Belowground allocation and dynamics of recently fixed plant carbon in a California annual grassland soil

Plant roots and the organisms that surround them are a primary source for stabilized organic C, particularly in grassland soils, which have a large capacity to store organic carbon belowground. To quantify the flow and fate of plant fixed carbon (C) in a Northern California annual grassland, we tracked plant carbon from a five-day 13CO2 pulse field labeling for the following two years. Soil and plant samples were collected immediately after the pulse labeling, and again at three days, four weeks, six months, one year, and two years. Soil organic matter was fractionated using a sodium polytungstate density gradient to separate the free-light fraction (FLF), occluded-light fraction (OLF), and heavy fraction (HF). Using isotope ratio mass spectrometry, we measured 13C enrichment and total C content for plant shoots, roots, soil, soil dissolved organic carbon (DOC), and the FLF, OLF, and HF. The HF was further analyzed by solid state 13C NMR spectroscopy. At the end of the labeling period, the largest amount of 13C was recovered in plant shoots (60%), but a substantial amount (40%) was already found belowground in roots, soil, and soil DOC. Density fractionation of 4-week soil samples (from which living roots were removed) indicated that the highest isotope enrichment was in the mineral-rich heavy fraction, with similar enrichment of the FLF and OLF. At the 6-month sampling, after the dry summer period during which plants senesced and died, the amount of label in the FLF increased such that it was equal to that in the HF. By the 1-year sampling, 13C in the FLF had declined substantially and continued to decline by the 2-year sampling. 13C recovery in the OLF and HF, however, was qualitatively stable between sampling times. By the end of the 2-year experiment, 69% of remaining label was in the HF, 18% in the FLF and 13% in the OLF. While the total 13C content of the HF did not change significantly from the 4-week to the 2-year sample time, 13C NMR spectroscopic analysis of spring HF samples from 2018, 2019, and 2020 suggests that the relative proportion of aliphatic/alkyl functional groups declined in the newly formed SOC over the 2-year period. Simultaneously, aromatic and carbonyl functional groups increased, and the proportion of carbohydrate groups remained relatively constant. In summary, our results indicate that initial associations between minerals and root-derived organic matter are significant and form rapidly; by 4 weeks, a substantial amount (17%) of the total plant-derived 13C had become associated with the heavy fraction (HF) of soil. While the majority of annual C input cycles rapidly (<2-year timescale), a sizeable proportion (~12% of the original inputs) persisted for 2 years.

Yellow-billed Magpie Population Status and Habitat Characteristics in Urban Sacramento, California

Most research on the ecology of the Yellow-billed Magpie (Pica nuttalli) has been focused in oak woodlands and savannas in California’s Coast Ranges; urban and suburban populations, some of which are sizable, have received little attention. In 2020, we studied eight colonies in six parks around Sacramento and in 2021 expanded the survey to 43 sites, detecting 827 breeding magpies. Population estimates based on nest counts were generally higher than those from direct counts, and nest counts were more repeatable and efficient. Counts of recently fledged young in family groups yielded reproductive rates similar to those observed near the coast before arrival of West Nile virus in 2003, suggesting that the virus is not currently affecting nestlings’ survival. Sacramento magpies nested in the upper canopy of a wide variety of large trees, both native and non-native. They foraged preferentially in low herbaceous habitat—irrigated turf and unirrigated annual grassland that was mowed or grazed. The presence of rivers and streams influenced occupancy strongly. Colony size was strongly related to the amount of low herbaceous foraging habitat within 0.5 km of colony sites with nearby flowing water. Our results suggest that at least 4 ha of low herbaceous foraging habitat is needed to support a small nesting colony. Retention of herbaceous habitat near large trees and flowing water, plus mowing or grazing to keep herbaceous growth low, should benefit urban Yellow-billed Magpies.

The elevational ascent and spread of exotic annual grasslands in the Great Basin, USA

In the Great Basin of the U.S., sagebrush (Artemisia spp.) and salt desert shrublands are rapidly transitioning to exotic annual grasslands, a novel and often self-reinforcing state that threatens the economic sustainability and conservation value of western grazing lands. Climate change is predicted to directly and indirectly favor annual grasses, potentially pushing annual grassland transitions into higher elevations. We used recently developed remote sensing-based rangeland vegetation data to retrospectively quantify expansion and elevational range shift of annual grassland transitions in the Great Basin from 1986–2019. During this period, we document an alarming six-fold increase in annual grassland area (to >75,000 km2) occurring at a rate of 1,950 km2 yr-1. Annual grasslands now occupy one fifth of Great Basin rangelands. This rapid expansion has been in part facilitated by a broadening of elevational range limits, with the leading edge of annual grassland transitions moving upslope at 60–110 m decade-1. Accelerated intervention is critically needed to conserve the fragile band of rangelands being compressed between annual grassland transitions at lower elevations and woodland expansion at higher elevations.SignificanceExotic annual grasses became widespread throughout the western U.S. Great Basin in the last century and now rank among the most vexing challenges facing western rangelands. Once established, these invaders can transform native sagebrush (Artemisia spp.) and salt desert shrublands into virtual monocultures of highly flammable exotic annual grasses with severely diminished biological and economic value. Capitalizing on a recently developed remote sensing vegetation product providing continuous spatial and annual temporal coverage of western US rangelands, we map the expansion of exotic annual grasslands over the past three decades. Our analysis reveals the alarming pace at which native shrublands are transitioning to annual grasslands, and confirms the movement of these transitions into ever higher elevations as the climate of the western U.S. warms.