Will nutrient-efficient genotypes mine the soil? Effects of genetic differences in root architecture in common bean (Phaseolus vulgaris L.) on soil phosphorus depletion in a low-input agro-ecosystem in Central America

Crop genotypes with root traits permitting increased nutrient acquisition would increase yields in low fertility soils but have uncertain effects on soil fertility in the long term because of competing effects on nutrient removal vs. the soil conserving effects of greater crop biomass. This study evaluated the relative importance of phosphorus loss in crop extraction vs. phosphorus loss in soil erosion as influenced by genetic differences in root shallowness and therefore phosphorus uptake in common bean (Phaseolus vulgaris L.). Six recombinant inbred lines of varying root architecture and two commercial genotypes of bean were grown in unfertilized, steeply sloped (32%), low phosphorus (5.8 mg kg^-1, Fe-strip) Udults in Costa Rica. Fertilized (60 kg total phosphorus ha^-1) plots of commercial genotypes were also included in the study. Runoff was monitored throughout the bean growing season in 2005 and 2006, and in 2006, monitoring continued through the maize growing season. Phosphorus removed in plant biomass at harvest through the 2006 bean-maize crop cycle averaged 7.3 kg ha^-1 year^-1, greatly exceeding phosphorus loss due to erosion (0.15-0.53 kg ha^-1 year^-1) in unfertilized plots. In fertilized bean plots, total biomass phosphorus averaged 6.32 kg ha^-1 year^-1 and total eroded phosphorus averaged 0.038 kg ha^-1 year^-1, indicating rapid sorption of fertilizer phosphorus. Shoot growth of several recombinant inbred lines under low phosphorus was comparable to that of fertilized commercial genotypes, illustrating the effectiveness of selection for root traits for improving plant growth in low-phosphorus soils. Genotypic differences in root architecture of recombinant inbred lines led to 20-50% variation in groundcover by shoots, which was associated with 50-80% reduction in sediment loss. This study demonstrates that root architecture traits can affect nutrient cycling at the agro-ecosystem level, and that integrated nutrient management strategies are necessary to avoid soil nutrient depletion.