Intercropping of Leguminous and Non-Leguminous Desert Plant Species Does Not Facilitate Phosphorus Mineralization and Plant Nutrition

Soil microbial functional profiles of P-cycling reveal drought-induced constraints on P-transformation in a hyper-arid desert ecosystem

Soil water conditions are known to influence soil nutrient availability, but the specific impact of different conditions on soil phosphorus (P) availability through the modulation of P-cycling functional microbial communities in hyper-arid desert ecosystems remains largely unexplored. To address this knowledge gap, we conducted a 3-year pot experiment using a typical desert plant species (Alhagi sparsifolia Shap.) subjected to two water supply levels (25 %-35 % and 65 %-75 % of maximum field capacity, MFC) and four P-supply levels (0, 1, 3, and 5 g P m-2 y-1). Our investigation focused on the soil Hedley-P pool and the four major microbial groups involved in the critical phases of soil microbial P-cycling. The results revealed that the drought (25 %-35 % MFC) and no P-supply treatments reduced soil resin-P and NaHCO3-Pi concentrations by 87.03 % and 93.22 %, respectively, compared to the well-watered (65 %-75 % MFC) and high P-supply (5 g P m-2 y-1) treatments. However, the P-supply treatment resulted in a 12 %-22 % decrease in the soil NH4+-N concentration preferred by microbes compared to the no P-supply treatment. Moreover, the abundance of genes engaged in microbial P-cycling (e.g. gcd and phoD) increased under the drought and no P-supply treatments (p < 0.05), suggesting that increased NH4+-N accumulation under these conditions may stimulate P-solubilizing microbes, thereby promoting the microbial community's investment in resources to enhance the P-cycling potential. Furthermore, the communities of Steroidobacter cummioxidans, Mesorhizobium alhagi, Devosia geojensis, and Ensifer sojae, associated with the major P-cycling genes, were enriched in drought and no or low-P soils. Overall, the drought and no or low-P treatments stimulated microbial communities and gene abundances involved in P-cycling. However, this increase was insufficient to maintain soil P-bioavailability. These findings shed light on the responses and feedback of microbial-mediated P-cycling behaviors in desert ecosystems under three-year drought and soil P-deficiency.

Combining different species in restoration is not always the right decision: Monocultures can provide higher ecological functions than intercropping in a desert ecosystem

Vegetation restoration in deserts is challenging due to these ecosystems' inherent fragility and harsh environmental conditions. One approach for active restoration involves planting native species, which can accelerate the recovery of ecosystem functions. To ensure the effectiveness of this process, carefully selecting species for planting is crucial. Generally, it is expected that a more diverse mix of species in the plantation will lead to the recovery of a greater number of ecosystem functions, especially when the selected species have complementary niche traits that facilitate maximum cooperation and minimize competition among them. In this study, we evaluated the planting of two native species from the hyper-desert of Taklamakan, China, which exhibit marked morpho-physiological differences: a phreatophytic legume (Alhagi sparsifolia) and a halophytic non-legume (Karelinia caspia). These species were grown in both monoculture and intercrop communities. Monoculture of the legume resulted in the highest biomass accumulation. Intercropping improved several ecosystem functions in the 50 cm-upper soil, particularly those related to phosphorus (P), carbon (C), and sulfur (S) concentrations, as well as soil enzyme activities. However, it also increased soil sodium (Na+) concentration and pH. Halophyte monocultures enhanced ecological functions associated with nitrogen concentrations in the upper soil and with P, S, C, and cation concentrations (K+, Ca2+, Mg2+, Cu2+, Fe2+, Zn2+, Co2+, Ni2+), along with enzyme activities in the deep soil. It also maximized Na+ accumulation in plant biomass. In summary, we recommend legume monoculture when the primary goal is to optimize biomass accumulation. Conversely, halophyte monoculture is advisable when the objective is to extract sodium from the soil or enhance ecosystem functions in the deep soil. Intercropping the two species is recommended to maximize the ecosystem functions of the upper soil, provided there is no salinization risk. When planning restoration efforts in desert regions, it is essential to understand the impact of each species on ecosystem function and how complementary species behave when intercropped. However, these interactions are likely species- and system-specific, highlighting the need for more work to optimize solutions for different arid ecosystems.

Fine-root traits are devoted to the allocation of foliar phosphorus fractions of desert species under water and phosphorus-poor environments

Traits of leaves and fine roots are expected to predict the responses and adaptation of plants to their environments. Whether and how fine-root traits (FRTs) are associated with the allocation of foliar phosphorus (P) fractions of desert species in water- and P-poor environments, however, remains unclear. We exposed seedlings of Alhagi sparsifolia Shap. (hereafter Alhagi) treated with two water and four P-supply levels for three years in open-air pot experiments and measured the concentrations of foliar P fractions, foliar traits, and FRTs. The allocation proportion of foliar nucleic acid-P and acid phosphatase (APase) activity of fine roots were significantly higher by 45.94 and 53.3% in drought and no-P treatments relative to well-watered and high-P treatments, whereas foliar metabolic-P and structural-P were significantly lower by 3.70 and 5.26%. Allocation proportions of foliar structural-P and residual-P were positively correlated with fine-root P (FRP) concentration, but nucleic acid-P concentration was negatively correlated with FRP concentration. A tradeoff was found between the allocation proportion to all foliar P fractions relative to the FRP concentration, fine-root APase activity, and amounts of carboxylates, followed by fine-root morphological traits. The requirement for a link between the aboveground and underground tissues of Alhagi was generally higher in the drought than the well-watered treatment. Altering FRTs and the allocation of P to foliar nucleic acid-P were two coupled strategies of Alhagi under conditions of drought and/or low-P. These results advance our understanding of the strategies for allocating foliar P by mediating FRTs in drought and P-poor environments.

Different Responses of Soil Bacterial and Fungal Communities in Three Typical Vegetations following Nitrogen Deposition in an Arid Desert

The effects of increased nitrogen (N) deposition on desert ecosystems have been extensively studied from a plant community perspective. However, the response of soil microbial communities, which play a crucial role in nutrient cycling, to N inputs and plant community types remains poorly understood. In this study, we conducted a two-year N-addition experiment with five gradients (0, 10, 30, 60, and 120 kg N ha-1 year-1) to evaluate the effect of increased N deposition on soil bacterial and fungal communities in three plant community types, namely, Alhagi sparsifolia Shap., Karelinia caspia (Pall.) Less. monocultures and their mixed community in a desert steppe located on the southern edge of the Taklimakan Desert, Northwest China. Our results indicate that N deposition and plant community types exerted an independent and significant influence on the soil microbial community. Bacterial α-diversity and community dissimilarity showed a unimodal pattern with peaks at 30 and 60 kg N ha-1 year-1, respectively. By contrast, fungal α-diversity and community dissimilarity did not vary significantly with increased N inputs. Furthermore, plant community type significantly altered microbial community dissimilarity. The Mantel test and redundancy analysis indicated that soil pH and total and inorganic N (NH4+ and NO3-) levels were the most critical factors regulating soil microbial communities. Similar to the patterns observed in taxonomic composition, fungi exhibit stronger resistance to N addition compared to bacteria in terms of their functionality. Overall, our findings suggest that the response of soil microbial communities to N deposition is domain-specific and independent of desert plant community diversity, and the bacterial community has a critical threshold under N enrichment in arid deserts.

Strigolactones can be a potential tool to fight environmental stresses in arid lands

BACKGROUND

Environmental stresses pose a significant threat to plant growth and ecosystem productivity, particularly in arid lands that are more susceptible to climate change. Strigolactones (SLs), carotenoid-derived plant hormones, have emerged as a potential tool for mitigating environmental stresses.

METHODS

This review aimed to gather information on SLs' role in enhancing plant tolerance to ecological stresses and their possible use in improving the resistance mechanisms of arid land plant species to intense aridity in the face of climate change.

RESULTS

Roots exude SLs under different environmental stresses, including macronutrient deficiency, especially phosphorus (P), which facilitates a symbiotic association with arbuscular mycorrhiza fungi (AMF). SLs, in association with AMF, improve root system architecture, nutrient acquisition, water uptake, stomatal conductance, antioxidant mechanisms, morphological traits, and overall stress tolerance in plants. Transcriptomic analysis revealed that SL-mediated acclimatization to abiotic stresses involves multiple hormonal pathways, including abscisic acid (ABA), cytokinins (CK), gibberellic acid (GA), and auxin. However, most of the experiments have been conducted on crops, and little attention has been paid to the dominant vegetation in arid lands that plays a crucial role in reducing soil erosion, desertification, and land degradation. All the environmental gradients (nutrient starvation, drought, salinity, and temperature) that trigger SL biosynthesis/exudation prevail in arid regions. The above-mentioned functions of SLs can potentially be used to improve vegetation restoration and sustainable agriculture.

CONCLUSIONS

Present review concluded that knowledge on SL-mediated tolerance in plants is developed, but still in-depth research is needed on downstream signaling components in plants, SL molecular mechanisms and physiological interactions, efficient methods of synthetic SLs production, and their effective application in field conditions. This review also invites researchers to explore the possible application of SLs in improving the survival rate of indigenous vegetation in arid lands, which can potentially help combat land degradation problems.

Alhagi sparsifolia: An ideal phreatophyte for combating desertification and land degradation

The upcoming launch of the post-2020 biodiversity framework entails elucidating the future priorities and strategies for conserving biodiversity on a regional and global scale. Desertification has caused significant environmental and socio-economic problems in many arid and semiarid areas of the world. Phreatophytes increase nutrient levels and water reserves in the ground below them and are essential resources for other plants and animals. Factors that can enhance or reduce the island effect of phreatophytes are worthy of research since they can contribute to the survival and growth of this group of plants. Furthermore, these plants substantially impact the whole environment and boast necessary evolutionary adaptations for sustaining islands of life in the arid landscape to prevent complete desertification. Alhagi sparsifolia is a semi-shrub legume and a typical desert phreatophyte. The main objective of this review is to disseminate research-based knowledge and improve awareness of this important plant species and provide a framework that could effectively improve regional socio-ecological environments, particularly in controlling desertification, thereby enhancing human welfare. First, we synthesized knowledge of the physiological acclimation of this species, above all in terms of water and nutrient use, the impact of salinity, and its response to different light environments and temperatures. Then, we analyzed the socio-ecological services that A. sparsifolia offers, highlighted the anthropogenic threats to its habitats, and strategies for its sustainable utilization and management. We conclude that more knowledge of how A. sparsifolia interacts with groundwater will contribute significantly to elucidating the possible effects of climate change and human activities on the sustainable use of groundwater. Protected areas must be set up, and key knowledge shared to combat desertification and improve people's welfare as a means of guaranteeing sustainable development. An integrative socio-ecological approach must be adapted to improve understanding of this valuable phreatophyte's importance and sustainable use and fulfill Sustainable Development Goals (SDGs).