Soil microbial biomass nitrogen (MBN) contains the largest proportion of biologically active nitrogen (N) in soil, and is considered as a crucial participant in soil N cycling.  Agronomic management practices such as crop rotation and mono-cropping systems, dramatically affect MBN in agroecosystems.  However, the influence of crop rotation and mono-cropping in agroecosystems on MBN remains unclear.  A meta-analysis based on 203 published studies was conducted to quantify the effect of crop rotation and mono-cropping systems on MBN under synthetic N fertilizer application.  The analysis showed that crop rotation significantly stimulated the response ratio (RR) of MBN to N fertilization and this parameter reached the highest levels in upland-fallow rotations.  Upland mono-cropping did not change the RR of MBN to N application, however, the RR of MBN to N application in paddy mono-cropping increased.  The difference between crop rotation and mono-cropping systems appeared to be due to the various cropping management scenarios, and the pattern, rate and duration of N addition.  Crop rotation systems led to a more positive effect on soil total N (TN) and a smaller reduction in soil pH than mono-cropping systems.  The RR of MBN to N application was positively correlated with the RR of mineral N only in crop rotation systems and with the RR of soil pH only in mono-cropping systems.  Combining the results of Random Forest (RF) model and structural equation model showed that the predominant driving factors of MBN changes in crop rotation systems were soil mineral N and TN, while in mono-cropping systems the main driving factor was soil pH.  Overall, our study indicates that crop rotation can be an effective way to enhance MBN by improving soil N resources, which promote the resistance of MBN to low pH induced by intensive synthetic N fertilizer application.

 

Biochar has been widely applied as a soil amendment in many parts of the world, which can enhance soil organic carbon (SOC) sequestration and nutrient availability beyond the fertilizer effect.  Enriched in poly-condensed aromatic C forms, biochar-C is difficult to be degraded and can hold carbon in soils for hundreds to thousands of years.  Biochar production and incorporation in soil would play critical roles in climate change mitigation.  Over the last decades, numerous researches have highlighted the beneficial roles of biochar in soil amendments.  The specific ecological functions of biochar have not been clearly described because of its diversities and complexities.  For example, biochar helps to ameliorate soil acidity and increase soil aggregated carbon, but the role of microbial interactions between biochar and soil has not been well understood.  In this special issue, we selected four papers contributed by Yuan et?al. (2019), Chen et?al. (2019), Guan et?al. (2019) and Shi et?al. (2019), as a window to reflect the latest research progress of this field in China.  Yuan et?al. (2019) investigated the diversity of microbial carbon use patterns in paddy soils amended with straw or straw-derived biochar in a 3-year field experiment; Chen et?al. (2019) reported that steam explosion of crop straws could improve the characteristics of biochar; Guan et?al. (2019) evaluated the effects of uncharred maize straw and charred maize straw amendments on organic carbon contents in bulk soil and in various soil aggregate-size and density fractions; Shi et?al. (2019) summarized the positive effects and mechanisms involved in the correction of soil acidity, the alleviation of aluminum toxicity and the increase of soil pH buffering capacity by applying crop residue biochar.  Through this special issue, we hope to further boost the basic and applied researches with related to the ecological functions of biochar from different aspects in soil in China and abroad. 

Seven inorganic amendment materials were added into arsenic (As) contaminated soil at a rate of 0.5% (w/w); the materials used were sepiolite, red mud, iron grit, phosphogypsum, ferrihydrite, iron phosphate, and layered double oxides (LDO). Plant growth trials using rape (edible rape, Brassia campestris L.) as a bio-indicator are commonly used to assess As bioavailability in soils. In this study, B. campestris was grown in a contaminated soil for 50 days. All of the inorganic amendments significantly inhibited the uptake of As by B. campestris. Following soil treatment with the seven aforementioned inorganic ammendments, the As concentrations in the edible parts of B. campestris were reduced by 28.6, 10.5, 8.7, 31.0, 47.4, 25.3, and 28.8%, respectively, as compared with the plants grown in control soil. The most effective amendment was ferrihydrite, which reduced As concentration in B. campestris from 1.84 to 0.97 mg kg–1, compared to control. Furthermore, ferrihydrite-treated soils had a remarkable decrease in both non-specifically sorbed As and available-As by 67 and 20%, respectively, comparing to control. Phosphogypsum was the most cost-effective amendment and it showed excellent performance in reducing the water soluble As in soils by 31% and inhibiting As uptake in B. campestris by 21% comparing to control. Additionally, obvious differences in As transfer rates were observed in the various amendments. The seven amendment materials used in this study all showed potential reduction of As bioavailability and influence on plant growth and other biological processes still need to be further explored in the long term.

It remains unclear whether biochar applications to calcareous soils can improve soil fertility and crop yield. A long-term field experiment was established in 2009 so as to determine the effect of biochar on crop yield and soil properties in a calcareous soil. Five treatments were: 1) straw incorporation; 2) straw incorporation with inorganic fertilizer; 3), 4) and 5) straw incorporation with inorganic fertilizer, and biochar at 30, 60, and 90 t ha-1, respectively. The annual yield of either winter wheat or summer maize was not increased significantly following biochar application, whereas the cumulative yield over the first 4 growing seasons was significantly increased. Soil pH, measured in situ, was increased by a maximum of 0.35 units after 2 yr following biochar application. After 3 yr, soil bulk density significantly decreased while soil water holding capacity increased with adding biochar of 90 t ha-1. Alkaline hydrolysable N decreased but exchangeable K increased due to biochar addition. Olsen-P did not change compared to the treatment without biochar. The results suggested that biochar could be used in calcareous soils without yield loss or significant impacts on nutrient availability.

Innovative measure is a urgent requirement for managing the huge volume of municipal sewage sludge. The hydrothermal carbonation (HTC) shows some potential advantages for using hydrochar as a soil conditioner. The aim of this work was to investigate the properties of hydrochars, by means of the HTC of municipal sewage sludge under different temperatures (190 and 260°C) and reaction hours (1, 6, 12, 18 and 24 h). The HTC led to the decreases of N, O and H contents by more than 54.6, 37.9 and 10.0%, respectively, and slight changes of C content. The Py-GC-MS analysis showed that a large proportion of fatty acids, in particular hexadecanoic acid, transferred into alkenes, olefins and aromatic compounds. The 13C-NMR and fourier transform infrared spectra (FTIR) confirmed the transformation and changes in chemical structure in which hydrochar contained lower oxygen-containing organic C of O-alkyl, carboxylic and carbonyl C and aliphaticity, but higher aromatic C and aromaticity. The rich hydrophobic functions induced in high positive charges in the charred sludge. The HTC facilitated the pore structure development, proved by higher specific surface area and specific pore volume, with a maximum of 17.30 and 0.83 cm3 g-1, respectively. The availabilities of N, P and K markedly reduced during HTC treatment. The activities of most heavy metals were depressed though they accumulated in the hydrochar. Further work is required to investigate the values and risk of the charred sludge amended to soil.