wheat straw combined with mineral fertilizers, a larger microbial population is found because of the priming effect. Consequently, more C is respired by soil microbes (Courtier-Murias et al, 2013). Secondly, by a lower soil pH; in the wheat straw treatments, the pH was significantly lower compared to the CK, CNPK and DNPK treatments.…”

“…Firstly, by the priming effect, which is defined as a short-term change in the turnover of SOM caused by the addition of organic carbon into soil (Kuzyzkov et al, 2000). Previous research surveyed that soil microbes played an important role in regulating SOM (Courtier-Murias et al, 2013). Following a high input of wheat straw combined with mineral fertilizers, a larger microbial population is found because of the priming effect.…”

“…We cannot exclude the possibility that organic matter is less stabilized and less bound to soil mineral surfaces if pH is relatively low (Liu et al, 2014). Also, soil pH has a positive effect on soil microbial biomass, which is positively related to the stabilization and accrual of SOM, because microbial biomass and microbial byproducts resulting from microbial growth and degradation processes are major sources for SOM (Heinze et al, 2010;Miltner et al, 2012;Courtier-Murias et al, 2013;Wallenstein et al, 2013). Thirdly, low soil nutrients availability.…”

HRMAS NMR spectroscopy applications in agriculture

“…The application of T 2 -based (namely CPMG or spinecho) NMR filters to reduce signals heterogeneity in 1 H spectra of humic substances allowed to develop cluster analyses to differentiate humic matter as a function of soil treatments [25]. In the same line, conventional and diffusion-edited HRMAS proton spectra were applied to evaluate the long-term stabilization mechanisms of organic materials in soils undergone solid cattle manure and crop residues amendments and fractionated into free, intra-macroaggregate, intra-microaggregate, mineral-associated, and dissolved organic matter [26]. The authors compared the peaks detected in 1D HRMAS spectra of HF-treated soil with those of a bovine albumin protein and cultured soil microbes and suggested that the material associated with the clay mostly resembles the molecular components derived from microbial biomass [26].…”

“…In the same line, conventional and diffusion-edited HRMAS proton spectra were applied to evaluate the long-term stabilization mechanisms of organic materials in soils undergone solid cattle manure and crop residues amendments and fractionated into free, intra-macroaggregate, intra-microaggregate, mineral-associated, and dissolved organic matter [26]. The authors compared the peaks detected in 1D HRMAS spectra of HF-treated soil with those of a bovine albumin protein and cultured soil microbes and suggested that the material associated with the clay mostly resembles the molecular components derived from microbial biomass [26]. Such finding was accompanied by a further study in which diffusion-edited HRMAS NMR spectra of HF-treated soil aggregates of different dimensions were related to the role of microbes in conservation tillage systems [27].…”