Soil Organic Carbon and Nitrogen Fractions and Sugar Beet Sucrose Yield in Furrow-Irrigated Agroecosystems

Soil organic matter (SOM) fractions were determined using extraction-, incubation-, and density-based fractionation techniques on samples collected from a range of furrow-irrigated sugar beet (Beta vulgaris L.) based rotations on the same soil series on farmers’ fields in Wyoming. We hypothesized that extending the period of time between sugar beet crops in rotations beyond the 2-yr sugar beet–barley (Hordeum vulgare L.) (SB-BA) rotation by adding perennial or annual legumes would lead to higher levels of surface-soil (0–15-cm) organic C and N. Four rotations were compared: SB-BA, sugar beet–dry bean (Phaseolus vulgaris L.) (SB-DB), sugar beet– barley–dry bean (SB-BA-DB), and sugar beet–sugar beet–alfalfa (Medicago sativa L.)–alfalfa (SB-SB-Alf-Alf). Soils under SB-BA and SB-DB rotations on average contained 607 g soil organic C (SOC) m−2 in the upper 15 cm, or 46% of the SOC found within SB-BA-DB and SB-SB-Alf-Alf soils. Potentially mineralizable C and N and microbial biomass C (MBC) were lower in SB-BA and SB-DB soils than SB-BA-DB and SB-SB-Alf-Alf soils, but, when normalized by SOC and total soil N (TSN), these labile C and N fractions were >1.5 times higher in SB-BA and SB-DB soils, suggesting greater SOM mineralization. Moreover, light-fraction C in SB-BA and SB-DB soils was about half that of SB-SB-Alf-Alf soils. Sugar beet sucrose yield was also higher in the SB-SB-Alf-Alf than any other rotation. There were strong linear relationships (r2 = 0.50–0.84) between sugar beet sucrose yield and TSN, SOC, and MBC across all four rotations. To conserve high surface-soil organic C and N fractions on furrow-irrigated farm fields without sacrificing sugar beet sucrose yield, extending the 2-yr SB-BA rotation by adding 2 yr of alfalfa is recommended.