Storage and turnover of organic matter in soil

Historically, attention on soil organic matter (SOM) has focused on the central role that it plays in ecosystem fertility and soil properties, but in the past two decades the role of soil organic carbon in moderating atmospheric CO{sub 2} concentrations has emerged as a critical research area. This chapter will focus on the storage and turnover of natural organic matter in soil (SOM), in the context of the global carbon cycle. Organic matter in soils is the largest carbon reservoir in rapid exchange with atmospheric CO{sub 2}, and is thus important as a potential source and sink of greenhouse gases over time scales of human concern (Fischlin and Gyalistras 1997). SOM is also an important human resource under active management in agricultural and range lands worldwide. Questions driving present research on the soil C cycle include: Are soils now acting as a net source or sink of carbon to the atmosphere? What role will soils play as a natural modulator or amplifier of climatic warming? How is C stabilized and sequestered, and what are effective management techniques to foster these processes? Answering these questions will require a mechanistic understanding of how and where C is stored in soils. The quantity and composition of organic matter in soil reflect the long-term balance between plant carbon inputs and microbial decomposition, as well as other loss processes such as fire, erosion, and leaching. The processes driving soil carbon storage and turnover are complex and involve influences at molecular to global scales. Moreover, the relative importance of these processes varies according to the temporal and spatial scales being considered; a process that is important at the regional scale may not be critical at the pedon scale. At the regional scale, SOM cycling is influenced by factors such as climate and parent material, which affect plant productivity and soil development. More locally, factors such as plant tissue quality and soil mineralogy affect decomposition pathways and stabilization. These factors influence the stability of SOM in part by shaping its molecular characteristics, which play a fundamental role in nearly all processes governing SOM stability but are not the focus of this chapter. We review here the most important controls on the distribution and dynamics of SOM at plot to global scales, and methods used to study them. We also explore the concepts of controls, processes, and mechanisms, and how they operate across scales. The concept of SOM turnover, or mean residence time, is central to this chapter and so it is described in some detail. The Appendix details the use of radiocarbon ({sup 14}C), a powerful isotopic tool for studying SOM dynamics. Much of the material here was originally presented at a NATO Advanced Study Institute on 'Soils and Global Change: Carbon Cycle, Trace Gas Exchange and Hydrology', held June 16-27, 1997, at the Chateau de Bonas, France

Soil Carbon and Nitrogen Changes Under Douglas‐fir With and Without Red Alder

We sampled pure Douglas-fir (DF) [Pseudotsuga menziesii (Mirb.) Franco] end mixed red alder (Alnus rabra Bong.)(RA) and DF (RA/DF) stands in 1980 and in 1999 to investigate the influence of RA on soil C and N pools. In RA/DF plots with 25% RA, the soil N pool to a 45-cm depth increased significantly (P < 0.05) by 190 g N m(-2), corresponding to 10 g N m(-2) yr(-1) accretion: The average between treatment soil N difference in 1999 was 166 g m(-2), representing N accretion of 8.7 g m(-2) yr(-1). In pure DF plots, the soil N pool remained nearly constant. Resin N mineralization in RA/DF plots was about ten fold greater than on pure DF plots, but the enhanced resin N availability did not affect DF foliar N concentration. Temporal plot pairing was necessary within this landscape with high spatial variability to detect significant changes in soil N pools, and only large effects, such as N addition by RA, could be identified with statistical significance. Minimum detectable difference (MDD) estimates for mean total soil C differences in RA/DF plots showed that it would require about 30 more years of C accretion to detect differences at P < 0.05. Conversely, total soil N accretion in RA/DF plots was 28% greater than the MDD after 19 yr