Long-term tillage, rotation and perennialization effects on particulate and aggregate soil organic matter

• We test particulate and aggregate organic matter on a Mollisol after 20 years of crop management.
• Perennials in rotation appear to increase particulate organic matter C and N.
• Aggregate formation and organic matter storage are reduced in an organic rotation.
• Belowground C inputs are positively related to particulate and aggregate C and N.

Perennialization and reducing tillage have increased soil organic matter (SOM) in both aggregate and particulate organic matter (POM) in short-term and small scale experiments, but there is a need for investigations into the long-term effects of agroecosystems on these dynamic pools of SOM. The objectives of this study were to investigate how management varying in crop rotation, tillage intensity and organic management from 1990 to 2013 has affected POM and aggregate C and N, and assess the relationship between these SOM fractions and biomass C inputs. We hypothesized that tillage, low biomass inputs, and annual crops in rotation would be associated with decreased POM and aggregate C and N. Soil from six systems from the Wisconsin Integrated Cropping Systems Trial (WICST) on a Phaeozem, or Mollisol, was sampled in 2013: Continuous Maize (Zea mays L.), Maize–Soybean [Glycine max (L.) Merr.], Organic Grain (including maize, soybean, and wheat [Triticum aestivum L.] sequentially seeded with oats [Avena sativa L.] and berseem clover [Trifolium alexandrinum L.]), Conventional Forage (three years alfalfa [Medicago sativa L.] followed by maize), Organic Forage (two years’ alfalfa with oats nurse crop followed by maize), and Pasture (rotationally grazed, seeded to a mixture of red clover [Trifolium pratense L.], timothy [Phleum pretense L.], smooth bromegrass [Bromu sinermis L.] and orchardgrass [Dactylis glomerata L.]). Among all systems at 0–25 cm depth, we found significantly greater concentrations of POM-C in the Pasture (4.4 g C kg−1 soil) and POM-N in Pasture (0.30 g N kg−1 soil) and Organic Forage (0.25 g N kg−1 soil). The Organic Grain system had lower concentrations of macroaggregates and lower stocks of C and N within macroaggregates. Across all systems, belowground biomass C input was significantly positively correlated with POM-C, POM-N, and aggregate C and N. The data supported our hypothesis in part, as results indicate that frequent cultivation in the form of tine weeding and rotary hoeing for weed control in Organic Grain rotation is likely disrupting formation of aggregates and storage of C and N therein. However, in systems that were chisel plowed every one to three years, high biomass C inputs maintain POM-C and POM-N and soil aggregation equivalent to the fully perennial system.