PNNL

Abstract

Most soil carbon (C) is in the form of soil organic matter (SOM), the composition of which is controlled by of the plant-microbe-soil continuum. While plants provide most fresh SOM inputs to soils, research suggests that the majority of SOM is of microbial origin in the form of microbial residues. Certain microbial residues are considered especially persistent, including amino sugars, proteins, and lipids. Their contribution to persistent SOM has been linked to edaphic properties such as mineralogy and aggregation. However, it is unknown how variation in plant inputs, microbial community structure, and soil physical and chemical attributes interact to influence SOM production and persistence. We used two long-term biofuel feedstock field experiments to test the influence of cropping systems (corn and switchgrass) and soil characteristics (sandy and silty loams) on microbial selection and SOM chemistry. Cropping system had a strong influence on water-extractable organic carbon chemistry with perennial switchgrass generally having a higher chemical richness than the annual corn cropping system. Nonetheless, cropping system was a less influential driver of soil microbial community structure and overall carbon chemistry than soil type. Soil type was especially influential on fungal community structure and the chemical composition of the chloroform-extractable carbon. Although plant inputs strongly influence decomposition and SOM formation, total carbon and nitrogen did not differ between cropping systems within either site, likely due to enhanced microbial activity under the perennial cropping system. Silty soils also had a higher activity of phosphate and carbon liberating enzymes. After 8 years, silty loams still contained twice the total carbon and nitrogen as sandy loams, with no significant response to biofuel cropping system inputs. Together these results demonstrate that initial site selection is critical to plant-microbe interactions and substantially impacts the potential for long term carbon accrual in soils under biofuel feedstock production.