Long-term alternative dairy manure management approaches enhance microbial biomass and activity in perennial forage grass

Removing solids from liquid dairy manure slurry reduces manure phosphorus (P) and increases the available (mineral) fraction of nitrogen (N) but also decreases the organic matter content of the manure. While this novel treatment reduces environmental concerns associated with excess N and P application to soils, it may also reduce microbial biomass and activity in soil. This study evaluated the long-term effects of this alternative manure treatment compared to more typical nutrient applications in a perennial grass sward (tall fescue, Festuca arundinacea Schreb.) on soil microbial biomass, community composition, hydrolytic enzyme activity, and forage yield. Nutrient treatments for this long-term field experiment in Agassiz, British Columbia, Canada were started in 2003. The treatments included liquid dairy manure slurry, liquid dairy manure with solids removed, commercial fertilizer, a combination of fertilizer and dairy manure, and a control. All treatments were applied at 400–600 kg total N ha^?1 year^?1 in four equal doses. Soil microbial community composition (phospholipid fatty acid analysis) and activity (hydrolytic enzyme activity) were determined several times during the 2013 and 2014 growing seasons to a depth of 15 cm. Time of sampling (date) had a strong influence on microbial biomass, community composition, and activity, while the response to soil properties and yield was more varied. All manure treatments (dairy manure slurry, liquid fraction, and the combination) increased microbial biomass (by 19–32%) and the potential activity of cellulose-degrading enzymes (by 31–47%) compared to commercial fertilizer and unamended plots. The commercial fertilizer and liquid fraction lowered fungal/bacterial ratios compared with both whole manure and unamended plots. Our results indicate that separating the solid from the liquid fraction of manure, to improve crop yield and reduce P loading, did not reduce microbial community size and activity and that all manure treatments increased microbial biomass and activity compared to mineral fertilizer application.