Abstract:
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.
