Soil microbial functional diversity and root growth responses to soil amendments contribute to CO2 emission in rainfed cropland

Abstract

Carbon (C) losses induced by soil amendments have been shown to trigger soil conservation concerns. However, the mechanisms of fertilizer-induced CO2 emissions are still unclear. Here, we investigated the impact of soil nitrogen (N) amendments at an upland site on soil C stocks, CO2 emission, microbial functional diversity, and soil enzymatic activities during the stem elongation and harvest stages of rapeseed in 2016–2018. Five treatments were designed including non-N fertilizer (N0), conventional urea (U), monotypic CRU fertilization (CRU), co-application of U with CRU (CRC), and CRU plus organic fertilizer (CRO). The key finding is that the significant increases in CO2 emissions from CRO soils were ephemeral compared with the N0 treatment. Root biomass and root: shoot ratio partly explained the significant effect of N addition to CO2 production. Further, improved amylase activities and increasing consumptions of carboxylic acids, polymers and miscellaneous by soil microbes were mainly responsible for enhanced soil CO2 emissions in both growing seasons (P < 0.01). This suggests that fertilizer-induced CO2 emission could be regulated by soil C and N availability, which is attributed to the dynamic changes in soil carbon metabolism. During the stem elongation of 2016–2017, the consumption of carbohydrates, carboxylic acids, polymers, amines, and miscellaneous in CRO treatment during the first sampling phase was 21.65%, 41.36%, 17.98%, 27.61% and 49.52% higher than those in traditional urea fertilizer, respectively. Moreover, compared to the CRU, U, and N0 treatments, the CRO treatment significantly increased the McIntosh index during the stem elongation of both seasons. The results indicate that a sensitive and rapid indicator to finding a good soil N management is short-term alterations in the microbial functional diversity. Additionally, a broad spectrum of the utilization of substrates by soil bacteria was showed in CRO plots, irrespective of interannual variations of precipitation from 2016 to 2018. In contrast, non-N and conventional urea treatments produced a greater disparity in the utilization of carbon substrates from across all sampling times compared with CRU amended treatments, and both of them were less correlated with the most C-sources. Overall, single basal CRO fertilization, as a stable N source, can satisfy plant and soil microbial C and N demands, hence mitigating soil CO2 emission.