Soil Quality as An Indicator of Sustainable Biofuel Feedstock Production.

Sustainable production of cellulosic feedstocks for second-generation biofuels must not degrade soil, water or air resources. Critical functions such as (i) sustaining biological productivity, (ii) regulating and portioning soil water, (iii) storing and cycling nutrients, and (iv) filtering and buffering must be maintained or solving our energy problem with renewable fuels could simply create other soil degradation and water quality problems. Without addressing the all aspects of soil resource management, removing a potential feedstock such as corn stover could significantly decrease soil carbon or upset nutrient balance. Compared to no removal, harvesting an average of 7.1 Mg ha^-1 of stover near Ames, IA resulted in a 30% yield reduction in soybean grain yield (2.21 vs 3.20 Mg ha^-1) the next year. This was not expected, but occurred because stover harvest had occurred on a site where the soil-test P and K levels were rated as being low to very low. It is also essential to monitor soil carbon pools, especially the active fraction, when feedstocks are harvested since it is well documented that if soil carbon falls to below a critical root-zone level agronomic productivity and input use efficiencies decrease dramatically. Opportunities for using the Soil Management Assessment Framework (SMAF) for monitoring soil quality at feedstock harvest sites will also be discussed.