A Proposal on the Restoration of Nostoc flagelliforme for Sustainable Improvement in the Ecology of Arid Steppes in China

Abstract

Nostoc flagelliforme, a filamentous nitrogen-fixing cyanobacterium, is widely distributed in arid steppes of the west and northwestern parts of China. However, as a food delicacy this species has been overexploited from 1970 to 2000. Moreover, overgrazing, land reclamation and the removal of medicinal herbs have caused severely reduced vegetation coverage there. In this communication, a badly damaged but slowly rehabilitating N. flagelliforme-inhibiting steppe is described, and the rehabilitation of desertified steppes by the renewed growth of N. flagelliforme is proposed. The restoration of this dominant nitrogen supplier would be an ecologically sustainable solution for supplementing current measures already taken in the desertified regions. In addition, a goal of 50%–60% vegetation coverage is proposed by the N. flagelliforme restoration.

1. Reduced Biomass of Nostoc flagelliforme as an Indicator of Desertification

Terrestrial cyanobacteria are the dominant population of algae in ecologically fragile desert steppes. They contribute organic nitrogen (N) and carbon (C) resources for the nutrient-poor soils [1]. In the arid steppes of the west and northwestern parts of China, some of them, such as Microcoleus species, participate in the formation of biological soil crusts (BSCs), which are both widespread and important for sand fixation [2,3]. Another widespread cyanobacterium is Nostoc flagelliforme, which appears as a hair-like colony form on the soil surface [4]. As a popular food delicacy, N. flagelliforme had been excessively exploited by raking from 1970s to 2000 in China [5]. A good example is Ningxia Province, in which the producing area for N. flagelliforme was about 360.7 × 10⁴ hm² in the 1960s, but was reduced to 173.6 × 10⁴ hm² in the mid-1980s, while the biomass was reduced from 3 to 7.5 kg/hm² in the 1970s to 0.15 kg/hm² in the 1990s [6]. In order to rehabilitate desertified regions, some preventative measures were implemented around 2000 in China, including prohibition from grazing, returning sloping field to grassland and prohibiting the trade of N. flagelliforme. This has resulted in some positive effects [7,8].

In arid steppes, soil N is usually considered as being secondary to water content in limiting ecological processes. The natural ecological recovery of the N supplier N. flagelliforme as well as vegetation coverage should be a very long process. N. flagelliforme has less than a 6% annual growth rate [9]. However, we found that some local governments were more inclined to accept immediate measures such as grass planting or afforestation for combating desertification, rather than the restoration of N suppliers such as N. flagelliforme. Such a “success” may be temporary or unsustainable [10]. In addition, it was reported that exogenous N additions exhibited neutral or even inhibitory effects on plant abundance and litter decomposition [11,12]. Therefore, an understanding of the dynamic equilibrium of C and N suppliers in arid steppes is of importance for sustained rehabilitation of damaged soils. In this communication, we describe a badly damaged but slowly rehabilitating arid steppe, while also emphasizing the great importance of N. flagelliforme restoration for achieving combined recovery of this cyanobacterium and xerophilic plants for ecological improvement in the arid steppes.

2. A Badly Damaged but Slowly Rehabilitating N. flagelliforme-Inhabiting Steppe

A scene of this rehabilitating steppe in winter is shown in Figure 1. Dwarfish shrubs were scattered throughout the whole steppe, with less than 5% vegetation coverage (Figure 1a). The vegetation coverage in the rainy season (summer) is about 15%–30%, which indicates moderate desertification [13]. Horse feces were occasionally found, suggesting that there is grazing disturbance (Figure 1a). The soils are calcic sierozem and light chestnut with pH of 8–9.5 [4,6], as implied by the weathered stones (Figure 1b). The most abundant two plants in this steppe were Ephedra lepidosperma C.Y. Cheng (Figure 1c) and Stipa breviflora Griseb (in Figure 1b). Approximately 10–20 withered plant species on the surface soil were found. In other regions with N. flagelliforme distribution, plant diversity may be different [6]. Around 40 or more plant species could be found in the rainy summer season in Ningxia Province (Figure S1). These higher plants serve as the main C supplier in arid steppes, since soil C was positively correlated to vegetation coverage and above-ground biomass, especially surface litter [14]. Traces of rodent activity were found as suggested by their holes (Figure 1d). Surveys showed that Meriones unguieulatus and Meriones meridianus were the two dominant rat species [15]. Proper animal activity, including controlled grazing intensity, was conducive to the virtuous cycle of ecology [16,17].

In addition, colonizing pioneers (also N suppliers) such as lichens (Figure 1e), BSCs (Figure 1f–g), and N. flagelliforme colonies (Figure 1h–l) were observed. Lichens were occasionally found, adhering to bare stones (Figure 1e). N. flagelliforme colonies were tightly (Figure 1h) or loosely (Figure 1i) aggregated, or mostly stretched by surface runoff (Figure 1j–l), entwining around stones or grasses. Most of these N. flagelliforme colonies were about 10–20 cm long, far from the length of 50 cm or more that they can grow up to. On the whole, small BSCs and N. flagelliforme colonies were widely but sparsely spread, suggesting that many had been destroyed or collected.

N. flagelliforme yield had reduced by at least 95% because of the over-collection. In addition, raking N. flagelliforme from the soil surface would have severely affected the development of BSCs. Overgrazing, land reclamation and the removal of medicinal herbs (e.g., Glycyrrhiza uralensis and Herba ephedrae) also accounted for the severe reduction of vegetation coverage and quality [18,19,20]. Therefore, this desolate scene reflects the comprehensive damage to N. flagelliforme, BSC and vegetation on the arid steppe. It also hints that soil C–N dynamics is currently at a weak equilibrium. Instead, a combined recovery of C suppliers (shrubs and grasses) and N suppliers (N. flagelliforme and BSCs) may be necessary for steppe rehabilitation.

3. The Potential of the Restoration of N. flagelliforme for Ecology Improvement

In the dynamic soil C–N cycle in arid steppes, xerophilic plants contribute to the majority of C storage [14,21]. Soil microorganisms and enzyme activities (e.g., polyphenol oxidase, cellulase, β-glucosidase, nitrate reductase and urease), which play crucial roles in the cycle, varied in correlation with the vegetation type and coverage [22,23]. However, the current low N level still remains a limiting factor for plant growth. Soil N level is only 0.7 g/kg in the aforementioned steppe and the average level in Ningxia is 0.47 g/kg [6]. An interesting finding is that the biomass of N. flagelliforme in soils with relatively rich organic matter (e.g., 1.0%) is higher than in those with less organic matter (e.g., 0.02%) [24,25]. Laboratory cultivation found that N. flagelliforme has the capability to use organic carbon sources for heterotrophic growth in darkness [26,27]. Thus, the organic products decomposed by soil bacteria from plant litter may sustain the relatively superior growth of N. flagelliforme. The development of BSCs could also be promoted by plantation establishment [28]. Therefore, an interactive promotion in growth between C and N suppliers actually exists. In the N-poor and relatively C-rich soils, it is possible that N. flagelliforme shifts its two roles (C and N suppliers) to one main role (N supplier) to collaborate with its surrounding biological circumstances.

An engineering experiment in Inner Mongolia, China, has proved that the “algae–grass–shrub” strategy, using the restoration of BSC, is feasible for accelerating the reversal of desertified land [29,30]. Similarly, a dynamic combined recovery of N. flagelliforme resource and vegetation coverage could be imagined by the restoration of N. flagelliforme. According to historical data [13], summer vegetation coverage of 50%–60% is expected to be steadily achieved through N. flagelliforme restoration coupled with other protective measures. N. flagelliforme is photophilic, since photosynthesis of rewetted N. flagelliforme is saturated at 1000 µmol photons m⁻²·s⁻¹ and no photoinhibition is recognized until 1800 µmol photons m⁻²·s⁻¹ [4]. Owing to this feature, the dispersed density (not biomass) of N. flagelliforme colonies would be affected at higher vegetation coverage. However, temperate soil algae and shrubs would appear more in those circumstances as suggested by Liu [29], thus maintaining a higher level of soil C–N cycling. An enhanced soil C–N cycle will also serve to maintain an ecological stability upon climate changes [31].

Compared to the restoration of BSC for ecological improvement, the use of N. flagelliforme propagules may be more convenient for dispersion and development on surface soils. Several ways have been suggested for reproduction of N. flagelliforme: (1) single cells or small filaments fragmented from big filaments forming new colonies; (2) via akinetes; (3) hormogonia dispersion and formation of new colonies [4]. The fragmentation of filaments to a very small size is an easy way to prepare propagules for direct application in natural habitats, but this will consume a huge amount of natural colonies.

Alternatively, mass cultivation of N. flagelliforme under aquatic conditions is an efficient way of preparing a large number of propagules [32,33]. However, a significant problem for these liquid-cultured propagules is that they are not very resistant to environmental stresses, such as desiccation stress, and may need extra processing, such as wrapping them with stabilizing components [33]. The solution of this key problem will greatly accelerate the application of N. flagelliforme for the improvement of ecology in arid steppes. According to the 6% annual growth rate, the growth of the filaments from 1 cm to the expected 50–60 cm in length still needs about 70 years. Therefore, the restoration of N. flagelliforme and the whole ecosystem is a long-term process.

4. Conclusions

This communication has proposed the restoration of N. flagelliforme for ecological improvement of arid steppes. This restoration may accelerate the recovery of vegetation cover and its quality, while it should be more ecologically sustainable compared to only planting xerophilic plants. In the current desertification situation, this restoration may also provide a valuable supplement for the measures already taken in desertification rehabilitation. In addition, the existing problem is that we have not completely mastered the biotechnology for cultivating N. flagelliforme propagules that are resistant to environmental stresses. Once this problem is overcome, it would be important to run field experiments or even to attempt large-scale application of N. flagelliforme propagules.