Soil Carbon Dynamics and Nutrient Cycling - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 98

7

Soil Carbon Dynamics and
Nutrient Cycling

David Powlson*, Zucong Cai and Philippe Lemanceau

Abstract
The quantity of organic carbon in soil and the quantity and type of organic inputs have profound impacts
on the dynamics of nutrients. Soil organic matter itself represents a large reservoir of nutrients that are
released gradually through the action of soil fauna and microorganisms: this is especially important for
the supply of N, P and S to plants, whether agricultural crops or natural vegetation. Organic matter also
modifies the behaviour and availability of nutrients through a range of mechanisms including increas-
ing the cation exchange capacity of soil, thus leading to greater retention of positively charged nutrient
ions such as Ca, Mg, K, Fe, Zn and many micronutrients. Carboxyl groups in organic matter, and in root
exudates or microbial metabolites, form complexes with various metal ions, usually increasing their
availability to plants. In some cases, the formation of stable complexes has a detoxifying effect, for ex-
ample by making Al and Cu less available to plants or microorganisms. Organic matter influences soil
physical conditions greatly, especially through the formation or stabilization of aggregates and pores;
this indirectly influences the availability of water and dissolved nutrients to plant roots. Organic matter
and organic inputs are the source of energy for heterotrophic soil organisms, variations in organic carbon
content and composition, impacting biome size, diversity and activities. These complex interactions
between organic carbon and the soil biome require additional research to be fully understood. The
implications for nutrient dynamics differ between nutrient-rich situations such as agricultural topsoils
and nutrient-poor environments such as subsoils or boreal forests. In agricultural soils, excessive in-
puts of organic matter in manures can lead to pollution problems associated with losses of N and P.

Introduction

decomposition. But it is also because the C
in soil organic matter is a source of energy
for soil organisms, which are mostly hetero-
trophs, and thus acts as the driver for vari-
ous biologically mediated processes involved
in nutrient transformations. In addition to
being a source of nutrients, and a controlling
factor in nutrient transformations, organic C
in soil contributes to soil cation exchange

Soil carbon (C) plays a major role in regulat-
ing the cycling of plant nutrients, especially,
but not limited to, nitrogen (N), phosphorus
(P) and sulfur (S). This is partly because the
organic entities in soil contain these elem-
ents combined with C, and thus act as a source
of nutrients as organic matter undergoes