Quantification of the effects of various soil fumigation treatments on nitrogen mineralization and nitrification in laboratory incubation and field studies

Encapsulated allyl isothiocyanate improves soil distribution, efficacy against soil-borne pathogens and tomato yield

BACKGROUND

Crop quality, yield and farmer income are reduced by soil-borne diseases, nematodes and weeds, although these can be controlled by allyl isothiocyanate (AITC), a plant-derived soil fumigant. However, its efficacy against soil-borne pathogens varies, mainly because of its chemical instability and uneven distribution in the soil. Formulation modification is an effective way to optimize pesticide application. We encapsulated AITC in modified diatomite granules (GR) and measured the formulation's loading content and stability, environmental fate and efficacy against soil-borne pathogens, and its impact on the growth and yield of tomatoes.

RESULTS

We observed that an AITC loading content in the granules of 27.6% resulted in a degradation half-life of GR that was 1.94 times longer than 20% AITC emulsifiable concentrate in water (EW) and shorter than AITC technical (TC) grade. The stable and more even distribution of GR in soil resulted in relatively consistent and acceptable control of soil-borne pathogens. Soil containing AITC residues that remained 10-24 days after GR fumigation were not phytotoxic to cucumber seeds. GR significantly reduced soil-borne pest populations, and tomato growth and yield increased as AITC dosage increased. GR containing an AITC dose of 20 g m-2 effectively controlled pathogens in soil for about 7 months and improved tomato yield by 108%.

CONCLUSION

Our research demonstrates the benefits of soil fumigation with loaded AITC over other formulations for effective pest control, and improved tomato plant growth and fruit yield. Fumigant encapsulation appears to be a useful method to improve pest and disease control, environmental performance and fumigant commercial sustainability. © 2024 Society of Chemical Industry.

Soil amendments promoting nitrifying bacteria recovery faster than the denitrifying bacteria at post soil fumigation

Chloropicrin (CP) is a soil fumigant that not only reduces disease-causing pathogenic microbes but regrettably also those that benefit soil quality and crop health. However, we have few knowledge on rapidly restoring populations of beneficial microbes suppressed by CP fumigation. Here we used genetic sequencing technology to monitor changes in the recovery of soil bacteria in response to ammonium sulfate added to the soil following CP fumigation. The results showed that regardless of the N fertilizer addition rate, the accumulated NH4+-N in CP fumigated soil was rapidly consumed within 42 d. The rapid reduction in NH4+-N coincided with the observed recovery nitrogen-cycling microorganisms, especially the nitrification bacteria AOA and AOB that contributed to the formation of NH4+-N. Additionally, we further observed that the resilience index of nitrifying bacteria (AOB and AOA) was greater than the resilience index of denitrifying bacteria that contain the denitrification genes nirS, nirK and nosZ (0.12 to 0.55 vs. -0.27 to 0.073). These results revealed that N fertilizer stimulated the recovery of nitrifying bacteria more than denitrifying bacteria. Our research suggests that ammonium sulfate applied to CP fumigated soil could be used commercially to improve soil health as a result of an increase in beneficial microbes.

Soil bacterial communities are influenced by soil chemical characteristics and dispersal limitation in commercial strawberry production systems

Bacterial communities play multiple functional roles in soil that have positive and negative feedbacks on plant health. However, relatively few studies have focused on the ecology of soil bacterial communities in commercial strawberry production systems. The objective of this study was to determine if ecological processes influencing soil bacterial communities are consistent among commercial strawberry production locations and plots within the same geographic region. Soil samples were collected using a spatially explicit design from three plots in two commercial strawberry production locations in the Salinas Valley region of California. Soil carbon, nitrogen, and pH were measured for each of the 72 soil samples and bacterial communities were characterized using 16 S rRNA sequencing. Multivariate analyses showed bacterial community composition was differentiated between the two strawberry production locations. Analyses of communities within plots demonstrated soil pH and nitrogen were significant predictors of bacterial community composition in one of the three sampled plots. Bacterial communities displayed spatial structure in two plots at one location based on a significant increase in community dissimilarity with increasing spatial distance. Null model analyses identified a lack of phylogenetic turnover among bacterial communities in all plots, but a greater frequency of dispersal limitation in the two plots where spatial structure was also observed. Overall, this work suggests that ecological factors influencing soil bacterial communities are not consistent among different strawberry production locations or plots which may impact the ability to predict or manage the effect of soil microbiomes on strawberry health.

Non-target impacts of pesticides on soil N transformations, abundances of nitrifying and denitrifying genes, and nitrous oxide emissions

Agriculture is the leading contributor to global nitrous oxide (N₂O) emissions, mostly from soils. We examined the non-target impacts of four pesticides on N transformations, N cycling genes and N₂O emissions from sugarcane-cropped soil. The pesticides, including a herbicide glyphosate (GLY), an insecticide imidacloprid (IMI), a fungicide methoxy ethyl mercuric chloride (MEMC) and a fumigant methyl isothiocyanate (MITC), were added to the soil and incubated in laboratory at 25 °C. The soil microcosms were maintained at two water contents, 55 % and 90 % water holding capacity (WHC), to simulate aerobic and partly anaerobic conditions, respectively. Half of the soil samples received an initial application of KNO₃ and were then maintained at 90 % WHC for 38 d, whilst the other half received (NH₄)₂SO₄ and were maintained at 55 % WHC for 28 d followed by 10 d at 90 % WHC to favour denitrification. Responses of individual functional genes involved in nitrification and denitrification to the pesticides and their relationships to N₂O emissions varied with time and soil water. Overall, MITC had pronounced repressive effects on AOA and AOB amoA gene abundances and gross nitrification. Under 55 % WHC during the initial 28 d, N₂O emissions were low for all treatments (≤62 μg N kg^-1 soil). However, under 90 % WHC (either during the first 28 d or the increase in water content from 55 to 90 % WHC after 28 d) the cumulative N₂O emissions increased markedly. Overall, under 90 % WHC the cumulative N₂O emissions were 19 (control) to 79-fold (MITC) higher than under 55% WHC; with the highest emissions observed in the MITC treatment (3140 μg N kg^-1 soil). This was associated with increases in gross nitrate consumption rates and abundances of denitrifying genes (nirK, nirS and qnorB). Therefore, to minimise N₂O emissions, MITC should not be applied to field under wet conditions favouring denitrification.

Impact of fumigants on non-target soil microorganisms: a review

Fumigants have been used for decades to control soil-borne pathogens of high-value crops, and increasing evidence indicates they can affect non-target soil microbial communities. Understanding the impacts of these products on soil microorganisms is of critical importance not only for evaluating their environmental safety, but also because soil microbial communities have a central role in soil quality and nutrient cycling, plant growth, and crop production. Thus, we conducted a systematic review and metanalysis study of fumigant impacts on non-target soil microorganisms. In general, we found that fumigation decreases the bacterial diversity and abundance of total bacteria and nitrogen-cycling genes by approximately 10-50% during the first four weeks after application compared to non-treated soils. These decreases appear transient and tend to diminish or disappear after four weeks. Increases in bacterial diversity and abundance can occur after fumigation but are less common. Fumigant application can also alter bacterial community composition during the first six weeks after treatment by significantly increasing and/or decreasing the relative abundance of bacterial taxa involved in key soil functions such as N-cycling and plant-growth promotion. Knowledge gaps and areas where future research efforts should be prioritized to improve our understanding of the impact of organic fumigants on non-target soil microorganisms are discussed.

Impact of Biochar on Rhizosphere Bacterial Diversity Restoration Following Chloropicrin Fumigation of Planted Soil

Chloropicrin (CP) can effectively combat soil-borne diseases but has significant side effects on nontarget microorganisms. The rhizosphere microflora play a crucial role in promoting plant growth and protecting plants from infection by soil-borne pathogens. We conducted a laboratory pot experiment to evaluate the effect of CP on the rhizosphere soil bacterial flora and the effect of biochar amendments on the reconstruction of microbial communities. Our results show that CP fumigation and biochar additions promoted the growth of cucumber plants in the later stage of the pot experiment. CP significantly inhibited the rhizobacterial diversity and changed the community composition. Biochar amendments after CP fumigation shortened the time for the rhizobacterial diversity to recover to unfumigated levels. Biochar amendments promoted the transplantation of new populations to empty microbiome niches that were caused by CP and, in particular, stimulated many beneficial microorganisms to become the predominant flora. The relative abundances of many functional taxa related to plant-disease suppressiveness and pollutant bioremediation increased, including Pseudomonas, Stenotrophomonas, Bacillus, Massilia, Acinetobacter, Delftia, Micromonospora, Cytophagaceae, and Flavisolibacter. These changes stimulated by biochar amendments would promote multifunctionality in the soil rhizosphere and benefit plant growth and disease resistance.

The remediation effects of microbial organic fertilizer on soil microorganisms after chloropicrin fumigation

Soil fumigation with chloropicrin (CP) is an effective means of overcoming continuous cropping obstacles (CCO) in Panax notoginseng and improving its yield and quality. CP fumigation can change the microbial community of soil. Therefore, a key step after CP fumigation is the rapid restoration of soil microorganisms and the promotion of beneficial microorganism proliferation as the dominant flora. In this study, continuously cropped soil of P. notoginseng was fumigated with CP, and general organic fertilizer (GOF) or microbial organic fertilizer (MOF) was used to restore soil microorganisms after fumigation. Soil physical and chemical properties, soil microorganisms, and quality of P. notoginseng were investigated. The application of MOF and GOF after CP fumigation promoted increases in soil nitrogen (9.88% and 8.21%, respectively), phosphorus (21.39% and 11.57%, respectively), potassium (7.99% and 2.75%, respectively), and the quality of P. notoginseng; it also promoted the accumulation of saponins in the main roots (23.62% and 9.12%, respectively). Application of MOF and GOF can restore the diversity of microorganisms in the soil. MOF increased the relative abundance of the beneficial soil microorganisms Glomeromycota, Mortierellomycota, Humicola and Bacillus, thereby lowering the relative abundance of the harmful Ascomycota and Fusarium relative to GOF. In summary, CP fumigation reduces the diversity of microorganisms in the soil. The addition of organic fertilizer can promote microbial diversity and increase the relative abundance of beneficial species. Moreover, the promotion effect of MOF is better than that of GOF, thereby improving soil fertility and ultimately promoting the quality and yield of P. notoginseng.

Effects of biocontrol Bacillus and fermentation bacteria additions on the microbial community, functions and antibiotic resistance genes of prickly ash seed oil meal-biochar compost

This study evaluated the effects of biocontrol Bacillus and fermenting bacteria addition on the microbial community, metabolic functions and antibiotic resistance genes (ARGs) of new prickly ash seed oil meal (PSOM)-biochar composting. The results showed that the addition of Bacillus subtilis and fermentation bacteria significantly increased the NH₄⁺-N, bacterial abundance and fungal diversity of compost while decreasing the relative abundances (RAs) of carbon metabolism genes in mature compost. NH₄⁺-N was significantly correlated with microbial abundance and diversity, and its increase was closely related to microbial amino acid metabolism. The addition of biocontrol and fermenting bacteria changed the RAs of ARGs, which was caused by changes in the potential hosts Proteobacteria, Bacteroidota and Firmicutes in the compost. Consequently, adding Bacillus and fermenting bacteria into PSOM to make composting was suggested as an effective method to promote nutrient transformation, regulate microbial activity and decrease RAs of tetracycline and vancomycin ARGs.

Effects of sulfoxaflor on greenhouse vegetable soil N2O emissions and its microbial driving mechanism

The wide application of pesticides ensures the safety of food production, but it also has a serious impact on soil ecosystem. Although sulfoxaflor as a pesticide has great potential for application due to its excellent insecticidal activity and low crossresistance, little is known about its soil environmental safety risks. In this study, the effects of sulfoxaflor on N₂O emissions and microorganisms in greenhouse vegetable soils were studied by indoor simulation culture experiments. Dynamic changes of soil main inorganic N and N₂O emission rate were tested, and the abundance and community of total bacteria and microorganisms related to N cycle were analyzed. The results indicated that soil microorganisms rapidly degraded sulfoxaflor, and the N₂O emissions rate and ammonium nitrogen (NH₄⁺-N) content significantly increased, while nitrate nitrogen (NO₃^--N) content was significantly decreased. Sulfoxaflor significantly changed the abundance and community of total bacteria, nitrite reducing and nitrous oxide reducing bacteria, but had no significant effect on ammoxidation microorganisms. The N₂O emission rate was positively correlated with gene abundance of denitrifying microorganisms. Under 65% soil maximum water holding capacity, sulfoxaflor may broke the dynamic balance of N₂O production and consumption in the denitrification process, which caused a significant increase in N₂O emission. Therefore, the application of sulfoxaflor had a certain effect on N cycling and utilization in greenhouse vegetable soil.

Effects of Dazomet Fumigation on Soil Phosphorus and the Composition of phoD-Harboring Microbial Communities

The soil phosphorus (P) cycle and P transformation are largely driven by the soil bacterial microbial community. However, little is known about the effects of dazomet (DZ) soil fumigation on soil P and soil microbial communities associated with P transformation. This research investigated P released from three farm soils as a result of DZ fumigation and changes in enzyme activity, gene abundance, and the encoding alkaline phosphatase PhoD microbial community. After DZ fumigation, we observed a briefly significant increase in the available P and the active P fractionation. The soil ALP activity, 16s rRNA abundance, and the phoD gene decreased significantly after DZ fumigation. The abundance and diversity of phoD-harboring microbes also decreased shortly after fumigation, increased significantly 14-28 days later, and then decreased again toward the end of the experimental period of 86 days. The shared OTUs between treatments became fewer with increasing time after fumigation. The changes in available P and the active P fractionation after DZ fumigation were significantly correlated with the abundance of the dominant phoD-harboring microbes. DZ fumigation promoted P mineralization in these farm soils and changed the composition of phoD-harboring microbial communities, which will benefit crops able to absorb and use P.

Chloropicrin fumigation alters the soil phosphorus and the composition of the encoding alkaline phosphatase PhoD gene microbial community

The transformation of phosphorus (P) compounds in soil depends largely on soil microbial communities and is sensitive to agricultural practices. However, the effects of soil fumigation on soil P, and microbes involved in P transformation, are unknown. Our results showed that chloropicrin (CP) fumigation significantly increased the available-P, Leached-P and active-P fractionation (inorganic P extracted from H₂O, NaHCO₃ and NaOH) in Shangdong and Miyun soils in the early stages of culture, while soil alkaline phosphatase (ALP) activity and phoD gene abundance decreased significantly. Leached-P in fumigated soil was positively correlated with increased active-P fractionation, indicating that it was an important source of soil Leached-P after fumigation. The changes in P-fractionation, Leached-P and ALP after fumigation were also significantly correlated with the composition of the microbial communities. CP fumigation briefly stimulated an increase in the abundance and diversity of phoD-harboring microbial communities and promoted the mineralization process of soil P. PICRUSt metagenomic analysis showed an increase in the relative abundance of microorganisms with involved in carbohydrate/lipid transport and metabolism functions after fumigation. These results suggest CP fumigation altered soil P transformation and phoD-harboring microbes that might lead to an increased risk of P enrichment in waterways.

Comparative analysis of the effects of five soil fumigants on the abundance of denitrifying microbes and changes in bacterial community composition

Soil fumigation is currently the most effective method for controlling soil-borne pests and diseases in high-value crops. To better understand the effect of chloropicrin (CP), dazomet (DZ), dimethyl disulfide (DMDS), allyl isothiocyanate (AITC) and 1,3-dichloropropene (1,3-D) fumigants on soil microorganisms, this study monitored changes in the diversity and community composition of soil bacteria involved in denitrification using real-time PCR and high-throughput gene sequencing techniques. These five fumigants significantly decreased the bacterial population size in some phyla including Proteobacteria, Chloroflexi and Acidobacteria, and increased the bacterial population size in other phyla such as Firmicutes, Gemmatimonadetes, Actinobacteria, Verrucomicrobia, Saccharibacteria and Parcubacteria. Although bacterial diversity declined after CP fumigation, it was briefly stimulated by the other four fumigants. Meanwhile, all five fumigants temporarily decreased populations of denitrifying bacteria containing the napA, narG, nirS or nirK enzyme-encoding genes. Denitrifiers bearing the cnorB, qnorB or nosZ genes were relatively stable following DZ and DMDS fumigation. However, cnorB and nosZ decreased initially following CP, AITC and 1,3-D fumigation. Simultaneously, the abundance of qnorB significantly increased in AITC and 1,3-D fumigated soils. These results showed that soil fumigation significantly shifted the abundance and community structure of denitrifying bacteria. This study will help to predict the response of different phyla of denitrifying bacteria to soil fumigation.

Adsorption isotherms, degradation kinetics, and leaching behaviors of cyanogen and hydrogen cyanide in eight texturally different agricultural soils from China

Cyanogen (C₂N₂) is a new and effective alternative soil fumigant to methyl bromide. The effects of soil properties on the fate of C₂N₂ and its degradation products, including hydrogen cyanide (HCN), are not fully understood. The objectives of this study were to determine the adsorption kinetics, adsorption isotherms, and degradation kinetics of C₂N₂ and HCN in texturally different soils and evaluate their leaching potentials using soil columns. Eight agricultural soils were collected throughout China: Luvisols (Hebei Province), Phaeozems (Heilongjiang Province), Gleysols (Sichuan Province), Anthrosols (Zhejiang Province), Ferralsols (Jiangxi Province), Lixisols (Hubei Province), Alisols (Shandong Province), and Plinthosols (Hainan Province). The adsorptions of C₂N₂ and HCN in C₂N₂-fumigated soils were positively correlated with organic matter and clay contents. For a C₂N₂ dose of 100 mg kg^-1, the adsorptions of C₂N₂ and HCN were highest in Phaeozems and lowest in Gleysols according to their adsorption coefficients (15.744 and 3.119, respectively). No significant difference in the half-life of C₂N₂ and HCN was observed between sterilized and unsterilized soils, indicating that abiotic degradation was predominant in the degradation of C₂N₂ and HCN. After leaching, the residual C₂N₂, HCN, NH₄⁺-N, and NO₃^--N concentrations in C₂N₂-fumigated Phaeozems were highest within 15 cm of the soil surface (30, 20, 19.68, and 10.41 mg kg^-1 soil, respectively). The results indicate that C₂N₂ and HCN have short lifetimes and low leaching potentials in agricultural soils, even under heavy rainfall conditions. The findings demonstrate that C₂N₂ and HCN resulting from fumigation will not accumulate in the soil and are not likely to contaminate groundwater.

Soil fumigation alters adsorption and degradation behavior of pesticides in soil

Many crops are produced using soil fumigation and chemical pesticides to control soil-borne fungi and bacterial diseases, nematodes and weeds. Fumigation of soil, however, may alter its ability to adsorb, degrade and volatilize pesticides, which can then change the potential for pesticides to leach into groundwater. Soil adsorption kinetics, Freundlich isothermal adsorption and pesticide degradation techniques were used to determine the potential for pesticides to pollute groundwater in fumigated soil. The effect on soil pesticide adsorption in different types of chloropicrin (CP) fumigated soils was also examined. We observed that the equilibrium adsorption (q_e) decreased significantly at 24 h. Soil fumigation decreased the Freundlich K_f and K_foc values, and increased the Freundlich exponent ¹/_n values, for pesticides in fumigated Beijing soil. Soil fumigation influenced the K_f of pendimethalin, oxyfluorfen and abamectin the most, which themselves had a larger K_f in untreated soil. This indicated that the greater the soil pesticide adsorption the greater the influence of a fumigation treatment on that pesticide. The K_f was decreased more in the Heilongjiang and Beijing CP-fumigated soils that had high organic carbon content compared to Hunan soil. Fumigation of the soil with CP extended the half-life values of fosthiazate (from 34.3 to 43.1 days) and azoxystrobin (from 52.9 to 64.2 days), which increased their potential to leach into groundwater. Famers should minimize the quantity of some pesticides applied to fumigated soil, or apply some pesticides 60 days after fumigation, in order to avoid ground water pollution when crops are grown in fields or greenhouses.

Changes in the abundance and community composition of different nitrogen cycling groups in response to fumigation with 1,3-dichloropropene

The fumigant 1,3-dichloropropene (1,3-D) is widely-used to control pathogenic bacteria, fungi, nematodes and insects in soil before a crop is planted. Although fumigants in general have been reported to have a 'fertilizer effect' in the soil by increasing nitrogen availability, little is known of how a specific fumigant such as 1,3-D affects available nitrogen. This study used real-time quantitative PCR (qPCR) and 16S rRNA gene amplicon sequencing techniques to investigate the effects of 1,3-D on microorganisms involved in nitrogen cycling that were present in 2 soils: Jiangxi lateritic red soil and Beijing fluvo-aquic soil. The fumigant 1,3-D temporarily decreased the abundance of 11 functional genes involved in nitrogen-fixing, nitrification and denitrification in both soil types. Different nitrogen cycling groups recovered to the unfumigated level in various incubation phases. Microorganisms containing nifH, nxrB, napA and qnorB genes were most vulnerable to 1,3-D fumigation. However, a stronger and longer inhibition effect of 1,3-D on these 11 functional genes was observed in Jiangxi soil than in Beijing soil. At the same time, the abundance of nifH, AOBamoA, nirS, qnorB and cnorB genes was significantly increased 59 days after 1,3-D fumigation. Fumigation with 1,3-D significantly reduced the nitrogen-fixing bacteria Azospirillum and Paenibacillus; the nitrifiers Nitrosomonas and Nitrospira; and the denitrifiers Pseudomonas, Paracoccus and Sphingomonas. Conversely, fumigation with 1,3-D increased the nitrogen-fixing bacteria Bradyrhizobium and Rhizobium; the nitrification bacteria Nitrosospira and Nitrolancea; and the denitrification bacteria Sphingobium, Alcanivorax, Bacillus, Streptomyces and Aeromonas. Fumigation with 1,3-D therefore caused significant shifts in the species composition and number of microbes directly involved in nitrogen cycling in the short-term. These results contribute toward a better understanding of the impact of 1,3-D fumigation on various types of soil nitrogen-cycling groups.

Does soil amendment alter reactive soil N dynamics following chloropicrin fumigation?

Chloropicrin fumigation had strong inhibitory effect on soil N cycling. Knowledge gap existed about the performance of reactive N in soil applied with different amendments used to improve the fumigation function or soil quality. In this study, we employed four amendments, i.e., wheat straw residue, manure, biochar and ammonium thiosulfate, incorporated into soil at the regular application rate. Simultaneously, bare soil was selected as control (CK). Based on a three months incubation assay, soil reactive N and activity of three enzymes governing N-mineralization was measured, i.e., protease, arylamidase and l-glutaminase, as well the soil fluorescein diacetate (FDA) hydrolysis, basal soil respiration, and dissolved soil organic carbon (DOC). Result showed that, compared with the bare soil, the addition of straw or manure to soil markedly enhanced the FDA and the resistance of arylamidase and l-glutaminase to the fumigation, while significantly decreased the concentration of DON, NH₄⁺N and NO₃^--N. The addition of biochar to soil had no effect on the reactive N, but contrasting effects on the three enzymes, i.e., suppressed protease activity, and enhanced arylamidase activity. The ammonium thiosulfate showed an inert effect on the measured microbiological indices and reactive N except the enhanced concentration of NH₄⁺N. DOC content of amendments governed microbial activity under fumigation condition. In synthesis, our findings suggested that under chloropicrin fumigation the use of straw or manure enhanced the microbial abundance and the activity of N-mineralization enzymes, which may lead to low reactive N by the microbial N immobilization for a longer period.

Effect of soil fumigants on degradation of abamectin and their combination synergistic effect to root-knot nematode

BACKGROUND

Root-knot nematode (Meloidogyne spp., RKN) causes a disease that significantly reduces the yield of greenhouse cucumber crops year after year. Chemical control based on a single pesticide is now unreliable mainly due to pest resistance. Fumigant and non-fumigant pesticide combinations can potentially result in effective and economic RKN control.

RESULTS

Combining the insecticide abamectin (ABM) with fumigants dazomet (DZ) or chloropicrin (CP) significantly extended the half-life of ABM by an average of about 1.68 and 1.56 times respectively in laboratory trials, and by an average of about 2.02 and 1.69 times respectively in greenhouse trials. Laboratory experiments indicated that all the low rate ABM combination treatments controlled RKN through a synergistic effect. ABM diffused into the nematode epidermis more rapidly when ABM was combined with DZ and CP, giving effective nematode control and an increase cucumber total yield, compared to the use of these products alone. ABM combined with CP or DZ produced significantly higher total cucumber yield than when these products were used alone.

CONCLUSIONS

A low concentration of ABM combined with DZ in preference to CP would be an economic and practical way to control nematode and soilborne fungi in a greenhouse producing cucumbers.

Effects of hexaconazole application on soil microbes community and nitrogen transformations in paddy soils

The ecological risks of widely used hexaconazole on soil microbes remain obscure. Thus, a 3-month-long experiment using two typical paddy soils in China (red soil and black soil) was conducted to assess the effects of hexaconazole (0.6 (T1) and 6 (T10) mgkg^-1 soil) on the overall microbial biomass, respiratory activity, bacterial abundance and community structure, and nitrogen transformations. Soil was sampled after 7, 15, 30, 60, and 90days of incubation. The half-lives of the two doses of hexaconazole varied from 122 to 135d in the black soil and from 270 to 845d in the red soil. Both dosages of hexaconazole did not affect NH⁺₄-N content, N₂-fixing bacterial populations, total bacterial diversity, and community structure, but transitorily decreased the populations of total bacteria in both soil types. In the black soil, T10 negatively affected microbial biomass carbon (MBC) and soil basal respiration (R_B), but transitorily increased NO^-₃-N concentration and ammonia-oxidizing bacteria populations, while T1 had almost no effect on most of the indicators. As for red soil, both concentrations of fungicide significantly, but transitorily, inhibited MBC and R_B, while only T10 had a relatively long stimulatory effect on NO^-₃-N concentration and ammonia-oxidizing archaea populations. This study showed that over application of hexaconazole is indeed harmful to soil microorganisms and may reduce soil quality and increase the risk of nitrogen loss in paddy soils.

Effects of fumigation with metam-sodium on soil microbial biomass, respiration, nitrogen transformation, bacterial community diversity and genes encoding key enzymes involved in nitrogen cycling

Metam-sodium (MS) is widely used as a soil pre-plant fumigant as methyl bromide is phased out of agriculture. However, the information about how fumigation with MS affects the soil microbial community is still limited. In this study, a 66-day-long experiment was conducted to ascertain the effects of MS on soil substrate-induced respiration (SIR), microbial biomass nitrogen (MBN), NH₄⁺-N and NO₃^--N concentrations, as well as the abundance of the total bacteria and fungi and the expression of genes involved in nitrogen cycling. In addition, 16S rRNA amplicon sequencing was used to investigate the effect of MS on the soil bacterial community. The half-lives of high and low doses of methyl isothiocyanate (MITC) are 10.51h and 9.93h, respectively. MS caused a short-term inhibition of SIR, MBN; had an accumulation effect on NH4⁺-N concentration in the short term; reduced the abundance of the total bacteria and fungi; and suppressed the expression of the nifH, AOA-amoA, anammox bacteria, nosZ, nirS, and narG. In addition, under the influence of MS, soil bacterial diversity decreased significantly in the long term, bacterial community structure was affected, and there was a shift in the predominant population; for example, some genera, such as Paenibacillus and Luteimonas, significantly increased in number. These changes in bacterial flora may be closely related to the growth of crops. Our study provides useful information for environmental safety assessments of MS in China.

Effect of fumigation with chloropicrin on soil bacterial communities and genes encoding key enzymes involved in nitrogen cycling

Chloropicrin (CP) is a potential alternative for methyl bromide as a soil fumigant given that the use of methyl bromide has become limited. However, little is known about how fumigation with CP affects the condition of the soil microbial community. In this study, 16S rRNA amplicon sequencing and quantitative PCR were combined to investigate the effect of CP on soil bacterial community. In total, 938,922 effective reads were obtained from 18 samples and clustered into 58,662 operational taxonomic units at a similarity cut-off of 97%. Both approaches showed that the primary structure of bacterial community in soil did not significantly change at the phylum level after fumigation, but CP had a significant impact on the abundance of the bacterial microbiome that was recovered and identified. Additionally, bacterial community diversity decreased significantly, and there was a shift in the predominant populations. Staphylococcus, Actinomadura, Acinetobacter and Streptomyces significantly decreased in number or disappeared, and Bacteroides, Lachnoclostridium, Pseudoalteromonas, Colwellia, Idiomarina and Cobetia became the new predominant populations. In addition, some species associated with biodegradation, such as Sphingomonas spp. and Rhodococcus spp., significantly increased in number. The abundance of ammonia-oxidizing archaea (AOA) were significantly inhibited, yet the abundance of ammonia-oxidizing bacteria (AOB) significantly increased, and denitrification was significantly promoted. These changes in bacterial flora can considerably impact soil function and health and lead to negative effects on the environment surrounding fumigated soils, indicating the need for proactive risk management. Our study provides useful information for environmental safety assessments of CP in China.

Analysis of the inhibitory effects of chloropicrin fumigation on nitrification in various soil types

Chloropicrin retards the conversion of ammonia to nitrite during the nitrification process in soil. In our study, the dynamic effect of chloropicrin fumigation on soil nitrification was evaluated in five different soil types to identify relationships between soil properties and the effect of fumigation on nitrification. Chloropicrin significantly inhibited nitrification in all soils; however, the recovery of nitrification varied greatly between the soils. Following chloropicrin fumigation, nitrification recovered to the control level in all soils, except in the acidic Guangxi soil. Nitrification recovered faster in fumigated sandy loam Beijing soil than in the other four fumigated soils. Soil texture and pH were two important factors that influenced chloropicrin's inhibitory effect on nitrification. An S-shaped function was fitted to soil NO₃^--N content to assess the nitrification recovery tendency in different soils. The time taken to reach maximum nitrification (t_max) ranged from 2.4 to 3.0 weeks in all unfumigated soils. Results demonstrated that t_max was greater in all fumigated soils than in untreated soils. Correlation calculations showed that t_max was strongly correlated to soil texture. The correlation analysis results indicated that the recovery rate of nitrification after chloropicrin fumigation is much faster in sandy loam soil than silty loam soil.

Effect of Soil Fumigation on Degradation of Pendimethalin and Oxyfluorfen in Laboratory and Ginger Field Studies

Herbicides are usually applied to agricultural fields following soil fumigation to provide effective weed control in high-value cash crops. However, phytotoxicity has been observed in ginger seedlings following the application of herbicides in fumigated fields. This study tested a mixture of herbicides (pendimethalin and oxyfluorfen) and several fumigant treatments in laboratory and field studies to determine their effect on the growth of ginger. The results showed that soil fumigation significantly (P < 0.05) extended the degradation period of these herbicides in the field and in laboratory studies. The half-life of pendimethalin was extended by an average of approximately 1.29 times in the field and 1.74 times in the laboratory. The half-life of oxyfluorfen was extended by an average of about 1.19 times in the field and 1.32 times in the laboratory. Moreover, the extended period of herbicide degradation in the fumigant and nonfumigant treatments significantly reduced ginger plant height, leaf number, stem diameter, and the chlorophyll content. The study concluded that applying a dose below the recommended rate of these herbicides in chloropicrin (CP) or CP + 1,3-dichloropropene fumigated ginger fields is appropriate, as application of the recommended herbicide dose in fumigated soil may be phytotoxic to ginger.

Response of microbial community to a new fungicide fluopyram in the silty-loam agricultural soil

The impacts of fluopyram on a soil microbial community were studied at three application rates: at the recommended field rate (T1, 0.5mg/kg soil), three-fold recommended field rate (T3, 1.5mg/kg soil) and ten-fold recommended field rate (T10, 5mg/kg soil). Soil samples were taken after 7, 15, 30, 45, 60 and 90 days of application to determine the fluopyram residue and microbial properties (i.e., basal respiration, substrate-induced respiration, microbial biomass carbon, microbial community function and structure). The half-lives of the fluopyram at levels of 0.5, 1.5 and 5mg/kg in soil were calculated to be 64.2, 81.5 and 93.6 days, respectively. The results demonstrated that fluopyram treatment (T1, T3 and T10) decreased microbial biomass C but increased the basal respiration, substrate-induced respiration, and ecophysiological indices (qCO2). Average well color development (AWCD) represents the oxidative capacity of soil microorganisms cultivated in the BIOLOG micro-plates and usually indicates the overall microbial metabolic capacity. The BIOLOG results revealed that the AWCD in the soil treated with 1.5 and 5mg/kg fluopyram (T3 and T10) was significantly lower than that of the control during the incubation period. A similar variation in the diversity indices (Simpson index and McIntosh index) was observed. Phospholipid fatty acid (PLFA) analysis revealed that the addition of fluopyram decreased the total amount of PLFAs, bacterial biomass (both Gram-positive (GP) bacteria and Gram-negative (GN)), fungal biomass, the ratios of the GN/GP and fungi/bacteria at all incubation times. Principal component analyses (PCA) suggested that the addition of fluopyram shifted the soil microbial community structure and function. Hence, fluopyram has a harmful effect on overall soil microbial activity, and changed soil microbial community structure and function.

Review of the application of near-infrared spectroscopy technology to determine the chemical composition of animal manure

Animal manure contains a variety of chemical constituents that are highly valuable to agriculture, including nitrogen, phosphorus, potassium, and metal micronutrients. Although appropriately applied manure has numerous positive attributes, the excessive application of manure may lead to pollution of the atmosphere, water, or soil. To reconcile precision agriculture and the potential negative environmental influences of animal manure, it is necessary to develop rapid and robust methods to evaluate the chemical composition of animal manure. This paper summarizes recent advances in near-infrared reflectance spectroscopy (NIRS) in predicting moisture, dry matter, organic matter, nitrogen, phosphorus, carbon, and metal content in animal manure. The results indicate the high potential of NIRS as an efficient tool for monitoring the chemical composition of animal manure. Future prospects and needs related to increasing the feasibility of the industrial application of NIRS and improving NIRS prediction precision in determining the chemical composition of animal manure are discussed.