Phosphorus Dynamics in a Changing Agroecosystem Landscape
Restricted (Penn State Only)
- Author:
- Mcconnell, Curt
- Graduate Program:
- Ecology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 06, 2023
- Committee Members:
- Kate Freeman, Outside Unit Member
Charlie White, Outside Field Member
Jason Kaye, Co-Chair & Dissertation Advisor
Jason Kaye, Program Head/Chair
Armen Kemanian, Co-Chair & Dissertation Advisor - Keywords:
- phosphorus
biogeochemistry
agroecosystems
modeling
isotope chemistry - Abstract:
- The availability of soil phosphorus (P) for plants changes as soil develops, limiting plant growth in early successional stages as P slowly releases from parent material and doing so again in late soil weathering stages as the little remaining P in system is slowly fixed by the soil matrix. Managing P deficiencies in many agricultural systems requires using P fertilizers, as grain and hay harvest export nutrients from the soil-plant systems. However, in the post-Green Revolution world, P excesses from over-fertilization are ubiquitous. Phosphorus removed with grain from one region is sometimes redeposited as manure in a different region, which gradually increases the soil P content and that which is lost from hydrologic transport. Agricultural P pollution is the principal driver of aquatic ecosystem eutrophication; optimizing on farm soil P levels is therefore essential to sustainable food and fuel production. Sustainable production systems are ever-more reliant on precision technology, predicated on analytical procedures that require a strong theoretical framework of the biogeochemical P cycle. Knowing when and where P will be in excess or deficient and how it cycles in varying soils and conditions are integral steps to building the most reliable nutrient models and decision support tools. Gaps in the understanding of P are reflected in poor model conceptualizations and in uncertainties of model results. Research efforts in this work towards filling these gaps (Chapter 2) include increasing the resolution of vertical P distribution measurements (Chapter 3), exploring the controls on the microbial cycling of P using oxygen-18 tracers (Chapter 4), and applying models to test new production systems that will ultimately shape the P cycle (Chapter 5). One example of a gap in understanding soil P dynamics was the failure to predict increased soluble P losses from no-till agricultural systems, despite long knowing the practice stratifies P at the surface. Prior to this research, measurements of stratification were taken in 5-10 cm increments, which may obscure the distribution of P at the surface extremes and thereby underestimate surface P loss potential. I designed a new sampling tool was designed to extract thin cross sections of the surface soil in 1-cm increments. It revealed that most nutrients are more stratified within the top 5-cm, a pattern typically obscured by routine sampling. Understanding the cycling of P has also been slowed by the absence of stable isotope P tracers. To overcome this limitation, using oxygen-18 to trace P dynamics has gained traction over the past decade. As microbes process phosphate labeled with oxygen-18 (18OPO4), the oxygen in the phosphate molecule reaches isotopic equilibrium with soil water; thus, the extent of the microbial P cycling can be tracked by following the oxygen-18 in phosphates. However, based on the research reviewed and conducted in this dissertation to test whether P saturation and P content affected microbial P use, there was no measured equilibration of 18OPO4. This points to either slower turnover of soil P than previously thought, a portion of the Mehlich-3 pool untapped by microbes, a lack of measurement sensitivity, or issues with divergent equilibration depending on the methods of 18O tracer introduction. These limitations have only been vaguely addressed in the literature before. Clarifying such limitations is a necessary step to improving the use of oxygen-18 as a P tracer, our understanding of P biogeochemical cycling, and ultimately the representation of P cycling in models. Systems modeling can represent complex, large scale processes without in-field experimentation that become unfeasible at the scope of watersheds. P losses from erosion, runoff, and through tile drains in agricultural systems are a significant contributor to P pollution. Planting cover crops is an effective means of controlling nutrient losses, but the earlier onset of winter in northern latitudes can hamper establishment of cover crops planted after the cash crop. Interseeding cover crops between the rows of growing cash crops can help improve cover crop establishment and provide ecosystem services. I used the Cycles model to simulate overall crop yields, interspecific competition, and nutrient losses and to determine at what latitude the tradeoffs of interseeding could be minimalized. The model showed this to be around latitude 41°N, where the benefits of interseeding outweigh the potential corn yield drag. The constant feedback between model performance and experimental results is what improves our understanding biogeochemical P cycling in agroecosystems. Knowing how P is distributed, how it is cycled, and how the surrounding landscape is changing due to aggressive human alteration of the P cycle globally, regionally, and within soil profiles, will enable better modeling and implementation of sustainable management practices.