文獻信息：
Ferrari Machado P. V., Farrell R. E., Bell G., Taveira C. J., Congreves K. A., Voroney R. P., Deen W., Wagner-Riddle C*. Crop residues contribute minimally to spring-thaw nitrous oxide emissions under contrasting tillage and crop rotations. Soil Biology and Biochemistry, 2020. doi: 10.1016/j.soilbio.2020.108057
摘 要：作物殘體是收穫後碳(C)、氮(N)的來源，其通過礦化釋放無機氮或通過氮素固持以維持土壤氮素水平。而無機氮控制氧化亞氮(N2O)的排放，N2O是一種來自農業生產的強力的溫室氣體(GHG)。因此，作物殘體在國家GHG排放清單中被列為N2O來源。然而，對於因凍融而導致收穫後N2O排放的地區，尚不清楚作物殘體是否會導致N2O排放，此外，耕作或殘體類型是否會影響這一貢獻，這是一個值得關注的問題，因為作物殘體和凍融導致的排放因子（emission factor，EF）可能是「重複計算」的N2O來源。本研究在兩個非生長季(NGS)進行了一項長期玉米、大豆和冬小麥試驗，以(i)比較免耕(NT)或常規耕作(CT)下不同作物殘體結合單作或多種作物輪作過程中N2O排放；(ii)確定地上和地下作物殘體對春季解凍N2O排放的重要性，以及其受輪作和耕作的影響。一項15N作物殘體富集研究被用來直接追蹤地上和地下殘體15N轉為15N2O通量及EF。不論是否輪作，CT處理的N2O排放量比NT高27%。大豆殘體比玉米殘體誘導更高的N2O排放量，相同的作物殘體(如玉米或大豆殘體)在長期不同作物輪作中表現出更高的N2O排放趨勢。在所有情況下，作物殘體對春季解凍N2O排放量的貢獻最小(< 2%)，這意味著N2O排放量的差異是由於耕作和輪作對土壤氮有效性的影響，而不是通過影響作物殘體釋放來進而導致N2O產生的。作物殘體的NGS六個月有效濃度從未超過0.05%，這是被推薦的年有效濃度0.6%的極小部分。寒冷氣候下排放清單的改進應側重於作物殘體以外的其他來源對凍融過程中基質釋放引發的N2O產量，而非重點關注作物殘體這一來源。
Table 1 Average dry biomass, total N content, and 15N atom% excess of the above- and below-ground crop residue pools, prior to measuring N2O emission overwinter. C = corn, S = soybean, Wrc = winter wheat + red clover, and, N.A. = not available. Tillage was significant for soil15N atom% excess (p < 0.05), with higher values under NT than CT soil.
a crop phase studied is shown underlined and bold.
b source: Taveira et al. (2020).
c source: Bolinder et al. (2007).
d source: Kriaučiūnienė et al. (2012) and Thiagarajan et al. (2018).
Fig. 1 Weather data for the non-growing seasons in (A) Dec 2017 to May 2018 (Year 1) and (B) Oct 2018 to Apr 2019 (Year 2). The solid black line indicates average daily air temperature, with grey shading showing the minimum and maximum temperature ranges. Blue bars indicate total daily precipitation (mm), and the dashed line with blue shading shows the depth of snow on the ground (cm). The small blue and brown arrows at the top of each panel indicate the dates when gas (↓) and soil (↓) samples were collected, respectively. W = winter, ST = spring-thaw, PP = pre-planting, and, PH = post-harvest.
Fig. 2 Average soil water-filled pore space (WFPS) at 0–12 cm soil depth for no-tillage and conventional tillage plots during the non-growing seasons of (A) 2017/18 (Year 1) and (B) 2018/19 (Year 2). The blue shading around lines illustrates the standard error of the measurement. No WFPS measurements were performed during the winter; and there was no PP period in 2019. ST = spring-thaw, PP = pre-planting, and, PH = post-harvest.
Fig. 3 Nitrous oxide emissions for simple (CCSS or SSCC) and diverse rotations (CCSWrc or SWrcCC) under no-tillage (NT) (top graph) and conventional tillage plots (CT) (bottom graph) during periods of the non-growing season (ST-spring-thaw, PP-pre-planting, PH-post harvest, W-winter) of Years 1 and 2. C = corn, S = soybean, and Wrc = winter wheat + red clover. Results for samples taken on Dec. 9, 2017 are not shown.
Table 2 P-values for the main effects and interaction (bolded values meaning p < 0.05), for periods of Years 1 and 2 (ST-spring-thaw, PP-pre-planting, PH-post-harvest), and mean cumulative N2O–N emissions for the significant main effects and interaction. The studied crop phase is underlined and bolded. CT and NT are conventional and no-tillage, respectively and letters indicate Duncan's classes.
a Lowercase letters show a difference within the main effect (i.e. tillage or rotation) per period.
b Lowercase letters shown a difference in rotation within a tillage treatment (i.e. within a row), and uppercase letters show differences in tillage within a rotation treatment (within a column).
c Marginal significance for tillage within the rotation treatment (within a column) (p < 0.10).
Table 3 Nitrous oxide emission factors (%) of above- and below-ground crop residues in response to tillage and rotation for the spring-thaw of Years 1 and 2. The crop phase studied is underlined and bold.
a Rotation, tillage, and, rotation x tillage not significant (p > 0.05).
b Tillage, and, rotation x tillage not significant (p > 0.05). Letters are Duncan's classes between rotation (p < 0.05).
Table 4 Contribution expressed as g N ha-1, of above- (AG) and below-ground (BG) residues, and indigenous soil-N to cumulative N2O emitted during the spring-thaw of Years 1 and 2. The percentage of total N2O emission derived from each source is shown in brackets. Letters indicate Duncan's classes, and an asterisk (*) indicates a significant difference between CT and NT (p < 0.05).
a Main tillage and rotation effects were significant for Indigenous Soil-N (p < 0.05), but no effects were found for AG and BG.
b Significant tillage effect for AG only, and rotation effect for BG only (p < 0.05). Significant tillage by rotation interaction for Indigenous soil –N.
c Interaction presented where lowercase letters show differences between rotations within each tillage treatment (i.e., within a row), and uppercase letters show differences between tillage practice within a rotation treatment (i.e., within a column). Within CCSS and SSCC, the tillage effect was marginally significant (p < 0.10).