Soil aggregates. Soil aggregates were dominated by mega-aggregates (> 2 mm) and silt + clay (< 0.053mm), accounting for about 78% (Table 1). In addition, there was no significant difference in the content of the 0.25-2mm and < 0.053mm aggregates among the treatments. Compared with CK, the content of > 2 mm aggregates increased by 7.25% in F and 10.64% in SF. However, the content of 0.053–0.25 mm aggregates decreased by 17.42% in F and 34.15% in SF compared with CK. The content of > 2 mm SF aggregates increased by 3.17% and that of 0.053–0.25 decreased by 20.27% compared with F. Our results revealed that both single compound fertilizer application and straw returning with compound fertilizer changed the composition of soil aggregates, increasing the content of mega-aggregates and decreasing the content of micro-aggregates. And yet, straw returning with compound fertilizer had a greater impact on the composition of soil aggregates, facilitating the conversion of micro-aggregates to the largest ones.
Table 1
Contents of soil aggregates in different treatments. Note within columns, means followed by the same letter are not significantly different according to LSD (0.05).
Treatments | Composition of soil aggregate fractions (%) |
| > 2 mm | 0.25-2mm | 0.053-0.25mm | < 0.053mm |
CK | 42.51b | 8.05a | 17.21a | 32.23a |
F | 45.59a | 7.70a | 14.21b | 32.51a |
SF | 47.03a | 7.03a | 11.33c | 34.61a |
Distribution of carbon, nitrogen and phosphorus in soil aggregates. As shown in Table 2, SOC and TN contents of < 0.053mm aggregates were significantly lower than other particle size aggregates, from > 2mm to 0.053-0.25mm aggregates, the smaller the aggregates were, SOC and TN contents gradually increased, and SOC and TN contents of 0.053-0.25mm aggregates were higher (except CK). In the same level of aggregates, SOC and TN contents of soil aggregates at each particle level were the highest in the straw returning with compound fertilizer (except < 0.053mm aggregates), while increased TP content in the aggregates at each grain level. Compared with CK, SOC content of 0.053–0.25 mm aggregates increased by 12.71% in F; SF significantly increased SOC content of 0.25-2 mm and 0.053–0.25 mm aggregates by 18.16% and 35.36%, respectively. Compared with F, SF significantly increased SOC content of 0.25-2 mm and 0.053–0.25 mm aggregates by 22.56% and 20.09%, respectively. In sum, straw returning with compound fertilizer can increase SOC content (except for < 0.053mm), but TN and TP contents in the aggregates at each grain level were not significantly different between treatments.
Table 2
Effects of different treatments on carbon, nitrogen and phosphorus in soil aggregates. Note within columns, means followed by the same letter are not significantly different according to LSD (0.05).
Soil variables | Treatments | Aggregate size |
| | > 2 mm | 0.25-2mm | 0.053-0.25mm | < 0.053mm |
SOC(g·kg− 1) | CK | 18.42a | 28.03b | 24.94c | 14.68a |
| F | 19.33a | 27.03b | 28.11b | 14.38a |
| SF | 19.95a | 33.12a | 33.75a | 13.79a |
TN(g·kg− 1) | CK | 1.70a | 2.42a | 2.19a | 1.56a |
| F | 1.77a | 2.26a | 2.60a | 1.51a |
| SF | 1.86a | 2.93a | 3.13a | 1.50a |
TP(g·kg− 1) | CK | 0.56b | 0.70a | 0.78a | 0.55a |
| F | 0.83ab | 0.77a | 0.67a | 0.49a |
| SF | 0.87a | 0.80a | 0.90a | 0.61a |
Enzyme activities in soil aggregates. Both F and SF treatments reduced CAT and INV in > 2 mm and < 0.053 mm aggregates compared with CK (Figure.1). SF increased CAT in > 2 mm aggregates compared with F treatment. As shown in Figure.1a, compared with CK, CAT significantly reduced in > 2 mm aggregates of SF and F by 19.42 and 14.85%, respectively, while they increased in 0.25-2 mm aggregates by 8.80 and 9.72%, respectively. Compared with F, SF increased and decreased CAT in > 2 and < 0.053 mm aggregates by 5.67 and 5.60%, respectively. As shown in Figure.1d, compared with CK, INV of each particle sized soil aggregate in F decreased by 27.98%~61.28%, while that of SF decreased by 53.55%~68.30%. Compared with F, SF reduced INV of soil aggregates by 7.71% ~ 40.66%. In short, straw returning with compound fertilizer reduced INV and CAT (except for > 2 mm CAT) compared to single compound fertilizer application.
Both F and SF treatments increased ACP and URE in each particle sized aggregate compared to CK (Figure.1). Compared with CK, F increased ACP in aggregates of different sizes by 15.63% ~ 68.87%, while SF increased ACP in 0.25-2 mm to < 0.053 mm aggregates by 20.15% ~ 49.85% (Figure.1b). Compared with F, SF decreased ACP in aggregates of different sizes by 9.38% ~ 13.82%. As can be seen from Figure.1c, compared with CK, F and SF increased URE each particle sized soil aggregates by 21.18% ~128.57% and 26.08%~261.86%, respectively. Compared with F, SF increased and decreased URE in > 2mm to 0.053-0.25mm aggregates (30.01% ~ 58.31%) and < 0.053 mm (35.53%), respectively. Overall, compared with single compound fertilizer application, straw returning with compound fertilizer reduced ACP, while enhanced URE (except for < 0.053 mm).
Bacterial diversity in soil aggregates. The bacterial alpha diversity of aggregates under different treatments is shown in Table 3. Compared with CK, F increased Chao index in > 2 mm aggregates by 2.71% and decreased Chao index in 0.25-2 to < 0.053 mm aggregates by 1.73% ~ 9.53%; SF decreased Chao index in > 2 mm to 0.053-0.25mm aggregates by 0.51% ~4.36%; F increased Shannon index of > 2 mm aggregates by 2.30%; SF decreased Shannon index of 0.053-0.25mm aggregates by 1.25%. Compared with F, SF decreased and increased Chao indices in > 2 mm and 0.25-2 to < 0.053 mm aggregates, respectively, by 5.60% and 1.25%% ~ 5.71%, respectively. In conclusion, compared to single compound fertilizer application, straw returning with compound fertilizer significantly increased Chao indices.
Table 3
Analysis of soil bacterial community alpha diversity at soil aggregates. Note within columns, means followed by the same letter are not significantly different according to LSD (0.05).
Diversity index | Treatments | Aggregate size |
| | > 2 mm | 0.25-2mm | 0.053-0.25mm | < 0.053mm |
Chao | CK | 5018.74b | 6266.74a | 6008.35a | 6033.56a |
| F | 5154.94a | 5754.40c | 5904.34c | 5458.59c |
| SF | 4866.33c | 6027.06b | 5977.90b | 5770.28b |
Shannon | CK | 10.34b | 10.96a | 10.77a | 10.79a |
| F | 10.58a | 10.93b | 10.77a | 10.68b |
| SF | 10.27c | 10.88c | 10.64b | 10.68b |
Bacterial community structure in soil aggregates. The bacterial communities of soil aggregates at the phylum level were ranked by the abundance as Proteobacteria, Chloroflexi, Acidobacteriota, Bacteroidota, Actinobacteriota, Planctomycetota, Myxococcota, Desulfobacterota, Gemmatimonadota, Nitrospirota, etc. The top 10 categories accounted for 83.36–90.44% of the total bacteria, with the dominant bacterial phylum (abundance > 7%). For all particle-sized soil aggregates, Proteobacteria (15.48%~26.69%) had the highest abundance followed by Chloroflexi (18.12%~25.41%), Acidobacteriota (14.20%~18.26%), Bacteroidota (3.79%~6.85%), Planctomycetota(4.24%~5.52%), Myxococcota (3.36%~4.82%), Desulfobacterota (3.16%~4.75%), Gemmatimonadota (2.54%~6.78%), and Nitrospirota (2.16%~5.72%) (Figure.2). Compared with CK, F increased the relative abundance of Proteobacteria in > 2 to < 0.053 mm aggregates by 4.14–44.49%; SF decreased and increased the relative abundance of Proteobacteria in > 2 mm and 0.25-2 to < 0.053 mm aggregates, respectively, by 5.35% and 6.33%~ 31.76%, respectively. Compared with F, SF decreased the relative abundance of Proteobacteria in > 2 mm, 0.053-0.25mm, and < 0.053 mm aggregates by 7.92%~28.47%, and increased that in 0.25-2 mm aggregates by 8.22%. Compared with CK, both F and SF decreased the relative abundance of Acidobacteriota in all particle-sized soil aggregates by 0.72%~13.02% and 6.33%~ 8.40%, respectively. SF increased the relative abundance of Acidobacteriota in all particle-sized soil aggregates by 0.92%~5.30% compared with F. Compared with CK, F reduced the relative abundance of Chloroflexi in > 2 mm to < 0.053 mm aggregates by 11.83%~28.67%; SF increased and decreased the relative abundance of Chloroflexi in > 2 mm and 0.25-2 to < 0.053 mm aggregates by 8.37 and 2.92%~10.12%, respectively. Compared with F, SF increased the relative abundance of Chloroflexi in > 2 and < 0.053 mm aggregates by 10.11% ~ 26.01%. Compared with CK, F increased the relative abundance of Gemmatimonadota in > 2 and < 0.053 mm aggregates by 1.30%~72.17%; SF decreased the relative abundance of Gemmatimonadota in > 2 and 0.25-2 mm aggregates by 3.74% and 7.02%, respectively, and increased that by 33.73% and 22.78% in 0.053–0.25 and < 0.053 mm aggregates, respectively. Compared with F, SF decreased the relative abundance of Gemmatimonadota in > 2, 0.25-2, and < 0.053 mm aggregates by 8.21%~44.09%, and increased that in 0.053-0.25mm aggregates by 7.57%. Compared with CK, F decreased the relative abundance of Nitrospirota in > 2, 0.053–0.25, and < 0.053 mm aggregates by 14.54%~44.82%, and increased that in 0.25-2 mm aggregates by 7.78%; SF decreased the relative abundance of Nitrospirota in > 2 and < 0.053 mm aggregates by 5.21% and 9.30%, respectively, and increased that in 0.25-2 and 0.053–0.25 mm aggregates by 29.21% and 24.79%, respectively. Compared with F, SF increased the relative abundance of Nitrospirota in > 2 and < 0.053 mm aggregates by 6.13%~ 71.77%. Altogether, our results suggested that straw returning with compound fertilizer altered the enrichment of bacterial communities in soil aggregates of Acidobacteriota, Chloroflexi, and Nitrospirota, etc.
Linkage between dominant bacterial phylum and carbon, nitrogen, phosphorus and enzyme activity in soil aggregates. Herein, Pearson correlation analysis was performed to discern the linkage between the dominant bacterial communities and carbon, nitrogen, phosphorus and enzyme activities (Figure.3). The correlation between the dominant bacterial population and enzyme activity, SOC, TN and TP in > 2 mm aggregates did not reach a significant level (Figure.3a). In 0.25-2 mm aggregates (Figure.3b), Proteobacteria and Nitrospirota displayed positive correlations with SOC and URE, were negatively INV, whereas Gemmatimonadota is positive INV, negatively correlated with SOC and URE. Chloroflexi was negatively correlated with ACP. Acidobacteriota was positively correlated with INV, but negatively correlated with URE, CAT and ACP. In 0.053–0.25 mm aggregates (Figure.3c), ACP and INV negatively correlated with Chloroflexi and Acidobacteriota, while Proteobacteria was positived with ACP and INV. Gemmatimonadota was positively correlated with SOC, TN, URE and ACP. The correlation between Nitrospirota and enzyme activity, carbon, nitrogen and phosphorus did not reach significant level. In < 0.053 mm aggregates (Figure.3d), Proteobacteria and Gemmatimonadota displayed significant positive correlations with URE and ACP, negatively correlated with INV, but Chloroflexi and Nitrospirota were positively correlated with INV, negative with URE and ACP. Acidobacteriota was negatively correlated with URE. The correlation between the dominant bacterial population and SOC, TN and TP did not reach a significant level in < 0.053 mm aggregates. To sum up, ACP, URE and INV are the main factors affecting the dominant bacterial community of soil aggregates, followed by CAT and SOC.