Edaphic factors and plants influence denitrification in soils from a long-term arable experiment

Abstract

Factors influencing production of greenhouse gases nitrous oxide (N₂O) and nitrogen (N₂) in arable soils include high nitrate, moisture and plants; we investigate how differences in the soil microbiome due to antecedent soil treatment additionally influence denitrification. Microbial communities, denitrification gene abundance and gas production in soils from tilled arable plots with contrasting fertilizer inputs (no N, mineral N, FYM) and regenerated woodland in the long-term Broadbalk field experiment were investigated. Soil was transferred to pots, kept bare or planted with wheat and after 6 weeks, transferred to sealed chambers with or without K¹⁵NO₃ fertilizer for 4 days; N₂O and N₂ were measured daily. Concentrations of N₂O were higher when fertilizer was added, lower in the presence of plants, whilst N₂ increased over time and with plants. Prior soil treatment but not exposure to N-fertiliser or plants during the experiment influenced denitrification gene (nirK, nirS, nosZI, nosZII) relative abundance. Under our experimental conditions, denitrification generated mostly N₂; N₂O was around 2% of total gaseous N₂ + N₂O. Prior long-term soil management influenced the soil microbiome and abundance of denitrification genes. The production of N₂O was driven by nitrate availability and N₂ generation increased in the presence of plants.

Figure 1

Relative mean abundance of prokaryotic phyla/sub-phyla in soils of origin on collection from the field. Phyla with at least 0.1% of the total community present in at least one soil treatment are included. Proteobacteria sub-phyla: a = alpha, b = beta, d = delta, g = gamma; s.e.d. for each group is shown; letters indicate mean significantly different means within each group (P = 0.05, according to Tukey’s post-hoc test on ANOVA). Insert top right shows NMDS plot of OTU for prokaryotic communities – PERMANOVA F = 9.477, P (same) = 0.0001.

Figure 2

Gene abundance from qPCR at the end of the experiment, pooling all treatments for each soil of origin (n = 12); letters denote significantly different values within each set of genes (P = 0.05) according to Tukey’s post-hoc test in ANOVA; s.e.d. = standard errors of difference of means; note that 16S and nirK are plotted as 10^–9, the other genes as 10^–6 copies g⁻¹ dw soil.

Figure 3

Soil properties at the end of the experiment. (a) concentration of NH₄⁺-N; (b) NO₃^–-N; (c) % wfps; means for soils with all treatments (n = 3); different letters denote significantly different values according to Tukey’s post-hoc test in ANOVA (P = 0.05), s.e.d. = standard errors of difference of means for all samples.

Figure 4

Mean gas production over 4 days. (a) N₂O-N, all treatments (48 pots); (b–e) K¹⁵NO₃-fertilized treatments only (24 pots). (b) ¹⁵N atom % measured in N₂O; (c) N₂O-N indicating %¹⁵N (upper s.e.d. relates to N₂O-N; lower bar relates to ¹⁵N atom %), (d) N₂-N, (e) N₂O-N as % (N₂-N + N₂O-N). Different letters denote significantly different values according to Tukey’s post-hoc test in ANOVA (P = 0.05), s.e.d. = standard errors of difference of means for all samples.