Soil Carbon Fractions in Response to Long‐Term Crop Rotations in the Loess Plateau of China

Core Ideas Long‐term (30 yr) diversified crop rotations had a positive impact on soil C fractions. Carbon fractions increased with increased rotation length. Diversified rotations enhanced soil C sequestration and microbial biomass and activity. Diversified crop rotations may enhance C fractions and soil quality by affecting the quality and quantity of crop residue returned to the soil compared with monocropping and fallow. We evaluated the effect of 30‐yr‐old diversified crop rotations on soil C fractions at 0‐ to 15‐, 15‐ to 30‐, and 0‐ to 30‐cm depths in the Loess Plateau of China. Crop rotations were continuous winter wheat (Triticum aestivum L.) (W); 3‐yr rotations of corn (Zea mays L.)–winter wheat–winter wheat–millet (Eleusine coracana L.) (CWWM), pea (Pisum sativum L.)–winter wheat–winter wheat–millet (PWWM), and sainfoin (Onobrychis viciifolia Scop.)–winter wheat–winter wheat–sainfoin (SWWS); 4‐yr rotation of pea–winter wheat–winter wheat–corn (PWWC); and 8‐yr rotation of alfalfa (Medicago sativa L.) (4 yr)–potato (Solanum tuberosum L.) (1 yr)–winter wheat (3 yr) (A4PoW3). A fallow (F) treatment was also included for comparison. Carbon fractions were soil organic C (SOC), particulate organic C (POC), potential C mineralization (PCM), and microbial biomass C (MBC). At all depths, SOC and POC were greater in A4PoW3 than F, W, and PWWC. The PCM and MBC were greater with SWWS than most other crops rotations. Soil C fractions increased linearly with increased belowground crop residue returned to the soil. Both SOC and POC increased linearly with increased rotation length. All C fractions were also correlated with mean annualized crop grain yield. Diversified crop rotations with increased belowground residue production enhanced soil C sequestration and microbial biomass and activity as well as crop yield compared with monocropping.