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PurposeSoil microbial biomass (SMB), as the source and sink of soil nutrients, and its stoichiometry play a key role in soil organic carbon (SOC) and nitrogen (N) mineralization. The objective of this study was to investigate the responses of SOC and N mineralization to changes in microbial biomass and SOC, N, and phosphorus (P) stoichiometry resulted from long-term fertilization regimes.Materials and methodsSoil was sampled from a rice-wheat rotation system subjected to 37 years of nine fertilization treatments with different nutrient input amounts: control (CK), N alone, N combined with mineral phosphorus (NP), NP plus potassium (NPK), manure alone (M), and M combined with N (MN), NP (MNP), NPK (MNPK), and a higher rate of M with NPK (hMNPK). The sampled soil was incubated for the determination of SOC and N mineralization, C, N, and P stoichiometry of soil and SMB, and associated soil enzymes related to C and N cycling.Results and discussionRelative to the CK and treatments with mineral fertilizers, treatments with manure (M, MN, MNP, MNPK, and hMNPK) significantly increased SOC and N mineralization by 48–78% and 54–97%, respectively. Microbial metabolic quotient (qCO2) decreased by 32–55% in treatments with manure compared to the N and NP treatments, but showed no effect on the qCO2 when compared to the NPK treatment. The leucine amino peptidase (LAP) enzyme showed significant positive correlation with SOC and N mineralization, and negatively related to the qCO2. Significantly negative correlations were also observed between SOC and N mineralization and soil C:P and N:P ratio, as well as microbial biomass SMBC:SMBP and SMBN:SMBP stoichiometry, respectively. However, the availability of N and P had limited effects on the qCO2 after reaching a certain value (0.69–0.72 mg CO2-C g−1 MBC h−1).ConclusionsLower soil elemental (C:P and N:P) and microbial biomass stoichiometry (SMBC:SMBP and SMBN:SMBP) and increase of LAP resulted from combined application of manure and mineral fertilizers, accelerated SOC, and N mineralization. Mineral nutrient input with manure amendments could be an optimal strategy to meet the microbial stoichiometric demands and enhance nutrient availability for crops in agricultural ecosystems.

PurposeThe multiple benefits of biochar use as a soil amendment has garnered global attention. Biochar addition is a crucial factor to improve soil biomass, soil enzyme activities, microbial biomass and improve soil nutrient utilization rate. However, the precise mechanism of effects of biochar addition on microbial community structure and diversity, as well as enzyme activity, remains unclear, especially for biochar obtained from different pyrolysis temperatures and variable quantities in which it is applied to soil.Materials and methodsWe compiled and summarized the existing literature on the impacts of biochar on microorganisms and enzymes, with a specific on articles published over a five-year period (2018–2022). This review provides a comprehensive review of the relevant literature on enzyme activity, microbial diversity, community structure and abundance following biochar amendment in soil, and further elucidates the underlying mechanisms of biochar-induced effects on various factors.Results and discussionThe impact of biochar on soil microorganisms could be categorized into three aspects: (1) biochar, due to its porous structure and high surface area, functions as a sanctuary for soil microorganisms; (2) biochar provides essential elements such as carbon (C) and nitrogen (N) sources to soil microorganisms, and finally (3) biochar improves the survival conditions of soil microorganisms by modifying soil pH, CEC, aggregation, and enzyme activity. Importantly, biochar produced at lower pyrolysis temperatures provides valuable C and N for soil microorganisms. Whereas biochar obtained at higher pyrolysis temperatures contains much less active C and N. However, it still contributes to microbial nutrition through diverse mechanisms, e.g., nutrient immobilization and increased nutrients residence time through its bonding with soil labile C.ConclusionsThis review found that the type of source material and pyrolysis temperature were the primary determinants in the impacts of biochar on soil microbial abundance, community structure, and diversity.

Purpose Soil acidification from chemical N fertilization has worsened and is a major yield-limiting factor in the red soil (Ferralic Cambisol) of southern China. Assessment of the acidification process under field conditions over a long term is essential to develop strategies for maintaining soil productivity. The objective of this study was to quantify soil acidification rates from chemical fertilizers and determine the amount of manure needed to inhibit the acidification process. Materials and methods A long-term experiment with various fertilizations was carried out during 1990–2008 in a wheat–corn cropping system in the red soil of southern China. Treatments included non-fertilized control, chemical N only (N), chemical N and P (NP), chemical N, P and K (NPK), pig manure only (M), and NPK plus M (NPKM; 70 % total N from M). All N treatments had an input of 300 kg N ha −1  year −1 . Annual soil sampling was carried out for pH measurement and acidity analysis. Results and discussion Soil pH decreased sharply from an initial pH of 5.7 and then stabilized after 8 to 12 years of fertilization in the N, NP, and NPK treatments with a final pH of 4.2, 4.5, and 4.5, respectively. These three treatments significantly increased soil exchangeable acidity dominated by Al, decreased soil exchangeable base cations (Ca 2+ and Mg 2+ ), and elevated acidification rates (3.2–3.9 kmol H + ha −1  year −1 ). In contrast, the manure applications (M or NPKM) showed either an increase or no change in soil pH and increases in soil exchangeable base cations. Conclusions Urea application to the intensive cropping system accelerated acidification of the red soil during the 18-year field experiment. As 70 % or more total N source, continuous manure application can fully prevent or reverse red soil acidification process. As an effective animal waste management tool, manure incorporation into the acidic soil can promote the overall agricultural sustainability.

PurposeSoil organic carbon (SOC) is an important parameter determining soil fertility and sustaining soil health. How C, N, and P contents and their stoichiometric ratios (C/N/P) regulate the nutrient availability, and SOC stabilization mechanisms have not been comprehensively explored, especially in response to long-term fertilization. The present study aimed to determine how the long-term mineral and manure fertilization influenced soil C/N/P ratios and various protection mechanisms underlying the stabilization of OC along with profile in a cropland soil.Materials and methodsThe soil was sampled from five depths, viz., 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, and 80–100 cm, from plots comprising wheat-maize-soybean rotation system subjected to the long-term (35 years) manure and mineral fertilizer applications.Results and discussionResults revealed that the soil C, N, P stoichiometry and their contents in topsoil depths (0–20 and 20–40 cm) and subsoil depths (40–60, 60–80, and 80–100 cm) varied significantly (p < 0.01) among the soil layers. Compared with CK, the C, N, and P contents were significantly higher (p < 0.05) in NPKM in the topsoil layers, while M alone increased these contents throughout the subsoil. Overall, the C, N, and P contents and their stoichiometry decreased with the increase in depth. Regression analysis showed that C/N, C/P, and N/P ratios associated significantly with the OC fractions in the topsoil layers only. These negative correlations indicated that these ratios significantly influence the C stabilization in the surface layers. However, the results warrant further investigations to study the relationship between soil and microbial stoichiometry and SOC at various depths.ConclusionsLong-term manure applications improved the C sequestration not only in the topsoil but also in the deep layers; hence, these facts can be considered relevant for fertilizer recommendations in cropping systems across China.

PurposeThe stability of soil organic matter is a key predictor of changes in management practices due to the progressive decomposition of organic compounds. However, the dynamics of soil compounds and to what extent the composition of microbes controls the process are still unclear.MethodsBlack soils from Northeast China without fertilizer (CK), soil treated with chemical fertilizer (NPK), NPK plus maize straw (NPKS), and NPK plus cattle manure (NPKM) were collected and separated into the labile fraction (macroaggregates > 150 μm) and the recalcitrant fraction (microaggregates < 150 μm and mineral fraction > 150 μm) for a 14-day incubation.ResultsThe net carbon (C) and net nitrogen (N) mineralization potentials of the labile fraction were 15.87–26.11 and 0.74–1.48 mg kg−1 soil day−1, respectively, which were between 1.4 and 2.3 times higher than those of the recalcitrant fraction. Compared with CK, NPKS and NPKM significantly increased C and N mineralization in the labile fraction but not in the recalcitrant fraction. Boosting regression tree analysis suggested that total nitrogen (TN), microbial biomass nitrogen (MBN), and bacterial abundance accounted for 48.9%, 31.7%, and 14.0% of the total net C mineralization, respectively. Additionally, soil organic carbon (SOC), microbial biomass carbon (MBC), and bacterial abundance accounted for 39.4%, 37.3%, and 22.1% of the total net N mineralization, respectively. Path analysis showed that TN, MBN, and Brevundimonas positively influenced soil net C mineralization. SOC and MBC positively affected soil net N mineralization, whereas unclassified Comamonadaceae had a negative impact.ConclusionsThe mineralization of soil C and N was affected by the inputs of external inorganic nutrients and organic materials and was attributed to the bacterial community, in which Brevundimonas positively responded and contributed to mineralization, while unclassified Comamonadaceae responded negatively.

Purpose The application of bio-fertilizers is one of the management practices that can help to maintain or increase the content of organic matter (OM) and improve soil fertility in arable soils. While some results have been obtained in relation to the influence of bio-fertilizers on organic matter content, less in known about the fractional composition of humus. Materials and methods The aim of this study was to determine the effects of the bio-fertilizer UGmax on soil total organic carbon (TOC), dissolved organic carbon (DOC), and the fractional composition of organic matter (C of humic acids (CHAs), C of fulvic acids (CFAs), and C in humins) in the humus horizon of an arable field. Measurements were taken in 2005 before the application of UGmax and in 2008, 3 years after its application, which was done in 2005, 2006, and 2007. Forty soil samples were taken in 2005 (the control year without UGmax), while 20 samples were taken after UGmax treatment and 20 from the control in 2008. Samples were always collected after the plants were harvested. Results and discussion After the 3-year period of the experiment, the TOC content was 6.3 % higher in plots on which UGmax was applied in comparison to the control, while the DOC content was 0.19 percentage points lower after 3 years of bio-fertilizer use as compared to the initial year of the experiment. The contribution of DOC to TOC decreased significantly after the application of UGmax in comparison with the control. The content of CFAs and its contribution in the TOC pools in soil without UGmax was higher at the end of the experiment compared to the beginning, while there was an inverse relationship in the soil with the bio-fertilizer. In comparison with the control, organic matter in the soil treated with UGmax had a higher content of C of humic acids, C in humins, and higher CHAs/CFAs ratio. Conclusions We conclude that the use of a bio-fertilizer that increases the stable fractions of organic matter provides evidence of an increase in the soil OM stability. In turn, the contribution of the organic matter fractions that are more resistant to decomposition is crucial for increasing soil carbon sequestration.

PurposeThis study aims to explore the spatio-temporal variation of soil nutrients as well as soil organic matter (SOM), and clarify the role of environmental and soil management factors in determining soil nutrients and SOM in farmland over Jiangxi Province of Southern China.Materials and methodsBetween 2005 and 2012, we collected 16,504 surface soil samples (0–20 cm) from farmland across Jiangxi Province. Based on this soil dataset, we summarized the changes in SOM, alkali-hydrolyzable nitrogen (available N), available phosphorus (P), available potassium (K), pH, and cation exchange capacity. Then, we used the geostatistical method to explore and map the spatio-temporal variability of SOM, and available N, P, and K. Finally, the random forest algorithm was used to identify the main factors controlling the variation of SOM and available N, P, and K.Results and discussionOur results revealed a clear right-skewed trend for the histogram of available P and K, pH, and cation exchange capacity in farmland soil of Jiangxi Province. From 2005 to 2012, the average concentrations of SOM and available P showed an insignificant decreasing temporal trend in the farmland of Jiangxi Province. The average concentrations of available N and available K showed a significant increasing trend between 2005 and 2012. In general, most of the soil samples had SOM, available N and P content at or above the level of Class 3 (high grade), and available K at or below the level of class 4 (moderate grade). The apparent lack of K fertility was detected. Regarding the spatio-temporal variation pattern, noticeable changes in the concentrations of SOM and available N, P, and K were detected in most of the region in 2012 when compared with 2005.ConclusionFarmland soils in Jiangxi Province had good fertility, and soil nutrients and SOM in farmland showed strong spatial variability. Overall, the climate (e.g., mean annual precipitation and mean annual temperature) and soil management (e.g., straw return and chemical fertilizer application) had dominant effects on soil nutrients and SOM, while other factors such as relief and soil properties had slight effect. The straw return is a sustainable way to improve soil fertility. Moreover, soil pH has a slight impact on soil nutrients and SOM. Great efforts are needed to prevent farmland soils from further acidification.

PurposeAccurately assessing soil organic carbon (SOC) content is vital for ecosystem services management and addressing global climate challenges. This study undertakes a comprehensive bibliometric analysis of global estimates for SOC using remote sensing (RS) and machine learning (ML) techniques. It showcases the historical growth and thematic evolution in SOC research, aiming to amplify the understanding of SOC estimation themes and provide scientific support for climate change adaptation and mitigation.Materials and MethodsEmploying extensive literature database analysis, bibliometric network analysis, and clustering techniques, the study reviews 1,761 articles on SOC estimation using RS technologies and 490 articles on SOC employing both RS and ML technologies.Results and DiscussionThe results indicate that satellite-based RS, particularly the Landsat series, is predominant for estimation of SOC and other associated studies, with North America, China, and Europe leading in evaluations with Africa is having low evaluations adopting RS technology. Trends in the research demonstrate an evolution from basic mapping to advanced topics such as carbon (C) sequestration, complex modeling, and big data utilization. Thematic clusters from co-occurrence analysis suggest the interplay between technology development, environmental surveys, soil properties, and climate dynamics.ConclusionThe study highlights the synergy between RS and ML, with advanced ML techniques proving to be critical for accurate SOC estimation. These findings are crucial for comprehensive ecosystem SOC estimation, informed environmental management and strategic decision-making.

PurposeSoil microbial biomass and extracellular enzymes are involved in the decomposition of soil organic matter, a temporary nutrient pool for plants. This study was conducted to evaluate the mineralization potentials of carbon (CMP) and nitrogen (NMP), and the relationship of CMP and NMP with microbial biomass and extracellular enzymes in soil fertilized with mineral and manure amendments for 3 decades.Materials and methodsA long-term experiment (37 years) consisted of seven fertilizer treatments, including unfertilized control (CK); nitrogen, phosphorus, and potassium (NPK); manure (M); manure plus phosphorus and potassium (PKM); nitrogen and potassium plus manure (NKM); nitrogen and phosphorus plus manure (NPM); and NPK plus manure (NPKM), was selected.Results and discussionCompared to the CK treatment, the long-term mineral fertilizer (NPK) and combined manure plus mineral fertilization (e.g., M, PKM, NKM, NPM, and NPKM) significantly increased soil microbial biomass carbon (SMBC) by 58% and 131% and nitrogen (SMBN) by 66% and 161%, respectively. Similarly, soil microbial biomass phosphorus (SMBP) also significantly increased by 67–156% in combined manure and mineral fertilizer treatments than the CK and NPK treatment. Higher values of β-glucosidase and cellobiohydrolase activities were observed with the application of PKM. In comparison with CK, the application of combined manure and mineral fertilization significantly increased the leucine aminopeptidase and N-acetyl-glucosaminidase enzyme by 129% and 175%, respectively. Soil CMP and NMP significantly increased by 231–307% and 176–289% in combined treatments than the CK. A significant positive correlation was also observed between the microbial biomass, extracellular enzymes, and the CMP and NMP (R2 = 0.66–0.84, 0.44–0.76, P < 0.0001).ConclusionsThis study suggested that long-term manure application greatly increased the microbial biomass and enzyme activities, playing an important role in co-mineralization potentials of organic carbon and nitrogen for improving soil fertility.

PurposeClimate warming and sea level rise have the potential to change the salt level of soil in tidal wetlands. And it is important to clarify the process and the mechanism of decomposition of soil organic carbon in a tidal wetland under varying salinities. The aim of this study was to evaluate the impacts of soil salinity on the decomposition rate of organic carbon (DROC) and dissolved organic carbon (DOC) in a tidal wetland.Materials and methodsTwo types of soil (surface soil in Suaeda salsa and bare tidal flat) were collected, air-dried, and homogenized. After adding different content of salt (0 g/L, 3 g/L, 6 g/L, 9 g/L, and 12 g/L), the soils were incubated for 28 days at stable room temperature (25 ± 2 °C) and added by deionized water to maintain the stability of soil moisture. The gases (CO2 and CH4) emission rates of each salt treatment were measured during 28-day incubation. DROC was determined by the sum of daily CO2-C emission rates and daily CH4-C emission rates in this study.Results and discussionSalt addition inhibited the process of gas emissions and DROC. Gases emission rates and DROC of two types of soil showed similar temporal trends, with distinctive drop in the beginning of experiment and no significant decrease followed. Significant difference of DOC among salt treatments was found in the bare tidal flat soil. Variations of partial correlation between DROC and soil salinity and DOC showed similar trends (e.g., in days 9–18, the positive effect of DOC on DROC was greatly promoted (R2 = 0.80, p < 0.001), and the negative effect of soil salinity was highly improved (R2 = 0.93, p < 0.001)). Soil properties, in particular DOC, may be primary factors accounting for the discrepancy of gases emission rates and DROC of two types of soil.ConclusionsIncreased soil salinity had a negative effect on DROC during 28-day incubation. The impact of soil salinity and DOC on DROC were varied in different phases of laboratory experiment (soil salinity generally had increasingly negative relationship with DROC, but DOC showed most significantly positive relationship in the middle stage of incubation). Both the formation and consumption of DOC may be valuable for more detail research regarding to decomposition of soil organic carbon.