Use of lake-sediment accumulation to assess how conversion of cropland to grassland affects watershed-scale erosion in the corn belt of Minnesota
Conservation grasslands, or “set-aside” lands, here comprise those lands enrolled in incentivized programs administered by federal and state agencies, such as the Conservation Reserve Program (CRP). These programs replace cultivated cropland with perennial vegetation, commonly grassland in our study region of southern and western Minnesota, thereby reducing field-scale erosion. How do such field-scale reductions in sediment and nutrient loads translate into watershed-scale delivery to receiving waters? This study used sediment accumulation rates in 26 lakes in southern and western Minnesota as a measure of the delivery of eroded soil and phosphorus from watershed uplands to the lakes. Accumulation rates were calculated for the periods 1963-1986 and 1986-2007 to characterize sediment and phosphorus delivery before and after 1986, when many agricultural lands were enrolled in CRP.
Inorganic sediment accumulation rates decreased with increasing area of conservation grassland in the watershed. This linear relation explained only about 20% of the variance, leaving substantial unexplained scatter. The relation predicted that sediment accumulation would decrease by 3-4% for every 10% of cropland converted to grassland. Consideration of wetland sediment traps within the watershed did not measurably improve the relationship, nor did consideration of soil erodibility, slope length, and steepness factors. The decrease in sediment phosphorus accumulation rates as a function of increasing grassland area was not statistically significant at the p = 0.05 level. In the absence of substantial land-cover change, inorganic sediment accumulation increased by about 20% and sediment phosphorus increased by about 35%, indicating that other factors were influential. These factors could include changes in annual rainfall, artificial drainage, in-lake sediment transport processes, and lag effects in transport from uplands to lowlands.