Effects of 15 years of manure and mineral fertilizers on enzyme activities in particle-size fractions in a North China Plain soil

Soil organic matter (SOM) and enzymes are essential for nutrient cycling, and are considered as important indicators of soil quality. The effects of organic and mineral fertilization on soil organic carbon (SOC), total nitrogen (TN) and enzyme activities in bulk soil and particle-size fractions were investigated under a winter wheat/maize cropping system in the North China Plain. The experiment established in 1993 includes three treatments: (1) unfertilized control (CK); (2) mineral fertilizers (MF); and (3) farmyard manure (FYM). Application of FYM significantly increased SOC and TN contents and activities of six enzymes: invertase, β-glucosidase, urease, acid and alkaline phosphatases and dehydrogenase in bulk soil and in all particle-size fractions as compared to those in MF and CK. Highest contents of SOC and TN were found in coarse sand and lowest in the silt fraction. The C/N ratios decreased with decreasing particle-size fractions. β-Glucosidase and acid phosphatase activities predominated in coarse sand fraction, reflecting high substrate availability. The urease activity was highest in clay-size fractions, depending on mineral sorption processes. The SOM and enzyme activities in the coarse sand were the most sensitive to fertilization. The smallest response of SOM in the clay fraction to fertilization confirmed that SOM on clay is the most stable C pool. The 15-year fertilization experiment clearly showed that FYM represented the best management practice for improving soil quality and microbial activity.