Effects of carbon input quality and timing on soil microbe mediated processes

Compost application is viewed as an eco-friendly and sustainable method to improve agricultural soil fertility. However, composts vary in lability and recalcitrance, which drives energy supply for microbial biomass formation and microbial processes such as soil respiration, nitrogen mineralization, and enzyme production. To provide further insights into the issue, this study aimed to elucidate how microbial activities are affected by substrate quality, amendment volume, and frequency. We proposed three hypotheses: i) microbial biomass and microbial activities increase as soon as C substrate is amended compared to control soil; ii) microbe-mediated processes are affected by substrate lability as incubation progresses; iii) repeated amendment of smaller split C source sustains a more gradual increase of microbial activities as compared to a single input. To test these hypotheses, we incubated an agricultural soil at 25 °C, adding substrates of various assimilability (glucose, cellulose and lignin) once or in 2–3 split additions. Soil respiration was determined every 2–3 days throughout incubation, while microbial biomass (MBC, MBN), inorganic N (NO₃-N and NH₄-N) concentration, and potential enzyme activities were assayed at day 4, 32, 82, and 186 following substrate amendment. Regardless of C source amendments, soil respiration increased twofold in amended soils while microbial biomass C (MBC) was 1.5 times higher compared to control soil, indicating that microbial growth was C-limited in this agricultural soil. The association between high NO₃-N concentration and low microbial biomass at the end of the incubation, regardless of amended substrate, suggested increased microbial C turnover due to C exhaustion. The addition of cellulose mostly enhanced enzyme activities of β-1,4-glucosidase (BG), β-1,4-N-acetyl glucosaminidase (NAG) and tyrosine amino peptidase (TAP) while the additions of glucose and lignin either reduced or did not affect enzyme activities compared to control at respective measuring time. Based on the result, we supposed that the selective synthesis of different enzymes by microbial community was C-source lability dependent. In conclusion, substrate quality rather than substrate amendment volume and timing had greater impacts on soil microbial activities, and hence indirectly influenced soil fertility.