Amorphous silica reduces N2O emissions from arable land at the field plot scale

Introduction: Increasing greenhouse gas emissions pose a strong threat due to accelerating global warming. N₂O emissions are highly important in this regard as N₂O is a very powerful greenhouse gas. Agriculture is the main human-induced source for N₂O emissions, contributing roughly 60% to total N₂O emissions. Soil amorphous silica (ASi) contents are reduced in arable soils due to yearly exports by crop harvest as most crops are silicon accumulator plants. Most recently it has been shown that ASi is increasing water and nutrient availability in soils. Both factors are known to directly and indirectly affect N₂O emissions from agroecosystems.

Methods: In this study we conducted a field plot trial on arable soil depleted in ASi and fertilized this soil to its pre-agricultural ASi level.

Results: Our data clearly shows that increasing soil ASi to a pre-agricultural level decreased seasonal N₂O emissions by ∼30%.

Discussion: This reduction of N₂O emissions due to ASi might be of global relevance as agricultural practice has reduced the ASi content in agricultural soils. If future studies confirm the effect of ASi on N₂O emissions, the soil ASi depletion by agricultural practice in the last decades may have led to a substantial increase of N₂O emissions.

1 Introduction

Nitrogen (N) is the nutrient that most often limits productivity of terrestrial ecosystems (Vitousek and Howarth, 1991; LeBauer and Treseder, 2008) though excess N has various negative consequences for the environment (Sutton et al., 2011) such as production of greenhouse gases and leaching of nitrate (NO₃⁻) into groundwater (Galloway et al., 2013; Van Groenigen et al., 2015). Soil N transformations and losses are controlled by several, microbial mediated and simultaneously occurring processes (Myrold and Tiedje, 1986; Hart et al., 1994). The amount of N that is available for plant uptake is determined by the balance between microbial transformation of soil organic N (SON) to soil mineral N and the microbial immobilization of soil mineral N (Hart et al., 1994; Liu et al., 2019). In agricultural systems, mineralized ammonium (NH₄⁺) is rapidly transformed into nitrate (NO₃⁻) during nitrification, a process during which nitrous oxid (N₂O) is produced as a side product. Additionally, N₂O production occurs when nitrate undergoes denitrification (Norton and Schimel, 2011). N₂O production is a major concern within the N cycle because N₂O is both a powerful greenhouse gas (Badr and Probert, 1993) as well as the main contributor to destruction of stratospheric ozone (Ravishankara et al., 2009).

Agriculture stands out as the primary human-induced source contributing to the escalating concentration of atmospheric N₂O, accounting for roughly 60% of the total N₂O emissions (IPCC, 2013; Mbow et al., 2019). The increasing global demand for food, fodder, and fuel is driving higher N₂O emissions through intensified agricultural practices that involve a growing reliance on both mineral and organic N fertilizers (Reay et al., 2012; Pradhan et al., 2015; Lassaletta et al., 2016). The rate of N₂O emission is closely tied to the quantity of N fertilization. As N fertilization rises, so does the concentration of ammonium (NH₄⁺) and nitrate (NO₃⁻) in the soil. Those substrates are crucial for microbial processes like nitrification and denitrification, which play a pivotal role in N₂O formation (Bouwman et al., 2002; Shcherbak et al., 2014; Pareja-Sánchez et al., 2020). Additionally, high N₂O emissions are favored in low pH, high C availability, high temperatures and high moisture levels (60%–70% WFPS) (Butterbach-Bahl et al., 2013).

It has recently been shown that amorphous silica (ASi) can significantly reduce water stress under drought conditions, increasing soil moisture. Particularly, Schaller et al., 2020a; Schaller et al., 2020b) showed that ASi significantly enhances soil water holding capacity and water availability, both enabling better plant performance (Schaller et al., 2021b). Soils used for agriculture are depleted in ASi, with natural soils exhibiting an ASi content between 1% and 6% (dry weight) whereas agricultural soils exhibiting an ASi content always below 1% and in most cases zero (Schaller et al., 2021a). This depletion occurs because most crops are Si-accumulators (uptake of monosilicic acid and accumulation of ASi >1% in biomass) (Ma and Takahashi, 2002; Katz et al., 2021) and large amounts of ASi are removed from the fields during harvest (Puppe et al., 2021). A difference in ASi content of just 1% is responsible for a change in the amount of water available to plants by up to 40% as higher ASi contents will lead to higher water holding capacities and therefore more plant available water (Schaller et al., 2020a). The increase in soil water availability by ASi can be explained by a very high water holding capacity of ASi forming silica gels (Schaller et al., 2020a). Additionally, ASi was shown to enhance mobility of soil C and N, as silicic acid competes with C and N for binding at the surface of soil particles (Reithmaier et al., 2017). Both, the increase in water availability but also the increase in soil C and N availability caused by ASi fertilization is likely to affect the release of N₂O from soil. This impact is anticipated to be more noticeable, especially in sandy soils characterized by low water holding capacities, prevalent in the northeast region of Germany. If ASi fertilization results in substantial increases in soil water content, reaching levels conducive to N₂O emissions, it is reasonable to anticipate that ASi fertilization promotes N₂O emissions. Furthermore, the augmented availability of C and N resulting from ASi fertilization may contribute to an increase in N₂O emissions. Conversely, ASi fertilization can improve plant growth by optimizing water conditions. This enhanced growth increases N uptake, which reduces available N and, consequently, N₂O emissions.

Up to date, it has not been investigated to which extent ASi addition to the soil affects soil N cycling and N₂O emissions. We therefore conducted a field trial during 2023/2024 to assess N₂O emissions in barley following fertilization (covering a growing and non-growing season period), comparing plots with and without the addition of ASi. We hypothesized that i) ASi fertilization supports favorable microclimatic conditions through increasing soil moisture, ii) ASi fertilization reduces the available N, and consequently N₂O emission, by increasing crop N uptake during the crop growth period; iii) Depending on which effect dominates, N₂O emissions are increased or decreased under ASi fertilization.

2 Materials and methods

2.1 Study site and experimental design

The study site is located near the city of Müncheberg in NE Germany (52.5176° N, 14.1300° E), at the experimental fields of the Leibniz-Centre for Agricultural Landscape Research (ZALF). The climate is characterized as temperate, with a mean annual air temperature and precipitation of 9.9°C and 509 mm, respectively (ZALF weather station, 2010–2024). The highly heterogeneous soil pattern in this area has been developed from aeolian and glaciofluvial sands overlying a thin layer of glacial till. Due to the strongly variable, upper boundary of the clay-enriched Bt horizon (80–120 cm), soils classified either as Albic Luvisol (Arenic, Aric, Neocambic) or as Albic, Lamellic Arenosol (Aric). The soil texture of the at least upper 80 cm is dominated by medium and fine sand with intercalated clay lamellae of 2–4 cm thickness (Schaller et al., 2021b). In 2020, in total six plots (3 × 4 m² 100) have been established in a strip design on one of the experimental fields, representing a onetime ASi incorporation treatment to refill the depleted soil silica content (1%ASi (Aerosil 300, Evonik Industries, Germany) incorporated into the plough horizon (30 cm depth) by cultivating (see Schaller et al. (2021b) for details); each (n = 3) and a control (n = 3). Soil characterization of the experimental field (18 × 6 m²) is given in Table 1. To investigate the effect of ASi fertilization on N₂O emissions during both the growing and non-growing period, weekly N₂O emission measurements were performed for the control and 1% ASi treatment for rainfed spring barley (Hordeum vulgare L.; variety: Planet) during the 2023 crop growth period and the subsequent fallow period of 2023/2024. Since ASi incorporation in 2020, spring wheat (2x) and potatoes have been cultivated on the same plots. After seed bed preparation on 21.03.2023 using a full turn plow (plowing depth: 23 cm), spring barley was sown (361 grains per m²; row distance: 12.5 cm) in all plots on 30.03.2023 using a top-mounted seed drill with rotary harrow. Fertilization with N and sulfur was done at tillering (85 kg ha⁻¹ for N and 20 kg ha⁻¹ a⁻¹ for sulfur) and at heading stage (40 kg ha⁻¹ and 10 kg ha⁻¹ a⁻¹ for sulfur) each PIASAN-S 25/6 (SKW Piesteritz, Germany). Fertilizer application was done using a trailed sprayer with a five-nozzle system (HARDI Ranger Pro VHP, HARDI, Denmark). To protect the plants against herbivory all plots were covered with a fleece until crop emergence (12/04/2023) and treated against weeds with Ariane ™ C (1.5 L ha⁻¹) at stem extension stage.

Table 1

Table 1. Cumulative soil N₂O and CO₂ emissions, total crop biomass and grain yield, crop N, P and Si uptake (summer 2023) as well as soil mineral N, available P, total carbon (C_t), total nitrogen (N_t), and pH (summer 2023 and winter 2023/2024). Given uncertainty represents calculated uncertainty estimates (N₂O and CO₂) as well as ±SE.

2.2 Soil and plant and analyses

Soil samples were collected at ASi fertilization (18/03/2020), at 20/06/2023 and at 29/01/2024 (depth 0–15 cm). Soil samples were taken randomly at five locations within each plot using an auger (diameter 5 cm), avoiding field and treatment borders. Soil samples were subsequently analyzed for mineral N (NH₄⁺ + NO₃⁻) (VDLUFA MB Bd. 1 Kap. 6.1.4.1, CFA-SAN, Skalar GmbH, Germany) at the central laboratory of ZALF. To determine aboveground biomass and grain yield, three biomass samples from an area of 0.4 m × 0.4 m (0.16 m²) were collected for each treatment at the end of the crop growth period. Each sample was separated for straw and grain, weighed to obtain fresh weight, and oven dried at 50°C for 48 h to determine dry weight. Dried biomass samples were subsequently analyzed for N_t DIN-ISO-10694 (1995), CNS928-MLC, Leco Instruments GmbH, Germany) at the central lab of ZALF. Initial soil ASi content was analysed using 0.1 M Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid (disodium salt), C₆H₄Na₂O₈S₂; Roth, Karlsruhe, Germany) as extractant (Kendrick and Graham, 2004). Plant Si was extracted from 0.03 g biomass samples using 30 mL Na₂CO₃ at 85°C for 5 hours and filtrated at pore-size of 0.2 mm (Puppe et al., 2023). For P analysis, 0.1–0.2 g biomass samples was digested in a closed vessel microwave digestion system (CEM-Mars5, CEM Corporation, Matthews, NC, United States) at 180°C with 3 mL HNO₃ and 2 mL H₂O₂. Si and P concentrations in the extracts were measured at an ICP-OES (Varian, Vista-Pro radial, Palo Alto, California, United States).

2.3 N₂O and CO₂ flux measurements and flux calculation

N₂O and CO₂ fluxes were measured using a non-flow-through non-steady-state (NFT-NSS) manual closed chamber system (Livingston and Hutchinson, 1995). The round (Ø: 16 cm) opaque PVC chambers had a volume of 0.005 m³ and covered a basal area of 0.02 m². To assure airtight closure and prevent leakage, chambers were sealed at the bottom using rubber gaskets. A pressure vent was installed on top of each chamber. Due to the rather small volume and height, chambers were not ventilated, assuming a sufficient headspace homogenization during chamber closure (Hoffmann et al., 2018). A closable valve at the top of the chamber allowed the mounting of pre-evacuated glass bottles (volume: 60 mL) for air sampling. We performed N₂O and CO₂ flux measurements in parallel for all six plots (three per treatment) by deploying the chambers on a PVC frame inserted 5 cm deep into the soil. Chambers were opaque and no plants were present in the small area covered with the frames for N₂O flux measurements. For both measurement periods, frames were installed at least 3 days prior to the first measurement, to ensure a sufficient connection between the frame and the surrounding soil (Charteris et al., 2020). Chamber deployment time was 45 min (Chadwick et al., 2014), with gas samples taken twice per chamber measurement, once at the beginning and once at the end of each measurement. Hence, 6 gas samples were taken per treatment (12 in total) during one measurement campaign. Measurement campaigns were carried out once to twice a week throughout both the crop growing as well as non-growing period. Additional measurement campaigns were conducted to more accurately cover events with expected peaks in N₂O fluxes ⁵⁰, including multiple N₂O and CO₂ flux measurements per week following mineral N fertilization, as well as after heavy rain and frost-thaw events. Collected gas samples were analyzed for N₂O and CO₂ concentrations using a gas chromatograph (GC-14A and GC-14B, Shimadzu Scientific Instruments, Japan), coupled with an electron capture detector (Loftfield et al., 1997). Resulting N₂O concentrations were subsequently checked for reliability using an expected CO₂ concentration increase (opaque chamber; ecosystem respiration) during chamber closure as a quality criterion. In the case of a non-positive (<0 ppm) CO₂ concentration increase during chamber closure, measurements were considered biased and excluded from further analysis. Finally, N₂O and CO₂ fluxes (F; μmol m⁻² s⁻¹) were calculated according to the ideal gas law (Equation 1) and based on the often-used assumption of a linear concentration increase during chamber closure ^50,52,53.

$\mathrm{F}=\text{pV}/\text{RTA\hspace{0.17em}}\mathrm{x}\Delta \mathrm{c}/\Delta \mathrm{t}(1)$

where p represents ambient air pressure (Pa), V denotes chamber volume (m³), R is the gas constant (8.314 m³ Pa/K mol⁻¹), T denotes air temperature (K), A represents chamber basal area and dc/dt denotes the N₂O or CO₂ concentration change in chamber headspace over measurement time. N₂O and CO₂ emission estimates were derived through simple linear interpolation of measured N₂O and CO₂ fluxes, thus assuming a linear change between fluxes of two consecutive measurement campaigns. Emission estimates were derived through unit conversion of measured N₂O and CO₂ fluxes (µmol m^-2 s⁻¹) to g ha⁻¹ d⁻¹. Subsequently, daily emissions were linear interpolated, assuming a linear change between fluxes of two consecutive measurement campaigns.

Agronomic water use efficiency (AWUE) was calculated as the ratio of yield (g m⁻²) to total precipitation (mm) during the growing season Tallec et al. (2013). N₂O intensity was determined following Mosier et al. (2006) through dividing the cumulative N₂O emission (mg m⁻²) by yield (g m⁻²).

2.4 Auxiliary measurements

Nearby the experimental field, a weather station measured air temperature in 2 m height, wind speed and direction, relative humidity, air pressure, global radiation, and precipitation in an hourly interval. Additionally, microclimate loggers (TMS-4, TOMST, Czech Republic) were placed next to each frame for soil humidity and temperature, as well as air temperature measurements in a 15 min frequency throughout both measurement periods with measurement depth of 0–8 cm. Plant development stages were visually determined during crop management measures.

2.5 Statistical analysis

After testing for normal distribution using the Kolmogorov-Smirnov test (p < 0.05), significant differences in campaign-wise averaged N₂O and CO₂ fluxes between control and ASi plots were determined through performing paired two sample Wilcoxon-tests. The same tests were performed to test for significant differences between obtained biomass and soil parameters. In addition, effect sizes for the comparisons between ASi and Control were calculated using Cohen´s d, where values >0.2, >0.5 and >0.8 indicate a small, medium and strong effect, respectively. We estimated the uncertainty of the measured N₂O and CO₂ emissions by using the error prediction algorithm described in detail by Huth et al. (2018). All statistical analyses were performed using R 4.2.2.

3 Results

3.1 Environmental conditions

Environmental conditions at the study side during the crop growth period in 2023 and following fallow period 2023/2024 are shown in Figure 1. Both periods were characterized by an in average warmer air temperature (>2°C), when compared to the long term average of 15°C for the crop growth period and 2.1°C for the fallow period. During the crop growth period air and soil temperature followed a clear seasonal trend with increasing temperature from April till June and July. During the fallow period, air and soil temperature remained relatively stable with two distinct frost-thaw events occurring consecutively between the 6th and 26th of January 2023. With a precipitation totaling 198.0 mm and 249.2 mm during the crop growth and fallow period respectively, it is notable that only the fallow period experienced significantly higher rainfall compared to the long term average (2000-2020, ZALF), with in average 210.3 mm and 143.9 mm for the respective periods. While precipitation during the winter period was relatively evenly distributed, the crop growth period was primarily characterized by distinct heavy precipitation events (>30 mm per day). These heavy precipitation events coincided with times of fertilization in mid-April and June 2023 and were followed by periods of drying. This erratic pattern is particularly evident in the course of soil moisture development (Figure 1C). Compared to that, the development of soil moisture during the fallow period was relatively stable. However, strong changes in soil moisture was observed during frost, likely attributed to a decrease in soil moisture as water in not available in frozen under these conditions (Figure 1D). During both periods, measurements with the microclimate loggers (Figure 1C) next to each frame revealed that ASi was significantly warmer (<0.25°C; paired t-test: p-value <0.01) and less wet (VWC; <2%; paired t-test: p-value <0.01) than Control. The initial soil ASi content (control treatment) was 0.2%.

Figure 1

Figure 1. Course of soil temperature (°C; solid red line for C and dashed red line for Si) at 6 cm soil depth (A,B), soil moisture (volumetric water content (VWC) in %; solid blue line for C and dashed blue line for Si) and precipitation (mm d⁻¹) (C,D) for the crop growth and following fallow period in 2023 (A,C) and 2023/2024 (B,D) at the study site in Müncheberg. Dashed red vertical lines indicate the two fertilization events. Dotted blue lines represent soil thawing. Shaded areas represent the daily magnitude of measured soil temperature and soil moisture.

3.2 Crop yield, nutrient uptake and soil properties

Total crop biomass and grain yield were substantially higher for ASi fertilized plots (1070.0 ± 173.2 g DM m⁻² and 527.5 ± 93.1 g DM m⁻²) than control plots (427.5 ± 88.0 g DM m⁻² and 210.0 ± 44.0 g DM m⁻²) (Table 1). Similar differences between ASi and control were observed in the total crop N and P uptake due to only a slightly higher N and identical P concentration in biomass sampled for the control (Nt: 1.6%; P: 0.23%) compared to ASi (Nt: 1.4%; P: 0.23%). In case of Si content in straw, substantial differences were found between both treatments, with the control evidencing a Si content of near zero and ASi showing an Si content of 8.36 mg Si g⁻¹ (Table 1). A near zero Si content was found in either of both grain biomass samples.

Figure 2 shows the results of the three biomass sampling campaigns for total crop biomass, as well as crop N, P and Si uptake. No difference in total crop biomass and nutrient uptake was found during the beginning of the crop growth period on 02.05.2024. However, during the following biomass sampling campaigns on 02.06.2024 and 01.08.2024 increasing differences in total crop biomass and nutrient uptake were observed between the control and ASi.

Figure 2

Figure 2. Results of the three biomass sampling campaigns (02/05/2024, 02/06/2024 and 01/08/2024) during the crop growth period 2023 with (A) barley biomass yield [g dry mass (DM) m⁻²] and nutrient uptake for (B) N (g N m⁻²), (C) P (mg P m⁻²) and (D) silica (Si; mg Si m⁻²). Error bars indicate ±SE.

No differences were found in the case of soil mineral N (NH₄-N and NO₃-N), sampled during February 2024 at the end of the fallow period (Table 1).

3.3 Soil N₂O and CO₂ emissions

Temporal dynamics of N₂O (n = 198) and CO₂ emissions (n = 198) during the crop growth and following fallow period measured for the ASi and control treatment, respectively, are shown in Figures 3, 4. During the crop growth period, two distinct N₂O emission peaks were observed, following the first and second N fertilization event in the beginning of April and mid of June 2023 (Figure 3A). During the fallow period, relatively stable N₂O emissions were observed for ASi, while a slight increase in N₂O emissions was observed for C following the first and second frost-thaw events in January 2024 (Figure 3B). While cumulative N₂O emissions for the ASi treatment were more than 30% lower than for the control treatment during the plant growth period, significant differences between observed N₂O fluxes of both treatments were not obtained for the entire measurement duration but only during certain periods. In detail, N₂O emissions measured for both treatments only periodically differed significantly during both the crop growth and fallow period. Thus, N₂O emissions in the ASi-treatment were lower than in the control treatment approx. 3–4 weeks after fertilization and generally slightly higher compared to the control treatment during the fallow period before the first frost-thaw event (Figures 3C, D).

Figure 3

Figure 3. Results of periodically measured N₂O emissions (g N₂O-N ha⁻¹ d⁻¹) for C (light violet dots) and Si (dark violet dots) during the (A) crop growth and (B) following fallow period in 2023 and 2023/2024, respectively. Error bars indicate ±SE. (C, D) shows boxplots of N₂O emissions grouped by period after fertilization (summer period) and before vs. after frost-thaw (winter period). Significant differences (Wilcoxon-test) between the Treatments within each group are indicated by asterisks (p-value: *** <0.01, ** <0.05, * <0.1, n. s. > 0.1).

Figure 4

Figure 4. Results of periodically measured CO₂ emissions (kg CO₂-C ha⁻¹ d⁻¹) for C (light red dots) and Si (dark red dots) during the (A) crop growth and (B) following fallow period in 2023 and 2023/2024, respectively. Error bars indicate ±SE. (Figures 3C, D) shows boxplots of CO₂ emissions grouped by period after planting (summer period) and before (C) vs. after frost-thaw (D) (winter period). Significant differences (Wilcoxon-test) between the Treatments within each group are indicated by asterisks (p-value: *** < 0.01, ** < 0.05, * < 0.1, n.s. > 0.1).

During the crop growth period, CO₂ emissions exhibited a distinct seasonal trend, with substantially higher emissions as compared to the following fallow period (Figures 4A, B). Similar to observed N₂O emissions, CO₂ emissions were in general not significantly different between the ASi fertilized and control plots throughout the entire duration of the experiment. Hence, persistently higher, and significantly different CO₂ emission for ASi compared to the control were only found 3–4 months after planting of barley in early April 2023 (Figure 4C). Unlike for N₂O, no significant differences were observed for CO₂ emissions measured before the first and second frost-thaw event for control and ASi during the fallow period. After the frost-thaw events, however, slightly higher CO₂ emissions were obtained in case of ASi during January and February 2024 (Figure 4D).

4 Discussion

We found a reduction of more than 30% cumulative N₂O emissions for the ASi treatment compared to the control treatment at the field plot scale during the plant growth period, accompanied by increased crop N-uptake following ASi fertilization (Figures 2, 3). This confirms our hypothesis that ASi fertilization reduces N₂O emission during the crop growth period, due to an increased crop N uptake, which likely reduced the available N in the soil. Such reduction of N₂O emissions due to ASi addition are of global relevance as agricultural practice has reduced the ASi content in agricultural soils by this 1% (Saccone et al., 2007; Clymans et al., 2011; Schaller et al., 2021a). Hence, the ASi treatment represents soil ASi contents of natural soils not been used for agriculture. If future studies confirm this ASi effect on N₂O emissions, the soil ASi depletion by agricultural practice may have led to substantial increases of N₂O emissions.

ASi fertilization likely reduces N₂O emissions through a combination of mechanisms, including improved plant nutrient and water uptake due to its modification of soil properties. Firstly, it is known that ASi addition enhances plant-available water content in soil (Schaller et al., 2020a). The soil water content in turn plays a critical role in plant nutrient accessibility as nutrient transport within the soil and towards plant roots occurs through mass flow and diffusion (Marschner, 2003), both being demonstrably influenced by soil water content (Seiffert et al., 1995; Zarebanadkouki et al., 2019). Therefore, ASi fertilization, can be expected to accelerate nutrient transport via these mechanisms and to promote nutrient availability for plant uptake. This would be the case in principle for all soil nutrients and has been shown in this study for N and P whose uptake rates were higher under ASi fertilization compared to the control. Secondly, it has to be considered that N mineralization, the process controlling soil N availability, is microbial driven (Hart et al., 1994; Zhang et al., 2018) and as such, it strongly depend on the soil water status. Water stress limits soil microorganisms by limited diffusion and the reduced supply of resources, such as organic N, to soil organisms (Schimel, 2018). Increased soil water content after ASi fertilization can therefore be expected to increase plant N availability via increased microbial N turnover. Finally, ASi fertilization might directly affect N availability: ASi dissolution generates silicic acid (Si(OH)₄) that is likely to compete for binding sites with NH₄⁺, therefore increasing NH₄⁺ availability in soil. This competition could increase the concentration of bioavailable NH₄⁺ in the soil solution, similar to the observed mobilization of P by ASi (Schaller et al., 2019; Schaller et al., 2022). While nitrate (NO₃⁻) is often the dominant form of plant-usable N in agriculture, an increase in bioavailable NH₄⁺ through ASi application could still contribute to plant nutrition. Finally, we found a decrease in soil pH in the ASi fertilized plots. The soil pH of the main regulators of soil N₂O fluxes (Stehfest and Bouwman, 2006; Jamali et al., 2016; Hénault et al., 2019), affecting soil microbial community composition and activity. On the one hand, the moderately acidic conditions observed in ASi fertilization might have inhibited the nitrification and denitrification process and thus lowered the N₂O flux compared to the control. In acidic conditions, it is generally believed that the size and activity of nitrifying and denitrifying communities is reduced (Šimek et al., 2002; Park et al., 2018; Jadeja et al., 2021). It is likely that all three mentioned processes contributed to increased N and P availability in the present study, finally resulting in increased plant performance under ASi fertilization and greater depletion of the available N pool. Our data shows indeed a higher N uptake by crops following both fertilization events, which indicates less N availability in soils for microbial N₂O formation (Butterbach-Bahl et al., 2013). The observed reduction in N₂O emissions following ASi amendment started approximately 2 weeks after N fertilizer application. This time lag potentially reflects the time required for plants in the ASi treatment to enhance their N uptake, leading to a subsequent depletion of soil N available for microbial processes like nitrification and denitrification. While we cannot show reduced N availability directly, we indeed observed a higher N uptake by crops following both fertilization events (Figure 2B). The stronger reduction in N₂O emissions due to ASi after the first N-fertilization compared to the second N-fertilization event can be explained by the respective growing stage as the first N-fertilization took place at the time of maximum requirement of crops whereas the second N-fertilization took place at grain filling where crop N demand decreases (Delogu et al., 1998).

N₂O emissions during the fallow period were slightly higher for ASi compared to the control. However, summer exhibited clear distinctions, with ASi fertilization leading to reduced N₂O emissions compared to the control. This seasonal disparity suggests that plant performance may be a key driver of the observed differences in N₂O emissions. Supporting this, the time point coinciding with the highest plant N uptake in the ASi treatment also corresponded with the most pronounced reduction in N₂O emissions. Further, our study revealed a positive correlation between N₂O emissions and soil moisture during the summer months, but not during winter. This seasonal disparity can likely be attributed to the strong influence of soil temperature on microbial activity. Microbial processes responsible for N₂O production, such as nitrification and denitrification, are significantly temperature-dependent (Davidson et al., 1991). During winter, when soil temperatures typically decline, these microbial activities are likely suppressed, leading to minimal N₂O emissions regardless of soil moisture content. The difference in soil pH between ASi treatment and control might be explained by higher microbial activity (due to potentially more root exudates and faster decomposition of organic matter), increased nitrification, and to a minor share by release of the weakly buffered silicic acid.

We could not directly verify our initial hypothesis that ASi fertilization supports nitrification through increasing soil moisture. No enhanced soil moisture was found for the ASi treatment compared to the control treatment during the winter as well as summer period for volumetric (VWC) as well as gravimetric water content (GWC; Figure 1; Table 1). In fact, especially during the summer period, VWC as well as GWC were slightly lower for ASi compared to control. The reason for this is most likely the increased biomass growth at the ASi treatment also increasing water demand, thus lowering soil water content for ASi compared to Control. However, soil moisture is not directly related to plant available water as hydraulic conductivity is strongly affected by ASi enhancing the matric potential at any soil water content (Schaller et al., 2020a). However, plants seem to be able to get access to this water decreasing plant water stress (Zarebanadkouki et al., 2024) potentially increasing biomass production (Schaller et al., 2023).

Overall, N uptake per ha was around 150 kg for ASi -treated, and only 67 kg for control plots. Considering that 125 kg N ha-1 were fertilized and that in agricultural systems, only about 50% of the N taken up by annual crops is current-year fertilizer derived (Gardner and Drinkwater, 2009; Yan et al., 2020) this indicates that large amounts of N were derived from soil organic matter in the ASi treatment. It is possible that the increased plant growth under ASi fertilization enhanced root derived C input which in turn induced rhizosphere priming, i.e., a short term increase in soil organic matter (SOM) decomposition caused by addition of easily available C from the root to the soil (Kuzyakov, 2002). Priming is positively related to gross N mineralization and uptake (Holz et al., 2023) and in particular to uptake of SOM-derived N (Pausch et al., 2024). The observation of a significant proportion of SOM-derived N in ASi-treated plants could potentially be attributed to enhanced rhizosphere priming effects triggered by ASi application. While this study did not separate plant-derived CO₂ from soil-derived CO₂, crucial for calculating rhizosphere priming effects, the observed increase in total CO₂ emissions from ASi-fertilized plants suggests ASi fertilization stimulated soil carbon turnover, potentially including rhizosphere priming. The increased CO₂ emissions due to ASi fertilization showed highest difference compared to the control treatment during the time of shoot elongation (Figure 4). Such increased CO₂ emissions due to ASi can be explained by the higher biomass production as increased root activity will lead to increased root and microbial respiration. However, as the enhanced plant growth in the ASi treatment will also increase C input by the plants, this does not necessarily indicate a negative C budget for the ASi treatment compared to the control. In a similar study for wheat grown at the same site during 2022, it could be shown that ASi addition compared to no ASi addition actually even resulted in a small C sink (Schaller et al., 2023). In this study it was deduced that both, the storage of CO₂ as carbonates and the sorption or binding of organic carbon by aggregates are probably insufficient to cause such effects over this short investigation period. Similarly, the storage of occluded organic carbon in phytoliths was mentioned to be unlikely to be responsible. Instead, the most probable cause argued was the increased C input from root biomass and root exudates during plant growth, which can be inferred from the increased aboveground biomass formation following ASi fertilization as shown in the study. Future studies should hence focus on longer term investigations and C pool separation in order to quantify the effect of ASi fertilization on the C budget and its causes.

To obtain a more comprehensive understanding of the impact of ASi fertilization on N₂O emissions from these poor soils, future research should consider multi-year experiments. This would allow for the incorporation of interannual variability and potentially capture the post-harvest period. Also the inclusion of periodic no-N fertilization treatments alongside ASi application and control groups would provide valuable insights. By comparing N₂O emissions across these treatments, researchers could isolate the specific effect of ASi on N use efficiency and its subsequent influence on N₂O production. Additionally, the study design of the present study did not allow us to determine which nutrient was most limiting for plant growth in the control treatment or to isolate which nutrient responded more strongly to ASi application and by which processes the nutrient availability was affected. To elucidate these questions, a future experiment employing a three-factorial design with ASi, N, and P fertilization as separate factors would be necessary.

Overall, we showed that ASi fertilization reduced N₂O emissions from a poor agricultural soil by over 30% compared to control plots without ASi fertilization. This reduction can likely be explained by the influence, ASi has on soil properties, potentially including enhanced plant-available water and increased microbial N turnover. Peak reduction in N₂O coincided with periods of high crop N demand, highlighting the critical role of plant performance. To obtain a more comprehensive understanding of the impact of ASi fertilization on N₂O emissions from these poor soils, future research should consider multi-year experiments including in particular measurements soil microbial activity and microbial community dynamics related to N turnover. This would allow not only for the incorporation of inter-annual variability but also to verify in a detail the underlying mechanisms. Also the In review inclusion of periodic no-N fertilization treatments alongside ASi application and control groups would provide valuable insights. By comparing N₂O emissions across these treatments, researchers could isolate the specific effect of ASi on N use efficiency and its subsequent influence on N₂O production. Additionally, the study design of the present study did not allow us to determine which nutrient was most limiting for plant growth in the control treatment or to isolate which nutrient responded more strongly to ASi application and by which processes the nutrient availability was affected. To elucidate these questions, a future experiment employing a three-factorial design with ASi, N, and P fertilization as separate factors would be necessary. In conclusion, ASi fertilization is a promising management practice for sustainable agriculture. By mitigating N losses while at the same time improving plant growth, ASi fertilization could be beneficial for both, the environment but also for crop production.

Data availability statement

All data generated and analyzed during this study will be freely available through https://doi.org/10.4228/zalf-km5x-g581 repository. Researchers interested in accessing the data can retrieve it using the provided DOI link. Additional details or requests can be directed to the corresponding author.

Author contributions

MHf: Conceptualization, Data curation, Software, Supervision, Validation, Visualization, Writing–review and editing. OD: Formal Analysis, Methodology, Writing–review and editing. IZ: Formal Analysis, Methodology, Writing–review and editing. WA-H: Formal Analysis, Writing–review and editing. MD: Project administration, Writing–review and editing. MS: Formal Analysis, Writing–review and editing. MHl: Writing–original draft, Writing–review and editing. JS: Conceptualization, Funding acquisition, Project administration, Writing–review and editing.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This research has been supported by the Leibniz Society Germany for their funding through the Leibniz Cooperative Excellence program (K378/2021) awarded to JS.

Acknowledgments

Special thanks goes to Yvonne Ayaribil and Shrijana Vaidya for their support during gas exchange measurements and the ZALF Research Station Müncheberg for their consistent support by managing the established field trail. We also thank the team of the central lab of ZALF for soil and biomass analysis.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declare that no Generative AI was used in the creation of this manuscript.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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