Particle-size Fractions and their Use in Studies of Soil Organic Matter: II. Cultivation Effects on Organic Matter Composition in Size Fractions

Losses of carbon, nitrogen, and phosphorus from soil particle-size fractions during cultivation were examined and their implications on the nutrient-supplying potential of soils is discussed. Particle-size fractions were obtained by ultrasonic dispersion in water from three north American grassland soils (Cryoborolls) of different textures. Soils that had been cultivated under small grain-fallow rotations for between 4 and 90 years were compared with similar soils under native prairie. Four years of cultivation of a Blaine Lake silt loam resulted in an extensive depletion of organic matter associated with particles > 50 µm (43% of the initial C lost). This was considered a result of physical disintegration which led to accumulation of these materials in finer particle-size fractions. With continued cultivation this accumulated material was slowly transformed within the system, accompanied by net losses of 34% C and 29% N from the total soil over 60 years. Reductions in organic P content amounted to 20% and were accompanied by slight increases in inorganic P content. Fine clay (< 0.2 µm) associated organic matter was rapidly depleted during the first 60 years but showed little change thereafter, reaching a new equilibrium level at approximately one-half its original C content. Losses of fine silt (5 to 2 µm) and coarse clay (2 to 0.2 µm) associated organic materials were substantially less, and the proportion of total soil organic matter in these forms increased with time of cultivation. The relative abundance of these biologically resistant, more humified materials in cultivated soils indicated that the remaining organic matter may have a reduced capability for supplying nutrients by mineralization. The continuing, slow mineralization of these more resistant organic matter fractions of the Blaine Lake soil after 90 years of cultivation suggests that organic matter losses may not level off further in the near future. In addition, losses of organic matter from the B horizon were evident after 60 and 90 years. Organic matter losses during 65 years cultivation of a coarser sandy loam soil showed a similar distribution and magnitude across size fractions, while a heavy clay soil was characterized by a greater stability of the silt and clay associated organic matter, which lost only 10% C during 70 years of cultivation.