Date of Award

12-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Advisor(s)

Chris Johnson

Keywords

Calcium treatment, Elemental analysis, Hot water extractable organic matter, Molecular mixing model, NMR, Soil organic matter

Subject Categories

Civil and Environmental Engineering

Abstract

The northeastern USA has a long history of acid rain, which has impacted soil fertility and raised concerns about the sustainability of the forests in the region. To facilitate recovery of these forests, calcium addition, in the form of wollastonite, was evaluated as a remedial tool at Hubbard Brook Experimental Forests in New Hampshire, USA. In order to study the effects of these amendments on soil organic matter and soil microbes, I characterized the structural chemistry of soil organic matter (SOM) and its hot-water extractable organic matter (HWEOM) fraction for Hubbard Brook soils and studied the changes in the structure of SOM and HWEOM after short- and long-term calcium amendment. I found that the C:N ratios for these soils were 27.2 and 22.8 in Oi and Oe horizons, respectively, and ranged from 20.8 to 22.6 in the Oa horizon. Hot-water extractable organic matter (HWEOM), which was 3.39% of soil C in Oi horizon and 0.93-1.14% in Oe and Oa horizons, had much lower C:N ratios than whole soil values, at 19.1 in Oie horizon, 14.8 in Oe horizon, and 13.5-14.8 in Oa horizon. This pattern suggests that HWEOM contains an abundance of microbially derived organic matter. The HWEOM also exhibited higher H:C ratios and a position in van Krevelen diagram closer to carbohydrates compared to the whole soil. Nuclear Magnetic Resonance (NMR) analyses revealed that HWEOM was higher in O-alkyl C (50-70%) than the soil from which it was extracted (40-55%), further indicating its richness in carbohydrates, which was also confirmed by a molecular mixing model. The soil organic matter decreased in its O-alkyl C and carbohydrate proportion with soil depth due to decomposition and increased in its refractory fraction, represented by alkyl C and lipids proportion. These patterns were opposite in HWEOM, indicating an increased solubility of residual carbohydrate structures during the course of decomposition. The HWEOM was richer than soil in both labile organic matter and microbial biomass, though microbial biomass could account for no more than 40% of the extracted C.

In a short-term calcium treatment study, I found a decrease in the HWEOM content of soils treated with high amounts of calcium (4250 kg Ca/ha), but not in low Ca (850 kg Ca/ha) treatments, two years after the treatment. This decrease was attributed to a decreased extractability of labile water-soluble structures represented by HWEOM, and not due to significant changes in SOM structure, as revealed by a biodegradation study and NMR spectroscopy. The biodegradation study showed that the HWEOM extracted from high Ca treated plots showed no significant difference in degradation compared to the reference plots, indicating that once extracted, the HWEOM structures are bioavailable. NMR spectroscopy revealed no significant changes in O-alkyl structures in calcium treated plots for the whole soil but showed a decrease in the O-alkyl C content of HWEOM, again suggesting that only the extractability of the labile structures has been affected. C:N ratios also were not found to differ between reference and Ca treated plots. Phosphorous treatment, with or without extra calcium, did not bring any significant effects on the structural composition of the soil organic matter. In the long-term (7-9 years) calcium treatment effects were only visible in the high-elevation spruce/fir/zone and not at lower elevations.

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Open Access

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