Smith, L. C. 2017. Nitrogen conservation in perennial grasses managed for bioenergy production. Dissertation, University of Wisconsin-Madison, Madison WI.

Reducing nitrogen (N) pollution from agriculturer emains a major challenge. Using crops that are N-conservative and retains N in the plant-soil system is one method to help mitigate N that would otherwise be lost to the environment. Native warm-season grasses are inherently N-use efficient and could also be a source of cellulosic biofuel feedstocks for renewable energy. Management for perennial bioenergy crops often recommends N fertilizer to increase and maintain yields. However, N addition may actually compromise some of the biological mechanisms that help conserve N in the plant-soil system. I investigated some of the mechanisms that conserve N resources in perennial grasses, with special attention to the effect of N addition on N conservation mechanisms and microbial associations, given the current N recommendations for perennial bioenergy grasses and the lack of regulation and policies associated with N management.My objectives were 1) to understand the genetic and environmental controls and variation in the perennial plant N conservation strategies of N resorption (internal plant N recycling) and nitrogen-use efficiency, 2) assess the importance of the arbuscular-mycorrhizal fungal (AMF) associations in perennial grass yield and N uptake under varying soil N conditions by measuring AMF abundance and function and 3) quantify the amount of AMF-supplied N to plants under varying soil N conditions using 15N natural abundance techniques.Results showed that the N conservation mechanisms of N resorption and nitrogen-use efficiency were highly variable and the dominant strategy used by plants varied by species, ecotype, and site. N addition had no effect on the plant N conservation strategies measured, but soil nutrient ratios affected plant N conservation across eight restored grassland sites. N addition significantly decreased AMF abundance and function (i.e.nutrient transfer with host plant) in vii perennial grass systems, and in addition, plant N correlated with increased AMF allocation to nutrient transfer structures within host roots. Demonstrating these relationships in situ provides important evidence that AMF benefit perennial grasses by increasing N uptake. Differentiation of δ15N among plant, soil N and AMF fungal pools was higher than anticipated, leading to estimates of 34 to 100% of plant N transferred from AMF in the treatments receiving no N addition and a significant reduction in plant N transferred in high N addition treatments. Our results suggest that N addition decreases AMF N-transfer to plants. When N is limited, AMF are able to supply N to plants in amounts comparable to recommended N fertilizer rates, highlighting that N fertilizer may be unnecessary in the management of perennial grasses for bioenergy production. A high degree of variation in N conservation parameters also suggested high potential for selection and management of C4 grasses for continued and improved N conservation.

Associated Treatment Areas:

Switchgrass Nitrogen/Harvest Experiment - GLBRC

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