Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/1996
Publication Date: N/A
Citation: Interpretive Summary: Plant growth requires the absorption of soil nutrients and water by roots. Roots are also important sources of food for soil microorganisms and small animals like insects and earthworms. Because roots are vital to plant growth and soil ecology, it is imperative that we understand how roots grow and function. For example, we know that alfalfa can provide nitrogen to grasses growing with it and that alfalfa builds up soil organic matter levels in the soil. But it is not clear whether these transfers of nitrogen and organic material come from decaying roots or from other tissues. Knowing this will help us produce new alfalfa varieties that can provide more nitrogen to neighboring plants, so farmers do not need to purchase as much fertilizer. Plant root systems are very difficult to study, especially under field conditions. We used the new technique of photographing roots with a miniature video camera to discover the patterns of alfalfa root production and death. After examining over six thousand roots, we found that more roots died in the upper part of the soil than in deeper layers. About one-half of the roots at 4 inches decomposed by the end of the first year of alfalfa growth, compared to only one-third of those at 16 inches. There were few differences between alfalfa types, except that one type had more roots at 8 inches than other alfalfas. This alfalfa had been developed for having many fine roots, so the results demonstrate that our selection method was successful. We estimate that over 50 lb N/acre was released from the death of fine roots in these alfalfas.
Technical Abstract: Fine root production and loss affect ecosystem functioning in part because of impacts on plant nutrient and water uptake and subsequent release of assimilated C and nutrients to other organisms. In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality. Our objective was to determine the extent and patterns of fine-diameter root production and loss during alfalfa establishment. Horizontally installed minirhizotrons were placed directly under the seeded rows at 10, 20, and 40 cm depths on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA. Four alfalfa germplasms (Agate; Ineffective Agate, which is a non-N2-fixing near isoline of Agate; a germplasm with few fibrous roots and strong taprooted traits; and a germplasm with many fibrous roots and a strongly branched root morphology) were seeded in four replicate blocks in early June 1994. Images collected biweekly throughout the initial growing season were processed using CMAP- Roots software. Root production was similar among germplasms, except that the fibrous rooted selection produced 29% more fine roots at 20 cm than other germplasms. About 7% of fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). There was a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. At all depths, fine roots produced in the 4 weeks before harvest had shorter median lifespans than early-season cohorts. Fine root turnover released an estimated 830 kg C/ha and 60 kg N/ha, with no differences due to root system architecture.