Effects of biodegradable film mulching on bacterial diversity in soils.

The spread of biodegradable plastic films (BDFs) not only increases grain yield but also reduces environmental pollution from plastic film to a large extent. Soil microbes are considered to be involved in biodegradation processes. However, the study of microbe diversity in soil mulched with biodegradable plastic film remains limited. Here, we compared the diversity of microbes between soils with biodegradable film and nonbiodegradable film (NBDF) mulch. The results showed that BDFs affected total C, P and NH4+-N, especially organism C content, as well as microbe species richness (ACE; Chao1) and diversity (Simpson index; Shannon index). In terms of dominant phyla and genera, BDFs and NBDF can influence the abundance of disparate species. Furthermore, BDFs could also contribute to improving the richness of the important functional bacterial groups in soil, e.g., Pedomicrobium and Comamonas, both of which are involved in the degradation of plastic residues in soil. Finally, we found that BDFs improved the transformation of nitrogen by significantly increasing the abundances of Nitrobacter and Nitrospira. Our results highlight the impact of BDF mulch on the abundance of functional bacteria in the soil.

Plastic pollution in croplands threatens long-term food security.

Plastic pollution is a global concern given its prevalence in aquatic and terrestrial ecosystems. Studies have been conducted on the distribution and impact of plastic pollution in marine ecosystems, but little is known on terrestrial ecosystems. Plastic mulch has been widely used to increase crop yields worldwide, yet the impact of plastic residues in cropland soils to soil health and crop production in the long term remained unclear. In this paper, using a global meta-analysis, we found that the use of plastic mulch can indeed increase crop yields on average by 25%-42% in the immediate season due to the increase of soil temperature (+8%) and moisture (+17%). However, the unabated accumulation of film residues in the field negatively impacts its physicochemical properties linked to healthy soil and threatens food production in the long term. It has multiple negative impacts on plant growth including crop yield (at the mean rate of -3% for every additional 100 kg/ha of film residue), plant height (-2%) and root weight (-5%), and soil properties including soil water evaporation capacity (-2%), soil water infiltration rate (-8%), soil organic matter (-0.8%) and soil available phosphorus (-5%) based on meta-regression. Using a nationwide field survey of China, the largest user of plastic mulch worldwide, we found that plastic residue accumulation in cropland soils has reached 550,800 tonnes, with an estimated 6%-10% reduction in cotton yield in some polluted sites based on current level of plastic residue content. Immediate actions should be taken to ensure the recovery of plastic film mulch and limit further increase in film residue loading to maintain the sustainability of these croplands.

First Report of Leaf Spot Caused by Alternaria alternata on Sonchus asper in China.

Sonchus asper (Spiny sowthistle), belonging to the Asteraceae, is a problematic weed in grain crops, orchards as well as turf (Cho et al., 2019). In April 2016, about 90% of the S. asper plants infesting a pear orchard had symptomatic with black spots densely distributed on the leaves and stems in Da Yang Town, Luyang District, Hefei, Anhui (N31°55'43″, E117°11'40″). The foliar lesions were mostly circular (3.0 mm to 1.5 cm in diameter) or irregular in shape; lesions on the stem were elliptical to irregular in shape running along the vascular bundle and about 3 to 6mm long × 2 to 3mm wide. Severely affected plants had their leaves become completely blighted and eventually the plants died. Fifteen tissue pieces from five symptomatic leaves and stems were surface sterilized with 0.5% NaClO for two min, rinsed three times with sterile water, and then plated on 1/4-strength potato dextrose agar (PDA) medium. Fourteen fungal isolated were obtained from the tissues, the isolation frequency from the plant tissue pieces was almost 93%. Fungal colonies were circular, initially white, and eventually turned to dark olive or black along with profuse sporulation. The conidia were tawny to brownish green, obclavate to obpyriform, with a cylindrical or coniform short beak, ranging from 16.8 to 39.8 µm long × 6.3 to 14.7 µm wide with two to five transverse and zero to two longitudinal septa (n = 50) and were borne on branched conidiophores. These morphological characteristics were similar to those described for Alternaria alternata (Simmons, 2007). For one representative isolate, the internal transcribed spacer (ITS) region of rDNA, partial coding sequence of the actin (ACT) gene, partial Alt a 1 major allergen (ALT) gene and partial Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) gene were amplified and sequenced with primers ITS1/ITS4 (White et al., 1990), ACT512F/ACT783R (Carbone and Kohn, 1999), Alt-4for/Alt-4rev and gpd1/gpd2 (Lawrence et al., 2013) respectively. Sequences were submitted to GenBank (ITS: MH886523; ACT: MH892480; ALT: MN655781; GAPDH: MN655782). Based on a BLAST analysis, ITS, ACT gene and GAPDH gene had 100%-99.8% identity with the existing sequences of the ex-type CBS 916.96 of A. alternata (Fries) Keissler (ITS: AF347031; ACT: JQ671702; GAPDH: AY278808, respectively) and ALT gene showed 100% identity with the A. alternata isolate SCWC10 (MG199093). Thus the pathogen was identified as A. alternata based on its morphological and molecular characteristics. Pathogenicity tests were conducted on six to seven-leaf stage potted S. asper plants (one plant per pot). Five mm diameter fungal disks, which were excised from the edge of a three-day old A. alternata colony, were placed on healthy leaves of five plants. Same size sterile PDA disks were used as controls. In addition, a conidial suspension (107 conidia/ml) was also inoculated on healthy leaves of five plants, sterile distilled water was used as control. Both inoculation experiments were conducted in the greenhouse at 25 ± 2°C, 90 % relative humidity, 12 h light/dark photoperiod and repeated at least three times. Lesions were observed on all the leaves within 12 h after fungal disk inoculation. All pathogen inoculated plants developed lesions, similar to those observed on the symptomatic leaves in the field 12 days after conidial inoculation. No symptoms were observed on both control treatments. A. alternata was re-isolated from the inoculated leaves and confirmed by both morphological and molecular techniques, confirming Koch's postulates. To our knowledge, this is the first report of A. alternata causing leaf spot on S. asper in China. The pathogen could cause severe disease in S. asper, and has the potential to be further studied as a fungal weed biocontrol.

[Accumulation Characteristics and Sources of PAEs in Agricultural Soils in Gansu Province].

In order to understand the composition and accumulation characteristics of phthalates esters (PAEs) in agricultural soils in Gansu province, a total of 41 soil samples from four agricultural soils in Gansu province were collected, and the content of six PAEs compounds was analyzed using a gas chromatography-single quadrupole mass spectrometer (GC-MS). The results showed that the average value of PAEs compounds in agricultural soils in Gansu province was 432.4 µg·kg-1. The detection rates of DMP, DEP, DnBP, DEHP, and DNOP in the soil were 100%, and BBP was not detected. The order of the average value of PAEs content in the four agricultural soils in Gansu province was:greenhouse>farmland (open field)>forest>grassland. The exceeding rates of dibutyl phthalate (DnBP), dimethyl phthalate (DMP), and dimethyl phthalate (DEP) were 94%, 28%, and 27%, and the remaining three did not exceed the standard. The composition of PAEs in different agricultural soils was different due to their different sources. DEHP and DnBP components in the six different PAEs monomers accounted for a higher proportion and were the main pollutants of PAEs in agricultural soils in Gansu province. In this study, the contents of soil PAEs and DEHP were significantly positively correlated with the residual amount of mulch film in the farmland (P<0.05). In general, the content of soil PAEs in the Hexi area of Gansu province was significantly higher than that in the Longdong area.