Effects of Long Term Nitrogen Application On Arbuscular Mycorrhizal Fungi Abundance, Soil Aggregate Stability, and Soil Carbon Storage in a Continuous Maize, Tilled System.

With an increase in grain market prices, continuous maize systems have been a growing trend, making elevated nitrogen applications and tillage common practice.  Past research has shown that arbuscular mycorrhizal fungi (AMF) tend to be negatively correlated with nutrient availability, making the increases in nitrogen application a concern since AMF are mutualistic in relation to plants.  In addition, the byproduct of AMF and their hyphal network can aid in the stability of soil aggregates.  With increased aggregate stability, turnover rate of macroaggregates (>250 µm) is reduced and soil carbon storage can be enhanced through the production of stable microaggregates (53-250 µm).  Microaggregates, and their associated carbon, are highly resistant to natural and human induced disturbances, such as tillage.  This study investigated the long term effects of nitrogen application on AMF abundance, soil aggregation, and in turn soil carbon storage in a continuous maize, tilled system.  Experimental design included four nitrogen application rates (60, 120, 180, and 240 lbs N ac ^-1) and a control, applied to maize in a randomized complete block design in Boone Co., IA.   AMF abundance was measured as percent fine root colonization and was positively correlated with nitrogen application, increasing by 2-3% with each additional 60 lbs of nitrogen.  To observe possible influences of AMF abundance on soil properties, soil was wet sieved to fractionate stable aggregate classes.  Large macroaggregates (>2000 µm) showed similar trends as AMF abundance, and increased linearly with nitrogen application rates.  Increased stability of large macraggregates may reduce aggregate turnover and promote the formation of stable microaggregates.  Microaggregate formation is necessary for long term carbon storage, and therefore, AMF may indirectly sequester carbon.