4 DISCUSSION
Herein, we found that desertified land reclamation for viticulture was
associated with significant changes in soil microbial communities and
soil properties, consistent with our hypotheses. Such land reclamation
was associated with significant increases in SOC relative to that
observed in DL soil, consistent with the land conversion to vineyards
having increased soil organic matter content. Organic matter inputs
include winegrape litter, which can additionally contribute to the soil
nutrient content as well as fertilizer and manure applied in the context
of viticulture, thereby contributing to SOC and nutrient availability. C
soil samples exhibited the highest SOC levels (Table 1), likely because
the soil is less disturbed in these fields and because the plant leaves
contain secondary metabolites such as tannins, terpenes, and polyphenols
that slow the rate of litter decomposition. Yanget al. (2016) have also
previously reported reclamation-related increases in SOC and nutrient
contents. We found that DL reclamation for winegrape cultivation was
associated with a significant increase in rhizosphere soil pH (Table 1),
in contrast to prior study results (Djukic, Zehetner, Tatzber, &
Gerzabek, 2010; Franco-Otero, Soler-Rovira, Hernández, López-de-Sá, &
Plaza, 2012), potentially due to study site-specific differences.
Indeed, the soil alkalinity in the Hongsibu area of Ningxia in this
study was higher than in the previous studies.
Winegrape planting was associated with a reduction in soil volume that
may be attributable to the long-term plowing associated with the
viticulture process. Similar changes in TN and SOC were also observed,
whereas TP and TK did not differ significantly among the five analyzed
land-use applications, potentially owing to the high phosphorus and
potassium content of desertified soil minerals (Hendrix, 2000; Ma, Sun,
Sun, & Wang, 2012; Gong, He, & Liu, 2016). Soil available potassium
rose significantly following reclamation, consistent with SOC
accumulation. In contrast, soil available nitrogen and available
phosphorus content decreased significantly after reclamation, likely due
to the absorption of these nutrients by grape plants at a rate greater
than that at which fertilized nitrogen and phosphorus were applied
(Gong, He, & Liu, 2016). Soil nutrient contents observed in this study
were generally lower than those reported in other regions (Domínguez,
Panettieri, Madejón, & Madejón, 2020), consistent with the fact that
winegrapes are suited to cultivation in regions with harsh ecological
conditions and relatively poor soil conditions.
The composition and diversity of bacterial communities in a given
ecosystem can change in response to environmental variation (Cheng,
Chen, & Zhang, 2018). We therefore hypothesized that bacterial
community would differ significantly when comparing DL and reclaimed
soils, and confirmed that this was true at the taxa level (Figure 2;
Figure 3), through NMDS analyses (Figure 4), and via a hierarchical
clustering analysis (Figure 5). We observed distinct differences in the
relative abundance of top bacterial phyla among land-use types,
indicating that DL reclamation was linked to altered microbial community
composition. Actinobacteria were the most abundant bacterial
phyla in the present study, and Actinobacteria abundance was
significantly higher in CS, M, C, and IR soils relative to DL soils,
consistent with prior reports (Gao et al., 2020). While pesticide use in
vineyards can impact soil bacterial communities, Actinobacteria ,
which degrade hydrocarbons and pesticides (Alvarez et al., 2017), are
not impacted by pesticides. Proteobacteria were also dominant in
the present study, with higher relative Proteobacteria abundance
in reclaimed soil relative to DL soil. Relative Proteobacterialevels were significantly higher in M and IR soil samples relative to DL
soil samples, potentially due to the higher sulfur levels in vineyards.
Sustained livestock organic fertilizer application and the input of
organic matter in M and IR vineyards can contribute to these increases
in soil sulfur levels. Indeed, shifts in Deltaproteobacteriaabundance have been linked to changes in soil sulfur content (Wasmund,
Mußmann, & Loy, 2017). We also observed reclamation-related changes in
the abundance of Acidobacteria , which are frequently reported to
be regulated by soil pH (Ramirez et al., 2014). In line with the results
of a prior study (Lauber, Hamady, Knight, &Fierer 2009), we found that
relative Acidobacteria abundance declined with rising soil pH. We
additionally found that bacterial abundance (Chao1) and diversity
(Shannon) in C and IR soils were significantly increased relative to DL
soils (Table 2), owing to differences in soil chemical properties.
With respect to soil fungal community composition and diversity, we
found that Ascomycota was the dominant phylum among the analyzed
land-use patterns, followed by Basidiomycota , in line with prior
reports (Maestre et al., 2015; Porras-Alfaro, Herrera, Natvig, Lipinski,
& Sinsabaugh, 2011; Prober et al., 2015). We found that winegrape
cultivation was associated with increased fungal diversity, although
there were no significant differences in fungal diversity indices
associated with different land use types (Table 2). This may be because
Bordeaux and lime sulfur mixtures are applied to these vineyards to
prevent fungal diseases, thereby reducing fungal diversity, whereasAscomycota species can tolerate harsher conditions (Chen et al.,
2017). NMDS and hierarchical clustering results indicated that microbial
community structures differed significantly in cultivated soils relative
to DL soils, but the bacterial and fungal community structure was
similar in CS and M (Figure 4; Figure 5). This may be a result of
changes in land-use that led to similar root exudate and litter
properties in CS and M soils, thus inducing similar rhizosphere
bacterial and fungal community structures. However, the bacterial and
fungal community structures differed significantly in M and C samples,
both of which were 3 years old, indicating that the microbial community
structure differences in the context of short-term cultivation (for less
than 5 years) were more closely related to winegrape cultivar type. In
addition, LEfSe analyses directly assess differences at all taxonomic
levels at once, and can identify differences in species between groups
as biomarkers of a particular sample type (Figure 6). These biomarkers
may contribute to long-term sustainable reclaimed land use, although
further study of this topic is required.
Soil microbial diversity is crucial to ensuring long-term soil ecosystem
sustainability (Cheng et al., 2017; Maron et al., 2018). Assessing soil
microbial diversity in a reclamation ecosystem can thus offer
perspectives on long-term sustainable development strategies for
reclaimed desert land. Herein, we found that bacterial and fungal
ASVs/OTUs (Figure 2) and diversity index (Table 3) were higher in
reclaimed vineyard soils relative to DL soil samples. These changes were
attributed to increased organic matter input in the reclaimed soil owing
to cultivation-related fertilizer application and associated crop
residues. However, we did find that when analyzing fungal ASVs/OTUs, the
Chao1, and Observed species index values in C soils were lower than in
DL soils, suggesting a decrease in mycorrhizal dependence associated
with Chardonnay grape cultivation. Winegrape cultivation is thus
generally associated with increased fungal and bacterial diversity owing
to the increased diversity of plants in the vineyard rows (Kowalchuk,
Buma, de Boer, Klinkhamer, & Van Veen, 2002). Plant diversity and
associated rhizosphere carbon input can help drive increased soil
microbial diversity (Lange et al., 2015; Meng et al., 2019). Higher
microbial diversity is often associated commonly linked to increased
ecosystem stability (Delgado-Baquerizo et al., 2016). Therefore, a
viticulture model of interrow grass planting with higher plant diversity
should be adopted to facilitate the sustainable development of land use
in desertification areas.
Several prior reports have demonstrated the important link between soil
chemistry and microbial community composition in a variety of ecosystems
(Kerfahi, Tripathi, Dong, Go, & Adams, 2016; Nanganoa et al., 2019;
Tian et al., 2017). SOC, pH, and nutrient contents have been reported to
be the key determinants of bacterial community composition and diversity
in vineyards (Li, Chi, Li, Wu, & Yan, 2019). Our correlation analyses
revealed SOC, ammonium, nitrate, and bacterial diversity indices to be
positively correlated (Table S). As such, the increase in SOC levels in
reclaimed soil may contribute to elevated bacterial diversity in
reclaimed soils in this particular ecosystem, given that SOC is a key
resource necessary for the majority of terrestrial microbial communities
(Cai et al., 2016; Zumsteg et al., 2012). RDA analysis further revealed
that the relative abundance of the dominant soil bacteriaProteobacteria was positively correlated with SOC. This is
consistent with prior work demonstrating that bacterial community
composition in semi-arid agricultural ecosystems is driven by SOC (Cheng
et al., 2017). Soil pH is also thought to be an important determinant of
soil bacteria community structure, diversity, and richness in a range of
ecosystems (Fierer, & Jackson, 2006; Ramirez et al., 2014). We observed
significant positive correlations between bacterial community structure
(Chao1, Observed species, and Shannon indices) and soil pH (Table S).
Overall, soil pH explains 42.20% of the total variation of soil
bacterial communities at the phylum level (Figure 7), suggesting that pH
is of critical importance in shaping the structure of bacterial
communities in reclaimed desertified soil. In addition to SOC and pH,
elevated nutrient levels in cultivated land may ameliorate
nutrient-dependent constraints on bacterial growth and function in
otherwise nutrient-poor desert soils, thus driving increased bacterial
diversity in crop ecosystems.
Herein, Ascomycota species were the dominant fungi and were
positively correlated with SOC and TN (Figure 7). Soil fungal community
diversity has been reported to be associated with nitrate-nitrogen, TN,
SOC, and pH (Ferreira, Leite, de Araújo, & Eisenhauer, 2016; Huhe,
Chen, Hou, Wu, & Cheng, 2017), in line with our data. Relative to
bacteria or archaea, fungal survival is more dependent upon carbon and
nitrogen sources (Schmidt, Nemergut, Darcy, & Lynch, 2014). Soil
nitrogen and SOC regulate the structure of both bacterial and fungal
communities in the soil. Overall, our data suggest that soil
physicochemical properties are significantly altered following desert
land reclamation, thereby increasing fungal and bacterial diversity,
which is essential to ensure crop ecosystem stability and long-term
sustainable land use.