Development of sustainable, native grass-based bioenergy production systems in the prairie region of Minnesota: Soil nutrient response to fertilizer and harvest treatments

Background/Question/Methods

Conversion of cropland for the production of bioenergy feedstocks is a promising scenario for the upper Midwest as economic and social interests in bioenergy and low-carbon fuels grow.  Landowners are in the forefront of developing the necessary whole-farm management systems.  Progressive multipurpose land use approaches can balance the economic and environmental risk of bioenergy production by building on ecosystem services. Fitting with “Productive Conservation on Working Lands,” and “Chippewa10%” initiatives, we established a 10-species, native grass-forb mix, suitable for biomass production and grazing, in an on-farm 4-acre (1.6 ha), randomized split-plot block experiment in 2008, and monitored soil nutrients. We applied four fertilizer treatments (zero, composted cattle manure, half rate (30-10-30 NPK) and full rate (60-20-60 NPK)) to 16 whole plots, with first fertilization occurring in June of 2011.  Split plots were harvested in fall of 2010, 2011 and 2012. We measured soil nutrients, such as total and organic C, total N, and mineral N (NO₃ and NH₄), in 10 cm increments from 0 to 30 cm depth and in the 30 to 60 cm depth in fall-collected samples in 2010 and 2012 across the experimental area.

Results/Conclusions

Total organic C and N decreased with soil depth but were similar between 2010 and 2012, except for an increase at the 30-60 cm depth, with harvest and fertilization.  Mineral NH₄-N also decreased with depth but was lower in 2012 than 2010, with no influence from harvest or fertility treatments.  Mineral NO₃-N was similar among depths and between years, except for a substantial increase in the 0-10 cm depth in 2012, which was due to fertilization but no-harvest of treatment plots.  However, parsed out for all treatment combinations, NO₃-N did increase with increasing fertilizer at lower soil depths, particularly at 20-30 cm depth for harvested treatments. Many more years may be necessary to observe management impacts on soil organic matter content. Observable changes were primarily in measures of soluble mineral N, easily related to application of inorganic fertilizers. Dynamics of NO₃-N at depth are likely linked with the influence of fertility and harvest treatments on plant root dynamics.  Continued evaluation of plant and soil dynamics will aid development of bioenergy production systems that are beneficial for landowners and the environment.