Abstract
Intercropping forest plantations of Eucalyptus with nitrogen-fixing trees can increase soil N inputs and stimulate soil organic matter (OM) cycling. However, microbial indicators and their correlation in specific fractions of soil OM are unclear in the tropical sandy soils. Here, we examined the microbial indicators associated with C and N in the soil resulting from pure and intercropped Eucalyptus grandis and Acacia mangium plantations. We hypothesized that introduction of A. mangium in a Eucalyptus plantation promotes changes in microbial indicators and increases C and N concentrations on labile fractions of the soil OM, when compared to pure eucalyptus plantations. We determined the microbial and enzymatic activity, and the potential for C degradation by the soil microbial community. Additionally, we evaluated soil OM fractions and litter parameters. Soil (0–20 cm) and litter samples were collected at 27 and 39 months after planting from the following treatments: pure E. grandis (E) and A. mangium (A) plantations, pure E. grandis plantations with N fertilizer (E+N) and an E. grandis, and A. mangium intercropped plantations (E+A). The results showed that intercropped plantations (E+A) increase 3, 45, and 70% microbial biomass C as compared to A, E+N, and E, at 27 months after planting. The metabolic quotient (qCO2) showed a tendency toward stressful values in pure E. grandis plantations and a strong correlation with dehydrogenase activity. A and E+A treatments also exhibited the highest organic fractions (OF) and C and N contents. A canonical redundancy analysis revealed positive correlations between microbial indicators of soil and litter attributes, and a strong effect of C and N variables in differentiating A and E+A from E and E+N treatments. The results suggested that a significant role of A. mangium enhance the dynamics of soil microbial indicators which help in the accumulation of C and N in soil OF in intercropped E. grandis plantations. Our results are mostly relevant to plantations in sandy soil areas with low levels of OM, suggesting and efficient method for improving nutrient availability in the soil and optimizing eucalyptus growth and development.
Highlights
Brazil is the world’s largest producer of Eucalyptus spp., a species of fundamental ecological, social, and financial importance due to its effects in reducing pressures on native forests and generating direct and indirect jobs (ABRAF, 2013)
The vectors that discriminated A. mangium (A) and mixed system (E+A) from other treatments were the microbial indicators Cmic (λ = 5%, p < 0.0039), CO2-C (λ = 2%, p < 0.0039) and qMic-C (λ = 1%, p < 0.0410), which had a strong effect on soil and litter attributes including C-organic fractions (OF) (λ = 14%, p < 0.0017), N-OF (λ = 10%, p < 0.0018), N-OMinF1 (λ = 9%, p < 0.0010), total organic carbon (TOC) (λ = 2%, p < 0.0280), Total-N (Litter) (λ = 17%, p < 0.0019), and NH+4 -N (Litter) (λ = 10%, p < 0.0011)
The attributes that most contributed to differentiation of E. grandis treatments (E and E+N) were the microbial indicators qCO2 (λ = 2%, p < 0.0441), amidase (λ = 9%, p < 0.0019) and urease (λ = 1%, p < 0.0501), which were positively correlated with litter C/N ratio (λ = 3%, p < 0.001) and dehydrogenase (λ =7% p < 0.0033) (Table S3 and Figure 5A)
Summary
Brazil is the world’s largest producer of Eucalyptus spp., a species of fundamental ecological, social, and financial importance due to its effects in reducing pressures on native forests and generating direct and indirect jobs (ABRAF, 2013). The sustainability of Eucalyptus plantations has been intensely debated due to their concentration in south and central Brazil, where low fertility soils prevail, and Eucalyptus is mostly grown in mono-specific plantations (Gonçalves et al, 2013). Mineral fertilizers are employed to maintain productivity levels of Eucalyptus plantations, increasing both economic input and risk of environmental pollution. In this context, intercropped Eucalyptus grandis and Acacia mangium plantations represent a convenient forest management strategy as it minimizes environmental side-effects especially in sandy soils with low organic matter (OM) levels (Laclau et al, 2008). Several studies have revealed improvements generated by this association, especially in C and N dynamics, and wood productivity (Bouillet et al, 2008; Laclau et al, 2008; Voigtlaender et al, 2012; Paula et al, 2015; Fonseca et al, 2017)
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