Influence of Management and Rotational Crop Diversity on Soil Organic Carbon and Soil Structural Stability in Long-Term Integrated Nutrient Management Systems.

Understanding processes that ameliorate cropping system productivity and sustainability is particularly important in intensively managed row crop systems. Soil organic carbon (SOC) accumulation is known to be linked to cropping system productivity. However, SOC accrual and soil structural stabilization have rarely been determined along crop diversity gradients in integrated nutrient management systems. We investigated the role of nutrient management and rotation diversity through the manipulation of nutrient management and crop diversity in a 20 year study at Kellogg Biological Station, southwest Michigan.  Rotational diversity treatments included monoculture corn (C), Corn-soy biculture (CS), Corn-soy-wheat triculture (CSW), and a polyculture of corn-soy-wheat with a cover crop (CSWco) nested within Integrated Fertilizer (IF) and Integrated Compost (IC) management systems. We quantified SOC, Permanganate Oxidizable Carbon (POXC) and water stable aggregation at 3 different depths (0-5, 5-20 and 20-25 cm) to i) determine long term responses of the measures to nutrient management and rotational diversity, and ii) examine relationships between various soil measures to structural stability of fine loamy mixed, semi active, mesic Typic Hapludalf soils at the trial site.  Results indicated that SOC, POXC and water stable aggregate size fractions responded to long-term treatment differences. Over the two decades, management had stronger influence compared to diversity for most the measures examined, with IC management showing improved soil structural stability as evidenced by an increase (19%) in macroagregates (>2000 µm) for 0-5 cm depth (p<0.001). Overall, SOC and POXC were moderate predictors of aggregate stability (r² = 0.61, 0.58 respectively) across the plow depth (0-25 cm). Furthermore, biodiverse rotational systems had better aggregate stability compared to monoculture treatments across nutrient management systems (p<0.001).   The results indicate that improvements in soil structural stability are attainable under IC management in the long term, and that crop bio-diversification is an additional means of ameliorating soil structural stability.