**1. Introduction**

The semiarid Loess Plateau region in China is characterized by severe water scarcity, soil erosion, and nutrient-poor soils, which all greatly limit vegetation growth [1]. Grassland, with a size of ~2.7 <sup>×</sup> <sup>10</sup><sup>5</sup> km<sup>2</sup> , accounts for ca. 43% of the regional total land area and is the dominant vegetation type on the Plateau [2]. It provides essential ecosystem functions and services such as carbon sequestration, soil and water conservation, and biodiversity [3,4]. Grassland management and restoration are of great ecological and economic significance in the region. Fertilization, as an effective management practice to

**Citation:** Jin, Y.; Lai, S.; Chen, Z.; Jian, C.; Zhou, J.; Niu, F.; Xu, B. Leaf Photosynthetic and Functional Traits of Grassland Dominant Species in Response to Nutrient Addition on the Chinese Loess Plateau. *Plants* **2022**, *11*, 2921. https://doi.org/10.3390/ plants11212921

Academic Editors: Jianhui Huang and Giuseppe Fenu

Received: 11 June 2022 Accepted: 27 October 2022 Published: 30 October 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

increase grassland productivity and promote grassland restoration, is adopted in many natural/semi-natural grasslands, e.g., in the Inner Mongolian steppe [5], the alpine meadow on the Qinghai–Tibet Plateau [6] in China, and in the European alpine grasslands [7] and the North American Great Plains [8]. However, fertilization may also lead to undesirable consequences such as exacerbated eutrophication [9], reduced ecosystem resilience (e.g., increased grassland drought sensitivity [8,10]), and biodiversity loss [11–13], which subsequently alter community composition and structure, as well as decrease the effects of diversity stability on maintaining grassland productivity [14]. Soils in the semiarid Loess Plateau are commonly deficient in both nitrogen (N) and phosphorus (P) [15]. Previous studies in the region have shown that appropriate N and P fertilization could improve soil N and P availability and play a positive role in increasing grassland productivity and recovery of degraded grasslands [16–18]. On the other hand, the effects of the N and P addition on photosynthetic and leaf functional traits of dominant species, which are important for understanding underlying mechanisms of grassland community dynamics under N and P inputs, have only received limited attention in the regional grassland.

Plant photosynthesis is the basis of plant growth, and its diurnal patterns reflect the sustained carbon assimilation ability of plants and have been extensively studied across a wide range of arid and semiarid grassland species, particularly in North America (e.g., [19–22]). Previous studies have explored the photosynthetic diurnal dynamics—under elevated CO<sup>2</sup> conditions—in dry and wet years [22,23], the diurnal photosynthetic performance of grasses with contrasting functional types (e.g., C<sup>3</sup> vs. C4, invasive vs. native) [20,21], and biotic and abiotic controls of photosynthetic diurnal courses [19,24]. However, there is limited information on the impacts of nutrient addition on photosynthetic diurnal courses in dryland grassland species. As an essential element of all proteins in plants (e.g., nucleic acid, enzymes, and chlorophyll), N primarily determines plant photosynthetic performance [25,26]. Extensive studies have documented that N addition increases plant photosynthetic rate and promotes plant growth in grasslands [27,28], and this promotion may be mediated by soil moisture [10]. However, when N addition exceeds a threshold, it will not continue to increase plant photosynthesis or, even, inhibit plant growth [26,29]. P, as another essential macronutrient, is also vital for plant photosynthesis, and it is the main component of ATP, NADPH, and phospholipids, which all play important roles in regulating photosynthesis machinery and electron transport activities [30,31]. Apart from the direct regulation of N/P on plant photosynthesis, N and P addition could indirectly or interactively affect plant photosynthesis. For instance, P addition could improve photosynthesis by increasing leaf area and stomatal aperture, particularly under soil water deficit conditions [32,33]. P addition could increase the activity of N-fixing bacteria, nodule biomass, and nitrogenase activity in legumes, which subsequently increases leaf N and P content and photosynthetic rate [34]. The combined fertilization of N and P is, thereby, often considered an effective management tool for sustaining productivity in many grassland communities, while such effects need to be evaluated in the regional semiarid grassland on the Loess Plateau.

Besides photosynthetic characteristics, other important leaf functional traits, such as specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen mass (*N*mass), and leaf phosphorus mass (*P*mass), are also the intuitive representation of strategies adopted by plants to cope with environmental changes [35,36]. According to the leaf economic spectrum (LES) theory [37], angiosperm plants could generally be divided into a rapid/slow growing strategy according to a set of leaf functional traits, with the rapid-growing ones having low LDMC and high *P*n, SLA, *N*mass, and *P*mass; contrarily, the slow-growing ones have the opposite leaf traits. This kind of functional trait-based theory, from leaf to plant levels, provides great insights into understanding species resource utilization and species distribution [37]. Nevertheless, species-specific responses in these leaf economic traits exist under varied environmental (e.g., under different soil water and nutrient availability) conditions, which should be systematically assessed [38]. Efforts have been made to quantify the variation of LES of grassland species under differed soil nutrient availability, which confirmed species-specific patterns [39], while the assessment of species-specific

responses to varying N and P fertilization conditions is seldom conducted on the semiarid Loess Plateau, which is needed to better understand grassland community development. which confirmed species-specific patterns [39], while the assessment of species-specific responses to varying N and P fertilization conditions is seldom conducted on the semiarid Loess Plateau, which is needed to better understand grassland community development.

conditions, which should be systematically assessed [38]. Efforts have been made to quantify the variation of LES of grassland species under differed soil nutrient availability,

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The dominant species occupy important ecological niches and play vital roles in maintaining community structure and function [40,41]. Biomass increases after N/P additions tend to be achieved by decreasing species diversity or increasing the biomass of dominant species [42]. Quantifying physiological and growth characteristics of dominant species under N and/or P addition could, thereby, be important for the evaluation of community productivity and dynamics, as well as provide valuable information for grassland management and restoration. *Bothriochloa ischaemum* (L.) Keng (a C<sup>4</sup> perennial grass), *Stipa bungeana* Trin. (a C<sup>3</sup> perennial grass), and *Lespedeza davurica* (Laxm.) Schindl. (a C<sup>3</sup> N-fixing subshrub) are co-dominant species in the natural/restored grasslands on the semiarid Loess Plateau of China [43]. The previous study on a regional grassland community, targeting these species, has shown that the addition of N and P combined improved grassland productivity and decreased species diversity, primarily via effects on tall clonal and annual species [44], which, once again, suggested species or functional-type-specific responses within a community, while the variation of leaf functional traits in these dominant species, after N and P fertilization, have not been fully assessed. Thus, we examined photosynthetic diurnal change, SPAD value, and leaf economic traits, including *N*mass, *P*mass*, Nmass/Pmass* ratio, SLA, and LDMC of the three dominant species, following a three-year N and P addition experiment in a typical semiarid grassland community on the Loess Plateau. We tested the hypotheses that: (1) N/P addition would increase photosynthetic rates and alter the photosynthetic diurnal dynamics of the three dominant species in the peak growing season, and these photosynthetic responses would be related to species-specific shifts in leaf functional traits; (2) addition of N and P combined would further promote photosynthesis compared with N/P additions alone. The dominant species occupy important ecological niches and play vital roles in maintaining community structure and function [40,41]. Biomass increases after N/P additions tend to be achieved by decreasing species diversity or increasing the biomass of dominant species [42]. Quantifying physiological and growth characteristics of dominant species under N and/or P addition could, thereby, be important for the evaluation of community productivity and dynamics, as well as provide valuable information for grassland management and restoration. *Bothriochloa ischaemum* (L.) Keng (a C4 perennial grass), *Stipa bungeana* Trin. (a C3 perennial grass), and *Lespedeza davurica* (Laxm.) Schindl. (a C3 N-fixing subshrub) are co-dominant species in the natural/restored grasslands on the semiarid Loess Plateau of China [43]. The previous study on a regional grassland community, targeting these species, has shown that the addition of N and P combined improved grassland productivity and decreased species diversity, primarily via effects on tall clonal and annual species [44], which, once again, suggested species or functional-type-specific responses within a community, while the variation of leaf functional traits in these dominant species, after N and P fertilization, have not been fully assessed. Thus, we examined photosynthetic diurnal change, SPAD value, and leaf economic traits, including *N*mass, *P*mass*, Nmass/Pmass* ratio, SLA, and LDMC of the three dominant species, following a three-year N and P addition experiment in a typical semiarid grassland community on the Loess Plateau. We tested the hypotheses that: (1) N/P addition would increase photosynthetic rates and alter the photosynthetic diurnal dynamics of the three dominant species in the peak growing season, and these photosynthetic responses would be related to species-specific shifts in leaf functional traits; (2) addition of N and P combined would further promote photosynthesis compared with N/P additions alone.

#### **2. Results**

#### *2.1. Environmental Factors* **2. Results**

Photosynthetically active radiation (PAR) and air temperature (*T*a) showed a singlepeaked diurnal curve during the experimental period, and the maximum values appeared at 12:00 h and 14:00 h, with the values of 1854 <sup>µ</sup>mol·m−<sup>2</sup> ·s <sup>−</sup><sup>1</sup> and 30.2 ◦C, respectively (Figure 1). The relative humidity (RH) remained stable during the daytime (~13%) (Figure 1). *2.1. Environmental Factors*  Photosynthetically active radiation (PAR) and air temperature (*T*a) showed a singlepeaked diurnal curve during the experimental period, and the maximum values appeared at 12:00 h and 14:00 h, with the values of 1854 μmol·m−2·s−1 and 30.2 °C, respectively (Figure 1). The relative humidity (RH) remained stable during the daytime (~13%) (Figure 1).

**Figure 1.** Diurnal changes of photosynthetically active radiation (PAR), air relative humidity (RH), and air temperature (*T*a) during the measurement period (20–22 July 2019). **Figure 1.** Diurnal changes of photosynthetically active radiation (PAR), air relative humidity (RH), and air temperature (*T*a) during the measurement period (20–22 July 2019).

#### *2.2. Diurnal Changes in Photosynthesis 2.2. Diurnal Changes in Photosynthesis*

The diurnal changes of net photosynthetic rate (*P*n) and leaf instantaneous water use efficiency (WUE) of the three dominant species showed a double-peak curve under different N and P addition treatments. The first peak appeared at 10:00 h, the second at 14:00 h,

and the midday depression of the photosynthesis (so-called "noon break") appeared at around 12:00 h (Figure 2). The leaf transpiration rate (*T*r) of *B. ischaemum* mostly showed a double-peak diurnal course. While diurnal changes of *T*<sup>r</sup> in *S. bungeana* and *L. davurica* showed a single peak. h, and the midday depression of the photosynthesis (so-called "noon break") appeared at around 12:00 h (Figure 2). The leaf transpiration rate (*T*r) of *B. ischaemum* mostly showed a double-peak diurnal course. While diurnal changes of *T*r in *S. bungeana* and *L. davurica* showed a single peak.

The diurnal changes of net photosynthetic rate (*P*n) and leaf instantaneous water use efficiency (WUE) of the three dominant species showed a double-peak curve under different N and P addition treatments. The first peak appeared at 10:00 h, the second at 14:00

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**Figure 2.** Diurnal changes of net photosynthetic rate (*P*n), transpiration rate (*T*r), instantaneous water use efficiency (WUE), and stomatal limitation value (*L*s) of the three species under different N and P addition treatments. Vertical bars indicate LSD values. **Figure 2.** Diurnal changes of net photosynthetic rate (*P*n), transpiration rate (*T*r), instantaneous water use efficiency (WUE), and stomatal limitation value (*L*s) of the three species under different N and P addition treatments. Vertical bars indicate LSD values.

Compared with CK (i.e., N0P0), N addition alone and addition of N and P combined significantly increased the *P*n values at 10:00 h and 14:00 h in the three species (except under N50P40 and N50P80 treatments in *S. bungeana*). The greatest *P*n values appeared at 10:00 h under N and P combined treatments (i.e., N50P40 and N50P80) for the three species. The WUE of the three species significantly increased by N addition alone compared with CK (Table 1). The WUE of *L. davurica* increased significantly under all levels of P alone additions, while the WUE of the two grasses only significantly increased under N0P40 (Table 1). Under N and P combined addition, the maximum WUE values of *B. ischaemum*, *S. bungeana,* and *L. davurica* were 1.17, 1.09, and 1.47 μmol mmol−1, respectively Compared with CK (i.e., N0P0), N addition alone and addition of N and P combined significantly increased the *P*<sup>n</sup> values at 10:00 h and 14:00 h in the three species (except under N50P40 and N50P80 treatments in *S. bungeana*). The greatest *P*<sup>n</sup> values appeared at 10:00 h under N and P combined treatments (i.e., N50P40 and N50P80) for the three species. The WUE of the three species significantly increased by N addition alone compared with CK (Table 1). The WUE of *L. davurica* increased significantly under all levels of P alone additions, while the WUE of the two grasses only significantly increased under N0P40 (Table 1). Under N and P combined addition, the maximum WUE values of *B. ischaemum*, *S. bungeana,* and *L. davurica* were 1.17, 1.09, and 1.47 µmol mmol−<sup>1</sup> , respectively (Table 1).

(Table 1). N addition, alone, significantly increased the *L*<sup>s</sup> values of *B. ischaemum* and *L. davurica* (Table 1). P addition, alone, increased (*p* < 0.05) and decreased (*p* < 0.05) the *L*<sup>s</sup> of *L davurica* and *S. bungeana*, respectively, while only significantly increasing the *L*<sup>s</sup> of *B. ischaemum* under N0P80 treatment. N and P interaction significantly affected the *L*<sup>s</sup> values of the three species (Table 1). Under the addition of N50 combined with P, the *L*<sup>s</sup> of *L. davurica* increased, and those of *B. ischaemum* decreased (both *p* < 0.05). Under the addition of N100 combined with P, the *L*<sup>s</sup> of *B. ischaemum* significantly increased, while those of *S. bungeana* decreased significantly (*p* < 0.05; Table 1).

*L. davurica*

*B. ischaemum*


decreased significantly (*p* < 0.05; Table 1).

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species under different N and P additions (mean ± SD, *n* = 3).

**Species Treatment WUE (μmol mmol–1)** *L***<sup>s</sup>**

**Table 1.** Instantaneous water use efficiency (WUE) and stomatal limitation value (*L*s) of the three species under different N and P additions (mean ± SD, *n* = 3). N0 0.93 ± 0.01 C,b 1.36 ± 0.03 A,a 1.18 ± 0.01 B,b 0.45 ± 0.01 B,b 0.52 ± 0.01 A,a 0.51 ± 0.01 A,b N50 1.24 ± 0.00 C,a 1.33 ± 0.02 B,b 1.47 ± 0.06 A,a 0.48 ± 0.01 B,a 0.50 ± 0.00 A,a 0.52 ± 0.01 A,b N100 1.22 ± 0.02 B,a 1.36 ± 0.01 A,a 1.43 ± 0.03 A,a 0.51 ± 0.00 B,a 0.51 ± 0.02 B,a 0.55 ± 0.01 A,a

**Table 1.** Instantaneous water use efficiency (WUE) and stomatal limitation value (*L*s) of the three

**P0 P40 P80 P0 P40 P80** 

N0 0.95 ± 0.02 B,b 0.97 ± 0.02 A,b 0.81 ± 0.01 C,b 0.44 ± 0.00 B,c 0.45 ± 0.01 B,b 0.48 ± 0.00 A,a N50 1.15 ± 0.02 A,a 1.17 ± 0.01 A,a 1.10 ± 0.01 B,a 0.50 ± 0.01 A,a 0.46 ± 0.01 B,b 0.48 ± 0.01 B,a N100 1.10 ± 0.05 A,a 1.21 ± 0.02 A,a 1.11 ± 0.04 A,a 0.42 ± 0.00 B,b 0.50 ± 0.01 A,a 0.48 ± 0.01 A,a

N0 0.69 ± 0.01 B,b 0.78 ± 0.01 A,b 0.59 ± 0.01 C,b 0.68 ± 0.00 A,a 0.35 ± 0.01 B,a 0.33 ± 0.01 B,b N50 1.05 ± 0.01 A,a 0.97 ± 0.02 B,a 0.97 ± 0.01 B,a 0.36 ± 0.02 A,c 0.35 ± 0.01 A,a 0.32 ± 0.01 B,b N100 0.91 ± 0.02 A,a 1.09 ± 0.02 A,a 0.84 ± 0.02 B,a 0.57 ± 0.03 A,b 0.36 ± 0.00 B,a 0.37 ± 0.00 B,a

Data with different capital letters indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are *F*-values, while '\*\*' and '\*\*\*' indicate *p* ≤ 0.01, and *p* ≤ 0.001, respectively. creased, and those of *B. ischaemum* decreased (both *p* < 0.05). Under the addition of N100 combined with P, the *L*s of *B. ischaemum*significantly increased, while those of *S. bungeana*

N addition, alone, increased the *P*Nmax values of the three species (*p* < 0.05), while there was no difference between them under N50 and N100 (Figure 3). The increase in *P*Nmax in the two grasses was about two times larger than those of *L. davurica* under N addition alone (Figure 3). P addition, alone, increased the *P*Nmax values of *L. davurica* and *S. bungeana* (*p* < 0.05; Figure 3). N and P combined addition only significantly affected the *P*Nmax values of *S. bungeana* and *L. davurica* (*p* < 0.05; Figure 3). N addition, alone, increased the *P*Nmax values of the three species (*p* < 0.05), while there was no difference between them under N50 and N100 (Figure 3). The increase in *P*Nmax in the two grasses was about two times larger than those of *L. davurica* under N addition alone (Figure 3). P addition, alone, increased the *P*Nmax values of *L. davurica* and *S. bungeana* (*p* < 0.05; Figure 3). N and P combined addition only significantly affected the *P*Nmax values of *S. bungeana* and *L. davurica* (*p* < 0.05; Figure 3).

**Figure 3.** Leaf maximum net photosynthetic rate (*P*Nmax) of the three species under different N and P addition treatments. Values are mean ± SD. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are *F*-values, while '\*\*', and '\*\*\*' indicate *p* ≤ 0.01, and *p* ≤ 0.001, respectively. 'n.s.' indicates no significant difference. **Figure 3.** Leaf maximum net photosynthetic rate (*P*Nmax) of the three species under different N and P addition treatments. Values are mean ± SD. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are *F*-values, while '\*\*', and '\*\*\*' indicate *p* ≤ 0.01, and *p* ≤ 0.001, respectively. 'n.s.' indicates no significant difference.

#### *2.3. Leaf SPAD Value*

N addition, alone, significantly increased the SPAD values of the three species (Figure 4). P addition, alone, significantly increased the SPAD values of *B. ischaemum* and *L. davurica* (*p* < 0.05), while SPAD values only increased under low levels of P addition alone (i.e., N0P40) in *S. bungeana* (*p* < 0.05). Addition of N and P combined significantly increased the SPAD values of both *B. ischaemum* and *L. davurica* (Figure 4).

the SPAD values of both *B. ischaemum* and *L. davurica* (Figure 4).

*2.3. Leaf SPAD Value* 

**Figure 4.** Leaf SPAD values of the three species under different N and P addition treatments. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are *F*-values, while '\*' and '\*\*\*' indicate *p* ≤ 0.05 and *p* ≤ 0.001, respectively. 'n.s.' indicates no significant difference. **Figure 4.** Leaf SPAD values of the three species under different N and P addition treatments. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are *F*-values, while '\*' and '\*\*\*' indicate *p* ≤ 0.05 and *p* ≤ 0.001, respectively. 'n.s.' indicates no significant difference.

N addition, alone, significantly increased the SPAD values of the three species (Figure 4). P addition, alone, significantly increased the SPAD values of *B. ischaemum* and *L. davurica* (*p* < 0.05), while SPAD values only increased under low levels of P addition alone (i.e., N0P40) in *S. bungeana* (*p* < 0.05). Addition of N and P combined significantly increased

#### *2.4. Leaf N and P Concentration (Nmass and Pmass) and Nmass/Pmass Ratio*  N addition, alone, significantly increased the *Nmass* values of the two grasses, but it *2.4. Leaf N and P Concentration (Nmass and Pmass) and Nmass/Pmass Ratio*

had no effects on *Nmass* of *L. davurica*. The high level of P addition, alone (N0P80), significantly increased *Pmass* of all species except *L. davurica* (Figure 5). N and P interaction significantly affected *Nmass* of the three species, while it only significantly affected *Pmass* of the two grasses. Under the addition of N50 combined with P, the *Pmass* of *B. ischaemum* and *S. bungeana*, as well as the *Nmass* of *L. davurica*, increased significantly. Under the addition of N100 combined with P, the *Pmass* of *S. bungeana* increased significantly, while those of *L. davurica* decreased significantly (Figure 5). N addition, alone, significantly increased the *Nmass* values of the two grasses, but it had no effects on *Nmass* of *L. davurica*. The high level of P addition, alone (N0P80), significantly increased *Pmass* of all species except *L. davurica* (Figure 5). N and P interaction significantly affected *Nmass* of the three species, while it only significantly affected *Pmass* of the two grasses. Under the addition of N50 combined with P, the *Pmass* of *B. ischaemum* and *S. bungeana*, as well as the *Nmass* of *L. davurica*, increased significantly. Under the addition of N100 combined with P, the *Pmass* of *S. bungeana* increased significantly, while those of *L. davurica* decreased significantly (Figure 5). *Plants* **2022**, *11*, x FOR PEER REVIEW 7 of 17

**Figure 5.** Leaf nitrogen (N) and phosphorus (P) content (*N*mass and *Pmass*), as well as the *N*mass/*Pmass* ratio, of the three species under different N and P addition treatments. Values are mean ± SD. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are *F*-values, while '\*', '\*\*', and '\*\*\*' indicate *p* ≤ 0.05, *p* ≤ 0.01, and *p*≤ 0.001, respectively. 'n.s.' indicates no significant difference. N addition, alone, significantly affected the *Nmass/Pmass* ratios of all three species. The **Figure 5.** Leaf nitrogen (N) and phosphorus (P) content (*N*mass and *Pmass*), as well as the *N*mass/*Pmass* ratio, of the three species under different N and P addition treatments. Values are mean ± SD. Different capital letters above the column indicate significant differences among P additions under each N addition rate, while different small letters indicate significant differences among N additions under each P addition rate. Numbers in parentheses are *F*-values, while '\*', '\*\*', and '\*\*\*' indicate *p* ≤ 0.05, *p* ≤ 0.01, and *p*≤ 0.001, respectively. 'n.s.' indicates no significant difference.

*2.5. Specific Leaf Area (SLA) and Leaf Dry Matter Content (LDMC)* 

low level of N addition alone (N50P0) significantly increased the *Nmass/Pmass* of the two grasses; the high level of N addition, alone (N100P0), only increased the *Nmass/Pmass* of *S. bungeana* grass (*p* < 0.05, Figure 5). N and P addition interaction significantly affects the

combined with P, the *Nmass/ Pmass* of *B. ischaemum* and *S. bungeana* decreased significantly, while the *Nmass/Pmass* of *L. davurica* increased under N50P40 treatment (*p* < 0.05). Under the addition of N100 combined with P, the *Nmass/ Pmass* of *S. bungeana* decreased significantly, while the *Nmass/Pmass* of *B. ischaemum* and *L. davurica* had no significant changes (Figure 5).

N addition, alone, significantly increased the SLA values of the three species, and the low level of N addition alone (N50P0) decreased the LDMC of the two grasses (*p* < 0.05, Figure 6). P addition, alone, significantly increased SLA of *L. davurica*, while it had limited effects on the two grasses (Figure 6). N and P addition interaction significantly affected both the SLA and LDMC values of all three species. Under the addition of N50 combined with P, the SLA of *B. ischaemum* and *L. davurica* increased significantly (*p* < 0.05), and the SLA of *S. bungeana* increased under N50P40, while it decreased significantly under N50P80 (*p* < 0.05); LDMC was comparable between different levels of P additions in the two grasses, but it decreased in *L. davurica*. Under the addition of N100 combined with P,

N addition, alone, significantly affected the *Nmass/Pmass* ratios of all three species. The low level of N addition alone (N50P0) significantly increased the *Nmass/Pmass* of the two grasses; the high level of N addition, alone (N100P0), only increased the *Nmass/Pmass* of *S. bungeana* grass (*p* < 0.05, Figure 5). N and P addition interaction significantly affects the *Nmass/Pmass* of the two grasses but has no effect on the subshrub. Under the addition of N50 combined with P, the *Nmass/ Pmass* of *B. ischaemum* and *S. bungeana* decreased significantly, while the *Nmass/Pmass* of *L. davurica* increased under N50P40 treatment (*p* < 0.05). Under the addition of N100 combined with P, the *Nmass/ Pmass* of *S. bungeana* decreased significantly, while the *Nmass/Pmass* of *B. ischaemum* and *L. davurica* had no significant changes (Figure 5).
