Both soil organic carbon (SOC) and soil inorganic carbon (SIC) are important carbon reservoirs involved in the global carbon cycle. Fertilization especially organic amendments can increase SOC sequestration, while nitrogen fertilization-induced soil acidification leads to significant SIC loss in alkline soil. How fertilization changes SIC and SOC via altering abiotic and biotic properties remains unclear. Here, we investigated the effects of mineral fertilizer with/without organic amendments (manure, straw, green manure) on SIC and SOC in bulk soil and particle-size fractions, as well as their interrelationship based on a 38-year field trial. Results showed that compared to the unfertilized soil, mineral fertilization significantly decreased SIC by 13.38%-15.69%, primarily due to decreased soil pH. However, mineral nitrogen plus manure (NM) largely increased SIC by 26.55%, likely resulting from the CO 2À 3 reprecipitated by Ca 2+ and/or Mg 2+ compensation. Mineral nitrogen plus manure and straw (NM, NS) increased SOC by 11.12%-100.35%. For bulk soil and particle-size fractions >0.25 mm, SIC content was positively correlated with SOC. Random forest model revealed that SOC was regulated by soil nutrients (total nitrogen [TN], Olsen-P), microbial biomass and β-1,4-N-acetyl-glucosaminidase. The change in SIC was directly regulated by soil Mg 2+ , and indirectly by soil nutrients (e.g., TN, Olsen-P) which affected crop-derived C input. Our findings suggested that organic manure amendments can facilitate SOC accumulation and mitigate SIC loss by occluded more SIC in large soil particles. The results are of fundamental significance for understanding the role of optimal manure application played in SOC sequestration and SIC accumulation.
Soil net nitrogen mineralisation (Nt) is crucial for nitrogen availability and ecosystem productivity. However, the patterns and drivers of Nt remain unclear under different management practices. We examined the biotic and abiotic determinants of Nt, using 11 treatments in Northern China fluvo-aquic soil under wheat-maize rotation. Biotic properties, for example, soil microbial community, were determined based on phospholipid fatty acid (PLFA) together with high-throughput sequencing technologies. Abiotic properties were characterised by the content and stoichiometric ratios of soil nutrients.Animal manure applications (HNM, NM, M, FM) significantly increased the Nt (1.80-3.40 mg kg À1 ) and available phosphorus (Olsen-P) (46.3-199.3 mg kg À1 ), compared with treatments with plant residues (NG, NS) incorporation.Fallow with animal manure (FM) had the highest Gram-negative bacteria (G-, 29.3 nmol g À1 ), arbuscular mycorrhizal fungi (AMF, 4.57 nmol g À1 ) abundance, which was also significantly higher than that of the NG and NS. Structural Equation Modelling revealed that the content of total nutrient, including soil organic carbon (SOC), total nitrogen (TN) and total phosphorus (TP) rather than biotic properties, such as microbial community (λ = À0.26) and enzyme activities (λ = À0.16), had the strongest direct effect on Nt (λ = 0.85). Stoichiometric ratios of C, N, and P controlled Nt indirectly by mediating enzyme activities. Specifically, high Nt was associated with low Dothideomycetes, Tectomicrobia abundance that negatively correlated with SOC, TN, TP, Olsen-P, and BG activity. Random forest model indicated that SOC and Olsen-P contents were top-rated determinants of Nt. Our result indicated that the content and stoichiometric ratios of SOC and N, P directly drive Nt or via microbial ways. Our study highlighted the importance of P to improve Nt: animal manure was thus recommended for nitrogen availability. Highlights• Animal manure significantly enhanced Nt rather than plant residue incorporation • Nt is negatively correlated with the AN:Olsen-P ratio and Dothideomycetes abundance
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