Letter to the Editor

Burning straw, air pollution, and respiratory infections in China

Soil organic carbon (SOC) accumulation and its components are influenced by agricultural practices, which are interfered by soil microorganisms. A field experiment was conducted to investigate how straw amendment and soil tillage influenced SOC fractions and chemical components, and their interrelations with microbial communities evaluated through phospholipid fatty acids (PLFAs) in Mollisols. Density fractionation and Fourier-transform Infrared Spectroscopy were combined to characterize OC properties, including free and occluded light fractions (fLFC, oLFC), heavy fraction (HFC), aliphatic-C and aromatic-C functional groups. Compared with control soil, straw amendment caused surface SOC increased by 6.7%–12.4% (P < 0.05). The dissolved OC, microbial biomass carbon (SMBC), fLFC, oLFC, aliphatic-C, and total PLFAs also increased after straw amendment. Whereas the aromatic-C proportions in bulk soil, fLFC, and oLFC decreased. The OC components and microbial properties didn't obviously change in soil with straw ash amendment. Straw amendment to the subsurface layer only increased subsurface SOC and OC fractions (P < 0.05). Principal component analysis revealed that soil microbial community structure changed with the amounts of straw amendment, soil tillage further underpinned the separation. The separation via PC1 was associated with labile OC components, including SMBC, fLFC, oLFC, and aliphatic-C pointing to soils with higher amounts of straw amendment, and with recalcitrant aromatic-C and HFC pointing to soils without straw or straw ash. Higher amounts of straw amendment plus soil tillage affected soil microbial community structure, and consequently influenced OC sequestration by altering labile OC fractions and components in Mollisols.

Soil aggregates are an important controlling factor for the physico-chemical and biological processes such as ammonium (NH₄⁺) retention. Straw return to the field is increasingly recommended to promote soil carbon (C) sequestration and improve crop yields. However, the effects of straw return on NH₄⁺ retention at soil aggregate level in agricultural soils have seldom been investigated. This study aimed to evaluate the influences of long-term straw return on NH₄⁺ adsorption and fixation in microaggregates (<0.25 mm) with or without soil organic carbon (SOC) oxidization. Soil samples were collected from plots of three treatments, i.e., no fertilizer (CK), inorganic NPK fertilizers (NPK), and inorganic NPK fertilizers with rice straw return (NPKS), from a 20-year-old field trial with rice-wheat rotations in Taihu Lake Region, China. Soil aggregates were separated using wet-sieving method. The SOC of microaggregates was oxidized by H₂O₂. The results showed that long-term straw return significantly increased SOC and NH₄⁺ adsorption, but inhibited NH₄⁺ fixation in microaggregates. NH₄⁺ adsorption potential and strength - obtained from adsorption isotherms - increased, but NH₄⁺ fixation decreased along with increasing SOC in microaggregates, indicating the important role of SOC in NH₄⁺ adsorption and fixation. This was verified by the SOC oxidization test that showed a relative decrease in NH₄⁺ adsorption potential for the NPKS treatment and an increase in NH₄⁺ fixation in all three treatments. Therefore, long-term straw return influences NH₄⁺ adsorption and fixation by enhancing SOC content and could improve N availability for crop uptake and minimize applied N fertilizer losses in rice-wheat cropping systems.

The flame-retardant modification of rice straw fibers (RSFs) was carried out to solve the technical problem of poor fire-proof performance of RSFs. The physical and mechanical properties and thermal insulation properties of rice straw fiber cement-based composites (RSFCC) at high temperatures were characterized. The results show that the three flame retardants affected the chemical structure of RSFs. The thermal stability and flame retardancy of flame-retardant RSFs were significantly higher than that of non-flame-retardant RSFs. The crystallinity of several groups of modified RSFs increased in varying degrees compared to untreated fibers. The mass-loss rate of RSFCC increased with the increase of temperatures. The strength and thermal conductivity of RSFCC decreased with the rise of temperatures. The strength and thermal conductivity of the composite flame-retardant modified fiber RSFCC at high temperatures were significantly higher than those of the non-flame retardant modified fiber or the single flame retardant modified fiber RSFCC.

Straw incorporation is a global common practice to improve soil fertility and rice yield. However, the effect of straw incorporation on rice yield stability is still unknown, especially under high fertilization level conditions. Here, we reported the effect of straw returning on rice yield and yield stability under high fertilization levels in the rice–wheat system over nine years. The results showed that straw incorporation did not significantly affect the average rice yield of nine years. Straw incorporation reduced the coefficient of variation of rice yield by 25.8% and increased the sustainable yield index by 8.2%. The rice yield positively correlated with mean photosynthetically active radiation (PAR) of rice growth season and the effects of straw incorporation on rice yield depended on the PAR. Straw incorporation increased the rice yield by 5.4% in the low PAR years, whereas it did not affect the rice yield in the high PAR years. Long-term straw incorporation lowered soil bulk density but improved the soil organic matter, total N, available N, available P, and available K more strongly than straw removal. Our findings suggest that straw incorporation can increase rice yield stability through improving the resistance of rice plant growth to low PAR.

To solve the poor fireproof performance of plant fibers, one of the technical difficulties, and use more straw fibers as construction materials, wheat straw fibers (WSFs) have been subject to flame-retardant modification. Then, tests and analysis over the modified WSFs per the burning behavior and pyrolysis performance have been made. The physical and mechanical, and thermal insulation properties of wheat straw fiber cement-based composites (WSFCC) at high temperatures were characterized. The results are as follows: Ammonium polyphosphate (APP), magnesium hydroxide (MH), and aluminum hydroxide (ATH), three fire retardants, all affect the chemical structure of WSFs. P, Mg, and Al elements of the three retardants are successfully adsorbed onto the fibers' surface subject to modification. Flame-retardant WSFs significantly outperforms the non-flame-retardant ones per thermostability and flame retardancy. The heat release rate (HRR), total heat release (THR), effective heat of combustion (EHC), and mass loss rate (MLR) of most flame-retardant WSFs are obviously lower than those of non-flame-retardant ones. The APP-modified WSFs demonstrate higher total smoke production (TSP), while WSFs modified with composite flame retardants perform better in flame retardancy and smoke suppression. The MLR of WSFCC at high-temperature increases as the temperature goes higher. The compressive and flexural strengths of WSFCC decrease with the increase of temperature and the decrease mitigates after 350 °C. In terms of thermal conductivity, composite-flame-retardant fiber WSFCC are much higher than non-flame-retardant fiber and APP-modified fiber WSFCC. As the temperature rises, the thermal conductivity of composite-flame-retardant fiber WSFCC tends to decrease.

Long-term straw return is an important carbon source for improving soil organic carbon (SOC) stocks in croplands, and straw removal through burning is also a common practice in open fields in South China. However, the specific effects of long-term rice straw management on SOC fractions, the related enzyme activities and their relationships, and whether these effects differ between crop growing seasons remain unknown. Three treatments with equal nitrogen, phosphorus, and potassium nutrient inputs, including straw/ash and chemical nutrients, were established to compare the effects of straw removal (CK), straw return (SR), and straw burned return (SBR). Compared to CK, long-term SR tended to improve the yield of early season rice (P=0.057), and significantly increased total organic carbon (TOC) and microbial biomass carbon (MBC) in double-cropped rice paddies. While SBR had no effect on TOC, it decreased light fraction organic carbon (LFOC) in early rice and easily oxidizable organic carbon (EOC) in late rice, significantly increased dissolved organic carbon (DOC), and significantly decreased soil pH. These results showed that MBC was the most sensitive indicator for assessing changes of SOC in the double-cropped rice system due to long-term straw return. In addition, the different effects on SOC fraction sizes between SR and SBR were attributed to the divergent trends in most of the soil enzyme activities in the early and late rice that mainly altered DOC, while DOC was positively affected by β-xylosidase in both early and late rice. We concluded that straw return was superior to straw burned return for improving SOC fractions, but the negative effects on soil enzyme activities in late rice require further research.