Estimating Long-Term Compost Application Effects on Active Soil Carbon with 24-Hour Basal Respiration.

Labile soil carbon is the fraction of organic matter that is readily degradable (<5 years) and is therefore available as an energy source for soil microbes. Given that microbes drive soil nutrient mineralization, a measurement of active carbon could help predict the mineralization potential of soils and be used to inform fertilizer applications.  Twenty-four hour basal respiration has been correlated with N and P mineralization and this method for estimating active carbon has the potential to be adopted on a large scale in commercial soil labs.  The objectives of this study were to (1) investigate effects of long-term compost application on labile soil carbon at 0-10cm and 0-30 cm and (2) examine the effects of soil storage on 24-hour basal respiration. We measured 24-hour basal respiration from 3 field replicated treatments within the Long-term Organic Farming Systems Experiment at WSU Puyallup:  1) On-Farm Compost (15:1, C:N applied annually at 30 Mg ha^-1 for 8 years); 2) Chicken Manure Compost (11:1, C:N, applied annually at 5 Mg ha^-1 for 8 years; and 3) Low-Input Pasture (On-Farm Compost applied annually 2003-2005, currently rotated with pasture for 2 of every 3 years).    For the on-farm and chicken compost treatments, 24-hour basal respiration produced a mean of 143 mg CO₂ kg soil^-1 wk^-1.  Microbial biomass, measured via substrate-induced respiration, was significantly higher in the On-Farm compost treatment.  Assessing the sensitivity of 24-hr basal respiration to long-term compost treatments and the effect of soil storage on this method could affect how it is adopted in research and on a larger scale.