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No abstract is provided for this article.

Knowledge of belowground C input by rice plants and its fate is essential for managing C cycling and sequestration in paddy soils. Previous reviews have summarized C input and the pathways of root-derived C in upland soils by labeling with 14C or 13C (13/14C), while rice rhizodeposition and C input in paddy soils have not been comprehensively evaluated. Here, we analyzed the results of 13/14C pulse and continuous labeling studies using 112 datasets from 13 articles on the allocation and pathways of photosynthesized C by rice plants to assess C input, budget, and amount stabilized in paddy soils. Overall, 13/14C partitioning estimated by continuous labeling was 72% to the shoots, 17% to the roots, 10% to the soil, and 1.3% was recovered in microbial biomass. Pulse-labeling studies showed a similar C partitioning: 79%, 13%, 5.5%, and 2.1%, respectively. The total belowground C input estimated based on continuous labeling was 1.6 Mg ha−1 after one rice season, of which rhizodeposition accounted for 0.4 Mg C ha−1. Carbon input assessed by pulse labeling was slightly lower (total belowground C input, 1.4 Mg ha−1; rhizodeposition, 0.3 Mg C ha−1; 14 days after labeling). Rice C input after one cropping season was lower than that by upland plants (cereals and grasses, 1.5–2.2 Mg ha−1). In contrast to upland crops, most paddy systems are located in the subtropics and tropics and have two or three cropping seasons per year. We conclude that (1) pulse labeling underestimates the total belowground C input by 15%, compared with that by continuous labeling, and (2) rhizodeposition of rice accounts for approximately 26% of the total belowground C input, regardless of the labeling method used. Based on allocation ratios, we suggest a simple and practical approach for assessment of the gross C input by rice into the soil, for partitioning among pools and for long-term C stabilization in paddies.

ABSTRACT Microplastics (MPs) have emerged as an increasingly concerning soil contaminant. Although biodegradable plastics are good alternatives to non‐biodegradable plastics in croplands, they can influence soil organic matter (SOM) decomposition through a priming effect. We investigated how the biodegradable MPs‐induced priming effect responds to nitrogen (N) availability in soil. The impact of biodegradable MPs and mineral N on the priming effect was generalized by a meta‐analysis, and the mechanisms were investigated by 13 C isotope techniques coupled with 16S rRNA amplicon sequencing. By combining the meta‐analysis of data from 67 publications with an incubation experiment, we tested the MPs‐induced priming effect and their mechanisms depending on four levels of mineral N: 1.50, 0.75, 0.50, 0.30 mg N g −1 soil. The meta‐analysis suggested that the mineral N input decreased the priming effect induced by root exudates (effects size: −1.1) and MPs (effects size: −1.5), but increased the priming effect induced by biochar (effects size: 3.1). The effect size of mineral N input on the priming effect decreased with the increase of carbon‐to‐nitrogen ratio (C/N) between added organic carbon and mineral N. Due to the differences in MPs degradability, the range of the priming effects induced by polyhydroxyalkanoate was from 200% to 250%, while the priming effects induced by polylactic acid were negative (−22% to −5%). Mineral N primarily mitigated the MPs‐induced priming effect by reducing the abundances of microorganisms with K‐strategy (Acidobacteria and Basidiomycota), thereby reducing their N mining from SOM. Priming reduction by N fertilization was minimal when the C/N between added MPs carbon and mineral N was 10 (high N availability), and the abundance of r ‐strategists (Proteobacteria and Ascomycota) was large. We conclude that both r ‐ and K‐strategists collectively drive the intensity and direction of the MPs‐induced priming effect, which decreases with increasing C/N between added MPs carbon and mineral N.

Drainage increases soil organic carbon (SOC) mineralization, but waterlogging will not necessarily slow the SOC decomposition. We investigated the effects of water regime (anaerobic vs. aerobic conditions) on SOC mineralization in adjacent wetland and grassland soils with contrasting biochemical properties in the east Tibetan Plateau. Biochemical characteristics of SOC (compositions of non-cellulosic and amino polysaccharides) were analyzed, then soils were moistened to 70% of water-holding capacity (aerobic) or saturated (anaerobic) and incubated at 25 °C for 95 days. The non-cellulosic sugar content in SOC was similar in wetland and meadow soils, but the mass ratio of hexoses (galactose plus mannose or rhamnose plus fucose) to pentoses (arabinose plus xylose) was three-fold greater in meadows than wetlands. Furthermore, there was three times more accumulation of microbial residues (sum of glucosamine, mannosamine, galactosamine, and muramic acid) in SOC, and more fungal than bacterial necromass in the meadow than wetland soils. This suggests slower transformation of plant litter by microorganisms, and thus less accumulation of microbial products in the SOC of wetlands than meadows. Meadow soils had more microbial biomass and higher activities of β–glucosidase, phenol oxidase, and urease per unit of SOC than wetland soils. During the 95-day incubation, C loss (as CO2) per unit of SOC was 55–73% greater in anaerobic than aerobic meadow soils, but was 59–64% less from wetland soils under anaerobic conditions than aerobic conditions. Thus, SOC decomposition in anaerobic conditions was dependent on the degree of SOC degradation, which was related to the initial proportion of plant and microbial residues. We addressed the importance of biochemical nature in regulating SOC decomposition under anaerobic conditions. We conclude that both the wetland drainage and grassland waterlogging increase SOC mineralization (measured as CO2 evolution) in alpine areas.

Carbon (C) cycling in the atmosphere-plant-soil-microorganism system has been the focus of much recent attention, because carbon dioxide (CO2) is the most important greenhouse gas and significantly influences global climate change. It is crucial to understand the fate of newly assimilated C in plants as this process helps clarify complex underground processes, for example, the formation, decomposition, and sequestration of soil organic carbon (SOC), turnover of the microorganisms, and energy flow and matter cycling through the soil food web. Considering the cost effectiveness and feasibility of field operations, pulse labeling plants with 13C or 14C via exposure to artificially labeled 13CO2 or 14CO2 atmospheres is a more direct and efficient tool, in many cases, the only tool available to investigate C allocation to shoots, roots, and the soil and C utilization by microorganisms. Carbon allocated belowground may be (i) lost to the atmosphere by root respiration, (ii) temporarily stored in the roots, (iii) released in the form of rhizodeposition, (iv) partially stored in microbial biomass, necromass, and soil fauna, (v) respired by rhizosphere microorganisms and soil fauna, or (vi) stored for extended periods as SOC. Rhizodeposition by living roots is a vital C and energy source for rhizosphere microorganisms. Carbon flux in the rhizosphere bridges plants, soil, and microorganisms and modifies microbial SOC decomposition and, therefore, C sequestration. The objectives of this review are to (1) summarize the scope, advantages, and disadvantages of C isotope (13C and 14C) applications; (2) propose a protocol for in situ plant 13CO2 pulse labeling in which all labeling, chasing, sampling, analysis, and calculation steps are described in detail; and (3) discuss the use of combinations of 13C labeling with biomarkers such as phospholipid fatty acids (PLFAs), amino sugars, nucleic acids (DNA and RNA), and other isotope tracers such as 14C, 15N, and 33P. We believe that the review presents a reliable and standardized protocol for tracing C flow through analyzing isotope enrichment in various pools or biomarkers within atmosphere-plant-soil-microorganism systems.

Understanding the effects of straw return and N fertilization on soil organic matter (SOM) transformations will help maintain crop production and soil function, which can ultimately contribute to mitigating climate change. In this study, we conducted a 100-day soil incubation experiment with the addition of 13C-labeled maize straw and/or N fertilization. Soils that were used in the study included soil without fertilizers (Control), with mineral fertilizer alone (NPK), and with mineral fertilizer and straw (NPK+Straw). Compared with the control, the NPK- and NPK+Straw-treated soils showed higher straw decomposition by 59% and 55%, and SOM mineralization by 27% and 37%, respectively, although the priming effect was decreased by 59% and 39%, respectively. The priming effect (PE) was higher with increased N content and lower with decreased N additions because of an improved C/N ratio for microorganisms. Straw additions compared to without straw increased the bacterial and fungal abundance by 1.4- and 4.9-fold, respectively. N fertilization lowered C/N ratios resulting in decreased fungal diversity. Although the bacterial abundance decreased, the diversity increased with the duration of incubation as the bacteria preferred to utilize the labile organic compounds that were abundant in the initial stages of incubation. In addition to the depletion of labile organic compounds, the fungal abundance increased. Bacteria (Firmicutes, Actinobacteria, and Proteobacteria phyla) and fungi (Ascomycota, Basidiomycota, and Mucoromycota phyla) dominated straw and SOM decomposition. Firmicutes were mostly involved in straw and SOM mineralization during the first day after straw addition. The edge number and ratio for pairwise correlations between environmental factors and fungal taxa (22.1–24.6%) were greater than those with bacterial taxa (1.0–2.9%) in the microbial correlation network. Overall, straw combined with a low level of added N benefited soil C sequestration by decreasing the PE. Compared to bacteria, the functional role of fungi in SOM mineralization is more prominent and should be considered during agricultural management.

Nutrient acquisition through symbiotic ectomycorrhizal fungi is carbon (C) costly but fundamental for plant growth, community, and ecosystem functioning. Here, we examined the functions of roots and mycorrhiza with respect to nutrient uptake after artificially inducing C limitation-seven months after girdling of an ectomycorrhizal tree, Pinus taeda. Root physiological activity (measured as root nitrogen content and root exudation) declined after girdling and was accompanied with 110% and 340% increases in mycorrhizal colonization and extramatrical hyphal length, respectively. Fungi colonizing roots switched to a community characterized by higher C efficiency (lower C cost) of nutrient acquisition (CENA, the amount of nutrient acquisition per unit C cost) and lower network complexity, indicating a tradeoff between CENA and stability of the fungal community. Root transcriptome analysis suggested a shift in metabolic pathways from a tricarboxylic acid cycle decomposition of carbohydrate to lipid biosynthesis to maintain closer associations with mycorrhiza for nutrient cycling after the girdling. By integrating multi-level evidence, including root transcriptome, fungal composition, and network complexity data, we demonstrate an increased dependence on mycorrhiza for nutrient acquisition under the C limitation condition, which is likely due to a shift to fungal community with higher CENA at the cost of lower stability.

Biochar modifications and fortifications are used to increase soil nutrient retention and increase crop yields, but the effects on phosphate (Pi) availability for plants are unknown for saline-alkaline soils. We used peanut shells (Peanut-S) as the starting material for biochar (Biochar) and MgO-modified biochar (MgO-Biochar) to study the Pi adsorption mechanisms under field conditions. Compared with Biochar and Peanut-S, MgO-Biochar adsorbed Pi in aqueous solution 20% better and reached equilibrium at twice the rate at twice the rate. MgO-Biochar also showed better Pi adsorption in saline soils and the maximum Pi adsorption capacity was 1.46 times higher than Biochar. We found that electrostatic attraction, precipitation and exchangeable anions contributed to the adsorption of phosphate and MgO-Biochar possessed the greatest adsorption capacity. MgO-Biochar application increased the soil available P content and resulted in higher rice yields in field experiments in the saline soil conditions found in the Yellow river delta region. Overall, our study offers a theoretical and practical foundation for the remediation of saline soil using MgO-modified biochar.

No abstract is provided for this article.

Tree mycorrhizal associations have substantial consequences for soil organic carbon (SOC), but it remains unclear how nutrient availability will regulate the performance of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) trees, and then consequently affect SOC sequestration in tropical forest soils. This study characterized the performances of AM and ECM trees, SOC content, and soil microbial functions under variable soil nitrogen (N) and phosphorus (P) content across an intact tropical rainforest based on the spatial dataset from a 60-ha dynamic plot and fitted statistical models to examine the mycorrhizal and nutrient controls on SOC stocks. ECM trees showed a better performance in soils containing higher N in total or in NH4 + forms and enhanced SOC content via increases in both species richness and basal area, which led to an increase in SOC as soil N content evaluated. AM trees had a greater basal area at N-richer (i.e., available N and NH4 +) niches while a higher species richness under higher soil P levels (i.e., total and available P). The AM tree community patterns had inconsistent regulations on SOC, with basal area showing a positive while species richness exerting a negative effect on SOC content. Such counteracting effects from AM trees might attenuate SOC accumulation along the P gradient, resulting in a positive trend in SOC with soil total N:P ratios. As soil available P increased, species richness of AM trees increased, which was accompanied by a higher abundance of pathogens while a lower abundance of AM fungi. This indicated a decreased dependence of AM trees on mycorrhizal P acquisition, accompanied by the high susceptibility of roots to pathogen attacks, which may promote AM tree diversity. The performances of ECM trees were positively related to ECM fungi abundance, offering ECM trees a competitive P strategy and pathogen resistance. Summarily, our results suggest that both the basal area and species richness of mycorrhizal trees act as significant regulators for SOC sequestration along soil N or P gradient in tropical forests. Such findings provide a mechanistic understanding of soil C dynamics during vegetation changes under the rising global stoichiometric imbalance between N and P.

The available methods of microbial phosphorus (P) analysis do not allow full cell lysis during fumigation and complete P extraction. Consequently, a correction of microbial P (Pmic) extraction efficiency (Kp factor) is always necessary. Here we evaluated possible under- or overestimation of microbial P by comparing the direct determination of Kp for various soils with the Kp values obtained from the literature. We determined a soil-specific Kp value for Cambisol by coupling 33P labeling, anion exchange membranes (AEM) water extraction and liquid fumigation with CHCl3. The measured Kp for the Cambisol (0.69) was much higher than the Kp commonly used in the literature over the last 35 years (0.4). Experimentally measured Kp avoided overestimation of microbial P by more than 1.7 times. Therefore, a soil-specific Kp correction of extractability is a prerequisite for microbial P analysis. To improve Pmic accuracy without direct Kp determination, we conducted stepwise regression analysis between soil-specific Kp values from the literature and soil parameters. The Kp increased linearly with decreasing total P. An exponential increase of Kp with decreasing organic C (R2 = 0.45–0.76) revealed a threshold of 10 g Corg kg−1. Combining three soil parameters in multiple regression – Kp = 0.76–0.007*Corg - 0.56*Ptot + 0.004*Clay – enables an excellent Kp prediction (R2 = 0.99). We conclude that the Kp value of 0.4 commonly used for estimating microbial biomass P cannot be accepted as a constant. Thus, in the absence of soil-specific Kp, we recommend using the regression models considering the basic soil properties.