1. Introduction
The recent development of perennial grain breeding programs has highlighted the value of the wheat wild-relative ‘intermediate wheatgrass’ [
1] (
Thinopyrum intermedium (Host) Barkworth & D.R. Dewey) to support the transition to multifunctional agroecological systems [
2,
3]. In addition to its ability to produce an interesting forage-grain dual income [
4,
5], its regrowth capacity for several years would achieve substantial production at minimal soil and environmental costs [
6]. In contrast to the recurrent use of annual crops, the use of intermediate wheatgrass has been suggested to sustain soil fertility through the development of an extensive root system beneficial to a range of soil functions [
6,
7,
8]. The extended lifetime of perennial grains improves the capacity to access soil resources through higher colonization of deep soil horizons [
9,
10,
11] and increased resource allocation towards belowground plant growth [
12]. The use of perennial crops is likely to have a range of effects on ecosystem processes and the provision of ecosystem services due to the large differences in plant traits between annuals and perennials [
13,
14,
15]. These includes the promotion of the soil organic carbon pool [
13,
16,
17], better retention of nutrients [
18], higher water storage capacity and uptakes [
11], improved soil stabilization and aggregation [
19,
20], lower soil disturbance, and a shift of soil microbial communities [
13,
19,
21,
22].
Greater investment in belowground biomass with intermediate wheatgrass has been demonstrated several times [
12,
23], including greater root carbon and nitrogen content, and was associated with benefits observed on leaching reduction and enhanced soil microbiota [
9,
22,
24,
25]. In spite of the increasing literature on perennial grains and intermediate wheatgrass, studies have not thoroughly investigated issues associated with rooting patterns. Data on intermediate wheatgrass often involve aging stands (>two years old) rather than new plantings (<two years old). Compared to long-term grasslands [
13,
19], the integration of a perennial grain into grain crop rotation may be potentially implemented over a short timeframe (2 to 3 years as a maximum). This would likely lead to a smaller impact on soil processes and properties [
26]. Therefore, it remains uncertain if the short-term use of a perennial grain can effectively enable increased belowground productivity, influence soil microbiology, or allow the establishment of root-microbe symbiosis, which are needed to confer the benefits of intermediate wheatgrass on the soil. The value of integrating a perennial phase within a grain crop rotation is then strongly dependent on the rapidity of rooting system development and its capacity to sustain soil services.
The amount and timing of these benefits are currently the cornerstone to ensure desirable and profitable use of intermediate wheatgrass, especially as grain yields are much lower compared to annual counterparts and might counteract the potential benefits to the soil [
5,
9,
27]. The dynamic of perennial rooting systems in cropping systems is therefore critical to designing the ‘safe operating space’ [
26] that takes advantage of a maximum services while limiting drawbacks from disservices (e.g., grain yields penalties).
In studying rooting systems, morphological root traits (e.g., specific length, length density, tissue density, diameter, vertical distribution) are used and recognized as good indicators of plant–soil processes (e.g., exudation, water and nutrients uptakes, tissue decomposability) and are useful in discussing the ecosystem services likely to be provided by plant communities (e.g., soil aggregation, water retention, carbon storage) (
Figure 1) [
28,
29,
30,
31,
32]. Complementarily, soil microbial indicators have been used to inform about soil–root interactions. Microbial biomass, community structure and catabolic diversity are particularly used to investigate carbon and nitrogen cycles and used as proxies for inputs of litter and root-derived compounds to soil [
33,
34,
35,
36,
37]. Mycorrhizal fungi are beneficial root symbionts that participate in root system functioning and impact the soil through its hyphal network [
38,
39,
40].
Here, we investigated root system development and changes in root traits of young stands of intermediate wheatgrass, from establishment to the next cropping season, comparing them to a continual annual grains crop rotation, to assess the potential to improve soil components within a limited period after establishment. We determined how rooting pattern, early root development, and a range of root traits differ in perennial and annual crops, in the topsoil to deeper soil layers. We further tested the impact of root systems on soil microbial communities, including arbuscular mycorrhiza, through the evaluation of microbial indicators. More specifically, we hypothesized that intermediate wheatgrass would demonstrate, first, a denser and deeper rooting pattern as compared to annual grains, indicating higher belowground investments. Additionally, under young perennial plants there would already be an increase in microbial organism groups, indicating an improvement in soil functioning.