Soil type regulates carbon and nitrogen stoichiometry and mineralization following biochar or nitrogen addition

Straw incorporation and nitrogen fertilization regulate soil quality, enzyme activities and maize crop productivity in dual maize cropping system

BACKGROUND

Straw incorporation serves as an effective strategy to enhance soil fertility and soil microbial biomass carbon (SMBC), which in turn improves maize yield and agricultural sustainability. However, our understanding of nitrogen (N) fertilization and straw incorporation into soil microenvironment is still evolving. This study explored the impact of six N fertilization rates (N0, N100, N150, N200, N250, and N300) with and without straw incorporation on soil fertility, SMBC, enzyme activities, and maize yield.

RESULTS

Results showed that both straw management and N fertilization significantly affected soil organic carbon (SOC), total N, SMBC, soil enzyme activities, and maize yield. Specifically, the N250 treatment combined with straw incorporation significantly increased SOC, total N, and SMBC compared to lower fertilization rates. Additionally, enzyme activities such as urease, cellulase, sucrose, catalase, and acid phosphatase reached their peak during the V6 growth stage in the N200 treatment under for both straw management conditions. Compared to N250 and N300 treatments of traditional planting, the N200 treatment with residue incorporation significantly increased yield by 8.30 and 4.22%, respectively. All measured parameters, except for cellulase activity, were significantly higher in spring than in the autumn across both study years, with notable increases observed in 2021.

CONCLUSIONS

These findings suggest that optimal levels of SOC, soil total N (STN), and SMBC, along with increased soil enzyme activities, is crucial for sustaining soil fertility and enhancing maize grain yield under straw incorporation and N200 treatments.

Effects of nitrogen addition on rhizosphere priming: The role of stoichiometric imbalance

Nitrogen (N) input has a significant impact on the availability of carbon (C), nitrogen (N), and phosphorus (P) in the rhizosphere, leading to an imbalanced stoichiometry in microbial demands. This imbalance can result in energy or nutrient limitations, which, in turn, affect C dynamics during plant growth. However, the precise influence of N addition on the C:N:P imbalance ratio and its subsequent effects on rhizosphere priming effects (RPEs) remain unclear. To address this gap, we conducted a 75-day microcosm experiment, varying N addition rates (0, 150, 300 kg N ha-1), to examine how microbes regulate RPE by adapting to stoichiometry and maintaining homeostasis in response to N addition, using the 13C natural method. Our result showed that N input induced a stoichiometric imbalance in C:N:P, leading to P or C limitation for microbes during plant growth. Microbes responded by adjusting enzymatic stoichiometry and functional taxa to preserve homeostasis, thereby modifying the threshold element ratios (TERs) to cope with the C:N:P imbalance. Microbes adapted to the stoichiometric imbalance by reducing TER, which was attributed to a reduction in carbon use efficiency. Consequently, we observed higher RPE under P limitation, whereas the opposite trend was observed under C or N limitation. These results offer novel insights into the microbial regulation of RPE variation under different soil nutrient conditions and contribute to a better understanding of soil C dynamics.

Mineral composition controls the stabilization of microbially derived carbon and nitrogen in soils: Insights from an isotope tracing model

Evidence is emerging that microbial products and residues (necromass) contribute greatly to stable soil organic matter (SOM), which calls for the necessity of separating the microbial necromass from other SOM pools in models. However, the understanding on how microbial necromass stabilizes in soil, especially the mineral protection mechanisms, is still lacking. Here, we incubated 13 C- and 15 N-labelled microbial necromass in a series of artificial soils varying in clay minerals and metal oxides. We found the mineralization, adsorption and desorption rate constants of necromass nitrogen were higher than those of necromass carbon. The accumulation rates of necromass carbon and nitrogen in mineral-associated SOM were positively correlated with the specific surface area of clay minerals. Our results provide direct evidence for the protection role of mineral in microbial necromass stabilization and provide a platform for simulating microbial necromass separately in SOM models.

Effects of magnesium-modified biochar on soil organic carbon mineralization in citrus orchard

In order to investigate the carbon sequestration potential of biochar on soil, citrus orchard soils with a forest age of 5 years was taken as the research object, citrus peel biochar (OBC) and magnesium-modified citrus peel biochar (OBC-mg) were selected as additive materials, and organic carbon mineralization experiments were carried out in citrus orchard soil. OBC and OBC-Mg were applied to citrus orchard soils at four application rates (0, 1, 2, and 4%), and incubated at a constant temperature for 100 days. Compared with CK, the cumulative mineralization of soil organic carbon decreased by 5.11% with 1% OBC and 2.14% with 1% OBC-Mg. The application of OBC and OBC-Mg significantly increased the content of soil organic carbon fraction, while the content of soil organic carbon fraction was higher in OBC-Mg treated soil than in OBC treated soil. Meanwhile, the cumulative mineralization of soil organic carbon was significantly and positively correlated with the activities of soil catalase, urease and sucrase. The enzyme activities increased with the cumulative mineralization of organic carbon, and the enzyme activities of the OBC-Mg treated soil were significantly higher than those of the OBC treated soil. The results indicated that the OBC-Mg treatment inhibited the organic carbon mineralization in citrus orchard soils and was more favorable to the increase of soil organic carbon fraction. The Mg-modified approach improved the carbon sequestration potential of biochar for citrus orchard soils and provided favorable support for the theory of soil carbon sink in orchards.

The impact of pristine and modified rice straw biochar on the emission of greenhouse gases from a red acidic soil

With the growing awareness of environmental impacts of land degradation, pressure is mounting to improve the health and productivity of degrading soils, which could be achieved through the use of raw and modified biochar materials. The primary objective of the current study was to investigate the efficiency of pristine and Mg-modified rice-straw biochar (RBC and MRBC) for the reduction of greenhouse gases (GHG) emissions and improvement of soil properties. A 90 days' incubation experiment was conducted using treatments which included control (CK), two RBC dosages (1% and 2.5%), and two MRBC doses (1% and 2.5%). Soil physico-chemical and biological properties were monitored to assess the effects due to the treatments. Results showed that both biochars improved soil physicochemical properties as the rate of biochar increased. The higher rates of biochar (RBC_2.5 and MRBC_2.5) particularly increased enzymatic activities (Catalase, Invertase and Urease) in comparison to the control. Data obtained for phospholipid fatty acid (PLFA) concentration indicated an increase in the Gram-negative bacteria (G-), actinomycetes and total PLFA with the increased biochar rate, while Gram-positive bacteria (G+) showed no changes to either level of biochar. As regards fungi concentration, it decreased with the biochar addition, whereas arbuscular mycorrhizal fungi (AMF) showed non-significant changes. The release of CO₂, CH₄ and N₂O showed a decreasing trend over the time. CO₂ cumulative emission decreased for MRBC₁ (5%) and MRBC_2.5 (9%) over the pristine biochar treatments. The cumulative N₂O emission decreased by 15-32% for RBC₁ and RBC_2.5 and by 22-33% for MRBC₁ and MRBC_2.5 as compared to the control, whereas CH₄ emission showed non-significant changes. Overall, the present study provides for the first-time data that could facilitate the correct use of Mg-modified rice biochar as a soil additive for the mitigation of greenhouse gas emission and improvement of soil properties.

Synthesis of an environmentally friendly binding material using pyrolysis by-products and modified starch binder for slow-release fertilizers

Biochar-based slow-release fertilizers (BSRFs) are vital for the development of eco-friendly and sustainable agriculture. Considerable attention has been given to enhancing the efficiency of fertilizers (EEFs) by appropriate modification or binding to reduce nutrient waste and improve the slow-release effect on the growth of plants. In this study, sustained binding materials were presented for BSRF synthesis, including pyroligneous acids (PA), bio-oil (BO), and modified starch binder (MSB). The results show that the release ratio of phosphorus from PA + BO+MSB was 4.7%, 15.2%, and 21.2% slower than that of PA, BO, and MSB alone, respectively. The BSRFs were characterized by SEM, XRD, FT-IR, XPS, and EDS, and the release kinetic outcome revealed that PA + BO+MSB contributed to the formation of a satisfactory structure in the BSRFs. The MSB viscosity significantly influences the slow-release performance and accumulation of N, P, and K nutrients. Moreover, economic assessments showed that PA + BO+MSB exhibited the lowest cost.

Effect of dissolved solids released from biochar on soil microbial metabolism

Dissolved solids released from biochar (DSRB), including organic and inorganic compounds, may affect the role of biochar as a soil amendment. In this study, the effects of DSRB on soil microbe metabolism, especially CO₂ fixation, were evaluated in liquid soil extract. DSRB were found to be released in large amounts (289.05 mg L^-1 at 1 hour) from biochar over a short period of time before their rate of release slowed to a gradual pace. They increased the microbial biomass and provided energy and reducing power to microbes, while reducing their metabolic output of extracellular proteins and polysaccharides. DSRB inputs led to the redistribution of metabolic flux in soil microorganisms and an increased organic carbon content in the short term. This content gradually decreased as it was utilized. DSRB did not improve microbial CO₂ fixation but, rather, enhanced its release, while promoting specific soil microorganism genera, including Cupriavidus, Flavisolibacter, and Pseudoxanthomonas. These heterotrophic genera may compete with autotrophic microorganisms for nutrients but have positive synergistic relationships with autotrophs during CO₂ fixation. These results demonstrated that reducing the DSRB in biochar can improve its role as a soil amendment by enhancing soil carbon storage and CO₂ fixation capabilities.

Stover return and nitrogen application affect soil organic carbon and nitrogen in a double-season maize field

Cultivation techniques have an important influence on grain yield of maize. This experiment investigated the effect of stover return (SR) and different nitrogen (N) application rate on soil organic carbon (SOC) composition, soil nutrient and maize yield. Different nitrogen application rate 100 (N100), 150 (N150), 200 (N200), 250 (N250) or 300 (N300) kg ha^-1 applied to the maize field with stover return and without stover return traditional planting (TP) method. Nitrogen application rate and stover return affected the SOC, labile organic carbon (LOC), microbial biomass (MBC), NO₃ ^- -N, NH₄ ⁺ -N and maize yield. Soil N, soil carbon content and maize yield of SR were all higher than TP. The SOC content of SR and TP were 9.67 and 9.19 g kg^-1 , respectively. Nitrogen application was significantly and positively correlated with soil MBC, LOC, SOC, NO₃ ^- -N, NH₄ ⁺ -N and yield. The maximum values of SOC composition, soil nutrients and maize yield were reached at SR with 250 kg ha^-1 . Stover return with application of N 250 kg ha^-1 significantly increased the growth attribute and maize yield in subtropical region compared with traditional planting.

Influences of Environmental Variables and Their Interactions on Chinese Farmland Soil Organic Carbon Density and Its Dynamics

Farmland is one of the most important and active components of the soil carbon pool. Exploring the controlling factors of farmland soil organic carbon density (SOCD) and its sequestration rate (SOCDSR) is vital for improving carbon sequestration and addressing climate change. Present studies provide considerable attention to the impacts of natural factors and agricultural management on SOCD and SOCDSR. However, few of them focus on the interaction effects of environmental variables on SOCD and SOCDSR. Therefore, using 64 samples collected from 19 agricultural stations in China, this study explored the effects of natural factors, human activities, and their interactions on farmland SOCD and SOCDSR by using geographical detector methods. Results of geographical detectors showed that SOCD was associated with natural factors, including groundwater depth, soil type, clay content, mean annual temperature (MAT), and mean annual precipitation. SOCDSR was related to natural factors and agricultural management, including MAT, groundwater depth, fertilization, and their interactions. Interaction effects existed in all environmental variable pairs, and the explanatory power of interaction effects was often greater than that of the sum of two single variables. Specifically, the interaction effect of soil type and MAT explained 74.8% of the variation in SOCD, and further investigation revealed that SOCD was highest in Luvisols and was under a low MAT (<6 °C). The interaction effect of groundwater depth and fertilization explained 40.4% of the variation in SOCDSR, and fertilization was conducive to SOCD increase at a high groundwater depth (<3 m). These findings suggest that low soil temperature, high soil moisture, and fertilization are conducive to soil carbon accumulation. These findings also highlight the importance of agricultural management and interaction effects in explaining SOCD and SOCDSR, which promote our knowledge to better understand the variation of SOCD and its dynamics.

Mitigation of greenhouse gas emissions from a red acidic soil by using magnesium-modified wheat straw biochar

To mitigate greenhouse gas (GHG) emissions, different strategies have been proposed, including application of dolomite, crop straw and biochar, thus contributing to cope with the increasing global warming affecting the planet. In the current study, pristine wheat straw biochar (WBC) and magnesium (MgCl₂.6H₂O) modified wheat straw biochar (MWBC) were used. Treatments included control (CK), two WBC dosages (1% and 2.5%), and two MWBC doses (1% and 2.5%). After 90 days of incubation, WBC and MWBC improved the soil physiochemical properties, being more pronounced with increasing rates of biochar. MWBC_2.5 significantly decreased microbial biomass carbon (MBC), while microbial biomass nitrogen (MBN) increased when both biochar materials (WBC₁ and MWBC₁) were applied at low rate. Compared to control soil, Urease and Alkaline phosphatase activities increased with the increasing rate of WBC and MWBC. The activities of dehydrogenase and β-glucosidase decreased with the WBC and MWBC application, compared to CK. The fluxes of all the three GHGs evaluated (CO₂, CH₄ and N₂O) decreased with time for both biochar amendments, while cumulative emission of CO₂ increased by 58% and 45% for WBC, and by 54% and 41% for MWBC, as compared to CK. The N₂O cumulative emissions decreased by 18 and 34% for WBC, and by 25 and 41% for MWBC, compared to CK, whereas cumulative methane emission showed non-significant differences among all treatments. These findings indicate that Mg-modified wheat straw biochar would be an appropriate management strategy aiding to reduce GHG emissions and improving the physiochemical properties of affected soils, and specifically of the red dry land soil investigated in the current work.

Preparation of ammonium-modified cassava waste-derived biochar and its evaluation for synergistic adsorption of ternary antibiotics from aqueous solution

Mono- and co-sorption of the three antibiotics i.e., norfloxacin (NOR), sulfamerazine (SMR) and oxytetracycline (OTC), to raw and NH₄⁺-modified cassava waste biochar added to aqueous solutions were investigated. The NH₄⁺-modified biochar showed higher sorption affinity for both NOR and SMR than the raw biochar, while the raw biochar showed higher sorption affinity for OTC than the modified biochar. The highest sorption to both biochars in both the mono- and competitive sorption systems was found for OTC followed by NOR and SMR. Sorption equilibrium in all systems analyzed was reached within 15 h. Electrostatic interactions among the ionic antibiotics in the multicomponent solution increased NOR and SMR sorption to both biochars. Antibiotics' mono- and co-sorption to biochars decreased with increasing solution pH. The co-sorption of NOR and SMR to the two biochars was regulated by π-π electron-donor-acceptor (EDA) interactions; besides, electrostatic interactions and Hydrogen (H-) bonding played an important part. Cation bridging might have been a potential mechanism to contribute to SMR sorption to the raw biochar, and OTC sorption to the NH₄⁺-modified biochar. These observations will improve our understanding of the simultaneous removal of multiple antibiotics from water or wastewater.

Biochar protects hydrophilic dissolved organic matter against mineralization and enhances its microbial carbon use efficiency

A combination of biochar with exogenous organic material in soils is often used in practical farmland management. The objective of this study was to determine how biochar affects organic matter decomposition by studying the decomposition of ¹³C-labelled hydrophilic (Hi-) and hydrophobic (Ho-) dissolved organic matter (DOM) in acid and neutral soils during a 60-day incubation experiment. The proportions of carbon (C) mineralization in Hi-DOM with or without biochar addition were 32.6% or 34.5% in acid soil (P > 0.05) and 15.4% or 22.3% in neutral soil (P < 0.05), respectively. In contrast, those proportions of Ho-DOM-C mineralization with or without biochar addition were 20.0% or 21.4% in acid soil and 19.0% or 20.5% in neutral soil (P > 0.05), respectively. These results showed that biochar could protect Hi-DOM against mineralization in neutral soil but exhibited less effect on Ho-DOM mineralization in both acid and neutral soils. Additionally, biochar did not affect microbial incorporation of Hi- or Ho-DOM in acid and neutral soils. However, biochar notably improved the microbial carbon use efficiency (CUE) of Hi-DOM while it significantly reduced the CUE of Ho-DOM in neutral soil (P < 0.05), indicating that the effect of biochar on microbial CUE was related to organic matter type and soil pH. This study suggests that Hi-DOM can outperform Ho-DOM to decrease C loss and improve microbial CUE in neutral soil with biochar addition. This phenomenon could be due mainly to the different chemical compositions of Hi-DOM and Ho-DOM and their distinct microbial preference. These findings can provide references for biochar's ability to regulate the decomposition of organic matter.

Spatial variations in soil organic carbon, nitrogen, phosphorus contents and controlling factors across the "Three Rivers" regions of southwest China

Based on the data of China Geochemical Baselines project, geostatistical analysis was used to investigate the spatial variations in soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) contents across the "Three Rivers" regions of southwest China, and the factors affecting them were analyzed by the redundancy analysis (RDA) and Person's correlation. Results showed that, the average content of SOC, TN and TP in the study area were 7.20 g/kg, 0.84 g/kg and 0.55 g/kg, respectively. The SOC and TN contents showed an obvious enrichment characteristic with great spatial variability, while TP content was stable on regional scale. The SOC, TN and TP contents decreased with elevation increase in the northern frigid highland, but showed an opposite character in the southern tropical & subtropical, which actually reflected the control of temperature on them. Combined with that there were higher SOC, TN and TP contents in subalpine meadow soil and red earth-yellow earth of the middle latitude zone, these suggested that the suitable temperature was conducive to the accumulation of soil nutrients. The weak positive correlation between population density and soil nutrients, together with high level of soil nutrients in the vicinity of large cities, demonstrated that human activities had significantly increased the soil nutrients contents in the study area. The RDA results showed that soil nutrients in the northern frigid highland were mainly controlled by the environmental factors dominated by temperature and soil structural factors dominated by parent materials with the total explanatory power as high as 75.87%, while in the southern tropical & subtropical mainly by the environmental factors dominated by chemical and biological weathering and the biological factors dominated by vegetation with the total explanatory power as high as 88.53%. The above factors superimposing a certain degree of human activities converged to cause that the SOC and TP contents in the south were higher than that in the north while the TN content was lower than that in the north.

Patterns and Internal Stability of Carbon, Nitrogen, and Phosphorus in Soils and Soil Microbial Biomass in Terrestrial Ecosystems in China: A Data Synthesis

Inspired by the strict constraint ratio (relatively low variability) between carbon (C), nitrogen (N), and phosphorus (P) in global soils and soil microbial biomass, our study explores the biogeographic distribution of C:N:P stoichiometric ratios in soils and soil microbial biomass in China and seeks to identify areas with similar ratios. Our study also attempts to determine the impacts of soil and soil microbial biomass C:N:P in China and the factors determining the ratio. The element concentrations may vary in each phylogenetic group of soils and soil microbial communities in China’s terrestrial ecosystems, as they do in global terrestrial ecosystems. However, on average, the C:N:P ratios for soil (66:5:1) and soil microbial biomass (22:2:1) are highly constrained within China. Soil microbial biomass C, N, and P concentrations have relatively weak internal stability, while soil microbial biomass C:N, C:P, and N:P ratios do not have internal stability at the national scale and in different terrestrial ecosystems of China. Unlike plant N:P, which can be used as the basis for evaluations of nutrient restrictions, the use of soil or soil microbial biomass N:P to evaluate soil nutrients is not universal. Latitude is the main factor influencing the patterns of soil C, N, and P. Longitude is the main factor determining the patterns of soil microbial biomass C, N, and P. pH is the main nonzonal factor affecting the patterns of soil and soil microbial biomass C, N, and P. The findings of this study are helpful in understanding the spatial pattern of soils and soil microbial biomass and their influencing factors in regions with complex ecosystems.

Plant inputs mediate the linkage between soil carbon and net nitrogen mineralization

Plant residue inputs play a crucial role in regulating soil carbon (C) stock and nitrogen (N) availability in cropland. However, little is known regarding how plant inputs mediate the relationships between soil C and net N mineralization, causing additional uncertainty in predicting ecosystem C and N dynamics. This study investigated the influences of long-term deprivation of plant inputs, short-term addition of maize straw and experimental warming on soil C and net N mineralization and their relationships. We conducted an 815-day laboratory incubation experiment under 10 and 20 °C using soils from a long-term bare fallow plot (without plant inputs for 23 years) and its adjacent old field plot (with continuous plant inputs). Our results showed that long-term deprivation of plant inputs decreased soil net N mineralization (per unit total N or TN) by 56% on average, but had minor effect on soil C mineralization (per unit soil organic C). Soil C and net N mineralization rates were positively correlated in the old field soil under 20 °C. However, soil C and net N mineralization rates were not correlated in the bare fallow soil, mainly due to the low level of net N mineralization. Moreover, soil C and net N mineralization rates were significantly increased by the addition of maize straw in both land-use types. When net N mineralization was <162 (or 159) μg N g^-1 TN d^-1, soil C and net N mineralization rates were negatively correlated due to an increase of microbial N demand during plant litter mineralization. When net N mineralization was >162 (or 159) μg N g^-1 TN d^-1, soil C and net N mineralization rates were positively correlated owing to a greater microbial mining of N from soil organic matter (SOM). Further, elevated temperature increased soil C and net N mineralization rates, and changed the relationships between soil C and net N mineralization. Taken together, this study provides evidence that plant inputs mediate the relationships between soil C and net N mineralization, and is thus critical in controlling ecosystem C and N cycling.