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

Land use 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. As this trend continues, it remains unclear what the short-term impacts of intermittent cropping are to soil biological properties. This study examines the short-term response of soil microbial biomass and activity during grassland conversion to cropland using conventional tillage (CT) and no-till (NT) practices in comparison to a grassland control. Long-term grassland was converted to cropland for three consecutive years. Following crop harvest, soil samples were gathered to measure the ‘first-year’ impacts to microbial biomass and enzyme activity (beta-glucosidase [BG], beta-glucosaminidase [BGA] and phosphomonoesterase [PHOS]). Conversion of grassland to small grain production did not produce a statistically significant trend for total microbial biomass or bacterial biomass. However, there were strong environmental effects due to yearly fluctuations in precipitation on total microbial biomass, total bacterial biomass and total fungal biomass. The effect of land conversion via CT on fungal biomass was strong and resulted in decreased fungal to bacterial biomass ratios. CT significantly decreased fungal populations over both the grassland and NT by 29 %. All three enzyme activities in the top 7.5 cm were significantly reduced by land conversion via CT during the study year with the highest precipitation. On average, BG and BGA activity in the CT plots were approximately 20 % lower than either the grassland or NT plots. Phosphomonoesterase activity under CT was significantly decreased by 28 % and 18 % compared to grassland and NT, respectively. In the 7.5–15 cm depth interval, BG and BGA activities in the grassland control and NT plots showed vertical stratification, with the CT plots having the highest BG and BGA activities with higher precipitation. There was no effect of land conversion on PHOS activity at the 7.5–15 cm depth. Significant declines in soil microbial biomass and enzyme activity may occur in the first year of grassland conversion to cropland, particularly with tillage, implying reduced C and nutrient cycling potentials. These effects were the strongest for fungal biomass and occurred under conditions of higher precipitation in the semi-arid study site.