Linking soil microbial biomass and enzyme activities to long-term manure applications and their nonlinear legacy

Long-term manure application can build soil carbon and nutrient stocks and enhance enzyme activity, but its legacy on enzyme activity after discontinued application is unclear. We quantified soil microbial biomass carbon (MBC) and potential enzyme activities in fall, spring and summer in response to long-term manure application and its discontinuation under irrigated and rain-fed conditions. Relative to the non-amended control, 43 years of annual manure applications increased MBC by 38% and 33%, and beta-glucosidase, beta-N- acetyl-glucosaminidase (NAGase) and acid phosphomonoesterase activities by 35% and 33%, 137% and 135%, and 67% and 90%, in bulk and soybean rhizosphere soil, respectively, while arylsulphatase activity only increased in soybean rhizosphere by 13%. Fertilizer N had no effects on MBC or most enzyme activities, which were lowest in fall. In bulk and soybean rhizosphere soil, irrigation increased MBC, and NAGase and arylsulphatase activities by 17-33%, 19-54% and 39-57%, respectively, compared to rain-fed conditions, but beta-glucosidase and acid phosphomonoesterase activities increased by 25 and 12%, respectively, only in soybean rhizosphere. Legacy effects of manure applied 13 years previously were observed for MBC and all enzyme activities in bulk and soybean rhizosphere soil except MBC in bulk soil and acid phosphomonoesterase in soybean rhizosphere. Legacy effects of manure applied 29 years previously were observed for NAGase and arylsulphatase activities in bulk soil, and NAGase in soybean rhizosphere. Manure legacy effects on enzyme activities decreased with the number of years without manure, following quadratic patterns, and the effects lasted longer than those on MBC.