Aggregate Carbon and Nitrogen under Reestablished Grasses and Shrubs.

Minimal research has been conducted on aggregate size distribution and associated C and N concentrations in coarse textured soils used in mine land reclamation. Two chronosequences of reclaimed sites with sandy loam soils were sampled (<1, 5, 10 and 16 year old reclamation and an undisturbed native site) under shrub and grass dominated communities on a surface coal mine located in the Powder River Basin, WY. For each soil, aggregate size distribution, aggregate fractions and associated C and N concentrations were measured. We hypothesize greater aggregation, as well as C and N concentrations with time under grass compared to shrub communities because of the influence of very fine and extensive root systems associated with grasses. No definitive trends of increasing macroaggregates with reclamation age were observed for the shrub sites; however, macroaggregation was greater 16 years after reclamation under grasses than that observed in the native soil. Macroaggregate associated C and N concentrations were 65% lower and microaggregate concentrations 40% lower in the <1 year old reclaimed soils compared to the native. Soils under shrubs and grasses both showed increases in C and N concentrations approaching concentrations observed in the native soil. Rates of C accumulation within soil aggregates were higher under shrub compared to grass site soils, while rates of N accumulation were higher under grass relative to shrub site soils. Reclaimed shrub site soils had consistently higher C concentrations at every site age, even without aggregate recovery, than soils under grass communities. This was likely a result of other forms of OM protection, such as biochemical recalcitrance or chemical stabilization rather than by physical protection within soil aggregates.