Soil Carbon and Nitrogen (C/N) Cycling

Introduction..................................................................................................183

Bacteria and Fungi Control Soil C/N Cycles............................................184

Continuous Soil C/N Cycling Requires Diversity in Organisms and Substrates

Natural versus Anthropogenic Controls of C/N Cycles..........................186

Anthropogenic Factors Disrupting C/N Cycles: Land Conversion, Tillage, and Sylvie M. Brouder N Fertilizers

and Ronald F. Turco Conclusion....................................................................................................188

Purdue University References......................................................................................................188

Introduction

Carbon (C) and nitrogen (N) exist in soils in both inorganic and organic forms. Globally, only 780 petagrams (Pg; 10¹⁵g) of C are in the atmosphere while the top one meter of soil is estimated to contain approximately 2700 Pg of C, 57% of which is soil organic matter (SOM).t'l In contrast, N in soil is less than one-hundredth of one percent of the N content of the atmosphere but, as with soil C, most soil N is in organic versus inorganic forms.¹²¹ The inorganic C and N forms in soils include the gases carbon dioxide (CO_z) and dinitrogen (N,) in the air space of soil pores and soluble forms in soil water such as carbonates from the chemical breakdown or weathering of rocks and minerals (e.g., silicates, limestones) and nitrate (N0₃ ) and ammonium (NF1₄⁺) from the breakdown or mineralization of SOM. C and N occur in all living tissues: C in sugars, starches, lipids, and a staggering array of complex molecules and N in nucleic acids (DNA and RNA) and in amino acids that are assembled into proteins. Both C and N are an integral component of SOM, which is formulated from plant and animal residues in various stages of decomposition including particulates of various size fractions, dissolved OM in soil water, and humus or stable OM as well as the living biomass of soil microorganisms (e.g., bacteria and fungi).¹³¹ To a large degree, microorganisms control or drive global C and N (C/N) cycling. Even under relatively extreme environmental conditions, soil C and N are in a state of constant flux where microbiology transforms C and N among an array of molecules and drives C and N movement between the soil and the surrounding air and water. The goal of this entry is to summarize the state-of-the-art knowledge on the microbiology of soil C/N cycling and discuss how human activities including agriculture are causing large perturbations in soil C/N cycles for which outcomes are uncertain but anticipated to continue to contribute to global climate change.