Short-term impacts to the soil microbial population during grassland conversion to cropland

Authors: GRAHAM, CHRIS - South Dakota State University; RAMOS-PEZZOTTI, MARINA - South Dakota State University; Lehman, R - Michael

Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2020
Publication Date: 11/13/2020
Citation: Graham, C., Ramos-Pezzotti, M., Lehman, R.M. 2021. Short-term impacts to the soil microbial population during grassland conversion to cropland. Soil & Tillage Research. 206. Article 104839. https://doi.org/10.1016/j.still.2020.104839.
DOI: https://doi.org/10.1016/j.still.2020.104839

Interpretive Summary: Perennial grasslands which are marginally productive for cash cropping are increasingly cycled in and out of annual crop production due to variations in market conditions. The conversion of perennial grasslands to annual cropping systems on a permanent basis produces long-term effects on soil functions and ecosystems services. However, the short-term impacts of intermittent cropping on soil functions are not well-documented. In this study, we examined the time-scale of changes to soil biological properties when grassland is converted to annual crop production over a three-year period. We also compared the influence of tillage management during the conversion on these same soil biological properties which included total microbial biomass, bacteria biomass, fungal biomass; and microbial carbon, nitrogen, and phosphorus cycling activities. Total soil microbial biomass, bacterial biomass, and fungal biomass decreased with elapsed time following conversion of grassland. Biomass decreases were more rapid when grassland conversion occurred via conventional tillage in comparison to no-till. Carbon cycling activities decreased 60% with conversion via conventional tillage compared to either conversion with no-till or the grassland control. Likewise, nitrogen cycling activities were significantly decreased by conversion to cropping via conventional tillage in comparison to no-till or the grassland control. Phosphorus cycling activities were decreased by grassland conversion to annual cropping, regardless of whether conventional till or no-till was used during the conversion. We conclude that intermittent cropping of grasslands produced immediate decreases in the size of the soil microbial community and their carbon and nutrient cycling activities. Conversion of grasslands to annual cropping via no-till moderates these decreases compared to conventional tillage.

Technical Abstract: Conversion from grassland to cropland places unique constraints on ecosystem services and sustainability across the United States. In the northern Great Plains, this process is acute with marginal lands continuously cycling in and out of production. From federal programs to fluctuating crop prices, there are various reasons for intermittent production. While these trends are likely to continue into the future, it remains unclear what the short-term impacts of intermittent cropping are to the soil. This study examines the effect of grassland conversion to cropland with a focus on microbial community and activity through the use of tillage (CT) and no-till (NT) practices. Long-term grassland was converted to cropland for three consecutive years. Following harvest, soil samples were gathered to measure the ‘first-year’ impacts to microbial biomass and enzyme activity (beta-glucosidase (BG), beta-glucosaminidase (BGA) and phophomonoesterase (PHOS)). In general, total microbial biomass, bacterial biomass and fungal biomass decreased through the conversion of grassland and to a greater extent in the CT plots over the NT. Full study averages in the NT plots were significantly influenced by a very strong response in 2018, the wettest year of the trial. Enzyme activity showed similar variability between tillage and study year. Additionally, there was very strong vertical stratification in the grassland and NT plots, particularly for the BG and BGA enzymes. Between the 0-7.5 cm sample and the 7.6-15 cm sample BG activity was reduced by nearly 60% in these two treatments and over 40% in BGA activity. BGA activity in CT plots was roughly 60% of the NT or grassland treatment. NT and grassland were not significantly different in either study year. Likewise, CT plots showed lower BGA activity than both the grassland at NT plots. Whereas CT showed a decrease of 28% and 18% for grassland and NT, respectively, PHOS activity also decreased in NT soils relative to grassland by 13%.