Defoliating Insect Mass Outbreak Affects Soil N Fluxes and Tree N Nutrition in Scots Pine Forests.

Maren M Grüning,Heinz Rennenberg,Judy Simon,Anne L-M-Arnold

doi:10.3389/fpls.2017.00954

Abstract

Biotic stress by mass outbreaks of defoliating pest insects does not only affect tree performance by reducing its photosynthetic capacity, but also changes N cycling in the soil of forest ecosystems. However, how insect induced defoliation affects soil N fluxes and, in turn, tree N nutrition is not well-studied. In the present study, we quantified N input and output fluxes via dry matter input, throughfall, and soil leachates. Furthermore, we investigated the effects of mass insect herbivory on tree N acquisition (i.e., organic and inorganic 15N net uptake capacity of fine roots) as well as N pools in fine roots and needles in a Scots pine (Pinus sylvestris L.) forest over an entire vegetation period. Plots were either infested by the nun moth (Lymantria monacha L.) or served as controls. Our results show an increased N input by insect feces, litter, and throughfall at the infested plots compared to controls, as well as increased leaching of nitrate. However, the additional N input into the soil did not increase, but reduce inorganic and organic net N uptake capacity of Scots pine roots. N pools in the fine roots and needles of infested trees showed an accumulation of total N, amino acid-N, protein-N, and structural N in the roots and the remaining needles as a compensatory response triggered by defoliation. Thus, although soil N availability was increased via surplus N input, trees did not respond with an increased N acquisition, but rather invested resources into defense by accumulation of amino acid-N and protein-N as a survival strategy.

Highlights

In forest ecosystems, nitrogen (N) cycling is influenced by insect herbivory (e.g., Kurz et al, 2008; Le Mellec and Michalzik, 2008; Morehouse et al, 2008; Cobb et al, 2010) already at low to moderate levels via loss of foliage, tree growth, throughfall leaching, litterfall, and litter decomposition (Schowalter et al, 1991; Chapman et al, 2003; Cunningham et al, 2009)
In forest ecosystems, insect mass outbreaks result in the mobilization of large amounts of organic N previously stored in otherwise long-living needles, which reenter ecosystem N cycling within a relatively short amount of time as a consequence of foliage loss
Focusing on organic N, increasing amounts are based on two processes: (1) Partly eaten needle fragments are dropped by the feeding larvae, adding additional N to the needle litter

Summary

Introduction

In forest ecosystems, nitrogen (N) cycling is influenced by insect herbivory (e.g., Kurz et al, 2008; Le Mellec and Michalzik, 2008; Morehouse et al, 2008; Cobb et al, 2010) already at low to moderate levels via loss of foliage, tree growth, throughfall leaching, litterfall, and litter decomposition (Schowalter et al, 1991; Chapman et al, 2003; Cunningham et al, 2009). Compared to leaf litter, insect feces often have an altered chemical as well as physical quality due to larvae digestion which causes inhomogeneous and porous surfaces and a wider C:N ratio (Le Mellec et al, 2009) In the soil, this surplus of N in combination with quality changes can accelerate important processes in N cycling, i.e., (1) soil respiration (Reynolds and Hunter, 2001; Frost and Hunter, 2004), which might lead to increased soil CO2 emissions (Lovett and Ruesink, 1995; Michalzik and Stadler, 2000), (2) mineralization, especially in N-limited ecosystems (Belovsky and Slade, 2000; Chapman et al, 2003; Le Mellec and Michalzik, 2008; Heinzdorf, 2013), and (3) N leaching, of nitrate, causing further N losses to the system (Swank et al, 1981; Pitman et al, 2010; Le Mellec et al, 2011). This might be observed especially when organic material, such as feces, are supplied to the soil, indicating slowed decomposition rates due to limited N availability (Le Mellec et al, 2009; Katayama et al, 2014)

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Discussion

Conclusion