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Journal of Arid Land
Journal of Arid Land  2026, Vol. 18 Issue (5): 868-885    DOI: 10.1016/j.jaridl.2026.05.008     CSTR: 32276.14.JAL.20250098
Research article     
Grassland biomass production and plant species diversity in response to nitrogen and phosphorus addition in central and southwestern Tajikistan
Mekhrovar OKHONNIYOZOV1,2,3,4, FAN Lianlian1,*(), MA Xuexi1, Sino YUSUPOV4, Hikmat HISORIEV1,4, Abdullo MADAMINOV4, Fakher ABBAS1, TAO Ye1, LI Yaoming1,2,3
1 China-Tajikistan Belt and Road Joint Laboratory on Biodiversity Conservation and Sustainable Use, Xinjiang Institute of Ecology Geography, Chinese Academy of Sciences, Urumqi, 830011, China
2 Research Center for Ecology and Environment of Central Asia, Dushanbe 734063, Tajikistan
3 University of Chinese Academy of Sciences, Beijing 100049, China
4 Institute of Botany, Plant Physiology and Genetics, National Academy of Science of Tajikistan, Dushanbe 734017, Tajikistan
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Abstract  

Nitrogen (N) and phosphorus (P) are essential nutrients regulating plant growth, yet their long-term impacts on grassland ecosystems in Tajikistan remain poorly understood. This study conducted a five-year (2018-2022) field experiment across four grassland sites (Tabakqi, Balkhi, Luchob, and Ziddi) along an elevation gradient in central and southwestern Tajikistan to explore the effects of varying N (0, 30, and 90 kg N/(hm2∙a)) and P (0 and 30 kg P/(hm2∙a)) additions on aboveground biomass (AGB) and plant species diversity. Nutrient addition significantly increased AGB across all sites. Compared with the control (without N or P addition), AGB increased by 20%-80% under moderate N treatment (adding 30 kg N/(hm2∙a)) and by up to 190%-200% under high N and P addition treatment (adding 90 kg N/(hm2∙a) and 30 kg P/(hm2∙a)). In 2022, AGB at the low-elevation site (Tabakqi) increased from 494 g/m2 under the control to 650 g/m2 under high N and P treatment, while at the high-elevation site (Ziddi), it rose from 552 to 1614 g/m2. In contrast, biodiversity responses were elevation-dependent: species richness declined at mid-elevation grassland sites (Balkhi and Luchob) but showed little change at low-elevation (Tabakqi) and high-elevation (Ziddi) sites. Shannon-Wiener index, Simpson's dominance index, and Pielou's equitability index also varied, reflecting complex interactions among nutrient addition, precipitation, and temperature. The structural equation model (SEM) confirmed that nutrient addition directly enhance AGB but generally suppress plant species diversity, while precipitation promotes AGB, and temperature effects are inconsistent across sites. Overall, our findings demonstrate that nutrient enrichment can increase productivity but reduce biodiversity, with responses strongly mediated by elevation and climate. These results provide the first long-term experimental evidence from Tajikistan's grasslands and underscore the need to balance productivity gains with biodiversity conservation in sustainable grassland management.

Key words： aboveground biomass (AGB)      plant species diversity      species richness      nutrient addition      structural equation model (SEM)      Tajikistan     
Received: 11 March 2025      Published: 31 May 2026
Corresponding Authors: *FAN Lianlian (E-mail: flianlian@ms.xjb.ac.cn)
About author: Author contributions

Conceptualization: LI Yaoming, FAN Lianlian; Data curation: Mekhrovar OKHONNIYOZOV, Sino YUSUPOV, MA Xuexi; Formal analysis: Mekhrovar OKHONNIYOZOV, FAN Lianlian; Funding acquisition: LI Yaoming, FAN Lianlian; Methodology: Hikmat HISORIEV, Abdullo MADAMINOV; Resources: Mekhrovar OKHONNIYOZOV, LI Yaoming, FAN Lianlian; Software: Mekhrovar OKHONNIYOZOV, FAN Lianlian, Fakher ABBAS; Validation: LI Yaoming, TAO Ye, Abdullo MADAMINOV; Visualization: Mekhrovar OKHONNIYOZOV; Writing - original draft: Mekhrovar OKHONNIYOZOV, FAN Lianlian; Writing - review and editing: Mekhrovar OKHONNIYOZOV, FAN Lianlian, LI Yaoming. All authors approved the manuscript.

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Mekhrovar OKHONNIYOZOV
FAN Lianlian
MA Xuexi
Sino YUSUPOV
Hikmat HISORIEV
Abdullo MADAMINOV
Fakher ABBAS
TAO Ye
LI Yaoming
Cite this article:

Mekhrovar OKHONNIYOZOV, FAN Lianlian, MA Xuexi, Sino YUSUPOV, Hikmat HISORIEV, Abdullo MADAMINOV, Fakher ABBAS, TAO Ye, LI Yaoming. Grassland biomass production and plant species diversity in response to nitrogen and phosphorus addition in central and southwestern Tajikistan. Journal of Arid Land, 2026, 18(5): 868-885.

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http://jal.xjegi.com/10.1016/j.jaridl.2026.05.008     OR     http://jal.xjegi.com/Y2026/V18/I5/868

Parameter Tabakqi Balkhi Luchob Ziddi
Latitude 37°51′33′′N 38°14′03′′N 38°39′55′′N 39°02′11′′N
Longitude 68°57′34′′E 69°17′01′′E 68°39′23′′E 68°49′22′′E
Elevation (m) 650 1100 1250 2000
Slope (°) 19 29 23 24
MAT (°C) 17.5 17.3 13 6.3
MAP (mm) 252 447 1017 667
SOC (g/kg) 8.2 17.3 28.65 29.79
TN (g/kg) 0.74 1.57 2.85 2.6
TP (g/kg) 0.6 0.79 0.88 0.63
Soil pH 8.04 7.96 7.63 8.41
Dominant species Aegilops triuncialis L.,
Poa bulbosa L., and
Strigosella turkestanica (Litv.) Botsch.		 Aegilops triuncialis L. and
Cynodon dactylon (L.) Pers.		 Hordeum bulbosum L.,
Bothriochloa ischaemum
(L.) Keng,
Avena trichophylla K.Koch, and Cynodon dactylon (L.) Pers.		 Prangos pabularia Lindl. and
Geranium collinum Stephan ex Willd.
Table 1 Brief description of the four grassland sites in central and southwestern Tajikistan
Fig. 1 Variations in aboveground biomass (AGB) under various nutrient addition treatments at the four grassland sites in 2018 (a), 2019 (b), 2020 (c), 2021 (d), and 2022 (e). In each panel, groups labeled with the identical capital letters above them do not exhibit significant differences at a significance level of P<0.050 level according to Duncan's test. Among each set of five error bars, error bars with distinct letters exhibit significant differences with P<0.050 level according to Duncan's test. Error bar represents the standard error of the mean. CK indicates the control (without nitrogen (N) and phosphorus (P) addition), N30 indicates adding 30 kg N/(hm2∙a), P30 indicates adding 30 kg P/(hm2∙a), N30P30 indicates adding 30 kg N/(hm2∙a) and 30 kg P/(hm2∙a), and N90P30 indicates adding 90 kg N/(hm2∙a) and 30 kg P/(hm2∙a).
Source Tabakqi Balkhi Luchob Ziddi
F P F P F P F P
Year 725.68 0.000 32.05 0.000 292.24 0.000 12.62 0.000
Treatment 95.47 0.000 50.80 0.000 167.48 0.000 5.26 0.000
Year×Treatment 6.75 0.000 2.59 0.000 20.73 0.000 1.52 0.120
Table 2 Effects of treatment, year, and their interactions on the aboveground biomass (AGB) at the four grassland sites during 2018‒2022
Fig. 2 Impact of nutrient addition treatments on species richness at Tabakqi (a), Balkhi (b), Luchob (c), and Ziddi (d) during 2018‒2022. * indicates that there are significant differences between treatments at P<0.050; ns indicates that the observed differences are not statistically significant among treatments. The upper and lower boundaries of the box represent the 25th and 75th percentiles, respectively. The line in the box indicates the median. The upper and lower whiskers represent the maximum and minimum values within 1.5 times the interquartile range from the 25th and 75th percentiles, respectively. Dots represent individual replicate values for each treatment; overlapping points may appear as a single dot when replicate values are identical or very similar.
Source Tabakqi Balkhi Luchob Ziddi
F P F P F P F P
Year 0.10 0.980 4.43 0.000 5.57 0.000 20.89 0.000
Treatment 0.76 0.560 2.46 0.050 6.65 0.000 0.14 0.970
Year×Treatment 2.06 0.020 4.48 0.000 7.69 0.000 0.97 0.490
Table 3 Effects of treatment, year, and their interactions on species richness at the four grassland sites during 2018‒2022
Fig. 3 Variations in the Simpson index (a, d, g, j, and m), Shannon index (b, e, h, k, and n), and Pielou index (c, f, i, l, and o) under various nutrient addition treatments at the four grassland sites from 2018 to 2022. In each panel, groups labeled with the identical capital letters above them do not exhibit significant differences at a significance level of P<0.050. Different lowercase letters above the error bars represent significant differences (P<0.050) among nutrient enrichment treatments; ns means that differences are not significant. Error bar represents the standard error of the mean. Simpson index, Simpson's dominance index; Shannon index, Shannon-Wiene index; Pielou index, Pielou's equitability index.
Fig. 4 Relationships of AGB with the Simpson index (a-d), Shannon index (e-h), Margalef index (i-l), and Pielou index (m-p) at Tabakqi, Balkhi, Luchob, and Ziddi sites. Margalef index, Margalef's richness index. Only fitted curves with significant correlations (P<0.050) are shown.
Fig. 5 Structural equation model (SEM) for the effect of climatic factors and nutrient addition on AGB and plant species diversity at Tabakqi (a), Balkhi (b), Luchob (c), and Ziddi (d) sites. df, degrees of freedom; AIC, Akaike Information Criterion. Blue bold arrows represent significant positive pathway; red dashed bold arrows represent significant negative pathway; and thin arrows represent non-significant pathways. The values beside the arrows indicate standardized path coefficients; the proportion of variance explained (R2) appears alongside the response variable in the model. In Fisher's C test, the P-value evaluates the overall goodness-of-fit of the SEM, where P>0.050 indicates that the model fits the data well (i.e., no significant difference between observed and expected relationships). In regression analysis of the SEM, *, **, and *** indicate statistically significant effects at P<0.050, P<0.010, and P<0.001 levels, respectively.
Fig. S1 Monthly temperature and annual precipitation at Tabakqi (a and e), Balkhi (b and f), Luchob (c and g), and Ziddi (d and h) sites during 2018‒2022
Fig. S2 Growing season precipitation at Tabakqi (February‒April), Balkhi (February‒May), Luchob (March‒ June), and Ziddi (May‒August) sites from 2018 to 2022
Fig. S3 Experimental design in this study. CK indicates the control (without nitrogen (N) and phosphorus (P) addition); N30 indicates adding 30 kg N/(hm2∙a), P30 indicates adding 30 kg P/(hm2∙a), N30P30 indicates adding 30 kg N/(hm2∙a) and 30 kg P/(hm2∙a), and N90P30 indicates adding 90 kg N/(hm2∙a) and 30 kg P/(hm2∙a).
Fig. S4 Pearson correlation matrix of aboveground biomass (AGB) with species diversity indices and climatic factors across four grassland sites. Larger and darker squares represent larger absolute correlation coefficients. Shannon index, Shannon-Wiener index; Simpson index, Simpson's dominance index; Margalef index, Margalef's richness index; Pielou index, Pielou's equitability index.
Site Source Simpson index Shannon index Margalef index Pielou index
f P f P f P f P
Tabakqi Year 68.58 0.000 23.56 0.000 27.17 0.000 48.01 0.000
Treatment 20.48 0.000 9.77 0.000 0.49 0.746 23.92 0.000
Year×Treatment 4.31 0.000 4.64 0.000 2.24 0.010 4.24 0.000
Balkhi Year 25.82 0.000 7.04 0.000 6.39 0.000 14.42 0.000
Treatment 38.69 0.000 30.49 0.000 7.33 0.000 45.04 0.000
Year×Treatment 7.53 0.000 4.21 0.000 4.89 0.000 5.00 0.000
Luchob Year 20.03 0.000 8.69 0.000 8.88 0.000 24.78 0.000
Treatment 83.57 0.000 24.59 0.000 3.09 0.021 66.55 0.000
Year×Treatment 6.40 0.000 6.64 0.000 7.96 0.000 2.33 0.008
Ziddi Year 18.68 0.000 18.00 0.000 96.79 0.000 15.18 0.000
Treatment 1.11 0.359 0.66 0.623 0.72 0.583 0.66 0.619
Year×Treatment 1.27 0.242 1.27 0.239 0.92 0.554 1.20 0.292
Table S1 Summary of the linear mixed-effect model relating fixed factors (year and treatment) for the species diversity indices at the four grassland sites during 2018-2022
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