Soil microbial nutrient constraints along a tropical forest elevation gradient: a belowground test of a biogeochemical paradigm

A T Nottingham,R D Bardgett,J Whitaker,N J Ostle,N Salinas,N P Mcnamara,B L Turner,P Meir

doi:10.5194/bg-12-6071-2015

Abstract

Abstract. Aboveground primary productivity is widely considered to be limited by phosphorus (P) availability in lowland tropical forests and by nitrogen (N) availability in montane tropical forests. However, the extent to which this paradigm applies to belowground processes remains unresolved. We measured indices of soil microbial nutrient status in lowland, sub-montane and montane tropical forests along a natural gradient spanning 3400 m in elevation in the Peruvian Andes. With increasing elevation there were marked increases in soil concentrations of total N, total P, and readily exchangeable P, but a decrease in N mineralization determined by in situ resin bags. Microbial carbon (C) and N increased with increasing elevation, but microbial C : N : P ratios were relatively constant, suggesting homeostasis. The activity of hydrolytic enzymes, which are rich in N, decreased with increasing elevation, while the ratio of enzymes involved in the acquisition of N and P increased with increasing elevation, further indicating an increase in the relative demand for N compared to P with increasing elevation. We conclude that soil microorganisms shift investment in nutrient acquisition from P to N between lowland and montane tropical forests, suggesting that different nutrients regulate soil microbial metabolism and the soil carbon balance in these ecosystems.

Highlights

Tropical forests have a major influence on the global carbon (C) cycle, being the most productive ecosystems on Earth and containing 34–55 % of the C in forests worldwide (Beer et al, 2010; Pan et al, 2011)
It has been proposed that tropical forest elevation gradients are gradients of nutrient limitation on plant productivity, with P limitation prevalent in lowland forests (Vitousek and Sanford, 1986) and N limitation prevalent in montane forests land (Cleveland et al, 2002; Turner and Wright, 2014) and montane tropical forests sites (Corre et al, 2010; Cusack et al, 2011a)
Our findings from a 3400 m tropical forest elevation gradient in the Peruvian Andes provide evidence that this paradigm applies to soil microorganisms, with a gradual transition in investment in nutrient acquisition from P to N between lowland and montane tropical forests

Summary

Introduction

Tropical forests have a major influence on the global carbon (C) cycle, being the most productive ecosystems on Earth and containing 34–55 % of the C in forests worldwide (Beer et al, 2010; Pan et al, 2011). The exchange of C between the atmosphere and forests is mediated by the availability of mineral nutrients, so there is widespread interest in understanding how plant or microbial metabolic processes are constrained by the deficiencies of specific “limiting” nutrients (Cleveland et al, 2011; Wright et al, 2011), and how human alteration of these nutrient cycles may impact tropical ecosystems (Hietz et al, 2011; Townsend et al, 2011). Belowground processes remain relatively under-studied, despite evidence that they are limited by different nutrients to those limiting aboveground productivity in some ecosystems, including tropical forests (Sundareshwar et al, 2003; Turner and Wright, 2014). It is important to identify nutrient constraints to soil microbial process in tropical forests to understand how anthropogenic alteration of biogeochemical cycles will impact C storage in these ecosystems

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Conclusion