Conservation Reserve Program (CRP) land on which perennial warm‐season grasses are grown could be a resource for bioenergy feedstock production and C sequestration. A 4‐yr field experiment was conducted to determine the response of switchgrass (Panicum virgatum L.) and soil C sequestration to N fertility and harvest frequency on switchgrass‐dominated CRP land in eastern South Dakota. Soil at the site is an Egan silty clay loam. Three N rates (0, 112, and 224 kg ha−1) were applied as NH4NO3 (NH4NO3–N) and cattle (Bos taurus L.) manure (manure‐N). Switchgrass was harvested at anthesis every year (EY) or alternate years (AY) from 2001 to 2004. Soil samples were collected before starting the experiment (fall 2000) and after 4 yr (fall 2004) to determine C sequestration. Averaged across N rate, the proportion of switchgrass was higher with manure‐N (64.7%) than NH4NO3‐N (46.8%). Total (switchgrass plus other herbaceous material) biomass production tended to be higher when harvested EY (average 5.0 Mg ha−1 yr−1) compared with AY (average 4.0 Mg ha−1 yr−1). However, by 2004, the proportion of switchgrass was 75% higher in plots harvested AY compared with those harvested EY. The concentration of structural components was greater in biomass harvested AY, whereas total N and ash tended to be lower. Total‐N and ash concentrations in biomass were higher with NH4NO3‐N than manure‐N. Soil C was sequestered at a rate of 2.4 ± 0.9 and 4.0 ± 1.0 Mg C ha−1 yr−1 at the 0‐ to 90‐cm depth with NH4NO3–N and manure‐N, respectively. There were no changes in soil organic C without N fertilization. Manure could be used as an alternate N source for switchgrass biomass production on CRP land with an added benefit of increased C sequestration.

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Abstract. Surface mine reclamation laws in Texas permit the use of mixed overburden as a topsoil substitute. Overburden material must have a positive acid/base account (ABa) to remain within 4 feet of the reclaimed surface. Acid/base account is calculated by subtracting potential acidity (PA) plus exchangeable acidity (EA) from the inherent acid neutralization potential (NP) of the material. Material with a negative ABa may be limed to increase the AB a value. Two separate research studies were conducted to determine 1) the effects of siderite (FeC03), a frequently identified mineral in mixed overburden, on NP determination and 2) the long-term pH stability of potentially acidic mixed overburden limed to an AB a of zero. Siderite stoichiometrically weathers to produce acidity upon complete oxidation and hydrolysis of released ferrous iron. Siderite contributes to NP because of incomplete iron hydrolysis under the conditions used to determine NP in the laboratory. This "siderite error" can lead to a false positive AB a value when the current NP procedures are used on samples containing significant quantities of PA and siderite. The long-term pH stability of potentially acidic mixed overburden limed with CaC03 was evaluated using a simulated weathering study. The liming rates ranged from Oto 125% of the theoretical amount of lime needed to neutralize the measured AB a deficit. The data showed the addition of CaC0:3 significantly slowed the oxidation of FeS2-however, the dissolution of the applied lime proceeded at a steady rate with each percolating water front. The dissolution of applied lime was measured to be significantly faster than the release of PA in all treatments, thus the ABa values of all the treatments were found to decrease with time. Treatments which received less than a 50% lime rate became highly acidic within one year. The results indicate that all treatments would eventually become highly acidic.

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