Effects of wetting frequency and afforestation on carbon, nitrogen and the microbial community in soil

https://doi.org/10.1016/j.agee.2016.06.024Get rights and content

Highlights

  • We examined reduced wetting frequency on C & N dynamics and microbial community.

  • Heterotopic respiration was lower in soil from pastures than tree plantings.

  • Little difference in soil microbial community among any of the soils or treatments.

  • Pasture and tree planting soil responded similarly to a reduction in wetting frequency.

Abstract

Afforestation of agricultural land is increasing, partly because it is an important biological method for reducing the concentration of atmospheric CO2 and potentially mitigating climate change. Rainfall patterns are changing and prolonged dry periods are predicted for many regions of the world, including southern Australia. To accurately predict land-use change potential for mitigating climate change, we need to have a better understanding of how changes in land-use (i.e. afforestation of pastures) may change the soils response to prolonged dry periods. We present results of an incubation study characterising C and N dynamics and the microbial community composition in soil collected from two tree plantings and their adjacent pastures under a baseline and reduced frequency. While the concentration of soil C was similar in pasture and tree planting soils, heterotrophic respiration was significantly lower in soil from pastures than tree plantings. Although there was little difference in the composition of the soil microbial community among any of the soils or treatments, differences in N cycling could indicate a difference in microbial activity, which may explain the differences in heterotrophic respiration between pastures and tree plantings. Soils from pastures and tree plantings responded similarly to a reduction in wetting frequency, with a decrease in microbial biomass (measured as total PLFA), and a similar reduction in heterotrophic respiration from the soil. This suggests that the responses to changes in future wetting cycles may be less dependent on land-use type than expected.

Introduction

Afforestation of agricultural land is increasingly occurring for several reasons. For example, it is recognised as an important way of reducing levels of atmospheric CO2 to mitigate climate change (IPCC, 2007), or it can reduce soil erosion (e.g. Chirino et al., 2006). Globally forests store large amounts of carbon (C) in their soil, biomass and litter and deadwood (373, 383 and 116 Pg, respectively, Pan et al., 2011). Mixed-species tree plantings are increasingly established because they have additional ecological benefits over single-species tree plantations, such as habitat restoration and increasing biodiversity (Cunningham et al., 2015). In Australia, plantings of mixed native species (termed ‘environmental plantings) accounted for up to 20% of the 1.14 Mha of afforestation between 1990 and 2012 (Paul et al., 2015). Soil moisture dynamics are a key factor affecting the sequestration of C in soils (e.g., Austin et al., 2004). Precipitation patterns are predicted to change under future climate projections (IPCC, 2007). In Australia, dry periods are predicted to increase (Whetton et al., 2015), affecting the wetting frequency of soil. In turn, this will alter C and nitrogen (N) cycling in the soil (e.g., Borken and Matzner, 2009) and may affect the potential of afforestation to increase C sequestration in soils.

Soil C cycling is affected predominantly by moisture (Borken and Matzner, 2009), temperature (e.g. Frey et al., 2008), and the quantity and quality (e.g. C:N ratio, lignin content, etc) of organic matter (e.g. Bending et al., 2002), through their impact on the activity and composition of the soil microbial community. Wetting regimes (frequency and amount of rainfall) strongly affect the activity and mortality of the microbial population (Fierer et al., 2003, Borken and Matzner, 2009). When the soil dries, microbes produce osmolytes in order to reduce their internal water potential and prevent dehydration (Halverson et al., 2000). Soil microbes that are less adapted to water stress may die over the dry period. When the soil is rewetted, the surviving microbes rapidly excrete the osmolytes, before water enters the cells by osmosis and causes the microbial cells to rupture (Fierer and Schimel, 2003). A pulse in CO2 emission after rewetting of dry soil, is caused by rapid mineralization of the excreted osmolytes in the soil, as well as mineralization of microbial biomass from microbes that died during the dry period or re-wetting of the soil (Steenwerth et al., 2005, Sponseller, 2007). The magnitude of this CO2 pulse is dependent on the specific microbial community (e.g. bacterial vs fungal dominated) and how it responds to wetting and drying cycles (Aanderud and Lennon, 2011). In addition, swelling and shrinking of the soil may breakdown aggregates, which exposes organic material that was previously protected from microbial decomposition or chemical oxidation (van Gestel et al., 1991), which may contribute to the observed pulse of CO2 after re-wetting of soil (Fierer and Schimel, 2003).

Afforestation of agricultural land can alter the biomass, activity and composition of soil microbes (e.g., Singh et al., 2007, Carson et al., 2010). For example, the soil microbial communities of afforested soils and mature forests typically contain larger fungal biomass compared with pastures. This difference is attributed to fungi decomposing woody material quicker than bacteria (e.g., Fierer et al., 2009, Macdonald et al., 2009) and forest ecosystems generally experience lower levels of soil disturbance, favouring fungi over bacteria (Six et al., 2006). Furthermore, fungi are thought to sequester more C than bacteria, due to the higher C use efficiency of fungi (Bailey et al., 2002, Jastrow et al., 2007) and are thought to be less sensitive to drought stress than bacteria (Schimel et al., 2007, Blazewicz et al., 2014). Whether a drier climate leads to less C sequestration after afforestation of pastures has yet to be determined.

A meta-analysis on the effect of different types of land-use change on soil C sequestration showed a trend of higher soil C sequestration after afforestation in low rainfall areas compared with high rainfall area’s (Guo and Gifford, 2002). In addition, a study investigating woody plant invasion along a precipitation gradient also showed that dryer sites showed an increase in soil organic carbon, while wetter sites lost soil organic carbon after woody plant invasion (Jackson et al., 2002). A doubling of the length of a dry-period was found to reduce soil respiration flux after re-wetting by approximately 17% (Fay et al., 2000). Because of the changes in quantity and quality of organic matter inputs into the soil after afforestation of pastures, and consequently changes in soil microbial communities, soil from pastures and adjacent tree plantings may respond differently to wetting and drying cycles (Gordon et al., 2008, Zhao et al., 2010). While afforestation effects on soil C sequestration is widely studied, research on (interactive) soil responses to both prolonged dry periods and land-use change (afforestation of pastures) are rare (e.g., Fierer and Schimel, 2003).

Given that drying climates are predicted for many regions of the world, including southern Australia, we need to have a better understanding of how prolonged dry periods may affect C sequestration and the degree to which the response is dependent on the land-use context: do soils in pastures and tree plantings respond differently? Here, we present results of an incubation study that aimed to answer the question: “Are the effects of an increase in the time between wetting events on C sequestration and soil respiration context-dependent? And, specifically, do different land-use types exhibit different responses? We measured soil respiration during a 100-day laboratory-based incubation study in which soils collected from tree plantings and their adjacent pastures at two farms were subjected to two different wetting frequencies. We also characterize soil microbial community composition and C and N dynamics.

Fungal biomass was expected to be relatively higher in the afforested pastures than pastures. Consequently, we expected lower heterotrophic respiration in soils from tree plantings than the pastures due to the larger mass of low quality (high C:N) inputs and the higher C use efficiency of fungi than bacteria (Bailey et al., 2002, Jastrow et al., 2007). Finally, because of the lower sensitivity of fungi to wetting and drying cycles than bacteria (Schimel et al., 2007, Blazewicz et al., 2014) we hypothesize that there will be a (relatively) greater reduction in respiration with reduced wetting frequency in pasture soils than in afforested soils.

Section snippets

Soil collection

Soil was collected for the incubation experiment in the austral winter (June 2012), from two mixed-species, restoration tree plantings and their adjacent pastures. All sites were riparian, with the pasture plots immediately downstream of the tree planting (Fig. 1). The sites were located around Benalla (−36.74°S, 145.99°E and −36.70°S, 145.88°E), Victoria, Australia. Tree plantings were established in 1994 and 1990 (i.e. this study was conducted 18 and 22 years after planting) respectively by

Soil moisture

Soil moisture in the baseline wetting frequency treatment (expressed as% field capacity) reached a minimum of 29 ± 0.7% field capacity after each of the first four wetting events, and a minimum of 18 ± 0.7% field capacity after the fifth wetting event (Fig. 2a). In the reduced wetting frequency treatment, soil moisture content reached a minimum of 11 ± 0.5% field capacity (Fig. 2b), which was significantly lower than in the baseline treatment.

Heterotrophic respiration

The cumulative amount of CO2 respired over the 100-day

Discussion

Two decades after afforestation, soils under the tree plantings and their adjacent pastures differed in C:N ratio, potential mineralizable nitrogen (PMN) and heterotrophic respiration (Table 1, Table 2, Fig. 3). Despite these differences, the soils responded in a similar magnitude to a reduction in the frequency of wetting (Table 1). All soils respired significantly less CO2 in the reduced wetting treatment than the baseline wetting treatment, and the proportional magnitude of the reduction in

Conclusions

While the concentration of soil C did not change during the early development (ca 20 yr) of mixed-species plantings, heterotrophic respiration in the laboratory incubation was higher in soils from tree plantings than from the adjacent pastures. Rather than differences in microbial community composition between soils of tree plantings and pastures, the higher respiration after afforestation may be explained by differences in the activity of certain groups of microbes involved in N

Acknowledgements

This research was funded by the Australian Research Council Linkage Program (LP0990038), Goulburn Broken Catchment Management Authority (CMA), North Central CMA, Victorian Department of Sustainability and Environment, EPA Victoria and Kilter Pty. Ltd. T.R.C. (FT120100463), J.B. (FT110100602) and P.J.B. (FT120100715) were supported by Australian Research Council Future Fellowships. M.H. thanks the Holsworth Wildlife Research Endowment for additional funding for fieldwork and laboratory analysis.

References (62)

  • M.M. Mikha et al.

    Carbon and nitrogen mineralization as affected by drying and wetting cycles

    Soil Biol. Biochem.

    (2005)
  • K.M. Miranda et al.

    A rapid, simple spectrophotometric method for simultaneous detection of nitrate and nitrite

    Nitric Oxide

    (2001)
  • E.-L. Ng et al.

    Does the chemical nature of soil carbon drive the structure ad functioning of soil microbial communities

    Soil Biol. Biochem.

    (2014)
  • K.I. Paul et al.

    Improved models for estimating temporal changes in carbon sequestration in above-ground biomass of mixed-species environmental plantings

    For. Ecol. Manag.

    (2015)
  • W.J. Rawls et al.

    Effect of soil organic carbon on soil water retention

    Geoderma

    (2003)
  • R. Setia et al.

    Relationships between carbon dioxide emission and soil properties in salt-affected landscapes

    Soil Biol. Biochem.

    (2011)
  • M. Smith et al.

    Spatial patterns of, and environmental controls on: soil properties at a riparian-paddock interface

    Soil Biol. Biochem.

    (2012)
  • K.L. Steenwerth et al.

    Response of microbial community composition and activity in agricultural and grassland soils after a simulated rainfall

    Soil Biol. Biochem.

    (2005)
  • S.-R. Xiang et al.

    Drying and rewetting effects on C and N mineralization and microbial activity in surface and subsurface California grassland soils

    Soil Biol. Biochem.

    (2008)
  • M. van Gestel et al.

    Carbon and nitrogen mineralization from two soils of contradicting texture and microaggregate stability: influce of sequential fumigation, drying and storage

    Soil Biol. Biochem.

    (1991)
  • B. Zhao et al.

    Soil microbial biomass and activity response to repeated drying-rewetting cycles along a soil fertility gradient modified by long-term fertilization managment practices

    Geoderma

    (2010)
  • Z.T. Aanderud et al.

    Validation of heavy-water stable isotope probing for the characterization of rapidly responding soil bacteria

    Appl. Environ. Microbiol.

    (2011)
  • R. Aerts et al.

    The mineral nutrition of wildplants revisited: a re-evaluation of process and patterns

    Adv. Ecol. Res.

    (2000)
  • A.T. Austin et al.

    Water pulses and biogeochemical cycles in arid and semiarid ecosystems

    Oecologia

    (2004)
  • Bates, DM, Maechler, M, Bolker, B, Walker, S, 2013. Lme4: linear mixed-effects models using Eigen and S4. R package...
  • H.F. Birch

    The effect of soil drying on humus decomposition and nitrogen availability

    Plant Soil

    (1958)
  • S.J. Blazewicz et al.

    Growth and death of bacteria and fungi underlie rainfall-induced carbon dioxide pulses from seasonally dried soil

    Ecology

    (2014)
  • W. Borken et al.

    Reappraisal of drying and wetting effects on C and N minieralization and fluxes in soils

    Glob. Change Biol.

    (2009)
  • D.A. Bossio et al.

    Determinants of soil microbial communities: effects of agricultural management, season, and soil type on phospolipid fatty acid profiles

    Microb. Ecol.

    (1998)
  • J.R. Bray et al.

    An ordination of upland forest communitites of southern Wisconsin

    Ecol. Monogr.

    (1957)
  • Bureau of Meteorology, 2013. Climate Data Online. http://www.bom.gov.au/climate/averages/. Visited: September...
  • Cited by (0)

    1

    Present address: South Australian Research and Development Institute, Waite Campus, Urrbrae, 5064 SA, Australia.

    View full text