These native species were planted in Quincunx (4 pioneers, with one
climax in the center) at a spacing of 2.0 × 1.5 m, using seedlings
produced from seeds collected in fragments of Atlantic Forest
(Woodland). For Euc and Ap, the adopted spacing was 3 × 2 m.
Evaluation of soil CO2 efflux
To evaluate the soil CO2 efflux we installed
chloro-polyvinyl chambers (0.20 m x 0.30 m) in triplicate 42 months
after planting the Eu, Ap and Nat, as well as in the Woodland and NCov.
We installed chambers beneath and outside the litter collectors,
allowing to evaluate the soil CO2 efflux with and
without the input of litter from the aerial part of Euc, Ap and Nat. We
collected the CO2 using syringes in the periods of 0,
10, 20 and 40 minutes after closing the chambers. We collected soil
CO2 every two months between September 2014 and July
2015 and we analysed it in a cavity resonance spectrometer (CRDS -
cavity ring-down spectroscopy, G2131-i, Picarro, Sunnyvale, CA). During
the soil CO2 efflux measurements, we also assessed soil
temperature with a digital thermometer inserted at a depth of 5 cm, and
soil moisture by the gravimetric method. The CO2 efflux
was calculated using the equation:
\begin{equation}
Flux=\frac{(Q}{T)}\times M\times\frac{(PV}{TR)}\times\frac{1}{A}\nonumber \\
\end{equation}where Flux: CO2 (mg m-2h-1); ΔQ/ΔT: the angular coefficient of the adjusted
straight line (ppm/s) obtained by readjustment of the gas concentrations
during the time (T) of collection; M: the molar mass of
CO2 (g mol-1); P: internal pressure of
the chamber, assumed to be 1 atmosphere (atm); V: volume of the chamber
(L); R: universal gas constant (0.08205 L atm K-1mol-1); T: soil temperature (K); A: base area of the
chamber (m²).
Soil CO2 efflux partitioning
Aiming to identify the contribution of autotrophic (AR) and
heterotrophic respiration (HR) to the total soil respiration (TR), we
used the partitioning method that was carried in areas with Euc, Ap and
Nat. We also analysed the relative contribution of soil microbiota
respiration under the influence of litter (HRlitter) and
SOM (HRSOM).
\begin{equation}
TR=AR+HR\nonumber \\
\end{equation}\begin{equation}
AR=\text{TR}_{\text{nolitter}}-\text{TR}_{\text{Ncover}}\nonumber \\
\end{equation}\begin{equation}
HR=TR-AR\nonumber \\
\end{equation}\begin{equation}
\text{HR}_{\text{litter}}=\text{TR}_{\text{whithlitter}}-\text{TR}_{\text{nolitter}}\nonumber \\
\end{equation}\begin{equation}
\text{HR}_{\text{SOM}}=\text{HR}_{\text{total}}-\text{HR}_{\text{litter}}\nonumber \\
\end{equation}\begin{equation}
HR=\text{HR}_{\text{litter}}-\text{HR}_{\text{SOM}}\nonumber \\
\end{equation}Where, TRnolitter and TRwithlitter:
total soil CO2 efflux in the treatment with the absence
and presence of litter, respectively; TRNcover: total
soil respiration in the minded area without cover
Soil analysis and root density
Aiming to identify other variables that may influence the soil
CO2 efflux, we determined the main soil soil properties
and roots density. Soil samples from the 0-10 layer of each plot, ground
in a mortar and passed through a 60-mesh sieve (0.250 mm), were used to
determine total organic carbon (TC) and total nitrogen (TN). Total
organic carbon was quantified by wet oxidation (YEOMANS; BREMNER, 1988)
and TN was determined by the Kjeldahl method, modified by Tedesco
(1985). In addition, we analysed the soil bulk density (Ds), by the
volumetric ring method; soil particle density (Dp) by the volumetric
balloon method and total porosity (Pr). The microbial biomass carbon
(CMB) and nitrogen (NMB) of the soil were determined in the 0-10 cm
layer only. Samples (<2 mm) were placed in lidded plastic
flasks, incubated for 10 days at 25°C, with moisture corresponding to
80% of the moisture equivalent, in order to re-establish the microbial
community. After incubation, the CMB and NMB content were determined by
irradiation-extraction (ISLAM; WEIL, 1998).
To deternine the roots density, we collected blocks of soil with
dimensions of 20 x 20 x 20 cm in triplicate in all plots. After
collection, the roots were manually separated from the soil, washed and
divided into two classes, with a diameter smaller or larger than 2 mm.
After determining the wet weight, the roots were placed in plastic pots
with 25% alcohol and stored in a refrigerator for later evaluation. An
Epson XL 10000 scanner, equipped with an additional light unit (TPU),
together the WinRHIZO Pro 2009 software, was used to determine the
following morphological characteristics: the biomass of roots smaller
than 2 mm (BioRSm2), length of roots smaller than 2 mm (LengRSm2),
specific root length (SRL), sectional area of roots smaller than 2 mm
(SARSm2), mean root diameter and volume of roots smaller than 2 mm
(VRSm2). Following these evaluations, the roots were placed in paper
bags and dried in an oven at 60°C to obtain the dry weight. All results
of soil properties and roots area presented in the Appendix 1.
Statistical analysis
The data for soil CO2 efflux were submitted to analysis
of variance (ANOVA) and the mean values compared by Tukey’s (10%). The
influence of biotic and abiotic variables on the CO2efflux was evaluated using multivariate regression analysis.
Results
Variations in soil moisture and temperature
The soil presented the highest moisture content during November 2014,
March and May 2015 for most of the types of cover, while the highest
soil temperatures occurred in September 2014, and January and March 2015
(Fig. 1). Woodland showed the highest soil moisture and NCov the highest
soil temperature for the studied period.
3.2. Soil CO2efflux
The five treatments showed significant differences in
CO2 efflux (p <0.1) (Fig. 2), with NCov
presenting the lowest soil CO2 values. Woodland had the
highest values for CO2 efflux during January and March
2015, while the soil CO2 efflux in Euc, Ap and Nat did
not vary during the study period. Euc and Nat had a similar soil
CO2 efflux pattern, with a tendency for higher
CO2 efflux during November, March and May. The increase
of CO2 efflux in Euc, Ap and Nat showed high percentage
values compared to Woodland for all measurements, except in May 2015,
when the soil CO2 efflux presented the lowest
percentages (< 33%) (Fig. 2a). In November, the three forest
covers showed the same soil CO2 efflux compared to the
Woodland.
Temporal variation in
CO2 efflux
The highest CO2 effluxes were recorded in the months of
higher soil moisture (Fig. 3), i.e. November 2014, March and May 2015,
with Woodland presenting the highest and NCov the lowest values.
Although Euc showed a tendency with higher soil CO2efflux during the months of higher soil moisture when compared to Ap and
Nat, there were no significant differences between the three types of
cover (p>0,1). In the driest months (September 2014,
January and July 2015) Woodland, Euc, Ap and Nat showed no differences
for CO2 efflux, differing only from NCov.
All covers types showed similar soil moisture in the drier months
(September 2014, January and July 2015), except Woodland, which had
higher soil moisture (Fig. 4). In the rainy months (November 2014, March
and May 2015), the lowest soil moisture was found for Euc and the
highest for Woodland. Soil temperature was similar for Euc, Ap and Nat
in the months of lowest soil moisture, while the lowest and the highest
soil temperature were measured for Woodland and NCov respectively. In
the rainy season, the highest soil temperature was recorded for NCov.
Soil CO2 efflux
partitioning
The contributions of AR and HR to soil TR were statically significant
over the months (p <0.1), with HR presenting the highest
values (Fig. 5), These differences can be seen in November 2014, and
March and May of 2015, when HR contributed more to TR than AR. In
September 2014, and January and July 2015, there was no difference
between the contributions of AR and HR to soil TR. AR and HR did not
differ in the areas of Euc, Ap or Nat.
Grouping the values for AR and HR during the months of highest and
lowest soil moisture, (i.e. in the months of the highest and lowest
precipitation) for the moments before gas collection it was seen that
during the drier months there was no difference between the
contributions of AR and HR to TR, with HR accounting for around 55% of
TR. However, during the wettest months, HR contributed more to TR
(71%). The forest plantations did not differ during the dry months
concerning AR and HR, and during the rainy months, differences were only
seen between Ap and Nat for AR (Fig. 6).
Contribution of litter to
heterotrophic soil respiration
The litter contributed significantly more than the SOM to HR for the
cover’s types (p <0.1; Fig. 7). The greatest contribution to
HR by the litter was recorded in November 2014 for the three types of
forest cover. The contribution of litter to HR does not differ between
Euc, Ap and Nat, except for November 2015, when the HR was higher in Euc
than Nat.
Principal component analysis
The multivariate model with two principal components (PCA) explained
70.7% of the variation of the data set: PC1 (60.3%) and PC2 (10.1%)
(Fig. 8). The variables TC, CMB and LC together contributed 43.4%, TN
and NMB 25.9%, and DRsm and DRgr 19.7%. From the PCA analysis, we
identified three groups, with NCov and Woodland at the two extremes of
the scatter plot and the other forest types are allocated between in the
middle. In general, soil CO2 efflux (TR) was positively
correlated with soil moisture (Ms), total TC, TN and CMB.
Discussion