Inversion tillage with straw amendment is widely applied in northeastern China, and it can substantially increase the storage of carbon and improve multiple subsoil functions. Soil microorganisms are believed to be the key to this process, but research into their role in subsoil amelioration is limited. Therefore, a field experiment was conducted in 2018 in a region in northeastern China with Hapli-Udic Cambisol using four treatments: conventional tillage (CT, tillage to a depth of 15 cm with no straw incorporation), straw incorporation with conventional tillage (SCT, tillage to a depth of 15 cm), inversion tillage (IT, tillage to a depth of 35 cm) and straw incorporation with inversion tillage (SIT, tillage to a depth of 35 cm). The soils were managed by inversion to a depth of 15 or 35 cm every year after harvest. The results indicated that SIT improved soil multi-nutrient cycling variables and increased the availability of key nutrients such as soil organic carbon, total nitrogen, available nitrogen, available phosphorus and available potassium in both the topsoil and subsoil. In contrast to CT and SCT, SIT created a looser microbial network structure but with highly centralized clusters by reducing the topological properties of average connectivity and node number, and by increasing the average path length and the modularity. A Random Forest analysis found that the average path length and the clustering coefficient were the main determinants of soil multi-nutrient cycling. These findings suggested that SIT can be an effective option for improving soil multi-nutrient cycling and the structure of microbial networks, and they provide crucial information about the microbial strategies that drive the decomposition of straw in Hapli-Udic Cambisol.

Although returning crop residue to fields is a recommended measure for improving soil carbon (C) stocks in agroecosystems, the response of newly formed soil C (NFC) to the integrated supply of residue and nutrients and the microbial mechanisms have not been fully understood. Therefore, an 84-day incubation experiment was conducted to ascertain the microbial mechanisms that underpin the NFC response to inputs of residue and nitrogen (N), phosphorus (P), and sulfur (S) in two black soils. The results showed that adding residue alone accelerated microbial nutrient mining, which was supported by decreases of 8–16% in the ratios of C:N and C:P enzyme activities (relative to soils with nutrient inputs). The NFC amounts increased from 1155.9 to 1722.4 mg kg⁻¹ soil in Gongzhuling and increased from 725.1 to 1067.5 mg kg⁻¹ soil in Hailun as the levels of nutrient supplementation increased. Boosted regression tree analysis suggested that β-glucosidase (BG), acid phosphatase (AP), microbial biomass C (MBC), and Acidobacteria accounted for 27.8, 18.5, 14.7, and 8.1%, respectively, of the NFC in Gongzhuling and accounted for 25.9, 29.5, 10.1, and 13.9%, respectively, of the NFC in Hailun. Path analysis determined that Acidobacteria positively influenced NFC both directly and indirectly by regulating BG, AP, and MBC, in which MBC acquisition was regulated more by AP. The intensity of NFC was lower in Hailun soil than in Gongzhuling soil and was directly affected by AP, thereby indicating the importance of soil status (e.g., SOC and pH) in determining NFC. Overall, our results reveal the response of NFC to supplementation by N, P, and S, which depends on Acidobacteria and Proteobacteria, and their investment in BG and AP in residue-amended soil.

Certain agricultural management practices are known to affect the soil microbial community structure; however, knowledge of the response of the fungal community structure to the long-term continuous cropping and rotation of soybean, maize and wheat in the same agroecosystem is limited.  We assessed the fungal abundance, composition and diversity among soybean rotation, maize rotation and wheat rotation systems and among long-term continuous cropping systems of soybean, maize and wheat as the effect of crop types on fungal community structure.  We compared these fungal parameters of same crop between long-term crop rotation and continuous cropping systems as the effect of cropping systems on fungal community structure.  The fungal abundance and composition were measured by quantitative real-time PCR and Illumina MiSeq sequencing.  The results revealed that long-term continuous soybean cropping increased the soil fungal abundance compared with soybean rotation, and the fungal abundance was decreased in long-term continuous maize cropping compared with maize rotation.  The long-term continuous soybean cropping also exhibited increased soil fungal diversity.  The variation in the fungal community structure among the three crops was greater than that between long-term continuous cropping and rotation cropping.  Mortierella, Guehomyces and Alternaria were the most important contributors to the dissimilarity of the fungal communities between the continuous cropping and rotation cropping of soybean, maize and wheat.  There were 11 potential pathogen and 11 potential biocontrol fungi identified, and the relative abundance of most of the potential pathogenic fungi increased during the long-term continuous cropping of all three crops.  The relative abundance of most biocontrol fungi increased in long-term continuous soybean cropping but decreased in long-term continuous maize and wheat cropping.  Our results indicate that the response of the soil fungal community structure to long-term continuous cropping varies based upon crop types.

Black soil is one of the most precious soil resources on earth because it has abundant carbon stocks and a relatively high production capacity.  However, decreasing organic matter after land reclamation, and the effects of long-term inputs of organic carbon have made it less fertile black soil in Northeast China.  Straw return could be an effective method for improving soil organic carbon (SOC) sequestration in black soils.  The objective of this study was to evaluate whether straw return effectively increases SOC sequestration.  Long-term field experiments were conducted at three sites in Northeast China with varying latitudes and SOC densities.  Study plots were subjected to three treatments: no fertilization (CK); inorganic fertilization (NPK); and NPK plus straw return (NPKS).  The results showed that the SOC stocks resulting from NPKS treatment were 4.0 and 5.7% higher than those from NPK treatment at two sites, but straw return did not significantly affect the SOC stocks at the third site.  Furthermore, at higher SOC densities, the NPKS treatment resulted in significantly higher soil carbon sequestration rates (CSR) than the NPK treatment.  The equilibrium value of the CSR for the NPKS treatment equated to cultivation times of 17, 11, and 8 years at the different sites.  Straw return did not significantly increase the SOC stocks in regions with low SOC densities, but did enhance the C pool in regions with high SOC densities.  These results show that there is strong regional variation in the effects of straw return on the SOC stocks in black soil in Northeast China.  Additional cultivations and fertilization practices should be used when straw return is considered as an approach for the long-term improvement of the soil organic carbon pool.

Long-term continuous cropping of soybean (Glycine max), spring wheat (Triticum aesativum) and maize (Zea mays) is widely practiced by local farmers in northeast China. A field experiment (started in 1991) was used to investigate the differences in soil carbon dioxide (CO2) emissions under continuous cropping of the three major crops and to evaluate the relationships between CO2 fluxes and soil temperature and moisture for Mollisols in northeast China. Soil CO2 emissions were measured using a closed-chamber method during the growing season in 2011. No remarkable differences in soil organic carbon were found among the cropping systems (P>0.05). However, significant differences in CO2 emissions from soils were observed among the three cropping systems (P<0.05). Over the course of the entire growing season, cumulative soil CO2 emissions under different cropping systems were in the following order: continuous maize ((829±10) g CO2 m-2)>continuous wheat ((629±22) g CO2 m-2)>continuous soybean ((474±30) g CO2 m-2). Soil temperature explained 42-65% of the seasonal variations in soil CO2 flux, with a Q10 between 1.63 and 2.31; water-filled pore space explained 25-47% of the seasonal variations in soil CO2 flux. A multiple regression model including both soil temperature (T, °C) and water-filled pore space (W, %), log(f)=a+bT log(W), was established, accounting for 51-66% of the seasonal variations in soil CO2 flux. The results suggest that soil CO2 emissions and their Q10 values under a continuous cropping system largely depend on crop types in Mollisols of Northeast China.