The effect of nitrogen fertilizer on the soil community of biofuel switchgrass
Panicum virgatum (switchgrass) is a desirable biofuel crop because of low irrigation and fertilizer requirements—less energy input leads to higher net energy gain. The soil community supported by switchgrass systems may play a major role in low fertilizer requirements—especially the amount of mycorrhizal symbiosis. Any feedbacks between fertilizer additions and changes in the soil community have not been documented. These changes may cause the fertilizer needs of a switchgrass field to increase over the projected lifetime of the field (10-20 years). An established switchgrass research field in Freehold, New Jersey was sampled in 2013 and 2014 to test whether nitrogen fertilizer additions (100 lbs/year) affected unmanipulated soil communities. A greenhouse experiment complemented the field samples by directly testing the impact of soil community members on switchgrass growth at three levels of nitrogen fertilization (0 lbs, 50 lbs, and 100 lbs). Switchgrass plants were grown in sterilized soil from the field site with additions of fungal-feeding nematodes, mycorrhizal inoculum, and nitrogen fertilizer. Yields of above- and below-ground biomass were measured to identify impacts on growth due to soil additions. Mycorrhizal communities were also sampled to identify changes due to nematode feeding preferences.
Factors within the soil community from the field-collected data showed significant differences between fertilized and unfertilized plots. The soil arthropod groups that differed were Sminthuridae collembola, Hypogastruridae collembola and large predatory mesostigmatic mites. Mycorrhizal colonization and mycorrhizal structures differed between fertilized and unfertilized plots. Greenhouse results confirm that the soil community does impact biomass yields. Pots with a single addition (either nematodes or mycorrhizal inoculum) produced the least amount of biomass and pots with both mycorrhizae and mycophagous nematodes produced the highest amount of biomass. These results will help inform best management practices for sustainable growth of switchgrass and help ensure crop (and bioenergy) sustainability.