Long-term weed control practices in the rice-wheat cropping system of the Yangtze River basin show good control for various types of weeds, such as broadleaf and grasses (Zhang 2013), but the effect of weed control goes far beyond only influencing the weed community. After fourteen years’ treatments, weed control exerted profound effects on soil properties, AMF communities and crop yields during the wheat growing season. To some extent, the results agreed with our hypotheses that four weed control practices changed soil properties (e.g., WC, TN, SOC) and AMF diversity and composition compared with no weeding treatment; while the effects of four weed control practices on soil properties and AMF diversity and composition were different. We also found the AMF community diversity and composition were related to soil properties, and wheat grain yield was significantly correlated to AMF OTU composition. These findings suggest that rational weed control practice should be taken to sustain crop production and soil ecological environment of arable land in the rice-wheat double cropping region.
Effects of weed control on soil properties
Soil properties are an important axis for evaluating soil quality of arable land (Bünemann et al. 2018). Our findings suggest that weed control markedly alters soil properties, which is consistent with the results of weed control experiments in other regions (Hosseini Bai et al. 2012; Naeem et al. 2022; Promsakha Na Sakonnakhon et al. 2006; Wang et al. 2022), indicating that weed removal from the crop field is also an agricultural practice that could significantly change soil quality. Therefore, it is necessary to pay attention to changes in soil quality of arable land under long-term weed control for further understanding of weed-soil feedback mechanisms during long-term agroecosystem development for better crop protection (Patzold et al. 2020; Png et al. 2019).
In this study, TN and nitrogen fractions were significantly affected by weed control practices. Compared with CK, TN decreased in MH, WaM and WaMH, which may be because weed and crop association could effectively increase soil TN (Promsakha Na Sakonnakhon et al. 2006). However, TN increased in WaH, which might be mainly due to the intensity of weeding in WaH was moderate in this experiment (Zhang 2013), confirming that more intensive weed biomass is not better to maintain a higher nitrogen level in the field. This may be because most weeds are C4 plants in our experimental plots, which generally have higher nitrogen-use efficiency (Yuan et al. 2007), thus absorbing more nutrients than C3 crops. NH4+-N and NO3−-N are available inorganic nitrogen fractions, which can be directly assimilated by plants (Haynes and Goh 1978). In the present study, NH4+-N was significantly decreased in WaMH and increased in MH and WaM, and NO3−-N was significantly increased in WaH as compared with CK. Moreover, the current findings showed that NO3−-N content was much higher than NH4+-N content, and NO3−-N was significantly positively correlated with TN, which suggests that reasonable weed control practice could increase soil nitrogen level by improving nitrogen availability. As a biological nitrogen component, soil MBN can comprehensively reflect the effects of soil microorganisms on nitrogen mineralization and retention (Shen et al. 1984). Soil MBN was reduced by weed control treatments reduced soil MBN as compared with CK. This was because weed control could affect the most growing weeds, leading to an inhibition of the biomass of nitrogen-transforming microorganisms, and thus significantly change soil nitrogen cycling (Promsakha Na Sakonnakhon et al. 2006).
Weed control practices significantly affected SOC. Compared with CK, SOC decreased in MH, WaM, and WaMH (Table 1), possibly because weed control reduced carbon input from root exudates (Ibell et al. 2010). However, SOC content was significantly increased in WaH, suggesting that moderately intensive weed management could promote soil carbon sequestration, possibly by maintaining a higher biomass of wheat without reducing carbon input from root exudates, while by reducing carbon output from soil respiration (Hosseini Bai et al. 2012).
Weed control practices also significantly altered soil WC. compared with CK, WC decreased in MH, WaM and WaMH, indicating that the presence of weeds could preserve soil moisture (Zhang et al. 2020). However, WC increased significantly in WaH compared with CK, which may be due to the increased transpiration water loss induced by excessive weeds in CK (Sen et al. 2021). In addition, WC, TN and SOC showed significantly positive correlation between each other, which further indicated that moderate weed control intensity may achieve the synergistic improvement of water and nutrient‑use efficiency and soil carbon sequestration. Therefore, it is unnecessary to completely remove weeds in arable land. A sustainable weed management approach can be applied to improve soil quality and thus sustain soil ecological environment in agroecosystem (MacLaren et al. 2020; Wortman 2016).
Effects of weed control on AMF community
At present, soil rhizosphere and endosphere are the hotpots to study AMF community (Guzman et al. 2021; Xiao et al. 2022). Unfortunately, we did not investigate AMF community in these hotpots, but only in bulk soil. Non-rhizosphere soil is also a very important part for plant growth (Angst et al. 2016). AMF can help host plants absorb nutrients from non-rhizosphere soil through their mycelium expansion (Dierks et al. 2022; Wang et al. 2017). Thus AMF community structure in non-rhizosphere soil has also attracted attention (Luo et al. 2021). Moreover, our weed control treatment has been lasted for 14 years at the sampling date, indicating that weed control may exert a long-term impact on soil environment. However, the crop rhizosphere effect usually works for a short term on soil process, perhaps only a few months. Actually, most soil microorganisms need sufficient time to adapt to soil environmental changes (Reardon et al. 2014). However, rhizosphere microorganisms are mainly recruited from non-rhizosphere soil (Ling et al. 2022). Therefore, the long-term effects of weed control on AMF community structure could be better revealed by studying AMF community composition in non-rhizosphere soils.
The findings released by NMDS ordination analysis showed that long-term weed control largely altered AMF community structure. Glomus is generally considered to be widespread and the most abundant genus in most natural and artificial ecosystems (Oehl et al. 2003; Pereira et al. 2014), and is often found in rice-wheat cropping region in China (Qin et al. 2015; Zhu et al. 2018). However, the current study showed that Glomeromycetes_unclassified was the most dominant genus than Glomus, indicating that our experimental site has a unique AMF community, but long-term weed control does not change the dominant genena’s abundance in AMF community. Interestingly, the relative abundance of key OTU taxa of AMF community was significantly altered among weed control treatments. This suggested that weed control practices could influence the key indicator taxa of AMF as significantly as other crop cultivation practices, such as tillage and fertilization (Lang et al. 2022; Liu et al. 2022; Luo et al. 2021).
The abundance, richness and α-diversity indices of AMF community are also significantly affected by weed control. Previous studies showed that AMF species diversity is positively correlated with plant diversity (Burrows and Pfleger 2002; Guzman et al. 2021). Based on this, we can infer reasonably that AMF diversity should be the highest under no weeding treatment, which has the highest weed species diversity. However, our results showed that AMF diversity was the highest under WaH treatment. Which could be explained by the intermediate-disturbance hypothesis, indicating that that species diversity is higher with intermediate-disturbance than with frequent disturbance or no disturbance (Fox 1979). In this experiment, the intensity of weeding in the field with WaH is moderate. Based on the investigation of soil weed seed banks, it was found that the species richness of weed seed banks in WaH was moderate (WaMH < MH < WaH < WaM < CK) (Zhang 2013), thus possibly maintaining the highest diversity of AMF. These is findings are consistent with the findings obtained by Ba et al. (Ba et al. 2012) who observed that the highest richness and evenness of AMF in the moderately grazed areas. Therefore, maintaining a certain abundance of weeds may play an important role in protecting species diversity in arable land (van der Heijden et al. 2008). In addition, with the exception of MH treatment where AMF diversity was significantly reduced, no decrease in AMF diversity was observed in other pesticide application treatments (WaH and WaMH). Thus, through the current findings still not clear whether the application of pesticides could exert negative effects on AMF community (Edlinger et al. 2022; Ramos-Zapata et al. 2012), which need a further investigation.
This is well confirmed by our results here AMF community structure is strongly correlated with soil properties (Guzman et al. 2021; Wakelin et al. 2012). The abundance of dominant genera and diversity of AMF can be explained by soil properties for more than 90%, and AMF community composition can be explained by soil properties over 50%. Mantel test showed that TN and SOC were significantly positively correlated with AMF diversity, which is consistent with the results of Yang et al. (2021) and Xiao et al. (2022). However, Qin et al. (2015) and Liu et al. (2022) found a reverse trend. On the one hand, the above-mentioned paradoxical phenomenon might be caused by the possible threshold effect of TN and SOC on AMF diversity, which only increases AMF diversity within a certain range (Wu et al. 2023; Xiao et al. 2022). On the other hand, this might be due to the complex interactions among many ecological and environmental factors (Moebius-Clune et al. 2013). For example, compared to no-fertilization control, AMF diversity of both wheat and maize harvest stages significantly decreased in the topsoil (0–15 cm) while it was increased in the subsoil (15–30 cm) under long term fertilization in wheat-maize double cropping region of southwest China. This phenomenon was not attributable to changes of SOC and TN, possibly as AMF diversity was also controlled by temperature, crop biomass and other soil factors (Luo et al. 2021). In this study, TN and nitrogen fractions (NO3−-N, NH4+-N, and MBN) were closely related to AMF OTU composition, suggesting that weed control might influence AMF community composition through regulating soil nitrogen availability. The amount of available nitrogen in the soil can affect AMF spore density and root colonization (Li et al. 2010; Yang et al. 2011), thus possibly influencing the trade-off between AMF adaptation strategies and field environment conditions (Babalola et al. 2022; Bakhshandeh et al. 2017). Hence, weed control might indirectly affect AMF community characteristics through altering soil properties.
Effects of weed control on crop yield
The current study showed that weed control has significantly affected on wheat yield, in which wheat yield could increase by more than 50%. This is in line with results from similar weed control treatments at other sites (Zhang et al. 2021). The yield-increasing effect can be directly explained by inhibiting the growth of weed communities from weed control practices, which could avoid competition from weeds to wheat for light and nutrient resources.
Our findings suggest that wheat grain yield can be explained by AMF OTU composition, suggesting that AMF might play important roles in wheat grain formation. In agroecosystems, AMF can drive soil nutrient cycling to support crop growth (Kim et al. 2022; Sheteiwy et al. 2021a; Sheteiwy et al. 2021b). A similar finding also was confirmed by meta-analysis, which suggests that AMF can promote grain crop yield (Zhang et al. 2019). In this study, sampling was conducted at wheat grain filling, a critical stage for wheat to assimilate carbon dioxide into organic matter such as starch and protein, which is closely related to the high yield (Aranjuelo et al. 2013). However, wheat could be more inclined to cooperate with AMF at this stage, because the absorption function of wheat roots gradually declined during wheat grain development, leading to wheat being unable to rely on its own roots to complete nutrient supply (Sawers et al. 2018). In other words, the symbiotic relationship between AMF and wheat root system at the grain filling stage could increase wheat yield through improving nutrient utilization efficiency (Yang et al. 2022). In a sense, the potential relationship between AMF community and wheat yield could be effectively revealed by studying AMF community composition during wheat grain filling. Similarly, previous studies reported that wheat yield could directly or indirectly affected by AMF diversity and community structure (Liu et al. 2022; Zhu et al. 2018). Therefore, weed control could increase wheat grain yield through regulating AMF community. Moreover, we found that some key OTUs taxa were significantly associated with wheat yield, suggesting that special AMF groups might play an important role in regulating crop productivity.
Weeds, crops and AMF are known to interact with each other (Jordan et al. 2000). AMF could promote the growth of strong host crops, not only directly by suppressing weaker or non-mycorrhizal weeds, but also indirectly by enhancing competitive ability of host crops to weaken the growth of some strong host weeds (El Omari and El Ghachtouli 2021; Li et al. 2016). However, various experiments, including this study, showed that weed control practices could significantly affect the characteristics of AMF community to alter the interactions among AMF, weeds and crops (Brito et al. 2013; Ramos-Zapata et al. 2012). Therefore, it is necessary to reduce the competition between weeds and crops, while concomitantly improve the cooperation between AMF and crops to ensure high crop yield in agricultural practices.