Particulate Soil Organic‐Matter Changes across a Grassland Cultivation Sequence

Many models have been constructed in an attempt to describe the dynamics of soil organic‐matter (SOM) turnover, most of which include 2 to 3 kinetically defined organic‐matter pools. Physical and chemical definition of these conceptualized SOM pools has been difficult. We describe a simple method for dispersion of soil to isolate a particulate organic‐matter (POM) fraction that may represent an important SOM pool in grassland soils. The POM fraction was isolated by dispersing the soil in 5 g L −1 hexametaphosphate and passing the dispersed soil samples through a 53‐µm sieve. We compared POM and mineral‐associated C among three tillage treatments (20 yr under cultivation) and an undisturbed grassland at Sidney, NE. The POM C in the native sod represented 39% of the total soil organic C. Twenty years of bare‐fallow, stubble‐mulch, and no‐till management reduced the C content in this fraction to 18, 19, and 25%, respectively, of the total organic C. The mineral‐associated organic‐matter fraction showed no reduction in C content in the bare‐fallow treatment compared with the grassland soil but increased in the no‐till and stubble‐mulch treatments. Nitrogen dynamics generally mirrored those observed for C. Analysis of the POM fraction for lignin and cellulose content indicated that this fraction was 47% lignin and had a lignocellulose index of 0.7. The stable C‐isotope composition of the POM fraction suggests that wheat‐derived POM turns over faster than grass‐derived POM. We suggest the POM fraction closely matches the characteristics of a SOM pool variously described as slow, decomposable, or stabilized organic matter.