Fine root dynamics of alfalfa after multiple cuttings and during a late invasion by weeds

The dynamics of fine root systems for alfalfa (Medicago sativa L.) during seasonal harvests are important for plant regrowth, yet they are not well understood. The primary objective of this study was to determine fine root responses to multiple cuttings of an alfalfa crop for a 5-yr period and during an invasion of companion weed plants during the last 2 yr of the study. Root development, distribution, and turnover rates were evaluated by minirhizotron (MR) and microvideo camera methods on a Kalamazoo loam (fine-loamy, mixed, mesic Typic Hapludalfs). The development and distribution of alfalfa fine roots were modified more by season and age of the plants than by defoliation. New root development occurred primarily in the spring and immediately following rainfall interruptions of extended dry periods. Spatial accumulations of roots were observed at the interface of the Bt and 2Bt soil horizons, which also accumulated water and nutrients. More roots died with the progression of each season, especially when plants were 3 to 5 yr of age. New root growth decreased and root death rates increased at soil depths from 0 to 1.1 m, during the period between cuttings from July and August 1993 in the aging alfalfa stand, whose active region of root growth exceeded the depths of the MR observation tubes. New alfalfa root growth appeared at progressively greater depths, especially after 3 yr of cropping. Therefore, mature alfalfa root systems should be sampled to soil depths greater than 1 m. Root systems of encroaching weeds appeared to accumulate in the 0- to 0.6-m region of the soil profile.