Rotational and cover crop determinants of soil structural stability and carbon in a potato system
Understanding processes that ameliorate structural degradation in sandy soils is particularly important in intensively managed potato (Solanum tuberosum L.) systems. Seven 2-yr potato rotation systems were evaluated over 3 yr in an irrigated field trial comparing winter management systems bare (B) and cover crops: rye (Secale cereale L.; R), rye-hairy vetch (Vicia villosa Roth; RV) mixture and red clover (Trifolium pratense L.; C). Crops rotated with potatoes (P) were snap bean (Phaseolus vulgaris (L.); SB), wheat (Triticum aestivum L.; W) and sweet corn (Zea mays L.; SC). The systems consisted of: S1 PBSBB; S2 PRSBR; S3 PRSCB; S4 PWWR; S5 PWWCC; S6 PRVSBRV; and S7 PRVSCRV, both entry points evaluated each year. Carbon inputs above- and belowground were measured and systems grouped as low (S1 and S4), medium (S2 and S6), and high (S5, S3 and S7): 1.2, 2.0, and 2.8 Mg C ha–1, respectively. Response variables included water stable aggregate (WSA) size fractions, macroaggregates (0.25 mm) and microaggregates (<0.25 mm), mean weight diameter (MWD), soil C, nitrogen mineralization potential (NMP), and potato tuber yield. Systems with SC contributed twofold higher biomass than rotations with W or SB, and the presence of RC contributed higher amounts of carbon (1.2 Mg ha–1) compared to R (0.7 Mg ha–1). Only the entry year influenced macroaggregates in 2001; both entry year and cropping system influenced aggregate size classes in 2004. Over 3 yr the macro-WSAs declined by 13%, except for high carbon input systems. Residue C input was a moderate predictor of total soil C (31% of variability explained), whereas macro- and micro-WSAs were predictors of soil C, accounting for 58 and 72% of observed variability, respectively. Low levels of aggregation were observed in this sandy soil and the modest amounts of C inputs from winter cover crops posed a challenge to detecting treatment effects, which was in part overcome by georeferencing, to improve precision of sampling over time.