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The incorporation of earthworms and straw into cropland soil can greatly affect soil structure and soil organic C (SOC) dynamics. However, information regarding how earthworms affect both carbon (C) in its transition from residue into soil and SOC turnover under conventional tillage (CT) and conservation tillage is limited, and some results are even contradictory. Moreover, most of studies were conducted out based on sieved soil samples. In this study, intact soil samples were used to analyze the effects of earthworms (Eisenia foetida) and both of earthworms and straw addition on SOC fractions and soil CO2 emission in a 30-day incubation experiment with CT and no tillage (NT) treatments. The results showed that CT accelerated the straw decomposition and increased the content of soil active C. In contrast, NT resulted in slower decomposition of the original straw and reduction of the total amount of SOC and soil active C. The earthworms could inhibit the straw from decomposing in CT and stimulate the decomposition of straw in NT. Whatever in NT or CT, earthworms did not affect soil CO2 emission, but straw addition significantly increased soil CO2 emission. The results of this study provide the selection of the appropriate tillage practice as well as the amount of returned straw to fertilize the soil.

Limitations of traditional measurement methods have impeded progress in understanding the role of soil aggregation in protecting soil organic carbon (SOC) from decomposition by soil microbes living in pore spaces. In this paper, we used the Scanning Electronic Microscope (SEM) and X-ray micro Computed Tomography (micro-CT) to study the relationships of the aggregate pore structure and microbial distribution in the interior and exterior of soil aggregates, and thereby gained an insight into protection of carbon within macroaggregates of an undisturbed Mollisol in northeastern China. There were close relationships between soil pore structure and distribution of soil microbes and soil organic carbon (SOC), but they were different on the exterior and interior of soil aggregate. On the exterior of macroaggregates, there were negative relationships between soil porosities, the number of pores and SOC, especially for soil pores in the 10–30 μm and 30–100 μm classes, indicating these two pore sizes are unlikely to help sequester C. In contrast, there was a positive correlation between soil pores > 100 μm and SOC. Furthermore, soil pore structure had no impact on soil microbial biomass and density or on SOC contents in the interior of soil aggregates. This study provides a new method by combining SEM with micro-CT technology for linking soil structure and soil microbial properties with C sequestration and SOC changes.

Soil erosion is a complex process which causes loss of soil mineral material together with soil organic carbon (SOC). Little is known about the impact of soil water erosion on catchment exports of soil aggregates and associated SOC, especially at a small agricultural catchment. SOC delivered from a small catchment located in Northeast China was assessed for three erosive rainfall events in 2011. The quantity and size of the exported soil aggregates and their concentration of SOC were assessed along with runoff rate, sediment loss and rainfall characteristics to further the understanding of SOC erosion and its potential fate. Aggregate size distribution in sediments was different from that in source soil and was significantly correlated to sediment concentration in runoff. Fine size aggregate fractions in the sediments were the most abundant. The SOC was enriched in the sediments and indicated a high selectivity of the erosion processes. The SOC concentration differed among different sizes of aggregate fractions with coarse size aggregate fractions generally having the largest concentration. We concluded that the soil water erosion processes impact the aggregate size distribution and SOC enrichment in the delivered sediment, which subsequently influence the catchment exports of SOC. Additional measurements over several rainy seasons are required to further quantify the impact of soil water erosion on catchment exports of soil aggregates and associated SOC.

Selecting appropriate soil management practices (e.g. tillage treatments) provides an opportunity to sequester soil carbon (C), improve soil health, and achieve ample food production in an agricultural system. The responses of soil CO2 emission, soil microbial community, and crop yield to precipitation are crucial for assessing sustainability of conservation tillage. Soil CO2 flux, soil microbial phospholipid fatty acid (PLFA) contents and crop yield were measured in a long-term (15 years) tillage study in the Northeast of China under a normal year in 2013 and natural drought year in 2015. The results showed in a normal year with precipitation similar to the 30-year mean, soil temperatures at 5, 10 and 20 cm in June under mouldboard plough (MP) were higher by 0.9, 1.0 and 1.1 °C, respectively, than under no-tillage (NT); soil water content in the top 22 cm under MP was significantly lower than that under NT in 04 June and 31 July; and MP significantly increased maize (Zea mays L.) yield and C emission efficiency (the ratio of yield to annual soil CO2 emission) by 22% and 25%, respectively. In a dry year (2015), soil temperatures at 5, 10, and 20 cm in June and July under MP were higher by 2.0, 1.8 and 1.7 °C than under NT, respectively; soil water content in the top 22 cm under MP was significantly lower than under NT by 28% in July; moreover, MP significantly decreased maize yield and C emission efficiency by 47% and 63%, respectively. However, there were no significant differences in annual soil CO2 emission between NT and MP in the normal and dry years. Compared with MP, NT significantly increased soil microbial PLFA contents in pre-planting (April) and late segments of the growing season (August-September) of a normal year, but significantly increased soil microbial PLFAs content in pre-planting and all of early, mid and late segments of the growing season in a dry year. Structural equation modelling (SEM) revealed that soil water content, temperature, bacteria and fungi directly contributed to C emission efficiency. Tillage was indirectly associated with C emission efficiency through soil water content, temperature, soil fungi and bacteria. These results suggested that the higher maize yield and C emission efficiency under NT compared to CT were related to improved soil water content and soil microbial PLFA contents and the lower soil temperature in a dry year. Our results suggested that NT (a subset of conservation tillage) might be a positive adaptation strategy to cope with drought under monoculture maize in northeast China.

Gamma irradiation is becoming a promising technique in soil ecological studies because it has a particular advantage in selectively eliminating the target organism. But this selective sterilization technique is still in its initial exploratory stage and the subsequent impacts on soil carbon (C) and nitrogen (N) pools are relatively unknown for the majority of soils. Therefore, the responses of soil respiration, soil collembola, nematodes and microbial communities, and soil C and N pools (extracted with KCl, K 2 SO 4 and H 2 SO 4 ) to a range of gamma irradiation doses (0, 5, 10, 20 and 40 kGy) were determined under a clay loam soil in a 4-week incubation study. A flush of CO 2 was observed at the beginning of the incubation period (1–2 days) post-irradiation, and then strongly decreased relative to the unirradiated soils. At the middle of incubation period (11–14 days), there was a recovery of CO 2 efflux in the 5 kGy treatment. The effects of radiation on biological communities were dependent on taxa groups. The majority of collembola (>80%) and nematodes (>90%) were killed immediately in the higher doses (>5 kGy), but at lower doses of 5 kGy they were killed within 2 weeks after irradiation. The relative abundance of saprophytic fungi and protozoa decreased with increasing irradiation dose throughout the incubation period, while an opposite trend was found for some special bacterial taxa (19:1ω8c and i17:1ω9c). The resistance threshold of the entire microbial community to gamma irradiation was 10 kGy. The C and N contents in the KCl/K 2 SO 4 -extracted pools (except the dissolved organic C), in the H 2 SO 4 -extracted labile pool II (LP II) and in the recalcitrant pool (RP) decreased with increasing irradiation dose across the incubation period. The decreases in LP II and RP were accompanied by the increase in labile pool I. The variation in the level of all soil C and N pools was significantly affected by the radiation doses higher than 5 kGy. Our results indicate that the gamma dose between 5 and 10 kGy is sufficient to eliminate soil fauna without significant effect on microbial community compared to the unirradiated treatments. Moreover, a radiation dose of 5 kGy for selective defaunation has minor impacts on soil C and N pools of a clay loam soil. Our results also suggest that dose optimization is necessary due to high variability associated with irradiation levels effect on biological taxa in different soils.

Soil erosion is a serious problem around the world, in addition to soil loss, erosion is also considered to have a significant impact on SOC dynamics and CO2 release to the atmosphere. There is disagreement on the overall effect of soil erosion on CO2 emission at the landscape level. We measured the proportion of various aggregate size fractions at different slope positions which were affected by erosion. The cumulative CO2 emitted from bulk soil samples and different aggregate sizes fractions from five slope positions in a landscape was examined in a 128-d incubation study to estimate how the breakdown and dispersion of soil aggregates by erosion affect C loss to the atmosphere via CO2 emission. The proportion of coarse size aggregate fractions (> 0.25 mm) at eroded slope positions was greater than that at depositional positions. Bulk soil samples at the summit emitted the greatest cumulative CO2-C (0.49 ± 0.04 g C kg− 1 soil) among all the slope positions. During the initial 22 days of incubation, the CO2 emission rate from coarse size aggregate fractions (0.024 ± 0.009 g C kg− 1 soil d− 1) was six times higher than that from small size aggregate fractions (0.0038 ± 0.0011 g C kg− 1 soil d− 1) at the depositional toe-slope position. The CO2 emission rate from coarse size aggregate fractions at depositional slope positions (toe-slope and foot-slope) was significantly greater than that at eroded slope positions (summit, shoulder-slope and back-slope). We concluded that the breakdown and dispersion of aggregates by erosion impacts both aggregate size distribution and CO2 emission from the aggregates at different slope positions, thus, affects the C loss to the atmosphere.

Our understanding of seasonal dynamics of fungal communities under different agricultural management practices remains limited. In this study, phospholipid fatty acid (PLFA) analysis and Illumina MiSeq high-throughput sequencing targeting fungal ITS1 region were combined to assess the responses of soil fungal communities to different tillage practices and cropping systems during the crop growing season. Bulk soils were collected from two long-term tillage practices (conventional tillage, CT and no-tillage, NT) under two cropping systems (maize (Zea mays L.)-soybean (Glycine max Merr.) rotation and continuous maize) in April, June and August of 2015 at 0–5 cm and 5–15 cm depths. Our results clearly highlighted the importance of cropping system and tillage for soil fungi; seasonal effect, by contrast, was weaker. The fungal PLFA content, used as an indicator of fungal biomass, appeared higher under continuous maize system than maize-soybean rotation system at depths of 0–5 cm and 5–15 cm, and higher in NT than CT at 0–5 cm depth, irrespective of sampling time. DNA-based sequencing results showed that soil fungal diversity (i.e. Shannon) and evenness (i.e. Pielou) indices were higher under rotation cropping system than continuous maize system at both soil depths. Soil fungal richness (i.e. Chao1) index was higher in NT than in CT at 0–5 cm depth. None of these three α diversity indices were significantly affected by sampling time. Cropping system and tillage played the most important roles in shaping soil fungal community at both soil depths. At 0–5 cm depth, there was an obvious consistent trend of enrichment from Ascomycota in maize-soybean rotation system to Basidiomycota in continuous maize system over the growing season. At 5–15 cm depth, there was an obvious consistent trend of enrichment from Tausonia and Chaetomidium in CT to Leptodontidium, Ceratobasidium in NT over the crop growing season. Notably NT favored symbiotrophs throughout the growing season, irrespective of cropping system and soil depth.

Soil structure plays a key role in soil organic carbon (SOC) dynamics. To determine how soil structure and aggregate affects SOC, we collected undisturbed soil cores of 0-5 cm layer (Typic Hapludoll) at an experimental site in Northeast China. The site had been under continuous tillage treatments of conventional tillage (CT) and no tillage (NT) for 17 years. We measured SOC by elemental analysis, aggregate size distribution by wet sieving, and soil pore parameters of pore size distribution, pore average diameter, pore numbers, pore connectivity, pore anisotropy, and pore fractal dimension by X-ray computer tomography. SOC content was significantly correlated with aggregate-associated SOC and soil water-stable aggregate content. CT with residue removal and annual plowing and cultivation increased53 μm and 53-250 μm aggregates. CT decreased total SOC of 0-5 cm soil layer but increased aggregate-associated SOC of53 μm. NT with greater residue input increased total SOC of 0-5 cm soil layer by 26.0% and aggregate mean weight diameter by 111.8% and increased aggregates of 250-1000 μm and1000 μm. Soil under NT had a greater total number of micropores and greater connectivity whereas CT had a greater total number of macropores, average macropore diameter, anisotropy, and fractal dimension. Structural equation modeling showed that CT can decrease SOC of 0-5 cm soil layer by different paths, including increased anisotropy and macropore porosity, and NT can increase SOC of 0-5 cm soil layer by different paths, including increased mean weight diameter and connectivity. These results enhance our understanding of the relationship between soil structure and SOC, and could guide tillage management decisions to increase SOC.

Tillage practice can alter the content and distribution of soil organic matter by burying crop residue and soil disturbance, which would impact soil microbial properties. The objective of the present study was to evaluate the effects of tillage practices (no tillage, NT; ridge tillage, RT; mouldboard plow, MP) and sampling date (April, June, September) on soil microbial properties at different soil depths. Soil core samples were taken at 0–5, 5–10 and 10–20 cm depths from a long-term tillage experiment site (initiated in 2001) in black soils in northeast China. Microbial biomass and extractable organic C were determined using by chloroform-fumigation-extraction method. Microbial abundance and community structure were determined using the phospholipid fatty acid (PLFA) method. Principal response curve (PRC) analysis revealed the magnitude of the effect of sampling date on the PLFA relative abundance decreased with depth within the tilled layer, whereas increased with depth for tillage practice. Microbial biomass carbon, abundance and the F/B ratio varied with the variation in available substrates at the 0–5 cm depths. NT and RT improved microbial abundance (total, fungal and bacterial abundance) in the 0–5 cm depth soil, but they did not contribute to a higher F/B ratio in the 0–5 cm depth soil, and had a lower F/B ratio than MP in the deeper soils below 5 cm depth. These results demonstrated that long-term conservation tillage practice has potential for improving microbial properties in surface soil, but may not cause a shift of microbial community structure in Northeast China.

The severe soil deterioration and the accompanying decline in maize yield are the main factors jeopardizing the sustainability of agricultural production in the black soil region of Northeast China. Conservation tillage practices have been proposed as new practices to enhance soil fertility and to produce food from a dwindling land resource in this region, but wide adoption of these practices can occur only when the practices demonstrate a steady increase in agricultural production. To compare the effects of tillage on corn yield, the structural equation modeling (SEM) was used in analyzing the relationships between maize growth/yield and soil properties (physical and chemical characteristics) based on a long-term (12 years) tillage study that included no tillage (NT), ridge tillage (RT) and conventional tillage (CT) practices. Compared with CT, NT and RT had higher maize grain yield, soil carbon/nitrogen ratio and soil moisture, and lower soil temperature and seedling emergence rate. The advantages of NT and RT in maize yield were primarily ascribed to the favorable soil nutrients and soil water content, which could increase plant size at the later growing season and ultimately yield more maize grain. The lower soil temperature and subsequently lower emergence rates in NT and RT at early stages (sowing to emergence) of maize growth did not exert negative effects on maize yield. These adverse effects of low soil temperature and low emergence rates could be offset by postponing sowing date without compromising the maize yield. Our study indicated that both NT and RT practices could offer a potentially significant improvement over the current conventional tillage practice in the black soil region of Northeast China.

Soil food webs are important in maintaining agricultural productivity and ecosystem health. However, our understanding is still limited with respect to the influences of tillage transitions on soil food webs. The present study aimed to quantify the response of microflora and microfauna, and their linkage to different tillage treatments: no tillage (NT), ridge tillage (RT) and conventional tillage (CT). Soil samples were collected from 0 to 20 cm depth in April of 2011 after 10 years of conservation tillage. The abundance and richness of bacteria and arbuscular mycorrhizal fungi were greater in NT and RT than in CT. In case of microfauna also, similar patterns were observed with greater protozoa, bacterivores and omnivores–carnivores in NT and RT compared to CT. The connectance of the bacterial and predator–prey pathways was greater in NT and RT than in CT and that of fungal pathway was greatest in RT. The trophic relationship of the bacterial and predator–prey pathways was strengthened due to the higher water content of soil and the lower NO 3 – -N after the conversion of CT to NT and RT. Our study suggested that 10 years of conservation tillage can effectively enhance the structure and function of soil food webs through bottom–up effects in the black soil region of Northeast China.

The CO 2 flux from soils is an important part of the global carbon (C) cycle, whose dependence on erosion is as yet largely unknown. We hypothesized that erosion affects CO 2 flux from the soil surface because of its effects on soil temperature and soil moisture. The CO 2 flux, soil temperature, and soil moisture were monitored on summit, shoulder-slope, back-slope, and toe-slope on a sloping corn field in the black soil zone in northeast China once a day for 47 days between August 3 and September 18. The average CO 2 flux varied significantly with slope positions ranging from 2.5 ± 0.29 (mean ± standard deviation) μmol m −2 s −1 at summit to 1.5 ± 0.32 μmol m −2 s −1 at toe-slope in the initial 27 days, but no significant difference among slope positions was observed in the last 20 days. Soil moisture contributed largely to CO 2 flux in the initial 27 days, and soil temperature became the most important factor affecting CO 2 flux in the last 20 days. In our study, second-order and first order regressions were fit the best models to predict CO 2 flux at various slope positions based, respectively, on soil moisture content and soil temperature, with respective R 2 ranging from 0.532 to 0.661 and 0.234 to 0.448.