Surface Soil Organic C Response to Discontinued Manure Application after Receiving Beef Cattle Feedlot Manure for 30 Years.

Long-term beef cattle feedlot manure applications builds soil organic C stocks, but how organic C responds to discontinuation of manure applications is uncertain.  The objective of this study was to quantify how soil organic C responded to the discontinuation of beef cattle manure application after 30 annual applications between 1973 and 2002, with manure discontinuation in 2003. Beef cattle feedlot manure was applied annually in autumn from 1973 to 2002 to an irrigated agroecosystem at 60, 120 and 180 Mg (wet mass) ha^-1 yr^-1 to six replicated plots. In 2003, one half of the replicated plots stopped receiving manure, while the other half continued to receive annual manure applications. Soil organic C stocks were measured from 1973 to 2002 and five times after manure discontinuation between 2003 and 2013 in the 0 to 15 and 15 to 30 cm depths. Between 1973 and 2003 soil organic C increased linearly by 1.2, 2.2 and 2.9 g kg^-1 yr^-1 in the 0 to 15 cm depth at 60, 120 and 180 Mg ha^-1 application rates, respectively.  After manure discontinuation soil organic C decreased linearly by -2.0, -3.3 and -3.4 g kg^-1 yr^-1at 60, 120 and 180 Mg ha^-1 application rates in the 0 to 15 cm depth, respectively. In the 15 to 30 cm depth, soil organic C increased linearly by 0.4, 0.7, and 1.1 g kg^-1 yr^-1 at 60, 120 and 180 Mg ha^-1 application rates between 1973 and 2003, respectively. After discontinuation, soil organic C increased in 2004 and linearly decreased thereafter at the 60, 120 and 180 Mg ha^-1 application rates in the 15 to 30 cm depth. Our results suggest that soil organic C in the 0 to 15 cm depth depleted faster in the ten years after discontinuation than C accumulated during 30 annual manure applications