Publications

Silicon (Si) and arbuscular mycorrhizal fungi (AMF) increase plant resistance to various environmental stresses, including heavy metal (and metalloid) toxicity. Although Si and AMF each independently enhance plant tolerance, the nature of their interactions and their combined impacts on nutrient uptake, especially in the context of toxic elements such as arsenic (As), remains to be elucidated. This study investigated AMF-mediated regulation of plant nutrient uptake under As stress using rice, a model Si-accumulating plant. Experiments were conducted under As-free and As stress conditions, incorporating AMF inoculation and silicic acid application, with a focus on nutrient uptake and transporter expression. Without As, AMF inoculation increased shoot Si content by 44%, while invariance was common under As toxicity stress (10 μM of As(III)). Despite As presence, AMF increased Lsi1 expression with Si application, elevating As content in roots and shoots by 38% and 55%, respectively. Introduction of As stress amplified AMF role in phosphorus (P) uptake from 13% to 38%, correlating with up-regulated P transporter expression. Three-way ANOVA of interactions among As, Si, and AMF on P and As uptake by rice revealed that As amplified AMF potential to increase P uptake while weakening promotive effect on Si uptake. Silicon reduced As absorption, while AMF increased As uptake, but the elevated As were potentially retained within fungal hyphae, limiting transfer to rice plants. Overall, As toxicity stress had contrasting effects on P- and Si-promoting roles of AMF. These findings contribute to our understanding of plant-fungal interactions under heavy metal stress.

Soil zymography is a new technique developed to visualize two‐dimensional distributions of enzyme activities. The method consists of incubating a membrane saturated with an enzyme‐specific fluorogenic substrate on a surface of the soil sample, followed by recording the membrane image generated by a fluorescent product (e.g. MUF: methylumbelliferone) in ultraviolet light. Despite its relative ease of use, performing zymography involves multiple user‐made decisions that might affect the accuracy of enzyme activity estimates. Therefore, unification of the zymography methodology is required for correct estimations and comparisons of various studies. We evaluated the following methodological aspects of the implementation of zymography: (a) camera settings and image processing, (b) effects of evaporation and (c) calibration procedures. Camera settings (shutter speeds or exposure time) affected the intensity of background fluorescence and signal‐to‐noise ratios (SNR). However, because their combined effects varied depending on MUF concentrations, light and camera setting need to be optimized for the expected range of MUF concentrations prior to zymography. Evaporation of MUF solution from the membrane had no effect on fluorescence. Relations between MUF concentration and intensity of fluorescence during calibrations demonstrated a saturated pattern and were strongly affected by image noise outside the optimal range (e.g. 8–14 μ m MUF pixel −1 ). We developed a new calibration approach that is based on a piecewise linear regression. The new approach accounted for specific ranges of MUF concentration and uses nonuniformly saturated membranes, reflecting the real distribution of enzyme activities in soil. The new calibration algorithm eliminated biases of the standard calibration and resulted in greater accuracy in predicting MUF concentrations. Highlights We developed a new approach to calibration for 2‐D soil zymography. The approach accounted for spatial nonuniformity of soil zymograms. Standard calibration resulted in systematic underestimation of enzyme activity. Soil zymography requires pixel‐based calibration with nonuniformly saturated membranes.

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.

Climate warming leads to widespread permafrost thaw with a fraction of the thawed permafrost carbon (C) being released as carbon dioxide (CO2 ), thus triggering a positive permafrost C-climate feedback. However, large uncertainty exists in the size of this model-projected feedback, partly owing to the limited understanding of permafrost CO2 release through the priming effect (i.e., the stimulation of soil organic matter decomposition by external C inputs) upon thaw. By combining permafrost sampling from 24 sites on the Tibetan Plateau and laboratory incubation, we detected an overall positive priming effect (an increase in soil C decomposition by up to 31%) upon permafrost thaw, which increased with permafrost C density (C storage per area). We then assessed the magnitude of thawed permafrost C under future climate scenarios by coupling increases in active layer thickness over half a century with spatial and vertical distributions of soil C density. The thawed C stocks in the top 3 m of soils from the present (2000-2015) to the future period (2061-2080) were estimated at 1.0 (95% confidence interval (CI): 0.8-1.2) and 1.3 (95% CI: 1.0-1.7) Pg (1 Pg = 1015 g) C under moderate and high Representative Concentration Pathway (RCP) scenarios 4.5 and 8.5, respectively. We further predicted permafrost priming effect potential (priming intensity under optimal conditions) based on the thawed C and the empirical relationship between the priming effect and permafrost C density. By the period 2061-2080, the regional priming potentials could be 8.8 (95% CI: 7.4-10.2) and 10.0 (95% CI: 8.3-11.6) Tg (1 Tg = 1012 g) C year-1 under the RCP 4.5 and RCP 8.5 scenarios, respectively. This large CO2 emission potential induced by the priming effect highlights the complex permafrost C dynamics upon thaw, potentially reinforcing permafrost C-climate feedback.

The purpose of this study was to develop options for a more sustainable catchment management, resulting in a reduction of agricultural non-point pollution of water resources in South Korean agricultural catchments. Therefore, an N budget analysis was conducted, which related N inputs into soil under intensive agriculture to N outputs at both field and catchment scale in a mountainous catchment in South Korea. The N budget of all investigated crops was positive, with total N inputs exceeding N outputs by 2.8 times. Radish showed the highest N uptake efficiency (43–45%), whereas rice showed the lowest with 24–30%. At the catchment scale, agriculture contributed over 90% to the maximum N surplus (473Mg). Rice and radish, with over 100MgN surplus each, contributed the largest part. Comparing these results to the N export in the catchment outlet, it was found that N leaching and surface runoff were the dominant loss pathways, leading to a seasonal inorganic N export of 329Mg. Because fertilizer N was the major N input (>50%) for all crop types except soybean, its reduction was identified as the major scope of action for N savings at the field and catchment scale. The currently observed trend of land use change from annual to perennial crops additionally assists the reduction of N surplus but shows only a spatially limited applicability for the future. Further measures like split applications, application timing to match crop needs and cover crops during the fallow complement the attempt.