4.1. Effect of plant cover on soil CO2 efflux
Plant cover can highly influence soil CO2 efflux (Raich
& Tufekciogul, 2000) and removal of native forests and land use changes
during mining activities accelerate the TC losses by increasing
CO2 efflux (Lorenz & Lal, 2007; Polglase et al .,
2000; ). Planting trees is one of the most effective activities in
recovering TC stocks of degraded areas (Moghiseh et al., 2013).
The present study identified that CO2 efflux tends to
increase with the planting of forest cover. Higher diversity in plant
species, together with favorable climate conditions, may result in
abundance and diversity of soil microbiological community (Langeet al ., 2015). Higher microbial activity may affect the
decomposition of tree’s residues and SOM, and consequently, the
production of soil CO2.
Planting different types of forest cover led to an increase in
CO2 efflux due to the benefits of increasing SOM. Forest
species favor soil microbiological activity by contributing organic
material from the shoots and roots that contribute to nutrient cycling,
speeding up the ecosystem recovery (Shrestha & Lal, 2006), and which
may reflect in the increase in soil CO2 efflux. While
the treatment with no plant covers presented the lower values of
CO2 efflux, the Woodland showed higher values (Fig. 2).
Soils of mining areas pose a limitation on plant growth, but our results
show that planting forest species is the right way to achieve similar
soil CO2 efflux compared with the unmined Woodland.
The forest types Euc, Ap and Nat showed higher values for
CO2 efflux compared to NCov. Different forest covers can
influence the soil chemical and biological properties by the input and
composition of organic material from the trees (litter and roots),
resulting in an increase of TC and soil biological activity. Different
tree species contribute with organic material of different
characteristics (Craine & Gelderman, 2011; Martin et al ., 2009),
which may also affect the soil microbiological activity (Barretoet al ., 2008) and the increase of TC. Therefore, differences in
root biomass, soil organic matter content, and the spatial arrangement
of the trees may contribute to soil CO2 efflux
variations (Katayama et al ., 2009; Gomes et al ., 2016).
Optimum soil moisture and temperature conditions combined with higher
values for TC, TN, LC, C-MB, N-MB and root density (Table 1), may
explain the greater values for CO2 efflux in Woodland
(Figure 3). With favourable conditions of temperature and moisture, soil
microorganisms act more intensely and produce more CO2.
In addition, the higher root density in Woodland is associated with the
higher soil CO2 efflux for the same climate conditions.
Tree roots are responsible for a large flow of C and nutrients in soil,
with respiration accounting for a third or more of the total soil
CO2 efflux in tropical forests (Högberg & Read, 2006).
In addition, the amount and quality of available C and photosynthetic
activity also influence soil CO2 concentrations and
consequently the efflux (Kuzyakov & Cheng, 2001). Therefore, increasing
the diversity of forest species, promotes in time the development of
soil microorganisms and roots, resulting in higher rates of
CO2 efflux.
4.2. Autotrophic and heterotrophic
respiration
The contribution of soil heterotrophic respiration to total respiration
in the forest species was on average 55% during the driest period, and
72% during the wettest period (Figure 3). The relative contributions of
heterotrophic and autotrophic respiration are specific
context-dependent, since are influenced by local soil temperature and
moisture conditions (Liu et al. , 2009; Hanson et al .,
2000), and by intrinsic plant factors, such as the photoassimilates
allocation to roots (Epron et al ., 2011; Högberg & Read, 2006).
The Ap forest cover showed the highest values for the relative
contribution of HR (77.4%) during the rainy season. This result may be
related to the high decomposition rate of the leaflets from this type of
forest cover compared to the others (Gama-Rodrigues et al. , 1997)
(Figure 4). Wei et al . (2010) analysed a global data of forest
ecosystems and also found increases in HR in rainy seasons. Soil
microbiota is sensitive to soil moisture and may explain the higher HR
contribution of HR to total soil respiration during the rainy season
(Carbone et al ., 2011). Variations in AR are more dependent on
plant photosynthetic activity (Gomes et al ., 2016), responding
slowly to variations in soil temperature and soil moisture when compared
to HR (Bond-Lamberty et al ., 2004; Högberg et al ., 2008,
2009).
The specific partitioning of heterotrophic respiration showed that the
soil CO2 produced originates mainly from the
mineralization of plant residues (Figure 7). Plant residues are
relatively easier to be degraded by microorganisms compared to the SOM,
which is protected by clay-mineral associations and has been passed by
biological degradation. In areas in an initial process of recovery, the
microorganisms have a preference to use the most recent added forms of C
(Novara et al ., 2014).
4.3. Influence of biotic and abiotic
factors on CO2flux
The principal component analysis shows that the planted forests are
placed between NCov and Woodland, and follow the same pattern of soil
CO2 efflux (Figure 2). This indicates that the planted
forests are improving the soil properties comparing with the treatment
with no ground cover and are on the way to the soil quality presented in
the native forest.
Soil CO2 efflux is a complex process and is influenced
by biotic and abiotic factors. Soil temperature and moisture are the
main abiotic factors that influence these processes (Fenn et al. ,
2010; Wu et al ., 2010), affecting directly the biotic factors,
such as microorganisms activity and plant cover. An increase in soil
temperature leads to an increase in microorganisms activity (Atkinet al. , 2000) and root respiration (Schindlbacher et al .,
2011), leading to increases in soil CO2 efflux. Plant
cover with tree species creates a microclimate beneath the canopy that
reduces variations in soil temperature, and therefore their influence on
soil CO2 efflux (Gomes et al ., 2016). However, in
the present study, soil temperature did not vary much during the
measurements (Figure 1) and a significative response was not found
(Figures 9 and 10). Soil moisture is a very important factor in the
maintenance and development of microbial activity (Liu et al .,
2009). Our results demonstrated that soil moisture was the most
important variable and associated positively with soil
CO2 efflux. While low values of soil moisture at the end
of the dry season leading to low values of soil CO2efflux (Figure 2), the increase of precipitation in the rainy season
promoted the increase of soil CO2 efflux values. These fluctuations of
soil CO2 efflux are directly linked with microbial activity during the
dry and rainy seasons.
Conclusions
In this study, we found that afforestation increases total
CO2 efflux in RMDL compared to those with no vegetation
cover.
In afforested RMDL, heterotrophic respiration contributes more to total
soil respiration and litter decomposition is responsible for the highest
CO2 efflux.
The variation in moisture is the main factor responsible for the soil
CO2 efflux changes in the types of forest cover studied.
The plantations of Euc, Ap and Nat forest cover were efficient in
recovering the mined soils, with similar CO2 efflux
values found under the Atlantic Forest remnant.