Long-term continuous monoculture cultivation harms soil physicochemical and microbial communities in agricultural practices. However, little has been reported on the effect of continuous cropping of industrial hemp on bacterial community and diversity in the rhizosphere soil. Our study investigated the changes in physicochemical properties and bacterial communities of industrial hemp rhizosphere soils in different continuous cropping years. The results showed that continuous cropping would reduce soil pH and available phosphorus (AP), while electrical conductivity (EC), available nitrogen (AN), and available potassium (AK) would increase. Soil bacterial diversity and richness index decreased with continuous cropping years. At the same time, continuous cropping marked Acidobacteria, Bacteroidetes, and Gemmatimonadetes increase, and the Proteobacteria and Actinobacteria decreased. Moreover, we found that pH, AK, and AP were the critical factors associated with the changes in the abundance and structure of the bacterial community. Overall, our study first reported the effect of continuous cropping on the rhizosphere soil microflora of industrial hemp. The results can provide a theoretical basis for revealing the obstacle mechanism of continuous cropping of industrial hemp and contribute to the sustainable cultivation of industrial hemp in the future.

Substantial soil phosphorus (P) losses often occur in the northern temperate regions owing to soil freeze-thaw cycles (FTCs). Presumably, biochar amendment is an efficient method of conserving P and sustaining agricultural production in the black soil region of northeast China. However, how biochar interacts with FTCs to affect soil P adsorption and desorption is unclear. A simulated laboratory FTC experiment was conducted on untreated and biochar-amended soil with varying moisture content to assess their effects on P adsorption and desorption. Soil P adsorption and desorption values were fitted with Langmuir and Freundlich isotherms to determine the interaction of the frequency of FTCs with moisture content and biochar amendment. Higher soil moisture content increased soil P adsorption, whereas biochar amendment mitigated decreased P retention by decreasing soil P adsorption capacity. Biochar amendment significantly increased the desorption ratio (D_avg) under all the FTCs. The desorption ratio of soil and biochar-amended soil in saturated moisture content treatment was significantly higher than that of 12 FTCs. The FTCs decreased the P availability of biochar-amended soil by enhancing P desorbability. Our results suggest that biochar amendment in arable black soil should not be conducted during FTCs, particularly during snowmelt.

Freeze–thaw cycles stimulate the release of available soil phosphorus (P) in winter, and biochar as a soil amendment could improve P availability. Nevertheless, it is unclear how freeze–thaw cycles and biochar amendment interact to affect the soil P fractions and their availability in winter, particularly under different soil water conditions. We simulateda freeze–thaw cycle experimentto assess the effects of three factors on soil P fractions: soil moisture content (22%, 31%, and 45%), frequencies of freeze–thaw cycles (0, 1, 3, 6, and 12 times) and biochar amendment (soil and biochar-amended soil). Modified Hedley sequential P fractionation was conducted to measure the soil P fractions. Increasing the number of freeze–thaw cycles increased soil labile P fractions in the soil with the lowest moisture content (22%). After biochar amendment, the content of labile P decreased as the number of freeze–thaw cycles increased. Biochar amendment enhanced P availability in Mollisols owing to the direct effect of NaOH-P_o, which has a large direct path coefficient. Principal components analysis showed that moisture content was a major factor influencing the variation in the P fractions. The P fractions were separated by the interactive effects of biochar amendment and freeze–thaw cycles in soils with a higher moisture content (45%), indicating that the effects of freeze–thaw cycles on P availability appear to be more pronounced in biochar-amended Mollisols of higher water contents.

Biochar amendment to soils can improve soil P availability, but details on the optimum application of biochar to black soils in Northeast China are limited. Three types of biochar were produced at six pyrolysis temperatures (between 200 and 700 °C) and then added to black soil samples. P adsorption-desorption isotherms were fitted by the Langmuir model to evaluate the changes in soil P adsorption-desorption after biochar amendment. Changes in P adsorption and desorption depended on biochar feedstock type and pyrolysis temperature. When pyrolysis temperature increased up to 400 °C, P sorption maximum (Q_m) of soybean pod (SP) and soybean straw (SS) biochar-amended soils were enhanced from 855.65 and 428.84 mg kg^-1 to 1666.67 and 1547.62 mg kg^-1, respectively, while a further increase in the pyrolysis temperature lowered the adsorption capacity. However, P adsorption of corncob (CC) biochar amended soils declined from 1428.57 mg kg^-1 to 556.70 mg kg^-1 as pyrolysis temperature increased. Higher P desorption in SP and SS compared with CC indicated that SP and SS biochar produced at higher than 400 °C pyrolysis temperatures were considered to be the optimum biochar to enhance P availability in the black soils of Northeast China.

Thinning is an important management practice for reducing plant competition and improving wood production in forests. The residues from thinning can contain large amounts of carbon (C) and nitrogen (N), and the management methods applied directly after thinning can affect the input of nutrients to soil, change the availability of substrates to soil bacterial communities, and thus affect soil bacterial community structure. Our objective was to determine the effects of different thinning residue treatments on soil bacterial community structure and diversity. Illumina high-throughput sequencing technology was used to sequence the bacterial 16SrRNA V3–V4 variable region of the soil (0–10 cm) of a Larix olgensis plantation to compare the composition and diversity of soil bacterial communities following removal of thinning residues (tree stems plus tree crowns) (RM) and retention of thinning residues (crowns retained with stem removal) (RT) treatments. Total soil carbon (TC) and nitrogen (TN) content in the residue retention treatment were significantly greater than in residue removal treatments (p < 0.05). The relative abundance of the dominant soil bacteria phyla were, in descending order: Proteobacteria, Verrucomicrobia, Acidobacteria, Chloroflexi, Actinobacteria, Nitrospirae, Planctomycetes, Gemmatimonadetes, and Bacteroidetes, with a total relative abundance of more than 80%. Acidobacteria were enriched in the RM treatment, while Proteobateria, Actinobacteria and Bacteroidetes were greater in the RT treatment. Rhizobiales and Rhodospirillales (belonging to the α-Proteobacteria) were enriched in the RM treatment. Soil bacteria α diversity was not significantly different among different treatments. Spearman correlation analysis showed that the α diversity index was significantly negatively correlated with TC and TN. Lefse analysis revealed that 42 significant soil bacteria from phylum to genus were found in the two different thinning residue treatments. Redundancy analysis showed that soil TC and TN were the major drivers of variation in soil bacterial community structure. Overall, thinning residue retention increased the availability of resources to the soil bacterial community, thus changing bacterial community structure. This research provides a theoretical basis for the regulation of plantation forest soil fertility and quality.