Short-term effects of selected organic fertilizer sources on carbon dioxide fluxes and soil quality

Biogas slurry (BGS) has potential as an organic nutrient source, and understanding its effect on carbon (C) exchange in soils is important for clarifying the contribution of organic amendments to soil fertility and the C budget globally. This study evaluated the effect of BGS, cattle manure (CM), and chemical fertilizer (CF) on carbon dioxide (CO2) fluxes, microbial biomass C, and beta-glucosidase activity during the 2016-2017 and 2017-2018 maize (Zea mays L.) seasons, covering December-May, in South Africa. The experiment was arranged in a randomized complete block design with the treatments as (a) BGS, (b) CM, and (c) CF, applied at 40, 80, and 120 kg nitrogen (N) ha(-1). In both seasons, BGS and CM had higher CO2 fluxes than CF. Relative to CM, BGS resulted in higher CO2 flux earlier in the season and lower flux later in the season. The highest CO2 flux was in February but decreased as the season progressed. When expressed as a percentage of organic C present, CO2 fluxes increased with N rate for CM, decreased for BGS, and changed minimally for CF. At maize harvest, the microbial biomass C and beta-glucosidase activity were higher in CM than BGS at all N levels tested. These findings imply that relative to CF, applications of BGS and CM increase CO2 fluxes, soil organic C, microbial biomass C, and beta-glucosidase activity, and the two organic resources also differ in their contributions to CO2 fluxes, soil organic C, microbial biomass C, and beta-glucosidase activity depending on the part of the season, at least in the short term.