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, NH4-N, NO3-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 NH4-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, NO3-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.