Pore architecture and particulate organic matter in soils under monoculture switchgrass and restored prairie in contrasting topography

Bioenergy cropping systems can substantially contribute to climate change mitigation. However, limited information is available on how they affect soil characteristics, including pores and particulate organic matter (POM), both essential components of the soil C cycle. The objective of this study was to determine effects of bioenergy systems and field topography on soil pore characteristics, POM, and POM decomposition under new plant growth. We collected intact soil cores from two systems: monoculture switchgrass (Panicum virgatum L.) and native prairie, at two contrasting topographical positions (depressions and slopes), planting half of the cores with switchgrass. Pore and POM characteristics were obtained using X-ray computed micro-tomography (μCT) (18.2 µm resolution) before and after new switchgrass growth. Diverse prairie vegetation led to higher soil C than switchgrass, with concomitantly higher volumes of 30–90 μm radius pores and greater solid-pore interface. Yet, that effect was present only in the coarse-textured soils on slopes and coincided with higher root biomass of prairie vegetation. Surprisingly, new switchgrass growth did not intensify decomposition of POM, but even somewhat decreased it in monoculture switchgrass as compared to non-planted controls. Our results suggest that topography can play a substantial role in regulating factors driving C sequestration in bioenergy systems.

summer

Summer was a time for transplanting prairie wildflowers. We knew that we wanted to restore prairie on the cornfield in front of the Shack. How did we know where we could get these prairie species? Of course there were no commercial sources at all. We had heard that prairie species were especially prolific along railroad tracks, because in those days the railroad frequently burned them to control brush. So we would stop there during different parts of the summer and find the prairie species in bloom (so we could identify them), or along an old road cut where we felt we could dig up chunks of sod with the species, put them in a tub in the car, and transport these to the Shack, to spud them in to the old corn field (our future prairie). This included prairie grasses, legumes, asters, and a whole variety of perennial species. And of course these can reproduce. This means that in those days (and to some extent now) there were “idle spots” along each side of the railroad tracks, as Dad observed, where the cow, plow, and mower are absent and a profusion of wild prairie herbs persist and bloom vigorously. Some species had huge deep roots, like the beautiful compass plant. Dad collected their seeds and built a little plot on the hill to plant these along with a mix of seeds of prairie grasses. This was an experiment. As mentioned, he did not water them, but they came up and did beautifully. So we knew how to promote such species on our prairie. (See chapter 7.) Over the years our prairie became more diverse, and more beautiful. According to the Land Institute of Salinas, Kansas, these native perennial prairie herb species typically grow very deep roots. Some extend downward ten to eighteen feet below the land surface! So it is no wonder the prairie vegetation is so stable and tenacious during drought; they have unusual adaptations to reach moisture and minerals at depth.