Growing deeper: pathways to enhancing soil organic matter in annual and perennial dryland grain agroecosystems

Abstract

The story of agriculture and human civilization is one of loss: loss of soil structure, soil carbon, ecosystem function, and diversity. As we find ourselves at the nexus of intersecting global challenges of radically altered biogeochemical cycles and anthropogenic climate and productivity influence, we urgently need to alter our relationship with the soil and biosphere that sustain our human systems. In this dissertation I evaluate two management strategies for enhancing soil organic matter (SOM) in dryland, grain fields in the U.S.: legume integration and perennial grains. These strategies have been part of traditional farming practices, but they are not commonly utilized by commodity farmers for reasons I explore in Chapter 5. I conclude with policy recommendations for one way that might lead to systemic change that would value soils and their vital role in our human systems more appropriately. Here I provide a brief synopsis of each chapter: In the introduction (Ch. 1) I provide some historical context of human reliance on grain agriculture and the reasons that legumes and perennials might enhance SOM. I also describe the framework of SOM formation used in this research and provide an overview of the components of SOM I measured in this research. The first study (Ch. 3) is an observational study of conventional, dryland wheat farmers in semi-arid Colorado and Nebraska. I examine the 'soil carbon (C) dilemma' (Janzen 2006): How can SOM be increased, while also increasing the release of nutrients that accompanies decomposition? We specifically tested whether incorporating legumes into a continuous rotation influences the form and amount of SOM as well as productivity in farms of the central Great Plains region of the U.S. by contrasting three, no-till rotation systems: 1) conventional wheat-fallow; 2) continuous grain-only rotations, and 3) continuous grain rotations that incorporate a legume crop. We sampled on-farm fields and experimental agricultural research station plots that had received one of these rotations for at least eight years. We found that intensifying the rotation with continuous grains led to 1.5-fold increase in aggregate size but did not change SOC stocks. Incorporating a legume to the continuous grain rotation resulted in 1 Mg C ha-1 more SOC on average in surface soil compared to wheat-fallow rotations. In chapter 3, I use a similar approach to assess whether conversion from annual to perennial grains such as intermediate wheatgrass Kernza® could sequester soil organic carbon (SOC). We sampled three sites with paired fields under annual grains and converted to Kernza 5-17 years ago to 100-cm and compared their SOC stocks as distributed between mineral-associated (MAOM) and particulate organic matter (POM). POM-C was higher under Kernza cultivation but total and MAOM-C were similar. Our findings suggest Kernza increases SOC at depth as POM. Further study is needed to assess whether this will result in long-term SOC sequestration. In order to quantify the effect of legume incorporation and ability of Kernza to form SOC, I performed a mechanistic study to quantify the formation of SOM from Kernza and alfalfa tissues under contrasting N management (Ch. 4) Using continuously labeled 13C/15N plant residues, we tested the effect of litter inputs of contrasting composition (shoot and root material from Kernza® and alfalfa, a perennial legume) under management of Kernza where N was (1) not added, (2) added as urea, or (3) fixed by an alfalfa intercrop. We selected Kernza for its theoretical potential to build SOM due to deep root systems and long growing season. We hypothesized that the higher quality litter from alfalfa shoots would lead to greater MAOM formation due to its higher density of metabolic components promoting enhanced microbial C use efficiency, while root tissues may more likely become stabilized within aggregates as oPOM due to increased contact with soil surfaces. We predicted that the management with N addition may enhance MAOM-formation by alleviating microbial N-limitation and leading to enhanced microbial C use efficiency. We found that overall Kernza promoted greater SOM formation, in both MAOM and oPOM, with 20% of roots stabilized and 12% of shoot stabilized after 27 mo compared to 10% for alfalfa roots and shoots. Finally, in chapter 5, I propose a pilot crop insurance and research program in the U.S. Northern Plains to promote practices that enhance soil health, farm income, resilience, and mitigate climate change. Such a program could inform nationwide adoption of such practices.