Browsing by Author "Ma, Li"

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    Drought-induced soil microbial amino acid and polysaccharide change and their implications for C-N cycles in a climate change world
    Kakumanu, Madhavi L.; Ma, Li; Williams, Mark A. (Springer Nature, 2019-07-29)
    High microbial carbon (MBC) demand, a proxy for energy demand (cost), during soil microbial response to stressors such as drought are a major gap in understanding global biogeochemical cycling of carbon (C) and nitrogen (N). The dynamics of two dominant microbial pools (amino acids; AA and exopolymeric substances; EPS) in soils exposed to drying and C and N amendment to mimic both low and high nutrient soil habitats were examined. It was hypothesized that dynamics of EPS and AA (osmolytes) would be greater when soil drying was preceded by a pulse of bioavailable C and N. Drying reduced AA content, even as overall soil MBC increased (similar to 35%). The increase in absolute amounts and mol% of certain AA (eg: Taurine, glutamine, tyrosine, phenylalanine) in the driest treatment (-10 MPa) were similar in both soils regardless of amendment suggesting a common mechanism underlying the energy intensive acclimation across soils. MBC and EPS, both increased similar to 1.5X and similar to 3X due to drying and especially drying associated with amendment. Overall major pools of C and N based microbial metabolites are dynamic to drying (drought), and thus have implications for earth's biogeochemical fluxes of C and N, perhaps costing 4-7% of forest fixed photosynthetic C input during a single drying (drought) period.
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    Plant - Microbial and mineral contributions to amino acid and protein organic matter accumulation during 4000 years of pedogenesis
    Moon, Jinyoung; Ma, Li; Xia, Kang; Williams, Mark A. (2016-09)
    The dynamics and persistence of proteinaceous compounds during pedogenesis are major mechanisms of soil formation and determinants of organic matter (OM) turnover. We investigated the accumulation patterns of proteinogenic amino acids associated with minerals dominated by permanently negative charges (primary silica minerals) and related these to vegetative and belowground microbial succession during soil ecosystem development. Positively-charged amino acids (arginine, lysine, histidine), extracted from whole soil pool using 6 M HCl, showed clear patterns of accumulation, increasing similar to 65% during 4010 years of development, while negatively charged amino acids (glutamic acid, aspartic acid) decreased similar to 13%. In the mineral associated sub-pool, positively charged amino acids were approximately similar to 431% more enriched, while negatively charged amino acids were similar to 38% depleted as compared to the whole soil pool. The multivariate ordination of soil bacterial community structure based on a 16s ribosomal RNA gene analysis and that of the aboveground plant community structure predicted 71% (p < 0.0001) and 66% (p < 0.0001) of the amino acid dynamics, respectively, during soil ecosystem development. Ala-rich Actinobacteria abundance declined with the year of development, concomitant with the decrease of Ala content in soil (r(2) = 0.82, p = 0.0019). His-rich Acidobacteria and His in soil both increased with the year of development (r(2) = 0.92, p = 0.0022). In support of the main hypothesis, the relative distribution of proteinogenic amino acids changed during pedogenesis with evidence indicating that biological communities and minerals play roles as source and sink of OM in soil, respectively.
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    Soil Organic Nitrogen - Investigation of Soil Amino Acids and Proteinaceous Compounds
    Ma, Li (Virginia Tech, 2015-05-01)
    Soil carbon (C) and nitrogen (N) are predominantly in organic form. Proteins/ peptides, as an important organic form of N, constitute a substantial part of soil organic matter. On one hand, proteins/peptides are an important N source for plants and microorganisms, particularly in soils where inorganic N is limited. On the other hand, their stabilization in soils by forming organo-mineral associates or macromolecule complex reduces the C loss as CO2 into the atmosphere. Therefore, studies on the turnover, abundance, composition, and stability of proteins/peptides are of crucial importance to agricultural productivity and environmental sustainability. In the first part of this study, the bioavailability and distribution of amino acids, (building block of proteins/peptides), were investigated, in soils across the North-South and West-East transects of continental United States. The second part of this study aimed to understand the variations of organic C speciation in soils of continental United States. Previous investigations of the interactions between soil minerals and proteins/peptides were mostly limited to batch sorption experiments in labs, seldom of which gave the details at the molecular scales. Therefore, in the third part of this study, the molecular orientation of self-assembled oligopeptides on mineral surfaces was investigated by employing synchrotron based polarization-dependent Near Edge X-ray Adsorption Fine Structure Spectroscopy (NEXAFS) techniques. Specific aims of this study were: 1) to assess potentially bioavailable pool of proteinaceous compounds and the immediately bioavailable pool of free amino acids in surface and subsurface soils of various ecosystems; 2) to evaluate the relationship between environmental factors and levels/composition of the two pools; 3) to investigate the organic C speciation in soils of various land use; and 4) to understand molecular level surface organization of small peptides on mineral surfaces. The levels of free amino acids and hydrolysable amino acids which represent the potentially bioavailable pool of proteinaceous compounds in A-horizon soils were significantly high than in C-horizon soils due to the accumulation of organic matter in surface. On average, free amino acids accounted for less than 4 % of hydrolysable amino acids which represent the total proteinaceous compounds in soils. The composition of free amino acids was significantly different between surface soil and subsurface soil and was significantly influenced by mean annual temperature and precipitation. A relatively uniform composition of hydrolysable amino acids was observed irrespective of a wide range of land use. Significant variations were observed for the levels of free and hydrolysable amino acids along mean annual temperature and precipitation gradients, as well as among vegetation types of continental USA, suggesting levels of free and hydrolysable amino acids were associated with the above-ground biomass and root distribution. Organic C speciation investigation revealed the presence of carboxylic-C (38%), aliphatic-C (~ 22%), aromatic-C (~ 18%), O/N-alkyl-C (~ 16%), and phenolic-C (< 6%). Factors such as temperature and vegetation cover were revealed in this study to account for the fluctuations of the proportions of aromatic-C and phenolic-C, in particular. Phenolic-C may serve as a good indicator for the effect of temperature or vegetation on the composition of SOC. The average composition of soil organic C, over the continental scale, was relatively uniform over various soil ecosystems and between two soil horizons irrespective of surface organic C content. Polarization dependent NEXAFS analysis showed the oligopeptides tend to orient on mineral surface with an average tilt angle of 40 ° between the molecular chain and the mineral surface.