Regulation of soil organic carbon sequestration by microbial community under long-term no-tillage and nitrogen application rates

A primary challenge of our era lies in achieving soil organic carbon (SOC) sequestration to provide a diverse array of benefits to the natural environment. Conservation tillage practices have gained significant global recognition to tackle this issue, owing to their impact on the physical-biological characteristics of SOC. However, the soil microbial mechanism of nitrogen (N) fertilizer effect on SOC sequestration in bulk soil under conservation tillage is still unclear. Moreover, the regulation of microorganisms within aggregates on aggregate stability and SOC sequestration remains elusive. The soil aggregate pore properties are also seldom taken into account to regulate carbon sequestration potential by changing soil microbial properties.
In this study, we used a long-term field experiment located at the Dryland Farming Experimental Station in Shouyang, Shanxi Province, in northern China to analyze the soil microbial properties (e.g. microbial community structure, microbial diversity, microbial biomass, microbial interactions, keystone taxa). We also assessed how these soil microbial properties in bulk soil influence SOC sequestration, especially revealing the mechanism of how soil microbial within aggregates affect SOC sequestration by various strategies. To better understand the SOC sequestration potential, we further assessed the effects of pore structure within aggregates on microbial interactions and keystone taxa. We used a 16-year field experiment with no-tillage (NT) and conventional tillage (CT), both of which combined with 105 (N1), 180 (N2), and 210 kg N ha-1 (N3) N application. The main results of this thesis are as follows:
(1) Soil microbial properties (e.g. microbial community structure, microbial diversity, microbial biomass, microbial interactions, keystone taxa) were significantly influenced by tillage management, nitrogen application rates and soil depth. The bacterial and fungal diversities of NT were higher than CT and N application decreased their diversities in 0–10 cm bulk soil. NT increased microbial carbon use efficiency (CUE) compared with CT in the 0–10 cm. Microbial CUE increased with increasing N application rate. In addition, under NT, high N application rate reduced the total amount of phospholipid fatty acids (PLFAs) within all aggregates. The bacterial network complexity of NT was higher than CT at lower N application rates but reduced it at higher rates. Under NT, increasing N application rate have no impact on properties of soil bacterial co-occurrence networks. Compared to CT, NT increased fungal network complexity under the same N application rates. Increasing N application rate decreased fungal network complexity under two tillage treatments. Compared to CT, NT increased the keystone taxa of Acidobacteria, Planctomycetes, and Bacteroidete under lower N application rate, while reducing the keystone taxa of bacteria and fungi under higher N application rate. Under NT, high N application rate reduced the the keystone taxa of Acidobacteria and Bacteroidetes.
(2) The partial least squares path model showed that bacterial diversity had a positive influence on microbial CUE. Furthermore, particulate organic carbon (POC) and mineral-associated organic matter carbon (MAOC) under NT were higher than CT and they also increased with increasing N application rate. Increasing microbial CUE induced by N application had the potential to increase POC and MAOC.
(3) Under NT, high N application rate increased SOC by 2.1–3.7 g⋅kg-1 within mega- and macro-aggregates. Actinobacteria were recruited by straw under NT and their biomass increased 1.5–7.8 times within all aggregates compared with CT, where they might participate in aggregate formation via degradation of straw and increasing SOC within mega- and macro-aggregates. Conversely, desulfovibrio biomass within all aggregates was diminished under NT compared with CT, while desulfovibrio possibly directly inhibited soil aggregate formation and decreased SOC within mega- and macro-aggregates under CT. Moreover, under NT, arbuscular mycorrhizal fungi biomass increased by 0.4–1.6 nmol g-1 within all aggregates compared with CT in 0–10 cm, potentially indirectly contributing to soil aggregate formation via co-metabolic processes and increasing SOC within mega- and macro-aggregates.
(4) The irregular and elongated pore morphology increased pore connectivity and porosity, and further increased the bacterial network connectivity. The pore connectivity and porosity augments provide favorable conditions for aerobic bacteria survival of Acidobacteria, Planctomycetes and Gemmatimonadetes. Reducing the distribution of (0-10 μm) pores may reduce competition among fungal communities. The decrease in porosity and the distribution of (10-30 μm) pores resulted in a concomitant reduction in the bacterial network connectivity and key bacterial taxa. Furthermore, high N application rates could improve soil carbon(C) sequestration potential (functional gene cbbL) by increasing the C sequestration key microorganisms (Chloroflexi and Proteobacteria) under two tillage practices.
Overall, the thesis revealed the mechanism of how tillage and nitrogen management affect SOC by changing soil microbial properties. We found that SOC sequestration in bulk soil was mainly driven by microbial CUE. N addition can alter the effect of soil microbial diversity on CUE. Furthermore, SOC within soil aggregates was mainly driven by the various survival strategies of Actinobacteria, desulfovibrio biomass and arbuscular mycorrhizal fungi. High N application under long-term NT protects SOC within mega-aggregates by altering aggregate formation through different microbial strategies. In addition, SOC sequestration potential within soil aggregates was mainly driven by microbial keystone taxa. N addition can alter the effect of soil pore structure on microbial co-occurrence patterns and keystone taxa. These results improve our understanding of explaining and predicting the sequestration and potential of SOC within bulk soil and aggregates in tillage systems. From this, we conclude that higher N application rate is the most effective nitrogen management under NT treatments from the perspective of SOC sequestration.