| More than any other Mexican state, Chiapas dedicates the most hectares to certified organic coffee production, providing a means of survival for indigenous Mayan peasants. Organic coffee producers are restricted from using agrochemicals, thus rely in part on nitrogen fixation to provide nitrogen (N) to their crop.; Chapter one assessed farmer understanding of soil fertility enhancement processes that organic coffee producers use as a basis for decision-making and experimentation. Such processes include knowledge of leaf litter decomposition, composting, soil biology, and N2-fixation. Results indicate that despite training attempts, farmers still possess knowledge gaps regarding phenomena that they cannot see.; In chapter two I established a collection of isolates from Inga sp. nodules found on organic coffee farms, used phenotypic and genotypic approaches to characterize isolates, and examined cross-inoculation patterns of Inga strains. Great diversity existed among Inga isolates both between and within farms.; In chapter three I evaluated the symbiotic effectiveness of selected rhizobia isolates as inoculants for Inga oerstediana under greenhouse and field conditions, and described morphological and histochemical characteristics of I. oerstediana nodules. Tested isolates appeared not to be fixing N2 150 days after inoculation with isolated bacteria, and inoculated treatment nodules lacked leghemoglobin and Nif H mRNA transcript, however contained infected bacteroids.; The fourth study used the 15N natural abundance technique to quantify nitrogen fixation in 5–7 year and 1–3 year old I. oerstediana intercropped with coffee. In older plots, Inga oerstediana derived between 64–85% of its N through fixation of atmospheric N, while in 1–3 year old trees only 20% was fixed. Older plot δ15N values varied greatly, however younger trees were within expected ranges, with legume δ15N values significantly lower than reference species. It was concluded that older plots are likely to recycle fixed 15N, and that younger plots offer a more accurate quantification. |
