Amino acid production exceeds plant nitrogen demand in Siberian tundra

Birgit Wild,Peter Kuhry,Judith Prommer,Norman Gentsch,Jörg Schnecker,Mounir Takriti,Juri Palmtag,Nikolay Lashchinskiy,Anna Knoltsch,Olga Shibistova,Petr Čapek,Gustaf Hugelius,Robert Mikutta,Ricardo J Eloy Alves,Georg Guggenberger,Jiři Bárta,Andreas Richter,Tim Urich

doi:10.1088/1748-9326/aaa4fa

Abstract

Arctic plant productivity is often limited by low soil N availability. This has been attributed to slow breakdown of N-containing polymers in litter and soil organic matter (SOM) into smaller, available units, and to shallow plant rooting constrained by permafrost and high soil moisture. Using 15N pool dilution assays, we here quantified gross amino acid and ammonium production rates in 97 active layer samples from four sites across the Siberian Arctic. We found that amino acid production in organic layers alone exceeded literature-based estimates of maximum plant N uptake 17-fold and therefore reject the hypothesis that arctic plant N limitation results from slow SOM breakdown. High microbial N use efficiency in organic layers rather suggests strong competition of microorganisms and plants in the dominant rooting zone. Deeper horizons showed lower amino acid production rates per volume, but also lower microbial N use efficiency. Permafrost thaw together with soil drainage might facilitate deeper plant rooting and uptake of previously inaccessible subsoil N, and thereby promote plant productivity in arctic ecosystems. We conclude that changes in microbial decomposer activity, microbial N utilization and plant root density with soil depth interactively control N availability for plants in the Arctic.

Highlights

Arctic plant productivity is limited by low temperatures (Rustad et al 2001, Natali et al 2012, Sistla et al 2013, Leffler et al 2016) and low soil N availability (Haag 1974, Shaver and Chapin 1980, Gough et al 2002, Gough and Hobbie 2003)
Gross protein depolymerization rates significantly decreased with depth, from organic layers to mineral topsoils and further to subducted topsoils and mineral subsoils
Mineral subsoils of arctic permafrost soils are susceptible to plant-derived compounds in laboratory experiments (Wild et al 2014, 2016), and we suggest that deeper plant rooting could accelerate soil organic C losses from the deep active layer, and counteract the increased plant CO2 uptake, as indicated in a long-term tundra warming study (Sistla et al 2013)

Summary

Introduction

Arctic plant productivity is limited by low temperatures (Rustad et al 2001, Natali et al 2012, Sistla et al 2013, Leffler et al 2016) and low soil N availability (Haag 1974, Shaver and Chapin 1980, Gough et al 2002, Gough and Hobbie 2003). Nitrogen limitation of arctic plants has been attributed to slow depolymerization under the cold and moist conditions in arctic soils (Hobbie et al 2002), and an increase in plant productivity with warming to an increase in soil N availability for plant growth (Chapin 1983, Hartley et al 1999, Shaver and Jonasson 1999, Natali et al 2012). If subsoils contain relevant amounts of available N, deeper rooting could alleviate plant N limitation, and stimulate plant CO2 uptake in the Arctic

Methods

Results

Discussion

Conclusion