Tillage and soil ecology: Partners for sustainable agriculture

Abstract

Much of the biodiversity of agroecosystems lies in the soil. The functions performed by soil biota have major direct and indirect effects on crop growth and quality, soil and residue-borne pests, diseases incidence, the quality of nutrient cycling and water transfer, and, thus, on the sustainability of crop management systems. Farmers use tillage, consciously or inadvertently, to manage soil biodiversity. Given the importance of soil biota, one of the key challenges in tillage research is understanding and predicting the effects of tillage on soil ecology, not only for assessments of the impact of tillage on soil organisms and functions, but also for the design of tillage systems to make the best use of soil biodiversity, particularly for crop protection. In this paper, we first address the complexity of soil ecosystems, the descriptions of which vary between studies, in terms of the size of organisms, the structure of food webs and functions. We then examine the impact of tillage on various groups of soil biota, outlining, through examples, the crucial effects of tillage on population dynamics and species diversity. Finally, we tackle the question of the design of tillage systems to enhance biological control in cultivated fields. Identification of the optimal tillage system requires a global consideration of soil management, rather than an analysis focusing on tillage alone, taking into account soil ecology. Organic residue management, the prevention of compaction, crop rotation and the timing of cultivation must all be considered together, taking into account their impact on pest populations and on the natural enemies of pests and ecosystem engineers. This approach requires more detailed research and careful experimental design than traditional comparisons of conventional and reduced tillage systems. We propose the development of population modeling in cultivated fields, as the available ecological models rarely include parameters linked to the soil management system.

Introduction

Soil is the most diverse and important ecosystem on the planet. A tremendous number of biological processes continually active in soils are of crucial importance for the maintenance of other ecosystems in the continental biosphere. Most of the biodiversity of agroecosystems lies in the soil (Young and Crawford, 2004), and the functions performed by soil biota have large, direct and indirect effects on crop growth and quality, soil and residue-borne pests, disease incidence, the quality of nutrient cycling and water transfer and the sustainability of soil productivity. They also determine the resistance and resilience of agroecosystems to abiotic disturbance and stress (Brussaard et al., 2007).

The rationale of sustainable crop management systems is based on the achievement of multicriterion objectives: crop yield is only one of a number of factors to be considered when evaluating the functioning of a crop management system. Consequently, the soil is no longer seen purely as a medium for plant growth, but also as a habitat for a number of organisms. A fundamental consequence of this change in approach is that ecological concepts and theories are now required for the design of new tillage systems, together with a knowledge of soil science, agronomy, ecophysiology and soil mechanics.

Farmers use tillage, consciously or inadvertently, to manage soil biodiversity. However, several literature reviews (e.g. House and Parmelee, 1985a, House and Parmelee, 1985b, Stinner and House, 1990, Kladivko, 2001, Lakshman et al., 2006, Miura et al., 2008) have highlighted the difficulties involved when trying to identify trends concerning the effect of tillage on soil biota. The identification of keys to help us to understand and predict the relationships between tillage regime and soil ecology therefore remains an important challenge in tillage research. When taking up this challenge, two key points must be addressed.

• Firstly, improvements in the assessment of the impact of tillage on soil organisms and functions are required. Unlike above-ground biodiversity, soil biodiversity can mostly be managed only indirectly, through tillage and other cropping practices (crop rotation, organic and mineral fertilization), complicating the design of new crop management systems.

• Secondly, we need to determine which tillage systems make the best use of soil biodiversity. Given the large number of functions of soil biota, we require biodiversity to fulfill many services, and tillage must be designed such that those services are optimized, even if the intrinsic value of soil biodiversity is, in many cases, difficult to assess (Brussaard et al., 2007).

This paper will focus on these two points, after a short presentation of the conceptual framework for soil biota studies. We will not deal here with weed control by tillage, focusing only on the soil microflora and fauna and the biological control of crop pests.