4.1 Effects of forest conversion on soil quality
The soil C, N contents dropped by 83% and 59% after conversion,
respectively (Fig. 1), similar to previous studies that were focused on
rainforest and temperate forest conversion into monoculture plantations
(Guillaume et al., 2015; Yang et al., 2018). Firstly, soil erosion
increased when understory vegetation and litter layer were removed after
conversion, coupled with a large amount of precipitation in the region.
These factors caused a sharp drop in C and N contents (Guillaume et al.,
2015). Secondly, the decomposition of SOC increased due to nutrient
release from the dead or burnt biomass, and the labile organic matter
released from destroyed soil aggregates (Berhe, Harte, Harden, & Torn,
2007). Therefore, soil SOC content decreased following conversion.
Third, C and N were reduced in plantations because low plant diversity
prevented the formation of developed organic horizons, and reduced the
input of labile organic compounds, leading to a decrease in microbial
biomass (Mcguire et al., 2015). In contrast, soil
pH increased after conversion, which
was ascribed to three factors: 1) Conversion reduces plant species,
resulting in a decline in root and ectomycorrhizal exudates which
contain a lot of organic acids (Grayston, Vaughan, & Jones, 1997).
Thus, soil pH increased after conversion. 2) Soil erosion removes the
surface layer, which is usually more acidic than the subsoil. 3) The
increase of exchangeable cations results in an increase in soil pH, as a
fertilizer following conversion (Tripathi et al., 2016). In addition,
soil TP, AP, TK, AK were the highest in the Peach plantation (Fig. 1)
because of intensive application of mineral and organic fertilizers.
Compared with Forest, the enzymatic activity of SC, ACPT, GLS, UR, ACP
and ALP decreased in plantations (Fig. 1). Soil SC is a C-degrading
enzyme involved in organic matter hydrolysis. Thus, the SC activities
declined with the reduction of SOC after conversion. Soil ACPT, UR and
GLS are key enzymes involved in N mineralization. ACPT acts on
proteolysis and breaks down organic N such as proteins and peptides into
amino acids (Watanabe, 2010). UR converts organic N to mineral nitrogen
that can be used by plants and microorganisms (Fraser, Hallett, Wookey,
Hartley, & Hopkins, 2013). In the Forest, high SOC content accelerates
N mineralization, resulting in higher N mineralization activity,
including ACPT and UR (Yang, Yang, & Yu,
2018).
Soil GLS involves the hydrolysis of
L-glutamine to produce L-glutamic acid and NH3, and its
activity is positively correlated with organic C and total N
(Frankenberger & Tabatabai, 1991). Thus, the decrease in GLS activity
is attributed to the drop in SOC and TN contents in plantations. After
conversion, the phosphatase activity decreased with the increase of AP
content. Compared with plantations, lower AP stimulated the microbial
community to secrete phosphatase to
mobilize P from organic sources, leading to higher phosphatase activity
in the Forest (Wu et al., 2019).
The decline in soil quality after conversion, revealed by the SQI,
showed that GLS was the biggest contributor to SQI, followed by SOC and
TP (Fig. 2, Table S2). GLS is a key enzyme of N metabolism, as it
catalyzes the hydrolysis of NH3 group from amino acid
and degrades L-glutamine into L-glutamic acid and ammonium ions, which
maybe a reliable indicator of soil fertility (Kanazawa & Kiyota, 2000).
Hence, this enzyme could be used as a biological indicator of SQI in
subtropical areas. SOC is often used as a proxy of soil quality and
productivity due to its key role in multiple soil processes such as
nutrient cycling as well as plant and microbial growth (Raiesi, 2017). A
big decrease in GLS activities and SOC contents was responsible for the
sharp drop in SQI after conversion. Forest conversion accelerated soil
erosion, reduced organic matter inputs and C, N contents, resulting in a
decrease in SQI. TP was another key indicator that contributed to SQI
(Zhang et al., 2019). Among the four plantations, SQI of Peach and Berry
was superior than the
other two, indicating that fertilization was beneficial for soil quality
after conversion by increasing TP contents.
4.2 Effects of forest
conversion on fungal communities
After forest conversion, α-diversity of fungi increased in Berry and
Peach plantations (Fig. 3a-c), and there were significant differences in
community structure among the five types of plantations (Fig. 3d). The
increase of α-diversity was consistent with the conversion of which
forest to which plantation (Nakayama et al., 2019). We explain this as
follows: 1) Acidity reduced the taxonomic diversity of the arbuscular
mycorrhiza fungi (AM) by decreasing sporulation (Coughlan, Dalpé,
Lapointe, & Piché, 2000). Hence, the gradual increase in soil pH led to
a higher diversity in plantations (Fig. 3a-c, Fig. 8). 2) Fertilization
was responsible for the increase of α-diversity in Berry and Peach. The
continuous application of manure increased fungal biomass and diversity
by increasing substrate variety and C input (Kamaa et al., 2011). The
exogenous fungi from the manure (organic resources) may increase fungal
diversity of plantation (Sun et al., 2016). According to the
β-diversity, notable divergence was observed in the composition between
Forest and four plantations (Fig. 3d). Forest conversion affects fungal
community structure via changes in soil chemical properties (e.g., pH,
C, N contents), root exudates and litter input (Prescott & Grayston,
2013).
Due to specific functional attributes and survival strategies, the
abundance of dominant fungi responded differently to forest conversion
(Fig. 4, 6). For instance, the abundance of Basidiomycota decreased
while that of Ascomycota and Zygomycota increased.
We can characterize these fungi based
on Grime’s C-S-R (Competitor,
Stress-tolerant, Ruderal) life history strategy to explain succession
dynamics and the spatial structure of fungal assemblages (Chagnon,
Bradley, Maherali, & Klironomos, 2013; Grime, 1979). Zygomycota
(R-strategists) are primary
saprotrophic fungi with ruderal strategies, which are only active in
habitats with low levels of competition and stress.
They are characterized by a short
growth period, high reproductive potential, and effective use of
accessible substrates (Cooke & Rayner, 1984). Thus, Zygomycota are more
abundant in plantations with low C and N content. Basidiomycota and
Ascomycota are secondary saprotrophic fungi, and their cbhI gene
is a reliable indicator of cellulolytic ability. Both fungi groups can
degrade part of plant residues and promote soil C accumulation
(Stursova, Zifcakova, Leigh, Burgess, & Baldrian, 2012). By contrast,
most Basidiomycota (C-strategists) are white and brown rot fungi, which
are late-successional and oligotrophic species with slow growth and
favor resource-rich conditions and better substrate quality (high C/N).
They are usually involved in decomposition of lignified plant detritus
(Lundell, Makela, & Hilden, 2010). In addition, the lignin content in
old forest litter is high (Osono, 2006).
Therefore, C-strategists
Basidiomycota decrease with reduction of C/N ratio and litter after
conversion. Subsequently, Basidiomycota such as the typical
ectomycorrhizal group (Russula) and other genera belonging to the
Basidiomycota declined sharply. In contrast, Ascomycotas are S-strategy
fungi, with survival related stress tolerance characteristics, and use
resources more efficiently under low-nutrient conditions. So they
exhibit a more stress-tolerant strategy to C substrate than
Basidiomycetes (Wang et al., 2021). The Ascomycotas are mainly soft rot
fungi that can decompose complex organic matters such as cellulose,
hemicellulose and the lignin in the soil (Stursova et al.,
2012). As a fast-growing
copiotrophic fungi, the S-strategists Ascomycota increased with
stressful conditions (low C, N contents) in plantations. Thereby, the
abundance of genera affiliated Ascomycota:Pseudophialophora andRhytisma increased. Moreover, among the root-associated fungi,
the change in the abundance of Ascomycota related to Basidiomycota also
implied that the ratio of ericoid mycorrhizal fungi (mainly Ascomycota)
and ectomycorrhizal (mainly Basidiomycota) had changed (Sterkenburg,
Bahr, Brandstrom, Clemmensen, & Lindahl, 2015).