Agriculture is under increasing pressure to produce more food with less environmental impacts and in the face of a changing climate. Management practices capable of sequestering soil carbon (C) and improving overall soil health hold promise for sustainable intensification, as well as climate change mitigation and adaptation. As market and policy-based incentives develop to support these practices, however, it is critical that adequate sampling protocols, minimum viable data sets, and thresholds of management responses to soil health indicators are identified across the diversity of cropping systems and edaphoclimatic conditions.Much of the research into the impacts of agricultural management on soil C and soil health have been conducted in the Midwest, over the short-term, and to a shallow depth. Soil C dynamics and other soil health indicators are strongly influenced by climate and mineralogy, necessitating more research across a range of edaphoclimatic conditions. Further, detectable changes in soil C take decades to accrue, requiring long-term research. Proper accounting of changes in C stocks on a given acreage for climate mitigation strategies and economic incentive programs also necessitates sampling to a sufficient depth (minimum 1 meter or a root-limiting layer).
Using long-term, on-farm interventions, controlling for cropping system, climate and soil type, this work investigates the impact of soil health practices on soil C in surface and subsurface soils, as well as on a suite of physical, chemical, and biological soil properties commonly used to assess soil health. Deep soil cores at a long-term, industrial scale, agricultural research station in a Mediterranean-type climate indicated that 19 years of cover cropping with annual composted poultry manure applications (4t ha-1) increased soil C to a depth of 200 cm by +21.8 Mg ha-1 relative to a -4.8 Mg ha-1 loss under conventional management (Chapter 1). Trends also indicated potential losses of -13.4 Mg ha-1 under conventional management with cover cropping, despite increases of +1.4 Mg ha-1 in the surface 0-30 cm, stressing the importance of deep soil sampling for greenhouse gas accounting purposes.
Continuing the theme of deep soil C, a nearby regional survey of 10+ yr old hedgerows and adjacent cultivated fields across four soil types showed a strong impact of hedgerows on soil C to a depth of 100 cm, with an average difference of 3.85 kg C m-2 (0-100 cm) and few differences across the four soil types (Chapter 2). Most differences occurred in the surface 0-10 cm and the subsoil at 50-100 cm, indicating a dual role of surface management (litter accumulation, reduced disturbance) and deep, woody perennial roots. Soil type differences were only apparent in one of the four soil types, which differed substantially in parent material, mineralogy, and degree of weathering. Soil type did not influence the management effect and may indicate broad potential for hedgerows as a climate mitigation strategy. The magnitude of this strategy is limited, however, by the extent of hedgerows on a given farm/ranch.
Revegetation of field margins with hedgerows also had a positive impact on a broad suite of physical, chemical, and biological parameters (0-20 cm) commonly associated with soil health (Chapter 3). Hedgerow values were greater than cultivated fields for nearly every indicator in the surface 0-10 cm, commonly 2-3 times greater. Fewer, smaller differences were observed at 10-20 cm. Total soil C and N, available C, microbial biomass C, aggregate stability, and surface hardness were some of the most sensitive and least variable indicators of management type. Texture, pH, and bulk density were more indicative of soil type. A composite of variables was necessary to explain most of the variation in the data, indicating the complexity of soil health.
