The influence of heterogeneity on soil microbial processes in agroecosystems - Microbial Ecology in Sustainable Agroecosystems - page 48

chapter 3
The influence of heterogeneity on soil
microbial processes in agroecosystems
Theory, evidence, and opportunities
Terry D. Loecke
University of Nebraska-Lincoln
Contents
3.1 Introduction .......................................................................................................................... 49
3.2 Theoretical underpinnings................................................................................................. 50
3.3 Processes proceed under heterogeneous, not uniform, conditions ............................. 56
3.4 Opportunities to advance the understanding and management
of heterogeneity in agroecosystems .................................................................................. 59
References....................................................................................................................................... 60
3.1 Introduction
Soil heterogeneity is the norm. The complex physical structure of soil is a product of
and causal agent of interacting biological diversity and chemical gradients at widely
ranging spatial (nanometers to kilometers) and temporal (seconds to centuries) scales.
Heterogeneity creates the conditions to select for the myriad biological species present in
soil (Brussaard et al., 1997). Chemical gradients (e.g., O 2 , CO 2 , NO 3 − , metals, and H + ) persist
in agricultural soils despite frequent mixing through cultivation (e.g., Robertson et al.,
1993). Understanding heterogeneity of soil resources is clearly an important consideration
from an agronomic perspective, but the role of heterogeneity as a driver of soil microbial
processes and biogeochemical transformations remains unclear.
This chapter describes the theoretical underpinnings of how heterogeneity influ-
ences soil microbial processes and agroecosystem functioning. Throughout this analysis,
I highlight continuing challenges and opportunities for advancing our understanding of
soil microbial processes in light of heterogeneous conditions. I do this through examples
of how heterogeneity allows or promotes soil microbial processes to proceed that do not
occur at average resource concentrations or constant environmental conditions.
The study of soil heterogeneity cuts across many subdisciplines, the synthesis of which
could fill entire volumes. To narrow the scope of this chapter, the examples presented are
limited to soil microbial processes affecting carbon (C) and nitrogen (N) cycling in tem-
perate agroecosystems. The goal is not to provide an exhaustive review of the forms and
drivers of soil heterogeneity but to couple the theoretical underpinnings to process rates
using biogeochemically relevant examples. An emphasis is placed on processes associated
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