Response of C, N and P Ecological Stoichiometry in Plants and Soils During a Soybean Growth Season to O3 Stress and Straw Return in Northeast China

C, N and P ecological stoichiometry plays important roles on biogeochemical cycles in ecosystems, yet the relationship between plant and soil stoichiometry and stoichiometric effects on the growth of soybean root in response to the O 3 stress and straw return remain poorly understood. Methods Here, a pot experiment was conducted in open top chambers to monitor the response of C, N and P ecological stoichiometry of leaves, shoots, roots and soils during a growing season (branching, owering and podding stages) of soybean (Glycine max; a highly sensitive species to O 3 ) to background O 3 concentration (45 ± 10 ppb), O 3 stress (80 ±10 ppb) and straw treatment (no straw return and straw return). Results The O 3 stress signicantly decreased root biomass. The straw return signicantly increased root biomass under the O 3 stress at branching and owering stages. Generally, the O 3 stress and straw return showed signicant effects on the C, N, P concentrations of leaves and soils, and stoichiometric ratios of leaves, shoots and microbial biomass. C, N, P concentrations and stoichiometric ratios of leaves, shoots, roots and soils responses to the O 3 stress and straw return at branching stage were inconsistent with changes observed at the owering and podding stages. The P conversion eciency showed signicant relationship with root P concentration under the combined effects of O 3 stress and straw return. C, N, and P concentrations of soybean might be more important than stoichiometric ratios as a driver of defense against the O 3 stress in the case of straw return. P of ecological stoichiometry are critically important for prediction stress consequences in biochemical cycles and other ecological process. A handful of studies are about the effects of O 3 stress on leaf stoichiometry and nutrient resorption The study was conducted on soybean plants grown in OTCs from June 16 to August 30 in 2017. The OTCs, which were established in 2008, had a diameter of 1.15 m and a height of 2.4 m with a 45° sloping frustum; the minimum distance between any two chambers was 4 m. Each chamber was made with an iron framework, clad with standard horticultural glass, with a plenum incorporated just below the mouth of the chamber (at 2.4 m from ground-level) to reduce the entrance of ambient air (Bao et al. 2014). From June to September, mean temperature of the day was about 25.6 ± 3.7 ℃ and mean relative air humidity in the OTCs throughout the day was 50.6 ± 19.9%. The mean value of O 3 concentration was about 45 ± 5 ppb and the average value of AOT40 (the O 3 concentration accumulated over a threshold O 3 concentration of 40 ppb during daylight hours) was 3.5 ppm h -1 during clear sky conditions from June to September in the OTCs. The soybean cultivation was fumigated in the OTCs by O 3 for 2.5 months (from June 16 to August 30 in 2017). The experimental design was based on completely randomized plots including two O 3 treatments and tree replicates per O 3 treatment (overall 6 OTCs). Two O 3 treatments were carried out: (1) non-ltered air treatment (control, hereinafter referred to as CK, O 3 concentration 45 ± 10 ppb); (2) O 3 stress treatment, non-ltered air with addition of O 3 35 ppb (hereinafter referred to as O 3 , 80 ± 10 ppb). The O 3 was produced from pure oxygen with an O 3 generator (GP-5J Guolin Ltd., Qingdao, China) and then it was mixed with ambient air to achieve the target O 3 concentration, and the mixture regulated by ow controllers in each OTC. The top of the OTCs is open. The O 3 concentrations were continuously monitored by O 3 analyzers (S-900 Aeroqual Ltd., Auckland, New Zealand) every day during the whole day from June 16 to August 30 in 2017, and controlled by computers using a professional program for O 3 This study contributes to address the inuence of the O 3 stress and/or straw return on the C, N, P concentrations and stoichiometric ratios of plants and soils during a soybean growing season. The O 3 stress signicantly inhibited soybean root growth, and the straw return effectively prevented the O 3 damage on soybean root at branching and owering stages of soybean. Conrming the rst hypothesis, the signicant effects of the O 3 stress and/or straw return on C, N, P concentrations and stoichiometric ratios of leaf, shoot, root and soils varied greatly and depended on the growth stage of soybean. The root growth of soybean might be indirectly affected by the O 3 stress through the alterations of soil microbial biomass stoichiometry which were directly induced by the leaf stoichiometric ratios. The straw return had limited mitigation effect on the damage from the O 3 stress via the changes of C, N, P concentrations of soils and roots.


Introduction
Tropospheric O 3 (Ozone), which is an important greenhouse gas and a secondary air pollutant (Sitch et  Meanwhile, fewer studies found that the belowground ecological processes (such as the roots growth and soil processes) were indirectly in uenced by the O 3 stress (Nikolova et al. 2010;Pregitzer et al. 2008).
There is closely linked between the above-and below-ground ecosystem due to the circulation and feedback of nutrients (Peichl et al. 2012; Wang et al. 2009). Plant roots could provide substrates to the soils and microbes, and therefore changes in the C, N, P concentrations and stoichiometric ratios of the plants might affect soil C, N, P concentrations and stoichiometric ratios (Peichl et  For example, Agathokleous et al. (2018) found that O 3 stress (68.84 ± 1.12 nmol mol − 1 ) altered the leaf stoichiometry, including impacted in nutrient resorption e ciency. Fewer studies focus on the effects of O 3 stress on stoichiometry of soil and root (Cao et al. 2016;Shang et al. 2018; Li et al. 2021). Cao et al. (2016) found that O 3 stress (100 ppb and 150 ppb) signi cantly decreased the C/N ratios in individual tissues (foliar, stem, and root) of both of Yang (P. bournei) and Phoebe zhennan S. Lee et F. N. Wei (P. zhennan), and C/P and N/P ratios in individual tissues of P. bournei, and the individual tissues of P. zhennan did not show consistent variation tendency under the O 3 stress. Shang et al. (2018) showed that the O 3 stress (60 ppb) signi cantly increased both N and P concentrations of individual organs (leaves, stems and roots) of poplar while a reduction in the C/N ratio was observed. Much-limited studies were concerned with the cycling of ecological stoichiometry between above-and below-ground ecosystem exposed to elevated O 3 stress.
Straw return is a common tillage practice for crops cultivation (such as rice, wheat, maize and soybean) in China. The annual output of straw in China is about 750 million tons. More than 1/3 of the straw is discarded or burned, resulting in resource waste and environmental pollution (Ma et al. 2019). According to the results of meta-analysis of 142 experiments, Zhao et al. (2015a) found that crop yields in 92% of 131 experiments increased with increasing straw incorporation. Straw returning to the eld not only can improve the soil structure, reduce the soil bulk density, but also activate the soil organic phosphorus, increase the soil nitrogen and improve yields (Tan et al. 2007;Zhao et al. 2015a, b). The addition of organic matter might directly affect the soil C, N and P Zhao et al. 2016b), and plants might adjust their growth rates with the changed stoichiometric ratio of soil C, N and P (Daufresne and Loreau 2001; Moe et al. 2005). However, much-limited information is available on the cycling of ecological stoichiometry between above-and below-ground ecosystem under straw return and/or O 3 stress. Tropospheric O 3 is currently the most important secondary air pollutant, and O 3 is posing serious threats to forest ecosystems, agriculture, and human health (Ainsworth 2017;Feng et al. 2019). Considering that the straw return is a common tillage practice for crops cultivation in China, it is necessary to study the combined in uence of straw return and O 3 stress on the cycling of ecological stoichiometry between above-and below-ground in the cropland ecosystem, in order to be able to predict the impacts of elevated  Morgan et al. 2006). Furthermore, straw return, which is a common tillage practice for soybean cultivation in China, might increase C and nutrient input through both root and aboveground biomass. Such an increase might be relatively great, and subsequent changes in stoichiometric relationship between the plants and soils might be signi cant.
However, whether straw return could alleviate the damage from O 3 stress and whether the mitigation effect of straw return is due to the changes of stoichiometry between above-and below-ground in soybean cropland is still rarely evaluated. Thus, the objective of this study is to examine the effects of O 3 stress and/or straw return on C, N and P stoichiometry dynamic in a plant-soil system during a growing season of soybean. Furthermore, soybeans have naturally higher levels of oral and pod loss (Liu et al. 2003). Previous studies showed that soybean yield is mainly determined during the post-owering phase (R1 stage onwards; Fehr and Caviness 1977), throughout owering and pod setting (Board et al. 1995;Egli 2010). The subsequently changes in C, N and P stoichiometry caused by the O 3 stress and/or straw return might are greater during owering and podding development. Thus, it is necessary to evaluate the responses of nutrient utilization and C distribution at different developmental stages (such as branching stage, owering stage and podding stages) to the effects of O 3 stress and/or straw return.
We hypothesized that (1) signi cant effects of the O 3 stress and straw return on C, N, P concentrations and stoichiometric ratios of leaf, shoot, root and soils depended on the growth stage of soybean; and (2) straw return alleviated the damage from O 3 stress due to the changes of C, N and P stoichiometry between above-and below-ground in the soybean cropland. To test these hypotheses, branching, owering and podding stages of soybean were chosen as sampling times to evaluate the temporal variations in C, N, P concentrations and stoichiometric ratios of leaf, shoot, root and soils in response to the O 3 stress and/or straw return using a pot experiment in open top chambers (OTC).

Experimental site
The experimental site located in Shenyang Experimental Station of Ecology, Chinese Academy of Sciences (41°310′N, 123°220′E). This region has a continental monsoon climate with a mean annual temperature of 7.0-8.0°C, annual precipitation of 650-700 mm, and an annual non-frost period of 147-164 days. The soil at the study site is classi ed as an Al sol (US Soil Taxonomy) with 11.28 g kg -1 organic C, 1.20 g kg -1 total N, 0.41 g kg -1 total P, pH (H 2 O) 6.7 at 0-15 cm depth.

Experimental design and sampling
The study was conducted on soybean plants grown in OTCs from June 16 to August 30 in 2017. The OTCs, which were established in 2008, had a diameter of 1.15 m and a height of 2.4 m with a 45° sloping frustum; the minimum distance between any two chambers was 4 m.
Each chamber was made with an iron framework, clad with standard horticultural glass, with a plenum incorporated just below the mouth of the chamber (at 2.4 m from ground-level) to reduce the entrance of ambient air (Bao et al. 2014  Meanwhile, there were two straw treatments (tree replicates per straw treatment) for each O 3 treatments. Two straw treatments were carried out: (1) no straw return (hereinafter referred to as S-); (2) The total amount of straw is returned to each pot (hereinafter referred to as S+). There were 18 pots per OTC, 3 collected periods (branching stage, owering stage and podding stage) × 3 replications × 2 straw treatments.
According to the average soybean yield and potted area, soybean straw (20 g), which was subjected to O 3 fumigation in 2016, was crushed and applied in situ to 20cm depth per pot. Soil, used in each pot experiment, was collected from a cropland (at 0-15 cm layer) at the study site before crops were planted. The previous crop was soybean (Tiefeng 29) and the eld did not receive any N fertilization because of the fallow management before the beginning of the pot. After sieving (<2 mm), the soil was immediately used to prepare the pot experiment. The potted soybean cultivar was Tiefeng 29, which was seeded in each pot (26 cm long ×36 cm wide × 45 cm deep) on May 09 in 2017.
Before sowing, NH 4 H 2 PO 4 at 300 kg ha -1 was applied to each plot. The plants were irrigated daily to avoid water stress and appropriate measures were taken to keep the plants free from any stresses of biotic, disease and grass. Five plants were planted in each pot.
Soil samples (containing rhizosphere and bulk soil) was collected at branching stage (July 10, 2017), owering stage (August 03, 2017) and podding stage (August 30, 2017), respectively. Five soil samples from each pot were randomly collected at 0-10 cm depth by using soil-corer with an inner diameter of 4.5 cm and then they were pooled together to give one composite sample. The collected soil samples were immediately sieved (<2mm) to remove visible stones, roots and plant materials, and then divided into two sub-samples. One subsample was air-dried at 25°C for chemical analysis, one sub-sample was stored at 4°C for microbial biomass analysis. Furthermore, at harvest time, all plants were carefully removed from soil of each pot; plant sampling times were: branching stage (July 10, 2017), owering stage (August 03, 2017) and podding stage (August 30, 2017); soil particles attached to roots were removed by water washing. After harvest, the root biomass was weighed after drying in the oven at 65 °C for 48 h and then were chemical analyzed. Meanwhile, the leaf samples and shoot samples were oven-dried at 65°C for chemical analysis.
Leaf, shoot, root and soil analysis Total N and P concentration of leaf, shoot, root and soil were determined by elemental analyzer (Vario MAX CNS, Elementar Analysensysteme GmbH, Hanau, Germany) ). Soil organic carbon (SOC) and total C concentration of leaf, shoot, root and soil were analyzed by the K 2 Cr 2 O 7 -H 2 SO 4 calefaction and titration method (Nelson and Sommers 1982). The C/N and C/P ratios of leaf, shoot, root and soil were calculated from the values of SOC concentration, total C concentration, total N concentration and total P concentration.
Microbial biomass carbon (MBC) and Microbial biomass nitrogen (MBN) of soil cropped to soybean were analyzed by the chloroform fumigation-extraction method as described by Vance et al. (1987). Brie y, soil samples (25 g dry base) were fumigated with ethanol-free chloroform for 24 h at 25 °C. After removal of the chloroform, soluble C and N were extracted from fumigated and non-fumigated samples in 100 mL of 0.5M K 2 SO 4 for 30 min on an orbital shaker. Total organic C in the ltered extract was determined by the K 2 Cr 2 O 7 -H 2 SO 4 calefaction and titration method. Total organic N in the ltered extract was determined using the elemental analyzer (Vario MAX CNS, Elementar Analysensysteme GmbH, Hanau, Germany). We converted microbial C ush (the difference in extractable C between fumigated and non-fumigated samples) to MBC using a factor of 0.45 (Vance et al. 1987) and microbial N ush to MBN using a factor of 0.54 (Brookes et al. 1985). Microbial biomass phosphorus (MBP) of soil cropped to soybean was determined using a fumigation extraction method as described by Brookes et al. (1982). The pre-treatment was in accordance with MBC and MBN. Soluble P in fumigated and nonfumigated soil samples (5 g dry base) was extracted in 100 mL of 0.5M NaHCO 3 (pH 8.5) for 30 min on an orbital shaker. We converted microbial P ush to MBP using a factor of 0.40 (Brookes et al. 1982).
The e ciency of conversion of nutrients taken up by the plant into crop biomass was calculated as follows (Tittonella et al. 2008): Conversion e ciency of nutrient X = total aboveground biomass/total uptake of nutrient X, where, the total aboveground biomass is the sum of the leaf biomass and shoot biomass at different stages, expressed on a dry weight basis. The conversion e ciencies for N and P have the units: g DM mg N −1 , g DM mg P −1 taken up by the soybean per plant, respectively. The uptake of nutrients was calculated from measurements of N and P concentrations in leaf and shoot biomass.

Statistical analysis
The differences in C, N, P concentrations and stoichiometric ratios, root biomass, aboveground biomass and conversion e ciency of N and P between the two straw treatments and between the two O 3 stress were evaluated by one-way analysis of variance (ANOVA) according to

Results
The root biomass of soybean The O 3 stress signi cantly decreased the root biomass and aboveground biomass (Tables 1 and 2). Furthermore, the straw return showed signi cant positive effects on the root biomass at branching and owering stages and on the aboveground biomass at branching stage under the O 3 stress (Table 1). According to the results of general linear model (GLM), the growth stage × straw return, the growth stage × O 3 stress and the O 3 stress × straw return showed signi cant effects on the root biomass and aboveground biomass (all at P < 0.05; Table 3)     The O 3 stress showed signi cant effects on C/N, C/P and N/P ratios and insigni cant effects on the concentrations of C, N and P of soybean shoot (Fig. 2). The ratios of C/N and C/P of shoot were signi cantly decreased at the stages of branching and podding by the O 3 stress (Fig. 2B, D). The O 3 stress signi cantly increased the C/N ratio of shoot at podding stage (Fig. 2B). There were insigni cant differences in C/P ratio of shoot between the O 3 stress and non-ltered air treatment (CK) at podding stage (Fig. 2D). There were no signi cant effects of the O 3 stress × straw return on C concentration and C/P ratio of soybean shoot ( Fig. 2A and D). The straw return signi cantly increased C concentration and C/N ratio of soybean shoot under the O 3 stress at owering and podding stages, while there were insigni cant differences in C concentration and C/N ratio of soybean shoot under the O 3 stress between straw return treatment and no straw return treatment at branching stage ( Fig. 2A and B).

C, N, P concentrations and stoichiometric ratios of roots, soils and soil microbial biomass
Generally, the O 3 stress and the straw return × O 3 stress had signi cant effects on the C concentration and C/N ratio of soybean root according to GLM results (Fig. 3). The C concentration of root was signi cantly decreased at the stages of branching and podding by the O 3 stress (Fig. 3A). There were insigni cant differences in C concentration of root between the O 3 stress and non-ltered air treatment (CK) at owering stage. The O 3 stress signi cantly decreased the C/P ratio of root at the stages of branching and owering and increased the C/P ratio of root at the podding stage (Fig. 3A). There were insigni cant effects of the straw return × O 3 stress on the N and P concentrations and C/P and N/P ratios of soybean shoot (Fig. 3B, D, F). The straw return signi cantly increased P concentration and signi cantly decreased N/P ratio of soybean root under the O 3 stress at podding stage, while there were insigni cant differences in P concentration and N/P ratio of soybean root under the O 3 stress between straw return treatment and no straw return treatment at branching and owering stages ( Fig. 3E and F).
According to GLM results, the O 3 stress and the interaction effects of the O 3 stress and straw return showed signi cant effects on SOC and P concentration of soybean soil (Fig. 4A, E). The O 3 stress signi cantly decreased SOC and P concentration of soil at the stages of owering and podding (Fig. 4A, E). There were insigni cant differences in SOC and P concentration of soil between the O 3 stress and nonltered air treatment (CK) at branching stage. The straw return showed insigni cant effects on C/N and N/P ratios (Fig. 4B, F). The straw return signi cantly increased SOC, N concentration and C/P ratio under the O 3 stress at the branching stage, while there were insigni cant differences in SOC, N concentration and C/P ratio under the O 3 stress between straw return treatment and no straw return treatment at podding stage (Fig. 4A, C, D).
Generally, the O 3 stress, the straw return and the interaction effects of O 3 stress and straw return showed signi cant effects on soil microbial biomass according to GLM results (Fig. 5). The O 3 stress signi cantly decreased the ratios of MBC/MBP and MBN/MBP at the stages of branching and owering and increased the ratios of MBC/MBP and MBN/MBP at the podding stage (Fig. 5D, F). The MBC and MBC/MBN ratio were signi cantly decreased at the branching stage by the O 3 stress (Fig. 5A, B). There were insigni cant differences in

Relationships Between Plants And Soils
There were signi cant relationships between the conversion e ciency of N and root N concentration under the O 3 stress, and between the conversion e ciency of P and root P concentration under the combined effects of O 3 stress and straw return (Fig. 6). According to the results of the path analyses, the O 3 stress had signi cant positive correlation with leaf stoichiometric ratios (Fig. 8A). There were signi cant positive relationships between the stoichiometric ratios of leaf and shoot, and between the stoichiometric ratios of microbial biomass and root biomass, and between the stoichiometric ratios of root and root biomass. There were signi cant negative relationships between the soil stoichiometric ratios and root biomass, and between the stoichiometric ratios of microbial biomass and root. Considering the combined effects of the O 3 stress and straw return, the O 3 stress showed signi cant positive correlation with the C, N, P concentrations of leaf (Fig. 8B). There were signi cant positive relationships between the C, N, P concentrations of leaf and shoot, and between the C, N, P concentrations of shoot and root. The stoichiometric ratios of microbial biomass showed negative relationships with the C, N, P concentrations of root and root biomass.

Discussion
In the present study, the O 3 stress signi cantly inhibited root growth during the whole stage of soybean, consistent with our previous study . Furthermore, straw return signi cantly increased root biomass exposed to the O 3 stress at branching and owering stages. There were insigni cant differences in root biomass between straw return treatment and no straw return treatment at podding stage. Indicated that straw return promoted root growth during branching and owering stage, while straw return inhibited root growth at podding stage of soybean exposed to the O 3 fumigation. Thus, straw return effectively prevented the O 3 damage only at branching and owering stage.  (Temple and Riechers 1995). The inconsistent results are probably because the different nutrient availability of soils, different O 3 stress, or plant species-speci c differences (Shang et al. 2018). In the present study, leaf C/N and C/P ratios and conversion e ciency were used as the nutrient use e ciency. Generally, the O 3 stress signi cantly inhibited the conversion e ciency of N and P in the present study. Although the straw return signi cantly increased the conversion e ciency of N and P at the branching and owering stages, the conversion e ciency of N and P was still signi cant lower in the O 3 stress than that in the non-ltered air treatment during the whole stages of soybean. Thus, the straw return had limited mitigation effect on the damage from the O 3 stress. Meanwhile, the conversion e ciency of P showed signi cant relationship with root P concentration under the combined effects of O 3 stress and straw return. Indicated that the responses of P concentration of plants (above-ground) and roots (below-ground) in the soybean cropland were strongly linked. And the limited mitigation effect of straw return on the damage from the O 3 stress was might due to the changes of P concentrations between plants and roots.
Generally, the straw return signi cantly increased the SOC and N concentrations of the soil cropped to soybean, consistent with the previous studies Liu et al. 2014). Meanwhile, under O 3 stress, there were insigni cant differences in N and P concentrations of leaves between straw return treatment and no straw return treatment. Indicated that straw return might not alleviate the destructive effect of O 3 stress on leaves. However, under O 3 stress, N and P concentrations and C/P and N/P ratios of soil in straw return treatments were signi cantly higher than that in no straw return treatments in branching and owering stages, indicating that straw return might alleviate the destructive effect of O 3 stress on soil. Moreover, the path analyses results suggested that the straw return showed signi cant effects on C, N, and P concentrations of soils and roots. Thus, the limited mitigation effects of straw return on the damage from the O 3 stress might be due to the changes of C, N, P concentrations of soils and roots.
Meanwhile, the present study also found that the effects of the straw return under the O 3 treatment on the C, N and P concentrations varied greatly and depended on the growth stage of soybean, consistent with the rst hypothesis of the present study. For instance, the trends of C concentration and N concentration of soils and roots in the owering stage were inconsistent with the observed in the podding stage. The trends of MBC, MBN and MBP in the owering stage were inconsistent with the observed in the podding stage. Furthermore, the path analyses results indicated that the relationship between the responses of the plants and soils to the combined effects of the O 3 stress and the straw return were not mainly driven by the stoichiometric ratios but more generally by the chemical concentration of the plants and soils, contrary to the second hypothesis of the present study. In other words, plant chemical traits might be able to override ecological stoichiometric ratios as a driver of defense against the O 3 stress in the case of straw return.

Conclusions
This study contributes to address the in uence of the O 3 stress and/or straw return on the C, N, P concentrations and stoichiometric ratios of plants and soils during a soybean growing season. The O 3 stress signi cantly inhibited soybean root growth, and the straw return effectively prevented the O 3 damage on soybean root at branching and owering stages of soybean. Con rming the rst hypothesis, the signi cant effects of the O 3 stress and/or straw return on C, N, P concentrations and stoichiometric ratios of leaf, shoot, root and soils varied greatly and depended on the growth stage of soybean. The root growth of soybean might be indirectly affected by the O 3 stress through the alterations of soil microbial biomass stoichiometry which were directly induced by the leaf stoichiometric ratios. The straw return had limited mitigation effect on the damage from the O 3 stress via the changes of C, N, P concentrations of soils and roots.
Furthermore, plant C, N, and P concentrations might be more important than ecological stoichiometric ratios as a driver of defense against the O 3 stress in the case of straw return, contrary to the second hypothesis of present study. Collectively, these ndings of present study provide a further understanding and forecasting on nutrient utilization and feedbacks between plants and soils during a growth season of soybean.
Declarations Figure 1 C, N, P concentrations and stoichiometric ratios of soybean leaves in response to non-ltered air treatment (CK), O3 stress, straw return (S+) and no straw return (S-) at growth stages of branching, owering and podding. Data are shown as mean ± standard error (n=3). For each parameter, results of general linear model (GLM) are reported, with asterisks showing the signi cant effect of O3 stress (O3) and growth stage (stage) and their interaction: ** P < 0.01, * P < 0.05, n.s. not signi cant. Different letters above the bars represent signi cant differences from Tukey's multiple comparisons among the treatments (P<0.05).   C, N, P concentrations and stoichiometric ratios of soils cropped to soybean in response to non-ltered air treatment (CK), O3 stress, straw return (S+) and no straw return (S-) at growth stages of branching, owering and podding. Data are shown as mean ± standard error (n=3).
For each parameter, results of general linear model (GLM) are reported, with asterisks showing the signi cant effect of O3 stress (O3) and growth stage (stage) and their interaction: ** P < 0.01, * P < 0.05, n.s. not signi cant. Different letters above the bars represent signi cant differences from Tukey's multiple comparisons among the treatments (P<0.05). SOC, soil organic C.  Relationship between conversion e ciency of N and root N concentration (A) and between conversion e ciency of P and root P concentration (B). regression lines in panel: solid black = the combined effects of O3 stress and straw return; dotted black = the O3 stress.