Energy, structure, soil and self-regulation in plant/soil systems: a conceptual model

1989 Fall.Includes bibliographical references.Typescript (Photocopy).A new concept is presented which suggests that in stable plant/soil systems, plants control the soil environmental factors that affect plant growth and the interactions among those factors by controlling system structure. The concept is based on the plant-control hypothesis and rhizocentric model of soil structural development. The plant-control hypothesis declares that in plant/soil systems energy is the primary resource, and structure an essential regulator of energy flows. The rhizocentric model of soil structural development in grass-dominated plant/soil systems describes the process which results in plant-control of soil structure, and, consequently, of energy and nutrient flows for such systems. In conjunction, the plant-control hypothesis and rhizocentric model form a conceptual model of control in plant/soil systems. The conceptual model may help explain the self-regulatory capabilities of stable plant/soil systems, and the causes of instability in some agricultural plant/soil systems. Examination of published data from various sources has revealed no case in which application of the conceptual control model did not result in logically consistent, reliable prediction of experimental outcomes, plausible interpretation of previously uninterpretable results, and often, formulation of testable new hypotheses. It is concluded that the control model -- and the plant-control hypothesis and rhizocentric model which it implies -- has enough credibility to merit further critical examination as a potentially useful conceptual tool for soil and agricultural science, biology, and ecology

Investigating Rhizosphere Controls of Soil Organic Matter Dynamics in Forest Soils using a 13C Labelling Approach

Rising atmospheric CO2 concentration may increase plant productivity through the “CO2 fertilization effect”, which may in turn increase the input of carbon (C) to soils through rhizodeposition or plant residues. However, whether this increase in C input to soils results in greater soil C storage is not clear, as the decomposition of different forms of organic matter and the role of the rhizosphere in the decomposition process remain poorly understood. In this thesis, I investigated the interactions between plant C dynamics and soil microbial processes, and how these interactions control C and nutrient cycling in forest soils. I manipulated soil carbon supply from trees to the rhizosphere both in mesocosms and in the field through either canopy shading or soil trenching. This allowed me to investigate the effect of assimilate C supply on the decomposition of 13C-labelled substrates of varying chemical compositions and structural complexities (glucose, straw, fungal necromass or biochar), and their combined effect on soil organic matter (SOM) decomposition. I found that plant C supply to the rhizosphere had no significant effect on the decomposition of substrates. Similarly, the presence of roots and their associated mycorrhizal fungi had no significant effect on litter mass loss. However, it was found that supply of C from plant to the rhizosphere promoted SOM decomposition by up to two-fold in soils amended with substrates. Although, the addition of both simple and complex substrates stimulated the activities of C, N and P- degrading enzymes, I observed that the activities of these enzymes were significantly greater in soils where a labile substrate (glucose) had been added. The increased activities of C-degrading enzymes suggest that microorganisms were C limited, and the input of labile C substrate alleviated C and energy limitation of enzyme production, allowing microbial communities to mobilize nutrients from decomposition of native SOM. This thesis demonstrates that substrate quality influences SOM decomposition, and that increased availability of labile substrates to the rhizosphere may have implications on forest soil C stocks

ENGINEERING FUNGAL-MYCELIA FOR SOIL IMPROVEMENT

Most of the conventional materials, processes and techniques used by geotechnical engineers for ground improvement are associated with the emission of greenhouse gases. Global targets for reducing carbon emissions therefore pose a direct challenge to research and practice in the field of geotechnical engineering and has led to the development of interdisciplinary approaches seeking alternative low carbon technologies that are resilient to climate change. This thesis presents a novel, potentially low-carbon technique involving the use of fungal hyphal networks of Pleurotus ostreatus (P. ostreatus) for ground engineering applications. An investigation was carried out to understand the range of environmental conditions suitable for growth of P. ostreatus in sand. Subsequently, the influence of the growth of P. ostreatus on the hydraulic and mechanical behaviour of sand was explored, in particular the influence on soil wettability, soil water retention curve, infiltration behaviour, saturated hydraulic conductivity and the stress-strain behaviour of sands. In addition, the influence of growth of P. ostreatus on the erodibility of sand was assessed using the Jet erosion test. The results presented in this thesis demonstrate that the treatment of sand with P. ostreatus (i) induced extreme water repellency, (ii) caused a shift of the soil water retention curve increasing the air entry value from ~0.6 to 6 kPa (iii) reduced the rate of infiltration of water into sand (iv) lowered saturated hydraulic conductivity by one order of magnitude from 1.3 x 10-4 to 3.0 x 10-5 ms-1 (v) inhibited the development of dilatancy during shearing with an associated loss of peak shear strength and (vi) significantly improved the resistance of sand to erosion. These results provide for the first time, evidence of the influence of the growth of fungal hyphae on the hydro-mechanical behaviour of soils within a geotechnical engineering context. The findings of this thesis implies that there is the potential to deploy fungal hyphal networks as a low carbon technique in areas of ground improvement where resistance of surface erosion and/or the creation of a semi-permeable barrier is require

Agroecological Approaches for Soil Health and Water Management

In the last century, innovations in agricultural technologies centered on maximizing food production to feed the growing population have contributed to significant changes in agroecosystem processes, including carbon, nutrients, and water cycling. There are growing concerns regarding soil fertility depletion, soil carbon loss, greenhouse gas emissions, irrigational water scarcity, and water pollution, affecting soil health, agricultural productivity, systems sustainability, and environmental quality. Soils provide the foundation for food production, soil water and nutrient cycling, and soil biological activities. Therefore, an improved understanding of biochemical pathways of soil organic matter and nutrient cycling, microbial community involved in regulating soil health, and soil processes associated with water flow and retention in soil profile helps design better agricultural systems and ultimately support plant growth and productivity. This book, Agroecological Approaches in Soil and Water Management, presents a collection of original research and review papers studying physical, chemical, and biological processes in soils and discusses multiple ecosystem services, including carbon sequestration, nutrients and water cycling, greenhouse gas emissions, and agro-environmental sustainability. We covered tillage, nutrients, irrigation, amendments, crop rotations, crop residue management practices for improving soil health, soil C and nutrient cycling, greenhouse gas emissions, soil water dynamics, and hydrological processes