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Composting in china
Composting is an inexpensive, efficient, and sustainable treatment for solid wastes. In China, the composting industry has been growing rapidly, owing to a boom in the animal industry over the past decades. Because an immature compost applied to soil results in seed germination inhibition, root destruction, and a decrease in the O2 concentration and redox potential , assessing organic fertilizer maturity is critical. By the way, a main difference between common composts and commercial organic fertilizers is the complexity and unpredictability of the raw materials of the latter.
In recent decades, livestock numbers have increased dramatically in China. The quantity of manure generated by China’s livestock has increased significantly as a result of the rapid increases in livestock numbers. The quantity increased by at least fourfold between 1980 and 2005, to an annual estimated total of 3060 million tons (Mt, fresh weight of manure) in 2005. It was estimated that manure generation in 2010 was ca. 2800 Mt (fresh weight). In addition, organic fertilizer amendment has been shown to be an effective way of increasing soil organic matter (SOM) content and reducing environmental pollution. However, the mechanism of storage of SOM remains largely unknown. Recently, some investigators have shown that organic fertilizer amendments could enhance the production of highly reactive short-range ordered (SRO) minerals, which further benefit for SOM storage and soil fertility improvement.
In this chapter, compost process and status, novel spectroscopy techniques in assessing compost maturity, and improvement of soil fertility by organic fertilizer amendments in China are introduced.
2. Compost process and status in China
Traditionally, farmers in China were mainly depending on organic fertilizers, for example, animal manures and agricultural residuals. In 1950s, farmers also began to apply some of chemical fertilizers ). In 1980s, the application of chemical fertilizers and organic fertilizers had a very similar percentage. However, the application of chemical fertilizers in 2010 was over 90%. As a result, soil acidification is a major problem in soils of intensive Chinese agricultural systems. Two nationwide surveys showed that soil pH declined significantly (P < 0.001) from the 1980s to the 2000s in the major Chinese crop-production areas. Therefore, the replacement of chemical fertilizers by organic fertilizers in a certain percentage is urgent.
During the last decade’s development of composting in China, numerous large-scale animal farms with more than 10,000 pigs or 5000 cattle have been established. As a result, a large amount of animal manure is produced, which is a major pollutant if untreated . On the other hand, this is also a perfect resource of organic fertilizers. For example, more than 100 factories produce over 5000 tons of commercial organic fertilizers each year in Jiangsu Province, China (). Correspondingly, the Jiangsu government now subsidizes the composting factories with 200 RMB per ton. As a result, the price of commercial organic fertilizers has decreased from 550 to 350 RMB. Therefore, the farmers are pleased to replace chemical fertilizers with commercial organic fertilizers. Presently, the total amount of commercial organic fertilizers produced by subsidized composting facilities is more than 2 million tons per year in Jiangsu Province, China . Thereof, the government of Jiangsu Province plays a critical role in promoting the production and application of organic fertilizers by the farmers.
In China, trough composting system and windrow composting system are the main composting processes, with windrow composting system being more popular in Jiangsu Province . Windrow composting consists of placing the mixture of raw materials in long narrow piles that are agitated or turned on a regular basis. The turning mixes the materials during composting and enhances passive aeration. Generally, the heights of windrows are in a range of 90 to 180 cm. Correspondingly, the width of them varies in a range of 100 to 300 cm. In general, the size, shape, and spacing of the windrows are determined by the turning equipment. During aeration, the rate of air exchange depends on the porosity of the windrow. Therefore, the size of a windrow is determined by its porosity.
3. Assessment of compost maturity by spectroscopy techniques
Various parameters are commonly used to evaluate compost quality. In general, these parameters include germination index (GI), water-soluble organic carbon (WSOC), water-soluble organic nitrogen (WSON), pH, electrical conductivity (EC), moisture, and total organic matter (TOM) content. It is accepted that any sole parameter cannot determine compost maturity, which must be assessed by a combination of different physical, chemical, and biological properties . However, all these approaches are expensive or time-consuming when a large number of samples are involved. It is reported that spectroscopy techniques have many advantages over traditional chemical analyses, such as its ease of sample preparation, rapid spectrum acquisition, nondestructive analysis, and portability.
| Physical | Odor, color, temperature, particle size and inert materials | |
|---|---|---|
| Chemical | Carbon and nitrogen analyses | C/N ratio in solid and water extract |
| Cation exchange capacity | CEC, CEC/Total organic-C ratio, etc | |
| Water-soluble extract | pH, EC, organic-C, ions, etc | |
| Mineral nitrogen | NH4-N content, NH4-N/NO3-N ratio | |
| Pollutants | Heavy metals and organics | |
| Organic matter quality | Organic composition: lignin, complex carbohydrates, lipids, sugars, etc. | |
| Humification | Humification indices and humic-like substances characterization: elemental and functional group analyses, molecular weight distribution, E4/E6 ratio, pyrolysis GC-MS, spectroscopic analyses (NMR, FTIR, Fluorescence, Raman, etc.) | |
| Biological | Microbial activity indicators | Respiration (O2 uptake/consumption, self-heating test, biodegradable constituents) |
| Enzyme activity (cellulase, phosphatases, dehydrogenases, proteases, etc) | ||
| ATP content | ||
| Nitrogen mineralization-immobilization potential, nitrification, etc. | ||
| Microbial biomass | ||
| Phytotoxicity | Germination and plant growth tests | |
| Others | Viable weed seed, pathogen, and ecotoxicity tests |
Table 1.
Current criteria evaluated in the literature to characterize compost quality
Near-infrared reflectance spectroscopy (NIRS) has been shown to rapidly (within 1 min) assess compost quality. However, it is unclear whether NIRS can also be applied to rapidly determine the quality of commercial organic fertilizers, owing to the complexity and unpredictability of the raw materials of the commercial products. shows a distinct appearance of commercial organic fertilizers with composting samples. A total of 104 commercial organic fertilizers were collected from full-scale compost factories in Jiangsu Province, China. These factories treat organic matter from animal manure and other agricultural organic residues. These factories produce approximately 5000–150,000 tons of commercial organic fertilizers per year.
We can see that all the NIR spectra collected from commercial organic fertilizers in Jiangsu Province, China (Figure 5) were divided into two groups of signals with different slopes under 1400 nm: one group has an increased curvature with a significant absorbance peak at a wavelength of approximately 1420 nm, while another is more flat and has only a small absorption at this position. This is because the second significant spectral peak is mainly at approximately 1950 nm (Figure 5). The band at 1420 nm is associated with the O–H and aliphatic C–H, while that at 1950 nm is assigned to the amide N–H and O–H. Because the NIR spectrum contains all strength information of the chemical bond, chemical composition, electronegativity, etc, the absorption peaks are heavily overlapped. In addition, other interference information, such as scattering, diffusion, special reflection, refractive index, and reflected light polarization, also has an important influence on the NIR spectrum. Thus, the quantitative predictions are difficult directly through NIR spectra alone.
Multivariate analyses are required to discern the spectral characteristics of commercial organic fertilizers with the support of chemometric methods, for example, partial least squares (PLS) analysis in this study. The results of the NIRS calibration and validation for the quality indices of commercial organic fertilizers are listed in and and . The NIR calibrations allowed accurate predictions of WSON, TOM, pH, and GI (R2 = 0.73−0.93 and RPD = 1.47−2.96). However, the results were less accurate for moisture (R2 = 0.91, r2 = 0.79, RPD = 2.22), TN (R2 = 0.98, r2 = 0.80, RPD = 2.25), and EC (R2 = 0.99, r2 = 0.74, RPD = 2.27). In addition, the WSOC had the worst prediction (R2 = 0.88, r2 = 0.76, RPD = 2.10). Therefore, predictions were moderately successful for TOM, TN, WSON, pH, EC, GI, and moisture, but failed for WSOC.o
Table 2.
NIRS calibration and validation results for quality indices of commercial organic fertilizers
TOM, total organic matter; TN, total nitrogen; WSOC, water-soluble organic carbon; WSON, water-soluble organic nitrogen; EC, electrical conductivity; GI, germination index; PC, number of principal components; R2, the coefficient of determination for the calibration set; RMSECV, the root mean squared error in cross-validation; r2, the coefficient of determination for the validation set; RMSEP, room mean squared error of prediction; RPD, the ratio of the standard deviation in the validation set over the room mean squared error of prediction.
Similar to NIRS, fluorescence excitation–emission matrix (EEM) spectroscopy is extensively utilized to detect protein-like, fulvic-acid-like, and humic-acid-like substances. These materials are directly proportional to fluorescence intensity at low concentrations and thus are applied to assess compost maturity . Fluorescence spectroscopy has been widely used as a tool to assess compost maturity, owing to high instrumental sensitivity. However, analysis of fluorescence EEM has generally been limited to visual identification of peaks or development of ratios of fluorescence intensities in different regions of the spectrum. These techniques lack the ability to capture the heterogeneity of samples. It has been reported that as opposed to individual main peak positions analysis, analyzing the full fluorescence EEMs can provide much information. Additionally, the composition complexity of WEOM in compost samples often results in the overlapped fluorophores in the EEM spectra. As a result, the EEM spectra are difficult to interpret.
Recent work has demonstrated that parallel factor (PARAFAC) analysis can be used to decompose full fluorescence EEMs into different independent groups of fluorescent components. Therefore, EEM-PARAFAC analysis is able to assess compost maturity and also to be a potential monitoring tool for rapidly characterizing compost maturity. For this purpose, 62 full-scale compost facilities in nine Provinces of China, yielding compost from animal manures and other industrial organic residues of different maturities, were selected. Then, these compost samples were used to extract water-soluble OM (WEOM) and characterized by fluorescence EEM spectra. EEM-PARAFAC analysis was then conducted for assessment of compost maturity.
