Soil environmental conditions
It is clear that the Mix-n treatment had higher DOC and NO3–N than the other treatments under all soil environmental conditions. Due to the different C/N ratios of the different straw types, N degradation and mineralization were also different. The change in soil nutrients caused by straw mulching is mainly due to the role of soil organisms. Therefore, we can explain the difference in soil nutrients by the soil biological composition of different straw mulching treatments. In general, the specific genus of soil nematode in the mix treatment can characterize the particular soil nutrient status. Previous studies have shown that some nematodes are found more often in areas with similar environmental variables and that nematode genera within the same trophic group responded differently to environmental variables16. We found that the higher abundances of Prismatolaimus, Cephalobus and Eucephalobus corresponded to the higher soil NO3–N (Appendix 1). Our results are consistent with the observations of Song et al.17. Moreover, the Mix-n treatment had a higher density of Mesodorylaimus, Aphelenchoides and Thonus where the DOC was higher. This result is in agreement with the findings of Olatunji et al.18, in which Thonus, Aporcelaimus, Mesodorylaimus, Aphelenchoides, Criconemoides, Tylenchus, and Rhabditidae were positively associated with DOC.
Soil nematode communities
From the data in Table 2, it is apparent that the CK treatment had a higher total number of soil nematodes and a higher abundance of soil nematodes in different nutritional groups than any straw mulch treatment; that is, the number of soil nematodes after straw mulching was lower than that in the control. Blankinship et al.19 used a meta-analysis method to study the response of soil nematodes to temperature increase under different ecosystem types. It was found that soil nematodes were mainly affected by annual precipitation. When annual precipitation exceeded 626 mm, the increase in temperature had a positive effect on the number of soil nematodes19. In this study, the annual precipitation in this area (1033.9 mm) exceeded 626 mm, and straw mulching had a cooling effect during the growth period of young walnut trees. This could be a possible reason of higher abundance of soil nematodes in the CK treatment than that in any straw mulching treatment. Moreover, this finding is also contrary to our first hypothesis that different straw mulching treatments would increase the number of soil nematodes. The reasons are as follows: on the one hand, phenolic acids enter the soil through the secretions of walnut roots and the decomposition of a large amount of straw residues, which results in an increase in phenolic acids in the soil and a decrease in the total number of soil nematodes and other nematodes20. On the other hand, straw mulching returns pathogenic bacteria and parasite eggs to the field directly. At the same time, the nutrients released from straw in the soil provide a favorable environment for pathogenic bacteria and parasite eggs to increase in number, which significantly inhibits soil nematodes21.
In addition, a key finding was that fungal nematodes were more common than bacterial nematodes in the treatments with complete mulch coverage than in the n and 1.5n coverage treatments. When rice straw, rapeseed straw and mix straw were applied at n and 1.5n distances, the decomposition pathway was a bacterial channel; when the coverage distance increased to all n, the decomposition pathway gradually changed to decomposition equally distributed between bacterial and fungal decomposition pathways. In contrast, the CK treatment was dominated by the number of bacterivorous nematodes, suggesting that the bacterial channel was the main pathway of decomposition, which was consistent with the result of the distribution map of nematode fauna in Fig. 1. At the same time, this result indicates that the coverage distance changed the dominant community of nematode trophic groups.
The footprints of different nematode trophic groups are proxies for the carbon or energy flow entering the soil food web through their respective channels22. In our study, we found that the footprint and the carbon biomass of the omnivore-predator nematodes and all structure metabolic footprints showed higher values under all straw mulching treatments compared with those of the other soil nematode trophic groups (Table 3). This observation may be explained by the predator–prey trophic cascade effect: straw mulching stimulates higher carbon and nutrient inputs first to microorganisms and then to microbivorous nematodes, which stimulate the metabolic activity and abundance of omnivore-predator nematodes; omnivore-predator nematodes consume more prey and thus inhibit the abundance of soil nematodes at lower trophic levels23.
The maturity index of nematodes is one of the key indices of soil health. In our study, the MI values for rice straw and rapeseed straw treatment alone were not significantly higher than those for the CK treatment (Fig. 2c). However, the MI values for the mix straw treatments were significantly higher than those for the CK treatment, indicating that the structure of the nematode community is stable and that the complexity of the soil food web could increase under the mix straw treatment.
Combined with the ecological indices BI, which is related to soil properties and decomposition pathways24, we found that higher CI value for the three straw mulching treatments appeared in the whole-plot coverage treatments (all n). Our results contrast with those of other studies, which found that bacterial-dominated decomposition pathways were the most common pathways20. This discrepancy could be explained mainly by the observed variations in the abundances of bacterivores and fungivores among the different coverage distances. Specifically, bacterivore nematodes predominate in different soil nematode trophic groups when the coverage distance is n, while bacterivore nematodes and fungivore nematodes predominate in different soil nematode trophic groups when the coverage distance is increased to all n (Table 2). In addition, soil nematode decomposition pathway changed with the increase in coverage distance in the three straw mulching treatments, which may have been caused by the increase in contact area between straw and soil. The specific mechanism needs to be further studied in our next work.
Soil nematode faunal profile
The SI is considered to indicate the structure of the soil food web response to disturbance and during remediation, while the EI reflects soil food web responses to available resources and the resource response to the primary decomposers17,25.
In the present study, the rice straw mulching treatments and rapeseed straw mulching treatments with high EI and SI values at different straw mulching distances were in quadrant B, indicating that the structure of the food web was fairly mature, the N concentration was high, the C:N ratio was low, the decomposition pathways of fungi and bacteria was balanced, and the disturbance level of the soil environment was low to moderate. These conditions occurred is mainly because of the large amounts of dissolved organic carbon and dissolved organic nitrogen in the soil due to straw degradation and the straw mulching water retention effect making the soil moisture content higher than that found in the CK treatment (Table 1).
However, the mix straw mulching treatments with high SI and low EI values at different straw mulching distances were in quadrant C, which indicates a structured food web, medium soil enrichment, a moderately high C/N ratio, fungal decomposition channels, and no disturbance. Our previous research suggested that the mix straw mulching treatment had a moderate carbon nitrogen ratio (C:N) and that mix straw degrades more quickly than rice straw or rapeseed straw9. In addition, the mix straw may have provided stable moisture content and higher dissolved organic carbon and dissolved organic nitrogen than rice straw or rapeseed straw (Table 1), thus increasing nutrient availability and soil fertility levels. This result is supported by other agricultural management practices20,26,27. This evidence supported our hypothesis that the mix straw mulching treatment led to a more stable soil food web and higher soil fertility levels.
Environmental factors affecting soil nematode community variability
Straw mulching directly increases the mineral nitrogen and DON contents in the soil through decomposition, which significantly increases the content of nitrogen in the soil, thus increasing the amount of soil nutrients and soil organisms. Plant parasite and omnivore-predator nematode abundances were negatively correlated with NH4+–N and DON contents, but there was no significant correlation between the nematode community and soil DOC content. This finding indicates that nitrogen in the soil of the agroforestry ecosystem had a more significant impact on the nematode community than carbon. This result is also consistent with previous results28,29. Another possible explanation was that ammonium toxicity may occur when soil nematodes feed on root fluid, resulting in a negative correlation between omnivore-predator nematodes and NH4+–N30. Compared with the control condition, straw mulching significantly increased soil moisture content and soil anoxia, while soil total nematodes were negatively correlated with SM value. The results showed that the increase in soil moisture changed the soil environment, inhibited the growth of soil microorganisms, and inhibited the growth of total nematodes through changes in nutrient levels and the environment in the food chain.
In terms of straw coverage distance, our results showed that the decomposition pathway gradually changed from the bacterial decomposition channel to the bacterial/fungal decomposition channel when the coverage distance increased from a narrow coverage distance (n) to a wide coverage distance (all n) in the three straw mulching treatment groups. In terms of straw mulch types, the mix straw mulching treatment had a higher maturity index, a more stable soil food web and higher soil fertility levels than the rice straw or rapeseed straw mulching treatments. There was a significant negative correlation between plant parasite and omnivore-predator nematodes and NH4+–N and DON, but there was no significant correlation between the nematode community and the soil DOC content. This finding was unexpected and suggests that nitrogen in the soil of agroforestry ecosystems had a more significant impact than soil carbon on the nematode community. Recommendations for sustainable walnut orchard management based on the complexity and stability of nematode food webs should advocate the use of mix straw mulching (mix) covering the whole plot (all n) and thus promote the accumulation of soil dissolved organic nitrogen and carbon nutrients.