"The government of Zimbabwe no longer provides a subsidy for fertilizer, resulting in significantly lower corn yields" says Besmer, a doctoral candidate in ecology. "The old-fashion use of legumes (members of the bean family) in crop rotations may prove to be a solution to this problem because of their ability to fix nitrogen and, thus, provide nitrogen for subsequently grown corn. We have shown in Zimbabwe, however, that legume growth and nitrogen fixation can be limited by the availability of phosphorus in the soil."
In order to improve nitrogen fixation in legumes, somehow phosphorus availability has to be increased.
"Mycorrhizal fungi are common in nature. They colonize the roots of many plant species including legumes. These fungi live symbiotically with their hosts, absorbing phosphorus from the soil, and transporting it to the root systems. In preliminary tests we have shown that enhanced mycorrhizal colonization of a number of legumes grown in soil from Zimbabwe increases nitrogen content indirectly by increasing phosphorus uptake," she told attendees today (Aug. 7) at the annual meeting of the Ecological Society of America in Tucson, Ariz.
"We first used peanut because it is a legume that is commonly grown by subsistence farmers. Peanut growth and nitrogen content was strongly limited by phosphorus availability, and by amending the soil with mycorrhizal fungi, peanut nitrogen content was significantly increased," reported Besmer.
The Penn State researcher is working with Roger Koide, professor of horticultural ecology, and Robert Myers, soil scientist with the International Crops Research Institute for the Semi-arid Tropics (ICRISAT), to find ways to increase the abundance of the beneficial mycorrhizal fungi in Zimbabwean soils.
"In temperate agro-ecosystems, mycorrhizal fungal abundance can be increased by reducing fallow periods and tillage," explained Besmer. "We want to take these lessons learned from temperate systems and try to apply them appropriately to the semi-arid tropics to increase mycorrhizal fungal activity.
"While peanut was a logical crop to study, it may not be the best legume to use in Zimbabwe to enhance soil fertility for corn production because most of the nitrogen resides in the nut, which is harvested and removed from the soil. Another legume, commonly called lablab, looks promising because it grows more vigorously and its stems and leaves contain more nitrogen," he adds. "The best way to use any legume to increase soil fertility is to plow most of the plant back into the soil or let animals graze on the plants and allow their manure naturally to fertilize the field. For subsistence farmers in the semi-arid tropics, the proper selection of legumes coupled with simple practices to increase the abundance of naturally occurring mycorrhizal fungi could sustain or increase corn yields without fertilizers."
The National Geographic Society, Penn State and ICRISAT support the research of Ylva Besmer.