Soil Aggregation and Carbon and Nitrogen Pools under Rhizoma Peanut and Perennial Weeds

Roots of rhizoma peanut [( Arachis glabrata Benth.), a warm‐season perennial legume forage] and perennial weeds may influence soil aggregation and associated C and N pools. We compared aggregate‐size distribution and concentrations of organic C and N, NH 4 –N, NO 3 –N, potential C and N mineralization (PCM and PNM), microbial biomass C and N (MBC and MBN), and particulate organic C and N (POC and PON) in whole‐soil and aggregates under 10‐yr‐old rhizoma peanut and perennial weeds [dominated by henbit ( Lamium amplexicaule L.) and cut‐leaf evening primrose ( Oenothera laciniata Hill)]. Field plots were established on a Norfolk loamy fine sand in April 1991 in central Georgia. While soil aggregation and associated C and N pools were not influenced by treatments, whole‐soil NH 4 –N and PON at the 0‐ to 15‐cm, PCM at the 30‐ to 90‐cm, and MBC at the 0‐ to 30‐cm depth were 28 to 100% greater under peanut than under weeds. Under both treatments, the amount of soil present in the 4.75‐ to 2.00‐mm aggregate‐size class and mean‐weight diameter of aggregates were 21 to 47% greater at 15‐ to 60‐ than at 0‐ to 15‐cm, indicating improved aggregation in the subsurface compared with the surface soil. At 0 to 15 cm, concentrations of organic C and N, MBC, MBN, POC, and PON were greater in the <0.25‐mm than in the 4.75‐ to 0.50‐mm size class. At 0 to 30 cm, NO 3 –N was greater in the 4.75‐ to 0.85‐mm than in the 0.50‐ to 0.25‐mm size class. Although treatments did not influence soil organic C and N levels, rhizoma peanut may increase microbial activities and labile N pools compared with perennial weeds. Rhizoma peanut and perennial weeds may improve aggregation in the subsurface compared with the surface soil and C and N pools in aggregates <0.25 mm in diameter.