Analysis of carbon and nitrogen forms in soil fractions after the addition of 15N-compost by 13C and 15N nuclear magnetic resonance

Effect of cotton straw-derived materials on native soil organic carbon

Different types of crop straw and their derived biochars and compost treatments have huge potential for carbon sequestration to sustain crop productivity. In this study, cotton straw (straw), cotton straw-derived compost (compost) and cotton straw-derived biochar (biochar) with equivalent carbon (C) content were added to soil and incubated for 30 and 180 days. The C sequestration potential of these organic materials was determined by ¹³C isotope trace method. The structural characteristic of soil organic carbon (SOC) was analyzed by solid-state ¹³C NMR spectroscopy. The SOC concentration was measured by wet oxidation and dry combustion methods. The results showed that 50.84%, 41.03% and 38.55% of native SOC were replaced by biochar, compost, and straw, respectively. The carbohydrate C and methoxyl C contents were significantly higher in straw and biochar amendments respectively, while phenolic C and alkyl C were high in compost amendment and a higher proportion of aryl C occurred in biochar treatment. These findings revealed that straw material was easier to be decomposed, but compost and biochar showing better stability.

Molecular characterization of organic nitrogen in cattle manure compost by size-exclusion HPLC with chemiluminescent nitrogen detection

To characterize organic nitrogen present in cattle manure compost, a hot-water extract of the compost along with an acid-insoluble (humic acid-like) fraction either treated with alkali or laccase or untreated were separated by size-exclusion high-performance liquid chromatography (SE-HPLC). Nitrogen was then detected by chemiluminescent nitrogen detection (CLND), and humic acid-like substances were detected by measuring the absorbance at 420 nm. The acid-insoluble fraction comprised the higher molecular-weight region in the chromatogram of the whole extract, and its chromatogram pattern of CLND was similar to that of the absorbance at 420 nm. Changes in the molecular-weight distribution of the acid-insoluble fraction detected by CLND due to alkaline degradation and laccase polymerization were in agreement with those observed by measuring the absorbance at 420 nm. These results, together with other data, demonstrate that protein and humic acid-like moieties are present in the same molecule in compost organic matter.