Long-term agroecosystem experiments are essential for determining the relative sustainability of agricultural management systems.  Evaluations conducted over the long-term are critical to the success of agroecosystem experiments, as treatment effects often take many years before consistent and measurable changes in soils are detectable.  This is especially true for agroecosystems in semiarid regions, where low production levels contribute to a slow rate of change in soil properties.

The USDA-ARS Northern Great Plains Research Laboratory (NGPRL) near Mandan, North Dakota, USA (46º 46' 12” N, 100º 54' 57” W) was established in 1914, and currently manages three long-term agroecosystem experiments, each related to a different form of grazing management.  Two native vegetation pastures, a moderately grazed pasture (MGP) and heavily grazed pasture (HGP), were established in 1916.  A third pasture, seeded to crested wheatgrass [Agropyron desertorum (Fisch. ex. Link) Schult.] (CWP), was established in 1932.  Management of the pastures has been consistent over time.  The pastures were originally established to assess the effects of grazing intensity on vegetation characteristics and animal performance.  However, the age of the pastures and the consistency of the applied treatments over time make them an ideal setting to evaluate grazing management effects on soil.

Research findings from the long-term experiments at NGPRL have contributed to understanding the effects of grazing management on soil properties known to influence ecosystem function.  Most notably, increased grazing intensity of native vegetation in HGP has caused a shift in species composition toward blue grama [Bouteloua gracilis (H.B.K.) Lag. Ex Griffiths], resulting in an increase soil C and N in surface soil depths relative to MGP.  Fertilization of CWP has been found to decrease soil pH and cation exchange capacity in the surface 5 cm, but increase soil C at 30 to 60 cm compared to MGP and HGP.  Recent greenhouse gas flux research indicates MGP to sequester C at 0.29 Mg C ha^-1 yr^-1, while emitting N₂O at 1.2 kg N ha^-1 yr^-1.  Collectively, research findings from the long-term experiments at NGPRL have provided a foundation from which future soils-related research can be designed to better understand grazing management effects on environmental quality at multiple spatial scales.

As a supplement to the three long-term agroecosystem experiments, NGPRL possesses extensive soil sample archives dating back to 1918.  Sample archives were originally established in the 1940s when a branch of the original USDA Soil Survey Program was housed at NGPRL.  Archived samples are largely from the Great Plains and Intermountain West regions of the USA, and represent land uses from both pre- and post-cultivation time periods.  Given increased emphasis on global change research, the sample archives at NGPRL represent an important resource for scientists investigating temporal changes in soil properties.