Eucalypt harvest residue management influences microbial community structure and soil organic matter fractions in an afforested grassland

https://doi.org/10.1016/j.still.2020.104787Get rights and content

Highlights

  • HR decomposition was stimulated by bark retention, but not by mineral-N addition.

  • Microbial community structure and POM were most sensitive to HR management.

  • N addition had little influence on SOM pools.

  • Keeping bark increased POM-C and fungal:bacterial ratio and decreased δ13C-POM.

Abstract

Retaining harvest residues (HR) in the field is considered an option to recycle carbon (C) and nutrients in short-rotation plantations, but the fate of HR in the soil remains unclear. The effects of HR management and nitrogen (N) availability on soil organic matter (SOM) pools and microbial community structure of forest plantations represent a major knowledge gap. To fill this gap, we explored how management scenarios that differed in the amount and type of HR [removal of all residues (-R), residue retention without (+R/-B), or with bark (+R/+B)] and N availability [0 (-N) or 200 kg N ha-1 (+N)] influenced microbial activity and structure and SOM fractions in a recently afforested grassland. Specifically, we measured HR decomposition, carbon dioxide efflux (respired CO2), phospholipid fatty acid (PLFA), and determined changes in particulate (POM) and mineral-associated organic matter (MAOM) over 12 months using differences in δ13C natural abundance between Eucalyptus HR and grassland soils (δ13C ∼-28 ‰ and -13 ‰, respectively). Microbial respiration was stimulated by HR retention (+R). Bark retention reduced HR half-life by ∼70 days, on average, while N had little influence. Bacterial groups (Gram-positive and Gram-negative) were the primary decomposer of eucalypt HR, while Actinobacteria used more of the former soil organic carbon (SOC). +R/+B increased fungal biomarker PLFA concentration and fungal:bacterial ratio, suggesting a key role of Fungi in the fragmentation of woody HR. N influence on microbial community structure and SOM pools was dependent upon HR management. Retaining HR increased SOC concentrations, most significantly in the 0–1 cm soil depth and POM fraction, and when bark was included. +R/+B resulted in higher POM-C concentrations (up to +37%) and more depleted δ13C-POM compared with -R and +R/-B, respectively. Collectively, our results suggest it is feasible to increase POM-C and fungal abundance through HR management practices in the early stages of decomposition, which may potentially contribute to SOC stabilization in the long-term. Yet our findings remain to be tested in long-term studies, we provide quantitative evidence of the potential of a more conservative HR management to contribute to the sustainability of eucalypt plantations.

Introduction

The ever-increasing global wood demand requires sustainable intensification of production forestry. Plantation forests have expanded globally at an annual rate of ∼4.4 Mha over recent decades (Payn et al., 2015), particularly with exotic species managed as short-rotations in tropical and subtropical regions. Eucalyptus trees are the most widely planted hardwood and occupy an estimated area of 25 million ha across 90+ countries (Zhou et al., 2020). Eucalypt plantations cover 5.6 Mha in Brazil, and the high growth rates of these plantations favor expansion and place the country as a prominent supplier of pulp and industrial timber (Colodette et al., 2014). The rapid accumulation of carbon (C) in their biomass exerts a critical role in mitigating greenhouse gas emissions by acting as an efficient C sink and reducing pressure over native forests (Sanquetta et al., 2018). Additionally, active management of plantation forests may result in longer-term C sequestration through increasing soil organic matter (SOM) levels (Mayer et al., 2020), which is critical to the sustainability of production forestry in the tropics and subtropics (Gonçalves et al., 2013).

Short-rotation eucalypt plantations generate considerable amounts of plant litter and residues at harvest (Gatto et al., 2010). Retaining harvest residues (HR) in the field is considered an option to recycle C and nutrients back into the soil (Laclau et al., 2010; Oliveira et al., 2018). Nonetheless, the fate of HR in the soil is still unclear. Manipulating the intensity of harvesting changes the amount and quality of HR that remain in the field. The gross biomass of eucalypt HR is lignin-rich and has a high C:nitrogen (N) ratio (Ferreira et al., 2018b), but different components (i.e., leaves, branches, bark, treetops, and roots) may decompose at different rates because of differences in biochemistry (Walela et al., 2014). Particularly, tree bark is assumed to be recalcitrant because of its abundance in polymeric compounds (e.g., lignin, tannins, terpene) (Lima et al., 2013), so manipulating the presence of bark is expected to alter the decomposition of HR (Adamczyk et al., 2015; Souza et al., 2016) and to influence soil organic carbon (SOC) dynamics (Cotrufo et al., 2013; Souza et al., 2020). However, the relationship between litter quality, decomposition, and SOC accumulation is complex due to the interactive effects on biological process (Fanin et al., 2011; Keiblinger et al., 2010), including shaping microbial community structure (Baumann et al., 2009), and remains uncertain.

High productivity eucalypt plantations are commonly N fertilized at planting. Changes in N availability will likely alter HR decomposition, with possible implications for SOC storage (Du et al., 2014; Oliveira et al., 2018). Specifically, higher N availability alleviates C:N stoichiometric constraints (Koranda et al., 2014) and might accelerate decomposition in early stages (Wang et al., 2011), but inhibit the decomposition of lignin-rich compound during late stages (Carreiro et al., 2000). However, the response of HR decomposition and SOC dynamics to N addition may differ with different harvesting intensities. For instance, lignin and carbohydrate concentrations, which are abundant on eucalypt HR but differ between its components (Ferreira et al., 2018b), have interactive effects with N on soil microbial structure, e.g., soil fungal and bacterial proportions (Baumann et al., 2009). There is little information on how the microbial community responds to HR management practices and N availability in eucalypt plantations, and the lack of studies linking the soil microbial community and SOC dynamics in this increasingly common land use represents a major knowledge gap (Mayer et al., 2020).

In this study, we tested two hypotheses to investigate the effect of HR management and N availability on the dynamics of SOM pools:

  • 1)

    the bark retention induces changes in microbial community structure and increases SOC concentrations by increasing HR quantity and complexity;

  • 2)

    increasing N availability through mineral-N addition stimulates HR decomposition rates and favors soil microbial biomass and SOC accrual by ameliorating the effect of HR low N concentration during early stages of decomposition.

Isolating the HR-derived C (HR-C) from C sources of aboveground litter and root turnover originating during a multiyear rotation represents a technical research challenge in perennial crops (Binkley et al., 2004; Ferreira et al., 2018a). Thus, to test our hypotheses, different harvesting intensities and N additions were simulated in a field experiment in a recently eucalypt afforested grassland (C4-dominated), and differences in the 13C natural abundance were used to assess SOM changes (Maquere et al., 2008). Incorporation of HR-C (C3-C source) into phospholipid fatty acids (PLFA) and particle size fractions of the native C4-SOC topsoil was determined using 13C isotope mass spectrometry over one year, i.e., during early decomposition stages.

Section snippets

Site description

The study was conducted in a commercial eucalypt plantation located at São Gabriel, Rio Grande do Sul (RS) state, southern Brazil (30°26’S; 54°31’W). The site is representative of the region where eucalypt expansion has been occurring on Pampa native grasslands. The Pampa biome covers 63% of the RS state and holds a diversity of soils and a complexity of plants with the prevalence of C4 grassland species (Overbeck et al., 2007). The local climate is humid subtropical without a dry season and

Respiration rates as influenced by HR management

The mean respiration rates of treatments with HR removal (-R) were similar at 3 and 6 months (9.23 mg CO2-C kg soil-1 day-1), which were higher than the rate observed at 12 months (3.16 mg CO2-C kg soil-1 day-1) (Fig. 2; Table A.1). In both +R/-B and +R/+B, respiration rates increased from 3 to 6 months but had the lowest rate after 12 months of decomposition (∼6.30 mg CO2-C kg soil-1 day-1). The presence of HR (+R) stimulated respiration throughout the experiment; +R/+B caused an additional

Influence of bark management and N-addition on decomposition rates of eucalypt HR and respired CO2

Simulating typical HR management options in a recently afforested site wherein eucalypt species-specific decomposer communities are yet absent or transitioning could impact the decay rate of HR (home-field advantage (HFA) hypothesis; Ayres et al., 2009). However, HFA remains controversial in forest ecosystems (Bachega et al., 2016; Gama-Rodrigues and Barros, 2002; Veen et al., 2015). In our site, the average HR t0.5 was 483 days, which is within the range reported in second-rotation eucalyptus

Conclusion

Building and maintaining SOM is critical to the sustainability of production forestry. Manipulating harvesting intensity can reduce C and nutrient export and provide multiple benefits to forest soils. Our study suggests that understanding biological interactions with HR may offer a new tool for managing C cycling in eucalypt-afforested soils. Our results revealed contrasting dynamics between SOM pools with different mean residence times, particularly the sensitivity of the soil microbial

Funding

This work was supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES Finance Code 001 and grant number BEX 3725/14-6); UK Biotechnology and Biological Science Research Council (BBSRC) (project number BBS/E/C/00005214) and NUTREE research group.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgments

Thanks are due to the CMPC Celulose Rio Grandense for providing access to experimental area and assistence in the fieldwork. We also thank the following colleagues for their help with this work: Claire Horrocks and Adrian Joynes (technical support with PLFA analysis) and Dan Dhanoa (advice on statistical analyses) at Rothamsted Research, and João Milagres and Lucas Guimarães (technical support with SOM analyses) at Federal University of Viçosa.

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    Present address: Carbon Management Center, SRUC - Scotland’s Rural College, Edinburgh, Scotland, UK, EH9 3JG; and Geography, CLES - Amory Building. University of Exeter, Exeter, UK, EX4 4RJ.

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