Location: Soil and Water Conservation Research
Title: Diazotroph abundance and diversity in wheat-fallow and wheat-pea crop rotation Authors
Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 28, 2013
Publication Date: N/A
Interpretive Summary: Free living diazotrophs (nitrogen-fixers) contribute to the plant available nitrogen (N) pool by fixing dinitrogen into ammonia. Biological input of N from the atmosphere either through free-living diazotrophs or legume-associated rhizobia can help alleviate fertilizer use in agricultural systems. In this study, we investigated the effect of N fertilizer and winter pea crop on the diversity and abundance of free-living diazotrophs in a dryland winter wheat no-till crop production system near Pendleton, Oregon, USA. The two crop rotations studied were wheat-fallow and pea-wheat in which N fertilizer was banded during wheat planting at either 0 or 180 kg N ha-1 (160 lbs N ac-1). Soil was collected from the top 0-5 cm depth of plots during the summer of crop year 2011 (CY11) and CY12. Nitrogen-fixers and total bacteria were quantified from DNA extracts from the soil using primers specific for the nifH (nitrogenase) and 16S rRNA (ribosomal DNA) genes, respectively. Sample year and crop were significant factors in the abundance of nitrogen-fixers. The population of nitrogen-fixers was greater in CY12 than CY11 and correlated well to an overall increase in bacteria. In CY11, the nifH abundance was correlated to an increase in ammonium concentration whereas bacteria were negatively related to nitrate concentration in the soil. The abundance of nitrogen-fixers was greater in the pea phase of the rotation compared to wheat but only in CY12. In neither year did the nitrogen-fixers respond to fertilizer application nor changes in total nitrogen, total organic carbon or soil acidity. The community diversity of the nitrogen-fixers also responded differently between the two years with respect to crop and N fertilizer. In the first crop year, fertilizer application appeared to be the primary factor in which the communities sampled from either previously fertilized plots of fallow and pea or unfertilized plots of wheat and fallow were most closely related. In CY12, both fertilizer application and crop appeared to influence diversity in which the communities sampled from previously fertilized fallow and pea were most closely similar as were both fertilized and unfertilized wheat plots. In summary, the factors influencing the diversity and abundance of the diazotrophs varied from year-to-year and were related to either soil chemistry or microbial biomass. The year-to-year variability detected in this study indicates that multi-year studies are necessary to identify the ecological drivers of nitrogen-fixing communities.
Technical Abstract: Biological input of nitrogen (N) from the atmosphere either through free-living diazotrophs or legume-associated rhizobia can help alleviate fertilizer use in agricultural systems. In this study, we investigated the effect of N fertilizer and winter pea (Pisum sativum L.) crop on the diversity and abundance of free-living diazotrophs in a two year study of dryland winter wheat (Triticum aestivum L.) no-till production system in Eastern Oregon, USA. Based on quantification of the nifH gene, diazotroph abundance was strongly influenced by plant species and crop year when soil samples were collected. A greater amount of nifH copies was recovered in 2012 compared to 2011 either as copies per gram soil or normalized to the abundance of bacterial 16S rRNA genes. The quantity of genes was greater under pea than wheat in 2012 although no difference was observed in the preceding year. The nifH gene abundance was positively correlated to ammonium concentration in the 2011 and bacterial abundance in 2012. Nitrogen application did not influence diazotroph abundance in the top 0-5 cm; however the abundance was reduced by application at the lower 5-10 cm depths under wheat crop. Diazotroph diversity appeared to be influenced more by N fertilization rather than plant species with the exception of wheat in 2012. Collectively, these data suggest that year to year variability had a greater influence on diazotroph communities rather than specific parameters of plant species, fertilization, total N, total organic C, or soil pH. Multi-year studies are necessary to better define the specific drivers of diazotroph abundance, community diversity and function.